Nuclear Receptors and Gene Transcriptional Effects

Nuclear receptors are intracellular proteins that act as ligand-activated transcription factors: when a lipophilic messenger such as a steroid, thyroid hormone, or vitamin-derived ligand binds them, they change the transcription of specific genes. Drugs that target nuclear receptors therefore work by altering which proteins a cell makes. Because their effect depends on synthesizing new proteins, these drugs typically act slowly, over hours to days, in contrast to the rapid action of channel- and signalling-receptor drugs.

Definition

A nuclear receptor is an intracellular, ligand-activated transcription factor; drug action through nuclear receptors is the alteration of target-gene transcription that follows a drug binding such a receptor and changing its interaction with DNA and transcriptional coregulators.

Scope

This topic covers how drugs act through nuclear receptors: ligand binding to intracellular receptors, the resulting recruitment of coactivators or corepressors at DNA response elements, and the consequent change in gene transcription. It treats nuclear-receptor pharmacology as a molecular mechanism of drug action for reference, and does not give clinical guidance on any nuclear-receptor-directed drug.

Core questions

• Which nuclear receptor does the drug bind, and is it a steroid, thyroid, or adopted-orphan receptor?
• Does ligand binding recruit coactivators (activating transcription) or corepressors (repressing it)?
• Which response elements and target genes are regulated as a result?
• Why does the requirement for new protein synthesis make the effect slow in onset and offset?

Key concepts

• Ligand-activated transcription factor
• Hormone response element
• Coactivator and corepressor recruitment
• Steroid hormone receptors
• Adopted orphan receptors
• Genomic (transcriptional) action
• Delayed onset and offset of effect

Mechanisms

Nuclear receptors share a modular structure with a ligand-binding domain and a DNA-binding domain. A lipophilic ligand crosses the cell membrane and binds the receptor's ligand-binding domain, changing its conformation. The activated receptor binds specific DNA sequences (response elements) at target genes and recruits coactivator or corepressor complexes, which remodel chromatin and engage the transcriptional machinery to increase or decrease transcription of those genes. The altered transcription changes the cellular complement of proteins, which produces the pharmacological effect. Because this depends on transcription and translation, the response develops over hours to days and persists after the drug is gone until the affected proteins turn over — a hallmark that distinguishes genomic drug action from the rapid effects of channel and surface-receptor drugs (Mangelsdorf 1995; Moore 2006; Brunton 2018).

Clinical relevance

Nuclear-receptor-directed drugs include important classes acting through steroid, thyroid, and related receptors, and their slow, transcription-dependent action explains both their delayed onset and their lingering effects. Knowing that the mechanism is transcriptional helps explain why benefits and adverse effects can take time to appear. This topic describes the molecular basis of nuclear-receptor drugs for reference and education and does not provide dosing or treatment recommendations.

Evidence & guidelines

The structure and classification of the nuclear receptor superfamily are set out in foundational reviews (Mangelsdorf 1995), and the family's role as a source of drug targets is summarized in pharmacological reviews (Moore 2006). Target-class surveys place nuclear receptors among the established families of drug targets (Overington 2006), and standard pharmacology texts describe the transcriptional mechanism of these agents (Brunton 2018).

History

The molecular era of nuclear-receptor pharmacology began with the cloning of the steroid and thyroid hormone receptors in the 1980s, which revealed a large superfamily of related ligand-activated transcription factors, including many initially orphan receptors whose ligands were later identified (Mangelsdorf 1995). This framework recast hormone and vitamin action as ligand-controlled gene regulation and defined nuclear receptors as a distinct class of drug target (Moore 2006).

Debates

Can selective receptor modulators separate beneficial from harmful gene-regulatory effects?

Because a single nuclear receptor regulates many genes in different tissues, ligands that act as agonists in some tissues and antagonists in others (selective modulators) are sought to dissociate desired from undesired effects; how completely this can be achieved remains an open question.

Seminal works

• mangelsdorf-1995
• moore-2006

Frequently asked questions

Why do drugs that act on nuclear receptors work slowly?

Their effect depends on changing gene transcription and then synthesizing new proteins, which takes hours to days; this contrasts with channel- or surface-receptor drugs that change existing molecules and act within seconds to minutes.

How does a nuclear receptor change gene expression once a drug binds it?

Binding alters the receptor's shape so it attaches to specific DNA response elements and recruits coactivator or corepressor proteins, which then increase or decrease transcription of the associated genes.