WEBVTT 00:00:01.000 --> 00:00:04.000 So, when I first came to town, the Red Sox hadn't won for 42 years. 00:00:04.000 --> 00:00:08.000 Now it's been 86 years, so the whole town all of a sudden changes 00:00:08.000 --> 00:00:13.000 its psychology. A time for great celebration, 00:00:13.000 --> 00:00:17.000 at least for you Red Sox fans. At the, after last lecture I had an 00:00:17.000 --> 00:00:21.000 interesting conversation with one of you. One of you came up to me and 00:00:21.000 --> 00:00:26.000 said very politely, Professor Weinberg, do you have some 00:00:26.000 --> 00:00:30.000 lecture notes that you have written out what the important points of the 00:00:30.000 --> 00:00:35.000 lecture are that you could give me or that you could give us? 00:00:35.000 --> 00:00:39.000 And I said, you know, I, I'm not really interested in 00:00:39.000 --> 00:00:43.000 doing that. And the reason I'm not really interested in doing that is 00:00:43.000 --> 00:00:47.000 that the main function of this course is to enable you to listen to, 00:00:47.000 --> 00:00:51.000 when somebody is talking about conceptually complex things and to 00:00:51.000 --> 00:00:55.000 distil what's being said and to figure out in your own mind what's 00:00:55.000 --> 00:00:59.000 important and what is just another piece of drivel coming out of my 00:00:59.000 --> 00:01:04.000 mouth or Eric Lander's mouth. In fact, speaking for him, 00:01:04.000 --> 00:01:08.000 if I will allow, can allow myself, and myself, in the end we don't 00:01:08.000 --> 00:01:12.000 really care that much whether you know the difference between DNA and 00:01:12.000 --> 00:01:16.000 RNA or proteins and phospholipids. What we're really interested in 00:01:16.000 --> 00:01:20.000 doing is to use this course as a vehicle for pushing you to get your 00:01:20.000 --> 00:01:24.000 brains functioning even better than they already are. 00:01:24.000 --> 00:01:28.000 And, therefore, if you learn in this course how to, 00:01:28.000 --> 00:01:32.000 to think about very complex subjects and figure out what's really going 00:01:32.000 --> 00:01:36.000 on then this course will have more than paid for itself in terms of the 00:01:36.000 --> 00:01:41.000 energy you put into it. So, that's part of the reason why we 00:01:41.000 --> 00:01:46.000 don't provide you with lecture outlines. We want you to figure out 00:01:46.000 --> 00:01:51.000 what's important on your own. Being able to do so will itself be 00:01:51.000 --> 00:01:55.000 a major triumph. At the end of our discussion last 00:01:55.000 --> 00:02:00.000 time we talked about the cell cycle, the fact that it has, has four major 00:02:00.000 --> 00:02:05.000 active phases. G1, S, G2 and mitosis. 00:02:05.000 --> 00:02:09.000 We talked about mitosis. And we talked about the fact that 00:02:09.000 --> 00:02:14.000 when cells emerge from mitosis, from M phase in the absence of 00:02:14.000 --> 00:02:18.000 growth stimulatory factors then they go into G zero. 00:02:18.000 --> 00:02:23.000 And if they're provided with growth stimulatory factors then they'll go 00:02:23.000 --> 00:02:27.000 back into the active cell cycle. And this G zero phase we talked 00:02:27.000 --> 00:02:32.000 about is a quiescence phase, it's a resting phase. 00:02:32.000 --> 00:02:36.000 In fact, there are two kinds of quiescent cells in our bodies. 00:02:36.000 --> 00:02:40.000 Those that go into quiescence, non-growth reversibly, and are able 00:02:40.000 --> 00:02:44.000 to reemerge back into the active cell cycle, and those cells which 00:02:44.000 --> 00:02:48.000 have irreversibly retreated from the active cell cycle. 00:02:48.000 --> 00:02:52.000 So, for example, there are many cells in our brain where no matter 00:02:52.000 --> 00:02:56.000 what you do to them they will never be able to go back into the active 00:02:56.000 --> 00:03:00.000 cell cycle. And they are, in that sense, considered to be 00:03:00.000 --> 00:03:04.000 post-mitotic. Post-mitotic meaning that they will 00:03:04.000 --> 00:03:09.000 never again grow. It's really not clear which of the 00:03:09.000 --> 00:03:13.000 post-mitotic cells, which tissues in the body harbor, 00:03:13.000 --> 00:03:18.000 harbor post-mitotic cells. We used to think that most of the 00:03:18.000 --> 00:03:23.000 differentiated cells in our body are post-mitotic. And when I say 00:03:23.000 --> 00:03:28.000 differentiated, a topic we will not get into for the 00:03:28.000 --> 00:03:33.000 moment, what I mean is that different cells in different parts 00:03:33.000 --> 00:03:37.000 of the body have become differentiated by becoming very 00:03:37.000 --> 00:03:42.000 specialized to become neurons or becoming liver cells or to becoming 00:03:42.000 --> 00:03:46.000 skin cells and so forth. And it is probably the case that 00:03:46.000 --> 00:03:50.000 there are many kinds of differentiated cells in the body 00:03:50.000 --> 00:03:54.000 which once they differentiate will no longer enter into the active cell 00:03:54.000 --> 00:03:58.000 cycle. Now, getting back to this. 00:03:58.000 --> 00:04:02.000 As I also mentioned, if we look at a Petri dish then and 00:04:02.000 --> 00:04:06.000 we put cells like fibroblast, connective tissue cells in the 00:04:06.000 --> 00:04:10.000 bottom of the Petri dish, as I told you last time, if you 00:04:10.000 --> 00:04:14.000 provide those cells with medium which contains all the requisite 00:04:14.000 --> 00:04:18.000 nutrients, these cells will sit here happily for an extended period of 00:04:18.000 --> 00:04:22.000 time but will not proliferate. However, if you add to their serum, 00:04:22.000 --> 00:04:26.000 add their medium, you add serum to this medium. Serum usually comes 00:04:26.000 --> 00:04:30.000 from cows, so therefore it's called bovine serum. 00:04:30.000 --> 00:04:34.000 Then the serum, in addition to the nutrients present 00:04:34.000 --> 00:04:39.000 in the, in the medium will indeed provoke these cells to proliferate, 00:04:39.000 --> 00:04:43.000 and there are agents in the serum, in fact there are agents which are 00:04:43.000 --> 00:04:48.000 called growth factors which are contained within the serum which are 00:04:48.000 --> 00:04:53.000 responsible for inducing these, these cells to begin to proliferate. 00:04:53.000 --> 00:04:57.000 Well, it's instructive just for a moment to step back and ask 00:04:57.000 --> 00:05:01.000 ourselves what actually is serum? How do you get serum? 00:05:01.000 --> 00:05:05.000 And the way you get serum is you take blood and you allow it to clot. 00:05:05.000 --> 00:05:09.000 And when blood clots the platelets in the, in the blood, 00:05:09.000 --> 00:05:13.000 platelets are small cellular fragments, they have an intact 00:05:13.000 --> 00:05:16.000 plasma membrane but they're just, they're very tiny, they don't have a 00:05:16.000 --> 00:05:20.000 nucleus, and these A, A nuclear fragments, these platelets, 00:05:20.000 --> 00:05:24.000 when blood is induced to clot the platelets aggregate with one another 00:05:24.000 --> 00:05:28.000 and you form a big clump, and that, a clot settles to the 00:05:28.000 --> 00:05:32.000 bottom of the test-tube. But in the context of wounding, 00:05:32.000 --> 00:05:36.000 let's say you make a cut on your skin, what happens is that blood 00:05:36.000 --> 00:05:40.000 rushes into the site of, of the, the cut or the wound, 00:05:40.000 --> 00:05:44.000 clotting occurs in order to create coagulation. And why is there 00:05:44.000 --> 00:05:48.000 coagulation? In order to staunch the bleeding. In order to prevent 00:05:48.000 --> 00:05:52.000 there to be further hemorrhage, further loss of blood. But at the 00:05:52.000 --> 00:05:56.000 same time, as the platelets are aggregating to help form the 00:05:56.000 --> 00:06:00.000 structure of the clot that, that creates a physical barrier to 00:06:00.000 --> 00:06:04.000 prevent further bleeding, simultaneously the platelets are 00:06:04.000 --> 00:06:08.000 releasing a lot of growth factors into the medium around them. 00:06:08.000 --> 00:06:13.000 Why are they doing that? Because what's happening 00:06:13.000 --> 00:06:18.000 simultaneous with stopping the bleeding is that the platelets are, 00:06:18.000 --> 00:06:23.000 are releasing growth factors which are used in order to begin to 00:06:23.000 --> 00:06:28.000 reconstruct and heal the site of wounding. Consequently, 00:06:28.000 --> 00:06:33.000 what happens is that the platelets release growth factors. 