Signal Transduction Practice Questions

Q: Growth factors promote cell growth by acting on:

Tyrosine Kinase. ***Tyrosine Kinase*** - Growth factors typically bind to **receptor tyrosine kinases (RTKs)**, activating their enzymatic activity. - This activation leads to the **phosphorylation of tyrosine residues** on target proteins, initiating intracellular signaling cascades that promote cell growth, proliferation, and differentiation. *cAMP* - **Cyclic AMP (cAMP)** is a second messenger often associated with **G-protein-coupled receptors (GPCRs)**, not directly with growth factor signaling. - While cAMP plays a role in various cellular processes, it is not the primary direct target for growth factor-mediated cell growth. *G-protein-coupled receptor (GPCR)* - **GPCRs** are a large family of receptors that mediate responses to a wide range of extracellular signals, but they are generally not the primary receptors for **growth factors**. - Growth factors predominantly utilize **receptor tyrosine kinases** for their signaling. *cGMP* - **Cyclic GMP (cGMP)** is a second messenger involved in various signaling pathways, notably those regulated by **nitric oxide** and **natriuretic peptides**. - It is not the main signaling molecule directly activated by growth factors to promote cell growth.

Q: Which of the following binds to Tyrosine Kinase receptor?

Insulin. ***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.

Q: Products of Phospholipase C are -

Inositol triphosphate and Diacylglycerol. ***Inositol triphosphate and Diacylglycerol*** - **Phospholipase C (PLC)** cleaves **phosphatidylinositol 4,5-bisphosphate (PIP2)** into two important second messengers: **inositol 1,4,5-trisphosphate (IP3)** and **diacylglycerol (DAG)**. - **IP3** mobilizes intracellular **calcium**, while **DAG** activates **protein kinase C (PKC)**, both crucial for signal transduction pathways. *Lysophospholipid and fatty acid* - These are typically products of **Phospholipase A2**, which hydrolyzes a **fatty acid** from the *sn-2* position of a glycerophospholipid, leaving a **lysophospholipid**. - **Phospholipase C** acts on a different bond within the **phospholipid structure** and produces different second messengers. *Phosphatidate and IP2* - **Phosphatidate** is a precursor in lipid synthesis and is not directly produced by **PLC's primary action** on **PIP2**. - **IP2 (Inositol bisphosphate)** is a dephosphorylated product of **IP3** and not the direct initial product of **PLC** cleavage from **PIP2**. *Inositol and diacylglycerol* - **Inositol** itself is a cyclic alcohol and is part of the **inositol phosphate** structures. It is not directly released as a standalone product by **PLC** cleavage of **PIP2**. - It is **inositol trisphosphate (IP3)**, not free inositol, that is the second messenger formed alongside **diacylglycerol**.

Q: APAF-1 is involved in the activation of which of the following caspases?

Caspase 9. ***Caspase 9*** - **APAF-1** (Apoptotic Protease Activating Factor-1) is a key component of the **apoptosome**, which is formed in response to intrinsic apoptotic signals. - The apoptosome recruits and activates **pro-caspase 9**, leading to its proteolytic cleavage and activation into **caspase 9**. *Caspase 8* - **Caspase 8** is primarily involved in the **extrinsic apoptotic pathway**, activated by cell surface death receptors and DISC (Death-Inducing Signaling Complex) formation. - It is not directly activated by APAF-1, which is specific to the intrinsic pathway. *Caspase 10* - **Caspase 10** is also an initiator caspase often associated with the **extrinsic apoptotic pathway**, similar to caspase 8. - Its activation typically occurs within the DISC and not through the apoptosome complex involving APAF-1. *Caspase 3* - **Caspase 3** is an **effector caspase** that is activated by upstream initiator caspases (like caspase 9 or caspase 8) in both intrinsic and extrinsic apoptotic pathways. - While it's a crucial caspase in apoptosis, it is too far downstream and not directly activated by APAF-1; APAF-1 activates caspase 9, which then activates caspase 3.

Q: Select the correct sequence of events in the cAMP signaling pathway.

Adenylyl cyclase converts ATP to cAMP, which activates PKA.. ***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.

