Glycoprotein hormones, alpha polypeptide

Transcription

Captioning provided by Disability Access Services at Oregon State University. [classroom chatter] Ahern: Okay, folks, let's get started. Student: Let's get started! Ahern: I like that attitude. [class laughing] Ahern: I looked at the calendar today and realized that next Friday we have an exam. Also, that's dad's weekend so it's good to get this out of the way, huh? Maybe have dad come take your exam for you? Maybe not have dad come take your exam for you? [Ahern laughs] Okay, today I'm going to finish up signaling and I will get talking a little bit about the considerations for metabolic controls and this involves Gibbs free energy and I'll give you some things about that. The TAs have been going through and probably gotten through with you in recitations, the considerations and problem solving for Gibbs free energy and so, as always, if you have questions or problems or concerns, come see me and I'll be happy to work with you as well. Last time, I spent some time getting ready to talk about how it is that, how it is that the beta adrenergic receptor and epinephrine play very important roles in increasing blood glucose. And this is very important. We have an emergency, when we need to escape, we need to do something, or we need to have muscular contraction. Having a supply of glucose, excuse me, in our blood stream is very important. As I also referred to in class last time, glucose in our bodies is essentially a poison that when we have too much glucose in our blood stream, we have very severe side effects. People who have diabetes for example have an insulin response system that is either absent, in which case they have type 1 diabetes, or, and there's other manifestations besides what I'm going to tell you, or they have a cellular system in their body that is not responding properly to glucose. I'm sorry, not responding properly to insulin. So the normal response of the body to insulin is that binding of the insulin to the insulin receptor will cause cells to take in glucose. We'll see at the molecular level today how that happens and why that happens, or why it happens is because glucose is a poison. And so if we don't decrease our blood glucose levels after we've had a meal, then they go very high and as I mentioned last time, what this can cause is severe problems that people who have diabetes experience. May involve kidney failure, it may involve blindness, it may involve the longer you have this amputation. People who have diabetes over a long period of time not uncommonly have amputated limbs. So it's very, very severe consequence of having blood glucose level go high. So it's important then that we spend some time talking about how it is that insulin causes cells to take up glucose. And so not surprisingly, there is a signaling pathway that's involved. The signaling pathway, in fact the signaling pathways that I'm going to describe to you today do not, underline not, involve 7TMs. So 7TMs we remember were the 7 transmembrane domain proteins like the beta adrenergic receptor, like the angiotensin receptor that we're involved in causing cells to activate a G protein, that means that the things I'm going to talk to you about today do not involve G proteins. No G proteins involved. Okay, so insulin is a relatively simple molecule. What you see on the screen is a depiction of insulin. It's comprised of two chains that are covalently linked together by disulfite bonds. Disulfite bonds you can see right there and down here. And those disulfite bonds are what hold the two chains together. So first of all we can say that insulin has quaternary structure and interestingly the way that insulin is made is insulin is made as one long chain. Then it folds and the disulfite bonds form, then protease clips off some of the segments so that you're only left with two linear pieces, kind of like what you see on the screen here holding everything together. Now insulin manifests its effects on target cells by binding to a specific insulin receptor. So the insulin receptor is a protein that's located in the membrane of target cells and it has a structure that looks schematically like what you see on the screen. The top part of this image is the outer part of the cell. The bottom part of the image is the inner portion of the cell. The insulin receptor exists as a dimer normally. We'll see the epidermal growth factor receptor that I will show you in a little bit exists as a dimer only when it binds to the epidermal growth factor. The insulin receptor is different. It exists as a dimer but the binding of insulin to this dimer causes some drastic changes to happen to it that cause insulin to ultimately bring glucose into the cell. Now, like the other receptors we saw the other day, insulin as I mentioned is a hormone just like epinephrine is a hormone. Hormones don't make it into, at least the one's we're talking about, don't make it into target cells. So insulin doesn't make it into the cell. It causes all of its effects by causing some changes within the insulin receptor. Now the insulin receptor is a transmembrane protein as you can see here. It has some different components to it here. There's an alpha subunit, there's the beta subunit. And these work together to communicate the information into the