Q. No26. Which one of the following hormones initiates a signal cascade by directly binding to an intra-cellular receptor?

Progesterone initiates a signal cascade by directly binding to an intracellular receptor, distinguishing it from the other options which use cell surface receptors.​

This is a multiple-choice question relevant for CSIR NET Life Sciences exams on hormone signaling mechanisms. The correct answer is (C) Progesterone.

Option Analysis

Hormones are classified by solubility: lipid-soluble ones like steroids diffuse through the cell membrane to bind intracellular receptors, altering gene expression, while water-soluble ones bind cell surface receptors to trigger second messenger cascades.​

• (A) Insulin: Binds to a transmembrane receptor tyrosine kinase on the cell surface, leading to autophosphorylation and downstream signaling via IRS proteins and PI3K pathway.​

• (B) Gonadotropin (e.g., FSH, LH): Glycoprotein hormones that bind G-protein coupled receptors (GPCRs) on the gonadal cell membrane, activating adenylate cyclase and cAMP production.​

• (C) Progesterone: A steroid hormone that crosses the plasma membrane and binds intracellular progesterone receptors (PR-A/PR-B) in the cytoplasm or nucleus, forming a complex that acts as a transcription factor to regulate gene expression.​

• (D) Epinephrine: Catecholamine that binds adrenergic GPCRs (e.g., β-receptors) on the cell surface, activating G-proteins and second messengers like cAMP.​

Hormone signaling is a core topic in molecular biology, where the key phrase “hormone binds intracellular receptor” highlights steroid hormones like progesterone that directly enter cells. This contrasts with peptide hormones using membrane receptors.​

Intracellular vs. Membrane Receptors

Lipid-soluble hormones such as progesterone diffuse across the lipid bilayer to bind intracellular receptors, forming complexes that translocate to the nucleus and modulate transcription. Water-soluble hormones like insulin, gonadotropins, and epinephrine cannot cross the membrane, so they bind surface receptors to activate rapid cascades via second messengers.​

Progesterone Mechanism

Progesterone binds progesterone receptors (PR), causing conformational change, dimerization, nuclear entry, and binding to progesterone response elements (PRE) on DNA for gene regulation. This genomic action influences reproduction, with non-genomic effects also possible via membrane PR.​