Insulin Signaling Pathway — MCQs
Which of the following is the PRIMARY physiological action of insulin in the fed state?
Insulin acts through which receptor?
Which of the following is FALSE about insulin action?
Which of the following statements best describes the mechanism of action of insulin on target cells?
Two normal, healthy subjects volunteer for a study on insulin secretion. In Patient 1, blood glucose is increased to 150 mg/dL by direct intravenous infusion. In Patient 2, blood glucose is increased to 150 mg/dL by ingestion of oral glucose. The peak plasma insulin concentration produced in Patient 1 is 70 uU/mL while in Patient 2, it is 95 uU/mL. Which of the following best explains the higher insulin concentration in Patient 2?
Select the correct sequence of events in the cAMP signaling pathway.
Which of the following binds to Tyrosine Kinase receptor?
What is the primary effect of GLP-1 on insulin secretion?
What is the primary function of G-proteins in cellular signaling?
Which of the following genes is least likely to be involved in the development of carcinoma of the colon?
Insulin Signaling Pathway Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following is the PRIMARY physiological action of insulin in the fed state?
- A. Activation of key enzymes of glycolysis (Correct Answer)
- B. Stimulation of gluconeogenesis
- C. Increased amino acid entry in the cell
- D. Increased lipogenesis
Explanation: ***Activation of key enzymes of glycolysis*** - In the fed state, **insulin's primary and immediate action** is to promote **glucose utilization** through activation of glycolytic enzymes including **hexokinase, phosphofructokinase (PFK-1), and pyruvate kinase**. - This represents the **most direct and immediate metabolic response** to elevated blood glucose after a meal, allowing cells to use glucose for **immediate energy production**. - Insulin also promotes **GLUT4 translocation** to cell membranes in muscle and adipose tissue, enhancing glucose uptake, which directly feeds into **glycolysis**. - **Glycogen synthesis** (glycogenesis) occurs simultaneously as another primary action for glucose storage in liver and muscle. *Increased lipogenesis* - While insulin does stimulate **lipogenesis** (fatty acid and triglyceride synthesis) in the fed state, this is a **secondary action** that becomes significant primarily when **glycogen stores are replete**. - Lipogenesis represents **long-term energy storage**, but the primary immediate concern in the fed state is handling the glucose load through **direct utilization and glycogen storage**. - Only the excess glucose beyond immediate energy needs and glycogen storage capacity is converted to fat via lipogenesis. *Stimulation of gluconeogenesis* - This is **incorrect** - insulin **inhibits gluconeogenesis** in the fed state by suppressing key enzymes like **PEPCK and glucose-6-phosphatase**. - Stimulating gluconeogenesis would counteract insulin's primary function of **lowering blood glucose levels**. *Increased amino acid entry in the cell* - Insulin does promote **amino acid uptake** and **protein synthesis**, which is an important anabolic action. - However, in the context of the fed state's primary metabolic challenge (handling elevated blood glucose), **glucose utilization through glycolysis** takes precedence as the primary action.
Question 2: Insulin acts through which receptor?
- A. GPCR
- B. Enzyme-linked (Correct Answer)
- C. Intracellular
- D. Ion channel
Explanation: ***Enzyme-linked*** - Insulin binds to an **enzyme-linked receptor**, specifically a **receptor tyrosine kinase**, to initiate its cellular effects. - Upon binding, the receptor undergoes **autophosphorylation** and then phosphorylates intracellular substrate proteins, leading to a cascade of metabolic actions. *GPCR* - **G protein-coupled receptors (GPCRs)** are integral membrane proteins that, upon ligand binding, activate intracellular G proteins, transmitting signals through second messengers. - Hormones like **glucagon** and **epinephrine** commonly act via GPCRs, not insulin. *Intracellular* - **Intracellular receptors** are typically found in the cytoplasm or nucleus and are activated by small, lipid-soluble ligands that can pass through the cell membrane, such as **steroid hormones**. - Insulin is a large peptide hormone and cannot readily cross the cell membrane to act on intracellular receptors. *Ion channel* - **Ion channel receptors** are transmembrane proteins that open or close an ion channel in response to ligand binding, leading to changes in membrane potential or ion concentration. - Neurotransmitters like **acetylcholine** often act on ligand-gated ion channels, which is not the mechanism of action for insulin.
Question 3: Which of the following is FALSE about insulin action?