cell. So how does this process work? Well, it turns out that insulin receptor is a special kind of kinase. I talked before about a different kinase. I talked about protein kinase A, I talked about protein kinase C. And these were kinases that we found dissolved in the cytoplasm of the cell. The insulin receptor is a kinase as well. You can see it's imbedded in a membrane. And, in addition, this kinase is different than protein kinase A and protein kinase C and it is a tyrosine kinase. It's a tyrosine kinase. So it's a membrane bound tyrosine kinase. Now, a tyrosine kinase, as its name tells you, is a kinase that puts phosphates onto target tyrosine residues. It puts tyrosines onto target residues. Now, what's interesting and odd about the insulin receptor and many receptors that are membrane bound exist like the insulin receptor does, is that the insulin receptor is a tyrosine kinase but it's normally, when you see it in a state like you see it here, it's completely inactive. And this tyrosine kinase ends up activating itself. How does it do that? Well, the binding of insulin on the external part of the receptor causes a shape change like you've seen before. Now, before the binding of that insulin occurs, the tyrosine kinase portions are down here. Each side has a tyrosine kinase activity in it. But each side is unable to function because of the way that these catalytic sites are oriented with respect to each other. They're just sitting there doing nothing. Binding the insulin causes a shape change that allows one of the tyrosine kinases to phosphorylate the other one. So there's a shape change. This now places into the active site of one of the portions of the dimer. It puts the target tyrosine into there. Well, the phosphorylation, let's say we're phosphorylating the right in this case, the phosphorylation of the right one now causes it to become active. And so it turns around and phosphorylates the left one. So now they're both fully active. They're able to do their thing. As a result of that, there's a series of phosphorylations that happen up and down these beta subunits. So several target tyrosines will get phosphorylated on these beta residues. That's an essential component of the insulin signaling. So first of all, we have to jump start everything, we jump start it by putting one phosphate on, then we go back and fourth, back and fourth, back and fourth, and get phosphates all over there. Everybody with me? Now, what happens as a result, here's the tyrosine kinase first of all. There's the side chain of tyrosine, there's the addition of a phosphate, and again like we've seen before, this changes this guy which is largely an OH group into something that has a negative charge. Not surprisingly, that negative charge may change again itself the shape of the protein in some way. And that causes all the other changes to happen that I've been talking about. Now, you can see on this receptor right here that this phosphorylation induces a pretty big change in shape. Here is this guy before phosphorylation and look how far this has moved over here after phosphorylation. So the shape change that's happening as a result of the phosphorylation of those tyrosines is inducing a pretty good size movement inside of this protein. There's a term that we use for this, I haven't given it to you and I should give to you at this point. It's called receptor mediated tyrosine kinase, or RMTK. This is a receptor, the insulin receptor's receptor, meditated tyrosine kinase. And we will see, we won't actually go into them in this class, we'll talk about one other one. But there are many receptor mediated tyrosine kinases that we find in cells. Many, many. And they all play important roles in signaling. Well how does insulin signaling work? So far you've seen how the receptor gets activated. What is involved in signaling through the insulin receptor? Well, now you see this a little bit more clearly, hopefully. You can see there's a lot of the guys, lot of things that are involved here. First of all, we see that this is the receptor that has bound to insulin. And once it is bound to insulin, there's this cross phosphorylation that happens across the beta units of the insulin receptor. One of these phosphotyrosines, as you can see here, is a binding target for a protein known as IRS-1. That's not in internal revenue service. It does better things than the internal revenue service does. There's another one called IRS-2 that will also do this that's not shown here. But this guy, this is a protein, in fact everything you see on here are proteins. This protein binds to phosphotyrosine. It has a domain that we refer to as a SH2 domain. An SH2 domain is a common structure that we find in many proteins that is capable of recognizing and binding to phosphotyrosine. This is a phosphotyrosine. This now is a perfect target for IRS-1. Well, this bringing of IRS-1 in place allows it to become phosphorylated on its tyrosines as well, so again, we have have this phosphorylation picnic that's going on here as it were. And these phosphorylated sites become targets for another protein. It's another enzyme, as you can see it's another kinase, phosphoinositide 3-kinase. So when we had the beta adrenergic receptor, we saw movement. We saw this G protein moving back and fourth to adenylate