Up-regulated P21CIP1 expression is part of the regulation quantitatively controlling serum deprivation-induced apoptosis.

Among the many genes which have been suggested to be required by the molecular mechanism dictating apoptotic death, some have been shown to function as pacemakers to pave the way for cells either to liver or to die. Previously we have shown that immediate early gene expression associated with the G1 phase of cell cycle traverse are candidates for this function. Here we report that well-known key regulator for halting cell cycling at the G1/S border, the p21 protein known as WAF1, Cip1, Pic1, or Sdi1, is also involved in the execution of cells' suicidal death. p21 up-regulation is seen in quiescent mouse 3T3 fibroblasts stimulated to die by serum deprivation, at both message and protein levels, evidenced by increased protein presence in its targeted functional site, the nucleus. In addition, we show that this up-regulation of p21 is functionally related to the operational efficiency of the apoptotic process, in that when cells are stably transfected with an antisense construct to repress the endogenous p21-protein level, death is delayed. Quantitative protection from apoptosis with antisense p21 transfection is relatively proportional to the repressed level of this protein in the cells. Taken together, our results suggest that the apoptotic-dependent additional increase of p21 beyond the base level, seen in serum-deprived quiescent cells, may be involved in the molecular events precipitating a rapid program of cell demise, and that repression of this increase may obstruct the operation of this program and postpone the eventual death.