Q: Which second messenger is directly generated by G-protein-coupled receptors (GPCRs)?

cAMP. ***cAMP*** - Upon activation, certain **Gα subunits** (e.g., Gs) of GPCRs directly activate **adenylyl cyclase**, which then catalyzes the conversion of ATP to **cAMP**. - cAMP acts as a **second messenger** by activating **protein kinase A (PKA)**, leading to a cascade of phosphorylation events. *ATP* - **ATP** is the precursor molecule for **cAMP** synthesis, but it is not a second messenger itself in this pathway. - Its primary role in this context is as a substrate for adenylyl cyclase, not as a direct signaling molecule. *cGMP* - **cGMP** is a second messenger involved in other signaling pathways, often activated by **guanylyl cyclase** (e.g., in response to nitric oxide or atrial natriuretic peptide). - While it's a cyclic nucleotide like cAMP, it is not directly produced by GPCR activation in the same manner. *GTP* - **GTP** is crucial for the function of G-proteins; it binds to the **α subunit** of G-proteins, displacing GDP and leading to G-protein activation. - However, GTP itself is not generally considered a direct "second messenger" in the same way cAMP functions downstream of GPCRs.

Q: A mutation that leads to overactivity of phosphoinositide 3-kinase (PI3K) would most likely result in increased activation of which pathway?

mTOR pathway. ***mTOR pathway*** - **PI3K** activation leads to the production of **PIP3**, which recruits and activates **PDK1** and **Akt** (Protein Kinase B) to the plasma membrane. - Activated **Akt** directly phosphorylates and inhibits **TSC1/TSC2 complex**, a negative regulator of **mTORC1**, thereby promoting **mTOR pathway** activity. *MAPK/ERK signaling pathway* - The **MAPK/ERK pathway** is primarily activated by **Ras** small G-proteins, which are often downstream of **receptor tyrosine kinases** but operate largely independently of the **PI3K/Akt axis**. - While there can be crosstalk, **PI3K overactivity** directly and most prominently impacts **Akt** and its downstream targets, not typically directly enhancing **MAPK/ERK** cascade. *IP3/DAG signaling pathway* - This pathway is initiated by the cleavage of **PIP2** by **phospholipase C (PLC)** into **inositol triphosphate (IP3)** and **diacylglycerol (DAG)**. - **PI3K** phosphorylates **PIP2** to form **PIP3**, which is a distinct lipid signaling molecule and does not directly enhance the production of **IP3** and **DAG**. *cAMP/PKA signaling pathway* - The **cAMP/PKA pathway** is typically activated by **G protein-coupled receptors** leading to the activation of **adenylyl cyclase**, which produces **cAMP**. - **PI3K** activity does not directly regulate **adenylyl cyclase** or **cAMP** levels, making it unrelated to this signaling cascade.

Q: In a patient with suspected signaling defects, a mutation was found in the gene encoding a G-protein. Which of the following cellular processes is most likely to be directly affected?

Signal transduction. ***Signal transduction*** - G-proteins are central to **G-protein coupled receptors (GPCRs)**, which are a major class of receptors involved in **signal transduction** pathways. - Mutations in G-proteins directly impair the cell's ability to receive and transmit signals from outside the cell to initiate internal cellular responses. *Transcription regulation* - While signaling pathways eventually can influence **gene expression** and transcription, a direct and immediate effect of a G-protein mutation is not on the regulation of transcription itself. - Transcription regulation involves various **transcription factors** and DNA binding, which are downstream from initial G-protein signaling. *Cell membrane integrity* - G-proteins are peripheral or integral membrane proteins but they do not primarily contribute to the **structural integrity** of the cell membrane. - Cell membrane integrity is maintained by phospholipids, cholesterol, and various other structural proteins. *Protein folding* - Protein folding is a process that occurs in the **endoplasmic reticulum** and cytoplasm, assisted by chaperones, to achieve the correct tertiary structure of a protein. - While G-proteins themselves must be properly folded to function, a mutation in the G-protein gene affects its function in signaling, not the general cellular machinery for **protein folding**.

Q: Which hormones share the alpha subunit with hCG?