- A. Insulin promotes glycolysis
- B. Insulin promotes ketogenesis (Correct Answer)
- C. Insulin promotes glycogen synthesis
- D. Insulin promotes lipogenesis
Explanation: ***Insulin promotes ketogenesis*** - Insulin is an **anabolic hormone** that works to prevent excessive **fat breakdown** and the formation of **ketone bodies**. - High insulin levels actively **inhibit** enzymes involved in ketogenesis, such as **carnitine palmitoyltransferase-1 (CPT1)**, thereby reducing the transport of fatty acids into mitochondria for oxidation. *Insulin promotes glycolysis* - Insulin stimulates **glycolysis**, particularly in the liver and muscle, by increasing the activity of key enzymes like **glucokinase** and **phosphofructokinase-1**. - This promotes the breakdown of glucose for **energy production** and provides substrates for fat synthesis. *Insulin promotes glycogen synthesis* - Insulin is a primary regulator of **glycogen synthesis** in the liver and muscles. - It activates **glycogen synthase** and inhibits glycogen phosphorylase, thereby shunting glucose towards storage as **glycogen**. *Insulin promotes lipogenesis* - Insulin promotes **lipogenesis** (fat synthesis) in adipose tissue and liver. - It increases glucose uptake into adipocytes and stimulates enzymes like **acetyl-CoA carboxylase** and **fatty acid synthase**, converting excess carbohydrates into fatty acids and subsequently **triglycerides**.
Question 4: Which of the following statements best describes the mechanism of action of insulin on target cells?
- A. Insulin binds to a receptor on the outer surface of the plasma membrane, activating adenylate cyclase through the Gs protein.
- B. Insulin binds to a cytoplasmic receptor and is transferred as a hormone receptor complex to the nucleus to modulate gene expression.
- C. Insulin enters the cell and causes the release of calcium ions from intracellular stores.
- D. Insulin binds to a transmembrane receptor on the outer surface of the plasma membrane, activating the tyrosine kinase in the cytosolic domain of the receptor. (Correct Answer)
Explanation: ***Insulin binds to a transmembrane receptor on the outer surface of the plasma membrane, activating the tyrosine kinase in the cytosolic domain of the receptor.*** - **Insulin** is a **peptide hormone** and cannot freely pass through the lipid bilayer, thus it binds to a **transmembrane receptor** on the cell surface. - This binding leads to the activation of the receptor's intrinsic **tyrosine kinase activity** in the intracellular domain, initiating a signaling cascade. *Insulin binds to a cytoplasmic receptor and is transferred as a hormone receptor complex to the nucleus to modulate gene expression.* - This mechanism describes the action of **steroid hormones**, which are lipid-soluble and can cross the cell membrane, binding to **intracellular receptors**. - **Insulin** acts via a **cell surface receptor** and its downstream effects are mediated through signal transduction pathways, not direct nuclear translocation. *Insulin binds to a receptor on the outer surface of the plasma membrane, activating adenylate cyclase through the Gs protein.* - This mechanism is characteristic of **G-protein coupled receptors (GPCRs)**, which activate or inhibit enzymes like adenylate cyclase via G-proteins to produce second messengers like cyclic AMP. - The **insulin receptor** is a **receptor tyrosine kinase**, not a GPCR, and does not directly activate adenylate cyclase via Gs protein. *Insulin enters the cell and causes the release of calcium ions from intracellular stores.* - While some hormones and neurotransmitters can trigger the release of intracellular **calcium ions**, this is typically mediated by specific pathways (e.g., GPCRs linked to phospholipase C). - **Insulin** does not directly enter target cells to cause calcium release; its actions are primarily mediated through receptor tyrosine kinase signaling pathways.
Question 5: Two normal, healthy subjects volunteer for a study on insulin secretion. In Patient 1, blood glucose is increased to 150 mg/dL by direct intravenous infusion. In Patient 2, blood glucose is increased to 150 mg/dL by ingestion of oral glucose. The peak plasma insulin concentration produced in Patient 1 is 70 uU/mL while in Patient 2, it is 95 uU/mL. Which of the following best explains the higher insulin concentration in Patient 2?
