The Stress-activated Protein Kinases p38 and JNK1 Stabilize p21 by Phosphorylation*

Stress signals activate the SAPK/JNK and p38 MAPK classes of protein kinases, which mediate cellular responses, including steps in apoptosis and the maturation of some cell types. We now show that stress signals initiated by transforming growth factor1 (TGF1) induce G1 arrest through protein stabilization of the CDK inhibitor p21. TGF1 was previously shown to increase p21 protein levels, which in turn mediated G1 arrest through inactivation of the CDK2-cyclin E complex in HD3 cells (Yan, Z., Kim, G.-Y., Deng, X., and Friedman, E. (2002) J. Biol. Chem. 277, 9870–9879). We now demonstrate that the increase in p21 abundance is caused by a post-transcriptional, SMAD-independent mechanism. TGF1 activated p38 and JNK1, which initiated the phosphorylation of p21. TGF1 treatment increased the half-life of p21 by 3–4-fold. The increase in p21 stability was detected following activation of p38 and JNK1, and treatment of cells with the p38 inhibitor SB203580 prevented this increase in p21 stability. p38 and JNK1 phosphorylated p21 in vivo, and both p38 and JNK1 phosphorylated p21 at Ser in vitro. Peptide mapping demonstrated that both TGF1 and p38 induced phosphorylation of p21 at Ser in vivo, and mutation of Ser to alanine rendered p21 less stable than wild-type p21. TGF1 increased the stability of wildtype p21, but not the p21-S130A mutant. These findings demonstrate that SAPKs can mediate cell cycle arrest through post-translational modification of p21.