The Sin3-histone deacetylase (HDAC) corepressor complex is conserved from yeast to humans. Mammals possess two highly related Sin3 proteins, mSin3A and mSin3B, which serve as scaffolds tethering HDAC enzymatic activity, and numerous sequence-specific transcription factors to enable local chromatin regulation at specific gene targets. Despite broad overlapping expression of mSin3A and mSin3B, mSin3A is cell-essential and vital for early embryonic development. Here, genetic disruption of mSin3B reveals a very different phenotype characterized by the survival of cultured cells and lethality at late stages of embryonic development with defective differentiation of multiple lineages-phenotypes that are strikingly reminiscent of those associated with loss of retinoblastoma family members or E2F transcriptional repressors. Additionally, we observe that, whereas mSin3B ؊/؊ cells cycle normally under standard growth conditions, they show an impaired ability to exit the cell cycle with limiting growth factors. Correspondingly, mSin3B interacts physically with the promoters of known E2F target genes, and its deficiency is associated with derepression of these gene targets in vivo. Together, these results reveal a critical role for mSin3B in the control of cell cycle exit and terminal differentiation in mammals and establish contrasting roles for the mSin3 proteins in the growth and development of specific lineages.E2F ͉ histone deacetylase ͉ knockout ͉ quiescence T he Sin3-histone deacetylase (HDAC) corepressor has been physically and functionally linked to diverse transcriptional complexes governing many physiological processes (1, 2). The two highly related mammalian Sin3 proteins, mSin3A and mSin3B, use their multiple interaction domains to direct chromatin-modifying activities to specific sites in the genome, most typically via sequence-specific transcription factors and their cognate binding elements. Class I HDACs, HDAC1 and HDAC2, are the principal enzymatic activities of the mSin3 complex. In addition, there are several other mSin3-associated proteins, including mSds3, p33 ING1 ,.mSin3A has been shown to be essential for early embryonic development and for the growth and survival of cultured cells that may relate to its requirement for the regulation of multiple transcriptional programs (7). Of relevance to the current study, mSin3B is expressed in cells deleted for mSin3A, suggesting that, despite their structural relatedness, mSin3B is not functionally equivalent to mSin3A. Both yeast and mammalian Sds3 are required for the maintenance of Sin3-associated HDAC enzymatic activity (3,8). Nullizygosity for mammalian Sds3 results in early embryonic lethality and engenders marked chromosome segregation defects due to a failure in pericentric heterochromatin formation (9). mSin3A-null fibroblasts exhibited normal karyotypes (7).In mammalian cell cycle, the G 0 /G 1 -to-S transition is a highly regulated event whose disruption represents a prerequisite for essentially all human cancers. This critical phase of the cell cy...

Serial passage of primary mammalian cells or strong mitogenic signals induce a permanent exit from the cell cycle called senescence. A characteristic of senescent cells is the heterochromatinization of loci encoding pro-proliferative genes, leading to their transcriptional silencing. Senescence is thought to represent a defense mechanism against uncontrolled proliferation and cancer. Consequently, genetic alterations that allow senescence bypass are associated with susceptibility to oncogenic transformation. We show that fibroblasts genetically inactivated for the chromatin-associated Sin3B protein are refractory to replicative and oncogene-induced senescence. Conversely, overexpression of Sin3B triggers senescence and the formation of senescence-associated heterochromatic foci. Although Sin3B is strongly up-regulated upon oncogenic stress, decrease in expression of Sin3B is associated with tumor progression in vivo, suggesting that expression of Sin3B may represent a barrier against transformation. Together, these results underscore the contribution of senescence in tumor suppression and suggest that expression of chromatin modifiers is modulated at specific stages of cellular transformation. Consequently, these findings suggest that modulation of Sin3B-associated activities may represent new therapeutic opportunities for treatment of cancers. [Cancer Res 2009;69(16):6430-7]

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Supplementary Figure Legends 1-7 from Sin3B Expression Is Required for Cellular Senescence and Is Up-regulated upon Oncogenic Stress

Supplementary Figure Legends 1-7 from Sin3B Expression Is Required for Cellular Senescence and Is Up-regulated upon Oncogenic Stress