Intracellular messnegers: Hormones and Nitric Oxide - Online Tutor, Practice Problems & Exam Prep

Intra-cellular receptor signaling involves hormones and nitric oxide (NO). Steroid hormones easily cross the plasma membrane and bind to nuclear receptors, which then translocate to the nucleus to regulate gene expression. This binding triggers a conformational change, activating specific DNA regulatory sites. Similarly, NO diffuses across membranes and binds to guanylyl cyclase, producing cyclic GMP, which mediates various physiological effects, including muscle relaxation and the action of Viagra. Understanding these pathways is crucial for grasping cellular communication and hormonal regulation.

Hormones and NO

Okay. In this video, we're going to be talking about Intracellular Receptor Signaling, specifically focusing on hormones and Nitric oxide. So first, let's talk about hormones. Hormones don't need anything special to cross the membrane; they'll just cross the plasma membrane, but once they're inside the cell, then they can bind to an intracellular receptor. Let's focus on steroid hormones just because that's the one that your book is going to focus on, and that's the one everyone's most familiar with. Steroid hormones cross the membrane, they bind to nuclear receptors. Now we've talked about nuclear receptors before. They kind of have this misleading name because they're not actually, I mean, they can be found in the nucleus. They don't have to be found in the nucleus; they can be found in the cytosol, but once they're activated by binding to the hormone, that is going to trigger transfer into the nucleus, where all nuclear receptors act as transcriptional regulators in the nucleus. So they don't have to be present in the nucleus to, you know, just be floating around the cell doing nothing, but once they are activated, they get transferred to the nucleus to act in the nucleus as transcriptional regulators, which is why they're called nuclear receptors. Now, hormones activate nuclear receptors when they bind to them, and this binding triggers a conformational change and then activation. And so these are really specific molecules, so each hormone binds to a specific nuclear receptor. And that nuclear receptor will go on to only bind certain DNA regulatory sites, not everywhere in the DNA, not every DNA regulatory site. So they are really specific towards activating certain pathways. Here, you see this hormone, it is diffusing across the plasma membrane. Once it's inside, it's going to interact with this nuclear receptor, even though the nuclear receptor is in the cytoplasm. Once it binds, it's going to cause transfer into the nucleus where that is going to interact with the DNA, and that's going to affect transcription and eventually translation and gene expression. That's how hormones act.

So now, let's move on to Nitric oxide. Nitric oxide works in a very similar way, it doesn't anything special to get into the plasma membrane. It just dissolves across it. But once it's inside, it's going to bind to an intracellular signaling protein. Nitric oxide regulates a variety of different pathways. I'm not going to go over them; I'm just going to very briefly mention a couple. The first is Nitric oxide can be released by the body into blood vessels to stimulate muscle relaxation. And then, the second is actually through the process of how Viagra works. So nitric oxide can bind to guanylate cyclase, which is this protein that forms cyclic GMP, and this GMP goes on to exert the physical effects of Viagra. Here we have this example. Here's NO, this gas, Nitric oxide. It diffuses across the plasma membrane, binds to guanylate cyclase, which creates cyclic GMP, and then you get Viagra's effects. So I'm sure many people would think that Nitric oxide is a really important signaling pathway just for this function, even though it has a ton of other ones that are also really important. So, with that, let's now move on.

Steroid hormones signaling through binding….

• A. GTPases
• B. Nuclear receptors
• C. Kinases
• D. Phosphatases

Nitric Oxide signals by binding to which of the following molecules?

• A. GTPases
• B. Nuclear Receptors
• C. Adenyl cyclase
• D. Guanyl cyclase

Here’s what students ask on this topic:

How do steroid hormones cross the plasma membrane and what happens once they are inside the cell?

Steroid hormones can easily cross the plasma membrane without the need for any special transport mechanisms because they are lipophilic. Once inside the cell, they bind to intracellular receptors known as nuclear receptors. These receptors can be located in the cytoplasm or the nucleus. Upon binding to the hormone, the nuclear receptor undergoes a conformational change and translocates to the nucleus. In the nucleus, the activated receptor acts as a transcriptional regulator, binding to specific DNA regulatory sites to influence gene expression. This process ultimately affects transcription and translation, leading to changes in cellular function.

What role does nitric oxide play in intracellular signaling?

Nitric oxide (NO) is a small, diffusible gas that plays a crucial role in intracellular signaling. It easily crosses the plasma membrane and binds to intracellular signaling proteins. One of its primary targets is guanylyl cyclase, an enzyme that converts GTP to cyclic GMP (cGMP). The production of cGMP mediates various physiological effects, such as muscle relaxation and vasodilation. For example, NO is released by endothelial cells in blood vessels to stimulate smooth muscle relaxation, leading to vasodilation. Additionally, NO is involved in the mechanism of action of drugs like Viagra, which enhances the effects of cGMP to promote blood flow.

What is the mechanism of action of nuclear receptors in gene regulation?

Nuclear receptors are a class of intracellular receptors that regulate gene expression. When a hormone binds to a nuclear receptor, it triggers a conformational change in the receptor. This activated receptor then translocates to the nucleus, where it binds to specific DNA regulatory sites known as hormone response elements (HREs). By binding to these HREs, the nuclear receptor acts as a transcriptional regulator, either promoting or inhibiting the transcription of target genes. This regulation of gene expression ultimately affects various cellular processes, including growth, metabolism, and differentiation.

How does nitric oxide contribute to the physiological effects of Viagra?

Nitric oxide (NO) contributes to the physiological effects of Viagra by enhancing the production of cyclic GMP (cGMP). NO diffuses across the plasma membrane and binds to guanylyl cyclase, an enzyme that catalyzes the conversion of GTP to cGMP. Increased levels of cGMP lead to smooth muscle relaxation and vasodilation, particularly in the blood vessels of the penis. This vasodilation increases blood flow, facilitating an erection. Viagra works by inhibiting the enzyme phosphodiesterase type 5 (PDE5), which breaks down cGMP. By preventing the degradation of cGMP, Viagra prolongs its effects, thereby enhancing erectile function.

What are the similarities and differences between hormone and nitric oxide signaling pathways?

Both hormone and nitric oxide (NO) signaling pathways involve the diffusion of signaling molecules across the plasma membrane and their interaction with intracellular targets. Hormones, particularly steroid hormones, bind to nuclear receptors, which then translocate to the nucleus to regulate gene expression. NO, on the other hand, binds to guanylyl cyclase, leading to the production of cyclic GMP (cGMP) and subsequent physiological effects like muscle relaxation. The key difference lies in their targets and outcomes: hormones primarily affect gene transcription, while NO influences enzymatic activity and second messenger systems. Despite these differences, both pathways are crucial for cellular communication and regulation.