The mammalian SIN3 complex consists of histone deacetylases (HDAC1, HDAC2), several known proteins (SAP30, N-CoR) and as yet unidentified proteins. Here we show that the mouse tetradecanoyl phorbol acetate induced sequence 7 (TIS7) protein is a novel transcriptional co-repressor that can associate with the SIN3 complex. We have identified tis7 as a gene that is up-regulated upon loss of polarity in a mouse mammary gland epithelial cell line expressing an estrogen-inducible c-JunER fusion protein. In unpolarized cells, TIS7 protein levels increase and TIS7 translocates into the nucleus. Overexpression of tis7 causes loss of polarity and represses a set of genes, as revealed by cDNA microarray analysis. We have shown that TIS7 protein interacts with several proteins of the SIN3 complex (mSin3B, HDAC1, N-CoR and SAP30) by yeast two-hybrid screening and co-immunoprecipitations. TIS7 co-immunoprecipitated HDAC complex is enzymatically active and represses a GAL4-dependent reporter transcription. The transcriptional repression of endogenous genes by tis7 overexpression is HDAC dependent. Thus, we propose TIS7 as a transcriptional co-repressor affecting the expression of specific genes in a HDAC activity-dependent manner during cell fate decisions, e.g. scattering.

The tetradecanoyl phorbol acetate-induced sequence 7 gene (tis7) is regulated during cell fate processes and functions as a transcriptional coregulator. Here, we describe the generation and analysis of mice lacking the tis7 gene. Surprisingly, TIS7 knockout mice show no gross histological abnormalities and are fertile. Disruption of the tis7 gene by homologous recombination delayed muscle regeneration and altered the isometric contractile properties of skeletal muscles after muscle crush damage in TIS7 ؊/؊ mice. Cultured primary myogenic satellite cells (MSCs) from TIS7 ؊/؊ mice displayed marked reductions in differentiation potential and fusion index in a strictly cell-autonomous fashion. Loss of TIS7 caused the down-regulation of musclespecific genes, such as those for MyoD, myogenin, and laminin-␣2. Fusion potential in TIS7 ؊/؊ MSCs could be rescued by TIS7 expression or laminin supplementation. Therefore, TIS7 is not essential for mouse development but plays a novel regulatory role during adult muscle regeneration.Cell proliferation and differentiation are governed by different stimuli, including soluble growth factors, the extracellular matrix (1, 12), and direct cell-cell interactions (8). While each of these signals uniquely regulates mitogenic responses and gene activity, the proliferation, differentiation, or apoptosis of a cell is an integrated response to its adhesive and growth factor environments (18,19).The mouse tis7 (PC4) gene was identified as an immediateearly gene specifically induced by tetradecanoyl phorbol acetate, epidermal growth factor, and fibroblast growth factor in Swiss 3T3 mouse cells and cultured rat astrocytes (2, 20). Vietor et al. showed previously that TIS7 is up-regulated after c-Jun activation in epithelial cells, translocates into the nucleus, and acts as a transcriptional coregulator (22). TIS7 can interact with mSin3B and histone deacetylase (HDAC) 1 as well as other members of the HDAC complex and is capable of specific transcriptional repression in an HDAC-dependent manner (22).TIS7 was shown to be expressed in C2C12 myoblasts and in differentiated myotubes, with a transient decrease after the onset of differentiation (7). In vitro studies with C2C12 myoblasts and antisense tis7 DNA transfection or microinjection of anti-TIS7 polyclonal antibodies caused a delay in myoblast differentiation (7). To define the essential functions of TIS7 in vivo, we have generated mice lacking a functional tis7 gene by homologous recombination. TIS7 Ϫ/Ϫ mice are viable and fertile but develop an interesting muscle regeneration phenotype upon muscle crush damage (MCD). In addition, we identified several myoblast-specific genes involved in muscle regeneration as being regulated by TIS7. This phenotype can be recapitulated in vitro in TIS7Ϫ/Ϫ primary myogenic satellite cell (MSC) cultures and is characterized by an almost complete absence of fusion-competent MSCs. In addition, we identified several myoblast-specific genes involved in satellite cell function as being regulated by TIS7. These d...