00:06:33.000 --> 00:06:37.000 These growth factors stimulate cells right around the sides of the wound 00:06:37.000 --> 00:06:42.000 which are still viable and have not been destroyed to begin to 00:06:42.000 --> 00:06:46.000 proliferate in order to reconstruct and intact tissue. 00:06:46.000 --> 00:06:51.000 One of the most important of these, of these, of these factors that they 00:06:51.000 --> 00:06:55.000 release is platelet-derived growth factor. Remember GF is 00:06:55.000 --> 00:07:00.000 just growth factor. And platelet-derived growth factor, 00:07:00.000 --> 00:07:04.000 or as it's called in the trade PDGF, I mentioned it briefly last time, is 00:07:04.000 --> 00:07:09.000 a very potent mitogen. A mitogen is a growth stimulatory 00:07:09.000 --> 00:07:14.000 agent. It's an important mitogen for fibroblasts. 00:07:14.000 --> 00:07:18.000 Fibroblasts, as you recall, are the connective tissue, the 00:07:18.000 --> 00:07:23.000 connective tissue cells that are found throughout the body. 00:07:23.000 --> 00:07:28.000 And, therefore, if you were to, for example, add platelet-derived 00:07:28.000 --> 00:07:32.000 growth factor, PDGF to those fibroblasts that were 00:07:32.000 --> 00:07:37.000 sitting there in G zero, PDGF will stimulate the fibroblasts 00:07:37.000 --> 00:07:42.000 to begin to enter into the active cell cycle, to exit from G zero, 00:07:42.000 --> 00:07:47.000 to enter into G1, and therefore to complete a, a full cell cycle. 00:07:47.000 --> 00:07:51.000 And, by the way, recall what I said before, 00:07:51.000 --> 00:07:55.000 that after the cells leave the active, leave G zero and move 00:07:55.000 --> 00:07:59.000 throughout the active cell cycle and they have a lot of growth factors, 00:07:59.000 --> 00:08:03.000 they'll go all the way around the active growth cycle to mitosis. 00:08:03.000 --> 00:08:07.000 And when they emerge from mitosis, once again they'll ask themselves 00:08:07.000 --> 00:08:12.000 the question whether it's a good idea to continue to be in the active 00:08:12.000 --> 00:08:16.000 cell cycle or whether they should exit into G zero, 00:08:16.000 --> 00:08:21.000 perhaps doing so reversibly. Interestingly, if you look at the 00:08:21.000 --> 00:08:25.000 way that the cell cycle is organized then what you see is the following, 00:08:25.000 --> 00:08:30.000 if I can draw the cell cycle again. Here's G1. 00:08:30.000 --> 00:08:34.000 Here's S phase. And here's G2. And right at, 00:08:34.000 --> 00:08:38.000 at a distinct point toward the end of G1 is something called the 00:08:38.000 --> 00:08:42.000 restriction point, which is going to be very 00:08:42.000 --> 00:08:46.000 interesting shortly. And what happens is after cells 00:08:46.000 --> 00:08:50.000 emerge from G, from mitosis and they move 00:08:50.000 --> 00:08:54.000 throughout the, the most of G1 they're continually 00:08:54.000 --> 00:08:58.000 assessing their extracellular environment to determine whether or 00:08:58.000 --> 00:09:02.000 not there are enough growth factors around to justify their continuing 00:09:02.000 --> 00:09:07.000 the rest of the cell cycle. And ultimately they'll reach this 00:09:07.000 --> 00:09:11.000 restriction point, or as it's sometimes called the R 00:09:11.000 --> 00:09:15.000 point, and here they will make the final go versus no-go decision. 00:09:15.000 --> 00:09:19.000 So, if there have historically been enough growth factors from the 00:09:19.000 --> 00:09:23.000 beginning of G1 all the way to the R point then cells will commit 00:09:23.000 --> 00:09:27.000 themselves essentially irreversibly to going through the entire rest of 00:09:27.000 --> 00:09:31.000 the cell cycle, through M. Conversely, if cells reach up to 00:09:31.000 --> 00:09:35.000 this R point and they calculate that there are enough, 00:09:35.000 --> 00:09:39.000 there are not enough mitogenic growth factors to justify 00:09:39.000 --> 00:09:43.000 proliferation then they'll jump out of the active cell cycle and go back 00:09:43.000 --> 00:09:46.000 to G zero. What that means, in effect, is as follows. Once the 00:09:46.000 --> 00:09:50.000 cells have passed through the R point and they're over here and they 00:09:50.000 --> 00:09:54.000 are committed to complete the rest of the cell cycle then you can take 00:09:54.000 --> 00:09:58.000 growth factors out of their medium and they don't care because they 00:09:58.000 --> 00:10:02.000 only want to receive these stimulatory signals. 00:10:02.000 --> 00:10:06.000 They only care about in this window of time. Hereafter, 00:10:06.000 --> 00:10:11.000 they're committed essentially irreversibly to go through the rest 00:10:11.000 --> 00:10:15.000 of the cell cycle. There are, as it turns out, 00:10:15.000 --> 00:10:20.000 also growth inhibitory factors. So, here we've been talking about 00:10:20.000 --> 00:10:25.000 mitogens, the growth inhibitory factors. So, an important growth 00:10:25.000 --> 00:10:30.000 inhibitory factor is, for example, TGF beta. 00:10:30.000 --> 00:10:34.000 And TGF beta works exactly opposite to PDGF because it is a single which 00:10:34.000 --> 00:10:38.000 is present in extracellular space and tells the cell it should stop 00:10:38.000 --> 00:10:42.000 proliferating. And, therefore, 00:10:42.000 --> 00:10:47.000 TGF beta, if it's present in large amounts in this part of the cell 00:10:47.000 --> 00:10:51.000 cycle, if the cell experiences it in large amounts, 00:10:51.000 --> 00:10:55.000 that will influence the cell not to move through the restriction point. 00:10:55.000 --> 00:10:59.000 Conversely, if it's absent then obviously PDGF can have the, 00:10:59.000 --> 00:11:04.000 the undiluted tensions of the cell. And, therefore, 00:11:04.000 --> 00:11:08.000 what I'm trying to convey by this is to tell you that cell balances both 00:11:08.000 --> 00:11:12.000 its growth stimulatory and growth inhibitory signals that it's 00:11:12.000 --> 00:11:16.000 receiving from extracellular space, these decisions are weighed, and 00:11:16.000 --> 00:11:20.000 finally down here the cell with make the, the binary decision to go ahead 00:11:20.000 --> 00:11:24.000 or not to go ahead, depending on historically how many 00:11:24.000 --> 00:11:29.000 of these factors its recruited in this specific window of time. 00:11:29.000 --> 00:11:33.000 Recall, as we said last time, that once a growth factor like PDGF 00:11:33.000 --> 00:11:37.000 goes to the plasma membrane it encounters a receptor on the surface 00:11:37.000 --> 00:11:42.000 which let's say we call the PDGF receptor. And I'll just draw it 00:11:42.000 --> 00:11:46.000 like this for the moment. It's a transmembrane protein. 00:11:46.000 --> 00:11:51.000 The extracellular domain is on the outside. And I'm drawing two copies 00:11:51.000 --> 00:11:55.000 of the PDGF receptor here for reasons that will become 00:11:55.000 --> 00:12:00.000 apparent in a moment. And what happens is that PDGF which, 00:12:00.000 --> 00:12:04.000 for example, can be itself a dimer-, it can be a dimeric growth factor. 00:12:04.000 --> 00:12:09.000 So, it has two distinct subunits in it. They're both essentially 00:12:09.000 --> 00:12:13.000 equivalent to one another but it is dimeric. And this dimeric structure, 00:12:13.000 --> 00:12:18.000 PDGF, allows it to bind to two growth factor receptors 00:12:18.000 --> 00:12:22.000 simultaneously. Well, why is that interesting? 00:12:22.000 --> 00:12:27.000 It's interesting for the following reason. 