Signal Transduction — MCQs

Signal Transduction Indian Medical PG Practice Questions and MCQs

Question 131: Which of the following statements is NOT true regarding transforming growth factor-beta (TGF-beta)?

A. Proliferation of fibroblasts
B. Inhibition of T-cell proliferation
C. Activation of macrophages (Correct Answer)
D. Chemotaxis of fibroblasts

Explanation: ***Activation of macrophages*** - Transforming growth factor (TGF) **does not activate macrophages**; rather, it can have an immunosuppressive effect on their activity. - The primary role of TGF is in **regulating inflammation** and promoting tissue repair, which does not include direct activation of macrophages. *Proliferation of endothelial cells* - TGF is known to stimulate the **proliferation of endothelial cells**, contributing to new blood vessel formation (angiogenesis). However, TGF-β can also suppress endothelial proliferation and migration while enhancing ECM protein production [1]. - It plays a significant role in **vascular remodeling** during wound healing and tissue regeneration. *Proliferation for fibroblast* - TGF is a key mediator in **fibroblast proliferation**, promoting production of extracellular matrix components [2]. - This process is essential for tissue healing and fibrosis, contrary to the statement in the question. *Chemotaxis of fibroblasts* - TGF facilitates **chemotaxis of fibroblasts**, guiding them to sites of injury for tissue repair [2]. - This chemotactic property is significant in **fibrosis** and tissue remodeling processes. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, pp. 115-116. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, pp. 117-119.

Question 132: Which of the following statements about G protein-coupled receptors (GPCRs) is true?

A. The three subunits alpha, beta, and gamma must remain together as a complex for G protein to function.
B. G proteins can act as either inhibitory or excitatory based on the type of alpha subunit. (Correct Answer)
C. G proteins bind directly to hormones to become activated.
D. In the resting state, G proteins are bound to GTP.

Explanation: ***G proteins can act as either inhibitory or excitatory based on the type of alpha subunit.*** - Different classes of Gα subunits (e.g., **Gαs**, **Gαi**, **Gαq**) couple to diverse downstream effectors, leading to either **stimulation** (excitatory) or **inhibition** of cellular processes. - For example, **Gαs** activates adenylyl cyclase, while **Gαi** inhibits it, demonstrating their opposing roles. *The three subunits alpha, beta, and gamma must remain together as a complex for G protein to function.* - Upon activation, the **Gα subunit dissociates** from the **Gβγ dimer**, and both free units can then independently modulate effector molecules. - For the G protein to function in signal transduction, the α subunit often separates from the βγ dimer to interact with its target enzyme or ion channel. *G proteins bind directly to hormones to become activated.* - **GPCRs** (the receptors themselves) bind to hormones or other ligands on the **extracellular side** of the membrane. - The binding of the ligand to the GPCR induces a conformational change in the receptor, which then activates the associated G protein on the intracellular side. *In the resting state, G proteins are bound to GTP.* - In the **resting (inactive) state**, the Gα subunit of the trimeric G protein is bound to **GDP**. - Activation occurs when the GPCR facilitates the exchange of **GDP for GTP** on the Gα subunit.

Question 133: Thyroid hormone receptors bind to DNA as a heterodimer with which of the following?

A. Prolactin receptor
B. Growth hormone receptor
C. Retinoid X receptor (Correct Answer)
D. Insulin receptor

Explanation: ***Retinoid X receptor*** - Thyroid hormone receptors (TRs) bind to **thyroid hormone response elements (TREs)** on DNA as a **heterodimer** with the **retinoid X receptor (RXR)**. - This TR/RXR heterodimer is a crucial component for the transcriptional regulation of genes by thyroid hormones. *Prolactin receptor* - The prolactin receptor is a **cytokine receptor** that signals through the **JAK/STAT pathway** and is not involved in direct DNA binding as a heterodimer with TR. - It plays a role in breast development and lactation, distinct from thyroid hormone action. *Growth hormone receptor* - The growth hormone receptor is also a **cytokine receptor** that mediates its effects primarily through the **JAK/STAT signaling pathway**, without forming a heterodimer with TR for DNA binding. - Its main roles include promoting growth and metabolism. *Insulin receptor* - The insulin receptor is a **receptor tyrosine kinase** that, upon binding insulin, initiates a cascade of intracellular signaling events, primarily involving **phosphorylation**. - It does not form a heterodimer with the thyroid hormone receptor to bind to DNA, as its mechanism of action is distinct.

Question 134: Which is the central organelle in apoptosis?

