1. What is the biochemical basis of hormone action in the body?
Answer: Hormones exert their effects by binding to specific receptors either on the cell surface or inside the cell, depending on their chemical nature. Peptide and protein hormones typically bind to receptors on the cell surface, activating a cascade of intracellular signaling pathways, often involving second messengers like cAMP. In contrast, steroid hormones are lipophilic and pass through the cell membrane to bind to intracellular receptors, which then directly influence gene expression. This receptor-ligand binding alters the cell’s function, such as initiating metabolic processes or modulating gene transcription. These processes are often highly regulated to ensure appropriate physiological responses.
2. How do steroid hormones influence gene expression?
Answer: Steroid hormones, such as cortisol and estrogen, influence gene expression by binding to intracellular receptors in the cytoplasm or nucleus. Upon binding, the receptor-hormone complex undergoes a conformational change, allowing it to bind to specific DNA sequences known as hormone response elements (HREs) within the promoter region of target genes. This interaction promotes or inhibits transcription, resulting in the synthesis of specific proteins. These proteins often regulate various cellular functions, including metabolism, growth, and immune responses. Since this process involves altering the transcriptional activity of genes, it is relatively slow compared to the action of peptide hormones.
3. What are the roles of second messengers in hormone signaling?
Answer: Second messengers play a crucial role in amplifying and propagating the signal from the hormone receptor on the cell surface to intracellular targets. When a hormone binds to a cell surface receptor, it activates a G-protein or other signaling molecules, which in turn stimulate the production of second messengers like cyclic AMP (cAMP), inositol trisphosphate (IP3), and diacylglycerol (DAG). These second messengers activate various protein kinases, which phosphorylate target proteins and initiate cellular responses. Second messengers enable rapid, widespread, and regulated cellular responses to hormones, such as changes in enzyme activity, gene expression, and ion channel function.
4. Describe the mechanism of action of peptide hormones.
Answer: Peptide hormones, like insulin and glucagon, are hydrophilic and cannot cross the lipid bilayer of the cell membrane. Therefore, they must bind to receptors located on the cell surface. These receptors are typically G-protein-coupled receptors (GPCRs) or receptor tyrosine kinases (RTKs). Upon hormone binding, these receptors undergo a conformational change that activates intracellular signaling pathways. For example, binding of insulin to its receptor activates the receptor’s intrinsic tyrosine kinase activity, leading to the phosphorylation of tyrosine residues within the receptor itself and other downstream signaling proteins. This activation eventually leads to changes in cellular processes like glucose uptake and metabolism.
5. How do G-protein coupled receptors (GPCRs) mediate hormone action?
Answer: G-protein coupled receptors (GPCRs) mediate hormone action by linking the extracellular signal (hormone binding) to intracellular signaling pathways through G-proteins. When a hormone binds to a GPCR, the receptor undergoes a conformational change, which activates the associated G-protein by exchanging GDP for GTP. The activated G-protein then dissociates into its alpha, beta, and gamma subunits, which can interact with various intracellular enzymes or ion channels. A common result of this activation is the generation of second messengers like cAMP, which in turn activates protein kinases, leading to the phosphorylation of target proteins and resulting in cellular responses such as increased enzyme activity or gene transcription.
6. What is the role of protein kinase A (PKA) in hormone signaling?
Answer: Protein kinase A (PKA) is a crucial enzyme in hormone signaling, particularly in pathways that involve cyclic AMP (cAMP) as a second messenger. When a hormone binds to a receptor, such as the beta-adrenergic receptor, it activates adenylate cyclase, leading to an increase in cAMP levels. cAMP then binds to the regulatory subunits of PKA, causing a conformational change that releases the catalytic subunits of PKA. These catalytic subunits phosphorylate various target proteins within the cell, such as enzymes involved in metabolism, transcription factors, and other signaling proteins. The phosphorylation of these targets leads to various cellular responses, including alterations in metabolism, ion channel activity, and gene expression.