00:12:27.000 --> 00:12:31.000 These transmembrane PDGF receptors, like the ones I've indicated right 00:12:31.000 --> 00:12:35.000 here, they're anchored in the plasma membrane because there's a portion 00:12:35.000 --> 00:12:39.000 of their sequence right in here in the transmembrane domain, 00:12:39.000 --> 00:12:43.000 I'm indicating it here in the orange, which contains highly hydrophobic 00:12:43.000 --> 00:12:47.000 amino acids. And those hydrophobic amino acids obviously love to be in 00:12:47.000 --> 00:12:51.000 this hydrophobic environment of the lipid bilayer and their, 00:12:51.000 --> 00:12:55.000 by they don't, they have no effect at all on whether the PDGF receptors 00:12:55.000 --> 00:13:00.000 can move, can traverse in the plane of the plasma membrane. 00:13:00.000 --> 00:13:03.000 So, the PDGF receptors can move across the face of the plasma 00:13:03.000 --> 00:13:07.000 membrane. These ones can move to the right or the left, 00:13:07.000 --> 00:13:10.000 but they're not going to come in or out because they're anchored in this 00:13:10.000 --> 00:13:14.000 lipid bilayer by this stretch of hydrophobic amino acids. 00:13:14.000 --> 00:13:17.000 Now, what happens interestingly when PDGF, the dimeric receptor 00:13:17.000 --> 00:13:21.000 binds to two of these PDGF receptor molecules, which, 00:13:21.000 --> 00:13:24.000 as I told you, have lateral freedom to translate laterally on the plane 00:13:24.000 --> 00:13:28.000 of a plasma membrane, what happens is it will bind two of 00:13:28.000 --> 00:13:32.000 these receptors. And, in so doing, 00:13:32.000 --> 00:13:36.000 it will pull the two receptor molecules next to one another. 00:13:36.000 --> 00:13:41.000 Previously, they were just floating around in the plane of the plasma 00:13:41.000 --> 00:13:46.000 membrane having bound their ligand, recall that PDGF is considered a 00:13:46.000 --> 00:13:50.000 ligand for the PDGF receptor, having, it will cause these two 00:13:50.000 --> 00:13:55.000 receptor molecules to now become, become pulled very close to one 00:13:55.000 --> 00:14:00.000 another. So, I'll redraw it now like this. 00:14:00.000 --> 00:14:03.000 Now these two receptor molecules look like this, 00:14:03.000 --> 00:14:07.000 they're right, they're cheek by jowl, they're right next to one another, 00:14:07.000 --> 00:14:11.000 and this has some interesting consequences. Of great interest 00:14:11.000 --> 00:14:15.000 here is the affect this has on the ability of the PDGF receptor to emit 00:14:15.000 --> 00:14:18.000 signals into the cytoplasm. Because recall that the end game 00:14:18.000 --> 00:14:22.000 here is always how does the, how does the intracellular part of 00:14:22.000 --> 00:14:26.000 the cell know that this, there's been an encounter with the 00:14:26.000 --> 00:14:30.000 growth factor in the extracellular space? 00:14:30.000 --> 00:14:35.000 And this signal-emitting power to PDGF receptor comes from the fact 00:14:35.000 --> 00:14:40.000 that once these two domains are brought together, 00:14:40.000 --> 00:14:45.000 each of these two domains is able to modify the other and change the 00:14:45.000 --> 00:14:50.000 other, i.e., subunit A modifies subunit B, subunit B modifies 00:14:50.000 --> 00:14:55.000 subunit A. And how is this modification achieved? 00:14:55.000 --> 00:15:00.000 For the follow, it is achieved in the following way. 00:15:00.000 --> 00:15:04.000 That the, this domain, which I've been calling, 00:15:04.000 --> 00:15:08.000 I've just been writing like this, as a rectangle, is actually a 00:15:08.000 --> 00:15:13.000 catalytic agent. It's actually a tyrosine kinase. 00:15:13.000 --> 00:15:17.000 So, it's an enzyme. And a tyrosine kinase is an enzyme that takes the 00:15:17.000 --> 00:15:22.000 gamma phosphate from ATP, the high energy phosphate from APT, 00:15:22.000 --> 00:15:26.000 ATP and transfers it to tyrosine amino acids that are present 00:15:26.000 --> 00:15:31.000 on substrates. So, if here is an amino acid 00:15:31.000 --> 00:15:35.000 sequence in the single letter, letter code, and if we admit that Y 00:15:35.000 --> 00:15:40.000 is the, is the code for tyrosine then if here's a protein that it 00:15:40.000 --> 00:15:44.000 functions as a substrate for a tyrosine kinase, 00:15:44.000 --> 00:15:48.000 a tyrosine kinase will add a phosphate group to the side chain of 00:15:48.000 --> 00:15:53.000 the tyrosine, which I'm not drawing here, but tyrosine has a hydroxyl 00:15:53.000 --> 00:15:57.000 group in its side chain and, therefore, it will phosphorilate 00:15:57.000 --> 00:16:02.000 this tyrosine. That is to say will tetraphosphate 00:16:02.000 --> 00:16:06.000 group do it? It will phosphorilate this tyrosine. 00:16:06.000 --> 00:16:11.000 So, these two rectangles are, in fact, tyrosine kinases. And what 00:16:11.000 --> 00:16:16.000 happens, after the two subunits of the receptor have been brought 00:16:16.000 --> 00:16:20.000 together, is thereafter, what one finds is that each of these 00:16:20.000 --> 00:16:25.000 receptor subunits becomes multiply phosphorilated. 00:16:25.000 --> 00:16:30.000 And each of these lollipops that I'm indicating here are sites where 00:16:30.000 --> 00:16:35.000 there, a tyrosine residue has become phosphorilated. 00:16:35.000 --> 00:16:38.000 In fact, there's a tail of the PDGF receptor that extends even further 00:16:38.000 --> 00:16:42.000 to the cytoplasm which also acquires a number of different phosphates on 00:16:42.000 --> 00:16:46.000 it. And, again, I'd remind us that this 00:16:46.000 --> 00:16:50.000 phosphorylation is really what's often called transphosphorylation 00:16:50.000 --> 00:16:53.000 because each receptor molecule phosphorilates the tyrosine residues 00:16:53.000 --> 00:16:57.000 on the other. Obviously, when these two receptor molecules 00:16:57.000 --> 00:17:01.000 are far apart in the plain of the plasma membrane, 00:17:01.000 --> 00:17:05.000 this transphosphorylation cannot occur. 00:17:05.000 --> 00:17:09.000 But once the two tyrosine kinase residues, once the two tyrosine 00:17:09.000 --> 00:17:13.000 kinases have been brought together, pulled together by the dimerization 00:17:13.000 --> 00:17:17.000 of the receptor, now this cross-phosphorylation, 00:17:17.000 --> 00:17:22.000 on each phosphorylating the other can occur, and soon the receptors 00:17:22.000 --> 00:17:26.000 are highly phosphorilated. All of these phosphate groups, 00:17:26.000 --> 00:17:30.000 to repeat myself, being attached to tyrosine residues in their 00:17:30.000 --> 00:17:35.000 cytoplasmic domains. And this, in turn, 00:17:35.000 --> 00:17:39.000 creates interesting docking sites for a variety of other cytoplasmic 00:17:39.000 --> 00:17:44.000 signaling proteins. And we'll talk about some today and 00:17:44.000 --> 00:17:49.000 next time, but what I want to leave you with is the following impression. 00:17:49.000 --> 00:17:53.000 That after this phosphorylation actually occurs there are a number 00:17:53.000 --> 00:17:58.000 of molecules in the cytoplasm, signaling molecules that have 00:17:58.000 --> 00:18:03.000 affinity for binding these phosphotyrosines. 00:18:03.000 --> 00:18:07.000 When I say phosphotyrosine, obviously, I'm referring to the 00:18:07.000 --> 00:18:11.000 phosphorilated form of tyrosine that's been created by a tyrosine 00:18:11.000 --> 00:18:15.000 kinase enzyme. So, here's one molecule that can 00:18:15.000 --> 00:18:19.000 bind. Here's molecule A combined to one of these phosphates, 00:18:19.000 --> 00:18:23.000 another one combined to this phosphate specifically, 00:18:23.000 --> 00:18:27.000 and each of these molecules, once they're attracted to this 00:18:27.000 --> 00:18:31.000 phosphorilated receptor, can then emit downstream signals, 00:18:31.000 --> 00:18:35.000 send a variety of signals into the cell that ultimately end up in 00:18:35.000 --> 00:18:40.000 persuading the cell to proliferate. 