Cell cycle and hormonal control of nuclear-cytoplasmic localization of the serum- and glucocorticoid-inducible protein kinase, Sgk, in mammary tumor cells. A novel convergence point of anti-proliferative and proliferative cell signaling pathways

The serum- and glucocorticoid-inducible kinase (sgk) is a novel serine/threonine protein kinase that is transcriptionally regulated in rat mammary tumor cells by serum under proliferative conditions or by glucocorticoids that induce a G1 cell cycle arrest. Our results establish that the subcellular distribution of Sgk is under stringent cell cycle and hormonal control. Sgk is localized to the perinuclear or cytoplasmic compartment as a 50-kDa hypophosphorylated protein in cells arrested in G1 by treatment with the synthetic glucocorticoid dexamethasone. In serum-stimulated cells, Sgk was transiently hyperphosphorylated and resided in the nucleus. Laser scanning cytometry, which monitors Sgk localization and DNA content in individual mammary tumor cells of an asynchronously growing population, revealed that Sgk actively shuttles between the nucleus (in S and G2/M) and the cytoplasm (in G1) in synchrony with the cell cycle. In cells synchronously released from the G1/S boundary, Sgk localized to the nucleus during progression through S phase. The forced retention of exogenous Sgk in either the cytoplasmic compartment, using a wild type sgk gene, or the nucleus, using a nuclear localization signal-containing sgk gene (NLS-Sgk), suppressed the growth and DNA synthesis of serum-stimulated cells. Thus, our study implicates the nuclear-cytoplasmic shuttling of sgk as a requirement for cell cycle progression and represents a novel convergence point of anti-proliferative and proliferative signaling in mammary tumor cells.

Repression of glucocorticoid receptor transactivation and DNA binding of a glucocorticoid response element within the serum/glucocorticoid-inducible protein kinase (sgk) gene promoter by the p53 tumor suppressor protein

sgk is a novel member of the serine/threonine protein kinase family that is transcriptionally regulated by serum and glucocorticoids in Rat2 fibroblasts and in mammary epithelial cells. 5'-Deletion analysis of the sgk promoter, using a series of sgk-CAT. (chloramphenicol acetyltransferase) chimeric reporter gene plasmids, defined a glucocorticoid-responsive region that contains a glucocorticoid response element (sgkGRE) between -1000 and -975 bp. The sgkGRE is specifically bound by glucocorticoid receptors and is sufficient to confer glucocorticoid responsiveness to a heterologous promoter in several cell lines. Strikingly, cotransfection of either the murine or human wild type p53, but not a mutant p53, repressed the dexamethasone-stimulated transactivation of reporter plasmids containing either the sgkGRE or a consensus GRE. Gel shift analysis revealed that in vitro synthesized p53 prevented binding of the glucocorticoid receptor both to the sgkGRE as well as to a consensus GRE. The p53-mediated repression of dexamethasone-induced sgkGRE activity required both the DNA binding and transactivation functions of the p53 protein. Activation of endogenous p53, by exposure to UV light, repressed the glucocorticoid receptor transactivation of a consensus GRE-CAT reporter plasmid in transfected cells. Conversely, activated glucocorticoid receptors suppressed the transactivation function of p53, while transrepression by p53 was largely unaffected. The presented data demonstrate that sgk is a primary glucocorticoid-responsive protein kinase gene that implicates a new pathway of cross-talk between steroid receptor signaling and cellular phosphorylation cascades. In addition, our study provides the first evidence of mutual interference of transactivation functions of p53 and the glucocorticoid receptor, possibly through their direct interaction.

p53 stimulates promoter activity of the sgk. serum/glucocorticoid-inducible serine/threonine protein kinase gene in rodent mammary epithelial cells

sgk is a novel member of the serine/threonine protein kinase gene family that is transcriptionally regulated by serum and glucocorticoids in mammary epithelial cells. To functionally determine if the sgk promoter is regulated by the p53 tumor suppressor protein in mammary cells, a series of sgk promoter fragments with 5'-deletions were linked to the bacterial chloramphenicol acetyltransferase gene (sgk-CAT) and transiently co-transfected into nontumorigenic NMuMG or transformed Con8Hd6 mammary epithelial cells with p53 expression plasmids. Wild-type p53, but not mutant p53, strongly stimulated sgk promoter activity in both mammary epithelial cell lines. These effects were mediated by specific regions within the sgk promoter containing p53 DNA-binding sites. The sgk p53 sequence at-1380 to-1345 (site IV) was sufficient to confer p53-dependent transactivation to a heterologous promoter, and p53 was capable of binding to this sequence in vitro as assessed by gel shift analysis. In the nontumorigenic NMuMG epithelial cell line, cotransfection of wild-type p53 strongly stimulated the activities of both the sgk promoter and the well characterized p53-responsive p21/Waf1 promoter, whereas in Rat-2 fibroblasts, wild-type p53 repressed the basal activities of both promoters, revealing that sgk and p21/Waf1 are similarly regulated in a cell type-specific manner. Taken together, these results demonstrate that sgk is a new transcriptional target of p53 in mammary epithelial cells and represent the first example of a hormone-regulated protein kinase gene with a functionally defined p53 promoter recognition element.