PROTEASOME-DEPENDENT REGULATION OF P21(WAF1 CIP1) EXPRESSION/

Proteasome-dependent degradation of regulatory proteins is a known mec hanism of cell cycle control. We found that the proteasome-specific in hibitor lactacystin (LC) induced expression of the cell cycle inhibito r p21(WAF1/CIP1) in human cancer cells regardless of their p53 status. Both wild-type (wt) p53 and p21 protein levels increased by two hours in wt p53 containing cells, whereas mutant (mt) p53 levels decreased and the increase in p21 levels was delayed to 6 hr following inhibitio n of proteolysis by LC in me p53 expressing cells. We found that wt bu t not mt p53 expressing cells increased p21 mRNA and p21-promoter repo rter levels following LC exposure, suggesting transcriptional inductio n of p21. Inhibition of protein synthesis by cycloheximide demonstrate d increased p21 protein half-life in the presence of LC in mutant p53 containing cells. p21 induction was correlated with the cytostatic eff ects of LC. The results suggest that p21 protein expression could be i ncreased by transcriptional mechanisms as well as inhibition of proteo lysis by LC. (C) 1996 Academic Press, Inc.