kinase. And we saw the cyclick AMP moving in the cell. All these things are happening right here in this one site. We'll see right here a little bit of movement, but for our purposes, essentially everything is happening at the same place. Well what happens here? What is this protein? This protein is known as phosphoinositide 3-kinase. It also has a SH2 domain and it binds to a phosphotyrosine on IRS-1. So we're making kind of a big sandwich here if you want to think about it that way. This enzyme, as you can see, catalyzes the formation of a molecule called PIP3. Now PIP2 you've seen before. PIP2 was involved in the cleavage reaction of phospholipase C that I talked about on Monday. If I take PIP2 and instead of cleaving it, I put an additional phosphate on to it, I make PIP3. I've put an additional phosphate onto this molecule. And yes, PIP3 is acting as a second messenger. PIP3 is able to travel in the membrane, as is PIP2. They move in the membrane very readily. And it moves in the membrane and it itself is a target for binding by PDK1. PDK1 is PIP3 dependent protein kinase. So we see kinase, kinase, kinase, kinase. We see this cascade that we've talked about before. This was a tyrosine kinase that got activated. This is a phosphoinositide kinase that got activated. This is a kinase that's getting activated, and we'll see that this PDK1 phosphorylates this important protein known as AKT. Yeah? Student: That catalyzes the reaction of PIP2 to 3? Ahern: The green guy catalyzes the conversion of PIP2 into PIP3, you're exactly right. Yes, sir? Student: Is IRS-1 the only one [inaudible]? Ahern: IRS-1 is simply a bridge in this scheme. It's simply a bridge. Student: It's not important to [inaudible]? Ahern: Nope. Student: Is there an amplification that happens during this process or will it always be together? Ahern: A very good question. Is there any amplification that occurs in this process? The main amplification actually occurs right here where this guy can phosphorylate a lot of PIP2s, but you don't see the same sort of cascading amplification that we've talked about before. That's a very, very good question. Well, we've gone here, here, here, we've got a protein kinase that's active. This protein kinase is going to phosphorylate. This protein known as AKT. AKT plays many roles in the cell and mercifully not going to show you all the roles in the cell, nor am I going to show you the series of proteins that it phosphorylates, that phosphorylates, that phosphorylates, that phosphorylates, that phosphorylates. But, I will tell you what the end result of this phosphorylation is. AKT is a kinase as well. And this enzyme will stimulate ultimately a change in the trafficking of proteins in the cell. What does that mean? Well trafficking, it refers to the movement of proteins. When we talked about the endoplasm reticulum and the Golgi apparatus the other day, and I said that these glycoproteins have various license plates on them that tells the cell where they should go. Should they go to the membrane? Should they get exported out of the cell? That's trafficking. Those guys get moved into the cell according to instructions that are on them. This guy here is altering the trafficking. What does it do? It changes one important protein where it goes. The important protein that it changes is known as glut, G-L-U-T. And as we'll talk later, there are several gluts. Glut stands for glucose transporter. Now, what this pathway is doing is it's taking glut, which is found normally in the cytoplasm, and it's moving it to the membrane. And since glucose, I'm sorry, since glut has the property of transporting glucose, the cell starts taking up glucose. Now, that's a lot of steps that you needed to know. Yes, okay. You need to know the steps. But that's a lot of steps to get glucose inside of the cell. As a result of this, cells start taking glucose out of the blood stream, and when they take glucose out of the blood stream, they are reducing blood glucose, reducing the toxic effects of glucose, and getting it to the cell that might either burn it or store it in the form of glycogen. So insulin ultimately is countering the effects of epinephrine. It's countering. Epinephrine is increasing blood glucose, insulin is reducing blood glucose. We see that they're doing very different mechanisms, but those are the results of the action of those different hormones. And yes, insulin is a hormone. It's a peptide hormone, meaning it's a protein that's a hormone. Okay, so I'll stop and take questions at that point. Or give you a chance to catch your breath. Yes, ma'am? Student: Since the glut goes from the cytoplasm into the membrane, and it takes glucose and with it, it counteracts epinephrine you said? Ahern: Yes, so what her question was, 'Glut, because it's going to membrane, is taking in glucose and that taking in of glucose is countering the actions of epinephrine, the answer to that question was yes. Question? Student: Was it changed by AKT? Ahern: So her question is, "Is glut changed by AKT?" Glut's location is changed by the pathway that's stimulated by AKT. There's several kinases that act before we ever get to that change. And all that's happening is glut is having its location changed from the cytoplasm to the membrane. Question over here, Lawrence? Student: This PT table [inaudible]? Ahern: PDK1 phosphorylates AKT, that's correct. Student: And that of course, affects blood...? Ahern: I'll tell you what, everyone is curious about the steps, maybe I'll make you memorize them. No, I won't make you memorize them, but let me show you the overview of the pathway, okay? Student: No! Ahern: Yeah, so I've taken you down to, oh, they've changed it this time. I've taken you down to here. You can see that there's actually several steps that's involved ultimately in moving the transporter to surface. They used to have a figure in the old book that showed like 20 steps that got us down to there. You wouldn't want to know the 20 steps. Yeah? Student: So what does amplification mean here? Ahern: I'm sorry? Student: What does amplification mean? Ahern: What does amplification mean? Student: Yeah, in this diagram. Ahern: Here? Student: Yeah. Ahern: So amplification is simply, well, I think it's a little misleading here. If we activate the receptor, then we're essentially activating the phosphorylation of many, many things. For the figure I've shown you, we're only looking at one thing, that's why I'm saying there's not really an amplification there. The insulin receptor is involved in phosphorylating many things. We're looking at one at the moment. There's other things that it can phosphorylate and activate. We're not looking at those. So let's leave that amplification out for the moment. Yes, back here? Student: The cell has a way of releasing the insulin and stopping the whole phosphorylation process or? Ahern: Yeah, so how does the cell stop this process? That's a very good question. Just like we saw before, we have to have a way of getting insulin out of the membrane. The cell has to have a way of handling that insulin and yes it does. And that's, again, beyond the scope of what we're going to talk about here. Was there another question? I thought I saw a hand. That's what's involved in the insulin signaling pathway. As I said, the receptor is involved in many things. The insulin receptor is one that, if you take my molecular medicine class in the fall, I'm sorry in the winter term, I'll talk a little more about that. It is a very important receptor that's involved in a lot of things, including phenomena as diverse as aging and cancer. So the insulin receptor has its fingers in a lot of pies, an awful lot of pies. Haha, glucose, you see. Alright, I don't think we need to talk about that. Alright, so that's the insulin receptor and the insulin signaling pathway that we will talk about here. I want to talk about another receptor mediated tyrosine kinase. And this is one that binds to the epidermal growth factor. The epidermal growth factor is a hormone and like insulin, it has a receptor that it binds to. The receptor is membrane bound. And the receptor is a tyrosine kinase. So it binds to insulin, I'm sorry epidermal growth factor, or EGF, binds to the EGF receptor. There's a schematic diagram of it, I don't like the schematic diagram as much as I like this. Now, I earlier pointed out that the insulin receptor exists as a dimer all the time. The epidermal growth factor receptor does not. You see it in the dimer form only when the receptor has bound to epidermal growth factor. So we can see that here's one half of the receptor that's bound to epidermal growth factor. Here's another half the receptor that's bound to epidermal growth factor. And only after both of these guys have bound epidermal growth factor do they dimerize as we see here. Now, there's a figure that's in your book and I don't like the figure as much as I like this little schematic. You see this little red sort of loops that are here? These red loops are the major shape changes that occur upon binding of the epidermal growth factor. So before the epidermal growth factor binds to the receptor, this loop is sort of folded over onto this thing so they can't interact. But the binding of the receptor, I'm sorry, binding of the epiderm growth factor by the receptor causes them to literally stick out and touch with the next one. That's how they dimerize. So the system is set up so that the receptors don't dimerize until they have both bound to an epidermal growth factor. Well what happens with the binding? Upon the binding, very much like what we saw with the insulin receptor, these kinases, which are inactive, become active. One phosphorylates the other, phosphorylates the other, phosphorylates the other, phosphorylates the other, and you see that we get a series of tyrosines with phosphates on them. Those tyrosines with phosphates on them are targets for another protein known as Grb-2. And Grb-2 has a SH2 domain just like we saw before. It's recognizing and binding to a phosphorylated tyrosine. Grb-2, like we saw with IRS-1, serves as a bridge. Excuse me, the other side of Grb-2 binds to this protein known as Sos. Sos now, here's a G protein. It's not really a G protein like we saw before. It's a different kind of a G protein. So the beta adrenergic receptor had what we classify as a pure G protein. This protein called Ras is a very interesting protein. It's like a G protein but technically it's not the same thing. So I wasn't lying to you earlier when I said we don't have G proteins involved at this point. Ras is one of the most interesting proteins in your cells. You