Abstract P2-06-04: p300 acts as a tumor suppressor by inhibiting pro-metastatic microRNA pathways

Abstract

Abstract The histone acetyltransferase p300 is an important epigenetic factor known to regulate gene silencing, DNA repair, and cell cycle. Altered p300 expression is linked to cancerous phenotypes and has potential as both a biomarker and therapeutic target. Existing anti-cancer therapies such as histone deacetylase inhibitors (vorinostat, valproic acid, trichostatin A) are known to affect p300 levels and activity. Understanding how p300 affects cancerous and metastatic development is crucial to optimizing and advancing these treatments. However, whether p300 acts as a tumor suppressor or promoter is still controversial. To address this issue, we stably knocked down p300 levels in MCF-7, BT 474 and MDA MB 231 to 30-40% endogenous levels to directly examine p300's effects on tumorigenicity. Our experiments show loss of p300 significantly increased cell motility in the scratch assay (p<0.05) and increased motility and invasion in a cell chamber migration chamber by 50% (p<0.001). Xenograft tumors formed from KD-p300 shRNA MCF-7 cells were on average about three times larger at six weeks post implantation than that of non-silencing control NS-MCF-7 tumors. KD-p300 MCF-7 xenograft tumors demonstrate a more aggressive phenotype with increased vascularity. To identify targets of p300 that might regulate its anti-tumor effect, we observed that miR-let-7c was downregulated in KD-p300 MCF-7 by 70% (p<0.05). Consistent with this, the miR-let-7c target N-Ras was up regulated in KD-p300 MCF-7 cell lines and its xenograft tumors by three fold (p<0.05). Knockdown of p300 of MCF-7 cells also displayed downregulated miR-141 and miR-200a and upregulated their known target, ZEB1, by 40% (p<0.05). Gene expression profile assay showed that loss of p300 resulted in up regulation of ZEB1, laminin, collagen, IL-6, IFN, TGF-b, and components of the Wnt signaling pathway, which indicated a shift from epithelial to mesenchymal phenotype. Our data showed that loss of p300 was sufficient to induce a more invasive and potentially metastatic or EMT phenotype in breast cancer cells. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P2-06-04.