The permissive effect of p21 Waf1/Cip1 on DNA synthesis is dependent on cell type : Effect is absent in p53-inactive cells

Abstract

The cyclin-dependent kinase inhibitors (CKI) interact with cyclin-cdk complexes to arrest mitogen-stimulated transit through the cell cycle, but we and others have recently shown that these molecules can exert permissive effects on cell cycle transit as well. The p53 protein induces transcription of the p21 Waf1/Cip1 gene, but whether p53 has any effect on the stimulatory versus inhibitory state of p21 Waf1/Cip1 toward cell growth is not known. The focus of the current study was to examine the effect of p21 Waf1/Cip1 inhibition on growth in cells which possess an inactive p53 protein. We found that there was significant and specific inhibition of p21 Waf1/Cip1 protein transcription in human squamous carcinoma A431 cells after transfection of an antisense p21 Waf1/Cip1 oligodeoxynucleotide, yet there was no significant growth inhibition in these cells after stimulation with 10% serum or with PDGF-BB, in contrast to what was observed in vascular smooth muscle (VSM) cells. Furthermore, there was no attenuation of either cyclinD/cdk4 association or of Rb hyperphosphorylation after antisense p21 Waf1/Cip1 oligodeoxynucleotide transfection, suggesting that an alternate pathway exists to allow association and phosphorylation of these cell cycle components in the absence (or with lower levels) of p21 Waf1/Cip1. Thus, the permissive effect of p21 Waf1/Cip1 toward growth is dependent on cell type, and active p53 is likely required for this effect.