The retinoblastoma tumor suppressor protein (RB) is a potent inhibitor of cell proliferation. RB is expressed throughout the cell cycle, but its antiproliferative activity is neutralized by phosphorylation during the G 1 /S transition. RB plays an essential role in the G 1 arrest induced by a variety of growth inhibitory signals. In this report, RB is shown to also be required for an intra-S-phase response to DNA damage. Treatment with cisplatin, etoposide, or mitomycin C inhibited S-phase progression in Rb ؉/؉ but not in Rb ؊/؊ mouse embryo fibroblasts. Dephosphorylation of RB in S-phase cells temporally preceded the inhibition of DNA synthesis. This S-phase dephosphorylation of RB and subsequent inhibition of DNA replication was observed in p21Cip1 -deficient cells. The induction of the RB-dependent intra-S-phase arrest persisted for days and correlated with a protection against DNA damage-induced cell death. These results demonstrate that RB plays a protective role in response to genotoxic stress by inhibiting cell cycle progression in G 1 and in S phase.

The retinoblastoma tumour suppressor protein RB is cleaved by caspases during apoptosis. Here we have mutated the caspase cleavage site in the carboxy terminus of the murine Rb protein in the mouse germ line to create the Rb-MI allele. After endotoxic shock, expression of Rb-MI inhibits apoptosis in the intestines, but not in the spleen, and promotes the survival of male mice. Fibroblasts expressing Rb-MI protein are protected from apoptosis induced by the tumour-necrosis factor-alpha type I receptor (TNFRI) but remain sensitive to cell death induced by DNA damage. Correspondingly, the release of cytochrome c and the activation of caspase-3 induced by TNFRI, but not by DNA damage, are defective in cells expressing Rb-MI. Our results highlight the importance of Rb cleavage in TNFRI-induced apoptosis.

beta-catenin ͉ blast crisis ͉ CML ͉ S17 stromal cells ͉ tyrosine kinase

Tumor necrosis factor-␣ (TNF

© 2 0 0 2 L a n d e s B i o s c i e n c e . N o t f o r d i s t r i b u t i o n .[ Cell Cycle 1:3,[193][194][195][196][197][198][199][200] ABSTRACTIn response to ionizing radiation (IR), cell cycle checkpoints are activated to provide time for DNA repair. Several different checkpoint mechanisms have been elucidated. However, mechanisms that regulate the duration of cell cycle arrest are not understood. Previous studies have shown that the retinoblastoma tumor suppressor protein (RB) is required for radiation-induced G1 arrest. Working with primary fibroblasts derived from Rb +/+ and Rb -/-mouse embryos, we show that RB also regulates the duration of G2 arrest. The initial G2 checkpoint response is enhanced in Rb -/-cells due to a defect in G1 arrest. However, the permanent arrest in G2 induced by higher doses of IR does not occur in Rb -/-cells. Rb -/-cells either resumed proliferation or underwent apoptosis at IR doses that caused the majority of Rb +/+ cells to arrest permanently in G2. The prolongation of G2 arrest in Rb +/+ cells correlated with a gradual accumulation of hypophosphorylated RB. Thus, regulation of the RB function may be an important aspect in the maintenance of cell cycle checkpoints in DNA damage response.