Glucocorticoid receptors: ATP and cell cycle dependence, phosphorylation, and hormone resistance.

Current views of glucocorticoid receptors (GRs) and other steroid hormone receptors are that in unliganded form they are present as oligomers formed by noncovalent association of the hormone-binding receptor protein monomer the receptorwith an hsp90' (90 kDa heat shock protein) dimer, p59 or hsp56, and often hsp70 and other proteins (1-3). The oligomers are soluble and appear in cytosols from cells disrupted by conventional means. Like other members of the steroid hormone receptor superfamily, GRs have three principal domains (Figure 1):(1) an N-terminal domain; (2) a DNA-binding domain (DBD) with zinc finger structures that confer high affinity for glucocorticoid response elements (GREs); and (3) a hormoneor ligand-binding domain (HBD) at the C-terminus, with the latter two domains being separated by a so-called "hinge" region. Within the domains are regions that exert particular functions, as indicated in Figure 1 (4). The HBD, in addition to its hormone binding site, has transactivation functions, an hsp90 binding site, a GR dimerization region, and two nuclear localization signals. The DBD includes major transactivating functions. The N-terminal domain has no primary function like the other two, but is responsible for most of the transactivating activity of GRs. In the human GR over 90070 of this activity resides in an acidic region, called .1 (5), that in the rat GR corresponds roughly to residues 100-305. The C-terminal half of this region in the mouse GR has been found to reduce nonspecific binding to DNA (6). Most of the activity of T1 has been ascribed to a 41-residue "core" region (Figure 1) (5). GRs, like other steroid hormone receptors, are phosphorylated (7, 8). The phosphorylated sites have been identified in mouse GRs (9). All are in the N-terminal domain (Figure 1), most of them within the transactivation region corresponding to .1 of the human GR. Three heavily phosphorylated sites are in the core region of t l, Here we will describe some of the dynamic interactions and transformations that GRs undergo in normal cells, the requirement for ATP to maintain GR hormone-binding capacity, and the changes that take place in GR structure and glucocorticoid sensitivity through the cell cycle. The relevance of these phenomena to development of glucocorticoid resistance is then discussed.