Abstract 3613: P300 inhibition enhances cytotoxic effect of Gemcitabine through E2F1 activation in pancreatic cancer

Abstract

Abstract Background: Transcriptional cofactor, P300 has been reported to regulate cell cycle on G1/S transition. Previously, we shown that prolonged S-phase is associated with increased apoptosis induced by Gemcitabine (GEM) in pancreatic cancer cells. Thus, we hypothesized that targeting P300 enhances the cytotoxic effect of GEM on pancreatic cancer. Methods: We studied 2 human pancreatic cancer cell lines, Panc1 and MIAPaCa2. P300 was gene-silenced using siRNA, and P300 histone acetyltransferase (HAT) activity was inhibited by specific inhibitor C646. Cell viability was measured by WST-8 assay and GEM-induced apoptosis was measured using Caspase3 and PARP by Western blotting, Tunel assay by immunofluorescence staining, and Annexin V-PI staining by flow cytometry. CDK4/6 inhibitor Palbociclib was used to block S-phase entry. Results: Robust P300 expression was observed in pancreatic cancer cell lines and human pancreatic adenocarcinoma tissues. When P300 expression was depleted by specific P300 siRNA, cell viability after GEM treatment was decreased significantly compared with the control pancreatic cancer cells (43% decrease in MIAPaCa2 and 39% decrease in Panc1, respectively, p<0.05). Increased GEM-induced apoptosis was confirmed by increased cleaved forms of Caspase3 and PARP, increased tunel positive cells, and increased Annexin positive cells in P300-depleted cells compared to the control cells. When S phase entry was blocked by Palbociclib, the effect of gene silencing of P300 on GEM-induced apoptosis was abolished. As Rb-E2F1 complex is known as the key regulator of the S phase specific gene transcriptions, we evaluated the effect of P300 gene silencing on Rb and E2F1. While P300 gene-silencing did not affect Rb phosphorylation after GEM treatment, it increased E2F1 expression significantly (56% increase in MIAPaCa2 and 119% increase in Panc1, respectively, p<0.05). When pancreatic cancer cells were treated with combination of GEM and C646, P300 specific HAT inhibitor, the cell viability was reduced significantly compared with when treated with GEM alone (48% decrease for Panc1, and 58% for MIAPaCa2, respectively, p<0.05). Conclusion: Targeting P300 by siRNA or HAT inhibitor enhances the efficacy of GEM on pancreatic cancer cells. Our observation suggests the presence of Rb-independent E2F1 regulatory mechanism in pancreatic cancer cells. This pathway may be an important therapeutic target to potentiate GEM chemotherapy in the treatment of pancreatic cancer. Citation Format: Hiroaki Ono, Marc D. Basson, Hiromichi Ito. P300 inhibition enhances cytotoxic effect of Gemcitabine through E2F1 activation in pancreatic cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3613. doi:10.1158/1538-7445.AM2015-3613