Question

Which one of the following correctly describes the mode of action of Follicle Stimulating Hormone (FSH) and estrogen?

• A. FSH interacts with membrane-bound receptor and generates cyclic AMP, while estrogen interacts with intracellular receptor and regulates gene expression
• B. FSH interacts with intracellular receptor and generates cyclic AMP, while estrogen interacts with membrane-bound receptor and regulates cellular metabolism
• C. Both FSH and estrogen regulate gene expression via intracellular receptors
• D. Both FSH and estrogen regulate cellular metabolism via membrane-bound receptors

Hint:

Protein/Peptide Hormones (FSH, LH, Insulin, Glucagon) = Membrane receptors + Second messenger (cAMP, $IP_3$, $Ca^{2+}$).
Steroid/Thyroid Hormones (Estrogen, Progesterone, Testosterone, Thyroxine) = Intracellular/Nuclear receptors + Direct Gene Regulation.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question asks us to compare the mechanisms of action and signal transduction pathways of two different classes of hormones: Follicle Stimulating Hormone (FSH) and estrogen.

Step 2: Key Formula or Approach:
Hormones are classified biochemically into two main categories which dictate their mode of action:
1. Water-soluble hormones (e.g., proteins/peptides): Cannot cross the lipid bilayer of the cell membrane, so they bind to extracellular, membrane-bound receptors and utilize second messengers.
2. Lipid-soluble hormones (e.g., steroids): Readily diffuse through the cell membrane and bind to intracellular receptors to directly alter gene transcription.

Step 3: Detailed Explanation:

• Follicle Stimulating Hormone (FSH):
- FSH is a glycoprotein (peptide) hormone secreted by the anterior pituitary.
- Because it is large and hydrophilic, it cannot cross the hydrophobic plasma membrane of its target cells (granulosa cells in ovaries, Sertoli cells in testes).
- It binds to specific extracellular GPCRs (G-Protein Coupled Receptors) on the cell membrane.
- This binding activates the enzyme adenylyl cyclase, which catalyzes the conversion of ATP into cyclic AMP (cAMP), a second messenger that triggers a downstream kinase cascade.

• Estrogen:
- Estrogen is a lipophilic steroid hormone derived from cholesterol.
- It easily diffuses across the phospholipid bilayer of the target cell membrane.
- Inside the cell, it binds to specific intracellular receptors (specifically estrogen receptors located in the cytoplasm or nucleus).
- The hormone-receptor complex then translocates to the nucleus, where it binds directly to hormone response elements on DNA to regulate transcription and gene expression.

Step 4: Final Answer:
FSH acts via membrane-bound receptors and cyclic AMP, while estrogen acts via intracellular receptors regulating gene expression. This corresponds to option (A).