00:18:40.000 --> 00:18:45.000 And so these effects here of growth factors in the G1 phase of the cell 00:18:45.000 --> 00:18:50.000 cycle are mediated by this transmembrane signaling, 00:18:50.000 --> 00:18:55.000 by the activation of these, of this PDGF receptor, for example, 00:18:55.000 --> 00:19:00.000 and by the resulting a release of downstream signals into the cell 00:19:00.000 --> 00:19:05.000 which pursued the cell to proliferate or not to proliferate. 00:19:05.000 --> 00:19:08.000 To be sure the, when platelets clot, 00:19:08.000 --> 00:19:12.000 when platelets aggregate and they release PDGF, they also release 00:19:12.000 --> 00:19:15.000 other kinds of growth factors. For instance, there's another 00:19:15.000 --> 00:19:19.000 growth factor that's called IGF1, insulin-like growth factor, and that 00:19:19.000 --> 00:19:22.000 has its own receptor on the surface of cells. And there are, 00:19:22.000 --> 00:19:26.000 on a cell, hundreds to thousands of these PDGF receptors, 00:19:26.000 --> 00:19:30.000 there are IGF receptors, there are EGF receptors. 00:19:30.000 --> 00:19:34.000 And a cell often will require several distinct kinds of growth 00:19:34.000 --> 00:19:38.000 factor activations in order to proliferate. So, 00:19:38.000 --> 00:19:42.000 this is only a minor part of the entire exposure that a cell 00:19:42.000 --> 00:19:47.000 experiences in the G1 phase of the cell cycle. To elaborate on a point 00:19:47.000 --> 00:19:51.000 that I made last time, an important biological distinction 00:19:51.000 --> 00:19:55.000 between normal cells and cancer cells is the fact that cancer cells 00:19:55.000 --> 00:19:59.000 require relatively little growth factors or in the medium in order to 00:19:59.000 --> 00:20:04.000 proliferate. Normal cells have a very strong 00:20:04.000 --> 00:20:08.000 requirement for growth factors in their medium. And, 00:20:08.000 --> 00:20:13.000 therefore, what we can already imagine is the following kind of 00:20:13.000 --> 00:20:17.000 scenario. That cancer cells have someone deregulated this signaling 00:20:17.000 --> 00:20:21.000 pathway. Somehow they have become independent of the stimulation that 00:20:21.000 --> 00:20:26.000 is normally required, usually required for cells to 00:20:26.000 --> 00:20:30.000 proliferate. And, in fact, we know of several 00:20:30.000 --> 00:20:35.000 different ways by which cancer cells can acquire this independence. 00:20:35.000 --> 00:20:39.000 One of the most important ways, it's, it a really interesting one, 00:20:39.000 --> 00:20:43.000 is here's a cancer cell, which we'll talk about very shortly. 00:20:43.000 --> 00:20:47.000 And what you find in certain kinds of cancer cells is that the cancer 00:20:47.000 --> 00:20:52.000 cells themselves release growth factors into the medium. 00:20:52.000 --> 00:20:56.000 So, there are certain kinds of cancer cells that will release, 00:20:56.000 --> 00:21:00.000 let's say, a growth factor that's like EGF into the medium around it, 00:21:00.000 --> 00:21:05.000 around themselves. Well, you'll say that's kind of 00:21:05.000 --> 00:21:09.000 amusing. But so what. The important part here is that 00:21:09.000 --> 00:21:13.000 these same cancer cells have receptors for TG, 00:21:13.000 --> 00:21:17.000 for EGF on their surface. So, they're producing a growth 00:21:17.000 --> 00:21:21.000 factor and they can also respond to the same growth factor. 00:21:21.000 --> 00:21:25.000 And, therefore, this EGF, once it's released, can swim over 00:21:25.000 --> 00:21:29.000 here, activate the receptor and pursued the cell to start 00:21:29.000 --> 00:21:33.000 proliferating. This is, if you will, 00:21:33.000 --> 00:21:37.000 a positive feedback loop. But note here importantly that this, 00:21:37.000 --> 00:21:41.000 the growth of this cell is not being controlled by growth factors coming 00:21:41.000 --> 00:21:45.000 from cells elsewhere in the tissue or the body. Here we're not talking 00:21:45.000 --> 00:21:49.000 about different cells talking to one another. Here we're talking about a 00:21:49.000 --> 00:21:53.000 monologue where this cell is talking to itself. This is sometimes called 00:21:53.000 --> 00:21:57.000 autocrine signaling and refers to the fact that certain kinds of 00:21:57.000 --> 00:22:02.000 cancer cells are able to make growth factors to which they can respond. 00:22:02.000 --> 00:22:05.000 In fact, in normal tissues it's rare for a single cell type in a normal 00:22:05.000 --> 00:22:09.000 tissue to be able to make a growth factor and to be able to respond to 00:22:09.000 --> 00:22:13.000 the same growth factor. Why can it normally not respond to 00:22:13.000 --> 00:22:17.000 that growth factor? Because it won't make the receptor 00:22:17.000 --> 00:22:21.000 for the growth factor. For example, epithelial cells like 00:22:21.000 --> 00:22:25.000 the cells in your skin or the cells lining the gut, 00:22:25.000 --> 00:22:29.000 they can release PDGF, but they don't have a PDGF receptor 00:22:29.000 --> 00:22:32.000 on their surface. And, therefore, 00:22:32.000 --> 00:22:36.000 even though they release copious amounts of PDGF, 00:22:36.000 --> 00:22:40.000 that will not result in this auto stimulatory proliferation and, 00:22:40.000 --> 00:22:44.000 therefore, you don't have this decontrolled self-proliferation that 00:22:44.000 --> 00:22:48.000 you see often in cancer cells, this autocrine loop. In many kinds 00:22:48.000 --> 00:22:52.000 of cancer cells you have another alteration of this growth factor 00:22:52.000 --> 00:22:56.000 signaling pathway. And here what we see is the 00:22:56.000 --> 00:23:00.000 following. Instead of there being a small number of growth factor 00:23:00.000 --> 00:23:04.000 receptors on the cell surface, now there are ten or twenty or fifty 00:23:04.000 --> 00:23:08.000 times more than are normally present on the cell surface. 00:23:08.000 --> 00:23:11.000 In other words, this growth, the growth factor 00:23:11.000 --> 00:23:14.000 receptors are what is called overexpressed. 00:23:14.000 --> 00:23:23.000 And, therefore, 00:23:23.000 --> 00:23:27.000 a delicate balance is disrupted because normally, 00:23:27.000 --> 00:23:31.000 let's say, a cell with have on its surface 500 EGF receptors, 00:23:31.000 --> 00:23:35.000 but in many kinds of cancers probably 30% or 40% of all 00:23:35.000 --> 00:23:39.000 carcinomas, carcinomas are the tumors from epithelial tissues, 00:23:39.000 --> 00:23:42.000 you'll find overexpressed EGF receptor. Well, 00:23:42.000 --> 00:23:46.000 why is that interesting? It's interesting for the following 00:23:46.000 --> 00:23:50.000 reason. I told you before that the activation of a receptor depends on 00:23:50.000 --> 00:23:54.000 its ligand to persuading two receptor subunits to come together 00:23:54.000 --> 00:23:58.000 and start firing, as we just discussed. 00:23:58.000 --> 00:24:01.000 But if now all of a sudden the cell contains large amounts of this 00:24:01.000 --> 00:24:04.000 growth being expressed, of the growth factor receptor being 00:24:04.000 --> 00:24:07.000 expressed on the surface, ten or a hundred times more than 00:24:07.000 --> 00:24:11.000 normal, then these growth factor receptors are going to be rather 00:24:11.000 --> 00:24:14.000 densely packed on the cell surface. And now they may just come together 00:24:14.000 --> 00:24:17.000 because they happen to bump into each other very frequently. 00:24:17.000 --> 00:24:21.000 They don't need the growth factor to pull them together just because 00:24:21.000 --> 00:24:24.000 there are so many of them. So, there's random interactions, 00:24:24.000 --> 00:24:27.000 random bumping together And these two growth factor 00:24:27.000 --> 00:24:31.000 receptors may just bump together and thereby send signals into the cell 00:24:31.000 --> 00:24:35.000 persuading the cell that there's been some kind of growth factor in 00:24:35.000 --> 00:24:39.000 the extracellular domain that's been encountered when, 00:24:39.000 --> 00:24:42.000 in fact, all that's happened is that there are so many of these growth 00:24:42.000 --> 00:24:46.000 factor receptors around that they're constantly bumping into each other, 00:24:46.000 --> 00:24:50.000 and while they've collided with one another they can activate signaling 00:24:50.000 --> 00:24:54.000 and release growth stimulatory signals into the cell. 00:24:54.000 --> 00:24:58.000 There's another kind of, of alteration of growth factor 00:24:58.000 --> 00:25:03.000 receptors that's also seen in many kinds of human tumors. 