Sex Steroid Mechanism of Action

Sex steroids — androgens, estrogens, and progestins — act mainly by binding intracellular receptors that belong to the nuclear receptor superfamily. The hormone-bound receptor functions as a ligand-activated transcription factor, binding specific DNA response elements to regulate target-gene expression; this genomic mechanism accounts for the slow, lasting effects of sex steroids, alongside faster non-genomic actions.

Definition

The sex steroid mechanism of action is the means by which androgens, estrogens, and progestins exert effects, chiefly by binding nuclear-superfamily receptors that act as ligand-activated transcription factors regulating gene expression, supplemented by rapid non-genomic signalling.

Scope

The topic covers the nuclear receptor superfamily and its modular structure, the genomic mechanism of steroid action through DNA response elements and coregulators, receptor-specific signalling for estrogen and androgen, and the existence of rapid non-genomic effects. It is a physiology reference topic and does not provide clinical guidance.

Core questions

• How do steroid hormones change gene expression in target cells?
• What is the modular structure of nuclear steroid receptors?
• How do response elements and coregulators determine the transcriptional response?
• How do non-genomic actions differ from the classical genomic pathway?

Key concepts

• Nuclear receptor superfamily
• Ligand-binding and DNA-binding domains
• Hormone-response elements
• Coactivators and corepressors
• Estrogen receptors (ERalpha and ERbeta)
• Androgen receptor
• Genomic versus non-genomic action

Key theories

Nuclear receptor superfamily

Steroid receptors are members of a conserved superfamily of ligand-activated transcription factors with a modular domain structure (ligand-binding and DNA-binding domains), so that diverse steroids regulate genes through a common molecular logic.

Genomic gene regulation by steroid receptors

Hormone-bound receptors bind specific hormone-response elements in target genes and recruit coregulators to activate or repress transcription, the classical genomic mechanism underlying the durable effects of sex steroids.

Mechanisms

Lipophilic sex steroids cross the plasma membrane and bind intracellular receptors of the steroid/thyroid nuclear-receptor superfamily, which share a modular architecture with a central DNA-binding domain and a C-terminal ligand-binding domain (Evans, 1988; Mangelsdorf et al., 1995). Ligand binding activates the receptor, which binds specific hormone-response elements in the regulatory regions of target genes and recruits coactivator or corepressor complexes to enhance or repress transcription (Beato, 1989). Estrogen acts through two receptor subtypes, ERalpha and ERbeta, with distinct tissue distributions and target genes (Nilsson et al., 2001), and androgens act through the androgen receptor, whose mutations produce a spectrum of androgen-insensitivity phenotypes that illustrate the receptor's central role (Quigley et al., 1995). In addition to this genomic pathway, sex steroids trigger rapid non-genomic signalling that does not require new gene transcription.

Clinical relevance

Knowing how sex steroids act at the molecular level provides the physiological basis for understanding target-tissue responses and the consequences of receptor defects, such as androgen insensitivity. The entry is educational reference on mechanism; it is non-prescriptive and is not a basis for diagnosis or treatment.

History

Cloning of steroid receptors in the 1980s revealed that they belong to a large superfamily of ligand-activated transcription factors with a shared modular design (Evans, 1988), and the genomic mechanism of transcriptional regulation by hormone-response elements was articulated soon after (Beato, 1989). A review marking the superfamily's second decade consolidated the field (Mangelsdorf et al., 1995), and the discovery of a second estrogen receptor refined understanding of estrogen action (Nilsson et al., 2001).

Debates

How important are non-genomic relative to genomic actions?

The physiological weight of rapid, membrane-initiated non-genomic steroid signalling compared with the classical genomic pathway, and the identity of the receptors mediating it, remain areas of continuing investigation.

Key figures

• Ronald Evans
• Miguel Beato
• Jan-Ake Gustafsson
• David Mangelsdorf

Seminal works

• evans-1988
• beato-1989
• mangelsdorf-1995

Frequently asked questions

How do sex steroids change cell behaviour?

They bind intracellular nuclear-superfamily receptors that act as ligand-activated transcription factors, binding DNA response elements and recruiting coregulators to switch target genes on or off, which alters the cell's protein output.

Do sex steroids only work by changing gene expression?

The classical pathway is genomic transcriptional regulation, but sex steroids also produce rapid non-genomic effects that occur too quickly to depend on new gene transcription.