Nuclear Receptor Signalling
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Direct answer
Steroid hormones bypass second messengers entirely: their receptors are intracellular, ligand-activated transcription factors built on two conserved domains — a zinc-finger DNA-binding region and a C-terminal ligand-binding pocket. Type I (steroid) receptors — glucocorticoid, mineralocorticoid, progesterone, androgen and oestrogen receptors — sit in the cytoplasm bound to heat-shock protein complexes until hormone arrives, exposing nuclear-localisation signals, dimerising the receptors and delivering them to their hormone-response elements. Type II receptors — thyroid hormone, retinoic acid, vitamin D and PPAR receptors — already occupy DNA as heterodimers with RXR, actively repressing through corepressors until ligand swaps in coactivators. Partial agonists create the clinical pharmacology: tamoxifen blocks oestrogen receptors in breast yet activates them in uterus and bone.
What you must remember
- Domain map: variable N-terminal A/B domain (AF-1), DNA-binding domain with two zinc fingers (the P box reads the response element), hinge region, ligand-binding domain (AF-2) — a sketch worth practising.
- Type I behaviour: cytoplasmic-HSP90 complexes, ligand-induced translocation, binding palindromic (inverted-repeat) response elements — GRE, ERE, PRE.
- Type II behaviour: TR, RAR, VDR, PPAR, LXR, FXR all pair with RXR on direct-repeat elements; unliganded receptors recruit SMRT/NCoR corepressors and actively silence genes — thyroid hormone relieves repression as much as it activates.
- RXR is the shared partner: one retinoid X receptor heterodimerises with the whole type II family; 9-cis retinoic acid is its ligand.
- Coactivator switch: ligand shifts the AF-2 helix, releasing corepressors and docking SRC-1/p300-type coactivators with histone acetyltransferase activity — the link to chromatin biology.
- Clinical ligands to name: tamoxifen (SERM, ER), mifepristone (antiprogestin), thiazolidinediones (PPAR-gamma agonists, insulin sensitisers), fibrates (PPAR-alpha, triglyceride lowering).
- Receptor-resistance syndromes: androgen insensitivity from androgen-receptor mutations; hereditary vitamin D-resistant rickets from VDR mutations — receptor disease as endocrine mimicry.
- Genomic timing: hours, because transcription, translation and protein turnover intervene; any seconds-scale steroid effect is non-genomic (membrane-associated receptors) and should be labelled as such.
Tamoxifen, one molecule, three tissues
Why can a breast-cancer drug thicken the endometrium and protect bone at the same time? Because tamoxifen is a partial agonist, and the oestrogen receptor's transcriptional output depends on the balance of its two activation functions and the coactivator milieu of the target cell. In breast carcinoma cells, where AF-2 signalling dominates, tamoxifen's failure to recruit AF-2 coactivators makes it a functional antagonist — tumour growth slows. In endometrial cells, its AF-1 activity is sufficient for agonism, which is why long-term users carry a measurable rise in endometrial cancer risk and need gynaecological surveillance. In bone, partial agonism preserves density in postmenopausal patients. Raloxifene took the same principle and tuned it — antagonist in breast and uterus, agonist in bone — the "designated SERM" answer. The general lesson for the exam: receptor pharmacology is tissue pharmacology, because coactivator ratios differ by cell.
How the exam frames it
Two framings recur. First, the classification table: examiners give a receptor and demand type I or type II — the fastest route is asking "does it partner with RXR?" (type II) or "does it sit on HSP90 in the cytoplasm?" (type I). Students who memorise the lists without the mechanistic handles mix thyroid receptor, the perennial trap — it is type II, already nuclear and DNA-bound, repressing genes in the hypothyroid state. Second, the "how do steroids act" long answer: the expected chain is diffusion through the membrane, HSP90 dissociation, dimerisation, hormone-response-element binding, coactivator recruitment, transcription of specific genes, effect in hours. Candidates who invoke second messengers like cyclic AMP have answered a different question — peptide-hormone action — and the marks follow the membrane-crossing logic. A sharp addition: glucocorticoid anti-inflammatory action includes transrepression of NF-kappaB-driven genes, explaining potency without new-protein agonism.
Frequently asked questions
Which nuclear receptors heterodimerise with RXR?
The type II group — thyroid hormone, retinoic acid, vitamin D, PPAR, LXR and FXR receptors all bind DNA as heterodimers with the retinoid X receptor.
Why is tamoxifen tissue-selective?
As a partial agonist, its output depends on each tissue's coactivator-to-corepressor balance and AF-1/AF-2 usage — antagonistic in breast, agonistic in endometrium and bone.
Why do steroid hormones take hours to act?
They alter gene transcription: receptor binding, RNA synthesis, translation and protein effects must all complete, unlike second-messenger cascades acting in seconds to minutes.
What holds type I receptors inactive in the cytoplasm?
A heat-shock protein (HSP90) chaperone complex that dissociates on ligand binding, exposing the DNA-binding and nuclear-localisation domains.
Which drugs act as PPAR agonists, and for what?
Thiazolidinediones such as pioglitazone activate PPAR-gamma for insulin resistance; fibrates such as fenofibrate activate PPAR-alpha to lower triglycerides.