Scaffold attachment factor B2 (SAFB2)-null mice reveal non-redundant functions of SAFB2 compared with its paralog, SAFB1

Scaffold attachment factors SAFB1 and SAFB2 are multifunctional proteins that share >70% sequence similarity. SAFB1-knockout (SAFB1(-/-)) mice display a high degree of lethality, severe growth retardation, and infertility in male mice. To assess the in vivo role of SAFB2, and to identify unique functions of the two paralogs, we generated SAFB2(-/-) mice. In stark contrast to SAFB1(-/-), SAFB2(-/-) offspring were born at expected Mendelian ratios and did not show any obvious defects in growth or fertility. Generation of paralog-specific antibodies allowed extensive expression analysis of SAFB1 and SAFB2 in mouse tissues, showing high expression of both SAFB1 and SAFB2 in the immune system, and in hormonally controlled tissues, with especially high expression of SAFB2 in the male reproductive tract. Further analysis showed a significantly increased testis weight in SAFB2(-/-) mice, which was associated with an increased number of Sertoli cells. Our data suggest that this is at least in part caused by alterations in androgen-receptor function and expression upon deletion of SAFB2. Thus, despite a high degree of sequence similarity, SAFB1(-/-) and SAFB2(-/-) mice do not totally phenocopy each other. SAFB2(-/-) mice are viable, and do not show any major defects, and our data suggest a role for SAFB2 in the differentiation and activity of Sertoli cells that deserves further study.

Epigenetic reprogramming of HOXC10 in endocrine-resistant breast cancer

Resistance to aromatase inhibitors (AIs) is a major clinical problem in the treatment of estrogen receptor (ER)-positive breast cancer. In two breast cancer cell line models of AI resistance, we identified widespread DNA hyper- and hypomethylation, with enrichment for promoter hypermethylation of developmental genes. For the homeobox gene HOXC10, methylation occurred in a CpG shore, which overlapped with a functional ER binding site, causing repression of HOXC10 expression. Although short-term blockade of ER signaling caused relief of HOXC10 repression in both cell lines and breast tumors, it also resulted in concurrent recruitment of EZH2 and increased H3K27me3, ultimately transitioning to increased DNA methylation and silencing of HOXC10. Reduced HOXC10 in vitro and in xenografts resulted in decreased apoptosis and caused antiestrogen resistance. Supporting this, we used paired primary and metastatic breast cancer specimens to show that HOXC10 was reduced in tumors that recurred during AI treatment. We propose a model in which estrogen represses apoptotic and growth-inhibitory genes such as HOXC10, contributing to tumor survival, whereas AIs induce these genes to cause apoptosis and therapeutic benefit, but long-term AI treatment results in permanent repression of these genes via methylation and confers resistance. Therapies aimed at inhibiting AI-induced histone and DNA methylation may be beneficial in blocking or delaying AI resistance.

Regulation of HER2 oncogene transcription by a multifunctional coactivator/corepressor complex

Transcription of the HER2 oncogene can be repressed by estrogen (E2). We now show that, a splice isoform of the nuclear receptor coactivator AIB1, AIB1-Δ4, is able to reverse E2 repression of HER2 gene expression in breast cancer cells. The first 224 amino acids of AIB1 that are absent in AIB1-Δ4, bind a co-repressor, ANCO1. Using chromatin immunoprecipitation assay approaches in MCF7 and BT474 cell lines, we demonstrate that AIB1 and AIB1-Δ4 can bind to the E2 regulatory site in the first intron of the HER2 gene, after E2 treatment, but only full-length AIB1 recruits ANCO1. Consistent with E2-induced chromatin repression, the AIB1-ANCO1 complex recruits HDAC3 and HDAC4 to the intronic estrogen response element and the proximal promoter acquires the repressive chromatin mark H3K9me3 and loses H3K4me1. In contrast, AIB1-Δ4 does not recruit ANCO 1, HDAC3, or HDAC4 and the proximal promoter retains activation marks of H3K4me1. In cell lines with low levels of ANCO1 (T47D), E2 does not repress HER2 gene transcription but the repressive response can be restored by overexpression of ANCO1. ANCO1 can also repress other E2-responsive genes, indicating that AIB1, AIB1-Δ4 and ANCO1 are important determinants of endocrine and growth factor responsiveness in breast cancer.