7. How does the hormone insulin regulate glucose metabolism?
Answer: Insulin is a peptide hormone that regulates glucose metabolism primarily by facilitating the uptake of glucose into cells, particularly muscle and adipose cells. When insulin binds to its receptor, a receptor tyrosine kinase, it activates the receptor’s intrinsic kinase activity, leading to phosphorylation of tyrosine residues on the receptor and other downstream signaling proteins. One of the key signaling pathways activated is the phosphoinositide 3-kinase (PI3K) pathway, which promotes the translocation of glucose transporter proteins (GLUT4) to the cell membrane, allowing glucose to enter the cell. Additionally, insulin activates enzymes involved in glycogen synthesis and inhibits enzymes that break down glycogen, thus lowering blood glucose levels.
8. What is the difference between the action of steroid hormones and peptide hormones?
Answer: The main difference between the action of steroid and peptide hormones lies in their ability to cross the cell membrane. Steroid hormones are lipophilic (fat-soluble) and can easily diffuse across the lipid bilayer of the cell membrane to bind to intracellular receptors, which then modulate gene expression directly. On the other hand, peptide hormones are hydrophilic (water-soluble) and cannot pass through the lipid bilayer. Instead, they bind to receptors on the cell surface, which then activate intracellular signaling pathways, often involving second messengers like cAMP. This leads to changes in cellular activities such as enzyme activity, ion channel function, and gene expression.
9. What is the role of phosphodiesterase in hormone signaling?
Answer: Phosphodiesterase is an enzyme that plays an essential role in regulating the duration and intensity of hormone signaling. It acts by hydrolyzing cyclic nucleotides like cyclic AMP (cAMP) or cyclic GMP (cGMP), converting them into their inactive forms (AMP or GMP). This process terminates the signal transduction initiated by second messengers, thus ensuring that the hormonal response is not prolonged beyond the necessary duration. For example, in the cAMP signaling pathway, phosphodiesterase breaks down cAMP, reducing its ability to activate protein kinase A (PKA), thereby turning off the signaling cascade.
10. How does the hormone adrenaline (epinephrine) influence the fight-or-flight response?
Answer: Adrenaline (epinephrine) is a catecholamine hormone that plays a critical role in the fight-or-flight response. It binds to beta-adrenergic receptors on target cells, activating the G-protein coupled receptor (GPCR) pathway. This activation stimulates adenylate cyclase, leading to an increase in cAMP levels. cAMP activates protein kinase A (PKA), which in turn activates enzymes that promote the breakdown of glycogen into glucose, increase heart rate, and enhance the dilation of airways. These actions prepare the body for quick energy mobilization and increased oxygen intake, allowing for rapid responses to stressful situations.
11. What are the roles of nuclear hormone receptors in hormone action?
Answer: Nuclear hormone receptors are intracellular receptors that mediate the actions of steroid hormones, thyroid hormones, and certain other lipophilic hormones. These hormones pass through the cell membrane and bind to their respective nuclear receptors, which are located in the cytoplasm or nucleus. Once the hormone binds, the receptor undergoes a conformational change, and the hormone-receptor complex then binds to specific DNA sequences called hormone response elements (HREs) within the promoter regions of target genes. This interaction either activates or represses the transcription of these genes, resulting in the synthesis of proteins that regulate various cellular processes such as metabolism, growth, and differentiation.
12. How do phospholipase C and IP3 mediate hormone action?
Answer: Phospholipase C (PLC) is an enzyme that is activated by G-protein coupled receptors (GPCRs) upon hormone binding. Once activated, PLC catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) into two second messengers: inositol trisphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates the release of calcium ions from intracellular stores, which then activates various calcium-dependent enzymes and signaling pathways. DAG, on the other hand, activates protein kinase C (PKC), which also phosphorylates target proteins to elicit cellular responses. These signaling events are crucial for regulating processes like cell growth, secretion, and muscle contraction.
13. What is the significance of the hormone cortisol in stress response?
Answer: Cortisol is a steroid hormone released by the adrenal glands in response to stress. It plays a key role in the body’s stress response by increasing glucose availability, modulating the immune system, and altering metabolism. Cortisol binds to intracellular glucocorticoid receptors, which then translocate to the nucleus to regulate the expression of genes involved in glucose metabolism, inflammation, and immune function. Cortisol helps maintain blood glucose levels by promoting gluconeogenesis in the liver and inhibiting insulin signaling in peripheral tissues, ensuring that the body has an adequate supply of energy to handle stress.
