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Co-activators and co-repressors: mediators of gene activation by nuclear hormone receptors

Hormones may act through either membrane-bound or nuclear receptor proteins. Whereas peptide hormones bind to receptors on the cell surface, small lipophilic hormones and vitamins such as steroids, retinoids and l-triiodothyronine enter the cell and bind to their respective intracellular receptor protein. All members of the nuclear hormone receptor superfamily (steroid, vitamin D (VDR), retinoid (RAR, RXR), thyroid (TR) and peroxisome proliferator-activated (PPAR) receptors) exert their action on target genes by binding to short, but specific, regulatory DNA sequences in the promoter region of hormone-regulated genes, thereby increasing or decreasing their transcription rate. However, in contrast to steroid hormone receptors, the RAR, RXR, TR, VDR and PPAR are capable of binding to DNA even in the absence of their cognate ligand. The molecular details of ligand-induced transcriptional activation have remained enigmatic because DNA binding of these unliganded receptors does not increase transcription and in some cases even decreases basal transcription