- A. Ingested glucose increases duodenal secretion of gastric inhibitory peptide (GIP), increasing beta cell release of insulin (Correct Answer)
- B. Intravenous glucose increases islet cell secretion of somatostatin, inhibiting beta cell release of insulin
- C. Intravenous glucose increases islet cell secretion of glucagon, inhibiting beta cell release of insulin
- D. Ingested glucose activates a sympathetic reflex that increases beta cell release of insulin
Explanation: ***Ingested glucose increases duodenal secretion of gastric inhibitory peptide (GIP), increasing beta cell release of insulin*** - **Oral glucose ingestion** stimulates the release of **incretin hormones** like **GIP (glucose-dependent insulinotropic polypeptide)** and GLP-1 (glucagon-like peptide-1) from the small intestine. - These **incretins amplify glucose-stimulated insulin secretion** from pancreatic beta cells, explaining the **higher insulin response** in Patient 2 compared to Patient 1, who received intravenous glucose and thus bypassed the intestinal incretin release. *Intravenous glucose increases islet cell secretion of somatostatin, inhibiting beta cell release of insulin* - While somatostatin does inhibit insulin secretion, its release is typically stimulated by high nutrient levels or certain hormones, not directly by **intravenous glucose** in a way that would explain the difference between the two patients. - The primary physiological difference between oral and intravenous glucose administration regarding insulin response is the **incretin effect**, not differential somatostatin secretion. *Intravenous glucose increases islet cell secretion of glucagon, inhibiting beta cell release of insulin* - **Glucagon** is generally secreted in response to **low blood glucose** and works to raise it, not inhibit insulin release in this specific context. - Furthermore, **glucose stimulation typically suppresses glucagon secretion**, so an increase in glucagon with intravenous glucose is unlikely and wouldn't explain the lower insulin. *Ingested glucose activates a sympathetic reflex that increases beta cell release of insulin* - The **sympathetic nervous system** generally **inhibits insulin secretion** (via alpha-adrenergic receptors) and stimulates glucagon secretion, particularly during stress or exercise. - Therefore, an activation of a sympathetic reflex due to ingested glucose would more likely *decrease* or have a minimal effect on insulin release, rather than increasing it.
Question 6: Select the correct sequence of events in the cAMP signaling pathway.
- A. Adenylyl cyclase converts ATP to cAMP, which activates PKA. (Correct Answer)
- B. PKA is activated before cAMP is formed.
- C. Adenylyl cyclase activates PKA before producing cAMP.
- D. cAMP directly activates adenylyl cyclase to produce more cAMP.
Explanation: ***Adenylyl cyclase converts ATP to cAMP, which activates PKA.*** - **Adenylyl cyclase** is an enzyme that catalyzes the conversion of **ATP (adenosine triphosphate)** into **cyclic AMP (cAMP)**, a crucial second messenger. - Subsequently, **cAMP** binds to and activates **Protein Kinase A (PKA)**, which then phosphorylates various target proteins to mediate cellular responses. *PKA is activated before cAMP is formed.* - **cAMP formation** is a prerequisite for **PKA activation**; PKA cannot be activated independently before cAMP is produced. - The binding of **cAMP** to the regulatory subunits of **PKA** is what causes the dissociation and activation of its catalytic subunits. *Adenylyl cyclase activates PKA before producing cAMP.* - **Adenylyl cyclase's** sole function in this pathway is to synthesize **cAMP** from ATP; it does not directly activate PKA. - **PKA activation** is mediated by **cAMP**, not directly by adenylyl cyclase. *cAMP directly activates adenylyl cyclase to produce more cAMP.* - While **cAMP** is a critical messenger, it does not directly activate **adenylyl cyclase** to produce more of itself in a positive feedback loop. - **Adenylyl cyclase** is typically activated by **G-protein coupled receptors (GPCRs)** binding to their ligands, which then stimulate G proteins to activate adenylyl cyclase.
Question 7: Which of the following binds to Tyrosine Kinase receptor?
- A. Insulin (Correct Answer)
- B. Glucagon
- C. Prolactin
- D. Growth Hormone
Explanation: ***Insulin*** - **Insulin** is a classic example of a hormone that binds to and activates a **tyrosine kinase receptor**, leading to a cascade of intracellular signaling events for glucose uptake and metabolism. - The **insulin receptor** is a heterodimeric protein with intrinsic tyrosine kinase activity that phosphorylates itself and other proteins upon insulin binding. *Glucagon* - **Glucagon** primarily acts on **G protein-coupled receptors (GPCRs)**, specifically the glucagon receptor, to increase cyclic AMP (cAMP) and activate protein kinase A. - Its main roles are to stimulate **glycogenolysis** and **gluconeogenesis** in the liver. *Prolactin* - **Prolactin** binds to a receptor that is a member of the **cytokine receptor superfamily**, which lacks intrinsic enzyme activity. - Upon ligand binding, these receptors associate with and activate **Janus kinases (JAKs)**, leading to the JAK-STAT signaling pathway. *Growth Hormone* - **Growth hormone (GH)** also binds to a receptor belonging to the **cytokine receptor superfamily** (similar to prolactin), which then associates with and activates **JAKs**. - This activation subsequently initiates the **JAK-STAT signaling pathway**, mediating its diverse growth-promoting and metabolic effects.
Question 8: What is the primary effect of GLP-1 on insulin secretion?