Transcriptional repression of AIB1 by FoxG1 leads to apoptosis in breast cancer cells

The oncogene nuclear receptor coactivator amplified in breast cancer 1 (AIB1) is a transcriptional coactivator that is overexpressed in various types of human cancers. However, the molecular mechanisms controlling AIB1 expression in the majority of cancers remain unclear. In this study, we identified a novel interacting protein of AIB1, forkhead-box protein G1 (FoxG1), which is an evolutionarily conserved forkhead-box transcriptional corepressor. We show that FoxG1 expression is low in breast cancer cell lines and that low levels of FoxG1 are correlated with a worse prognosis in breast cancer. We also demonstrate that transient overexpression of FoxG1 can suppress endogenous levels of AIB1 mRNA and protein in MCF-7 breast cancer cells. Exogenously expressed FoxG1 in MCF-7 cells also leads to apoptosis that can be rescued in part by AIB1 overexpression. Using chromatin immunoprecipitation, we determined that FoxG1 is recruited to a region of the AIB1 gene promoter previously characterized to be responsible for AIB1-induced, positive autoregulation of transcription through the recruitment of an activating, multiprotein complex, involving AIB1, E2F transcription factor 1, and specificity protein 1. Increased FoxG1 expression significantly reduces the recruitment of AIB1, E2F transcription factor 1 and E1A-binding protein p300 to this region of the endogenous AIB1 gene promoter. Our data imply that FoxG1 can function as a pro-apoptotic factor in part through suppression of AIB1 coactivator transcription complex formation, thereby reducing the expression of the AIB1 oncogene.

Abstract P6-05-07: Amplified in breast cancer 1 and ankyrin repeat containing cofactor 1 mediate estrogen induced repression of the ErbB2 oncogene

The oncogene amplified in breast cancer 1 (AIB1/SRC3) is a nuclear receptor coactivator and plays an important role in the regulation of gene transcription. Additionally, AIB1 is important in malignant progression as AIB1 has been shown to be a gene amplified in breast cancer and is also over-expressed in a variety of cancer types. Previous work from our laboratory has identified a splice variant of AIB1 termed AIB1-Δ4. This splice variant arises from the splicing of exon 4 from the mature mRNA. This leads to the formation of an N terminally truncated form of the AIB1 protein, which is a more potent coactivator of steroid dependent transcription compared to the full length AIB1. This suggests that the N-terminal fragment of AIB1, which is not present in AIB1-Δ4, may act to dampen AIB1 mediated transcriptional activation. The presence of this N terminal region suggests this region may be a site of interaction with a repressive protein, which could interact with AIB1 and not AIB1-Δ4. We have identified ANCO1 as an N terminal AIB1 binding protein that acts to repress AIB1 mediated transcriptional activity. Co-immunoprecipitation experiments have indicated interaction between AIB1 and ANCO1 but not between AIB1-Δ4 and ANCO1 indicating ANCO1 binds in the N terminal region of AIB1. AIB1 has a clinical implications and association with the oncogene ErbB2, which is an estrogen repressed gene with hormone effect acting through an estrogen response element (ERE) in intron 1 of the ErbB2 gene. We wanted to determine if the AIB1/ANCO1 complex played a role in hormone regulation of ErbB2. Silencing of AIB1 and ANCO1 by shRNA led to a reversal of estrogen-induced repression of ErbB2 indicating this protein complex may play a role in mediating this repression. Examining binding of these proteins to the intronic ERE of ErbB2 we determined that AIB1 and ANCO1 are recruited in the presence of estrogen. Additionally, repressive proteins such as HDAC3, HDAC4 and N-CoR are recruited along with the AIB1/ANCO1 complex. While AIB1-Δ4 can still be recruited to the intronic ERE no additional repressive proteins are recruited. This suggests a mechanism by which the AIB1/ANCO1 complex can recruit repressive proteins to repress ErbB2 gene transcription, while AIB1-Δ4 can escape this repressive mechanism. This may potentially have interesting clinical implications, as high levels of AIB1-Δ4 may be responsible for increasing the expression levels of the ErbB2 oncogene.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P6-05-07.

