Regulation of the p21(Sdi1/Cip1/Waf1) DNA synthesis inhibitor in senescent human diploid fibroblasts

A large body of evidence has demonstrated that normal human fibroblasts have a limited division potential in culture and underwent senescence, a process whereby cells became arrested in the G₁ phase of the cell cycle and overexpressed a DNA synthesis inhibitor(s). Cyclin-dependent kinase two (Cdk2) is required for the promotion of the G₁-to-S phase transition in human cells. Senescent fibroblasts contain intact cyclin-Cdk2 complexes but cannot induce Cdk2 protein kinase activity in response to mitogen stimulation. Recently, we cloned p21(Sdi1), a potent inhibitor of DNA synthesis and Cdk2 kinase activity, from a senescent cell cDNA library and demonstrated that it was expressed at significantly higher levels in senescent cells than actively proliferating cells. In contrast to actively dividing cells, mitogen-stimulated senescent cells do not down-regulate the expression of p21(Sdi1) and do not express late G₁ phase gene products that are required for entry into S phase. We suggest that the inability of mitogen-stimulated senescent cells to down-regulate p21(Sdi1) levels contributes to the resulting lack of late G₁ gene expression and failure to traverse the G₁/S phase boundary.