Involvement of pRB-related p107 Protein in the Inhibition of S Phase Progression in Response to Genotoxic Stress

Abstract

pRB family pocket proteins consisting of pRB, p107, and p130 are thought to act as a set of growth regulators that inhibit the cell cycle transition from G 1 to S phases by virtue of their interaction with E2F transcription factors. When cells are committed to progressing through the cell cycle at the late G 1 restriction point, they are hyperphosphorylated by G 1 cyclin-cyclin-dependent kinase and are functionally inactivated. Consistent with such a G 1 regulatory role, pRB and p130 are abundantly expressed in quiescent cells. In contrast, p107 is present at low levels in the hypophosphorylated form in quiescent cells. As cells progress toward late G 1 to S phases, the levels of p107 increase, and the majority become hyperphosphorylated, suggesting a possible role of p107 in post-G 1 cell cycle regulation. In this study, we have demonstrated that a nonphosphorylatable and thus constitutively active p107 has the potential to inhibit S phase progression. The levels of the phosphorylation-resistant p107 required for the S phase inhibition are significantly less than those of endogenous p107. We further show herein that the exposure of cells to the DNA-damaging agent, cisplatin, provokes S phase arrest, which is concomitantly associated with the accumulation of hypophosphorylated p107. Furthermore, the S phase inhibitory response to cisplatin is augmented by the ectopic expression of wild type p107, although it is diminished by the adenovirus E1A oncoprotein, which counteracts the pocket protein functions. Because p107 is a major pRB family protein expressed in S phase cells, our results indicate that p107 participates in an inhibition of cell cycle progression in response to DNA damage in S phase cells.

Pocket proteins and cell cycle control

“…In addition, after S phase has begun, pRB and possibly p107 can inhibit S phase progression by repressing cyclin A, and thereby inhibiting Cdk2 and PCNA function (Karantza et al, 1993;Kondo et al, 2001;Sever-Chroneos et al, 2001). These transcriptional effects may be utilized to induce an intra-S phase block in response to DNA damage (Harrington et al, 1998;Knudsen et al, 2000;Kondo et al, 2001;Lan et al, 2002).…”

Proto-oncogene Activity of Melanoma Antigen-A11 (MAGE-A11) Regulates Retinoblastoma-related p107 and E2F1 Proteins

“…In contrast, mutations in p107 have not been reported in cancer (21,22), although p107 is important for cell cycle regulation (23,24). Loss of Rb-related protein activity is also achieved by cancer cells through the action of viral oncogenes that target the pocket region (25,26).…”

Developmental Activation of the Rb–E2F Pathway and Establishment of Cell Cycle-regulated Cyclin-dependent Kinase Activity during Embryonic Stem Cell Differentiation

“…This hypothesis was tested by expressing a mutant form of p107 that evades Cdk regulation through multiple amino acid substitutions in key phosphorylation sites Kondo et al, 2001). The first approach to address this question involved the generation of stable ES cell lines expressing wild-type p107 (p107 wt ) or a mutant that evades Cdk inhibition (p107 ⌬S/T-P ; Figure 5C).…”