p53-mediated accumulation of hypophosphorylated pRb after the G₁ restriction point fails to halt cell cycle progression

Abstract

This study analyses whether the inability of p53 to induce G₁ arrest after the restriction point relates to an inability to modulate pRb phosphorylation. Transient p53 overexpression in normal human diploid fibroblasts and p53-deficient cancer cells led to increased levels of the cyclin-dependent kinase inhibitor p21^{Cip1/Waf1/Sdi1} and an accumulation of hypophosphorylated pRb in cells growing asynchronously and in cells synchronized in late G₁ or M. Similarly, γ-irradiation of asynchronous, late-G₁, or S phase fibroblasts led to an increase in hypophosphorylated pRb. Experiments with fibroblasts expressing the HPV16 E6 protein indicated that accumulation of hypophosphorylated pRb required functional p53. Progression into and through S phase was not altered by the presence of hypophosphorylated pRb in late G₁, consistent with the failure of p53 to mediate G₁ arrest in cells that are past the restriction point. These data indicate that accumulation of hypophosphorylated pRb has significantly different effects on cell cycle progression in early G₁ versus late G₁ or S phase.