see that, like a G protein, it binds to GDP and like a G protein, when it gets activated, drops the GDP and picks up a GTP. So for all apparent purposes out here, it's functioning kind of like a G protein. Now, the G proteins we talked about before either activate phospholipase C or activated adenylate kinase. Ras instead activities a signaling pathway series of events. One of which ultimately stimulates a cell to divide. One of which ultimately stimulates a cell to divide. And Ras has many, many pathways it can affect. But one of those is stimulating the cell to divide. Yes? Student: So did Sos activate Ras? Ahern: Right, so the binding of the Sos to the Grb-2, good question, the binding of the Sos to the Grb-2 cause a shape change the in Sos? The shape change in the Sos caused the change in Ras, which was the dumping of the GDP and the replacement by GTP. And as a result, we have an activated Ras. So we can see in this pathway that here's a growth factor. A growth factor is a hormone, in this case it's a peptide hormone, that's stimulating a cell to divide. That's what growth is all about. Not surprising. Multi cellular organisms need to control their growth. I want my left leg to be at least approximately the length of my right leg. I know there's a little bit of difference in how long legs are but I want them to be approximately the same length. I want to have the control so that I'm determining when cell division in my bones is occurring. If I do that and I control that growth, then I will be reasonably symmetrical in my appearance. Now this protein Ras, as I said is one of the most interesting proteins that we find inside of cells. It is an example of a class of proteins of which there are a few hundred that play very critical roles in this decision to divide or not to divide. They're involved, these proteins that I'm getting ready to describe to you play very critical roles in signaling and usually in some level affect the decision to divide or not to divide. This class of proteins has a name, it's very important, they're called protooncogenes. Proto, P-R-O-T-O dash oncogene, O-N-C-O-G-E-N-E. Well what is a protooncogene? A protooncogene is a protein intimately involved in cellular control. Usually by a signaling pathway. That intimate nature of its action in controlling the cell is essential for the cell to function properly. It's essential for the cell to function properly. If it doesn't function properly, if the protooncogene doesn't function properly, it behaves as what we refer to as an oncogene. An oncogene has another name. It's a gene that causes cancer. Now, how does a protooncogene become an oncogene? The most common way in which that occurs is mutation. If we mutate the coding sequence for Ras, we may convert it so that it no longer performs its normal function. It may stimulate the cell to divide uncontrollably. When I mutate a protooncogene, I can make an oncogene. So the difference between a protooncogene and an oncogene is a mutation. Unmutated equals protooncogene. Mutated equals oncogene. It can lead to uncontrolled division. There are many examples, there are several hundred protooncogenes that are known. And normally, they function exactly as they're supposed to. They're supposed to control whether a cell divides or not divides in response to the signals that it's getting. But when they mutate, we can have real problems. That's why we worry about mutagens. Cigarette smoking, pollution in our air, pollution in our water, junk that we're eating in our food. These things may favor mutation, mutation of DNA in general, you're increasing the chances that you're going to cause a protooncogene to become an oncogene. Now in the case of Ras, I'm going to tell you exactly what happens. There are many examples though of different mutations that can happen. And I'll show you one other one after I finish with Ras. Ras, like the class of G protein, I don't want to say like other proteins, but like the class of G proteins, is a very bad enzyme. Remember I said that the G proteins were bad enzymes, bad in the sense that they're very inefficient at breaking down GTP. Ras is the same way. Ras will cleave GTP, and as we can see in the scheme, when GTP gets cleaved, Ras is no longer active, it goes back to here. As long as Ras is active, it's going to stimulate the cell to divide. One of the mutations in Ras that converts it from a protooncogene into an oncogene affects the ability of Ras to break down GTP. It affects the ability of Ras to break down GTP. Now in the case of Ras, it's a fairly small protein. There are two, it's actually three, but two that we focus on, two critical amino acids at the active site of Ras. Positions 11 and 12. You don't need to know those numbers. Mutations at either one of those amino acids that converts that into any other amino acid causes Ras to be unable to cleave GTP. Yowza. Any mutation can do that. That can involve a single base pair change in the coding sequence of Ras at that position. Now, if you want to think about why you want clean water and clean air and good food, and you don't want to smoke, and all of these various things, Ras is a really good thing to think about. There are animal systems that have been shown that they can induce a tumor by making a single base change in the coding of Ras. Now