14.4 Receptor Types & Intracellular Receptors

Two Broad Classes of Receptors

Every signaling pathway starts at a receptor, and receptors divide into two fundamental classes determined by the chemistry of their ligand. Hydrophilic ligands (peptide hormones, catecholamines, growth factors, cytokines) cannot cross the plasma membrane's lipid bilayer, so they bind cell-surface receptors — ion-channel-linked, G-protein-coupled, or enzyme-linked (kinase) receptors covered in section 14.5. Hydrophobic ligands (steroid hormones, thyroid hormone, retinoids, vitamin D) diffuse directly across the plasma membrane and bind intracellular receptors located in the cytosol or nucleus.

The PA-CAT Bulletin of Information, rev. 20240815 places Receptor Types and Intracellular Receptors within the Signal Transduction group, and the most testable distinction is the hydrophilic/hydrophobic ligand dichotomy paired with response kinetics.

Intracellular (Nuclear) Receptors: The Steroid Hormone Receptor Family

The prototypical intracellular receptors are the steroid hormone receptors: the glucocorticoid receptor (GR), mineralocorticoid receptor (MR), estrogen receptor (ER), progesterone receptor (PR), and androgen receptor (AR). They share a common modular architecture with distinct domains:

• N-terminal transactivation domain (AF1) — regulates transcription once the receptor is DNA-bound.
• DNA-binding domain (DBD) — contains zinc-finger motifs that recognize specific hormone response elements (HREs) in target gene promoters.
• Ligand-binding domain (LBD) — binds the steroid; contains the AF2 transactivation surface.
• Hinge region — contains the nuclear localization signal (NLS).

Mechanism — The Glucocorticoid Receptor as Model

In the absence of hormone, the glucocorticoid receptor resides in the cytosol in an inactive complex with heat-shock protein 90 (HSP90) and other chaperones. HSP90 masks the DNA-binding domain and the NLS. The sequence of activation is:

1. Diffusion: Cortisol (a hydrophobic steroid) crosses the plasma membrane.
2. Binding: Cortisol binds the receptor's LBD, inducing a conformational change.
3. HSP release: The receptor dissociates from HSP90, exposing the DBD and the NLS.
4. Dimerization and nuclear import: The ligand-bound receptor dimerizes and is imported through the nuclear pore.
5. DNA binding: The dimer binds a glucocorticoid response element (GRE) in the enhancer/promoter region of target genes.
6. Transcriptional regulation: AF1 and AF2 recruit coactivators (or corepressors) and the transcription machinery, increasing (or decreasing) transcription of target genes such as those encoding metabolic enzymes and anti-inflammatory peptides.
7. Termination: Hormone is metabolized (e.g., by 11β-HSD); the receptor can be recycled or degraded.

Receptors Already in the Nucleus

Not all intracellular receptors are cytosolic when inactive. Thyroid hormone receptors (TR) and retinoic acid receptors (RAR/RXR) sit on DNA even without ligand, usually as heterodimers with RXR, bound to corepressors that maintain basal repression. Ligand binding triggers corepressor release and coactivator recruitment — a switch from repression to activation without receptor translocation. This distinction (cytosolic vs. nuclear-resident) is a common PA-CAT comparison.

Response Times and the Logic of Slow Signaling

Because intracellular receptors regulate gene transcription, their responses are inherently slow: mRNA must be transcribed, exported, and translated into protein, so the biological effect appears over hours rather than seconds. This is the opposite of ion-channel receptors (milliseconds) and GPCRs (seconds to minutes). The trade-off is duration — steroid hormone effects persist long after the hormone is cleared, because the induced proteins accumulate and have their own half-lives. PA-CAT items frequently pair the slow-onset, long-duration profile with steroid hormones and the fast-onset, short-duration profile with peptide hormones acting through cell-surface receptors.

Clinical Relevance

Steroid receptor pharmacology is fertile PA-CAT territory because so many drugs target this family:

• Prednisone and dexamethasone — synthetic glucocorticoids that suppress inflammation via GR-mediated transcription of anti-inflammatory genes (e.g., IκBα) and repression of pro-inflammatory genes (e.g., cytokines).
• Tamoxifen — a selective estrogen receptor modulator (SERM) that antagonizes ER in breast tissue but agonizes it in bone and uterus.
• Flutamide — an androgen receptor antagonist used in prostate cancer.
• Spironolactone — a mineralocorticoid receptor antagonist used as a potassium-sparing diuretic.

The mechanism connecting these drugs to molecular biology is the same: each is a hydrophobic ligand that crosses the membrane, binds an intracellular receptor, and alters the receptor's activity as a transcription factor.

Why This Matters on the PA-CAT

This section directly addresses the Receptor Types and Intracellular Receptors leaves in the Bulletin's Signal Transduction group. Expect items that test: (1) which ligands reach intracellular versus surface receptors (hydrophobic vs. hydrophilic), (2) the domain architecture of steroid hormone receptors (DBD zinc fingers, LBD, AF1/AF2), (3) the role of HSP90 in keeping cytosolic steroid receptors inactive, (4) the receptor's dual function as ligand-regulated transcription factor, and (5) the slow, long-lasting response profile that distinguishes nuclear receptor signaling from GPCR or ion-channel signaling. Pair this section with 14.5, which returns to the cell-surface receptor classes — GPCRs and receptor kinases — and their faster signaling mechanisms.