The prototype GR isoform is composed of about 800 amino acids and can be divided into three functional domains (see Figure 2–6). The glucocorticoid-binding domain is located at the carboxyl terminal of the molecule. The DNA-binding domain is located in the middle of the protein and contains nine cysteine residues. This region folds into a “two-finger” structure stabilized by zinc ions connected to cysteines to form two tetrahedrons. This part of the molecule binds to the GREs that regulate glucocorticoid action on glucocorticoidregulated genes. The zinc fingers represent the basic structure by which the DNA-binding domain recognizes specific nucleic acid sequences. The amino-terminal domain is involved in the transactivation activity of the receptor and increases its specificity.
The interaction of glucocorticoid receptors with GREs or other transcription factors is facilitated or inhibited by several families of proteins called steroid receptor coregulators, divided into coactivators and corepressors. The coregulators do this by serving as bridges between the receptors and other nuclear proteins and by expressing enzymatic activities such as histone acetylase or deacetylase, which alter the conformation of nucleosomes and the transcribability of genes.
Between 10% and 20% of expressed genes in a cell are regulated by glucocorticoids. The number and affinity of receptors for the hormone, the complement of transcription factors and coregulators, and post-transcription events determine the relative specificity of these hormones’ actions in various cells. The effects of glucocorticoids are mainly due to proteins synthesized from mRNA transcribed from their target genes.
Some of the effects of glucocorticoids can be attributed to their binding to mineralocorticoid receptors. Indeed, MRs bind aldosterone and cortisol with similar affinity. A mineralocorticoid effect of the higher levels of cortisol is avoided in some tissues (eg, kidney, colon, salivary glands) by expression of 11β-hydroxysteroid dehydrogenase type 2, the enzyme responsible for biotransformation to its 11-keto derivative (cortisone), which has minimal action on aldosterone receptors.
The GR also interacts with other regulators of cell function. One such molecule is CLOCK/BMAL-1, a transcription factor dimer expressed in all tissues and generating the circadian rhythm of cortisol secretion (Figure 39–2) at the suprachiasmatic nucleus of the hypothalamus. CLOCK is an acetyltransferase that acetylates the hinge region of the GR, neutralizing its transcriptional activity and thus rendering target tissues resistant to glucocorticoids. As shown in Figure 39–2, lower panel, the glucocorticoid target tissue sensitivity rhythm generated is in reverse phase to that of circulating cortisol concentrations, explaining the increased sensitivity of the organism to evening administration of glucocorticoids. The GR also interacts with NF-κB, a regulator of production of cytokines and other molecules involved in inflammation.
pre-mRNA NucleusCytoplasm mRNA Protein Response GRE DNA (Editing) Transcription machinery (RNA poly-merase, etc) R R* R* R* R* R S (Unstable) S S S S S S S hsp90 hsp90 x CBG Steroid-receptor dimer (activated)