A. Mitochondria (Correct Answer)
B. Nucleus
C. Endoplasmic Reticulum
D. Golgi body

Explanation: ***Mitochondria*** - Mitochondria play a crucial role in the intrinsic pathway of **apoptosis** by releasing cytochrome c and other pro-apoptotic factors [1]. - They are involved in the **regulation of cellular energy** and initiation of apoptotic signaling cascades, making them central to the process [1][2]. *Nucleus* - While the nucleus is involved in gene expression and can signal apoptosis, it is not the **central organ** where the apoptotic process primarily initiates. - The nucleus contains **DNA** that may undergo fragmentation during apoptosis, but it is the mitochondria that trigger these changes [1]. *Endoplasmic Reticulum* - The Endoplasmic Reticulum is essential for **protein synthesis** and can trigger apoptosis under stress (ER stress), but is not central in overall apoptosis regulation. - It influences **calcium homeostasis**, but the mitochondria have a more pivotal role in cell death mechanisms [1]. *Golgi body* - The Golgi body is primarily involved in the **modification and transport** of proteins, not in apoptosis. - Although it may indirectly affect cell function, it does not play a direct role in initiating or regulating the apoptotic process. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death, pp. 64-65. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death, pp. 65-67.

Question 135: Which of the following is not classified as a chemokine?

A. IL-8
B. Eotaxin
C. MCP-1
D. IL-1 (Correct Answer)

Explanation: ***Histamine*** - Histamine is a **biogenic amine** involved in local immune responses, not classified as a chemokine [1]. - It functions primarily in **vasodilation** and **increased vascular permeability**, contrasting with chemokine roles. *IL-1* - IL-1 is a **cytokine** that plays a role in inflammatory responses but is not a chemokine [1]. - It primarily acts as a mediator for **fever** and **acute inflammation**. *IL-8* - IL-8 is a **chemokine** specifically known for attracting **neutrophils** to sites of inflammation [1]. - It plays a crucial role in **immune response** and is classified within the CXC chemokine family. *Eotaxin* - Eotaxin is a specific **chemokine** that primarily attracts **eosinophils** to sites of inflammation, especially in allergic reactions. - It is involved in the pathogenesis of **asthma** and other eosinophil-associated conditions. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, pp. 93-94.

Question 136: During growth factor-induced cellular regeneration, which transition during the cell cycle is controlled by the phosphorylation of the retinoblastoma protein?

A. S to G2
B. G2 to M
C. G0 to G1
D. G1 to S (Correct Answer)

Explanation: ***G1 to S*** - The **phosphorylation of the retinoblastoma protein (Rb)** is crucial for the **G1 checkpoint**, allowing progression into the S phase for DNA synthesis [1]. - This transition is a key regulatory point in the cell cycle, determining whether the cell commits to division or remains quiescent [2]. *G2 to M* - This transition involves the **activation of cyclin-dependent kinases (CDKs)**, but it is **not primarily regulated by Rb** phosphorylation. - Instead, it features the **cyclin B/CDK1 complex** which is crucial for mitosis initiation. *G0 to G1* - Transitioning from G0 to G1 typically involves signals that promote **cell re-entry** into the cell cycle, not the **phosphorylation of Rb**. - This phase is characterized by cellular **growth signals** but does not involve direct Rb regulation. *S to G2* - In this phase, cells are primarily focused on **DNA replication** and preparing for mitosis rather than Rb activity. - Rb is primarily active during the **G1 to S phase** transition, thus making its role in the S to G2 transition minimal [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 300-301. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 297-298.

Question 137: 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 138: Which of the following statements accurately describes G proteins?

A. Are associated with cellular membranes and play a crucial role in signal transduction. (Correct Answer)
B. Regulate second messengers like cyclic adenosine monophosphate (cAMP).
C. Play a role in the amplification of hormonal signals.
D. Consist of three subunits: alpha, beta, and gamma.

Explanation: ***Are associated with cellular membranes and play a crucial role in signal transduction.*** - **G proteins** are critical components of **G protein-coupled receptors (GPCRs)**, which are embedded in the **cellular membrane**. - They act as molecular switches, relaying signals from diverse extracellular stimuli (like hormones, neurotransmitters, and light) across the cell membrane into the cell's interior, thus initiating a **signal transduction pathway**. - This is the most comprehensive and accurate description of G proteins as a whole. *Regulate second messengers like cyclic adenosine monophosphate (cAMP).* - While G proteins do regulate second messengers such as **cAMP** by activating enzymes like **adenylyl cyclase**, this describes a **specific mechanism of action**, not a broad description of what G proteins are. - This statement is accurate but too narrow, describing one particular function rather than their fundamental role in membrane association and general signal transduction. *Play a role in the amplification of hormonal signals.* - G proteins are involved in signal transduction pathways that can lead to **signal amplification**, but this is a **downstream effect**, not their primary defining characteristic. - Mentioning their role only in amplification of hormonal signals is too narrow and doesn't capture their fundamental nature as signal transducers. *Consist of three subunits: alpha, beta, and gamma.* - This statement accurately describes **heterotrimeric G proteins** (the most common type involved in GPCR signaling), which do have three subunits (Gα, Gβ, Gγ). - However, this is **incorrect as a general description** because there are also **monomeric G proteins** (small GTPases like Ras, Rho, Rac, and Rab) that consist of a single polypeptide chain. - Since the question asks about "G proteins" in general without specifying heterotrimeric G proteins, this statement is incomplete and therefore incorrect.

