Distinctive roles of STAT5a and STAT5b in sexual dimorphism of hepatic P450 gene expression. Impact of STAT5a gene disruption

Stat5b gene disruption leads to an apparent growth hormone (GH) pulse insensitivity associated with loss of male-characteristic body growth rates and male-specific liver gene expression (Udy, G. B., Towers, R. P., Snell, R. G., Wilkins, R. J., Park, S. H., Ram, P. A., Waxman, D. J., and Davey, H. W. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 7239-7244). In the present study, disruption of the mouse Stat5a gene, whose coding sequence is approximately 90% identical to the Stat5b gene, resulted in no loss of expression in male mice of several sex-dependent, GH-regulated liver cytochrome P450 (CYP) enzymes. By contrast, the loss of STAT5b feminized the livers of males by decreasing expression of male-specific CYPs (CYP2D9 and testosterone 16alpha-hydroxylase) while increasing to female levels several female-predominant liver CYPs (CYP3A, CYP2B, and testosterone 6beta-hydroxylase). Since STAT5a is thus nonessential for these male GH responses, STAT5b homodimers, but not STAT5a-STAT5b heterodimers, probably mediate the sexually dimorphic effects of male GH pulses on liver CYP expression. In female mice, however, disruption of either Stat5a or Stat5b led to striking decreases in several liver CYP-catalyzed testosterone hydroxylase activities. Stat5a or Stat5b gene disruption also led to the loss of a female-specific, GH-regulated hepatic CYP2B enzyme. STAT5a, which is much less abundant in liver than STAT5b, and STAT5b are therefore both required for constitutive expression in female but not male mouse liver of certain GH-regulated CYP steroid hydroxylases, suggesting that STAT5 protein heterodimerization is an important determinant of the sex-dependent and gene-specific effects that GH has on the liver.     

Binding of [3H]triamcinolone acetonide-receptor complexes to chromatin from the B-cell leukemia line, BCL1

The binding characteristics of partially purified glucocorticoid receptor complexes from hormone sensitive, non-differentiating BCL1 cells to sequentially deproteinized BCL1 chromatin-cellulose was investigated. [3H]Triamcinolone acetonide (TA)-receptor complexes were purified (approx. 30-fold) from DEAE-cellulose columns by salt elution which allowed receptor activation only in the absence of molybdate. Addition of 10 mM molybdate completely blocked salt activation. The binding pattern of the activated [3H]TA-receptor complexes to chromatin-cellulose extracted with 0-8 M guanidine hydrochloride revealed three regions of increased binding activity (acceptor sites), at 2, 5 and 7 M guanidine hydrochloride. Acceptor site binding was markedly reduced for chromatin extracted with 3, 6 and 8 M guanidine hydrochloride. Non-activated receptor complexes demonstrated very low binding to deproteinized chromatin. It was also shown that chromatin binding required glucocorticoid receptors and that free ligand or ligand bound to other proteins did not bind significantly to chromatin. In addition, binding of [3H]TA-receptor complexes to partially deproteinized chromatin was competable by unlabeled TA-receptor complexes. Scatchard analysis demonstrated that chromatin from non-differentiating BCL1 cells possesses multiple, high-affinity binding sites which differ in their affinity for the glucocorticoid receptor. Partially deproteinized chromatin from lipopolysaccharide-stimulated BCL1 cells demonstrated a different pattern of receptor binding, i.e., receptor binding was significantly greater to chromatin previously extracted with 6-8 M guanidine hydrochloride. These results suggest that differentiation alters the state of chromatin and the interaction of non-histone protein/DNA acceptor sites with glucocorticoid receptors. These alterations may play a role in the acquisition of hormone resistance.