00:25:03.000 --> 00:25:08.000 For example, in, in glioblastomas, which is a brain tumor. And a 00:25:08.000 --> 00:25:13.000 glioblastoma has the following kind of, of receptor on the surface. 00:25:13.000 --> 00:25:18.000 It has truncated forms of the EGF receptor on the surface where a lot 00:25:18.000 --> 00:25:22.000 of the ectodomain is simply not present. So, here's the normal EGF 00:25:22.000 --> 00:25:27.000 receptor, here is a truncated EGF receptor where a lot of the 00:25:27.000 --> 00:25:32.000 extracellular domain, which I'm calling the ectodomain, 00:25:32.000 --> 00:25:37.000 has simply been lopped off. How has it be lopped off? 00:25:37.000 --> 00:25:41.000 Well, there's been a mutation it the gene which has, 00:25:41.000 --> 00:25:45.000 in effect, deleted segments in coding the N-terminus of the 00:25:45.000 --> 00:25:49.000 receptor protein, which normally sticks its head out 00:25:49.000 --> 00:25:54.000 of the cell. And now you have these truncated receptors. 00:25:54.000 --> 00:25:58.000 And such truncated EGF receptors are able to fire constitutively. 00:25:58.000 --> 00:26:02.000 Constitutively implies that these receptors are able to fire in a 00:26:02.000 --> 00:26:07.000 fashion that is no longer regulated by physiologic signals. 00:26:07.000 --> 00:26:10.000 It's a high steady rate. So now these receptor molecules, 00:26:10.000 --> 00:26:14.000 these truncated receptor molecules flood the cell with growth 00:26:14.000 --> 00:26:18.000 stimulatory signal and, for reasons that aren't really clear 00:26:18.000 --> 00:26:22.000 to this day, these two, these receptor, truncated receptor 00:26:22.000 --> 00:26:26.000 molecules can dimerize, they can come together even if 00:26:26.000 --> 00:26:30.000 there's no extracellular ligand present, even if there's no growth 00:26:30.000 --> 00:26:34.000 factor in the extracellular space. And we now realize, 00:26:34.000 --> 00:26:38.000 for example, that there are a variety of structurally altered 00:26:38.000 --> 00:26:42.000 receptors that fire constitutively into a cell in many kinds of human 00:26:42.000 --> 00:26:46.000 tumors. And in each case the cancer cell is deluded into thinking that 00:26:46.000 --> 00:26:50.000 some growth factor has been encountered out here when, 00:26:50.000 --> 00:26:54.000 in fact, there's not at all. Once again, what we see is a 00:26:54.000 --> 00:26:58.000 situation in which the cell, the cancer cell is being deluded 00:26:58.000 --> 00:27:03.000 into thinking there's growth factors present, extracellular space. 00:27:03.000 --> 00:27:07.000 None has been present at all. There have been a variety of drugs 00:27:07.000 --> 00:27:12.000 developed against, for example, lung cancer. 00:27:12.000 --> 00:27:17.000 And there are a variety of different kinds of lung cancers. 00:27:17.000 --> 00:27:22.000 One is called non-small cell lung carcinoma. We don't have to deal 00:27:22.000 --> 00:27:26.000 with the subsets of lung cancers. And it turned out, it turned out 00:27:26.000 --> 00:27:31.000 that one of these drugs, it's called Iressa, had very mixed 00:27:31.000 --> 00:27:38.000 effects on patients. 00:27:38.000 --> 00:27:42.000 In about 90% of these, of the class of lung cancers, 00:27:42.000 --> 00:27:47.000 patients that were treated, the drug Iressa, used over the last several 00:27:47.000 --> 00:27:51.000 years, had almost no effect on the tumor treatment and the patients 00:27:51.000 --> 00:27:56.000 continued to, to proceed to their death. It had, 00:27:56.000 --> 00:28:00.000 it really had no effect. But in 10%, in fact, there was some 00:28:00.000 --> 00:28:05.000 dramatic responses and tumors shrunk down. 00:28:05.000 --> 00:28:09.000 Now, normally a 10% response rate would be enough to cause a drug 00:28:09.000 --> 00:28:13.000 company to abandon all further development of the drug because it's 00:28:13.000 --> 00:28:17.000 just too low a response and who wants to take a drug where the 00:28:17.000 --> 00:28:21.000 chances of having a good response are as low as 10%? 00:28:21.000 --> 00:28:25.000 It's just not a good situation. But then some geneticists here in 00:28:25.000 --> 00:28:29.000 Boston, one group at the MGH and another over at the Dana Farber, 00:28:29.000 --> 00:28:33.000 began to look at the lung cancer cells that responded, 00:28:33.000 --> 00:28:37.000 i.e., from tumors of patients that responded and shrank in response to 00:28:37.000 --> 00:28:41.000 the drug and the lung, and the lung cancer cells of 00:28:41.000 --> 00:28:44.000 patients who didn't. It turns out that Iressa is an 00:28:44.000 --> 00:28:48.000 inhibitor of the tyrosine kinase of the EGF receptor. 00:28:48.000 --> 00:28:52.000 That's how it was designed. In other words, this drug, it's a 00:28:52.000 --> 00:28:56.000 low molecular weight drug and it goes into the tyrosine kinase domain, 00:28:56.000 --> 00:29:00.000 that rectangular thing I showed you before very schematically, 00:29:00.000 --> 00:29:04.000 and it shuts down the firing of the receptor. 00:29:04.000 --> 00:29:08.000 That was the motivation behind creating this drug. 00:29:08.000 --> 00:29:13.000 So, Iressa shuts down the EGF receptor and 10% of lung cancer 00:29:13.000 --> 00:29:18.000 patients, their tumor shrank, the other 90% didn't, weren't 00:29:18.000 --> 00:29:22.000 effected at all. And what these two groups of 00:29:22.000 --> 00:29:27.000 geneticists found over the last three or four months is that the 00:29:27.000 --> 00:29:32.000 patients whose tumors responded had tumor cells where the EGF receptor 00:29:32.000 --> 00:29:37.000 was mutated and therefore firing in a constitutively active fashion. 00:29:37.000 --> 00:29:40.000 That is to say there were actually structural alterations in the 00:29:40.000 --> 00:29:44.000 receptor. This is a massive structural alteration of the 00:29:44.000 --> 00:29:48.000 receptor here, this truncation. 00:29:48.000 --> 00:29:52.000 But, in fact, in certain patients the 10% of patients that responded, 00:29:52.000 --> 00:29:56.000 there were much more subtle changes in the cytoplasmic domain of the 00:29:56.000 --> 00:30:00.000 protein which allowed these receptors to constitutively dimerize, 00:30:00.000 --> 00:30:04.000 once again, in a ligand independent fashion. 00:30:04.000 --> 00:30:08.000 So, these subtle mutations mimic, in effect, the consequences of 00:30:08.000 --> 00:30:12.000 deleting or truncating the extracellular domain in that in both 00:30:12.000 --> 00:30:16.000 cases one gets a ligand independent receptor. In those cases where 00:30:16.000 --> 00:30:20.000 these, where the patients had a mutant EGF receptor, 00:30:20.000 --> 00:30:24.000 structurally altered EGF receptor that was firing constitutively, 00:30:24.000 --> 00:30:28.000 there were dramatic responses to the Iressa drug. 00:30:28.000 --> 00:30:31.000 In the 90% of patients where there was no effective response to the 00:30:31.000 --> 00:30:35.000 drug, the EGF receptor was wild-type, it was present in a wild-type 00:30:35.000 --> 00:30:39.000 configuration. It might have been slightly 00:30:39.000 --> 00:30:43.000 overexpressed but it wasn't, but it continued to, to function 00:30:43.000 --> 00:30:46.000 essentially as a normal EGF receptor. And this represents a major advance 00:30:46.000 --> 00:30:50.000 in cancer therapy because it suggests that one has to begin to 00:30:50.000 --> 00:30:54.000 understand what subsets of patients one should treat with a drug which 00:30:54.000 --> 00:30:58.000 can, on its own, have quite toxic effects on the 00:30:58.000 --> 00:31:01.000 patient. And, from now on, 00:31:01.000 --> 00:31:05.000 to state the obvious, when one gets lung cancer patients 00:31:05.000 --> 00:31:09.000 one will check quickly using various reactions, like the PCR reaction, 00:31:09.000 --> 00:31:13.000 to see whether or not their cancer cells have a mutated EGF receptor. 