Question 139: Phospholipid associated with the mechanism of hormone action is

A. Phosphatidylcholine
B. Phosphatidylethanolamine
C. Plasmalogen
D. Phosphatidylinositol (Correct Answer)

Explanation: ***Phosphatidylinositol*** (Correct) - **Phosphatidylinositol (PI)** and its phosphorylated derivatives, particularly **PIP2 (phosphatidylinositol 4,5-bisphosphate)**, are critical in signal transduction pathways activated by many hormones. - Hormones binding to **G protein-coupled receptors** can activate phospholipase C, which cleaves PIP2 into **inositol triphosphate (IP3)** and **diacylglycerol (DAG)**, leading to increased intracellular calcium and protein kinase C activation, respectively. *Phosphatidylcholine* (Incorrect) - **Phosphatidylcholine** is a major component of cell membranes and is involved in membrane structure and fluidity. - While it can be a source of signaling molecules like **lysophosphatidic acid**, it is not primarily associated with the initial intracellular signaling events of hormone action in the same way as phosphatidylinositol. *Phosphatidylethanolamine* (Incorrect) - **Phosphatidylethanolamine** is another abundant membrane phospholipid primarily involved in membrane structure and stability. - It can be a precursor for other lipids, but it does not directly participate in the **second messenger systems** triggered by most hormones as a primary signaling molecule. *Plasmalogen* (Incorrect) - **Plasmalogens** are a unique class of phospholipids containing an ether bond at the sn-1 position. - They are abundant in certain tissues, particularly nervous and cardiovascular tissues, and are thought to have antioxidant properties, but they are not directly involved in the initiating events of **hormone signaling pathways**.

Question 140: Many signaling pathways involve the generation of inositol trisphosphate (IP3) and diacylglycerol (DAG). These molecules are involved in the regulation of which cellular processes?

A. DNA replication and cell division
B. Protein synthesis and degradation
C. Lipid metabolism and fatty acid synthesis
D. Calcium signaling and protein kinase C activation (Correct Answer)

Explanation: ***Calcium signaling and protein kinase C activation*** - **Inositol trisphosphate (IP3)** binds to **IP3 receptors** on the **endoplasmic reticulum**, triggering the release of **calcium ions (Ca²⁺)** into the cytoplasm, thereby initiating **intracellular calcium signaling cascades**. - **Diacylglycerol (DAG)** remains in the **plasma membrane** and activates **protein kinase C (PKC)**, which phosphorylates various target proteins to regulate cellular responses. - Together, IP3 and DAG form a classic **second messenger system** downstream of **phospholipase C (PLC)** activation, mediating responses from **G-protein coupled receptors (GPCRs)** and **receptor tyrosine kinases (RTKs)**. - This is the **primary and direct function** of the IP3/DAG signaling pathway in cell biology. *Lipid metabolism and fatty acid synthesis* - While DAG is structurally a **lipid intermediate** and can serve as a precursor in lipid metabolism, this is **not its primary role in signaling pathways**. - The question specifically asks about their involvement in **signaling pathways**, where IP3 and DAG function as **second messengers**, not metabolic intermediates. - Their generation from **PIP2 cleavage** by phospholipase C is a **signaling event**, not a metabolic one. *DNA replication and cell division* - Cell division is regulated by **cyclins**, **cyclin-dependent kinases (CDKs)**, and **checkpoint proteins**. - While IP3/DAG signaling can have **downstream effects** on cell proliferation through PKC activation, this is not their primary or direct role. - DNA replication machinery is not directly regulated by IP3 or DAG. *Protein synthesis and degradation* - **Protein synthesis** is primarily regulated by the **mTOR pathway**, **eIF factors**, and **ribosomal proteins**. - **Protein degradation** is mediated by the **ubiquitin-proteasome system** and **autophagy-lysosome pathway**. - While PKC activation can have indirect effects on these processes, IP3/DAG are not primary regulators of protein synthesis or degradation.

Practice by Chapter

Cell Surface Receptors: Types and Functions

Practice Questions

G-Protein Coupled Receptors

Practice Questions

Enzyme-Linked Receptors

Practice Questions

Second Messengers in Signal Transduction

Practice Questions

cAMP and cGMP Signaling

Practice Questions

Calcium as Second Messenger

Practice Questions

Inositol Phosphate Pathway

Practice Questions

MAP Kinase Cascades

Practice Questions

JAK-STAT Signaling Pathway

Practice Questions

Insulin Signaling Pathway

Practice Questions

Nuclear Receptors and Gene Regulation

Practice Questions

Defects in Signal Transduction and Disease

Practice Questions

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