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


Uploaded by oharow on 09.11.2011

Transcript:
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]