Osmotic shock induces G(1) arrest through p53 phosphorylation at Ser(33) by activated p38(MAPK) without phosphorylation at Ser(15) and Ser(20)

Osmotic shock induced transient stabilization of p53, possibly due to incre ased degradation of Mdm2. Stabilized p53 was activated by p38(MAPK), result ing in G(1) arrest through induction of p21(WAF1). Among the postulated pho sphorylation sites involved in p53 stabilization or activation (Ser(15), Se r(20), Ser(33), and Ser(46)), only Ser(33) was phosphorylated. Furthermore, interaction of p53 with the transcriptional coactivator p300 was induced, and Lys(382) of p53 was acetylated. Although inhibition of p38(MAPK) did no t prevent nuclear accumulation of p53, phosphorylation of Ser(33) was marke dly suppressed by SB203580, a specific inhibitor of p38(MAPK). Under these conditions, acetylation of Lys(382) and induction of p21(WAF1) were also in hibited, and cells with elevated levels of p53 showed normal cell cycle pro gression. Activated p38(MAPK) phosphorylated endogenous p53 at Ser(33) in l iving cells. In stable transformants expressing dominant negative MKK6, an upstream protein kinase of p38(MAPK), p53 stabilization was induced normall y following osmotic shock, but phosphorylation of Ser(33), acetylation of L ys(382), and induction of p21(WAF1) were almost completely inhibited. These results suggest that phosphorylation at Ser(33) by p38(MAPK) is critical f or activation of p53 following osmotic shock. Phosphorylation of neither Se r(15) nor Ser(20) was needed in this activation.