Rapid dephosphorylation of p107 following UV irradiation

In response to UV irradiation, mouse NIH3T3 fibroblasts transiently arrest predominantly in the G1 phase of the cell cycle. Here, we investigate the r ole of the retinoblastoma-related pocket proteins in this biological proces s. We report here that UV induces an increase in p107/E2F complexes, shown previously to be repressors of E2F-dependent transcriptional activity. Seve ral lines of evidence indicate that the increase of p107/E2F complexes foll owing UV irradiation is a consequence of rapid dephosphorylation of p107. F irst, UV-mediated p107 dephosphorylation could be abolished by pretreatment of NIH3T3 fibroblasts with the serine/threonine phosphatase inhibitors cal yculin A and okadaic acid. Second, alteration of protein phosphatase 2A hol oenzyme composition by over-expression of specific B subunits interfered wi th UV-mediated dephosphorylation of p107, Consistent with this, p107 could be dephosphorylated in vitro with PP2A, Moreover, dephosphorylation of p107 was shown to be independent of the activity of p53 and p21, as it occurred also in UV-treated p53-null as well as p21-null mouse fibroblasts. We obse rved a close correlation between the UV dosages required for G1 cell cycle arrest and p107 dephosphorylation. Our data suggest a model in which UV rad iation-induced cell cycle arrest depends, at least in part, on the inductio n of a PP2A-like phosphatase that acts on p107.