00:31:13.000 --> 00:31:16.000 And, if they do, they will be candidates for Iressa treatment with 00:31:16.000 --> 00:31:20.000 the expectation that 60%, 80% or even 100% of them will have 00:31:20.000 --> 00:31:24.000 tumors that respond. And if they don't have a mutated 00:31:24.000 --> 00:31:28.000 EGF receptor then they will not be subjected to a treatment 00:31:28.000 --> 00:31:31.000 by this drug. This is the beginning of a new era 00:31:31.000 --> 00:31:35.000 of cancer drug treatment. It's called rational drug design, 00:31:35.000 --> 00:31:39.000 or rational treatment, where you don't just lump all the patients 00:31:39.000 --> 00:31:43.000 with a certain disease together and say let's give them all this drug 00:31:43.000 --> 00:31:46.000 and throw things up in the air and see what happens. 00:31:46.000 --> 00:31:50.000 Here one begins to do a genetic diagnosis of the genomes of the 00:31:50.000 --> 00:31:54.000 patient's cancer cells in order to determine whether or not they have 00:31:54.000 --> 00:31:58.000 certain mutated genes. In this case we're referring to one 00:31:58.000 --> 00:32:02.000 of these growth factor receptors. By the way, we're talking about lung 00:32:02.000 --> 00:32:06.000 cancer today, right? If you are smoking now, 00:32:06.000 --> 00:32:10.000 I always ask the class how many people are smoking, 00:32:10.000 --> 00:32:14.000 and nobody has the, has the moral fortitude to raise their hands. 00:32:14.000 --> 00:32:18.000 But if you are smoking now and you started at this age and you continue. 00:32:18.000 --> 00:32:22.000 And, by the way, if you start at your age and you 00:32:22.000 --> 00:32:26.000 continue smoke, stopping smoking is actually a bit 00:32:26.000 --> 00:32:30.000 more difficult, quite a bit more difficult than 00:32:30.000 --> 00:32:34.000 stopping heroine. That's pretty interesting, 00:32:34.000 --> 00:32:38.000 right? So, if you continue to smoke now you will be healthy for a pretty 00:32:38.000 --> 00:32:42.000 long period of time, probably another 20 or 30 years. 00:32:42.000 --> 00:32:46.000 And for you that sounds like forever, but when you get to be 00:32:46.000 --> 00:32:50.000 about 40 or 50 things are going to start falling apart. 00:32:50.000 --> 00:32:54.000 Soon you won't be able to be very athletic, soon your lungs are going 00:32:54.000 --> 00:32:58.000 to be able, are going to degrade, and by the time you reach your 00:32:58.000 --> 00:33:02.000 fifties, sixties or seventies what's going to happen is you will, 00:33:02.000 --> 00:33:06.000 on average, have a six to eight year shortened life expectancy. 00:33:06.000 --> 00:33:09.000 Now you say six to eight years is not that much, 00:33:09.000 --> 00:33:13.000 but it really is. You know, when you get to be 70 and 00:33:13.000 --> 00:33:17.000 you think you're going to die next year or you're going to die in six 00:33:17.000 --> 00:33:20.000 or eight years it makes a big difference. Six to eight years is 00:33:20.000 --> 00:33:24.000 an enormous difference in life expectancy. 20% of all people who 00:33:24.000 --> 00:33:28.000 died last year in this country, 20% of all deaths came from 00:33:28.000 --> 00:33:32.000 cigarette smoking. Imagine that. And when you die from cigarette 00:33:32.000 --> 00:33:36.000 smokes, smoking sometimes you get lung cancer. There probably were, 00:33:36.000 --> 00:33:40.000 I think, 600,000 people who died from smoking last year. 00:33:40.000 --> 00:33:44.000 Six hundred thousand. There were 55,000 American soldiers who died in 00:33:44.000 --> 00:33:48.000 Vietnam in the whole war, there were 220,000 American soldiers 00:33:48.000 --> 00:33:52.000 who died in all of World War II, and last year in this, and there 00:33:52.000 --> 00:33:56.000 were 3,000, or 2, 00 people who died in the World 00:33:56.000 --> 00:34:00.000 Trade Center. All right? Got all those numbers? 00:34:00.000 --> 00:34:05.000 So, last year 600, 00 people died premature deaths 00:34:05.000 --> 00:34:10.000 because they were smoking. How many people died last year from 00:34:10.000 --> 00:34:15.000 smoking marijuana? Maybe two or three, 00:34:15.000 --> 00:34:20.000 I don't know. [APPLAUSE] Am I urging you to do any kind of smoking? 00:34:20.000 --> 00:34:25.000 I'm not saying marijuana smoking is good for you, but I just want you to 00:34:25.000 --> 00:34:30.000 get these things in mind, the perspective. 00:34:30.000 --> 00:34:33.000 If you smoke, you know, in many countries, including this 00:34:33.000 --> 00:34:37.000 one, there isn't much tension by, given by the government to, 00:34:37.000 --> 00:34:40.000 dissuading people from smoking, and here's the reason why. If you 00:34:40.000 --> 00:34:44.000 smoke, and you going to get, get sick eventually, eventually the 00:34:44.000 --> 00:34:47.000 country is always going to have to pay for your medical costs, 00:34:47.000 --> 00:34:51.000 right? Sooner or later we all have to pay for the costs of people who 00:34:51.000 --> 00:34:54.000 get sick. It's all shared in one way or another. 00:34:54.000 --> 00:34:58.000 But it's not such a big problem for a government like the 00:34:58.000 --> 00:35:02.000 American government. Because if you smoke you're going to 00:35:02.000 --> 00:35:06.000 die early enough that you won't draw on social security. 00:35:06.000 --> 00:35:10.000 And, therefore, the government actually saves money by your smoking, 00:35:10.000 --> 00:35:14.000 because by the time they add up how much they get on the tobacco tax and 00:35:14.000 --> 00:35:18.000 how much they earn by your not living long enough to draw a pension, 00:35:18.000 --> 00:35:22.000 it's much better, it's much more money than how much 00:35:22.000 --> 00:35:26.000 it's going to cost to take care of you while you're dying from 00:35:26.000 --> 00:35:30.000 emphysema or bladder cancer or lung cancer or heart disease. 00:35:30.000 --> 00:35:33.000 Many more people die from heart attacks due to smoking than die from 00:35:33.000 --> 00:35:37.000 lung cancer, in fact. So, think about this. 00:35:37.000 --> 00:35:41.000 Think about this. If you smoke it's probably a good time to stop 00:35:41.000 --> 00:35:45.000 because if you continue at your age, especially if you're women, which is 00:35:45.000 --> 00:35:48.000 women, for some reason women have a harder time stopping than men, 00:35:48.000 --> 00:35:52.000 they can't say why. It's probably some physiological thing. 00:35:52.000 --> 00:35:56.000 If you, if you continue now at your age, it will be almost impossible to 00:35:56.000 --> 00:36:00.000 stop. If you live with smokers ask them to leave. [LAUGHTER] 00:36:00.000 --> 00:36:04.000 If you live at home with your parents and they smoke tell them 00:36:04.000 --> 00:36:08.000 it's time for them to leave. Throw them out of the house. 00:36:08.000 --> 00:36:12.000 Smoke, second-hand smoking killed probably between 60, 00:36:12.000 --> 00:36:16.000 00 and 80,000 people last year in this country. Second-hand smoke. 00:36:16.000 --> 00:36:20.000 And, by the way, if you want to see an interesting phenomenon, 00:36:20.000 --> 00:36:24.000 go to a veterinary hospital because there they with great frequently, 00:36:24.000 --> 00:36:28.000 frequency treat dogs who have lung cancer. And why do they 00:36:28.000 --> 00:36:32.000 have lung cancer? Not in 99% of the cases, 00:36:32.000 --> 00:36:36.000 in 100% of the cases these dogs live with owners who smoke. 