the formation of the tumor is a complex process. I'm not going to say in a human being that's necessarily what's going to happen. I can tell you that making Ras mutated is not a good career move. In general, mutating protooncogenes are not good career moves at all. You're asking for trouble if you start doing that. So be careful what you eat, be careful what you drink, think about the environment, think about your health, because these things really are very important in your survival. Yes, sir? Student: [inaudible] require 3 or 4 separate mutations that would disable like apoptosis and induce constitutive cell division? Ahern: So his question is, doesn't the formation of a tumor require several independent, separate mutations? And there are thousands, tens of thousands of mechanisms that can lead to a tumor. You are correct. That's why I say I'm not talking about necessarily in one sense, but at least in some animal systems, that has been shown to be possible to do. So you got to be careful. You don't know. I mean how many, is it 2, is it 3, is it 20? If there are some systems that you could do where you might take 2 or 3 of the right type of mutation, or maybe the wrong type of mutation, you don't want to mess with that. Student: But if a single cellular signal just activated Ras constitutively, wouldn't you still add a regular active like a P51 that would initiate apoptosis and... Ahern: Okay, so, let's talk about apoptosis later. What he's asking about is a phenomenon where cells commit suicide. And you are right, there are checking mechanisms in cells that will help prevent cells from becoming out of control growth. So the mutation of proto-oncogenes is a necessary step for formation of a tumor. So I'm only telling you one way by doing this. Apoptosis is one way of preventing that, but again, let's save that until we talk about apoptosis, okay? Because there's many factors to consider. But I want you to be left with the gravity of this, which is that mutating your protooncogenes is not the best thing to do. Yes, Neil? Student: How does the cell go into uncontrolled division? Ahern: How does a cell go into uncontrolled division? Well, okay, you guys really want to get into this here. So cells control their cell cycle. In multicellular organisms, we see the cell cycle that they go through, there's a synthetic phase, a mitotic phase, and there are resting phases, and there are specific proteins that will allow movement through those phases. So when we have uncontrolled growth, we do not have regulation of those phases. That can involve, again, multiple steps in the process. So I'm just talking about one mutation here, folks. So I'm not going to go through the whole cell cycle, but the point is that the more protooncogenes we mutate, the more likely we're going to have something that we don't want. Yes, sir? Student: So does the GTP play a role in the deactivating, so when it mutates the GTP is broken down...? Ahern: Okay, so I'm not sure I understand the question, but the point is that once it's bound to GTP, it's activated. So there's no role of GTP or GDP because all that we have to have is this activated. If the Ras cannot break it down, then it's always in the activated state. The only shut off mechanism is the breaking down the GTP. I'm sorry, maybe I didn't understand your question, but if I we can't break this down, it's on. It's on. Okay. So that's a pretty important, pretty cool system to understand. There's a long set of steps I didn't take you all the way through. There we activated Ras, Ras activities Raf, activities MEK, activities ERK, and phosphorylates transcription factors. Phosphorylates transcription factors. Transcription factors of proteins that bind to DNA that activate transcription. If we turn on the wrong genes, getting back to Neil's question back over here, if we turn on the wrong genes that are otherwise stopping cell cycle, now they're starting cell cycle, we can have uncontrolled growth. So I know I'm giving you a very sort of black box image of this, but the point is the to we lose control of the system here, everything else that follows can be a really big problem for us. Okay. Ba-da-ba-da. The last things I want to talk about with respect to signaling and then I'm only going to talk about one of these and that's this guy right here, bcr-abl. This one's an interesting one and it's interesting particularly for people who live in Oregon, interestingly enough. And this thing that you see on the screen is a way of making an oncogene from a protooncogene. Now I talked about well, we mutate. Maybe the DNA polymerase doesn't copy something properly. Another way of having changes happen that are the equivalent of mutation are to have recombination. You guys have learned about recombination in biology I'm sure. This happens when two DNAs that were not originally together get linked together by a cross over phenomenon. A very common, I shouldn't say very common, but a relatively common cross over that can occur that is a recombinational event that can occur, occurs between two genes known as bcr and abl. Abl is a receptor, I'm sorry, abl is a tyrosine kinase involved in signaling. It's a tyrosine kinase involved in signaling. Bcr is another gene that's up here on chromosome 22, abl is on chromosome 9. Cross over events