Co-repressor activity of scaffold attachment factor B1 requires sumoylation

Sumoylation is an emerging modification associated with a variety of cellular processes including the regulation of transcriptional activities of nuclear receptors and their coregulators. As SUMO modifications are often associated with transcriptional repression, we examined if sumoylation was involved in modulation of the transcriptional repressive activity of scaffold attachment factor B1. Here we show that SAFB1 is modified by both the SUMO1 and SUMO2/3 family of proteins, on lysine's K231 and K294. Further, we demonstrate that SAFB1 can interact with PIAS1, a SUMO E3 ligase which mediates SAFB1 sumoylation. Additionally, SENP1 was identified as the enzyme desumoylating SAFB1. Mutation of the SAFB1 sumoylation sites lead to a loss of transcriptional repression, at least in part due to decreased interaction with HDAC3, a known transcriptional repressor and SAFB1 binding partner. In summary, the transcriptional repressor SAFB1 is modified by both SUMO1 and SUMO2/3, and this modification is necessary for its full repressive activity.

SAFB1's multiple functions in biological control-lots still to be done!

The examination of scaffold attachment factor B1 (SAFB1) and its multiple functions and tasks in cellular processes provides insight into its role in diseases, such as cancer. SAFB1 is a large multi-domain protein with well-described functions in transcriptional repression, and RNA splicing. It is ubiquitously expressed, and has been shown to be important in numerous cellular processes including cell growth, stress response, and apoptosis. SAFB1 is part of a protein family with at least two other family members, SAFB2 and the SAFB-like transcriptional modulator SLTM. The goal of this prospect article is to summarize known functions of SAFB1, and its roles in cellular processes, but also to speculate on less well described, novel attributes of SAFB1, such as a potential role in chromatin organization. This timely review shows aspects of SAFB1, which are proving to have a complexity far greater than was previously thought.

Sumoylation of corepressor scaffold attachment factor b1-mechanism of action and biological relevance in breast cancer

Abstract #3046

Scaffold attachment factor B1 (SAFB1) was originally isolated due to its ability to bind Scaffold/Matrix attachment regions of genomic DNA. Subsequently, SAFB1 has also been shown to be involved in RNA processing, chromatin organization and stress response. Work from our laboratory has shown that SAFB1 can bind and modulate transcription of the HSP-27 promoter and repress activity of ERα. Repression of ERα occurs through an independent and transferable c-terminal repression domain. Further work has indicated that SAFB1 can interact with and repress transactivation of a host of nuclear receptors suggesting a general role for SAFB1 in transcriptional repression. Many proteins associated with transcriptional regulation are post translationally modified by small ubiquitin like modifier (SUMO). SUMO is a 98 amino acid mature polypeptide that is added post translationally to target proteins usually within a consensus sumoylation motif, YKXE (where Y is a large hydrophobic residue, K-lysine of modification and X any residue). SUMO modification of proteins associated with transcription has generally been associated with transcriptional repression. The mechanism by which this modification leads to transcriptional repression has yet to be defined. Work by our laboratory has indicated that SAFB1 is modified by sumoylation. Immunoprecpitation studies indicated modification of SAFB1 by SUMO1. To better understand the potential role sumoylation plays in SAFB1's function we mutated both lysines within the two consensus sumoylation sites to arginine creating a SAFB1 K2R mutant. Mutation of these sumoylation sites lead to loss of interaction between SAFB1 and SUMO1 by co-immunoprecipitation. Mutation of the sumoylation sites also lead to loss of transcriptional repression. Transient transfection ERE-Tk-Luciferase assays showed a loss of repression with overexpression of SAFB1 K2R compared to SAFB1 WT. To examine the effect of SAFB1 sumoylation on its intrinsic repressive capability transient transfection Gal4-DBD assays were performed. Loss of transcriptional repression was seen with SAFB1 K2R compared to SAFB1 WT indicating an effect of sumoylation on SAFB1's intrinsic repressive function. The mechanism by which SAFB1 is sumoylated and the extent of the functional consequences of this sumoylation has yet to be determined. Ongoing studies within the lab are aimed at addressing this question. Studies are focused to the conjugating and deconjugating enzymes responsible for SAFB1 sumoylation. Other work is aimed at determining possible changes in protein-protein interaction or subcellular localization with sumoylation of SAFB1. In relation to the potential role of SAFB1 sumoylation in breast cancer, DNA from clinical breast cancer specimens is going to be sequenced to examine possible mutation present within the SAFB1 sumoylatyion sites that may be important in clinical breast cancer specimens.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 3046.