We have characterized previously the nuclear matrix protein/scaffold attachment factor (SAFB) as an estrogen receptor corepressor and as a potential tumor suppressor gene in breast cancer. A search of the human genome for other potential SAFB family members revealed that KIAA00138 (now designated as SAFB2) has high homology to SAFB (now designated as SAFB1). SAFB1 and SAFB2 are mapped adjacent to each other on chromosome 19p13.3 and are arranged in a bidirectional divergent configuration (head to head), being separated by a short (<500 bp) GC-rich intergenic region that can function as a bidirectional promoter. SAFB1 and SAFB2 share common functions but also have unique properties. As shown previously for SAFB1, SAFB2 functions as an estrogen receptor corepressor, and its overexpression results in inhibition of proliferation. SAFB1 and SAFB2 interact directly through a C-terminal domain, resulting in additive repression activity. They are coexpressed in a number of tissues, but unlike SAFB1, which is exclusively nuclear, SAFB2 is found in the cytoplasm as well as the nucleus. Consistent with its cytoplasmic localization, we detected an interaction between SAFB2 and vinexin, a protein involved in linking signaling to the cytoskeleton. Our findings suggest that evolutionary duplication of the SAFB gene has allowed it to retain crucial functions, but also to gain novel functions in the cytoplasm and/or nucleus.Scaffold attachment factor B (SAFB, 1 also named HET and HAP in the literature) was originally identified based on its ability to bind to scaffold/matrix attachment regions (1) and as a protein binding to the small heat shock protein hsp27 gene promoter (2). SAFB is a large protein (130 kDa) with a number of putative functional domains. The function of many of these domains is still unclear but can be inferred from the roles of similar domains in other proteins. The N terminus contains an SAF box (3) (also called an SAP domain (4)), which is a homeodomain-like DNA binding motif. This motif is believed to play a role in chromatin organization and specifically in organizing the interaction between nuclear matrix proteins and scaffold/ matrix attachment regions. SAF boxes are found in proteins involved in very diverse processes such as transcription, RNA processing, apoptotic chromatin degradation, and DNA repair (3). Amino acids 409 -484 harbor an RNA recognition motif (RRM), which is often found in mRNA-processing proteins. SAFB can interact with a number of proteins from the RNAprocessing machinery, such as AUF1/hnRNP D, hnRNP A1, htra2-␤1, ASF/SF2, SRp30c, and CLK2 (5-7). Consistent with this, SAFB is able to alter the splice site selection of an E1A minigene (7). Because SAFB was also shown to interact with the C-terminal domain of RNA polymerase II, it has been suggested to be part of a "transcriptosome" complex, coupling transcription and RNA processing (7). SAFB has a nuclear localization signal (NLS), and using biochemical fractionation experiments, we and others have shown that SAFB is a nuclear protei...

Estrogen receptor α (ERα) has an established role in promoting breast cancer. Transcriptional activation by ERα is a complex and multistep process, and it is influenced by coactivator and corepressor proteins that can either positively or negatively modulate ERα-mediated transcriptional activity. Corepressors are proposed to provide a counterbalance to the estrogen-induced transactivation, and represent a potential mechanism employed by the cell to regulate hormonal responses. In this review, we present evidence from tissue culture, animal and clinical studies, supporting the hypothesis that corepressors are crucial regulators of ERα-mediated action, and that their loss could promote breast cancer development and resistance to endocrine therapy. We propose that ERα corepressors play an important biological role by controlling the magnitude of the estrogen response, mediating antiestrogen inhibition of ERα, repressing DNA-bound ERα in the absence of the ligand, and conferring active repression of ERα-downregulated genes. Different ERα corepressors regulate steroid receptor activity through a variety of mechanisms, including formation of multiprotein complexes that are able to affect chromatin remodeling, histone deacetylation, or basal transcription. Other mechanisms include competition with coactivators, interference with DNA binding and ERα homodimerization, alteration of ERα stability, sequestration of ERα in the cytoplasm, and effects on RNA processing. Most ERα corepressors can control the receptor's activity through more than one mechanism, and it is possible that the synergy between different pathways cooperates to fully inhibit ERα transcriptional activity, and create an integrated response to a variety of different cellular signaling pathways. We will discuss the role of corepressors in tumor suppression and the link they might present between ERα regulation and DNA repair. Finally, we will discuss major challenges in the field and speculate on the exciting findings that await us in the next few years.

Scaffold attachment factor B1 (SAFB1) is a multifunctional protein that can bind both DNA and RNA and is involved in RNA processing and stress response. In addition, SAFB1 contains a transcriptional repression domain and can bind certain hormone receptors and repress their activity. To assess the role of SAFB1 in vivo, we generated SAFB1 mutant mice through targeted deletion in embryonic stem cells. While viable homozygous mutant (SAFB1 ؊/؊ ) mice were obtained, genotypic distribution indicated that homozygous deficiency resulted in both prenatal and neonatal lethality. Mice lacking SAFB1 exhibited dwarfism, as a result of in utero growth retardation, and had low serum insulin-like growth factor 1 (IGF1) levels. In agreement with the previous characterization of SAFB1 as a corepressor for hormone receptors, we found that SAFB1 ؊/؊ mice displayed dramatic defects in the development and function of the reproductive system. Male SAFB1 null mice were infertile, apparently because of low circulating levels of testosterone. SAFB1؊/؊ testes were small and showed progressive degeneration of the germinal epithelium, increased apoptosis of germ cells, and Leydig cell hyperplasia. SAFB ؊/؊ female mice were subfertile and showed progressive infertility, in part because of defects in oviductal transport and reduced numbers of follicles. Immortalized SAFB1 ؊/؊ mouse embryonic fibroblasts showed cell-intrinsic defects including increased transcriptional estrogen receptor ␣ activity and enhanced responsiveness to IGF1. Together, these in vivo findings establish a critical role for SAFB1 in development, growth regulation, and reproduction.

Scaffold attachment factor B1 (SAFB1) is a multifunctional protein, which has previously been implicated in breast cancer. Here, we show that genetic deletion of SAFB1 in mouse embryonic fibroblasts (MEF) leads to spontaneous immortalization and altered expression of two proteins involved in immortalization and escape from senescence: low levels of p19 ARF and high levels of TBX2. Inactivation of TBX2 using a dominant-negative TBX2 resulted in up-regulation of p19 ARF in SAFB1 knockout MEFs. SAFB1 loss also caused lack of contact inhibition, increased foci formation, and increased oncogene-induced anchorage-independent growth. These findings suggest that SAFB1 is a novel player in cellular immortalization and transformation. (Cancer Res 2006; 66(16): 7859-63)