SGK integrates insulin and mineralocorticoid regulation of epithelial sodium transport

The epithelial Na+ channel (ENaC) constitutes the rate-limiting step for Na+ transport across tight epithelia and is the principal target of hormonal regulation, particularly by insulin and mineralocorticoids. Recently, the serine-threonine kinase (SGK) was identified as a rapidly mineralocorticoid-responsive gene, the product of which stimulates ENaC-mediated Na+ transport. Like its close relative, protein kinase B (also called Akt), SGK's kinase activity is dependent on phosphatidylinositol 3-kinase (PI3K), a key mediator of insulin signaling. In our study we show that PI3K is required for SGK-dependent stimulation of ENaC-mediated Na+ transport as well as for the production of the phosphorylated form of SGK. In A6 kidney cells, mineralocorticoid induction of the phosphorylated form of SGK preceded the increase in Na+ transport, and specific inhibition of PI3K inhibited both phosphorylation of SGK and mineralocorticoid-induced Na+ transport. Insulin both augmented SGK phosphorylation and synergized with mineralocorticoids in stimulating Na+ transport. In a Xenopus laevis oocyte coexpression assay, SGK-stimulated ENaC activity was also markedly reduced by PI3K inhibition. Finally, in vitro-translated SGK specifically interacted with the ENaC subunits expressed in Escherichia coli as glutathione S-transferase fusion proteins. These data suggest that SGK is a PI3K-dependent integrator of insulin and mineralocorticoid actions that interacts with ENaC subunits to control Na+ entry into kidney collecting duct cells.

Serum and glucocorticoid-inducible kinase (SGK) is a target of the PI 3-kinase-stimulated signaling pathway

Serum and glucocorticoid-inducible kinase (SGK) is a novel member of the serine/threonine protein kinase family that is transcriptionally regulated. In this study, we have investigated the regulatory mechanisms that control SGK activity. We have established a peptide kinase assay for SGK and present evidence demonstrating that SGK is a component of the phosphoinositide 3 (PI 3)-kinase signaling pathway. Treatment of human embryo kidney 293 cells with insulin, IGF-1 or pervanadate induced a 3- to 12-fold activation of ectopically expressed SGK. Activation was completely abolished by pretreatment of cells with the PI 3-kinase inhibitor, LY294002. Treatment of activated SGK with protein phosphatase 2A in vitro led to kinase inactivation. Consistent with the similarity of SGK to other second-messenger regulated kinases, mutation of putative phosphorylation sites at Thr256 and Ser422 inhibited SGK activation. Cotransfection of PDK1 with SGK caused a 6-fold activation of SGK activity, whereas kinase-dead PDK1 caused no activation. GST-pulldown assays revealed a direct interaction between PDK1 and the catalytic domain of SGK. Treatment of rat mammary tumor cells with serum caused hyperphosphorylation of endogenous SGK, and promoted translocation to the nucleus. Both hyperphosphorylation and nuclear translocation could be inhibited by wortmannin, but not by rapamycin.

Follicle stimulating hormone-regulated expression of serum/glucocorticoid-inducible kinase in rat ovarian granulosa cells: a functional role for the Sp1 family in promoter activity

Recently, a family of novel, serine/threonine protein kinases has been identified. One of these transcriptionally inducible, immediate-early genes encodes serum/glucocorticoid inducible-protein kinase, sgk. By in situ hybridization, we show that sgk expression in the rat ovary is selectively localized to granulosa cells. In culture, FSH or forskolin, activators of the protein kinase A (PKA) pathway, rapidly (2 h) and transiently increased sgk mRNA levels in undifferentiated granulosa cells. Sgk mRNA exhibited a biphasic expression pattern, with maximal levels observed at 48 h of FSH/forskolin as granulosa cells differentiate to the preovulatory phenotype. Deletion analyses using sgk promoter-reporter constructs (-4.0 kb to -35 bp) identified a region between -63 and -43 bp that mediated FSH and forskolin-responsive transcription in undifferentiated and differentiated granulosa cells. This G/C-rich region 1) conferred both basal and inducible transcription to the minimal -35 sgk promoter chloramphenicol acetyltransferase reporter construct, 2) specifically bound Sp1 and Sp3 present in granulosa cell extracts, and 3) bound recombinant Sp1. Mutation of 2 bp in this region not only prevented Sp1 and Sp3 binding, but also abolished the PKA-mediated transactivation observed when using the wild type construct. Sp1 and Sp3 DNA-binding activity and protein levels did not change significantly during sgk induction. Collectively, these data indicate that Sp1/Sp3 transactivation of the sgk promoter likely involves regulated, phosphorylation-dependent interaction with other factors. Thus the novel, biphasic induction of sgk that correlates with granulosa cell progression from proliferation to differentiation appears to involve sequential, coordinated actions of FSH, PKA, and transcription factors, including Sp1 and Sp3.

