p53-Independent elevation of p21 expression by PMA results from PKC-mediated mRNA stabilization

The p21 (cip1/waf1) protein induces cell cycle arrest through inhibition of the activity of cdk (cyclin dependent kinase)/cyclin complexes. Expression of p21 is induced in a p53-dependent manner by DNA damage. p21 can also be induced independently of p53 by phorbol ester or okadaic acid. In this stu dy, we have addressed the role of the PKC (protein kinase C) signaling path way in the induction of p21 in response to PMA (phorbol myristate acetate) and okadaic acid. Levels of p21 (protein and mRNA) rapidly increased (withi n similar to4 h) in U937 cells treated with PMA. The PKC-specific inhibitor s RO 31-8220 and GF109203X down-regulated PMA or okadaic acid-induced p21 e xpression. Following persistent PKC activation, p21 mRNA levels remained el evated, indicating an enhanced stability of the mRNA. Using actinomycin D t o measure mRNA stability and p21 promoter luciferase assays to measure acti vity, we provide evidence to support a role for the PKC signaling pathway i n p21 mRNA stability. Thus, PKC regulates the amount of p21 in U937 cells a t the level of mRNA accumulation and translation, (C) 2001 Academic Press.