00:36:36.000 --> 00:36:40.000 An average tobacco smoker goes through six or eight dogs in his or 00:36:40.000 --> 00:36:44.000 her lifetime. [LAUGHTER] It's true. It's absolutely true. 00:36:44.000 --> 00:36:49.000 So if you, if you think the dogs, if that's a fact for the, for the, 00:36:49.000 --> 00:36:53.000 for the dog owners, think about what's happening to the 00:36:53.000 --> 00:36:57.000 inside of your lungs. And so I'm going to take back what 00:36:57.000 --> 00:37:02.000 I said before. Before I told you that the most 00:37:02.000 --> 00:37:06.000 important thing for you to do in this course is to learn how to think 00:37:06.000 --> 00:37:10.000 clearly and to assess and to distil conceptually complex material, 00:37:10.000 --> 00:37:14.000 there's actually one more thing that's even more important to get 00:37:14.000 --> 00:37:18.000 out of this course, if you do, and that is to stop 00:37:18.000 --> 00:37:22.000 smoking. If you do that, if you do that it'll be vastly more 00:37:22.000 --> 00:37:26.000 important for the rest of your life than anything you learn here. 00:37:26.000 --> 00:37:30.000 So, write that down, vastly more important. You may think it's 00:37:30.000 --> 00:37:34.000 glamorous, you may think it's exciting, but keep in mind, 00:37:34.000 --> 00:37:38.000 people who stop smoking have vastly greater effects on reducing the 00:37:38.000 --> 00:37:42.000 morbidity and the mortality in this country than anything that cancer 00:37:42.000 --> 00:37:46.000 researchers can do. Keep that in mind. 00:37:46.000 --> 00:37:50.000 And if you start smoking now and you think that somehow the cancer 00:37:50.000 --> 00:37:53.000 researchers are going to be able to come up with some miracle cure by 00:37:53.000 --> 00:37:56.000 this time you start coughing and start spitting up blood, 00:37:56.000 --> 00:38:00.000 don't be so certain. They may not be able to save you, 00:38:00.000 --> 00:38:04.000 to pull your fat out of the fire. So, I don't know whether I, 00:38:04.000 --> 00:38:08.000 I gave this message in a very subtle way or I hit you over the head with 00:38:08.000 --> 00:38:12.000 it. [LAUGHTER] But think, think about that. Now, now we're 00:38:12.000 --> 00:38:16.000 going to focus on lung cancer, we're going to focus on cancer 00:38:16.000 --> 00:38:20.000 because it's one of the consequences of cigarette smoking, 00:38:20.000 --> 00:38:24.000 but it's a disease we want to talk about both this time and next time, 00:38:24.000 --> 00:38:28.000 and we want to relate it here to the cell cycle and, 00:38:28.000 --> 00:38:32.000 and how the growth of cell proliferation occurs. 00:38:32.000 --> 00:38:36.000 I told you last time that a human tumor is roughly-speaking about, 00:38:36.000 --> 00:38:40.000 a human body roughly carries three times ten to the thirteenth cells. 00:38:40.000 --> 00:38:44.000 So, that's quite a few cells. That's how many cells there are in 00:38:44.000 --> 00:38:48.000 the human body, plus or minus. A human tumor of one, 00:38:48.000 --> 00:38:52.000 let's say one cubic centimeter is roughly ten to the ninth cells. 00:38:52.000 --> 00:38:56.000 So, a tiny tumor this way already has a billion cells in it. 00:38:56.000 --> 00:39:00.000 And what I want to say is that those billion cells, 00:39:00.000 --> 00:39:04.000 or if the tumor grows larger to ten to a hundred billion cells, 00:39:04.000 --> 00:39:09.000 it's still not that much compared with the overall size of the body. 00:39:09.000 --> 00:39:14.000 But tumors of that size can kill you. And one, an interesting and 00:39:14.000 --> 00:39:19.000 important thing to realize about all the cancer cells in that tumor mass 00:39:19.000 --> 00:39:24.000 is that they all descend from a single progenitor. 00:39:24.000 --> 00:39:29.000 In other words, if we imagine a situation where here are a whole 00:39:29.000 --> 00:39:34.000 bunch of normal cells and here's the boundary between normalcy up here 00:39:34.000 --> 00:39:40.000 and malignancy, malignancy obviously refers to 00:39:40.000 --> 00:39:45.000 cancer, we could imagine where there are many cells which independently 00:39:45.000 --> 00:39:50.000 cross the boundary from one to the other and become the progenitors of 00:39:50.000 --> 00:39:55.000 a vast tumor mass. But that's not what happens. 00:39:55.000 --> 00:40:00.000 What happens, in fact, is that only one cell gets converted or becomes, 00:40:00.000 --> 00:40:05.000 as one says here, it becomes transformed from a normal cell into 00:40:05.000 --> 00:40:10.000 a cancer cell. And this transformation causes this 00:40:10.000 --> 00:40:15.000 one cell to become the progenitor, the ancestor of all the cells in the 00:40:15.000 --> 00:40:20.000 tumor mass. So, one important realization we have 00:40:20.000 --> 00:40:25.000 about looking at different tumors is that cancers are monoclonal growths, 00:40:25.000 --> 00:40:30.000 i.e., they form clonal populations. They're monoclonal in the sense that 00:40:30.000 --> 00:40:35.000 they all are genetically derived from a single common ancestor rather 00:40:35.000 --> 00:40:40.000 than being polyclonal. What else can we say about cancer 00:40:40.000 --> 00:40:44.000 cells or the cells in a tumor? If you take cells out of Petri dish, 00:40:44.000 --> 00:40:48.000 out of an animal or a human and put them on a Petri dish, 00:40:48.000 --> 00:40:52.000 excuse me, and you put them there, then what you'll see is following 00:40:52.000 --> 00:40:56.000 interesting behavior. If you put normal cells in a Petri 00:40:56.000 --> 00:41:00.000 dish they'll grow across the bottom of the Petri dish until they cover 00:41:00.000 --> 00:41:04.000 the entire bottom of the Petri dish. 00:41:04.000 --> 00:41:08.000 So, you can put a hundred cells in and they'll continue to proliferate. 00:41:08.000 --> 00:41:12.000 Let's look at the Petri dish from top down, so you might have a small 00:41:12.000 --> 00:41:16.000 number of cells here and here, and normal cells will continue to 00:41:16.000 --> 00:41:21.000 proliferate until they begin to touch one another, 00:41:21.000 --> 00:41:25.000 and then they'll stop growing. And this stopping of growing is, 00:41:25.000 --> 00:41:30.000 is the phenomenon that's called contact inhibition. 00:41:30.000 --> 00:41:33.000 So, a normal cell will indicate contact inhibition. 00:41:33.000 --> 00:41:37.000 And to state, to state the obvious, this phenomenon or this behavior of 00:41:37.000 --> 00:41:40.000 contact inhibition creates what's called a cell monolayer because if 00:41:40.000 --> 00:41:44.000 the cell stopped growing once they touch each other they're not going 00:41:44.000 --> 00:41:47.000 to be two or three or four layers of cells in the Petri dish. 00:41:47.000 --> 00:41:51.000 So, here we're looking at this Petri dish in cross-section and 00:41:51.000 --> 00:41:55.000 there's a monolayer of normal cells here. 00:41:55.000 --> 00:41:58.000 If you put a cancer cell in the Petri dish, or let's put here a 00:41:58.000 --> 00:42:02.000 cancer cell, we'll seed it amidst normal cells, what will happen is 00:42:02.000 --> 00:42:06.000 that the cancer cell lacks, has lost contact inhibition, 00:42:06.000 --> 00:42:09.000 and the cancer cell will continue to proliferate even after it's touched 00:42:09.000 --> 00:42:13.000 its neighbors. So, it has lost cancer, 00:42:13.000 --> 00:42:17.000 it has lost contact inhibition and will start growing on top of, 00:42:17.000 --> 00:42:20.000 the cancer cells will start growing on top of each other because they 00:42:20.000 --> 00:42:24.000 don't mind growing in spite of their having intimate contact with 00:42:24.000 --> 00:42:28.000 neighboring cells. And, in fact, what you can do is 00:42:28.000 --> 00:42:32.000 the following experiment. You can put cells in a Petri dish 00:42:32.000 --> 00:42:37.000 like this, a whole bunch of normal cells in a Petri dish, 00:42:37.000 --> 00:42:42.000 and then you can put into them some kind of transforming influence, 00:42:42.000 --> 00:42:47.000 which we'll talk about very shortly, i.e., you can influence some of 00:42:47.000 --> 00:42:52.000 these cells to become transformed. And how we do so we'll tell, we'll 00:42:52.000 --> 00:42:57.000 hold in abeyance just for a moment. So, we'll have this cell. We'll do 00:42:57.000 --> 00:43:02.000 this cell to become transformed and this cell to become transformed. 