that bring these two guys together happen as I say relatively commonly, not every day, but relatively commonly to make something that we call bcr-abl. What happens in this case is that the abl gene gets linked to a portion of bcr gene. So the bcr genes here, we see the bcr gene in red. We see this portion of the abl that gets linked to it. And we make essentially a new protein. Now if we completely alter the function of the protein, it probably wouldn't cause too much of a problem. However, this fusion keeps the tyrosine kinase activity of abl in the active form. This guy is still a tyrosine kinase and abl is involved in telling cells to divide or not to divide. The result of this fusion gives a phenomenon that's very interesting. When we talk next term about gene expression, we'll talk about how much transcription of a gene occurs. We can imagine that some genes might have on average, let's say 1,000 copies of its messenger RNA made. Another gene that's used a lot might have 20,000 copies of its messenger RNA made. Bcr, it turns out, has a lot more copies of its self made than abl does. Abl only has a few copies made normally. So what's happening as a result of this fusion is abl is being brought under the transcriptional control of the bcr gene. So now instead of having just a few messenger RNAs for abl, the cell is flooded with them. Well you've got, if you have thousands and thousands more than you would normally have, each one of those has more opportunity to get activated and to activate cellular division. So here's a case where the amount of a protein that we're making, the amount of the protein that we're making is affecting the cell's ability to control itself. Now we've got an awful lot of this stuff here. That's the bad news. This mutation happens in a type of leukemia. It happens in a type of leukemia known as CML. The good news is that there's a pretty darn good treatment for it. And the pretty darn good treatment was actually invented at OHSU. Now, it involves a drug that inhibits this enzyme. It is a tyrosine kinase inhibitor. In the back of your minds I hope you were thinking, Do tyrosine kinase inhibitors have effects on cells? And the answer is they can. Inhibiting this tyrosine kinase is one way of keeping this tyrosine kinase under control. Because if this guy doesn't have the ability to phosphorylate tyrosines, it's going to in fact not be stimulating that cell to divide. We have a better way of handling this mutation in this cell. The tyrosine kinase inhibitor that was invented at OHSU was known as Gleevec, G-L-E-E-V-E-C. It's very effective against this type of mutation, or this type of alteration, and interestingly enough, this Gleevec doesn't have many side effects. Why? Well, it turns out that it really binds to this fused protein very well and this fused protein isn't found in regular cells. So when we think about an anti-cancer drug and we think about something that we want few side effects, we would really like to be able to target something that occurs in cancer cells but doesn't occur in other cells and Gleevec actually does this quite well on this particular fusion. So in this case, the fusion actually gave us a unique target that a regular cell doesn't have. It's something we think of a magic bullet or a silver bullet that is targeted at a cell that is in trouble. Questions about that? I brought you guys to silence. Wow. Yes? Student: Will cellular systems still recognize like in this case, a new protein, that it will recognize it as foreign? Ahern: Are their cellular systems that recognize this as foreign? The cell would have no way of recognizing it's a foreign thing. When we think about recognizing foreign vs. natural, we're talking about the immune system which is working outside of cells. So no, there's not a way of recognizing this. Good question, though. Okay, so we're getting late. Maybe we should sing a song and call it a day. I've got a signaling song. Anybody here like Simon and Garfunkel? Alright. This is one of my favorite Simon and Garfunkel songs. I'm an old guy. Come on here. Oh, wrong one. It's called "the Tao of Hormones." It's to the tune of "the Sound of Silence." Lyrics: Biochemistry my friend It's time to study you again Mechanisms that I need to know Are the things that really stress me so Get these pathways planted firmly in your head Ahern said let's start with epinephrine. Membrane proteins are well known Changed on binding this hormone Rearranging selves without protest Stimulating a G alpha S To go open up and displace its GDP With GTP, got too high there Because of epinephrine Active G then moves a ways Stimulating ad cyclase So a bunch of cyclic AMP Binds to kinase and then sets it free All the active sites of the kinases await Triphosphate Because of epinephrine. Muscles are affected then Breaking down their glycogen So they get wad of energy In the form of lots of G-1-P And the synthases that could make a glucose chain All refrain Because of epinephrine. Now I've reached the pathway end Going from adrenaline Here's a trick I learned to get it right Linking memory to flight or fright So the mechanism that's the source of anxious fears reappears When I make epinephrine. I had a little bit of that fear at the end there. Alright, take care guys. [class clapping] [classroom chatter] [END]