Cellular mechanism of hormone action

After coursing throughout the blood vessels of our bodies, a hormone eventually meets a receptor that was created specifically for it by the target cells that it was sent to stimulate. And the way that hormone interacts with the receptor happens in one of two very characteristic ways. And I want to show you today how that happens. And so the first major mechanism of hormone action at the cell that I'm going to start with is by secondary messengers. And I'm going to start with secondary messengers because, historically, they're a little bit more confusing. But essentially what's happening is a hormone is binding to a receptor on a cell. So let me draw a cell and its receptor. And I'll draw a hormone binding to it. But the process of that binding, instead of just stimulating the effect, it really sets off this chain reaction that leads to secondary messengers inside the cell being released. So let me draw those. And you've got these secondary messengers being released, and these are actually what's stimulating whatever effect is desired, whether that may be insulin being released or glucose being taken up inside the cell, or any of the other countless things taking place in our bodies that are controlled by hormones. And so in order to give you a visual for how this might take place in the cell, I've pre-drawn some pictures in the key. So let me pull those in here. And I want you to be sure that these drawings are not to scale in any way. But really, this is the best I can at least do in explaining the process, because all of this is happening at the atomic level, and atoms are really tiny. But anyway, what we have in this picture is a receptor. And I've drawn that in pink, and it's located inside the cell membrane. So this is the phospholipid bilayer of the cell membrane, and we've got the inside of the cell, and the outside. And then also in the cell membrane, we've got a G protein, and I've drawn that in green. It's called a G protein because it binds to molecules that include the nucleotide guanine. And that's the same G from the DNA bases that you might be a little bit more familiar with. But it's currently bound to a molecule called guanine diphosphate. And then we'll see how that changes. But we also have this adenylate cyclase enzyme that's in the cell membrane. And remember that an enzyme speeds up reaction. So we're going to see how adenlyate cyclase speeds up a reaction. But what starts this process off is the hormone is going to bind to the receptor. So it's going to look like that. And you're going to have the hormone bound to the receptor. And once that hormone binds to the receptor, it's going to change shape. And that's going to allow it to interact with the G protein here. And so that looks like this. And what you saw happen was that as the G protein interacts with the receptor in a hormone complex, it's going to exchange that GDP that it started with, that guanine diphosphate for GTP. So essentially, it's exchanging a guanine bound to two phosphates for a guanine bound to three phosphates. And what happens is that enables the G protein to move through the cell membrane and interact with adenylate cyclase. And so that activates the adenylate cyclase. And as an enzyme, that activated adenylate cyclase facilitates the conversion of ATP, which is the energy currency of the cell, into cAMP. And cAMP stands for cyclic adenosine monophosphate. So we've taken this ATP, or adenosine triphosphate, and created a cyclic adenosine monophosphate. And we also have the additional two phosphates. But it is this cAMP molecule that activates the protein inside the cell whose effect is really being targeted by the hormone in the first place. And so eventually, this system resets, but not before several adenlyate cyclase enzymes were activated, resulting in a lot of cAMP being produced. And so we call this signal amplification. And what I mean by signal amplification is that, in theory, one hormone can bind to a receptor. And that process can set off a chain reaction that leads to a lot of cAMP being produced. And so it can mean that less hormone is required to ultimately activate the protein or the effect that's being desired. And so secondary messengers are a means in which hormones act on the cell. But really, if I'm being honest, this effect happens differently for a lot of cells. And all of the mechanisms of secondary messenger hormone action aren't known right now. And honestly, there are a lot of secondary messengers other than cAMP. But really, the takeaway is that for the majority of the hormones in your body, binding to the cell surface activates a series of reactions that initiate a response inside the cell. And so it's very similar to the use of a phone service provider. And because we as people-- let me draw us-- because we're unable to communicate directly with people sometimes, because of separation or convenience or lack of efficiency, we use a phone service to project our voice to them through a phone conversation. So through a phone conversation, we'll direct our voice to people that we want to communicate with. Or maybe we use the delivery of a text message. A text message will transmit some intended message that we have for people that we can't communicate directly with, for whatever reason. And so that's very similar to how secondary messengers assist hormones that can't communicate with a receptor directly inside of the cell. And so peptide hormones and catecholamines, both of which can't cross the cell membrane, use secondary messengers to communicate. And then the other major method of hormone action on a cell is as a primary messenger. Certain hormones, like steroids and thyroid hormones, can actually cross the cell membrane. And it eliminates this entire middleman system that we set up before. So let me pull in another cell membrane. And so the hormone crosses the cell membrane and it binds to a receptor that's located either in the cytosol or in the nucleus. And so we could imagine a nucleus, and there might be DNA inside. But when the hormone binds to the receptor that's either in the cytoplasm or inside the nucleus, that binding process is going to directly affect transcription in the nucleus, or translation in the cytoplasm, of the protein that's being activated by the hormone. And this process has quite a few less moving parts than the secondary messenger system did. But again, it stems back to the fact that these are steroid and thyroid hormones that are typically lipid-based and are capable of crossing through the cell membrane on their own. And so they don't need all of that extra machinery set up for them. But anyway, these are both primary messengers and secondary messengers. And those are the two main processes by which hormones act on the cell that they're created to target.