- A. Increased aldosterone secretion by adrenal
- B. Increased PTH secretion
- C. Increased insulin secretion from beta-cells of pancreas (Correct Answer)
- D. Increased testosterone secretion from Leydig cells
Explanation: ***Increased insulin secretion from beta-cells of pancreas*** - **Glucagon-like peptide-1 (GLP-1)** is an **incretin hormone** that stimulates **glucose-dependent insulin secretion** from pancreatic beta-cells. - This effect is crucial for maintaining **glucose homeostasis**, especially after a meal. *Increased aldosterone secretion by adrenal* - **Aldosterone secretion** is primarily regulated by the **renin-angiotensin-aldosterone system (RAAS)** and potassium levels, not directly by GLP-1. - Aldosterone's main function is to regulate **sodium and water balance** and **blood pressure**. *Increased PTH secretion* - **Parathyroid hormone (PTH)** secretion is primarily regulated by **serum calcium levels**. - Its main role is to maintain **calcium homeostasis** by affecting bone, kidney, and intestine. *Increased testosterone secretion from Leydig cells* - **Testosterone secretion** from Leydig cells is primarily regulated by **luteinizing hormone (LH)** from the pituitary gland. - GLP-1 has no direct significant role in **gonadal steroidogenesis**.
Question 9: What is the primary function of G-proteins in cellular signaling?
- A. Signal transducers (Correct Answer)
- B. Mediators of hormone action
- C. Molecules that bind hormones
- D. Intracellular signaling molecules
Explanation: ***Signal transducers*** - G-proteins act as **molecular switches**, converting extracellular signals received by G protein-coupled receptors (GPCRs) into intracellular responses. - They bind **GTP** in their active state and **hydrolyze it to GDP** to become inactive, regulating downstream effectors like enzymes and ion channels. *Mediators of hormone action* - While G-proteins are involved in the action of many hormones, this describes a *result* of their function rather than their fundamental role. - Their primary function is to transduce signals, which then mediates hormone effects. *Molecules that bind hormones* - **Receptors**, not G-proteins, are primarily responsible for binding hormones or other ligands. - G-proteins are activated *after* a receptor binds a ligand and undergoes a conformational change. *Intracellular signaling molecules* - This statement is true, but it's a broad category. **Signal transducers** specifically highlights their role in converting one form of signal to another. - Many molecules operate intracellularly, but G-proteins' unique role is in linking receptor activation to effector modulation.
Question 10: Which of the following genes is least likely to be involved in the development of carcinoma of the colon?
- A. K-ras
- B. Beta-Catenin (Correct Answer)
- C. APC
- D. Mismatch Repair Genes
Explanation: ***Beta-Catenin*** - While **beta-catenin protein accumulation** is critical in colorectal cancer pathogenesis (primarily through APC mutations), direct mutations in the **CTNNB1 gene** (encoding beta-catenin) are **rare in colorectal cancer** (~5% of cases) [1]. - Most colorectal cancers achieve beta-catenin activation indirectly through **APC inactivation**, making beta-catenin gene mutations the least likely mechanism among the listed options [1]. - This contrasts with other cancers (e.g., hepatocellular carcinoma, endometrial cancer) where direct CTNNB1 mutations are more common. *APC* - The **adenomatous polyposis coli (APC) gene** is mutated in approximately **80% of sporadic colorectal cancers**, representing the earliest and most common genetic alteration in the **adenoma-carcinoma sequence** [1]. - APC loss leads to beta-catenin accumulation and constitutive **Wnt pathway activation**, driving uncontrolled cell proliferation [2]. - Germline APC mutations cause **familial adenomatous polyposis (FAP)** [5]. *K-ras* - **K-ras oncogene** mutations occur in **30-50% of colorectal cancers**, typically as an intermediate event in the adenoma-carcinoma progression [1]. - These activating mutations lead to constitutive signaling through the **MAPK pathway**, promoting cell proliferation and survival independent of growth factor signals. *Mismatch Repair Genes* - **Mismatch repair (MMR) genes** (MLH1, MSH2, MSH6, PMS2) are involved in **15-20% of all colorectal cancers** [4]. - Germline mutations cause **Lynch syndrome (HNPCC)** (~3% of CRCs) [5]. - Sporadic **MLH1 promoter hypermethylation** accounts for 12-15% of colorectal cancers, leading to **microsatellite instability (MSI-high)** tumors [3]. - MMR deficiency represents an alternative, well-established pathway of colorectal carcinogenesis. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Gastrointestinal Tract, p. 819. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 304-305. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Gastrointestinal Tract, pp. 819-821. [4] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Alimentary System Disease, pp. 373-374. [5] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Gastrointestinal Tract, pp. 821-822.
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