14. How does thyroid hormone affect metabolism at the cellular level?
Answer: Thyroid hormones, including thyroxine (T4) and triiodothyronine (T3), regulate metabolism by binding to nuclear hormone receptors inside cells. Once inside the cell, T3 (the more active form of thyroid hormone) binds to thyroid hormone receptors in the nucleus, influencing the transcription of genes involved in energy production, thermogenesis, and metabolic regulation. The increased expression of genes involved in oxidative phosphorylation enhances the production of ATP, boosting overall metabolic rate. These effects increase the rate at which cells generate energy, thus elevating basal metabolic rate and influencing the growth and development of tissues.
15. What is the role of receptor tyrosine kinases (RTKs) in hormone signaling?
Answer: Receptor tyrosine kinases (RTKs) are a class of cell surface receptors involved in the signaling of many hormones, such as insulin and growth factors. When a hormone binds to an RTK, it causes the receptor to dimerize and activates its intrinsic kinase activity. This leads to the phosphorylation of tyrosine residues on the receptor itself and on downstream signaling proteins. These phosphorylated proteins serve as docking sites for other signaling molecules, which activate intracellular pathways like the MAP kinase pathway. These pathways regulate cell division, differentiation, and metabolism, and are essential for processes like growth and tissue repair.
16. How do hormones regulate the immune response?
Answer: Hormones, such as cortisol, thyroid hormones, and sex hormones, play significant roles in modulating the immune response. Cortisol, for example, has anti-inflammatory effects and suppresses immune system activation by inhibiting the production of pro-inflammatory cytokines and by promoting the production of anti-inflammatory cytokines. Estrogen and progesterone influence immune responses by modulating the activity of immune cells, such as T-cells and macrophages. These hormones can either enhance or suppress immune function, depending on the context, and are particularly important in regulating responses during pregnancy, infection, or autoimmunity.
17. What is the role of the cAMP pathway in regulating heart rate?
Answer: The cAMP pathway plays a crucial role in regulating heart rate, particularly in response to catecholamine hormones like adrenaline. When adrenaline binds to beta-adrenergic receptors on cardiac muscle cells, it activates a G-protein that stimulates adenylate cyclase to increase cAMP levels. Elevated cAMP activates protein kinase A (PKA), which in turn phosphorylates specific proteins, including ion channels. This leads to increased calcium influx into the cardiac cells, enhancing the contractile force and rate of heartbeats. The cAMP-mediated signaling thus accelerates heart rate, ensuring that the heart pumps more blood in response to stress or physical activity.
18. What are the cellular responses to hormones like aldosterone and how does it affect electrolyte balance?
Answer: Aldosterone, a steroid hormone secreted by the adrenal cortex, regulates electrolyte balance by acting on the kidneys. It binds to mineralocorticoid receptors in the cytoplasm of kidney cells, leading to the translocation of the receptor-hormone complex to the nucleus, where it influences the expression of genes involved in sodium reabsorption. Specifically, aldosterone increases the expression of sodium-potassium ATPases in the distal tubules of the kidney, which enhances sodium reabsorption and potassium excretion. This action helps maintain fluid and electrolyte balance, contributing to blood pressure regulation.
19. How does the hormone glucagon increase blood glucose levels?
Answer: Glucagon is a peptide hormone secreted by the alpha cells of the pancreas in response to low blood glucose levels. It acts primarily on the liver, where it binds to G-protein coupled receptors, activating adenylate cyclase and increasing cAMP levels. The elevated cAMP activates protein kinase A (PKA), which promotes the breakdown of glycogen into glucose (glycogenolysis) and stimulates the production of glucose from non-carbohydrate sources (gluconeogenesis). These actions increase blood glucose levels, providing energy to the body, especially during periods of fasting or intense physical activity.
20. What is the significance of the hormone oxytocin in childbirth and lactation?
Answer: Oxytocin is a peptide hormone released by the posterior pituitary gland that plays a vital role in childbirth and lactation. During labor, oxytocin stimulates uterine contractions by binding to oxytocin receptors on smooth muscle cells in the uterus. These contractions help expel the baby. After childbirth, oxytocin promotes milk ejection (letdown) by acting on the myoepithelial cells surrounding the milk glands in the breast. The release of oxytocin during breastfeeding also enhances maternal bonding. This hormone’s actions are critical for both the delivery process and the initiation of breastfeeding.
