Abstract 4882: Loss of p300 reduces miR-let-7c and enhances ER-positive breast cancer cell tumorigenicity

Abstract

Abstract Histone deacetylase (HDAC) inhibitors have been extensively studied as potential anticancer treatments and several are currently in clinical trials. p300 is a nuclear histone acetyltransferase that is an important coordinator of multiple transcription-dependent cellular processes. HDAC inhibitors may directly and indirectly increase the activity of p300. The role of p300 is dependent on cell context but most or all of its biological activity stems from its ability to acetylate histones or non-histone nucleoproteins including p53, HSP) which regulate cell proliferation, differentiation and apoptosis. Whether p300 acts as a tumor suppressor or promoter is controversial, and its role in estrogen positive breast cancer remains to be defined, however, structural studies indicate that it has significant potential as a target for small molecule therapy. To address this issue, we have used a lentiviral approach to achieve 60-70% stable knockdown (KD) of p300 levels in estrogen receptor-positive breast cancer lines MCF-7 and BT 474 respectively. Growth curve and FACS analysis showed that loss of p300 shortened doubling times and increased G2 phase cells in both lines. Xenograft tumors formed from p300 KD cells were on average about three times larger at six weeks post implantation than control tumors (p<0.05). H&E and CD31 staining showed that p300 KD xenograft tumors have a more aggressive phenotype with increased vascularity. In searching for targets of p300 that might regulate its anti-tumor effect, we observed that the microRNA miR-let-7c was downregulated in p300-depleted MCF-7 and BT474 cells. Consistent with this, the miR-let-7c target N-Ras was up-regulated in both p300 KD cell lines and in p300 KD xenograft tumors, and adenovirus-mediated over-expression of p300 caused downregulation of N-RAS. Finally, gene expression analysis showed that loss of p300 resulted in upregulation of E-cadherin, laminin, collagen, IL-6, IFN, TGF-β and components of the Wnt signaling pathway, indicating a shift from epithelial to mesenchymal phenotype. Our data show that p300 loss or inactivation results in altered transcription patterns and suppression of anti-tumor microRNAs, including let-7, and may induce a more invasive and potentially metastatic phenotype. We conclude that p300 likely acts as a tumor suppressor in breast cancer and that activation of p300 may contribute to the anti-tumor effects of TSA. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4882.