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.

Glucocorticoid-stimulated CCAAT/enhancer-binding protein alpha expression is required for steroid-induced G1 cell cycle arrest of minimal-deviation rat hepatoma cells

By genetic correlation with the growth-suppressible phenotype and direct functional tests, we demonstrate that the glucocorticoid-stimulated expression of the CCAAT/enhancer-binding protein alpha (C/EBP alpha) transcription factor is required for the steroid-mediated G1 cell cycle arrest of minimal-deviation rat hepatoma cells. Comparison of C/EBP alpha transcript and active protein levels induced by the synthetic glucocorticoid dexamethasone in glucocorticoid growth-suppressible (BDS1), nonsuppressible receptor-positive (EDR1) and nonsuppressible receptor-deficient (EDR3) hepatoma cell proliferative variants revealed that the stimulation of C/EBP alpha expression is a rapid, glucocorticoid receptor-mediated response associated with the G1 cell cycle arrest. Consistent with the role of C/EBP alpha as a critical intermediate in the growth suppression response, maximal induction of transcription factor mRNA occurred within 2 h of dexamethasone treatment whereas maximal inhibition of [3H] thymidine incorporation was observed 24 h after steroid treatment. As a direct functional approach, ablation of C/EBP alpha protein expression and DNA-binding activity by transfection of an antisense C/EBP alpha expression vector blocked the dexamethasone-induced G1 cell cycle arrest of hepatoma cells but did not alter general glucocorticoid responsiveness. Transforming growth factor beta induced a G1 cell cycle arrest in C/EBP alpha antisense transfected cells, demonstrating the specific involvement of C/EBP alpha in the glucocorticoid growth suppression response. Constitutive expression of a conditionally activated form of C/EBP alpha caused a G1 cell cycle arrest of BDS1 hepatoma cells in the absence of glucocorticoids. In contrast, overexpression of C/EBP beta or C/EBP delta had no effect on hepatoma cell growth. Taken together, these results demonstrate that the steroid-induced expression of C/EBP alpha is necessary to mediate the glucocorticoid G1 cell cycle arrest of rat hepatoma cells and implicates a role for this transcription factor in the growth control of liver-derived epithelial tumor cells.

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.

Glucocorticoids induce a G1/G0 cell cycle arrest of Con8 rat mammary tumor cells that is synchronously reversed by steroid withdrawal or addition of transforming growth factor-alpha

Con8 mammary tumor cells are an epithelial cell line derived from the 7,12-dimethylbenz(alpha)anthracene-induced 13762NF rat mammary adenocarcinoma. The synthetic glucocorticoid dexamethasone suppresses the growth of Con8 cells, and after 5 days of treatment with this steroid, Con8 cells undergo less than 0.5 population doublings. This growth arrest is accompanied by a 30-fold elevation in c-jun transcript levels, no change in c-fos expression, and a moderate increase in total AP-1 transcriptional activity. Dexamethasone inhibited DNA synthesis within one cell cycle, and flow cytometry of propidium iodide-stained nuclei demonstrated that dexamethasone growth-suppressed cells had a DNA content indicative of a specific cell cycle block in either G1 or G0. Consistent with a G1/G0 arrest of the cell cycle, dexamethasone did not prevent Con8 cells from entering the S phase after release from synchronization at the G1/S boundary by a double thymidine block. Analysis of [3H]thymidine incorporation and autoradiography of [3H]thymidine-labeled nuclei revealed that after either dexamethasone withdrawal or the addition of transforming growth factor-alpha (TGF alpha), Con8 cells synchronously reinitiate cell cycle progression. Northern blot analysis demonstrated that an induction of transcripts for the G1 marker genes c-myc and cyclin D1 occurs before cells enter the S-phase. After dexamethasone withdrawal, c-myc and cyclin D1 expression transiently peak at 2 and 4 h, respectively. In contrast, c-myc expression peaked at 0.5-1 h, whereas cyclin D1 expression was induced at 2 h and maintained at a high level after the addition of TGF alpha. Our results demonstrate that glucocorticoids induce a specific block of the cell cycle progression of a rat mammary tumor cell, and that after synchronous progression through the cell cycle, the temporal expression pattern for c-myc and cyclin D1 is distinct for dexamethasone release vs. the addition of TGF alpha to glucocorticoid-suppressed cells.

