| Progression through the mammalian cell cycle is a tightly regulated process that allows cells to replicate their genomes and divide properly. In growth factor-deprived conditions or in response to stress, the cell will exit the cell cycle either reversibly through quiescence, or permanently via senescence. Studies have shown that the SIN3 family of proteins plays a crucial role in these cell cycle exit processes. SIN3 proteins are highly conserved, and exist in mammals as two family members: SIN3A and SIN3B, which function as flexible scaffolding proteins to assemble co-repressor complexes. Our laboratory has recently implicated SIN3B as a critical mediator of each of these cell cycle exit processes. However, its mechanism of action and the consequences of its disruption pertaining to cancer progression have not been comprehensively elucidated. Here we demonstrate that SIN3B is required for the induction of senescence in a mouse model of prostate cancer, and thus prevents the progression to aggressive and invasive carcinoma. In addition, through interaction analysis, we uncovered a novel and robust association between SIN3B and the DREAM complex. The DREAM complex, comprised of p107/p130, E2F4/5, DP1 and the MuvB core complex, is responsible for the repression of hundreds of cell cycle-related transcripts during quiescence. We determined that the deletion of SIN3B resulted in the derepression of DREAM target genes during quiescence, but was not sufficient to allow quiescent cells to resume proliferation. However, the ectopic expression of APC/CCDH1 inhibitor EMI1 was sufficient for SIN3B deleted cells, but not wild-type cells, to reenter the cell cycle. These studies demonstrate a critical role for SIN3B in the senescence and quiescence programs, and provide important mechanistic insight into the molecular pathways that exquisitely regulate cell cycle exit. |
