SIRT3 is a genomically expressed, mitochondrial localized tumor suppressor protein where it directs multiple metabolic processes by deacetylating downstream protein substrates. Genetic deletion of Sirt3 in mice leads to the spontaneous development of mammary tumors starting at one year and decreased SIRT3 mRNA has been observed in several human tumors including breast malignancies. In this investigation we assessed SIRT3 expression in human breast cancer tissue microarray and examined the relationship between SIRT3 expression and outcome in breast cancer patients. SIRT3 protein expression is significantly lower in neoplastic compared to normal breast epithelium, including normal epithelium adjacent to tumors. Breast cancer patients in the lowest quartile of SIRT3 expression had a significantly shorter locoregional relapse free survival [Hazard ratio = 0.53 (0.47–0.61), log rank p=0]. Notably, low SIRT3 expression was associated with reduced locoregional relapse-free survival in all breast cancer subtypes analyzed, including ER+, ER−, HER2+, and basal subtypes (Hazard ratios =0.44 to 0.65; log rank p=0 to 0.0019). These results highlight the importance of the SIRT3 as a tumor suppressor protein in breast cancer and suggest that SIRT3 may be a potential molecular biomarker to identify high risk patients across all molecular subtypes of breast cancer.

Cooperativity between oncogenic mutations is recognized as a fundamental feature of malignant transformation, and it may be mediated by synergistic regulation of the expression of pro-and antitumorigenic target genes. However, the mechanisms by which oncogenes and tumor suppressors coregulate downstream targets and pathways remain largely unknown. Here, we used ChIP coupled to massively parallel sequencing (ChIP-seq) and gene expression profiling in mouse prostates to identify direct targets of the tumor suppressor Nkx3.1. Further analysis indicated that a substantial fraction of Nkx3.1 target genes are also direct targets of the oncoprotein Myc. We also showed that Nkx3.1 and Myc bound to and crossregulated shared target genes in mouse and human prostate epithelial cells and that Nkx3.1 could oppose the transcriptional activity of Myc. Furthermore, loss of Nkx3.1 cooperated with concurrent overexpression of Myc to promote prostate cancer in transgenic mice. In human prostate cancer patients, dysregulation of shared NKX3.1/MYC target genes was associated with disease relapse. Our results indicate that NKX3.1 and MYC coregulate prostate tumorigenesis by converging on, and crossregulating, a common set of target genes. We propose that coregulation of target gene expression by oncogenic/tumor suppressor transcription factors may represent a general mechanism underlying the cooperativity of oncogenic mutations during tumorigenesis.

Human tumors are heterogeneous and evolve through a dynamic process of genetic mutation and selection. During this process, the effects of a specific mutation on the incipient cancer cell may dictate the nature of subsequent mutations that can be tolerated or selected for, affecting the rate at which subsequent mutations occur. Here we have used a new mouse model of prostate cancer that recapitulates several salient features of the human disease to examine the relative rates in which the remaining wild type alleles of Pten and p53 tumor suppressor genes are lost. In this model, focal overexpression of c-MYC in a few prostate luminal epithelial cells provokes a mild proliferative response. In the context of compound Pten/p53 heterozygosity, c-MYC-initiated cells progress to prostatic intraepithelial neoplasia (mPIN) and adenocarcinoma lesions with marked heterogeneity within the same prostate glands. Using Laser Capture Microdissection and gene copy number analyses, we found that the frequency of Pten loss was significantly higher than that of p53 loss in mPIN but not invasive carcinoma lesions. c-MYC overexpression, unlike Pten loss, did not activate the p53 pathway in transgenic mouse prostate cells, explaining the lack of selective pressure to lose p53 in the c-MYC-overexpressing cells. This model of heterogeneous prostate cancer based on alterations in genes relevant to the human disease may be useful for understanding pathogenesis of the disease and testing new therapeutic agents.

This study examined the effects of 4 weeks of binge ethanol administration (BEAn) on the behavioral outcome in rats after lateral fluid percussion (FP) brain injury. Rats were intragastrically given 7.5 mL/kg of either 40% ethanol in 5% glucose solution (3 g ethanol/kg; binge ethanol group), or 5% glucose solution (vehicle group), twice on Thursday and Friday of 3 consecutive weeks. Then rats from both groups were subjected to either lateral FP brain injury of moderate severity (1.8 atm) or to sham operation. Postinjury behavioral measurements revealed that brain injury caused significant spatial learning disability in both groups. There were no significant differences in mean search latencies in the sham animals between the vehicle and binge ethanol groups. On the other hand, the mean search latency of the binge ethanol group was significantly higher than that of the vehicle group in trial blocks 2 and 4. There were no significant differences in the target visits (expressed as mean zone difference [MZD]) during the probe trial between the injured animals of binge ethanol and vehicle groups. However, there was only a minor trend towards worsened MZD score in the binge-injured animals. Histologic analysis of injured animals from both injured ethanol and vehicle groups revealed similar extents of ipsilateral cortical and observable hippocampal damage. These results suggest that 4 weeks of binge ethanol treatment followed by ethanol intoxication at the time of injury worsens some aspects of the spatial learning ability of rats. This worsening is probably caused by subtle, undetectable morphologic damage by binge ethanol administration.

Cooperativity between oncogenic mutations is recognized as a fundamental feature of malignant transformation, and may be mediated by synergistic regulation of the expression of pro- and anti-tumorigenic target genes. However, the mechanisms by which oncogenic mutations cooperatively regulate downstream target gene expression remain largely unknown. Here we used chromatin immunoprecipitation coupled with massively parallel sequencing (ChIP-seq) and gene expression analyses to identify direct targets of the prostate tumor suppressor Nkx3.1 in the mouse prostate. Our analysis showed that a significant fraction of Nkx3.1 targets are also targets of the oncoprotein c-Myc, suggesting that these two proteins may cooperate in prostate tumorigenesis by directly co-regulating a common set of target genes. Using ChIP, we confirmed binding of Nkx3.1 and c-Myc to the regulatory regions of shared target genes in mouse and human prostate epithelial cells. In conditional mutant mice, loss of Nkx3.1 cooperates with concurrent overexpression of c-MYC to promote prostate tumorigenesis. Furthermore, dysregulation of the shared NKX3.1/c-MYC target gene set was associated with relapse in human prostate tumors. We propose that co-regulation of target gene expression by oncogenic/tumor suppressor transcription factors may represent a general mechanism for the cooperativity of oncogenic mutations during tumorigenesis. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2983. doi:1538-7445.AM2012-2983