Ac. Maiyar et al., P53 STIMULATES PROMOTER ACTIVITY OF THE SGK SERUM GLUCOCORTICOID-INDUCIBLE SERINE/THREONINE PROTEIN-KINASE GENE IN RODENT MAMMARY EPITHELIAL-CELLS/, The Journal of biological chemistry, 271(21), 1996, pp. 12414-12422
sgk is a novel member of the serine/threonine protein kinase gene fami
ly that is transcriptionally regulated by serum and glucocorticoids in
mammary epithelial cells. To functionally determine if the sgk promot
er is regulated by the p53 tumor suppressor protein in mammary cells,
a series of sgk promoter fragments with 5'-deletions were linked to th
e bacterial chloramphenicol acetyltransferase gene (sgk-CAT) and trans
iently cotransfected 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, cotransfe
ction of wild-type p53 strongly stimulated the activities of both the
sgk promoter and the well characterized p53-responsive p21/Waf1 promot
er, whereas in Rat a fibroblasts, wild-type p53 repressed the basal ac
tivities of both promoters, revealing that sgk and p21/Waf1 are simila
rly regulated in a cell type-specific manner. Taken together, these re
sults demonstrate that sgk is a new transcriptional target of p53 in m
ammary epithelial cells and represent the first example of a hormone-r
egulated protein kinase gene with a functionally defined p53 promoter
recognition element.