Abstract C173: APE1/Ref-1-STAT3 dual-targeting synergize to effectively inhibit pancreatic cancer cell survival.

Pancreatic cancer remains a largely incurable disease, with patients facing the worst 5-year survival rate of any cancer. The challenge is to identify the molecular effectors that critically regulate the survival of pancreatic ductal adenocarcinoma (PDAC) cells, to devise effective molecular-targeted strategies that can prevent or minimize the selection of resistant tumor variants, and overcome the protective role of the tumor-associated fibrosis and stroma. Increasing evidence supports the need for strategies targeting multiple molecular effectors in PDAC. Thus, a strategy is to identify critical molecules that regulate multiple signaling mediators (as transcription factors), and on intracellular mechanisms with direct effects on multiple pathways (as antioxidant and redox mediators) critical for PDAC functions. APE1/Ref-1 is a dual function protein, which in addition to DNA repair activity also exerts redox control of transcription factors, including NF- B, p53, AP-1, CREB, HIF-1 and others. Treatment with E3330, a unique small molecule redox signaling inhibitor that recognizes an alternate, redox active conformation of APE1/Ref-1, markedly inhibits the DNA binding and transcriptional activity of NF-κB, AP-1, and HIF-1. Our previous work established APE1/Ref-1 as a potential molecular target in pancreatic cancer. We demonstrated that human adenocarcinoma and peri-pancreatic metastases exhibit increased APE1/Ref-1 expression, and blocking APE1/Ref-1 redox activity delays tumor progression in xenograft models of PDAC, including patient-derived tumor cells. STAT3 is a transcription factor that regulates critical cell functions and has been shown to play important roles in several cancers. STAT3 signaling has been implicated in pancreatic cancer biology, namely by mediating or regulating cell survival, proliferation, tumor angiogenesis and metastasis. Although STAT3 signaling can be engaged and modulated by different processes, the mechanisms regulating STAT3 transcriptional activity in PDAC cells are largely unknown, namely the impact of oxidative stress and its redox status. A recent report demonstrated that STAT3 activity is under redox control and identified the critical oxidation-sensitive cysteines in the STAT3 DNA binding domain. However, the modifier of STAT3 which converts it from an oxidized to a reduced form was not identified. It has been shown that APE1/Ref-1 physically interacts with STAT3 on the VEGF promoter and enhances IL-6-induced DNA binding activity of STAT3 in HepG2 cells. However, it is unknown whether APE-1/Ref-1 is involved in the redox control of STAT3 activity, and whether the cellular redox status affects STAT3 signaling in PDAC cells. Here, we demonstrate that APE1/Ref-1 redox activity regulates the STAT3 DNA binding and transcriptional activity using gene silencing, overexpression of WT or redox-defective APE1/Ref-1, and redox-selective pharmacological inhibition. Blockade of APE1/Ref-1 redox synergizes with STAT3 selective antagonists to markedly inhibit the proliferation and survival of human PDAC cells, inducing cell apoptosis. These studies identify the mechanism by which APE1/Ref-1 regulates STAT3 signaling, and establishes the rationale for the development of APE1/STAT3 dual-targeting strategies for the treatment of pancreatic cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C173.