Growth Hormone Activation of Stat 1, Stat 3, and Stat 5 in Rat Liver

Abstract

Intermittent plasma growth hormone (GH) pulses, which occur in male but not female rats, activate liver Stat 5 by a mechanism that involves tyrosine phosphorylation and nuclear translocation of this latent cytoplasmic transcription factor (Waxman, D. J., Ram, P. A., Park, S. H., and Choi, H. K. (1995) J. Biol. Chem. 270, 13262-13270). We demonstrate that physiological levels of GH can also activate Stat 1 and Stat 3 in liver tissue, but with a dependence on the dose of GH and its temporal plasma profile that is distinct from Stat 5 and with a striking desensitization following a single hormone pulse that is not observed with liver Stat 5. GH activation of the two groups of Stats leads to their selective binding to DNA response elements upstream of the c-fos gene (c-sis-inducible enhancer element; Stat 1 and Stat 3 binding) and the ␤-casein gene (mammary gland factor element; liver Stat 5 binding). In addition to tyrosine phosphorylation, GH is shown to stimulate phosphorylation of these Stats on serine or threonine in a manner that either enhances (Stat 1 and Stat 3) or substantially alters (liver Stat 5) the binding of each Stat to its cognate DNA response element. These findings establish the occurrence of multiple, Stat-dependent GH signaling pathways in liver cells that can target distinct genes and thereby contribute to the diverse effects that GH and its sexually dimorphic plasma profile have on liver gene expression. Growth hormone (GH)1 regulates a large number of metabolic and other processes in the liver, primary through its effects on gene transcription. GH exerts both stimulatory and inhibitory effects on the expression of a wide range of liver gene products, including cytochrome P450 (1, 2), glutathione S-transferase (3), and sulfotransferase enzymes involved in steroid and drug metabolism (4), in addition to several cellsurface receptors (5, 6), including GH receptor (7). Hepatic secretory products, such as insulin-like growth factor 1, serine protease inhibitor Spi 2.1 (8), and various urinary proteins (9, 10), are also expressed in a GH-dependent manner. Studies carried out in rodents demonstrate that many, although not all, of the effects of GH on liver gene expression are sex-dependent. This sex dependence is a direct consequence of a striking differential response of individual target genes to GH, depending on whether hepatocytes are stimulated by the intermittent plasma pulses of GH that are characteristic of adult male rats or whether the cells are exposed to GH on a more continual basis, as occurs in adult female rats. Prototypic examples of this sex-dependent GH regulation are the cytochrome P450 genes CYP2C11 and CYP2C12, which are transcribed in a male-and female-specific manner, respectively, in adult rat liver in direct dependence on the plasma GH profile (11,12). By contrast, other effects of GH, in particular the acute stimulation of insulin-like growth factor 1 (13), c-fos (14), and serine protease inhibitor (Spi 2.1) gene expression in liver (8), exhibit little or no sex dependence...

Requirement of STAT5b for sexual dimorphism of body growth rates and liver gene expression

“…1D), it alone is insufficient to maintain normal sexually dimorphic GH responses. Liver STAT1 and STAT3 also respond to GH (11,45,46); however, neither of these STATs exhibits the striking discrimination between intermittent and continuous GH stimulation seen with STAT5 (11), and the present study suggests that neither is likely to play an important role in the sexually dimorphic effects of GH. Interestingly, STAT1 protein expression was up-regulated in both male and female STAT5b Ϫ͞Ϫ mice in what may be an attempt at a compensatory response to the loss of STAT5b expression; however, the significance of this STAT1 increase and its possible contribution to the observed STAT5b Ϫ͞Ϫ phenotype are unclear.…”

“…These responses are very similar to those observed in GH-deficient Little mice and support our hypothesis (10) that STAT5b is the major, if not the sole, STAT protein that mediates the sexually dimorphic effects of GH on the liver and perhaps other target tissues. The closely related STAT5a [96% sequence identity; (17,18)] is a minor STAT5 form in rat liver (11,13). However, the present studies indicate that although STAT5a can be detected in an active form in the liver (Fig.…”

“…The simplest explanation is that STAT5b is necessary for (and perhaps directly mediates) the physiological effects of male plasma GH pulses on body growth rates and liver gene expression. This possibility is strongly supported by the direct response of liver STAT5b, but not other STAT forms, to the male pattern of plasma GH pulsation (10,11), which previously has been established as being both necessary and sufficient to achieve male body growth rates (8,40,41) and male liver gene expression (5)(6)(7)(8). One prediction of this hypothesis is that at least some of the sexually dimorphic liver gene products whose expression is altered in STAT5b Ϫ͞Ϫ males (Figs.…”

“…Liver cytosolic proteins prepared from 8-to 9-week-old wild-type and STAT5b Ϫ͞Ϫ mice by ultracentrifugation were analyzed on Western blots probed sequentially with anti-STAT antibodies (11). Western blots of mouse liver microsomes (25 g per lane) were probed with anti-rat CYP3A monoclonal antibody 2-3-2 (36) or anti-mouse CYP2D [anti-C-P450 16␣ , provided by M. Negishi, National Institute on Environmental Health Sciences (NIEHS), Research Triangle Park, NC (5)] as described (37).…”

“…This activation, which involves GH pulse-induced STAT5 tyrosine phosphorylation followed by serine or threonine phosphorylation (11,12), induces nuclear translocation of STAT5 and has been proposed to be a key event in mediating the effects of GH pulses on male-specific liver gene expression (10). Most of this STAT5 appears to be STAT5b (11,13), which belongs to a gene family, the signal transducers and activators of transcription, whose members are resident in the cytosol but become activated to nuclear DNA-binding proteins following the binding of cytokines and growth factors to their cognate cellsurface receptors. STAT5 was originally identified as a prolactin-inducible transcription factor in lactating mammary gland (14)(15)(16).…”