Activation of serum- and glucocorticoid-regulated protein kinase by agonists that activate phosphatidylinositide 3-kinase is mediated by 3-phosphoinositide-dependent protein kinase-1 (PDK1) and PDK2

摘要

pThe PtdIns(3,4,5)iP/isub3/sub-dependent activation of protein kinase B (PKB) by 3-phosphoinositide-dependent protein kinases-1 and -2 (PDK1 and PDK2 respectively) is a key event in mediating the effects of signals that activate PtdIns 3-kinase. The catalytic domain of serum- and glucocorticoid-regulated protein kinase (SGK) is 54% identical with that of PKB and, although lacking the PtdIns(3,4,5)iP/isub3/sub-binding pleckstrin-homology domain, SGK retains the residues that are phosphorylated by PDK1 and PDK2, which are Thrsup256/sup and Sersup422/sup in SGK. Here we show that PDK1 activates SGK iin iv/iitro/i by phosphorylating Thrsup256/sup. We also show that, in response to insulin-like growth factor-1 (IGF-1) or hydrogen peroxide, transfected SGK is activated in 293 cells via a PtdIns 3-kinase-dependent pathway that involves the phosphorylation of Thrsup256/sup and Sersup422/sup. The activation of SGK by PDK1 iin iv/iitro/i is unaffected by PtdIns(3,4,5)iP/isub3/sub, abolished by the mutation of Sersup422/sup to Ala, and greatly potentiated by mutation of Sersup422/sup to Asp (although this mutation does not activate SGK itself). Consistent with these findings, the Sersup422/supAsp mutant of SGK is activated by phosphorylation (probably at Thrsup256/sup) in unstimulated 293 cells, and activation is unaffected by inhibitors of PtdIns 3-kinase. Our results are consistent with a model in which activation of SGK by IGF-1 or hydrogen peroxide is initiated by a PtdIns(3,4,5)iP/isub3/sub-dependent activation of PDK2, which phosphorylates Sersup422/sup. This is followed by the PtdIns(3,4,5)iP/isub3/sub-independent phosphorylation at Thrsup256/sup that activates SGK, and is catalysed by PDK1. Like PKB, SGK preferentially phosphorylates serine and threonine residues that lie in Arg-Xaa-Arg-Xaa-Xaa-Ser/Thr motifs, and SGK and PKB inactivate glycogen synthase kinase-3 similarly iin iv/iitro/iand in co-transfection experiments. These findings raise the possibility that some physiological roles ascribed to PKB on the basis of the overexpression of constitutively active PKB mutants might be mediated by SGK./p