All of the above. ***All of the above*** - **Luteinizing hormone (LH)**, **follicle-stimulating hormone (FSH)**, and **thyroid-stimulating hormone (TSH)** all share a common **alpha subunit** with **human chorionic gonadotropin (hCG)**. - The distinct biological activities of these hormones are conferred by their unique **beta subunits**. *LH* - **Luteinizing hormone (LH)** shares the **alpha subunit** with hCG, FSH, and TSH. - Its specific actions, like stimulating ovulation and testosterone production, are derived from its unique **beta subunit**. *TSH* - **Thyroid-stimulating hormone (TSH)** contains the same **alpha subunit** as hCG, LH, and FSH. - The **beta subunit** of TSH is specific to regulating thyroid hormone production. *FSH* - **Follicle-stimulating hormone (FSH)** shares the common **alpha subunit** with hCG, LH, and TSH. - Its role in follicular development and spermatogenesis is dictated by its distinct **beta subunit**.

Q: Which of the following statements about VEGF is false?

Inhibits angiogenesis. ***Inhibits angiogenesis*** - **Incorrect statement**, as VEGF (Vascular Endothelial Growth Factor) actually **promotes angiogenesis**, fostering the development of new blood vessels. - It plays a crucial role in **vascular permeability** and is vital for **tumor growth and survival** [1]. *Highly specific for endothelium* - VEGF is indeed **specific for endothelial cells**, acting primarily on these cells to stimulate their proliferation and migration. - This specificity is central to its role in **angiogenesis**, targeting the endothelial cells of blood vessels [1]. *Hypoxia potentiates its expression* - Contrary to the statement, **hypoxia** (low oxygen levels) actually increases VEGF expression as part of the body's mechanism to enhance oxygen delivery through new vessel formation [1]. - This response is critical in **tumor biology** where rapid cell growth often leads to low oxygen environments. *Helps in tumor metastasis* - VEGF contributes to **tumor progression** by aiding in angiogenesis, but it does not directly cause tumor metastasis. - Metastasis is more related to **cellular adhesion** and **migratory** properties rather than just vascular growth. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 313-314.

Question 1

Growth factors promote cell growth by acting on:

Accepted Answer

Tyrosine Kinase

Answer

cAMP

Answer

G-protein-coupled receptor (GPCR)

Answer

cGMP

Question 2

Which of the following binds to Tyrosine Kinase receptor?

Accepted Answer

Insulin

Answer

Glucagon

Answer

Prolactin

Answer

Growth Hormone

Question 3

Products of Phospholipase C are -

Accepted Answer

Inositol triphosphate and Diacylglycerol

Answer

Lysophospholipid and fatty acid

Answer

Phosphatidate and IP2

Answer

Inositol and diacylglycerol

Question 4

APAF-1 is involved in the activation of which of the following caspases?

Accepted Answer

Caspase 9

Answer

Caspase 8

Answer

Caspase 10

Answer

Caspase 3

Question 5

Select the correct sequence of events in the cAMP signaling pathway.

Accepted Answer

Adenylyl cyclase converts ATP to cAMP, which activates PKA.

Answer

PKA is activated before cAMP is formed.

Answer

Adenylyl cyclase activates PKA before producing cAMP.

Answer

cAMP directly activates adenylyl cyclase to produce more cAMP.

Question 6

Which second messenger is directly generated by G-protein-coupled receptors (GPCRs)?

Accepted Answer

cAMP

Answer

ATP

Answer

cGMP

Answer

GTP

Question 7

A mutation that leads to overactivity of phosphoinositide 3-kinase (PI3K) would most likely result in increased activation of which pathway?

Accepted Answer

mTOR pathway

Answer

MAPK/ERK signaling pathway

Answer

IP3/DAG signaling pathway

Answer

cAMP/PKA signaling pathway

Question 8

In a patient with suspected signaling defects, a mutation was found in the gene encoding a G-protein. Which of the following cellular processes is most likely to be directly affected?

Accepted Answer

Signal transduction

Answer

Transcription regulation

Answer

Cell membrane integrity

Answer

Protein folding

Question 9

Which hormones share the alpha subunit with hCG?

Accepted Answer

All of the above

Answer

LH

Answer

TSH

Answer

FSH

Question 10

Which of the following statements about VEGF is false?

Accepted Answer

Inhibits angiogenesis

Answer

Hypoxia potentiates its expression

Answer

Helps in tumor metastasis

Answer

Highly specific for endothelium

Signal Transduction — MCQs

Signal Transduction — MCQs

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