#19 Biochemistry Signaling II Lecture for Kevin Ahern’s BB 450/550
Articles,  Blog

#19 Biochemistry Signaling II Lecture for Kevin Ahern’s BB 450/550


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]

2 Comments

  • F Rogers

    In the sketch of insulin action, why stop at glucagon production and not continue on to lipogenesis? “We can burn glucose or convert it to glucagon.” But when glucagon levels are high and glucose levels surge (carb rich meal followed by carb snack followed by carb rich meal, etc) things start to get very interesting (metabolically).

Leave a Reply

Your email address will not be published. Required fields are marked *