Characterization of sgk, a novel member of the serine/threonine protein kinase gene family which is transcriptionally induced by glucocorticoids and serum

A novel member of the serine/threonine protein kinase gene family, designated sgk, for serum and glucocorticoid-regulated kinase, was identified in a differential screen for glucocorticoid-inducible transcripts expressed in the Con8.hd6 rat mammary tumor cell line. sgk encodes a protein of 49 kDa which has significant sequence homology (45 to 55% identity) throughout its catalytic domain with rac protein kinase, the protein kinase C family, ribosomal protein S6 kinase, and cyclic AMP-dependent protein kinase. sgk mRNA is expressed in most adult rat tissues, with the highest levels in the thymus, ovary, and lung, as well as in several rodent and human cell lines. sgk mRNA was stimulated by glucocorticoids and by serum within 30 min, and both inductions were independent of de novo protein synthesis. The transcriptional regulation by glucocorticoids is a primary response, since the promoter of sgk contains a glucocorticoid response element consensus sequence 1.0 kb upstream of the start of transcription which is able to stimulate chloramphenicol acetyltransferase reporter gene activity in a dexamethasone-dependent manner. Antibodies that specifically recognize sgk-encoded protein on an immunoblot were generated. This protein was shown to increase in abundance with glucocorticoid treatment in a manner which paralleled the mRNA accumulation. This is the first report of a presumed serine/threonine protein kinase that is highly regulated at the transcriptional level by glucocorticoid hormones and suggests a novel interplay between glucocorticoid receptor signalling and a protein kinase of the second messenger family.

1,25(OH)2-vitamin D3, a steroid hormone that produces biologic effects via both genomic and nongenomic pathways

The hormonally active form of vitamin D is 1,25(OH)2-vitamin D3 [1,25(OH)2D3]. This seco-steroid is the key mediator of the vitamin D endocrine system which produces biological effects in over 28 target tissues. In these target tissues, the biological responses may be generated both by a signal transduction mechanism which involves a nuclear receptor for 1,25(OH)2D3 that modulates gene transcription or a signal transduction pathway which involves rapid opening of Ca2+ channels which are externally located on the plasma membrane. This paper reviews the evidence in support of the pleiotropic effects of this steroid hormone and presents evidence that the receptor of the genomic effects is likely to be separate from the receptor/membrane recognition element which initiates the rapid nongenomic biological effects.

Effects of sodium butyrate on 1,25-dihydroxyvitamin D3 receptor activity in primary chick kidney cells

The genomic effects of the steroid hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) are mediated by high affinity nuclear associated specific receptors that belong to the superfamily of ligand induced transcription factors. The carboxylic acid, sodium butyrate--a potent inhibitor of histone deacetylase--is known to modulate gene expression in a variety of systems. Specific binding of 1,25(OH)2D3 to its receptor was examined in primary chick kidney cells, the chick macrophage cell line HD-11, and other mammalian cell lines such as ROS 17/2.8, HT-29 and CV-1 cells, that were all cultured in the presence or absence of 1 mM sodium butyrate. Treatment with n-butyrate resulted in significant (4.0-4.5-fold) increases in 1,25(OH)2D3 receptor binding without changing binding affinity only in the primary cultures of chick renal epithelial cells and the chick macrophage cell line but not in the other heterologous receptor-positive cell lines. The maximum increase in receptor binding was evident at 1 mM butyrate concentration. This effect reached a maximum at 15 h treatment, beyond which there was slow attenuation in increased binding until 24 h. The butyrate induced increases in receptor activity was associated with increases in the 1,25(OH)2D3-mediated induction of calbindin-D28K protein only in primary chick kidney cultures but not in the macrophage cell line (HD-11). Similarly, calbindin-D28K promoter activity was enhanced only in butyrate-treated primary chick kidney cultures, transfected with chimeric plasmids containing the 5' flanking sequence of the calbindin-D28K promoter fused to the chloramphenicol acetyl transferase (CAT) reporter gene but not in HD-11 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Transfection of avian vitamin D-dependent calbindin-D28K 5' flanking promoter sequence in primary chick kidney cells

Expression of the vitamin D induced calbindin-D28K protein is transcriptionally controlled by the steroid hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in a tissue-specific manner in the intestine and kidney. In order to examine the cis-acting elements of the calbindin-D28K promoter and its modulation by 1,25-dihydroxyvitamin D3, chimeric plasmids containing 2.1 kb of 5' flanking region linked to the reporter gene chloramphenicol acetyl transferase (CAT) were transfected by lipofection into primary cultures of chick kidney cells. Transfected chick kidney cells exhibited a high basal expression of the chloramphenicol acetyl transferase gene, reflecting the strong activity of the calbindin-D28K promoter. Expression of the pCaBP2.1 reporter gene was increased 2-fold in the presence of the hormone 1,25(OH)2D3 in the primary kidney cells. Deletion of a 1.42 kb fragment ending -679 base pairs upstream from the transcription start site led to a 2-fold repression in the reporter gene activity by the hormone 1,25(OH)2D3 in primary chick kidney cultures. These preliminary results suggest that both positive and negative elements normally act to regulate the expression of the calbindin-D28K gene in primary chick kidney cells.