Intracellular Receptors (Steroid and Nuclear Receptors)

Core Concept

Hydrophobic signaling molecules — steroid hormones (cortisol, estrogen, testosterone, aldosterone), thyroid hormone, retinoids, and vitamin D — diffuse freely across the plasma membrane and bind intracellular receptors that are ligand-activated transcription factors. In the absence of ligand, many of these receptors are held inactive in the cytoplasm; on binding their hormone, they change conformation, translocate to the nucleus (or are already bound to DNA), dimerize, and bind specific hormone response elements (HREs) in DNA to turn target genes on or off. Because they act through transcription, nuclear-receptor responses are relatively slow but sustained — altering protein expression over hours to days.

Key Components

• Nuclear receptor superfamily: ~48 human receptors, all sharing a common domain architecture.
• DNA-binding domain (DBD): a highly conserved region with two zinc-finger motifs that reads the HRE sequence.
• Ligand-binding domain (LBD): binds the hormone; contains the activation function AF-2.
• N-terminal domain: variable, contains the ligand-independent activation function AF-1.
• Hinge region: flexible linker between DBD and LBD.
• Hormone response element (HRE): a specific DNA sequence (often a palindromic or direct repeat) recognized by the receptor dimer.
• HSP90 chaperone: holds steroid receptors in an inactive, ligand-ready state in the cytoplasm.
• Coactivators / corepressors: recruited complexes that open or close chromatin and regulate transcription.

Mechanism / How It Works

1. A lipophilic hormone (e.g., cortisol) enters the cell by passive diffusion through the membrane.
2. Type I (steroid) receptors (glucocorticoid, mineralocorticoid, estrogen, progesterone, androgen) sit in the cytoplasm bound to HSP90; hormone binding releases the chaperone, exposing the nuclear-localization signal.
3. The liganded receptor dimerizes, translocates to the nucleus, and binds its HRE in the promoter/enhancer of target genes.
4. Type II receptors (thyroid hormone, retinoid, vitamin D receptors) are already in the nucleus bound to DNA, typically as heterodimers with RXR, and repress transcription in the absence of ligand; ligand binding swaps corepressors for coactivators, activating transcription.
5. Recruited coactivators (histone acetyltransferases, chromatin remodelers, Mediator) open chromatin and recruit RNA polymerase II, changing the transcription of specific target genes.
6. New protein synthesis produces the physiological response (e.g., metabolic, anti-inflammatory, developmental effects).

Energy and Directionality

The response is ATP-dependent at the transcription step: chromatin remodeling and transcriptional initiation consume ATP (and GTP for nuclear import via the Ran gradient). Directionality is imposed by the hormone gradient and receptor specificity: the ligand is a passive diffuser, but the receptor converts its presence into a directed change in specific gene expression — an essentially irreversible (on the timescale of the response) commitment to a new transcriptional program that persists after ligand removal until the induced proteins decay.

Experimental Evidence / Technique

• Radiolabeled-hormone autoradiography: ³H-estradiol concentrates in the nuclei of target tissues, proving nuclear localization of the hormone–receptor complex.
• Cell-free transcription / reporter assays: a hormone response element fused to a reporter gene is activated only when hormone + receptor are present, demonstrating direct transcriptional control.
• Receptor domain deletion/mutation: deleting the DBD, LBD, or AF domains maps each function (DNA binding, ligand binding, transactivation).
• Chromatin immunoprecipitation (ChIP): shows liganded receptors bound to specific genomic HREs, and coactivator recruitment.
• Clinical pharmacology: glucocorticoids (dexamethasone) suppress inflammation via the glucocorticoid receptor; tamoxifen competes with estrogen at the ER — providing in vivo proof of receptor-mediated action.

How it works

1. A lipophilic hormone (e.g., cortisol) enters the cell by passive diffusion through the membrane.
2. Type I (steroid) receptors (glucocorticoid, mineralocorticoid, estrogen, progesterone, androgen) sit in the cytoplasm bound to HSP90; hormone binding releases the chaperone, exposing the nuclear-localization signal.
3. The liganded receptor dimerizes, translocates to the nucleus, and binds its HRE in the promoter/enhancer of target genes.
4. Type II receptors (thyroid hormone, retinoid, vitamin D receptors) are already in the nucleus bound to DNA, typically as heterodimers with RXR, and repress transcription in the absence of ligand; ligand binding swaps corepressors for coactivators, activating transcription.
5. Recruited coactivators (histone acetyltransferases, chromatin remodelers, Mediator) open chromatin and recruit RNA polymerase II, changing the transcription of specific target genes.
6. New protein synthesis produces the physiological response (e.g., metabolic, anti-inflammatory, developmental effects).

Common confusions

• "All hormones bind cell-surface receptors." — Steroid and thyroid hormones, retinoids, and vitamin D bind intracellular receptors.
• "The receptor enters the nucleus only when the ligand binds, in every case." — Type I receptors translocate on ligand; Type II receptors are already nuclear and DNA-bound (ligand changes their activity, not location).
• "Nuclear receptors only activate genes." — They can repress genes too (glucocorticoid transrepression of inflammatory genes; unliganded Type II receptors repress via corepressors).
• "The response is fast because it's a receptor." — It is slow (transcription + translation), in contrast to millisecond ion-channel or kinase responses.
• "Nitric oxide is a steroid-like nuclear-receptor ligand." — NO is a gas that acts on soluble guanylyl cyclase (a cytoplasmic enzyme), not a nuclear receptor.

Quick review

• Lipophilic ligands → intracellular receptors = ligand-activated transcription factors.
• Domains: DBD (zinc fingers), LBD (AF-2), AF-1, hinge.
• Type I: cytoplasmic + HSP90 → ligand → dimerize → nucleus → HRE. Type II: nuclear, RXR heterodimer, corepressor→coactivator swap.
• Slow, sustained, transcriptional responses (hours–days).
• Clinical: glucocorticoids, tamoxifen, PPARγ agonists.
• Techniques: autoradiography, reporter assays, domain mutagenesis, ChIP.