00:43:02.000 --> 00:43:05.000 And what will happen is that those cells will begin to form a very 00:43:05.000 --> 00:43:09.000 thick clump of cells, these blue ones, the ones that are 00:43:09.000 --> 00:43:13.000 transformed. Whereas, all the other cells will grow until 00:43:13.000 --> 00:43:16.000 they form a monolayer at which point they'll stop proliferating. 00:43:16.000 --> 00:43:20.000 So, the cancer cells keep piling up and the transformed cells, 00:43:20.000 --> 00:43:24.000 transformed by one or another agent, we won't talk about that yet, 00:43:24.000 --> 00:43:28.000 continue to proliferate long after the normal cells have stopped 00:43:28.000 --> 00:43:31.000 proliferating. The normal cells having stopped 00:43:31.000 --> 00:43:35.000 proliferating because they're contact inhibited. 00:43:35.000 --> 00:43:38.000 And, therefore, they'll form this clump of cells which we'll call a 00:43:38.000 --> 00:43:42.000 focus. And if you hold the Petri dish up to the light and you look at 00:43:42.000 --> 00:43:45.000 it and there are some transformed cells present, 00:43:45.000 --> 00:43:49.000 you can see the foci, they're very thick. Whereas, 00:43:49.000 --> 00:43:52.000 the thin monolayer of cells will just look pretty transparent. 00:43:52.000 --> 00:43:56.000 But this focus will look highly opaque by virtue of the multiple 00:43:56.000 --> 00:44:00.000 layers of cells that are involved in it. 00:44:00.000 --> 00:44:05.000 Now, in fact, we can begin to ask the question of how and why cells 00:44:05.000 --> 00:44:10.000 like this become transformed and exhibit this behavior. 00:44:10.000 --> 00:44:16.000 In fact, until the 1980s there wasn't really a clear understanding 00:44:16.000 --> 00:44:21.000 about how that happened. I've already given you some clues 00:44:21.000 --> 00:44:27.000 because I've already told you the fact that certain cancer cells carry 00:44:27.000 --> 00:44:32.000 mutant genes. What kind of mutant genes? 00:44:32.000 --> 00:44:36.000 Well, I gave you, in anticipation of discussion, 00:44:36.000 --> 00:44:40.000 the fact that certain cancer cells carry mutant genes that specify 00:44:40.000 --> 00:44:44.000 mutant growth factor receptors. And these mutant growth factor 00:44:44.000 --> 00:44:49.000 receptors, as I indicated, begin to push the cell the 00:44:49.000 --> 00:44:53.000 proliferate. And so that already anticipates a conclusion we're about 00:44:53.000 --> 00:44:57.000 to make, which is that the reason why cancer cells behave abnormally 00:44:57.000 --> 00:45:02.000 is that they carry mutant genes. Now, let's talk about the nature of 00:45:02.000 --> 00:45:07.000 these mutant genes because, if you look at these mutant genes, 00:45:07.000 --> 00:45:12.000 invariably they are the consequences of what we call somatic mutations. 00:45:12.000 --> 00:45:17.000 By that I mean, I mean the following. Let's say we all start 00:45:17.000 --> 00:45:22.000 out with a really good set of genes. And, thank the good Lord, we all do. 00:45:22.000 --> 00:45:27.000 But as we go through life through accidents or through intent we can 00:45:27.000 --> 00:45:32.000 damage these genes. We can muck them up in different 00:45:32.000 --> 00:45:36.000 ways. And these genes, this damage may occur to cells in 00:45:36.000 --> 00:45:41.000 the skin, they may occur to cells in your belly, they may occur to cells 00:45:41.000 --> 00:45:45.000 in the brain, and these are called somatic mutations in contrast to the 00:45:45.000 --> 00:45:50.000 germline mutations that affect one's offspring. Because, 00:45:50.000 --> 00:45:54.000 as you must realize by now, the only way you can have mutations 00:45:54.000 --> 00:45:59.000 that affect your descendents is if those mutations strike in the gonads 00:45:59.000 --> 00:46:04.000 and affect the genomes of either sperm or egg. 00:46:04.000 --> 00:46:08.000 But the mutations occurring everywhere else in the body outside 00:46:08.000 --> 00:46:12.000 of the gonads, because they occur in, 00:46:12.000 --> 00:46:16.000 I think I, this is somatic, excuse me, because they occur in the 00:46:16.000 --> 00:46:21.000 soma, the soma is defined as the entirety of the body outside of the 00:46:21.000 --> 00:46:25.000 gonads, outside of the germi, these somatic mutations might affect 00:46:25.000 --> 00:46:29.000 the tissues around them but they will not be transmitted from one 00:46:29.000 --> 00:46:34.000 organismic generation to the next. And, accordingly, 00:46:34.000 --> 00:46:38.000 we begin to imagine, in fact, correctly we begin to 00:46:38.000 --> 00:46:42.000 construct this model that one of the most important mechanisms of 00:46:42.000 --> 00:46:46.000 creating a cancer cell is to damage its genome. So, 00:46:46.000 --> 00:46:50.000 I'll tell you a story now, which I'm only going to finish on 00:46:50.000 --> 00:46:54.000 Monday. We have, let's imagine, a 55-year-old, 00:46:54.000 --> 00:46:58.000 this is a true story, 55-year-old man who's been smoking since he or 00:46:58.000 --> 00:47:02.000 she, he was 15 years old. So, he's been smoking for 40 years. 00:47:02.000 --> 00:47:06.000 And during those periods of 40 years, by the way, I'm not saying whether 00:47:06.000 --> 00:47:10.000 I'm for or against smoking, you understand that. During that 00:47:10.000 --> 00:47:15.000 period of 40 years this person has been introducing large amounts of 00:47:15.000 --> 00:47:19.000 tobacco smoke compounds into his lungs. Now, it turns out that these 00:47:19.000 --> 00:47:23.000 compounds, this tobacco smoke compounds are carcinogens. 00:47:23.000 --> 00:47:28.000 You know carcinogen means it causes cancer. But it happens also to be 00:47:28.000 --> 00:47:32.000 the case that a lot of carcinogens, cancer-causing compounds are also 00:47:32.000 --> 00:47:37.000 mutagens. That is to say they can mutate DNA. 00:47:37.000 --> 00:47:41.000 So, here we have a scenario that we're going to set up for next time. 00:47:41.000 --> 00:47:46.000 55-year-old man. Smokes for 40 years. Dumps a lot of carcinogens 00:47:46.000 --> 00:47:50.000 into his lungs. The carcinogens which are highly, 00:47:50.000 --> 00:47:55.000 which induce mutations very potently are passed from his lungs into his 00:47:55.000 --> 00:48:00.000 blood and there go from the blood into the kidneys. 00:48:00.000 --> 00:48:04.000 And from the kidneys they are excreted into the urine and then 00:48:04.000 --> 00:48:09.000 they sit around in the bladder for a while. And let's imagine now that 00:48:09.000 --> 00:48:13.000 the urine of this man has all of these highly mutation-active 00:48:13.000 --> 00:48:18.000 carcinogens in his urine, which, in principle, can begin now 00:48:18.000 --> 00:48:23.000 to strike out and attack the genomes of the cells lining the urinary 00:48:23.000 --> 00:48:27.000 bladder. In other words, by, by smoking cigarettes you can 00:48:27.000 --> 00:48:32.000 actually mutate the genomes, somatic mutation, the genomes of 00:48:32.000 --> 00:48:37.000 cells lining the bladder of the, the urinary bladder, or, of course, 00:48:37.000 --> 00:48:41.000 to state the obvious, you can also mutate the genomes of the cells 00:48:41.000 --> 00:48:46.000 lining the alveoli in the lungs. That's why you get lung cancer. 00:48:46.000 --> 00:48:50.000 And a consequence of this can be, with serious probability, that now 00:48:50.000 --> 00:48:54.000 some of these cells become mutated and that, in turn, 00:48:54.000 --> 00:48:58.000 will lead to a life-threatening tumor. So, on this very cheerful 00:48:58.000 --> 00:49:02.000 note and your having heard two major take-home lessons, have 00:49:02.000 --> 00:49:07.000 a great weekend. See you on Monday. Enjoy 00:49:07.000 --> 00:49:12.000 the parade tomorrow.