Abstract 2913: STAT3 inhibition overcomes CHOP-resistance in the activated B-cell subtype of DLBCL by a ROS-dependent mechanism.

Diffuse large B cell lymphoma (DLBCL) is the most common lymphoid malignancy in the adult population and can be subdivided into two main subtypes, e.g. GCB-DLBCL and ABC-DLBCL. Both subtypes are derived from normal germinal center (GC) B cells found in secondary lymphoid organs but differ in their B cell maturation stage, transformation pathway, and clinical behavior. When treated with either the combination chemotherapy CHOP or the immuno-chemotherapy R-CHOP, ABC-DLBCL patients typically have a significantly worse survival outcome compared to those carrying GCB-DLBCLs. The underlying mechanism for this disparity is unclear. We have previously reported that expression of the STAT3 gene is developmentally regulated in GC B cells; in addition, high level and constitutively activated STAT3 is prevalent in ABC-DLBCL but not in GCB-DLBCL. Although STAT3 activation in epithelial cancers is often associated with increased chemo-resistance, it is unclear if and how activated STAT3 contributes to therapy resistance in ABC-DLBCL. Since the principle cytotoxic component of CHOP is Doxorubicin (Dox), often described as a Topo II inhibitor, we began our investigation by examining DNA damage response (DDR) following Dox treatment in GCB-DLBCL and ABC-DLBCL cell lines. When applied at IC50 concentrations, Dox elicited a progressive and robust DDR in GCB-DLBCL cell lines starting from 1 h after treatment. In ABC-DLBCL cells, however, activation of several DDR markers was delayed until 24 h post treatment. Instead, Dox increased the levels of reactive oxygen species (ROS) in ABC-DLBCL cells with the most prominent increase in mitochondria superoxide as measured by MitoSox staining. The importance of ROS in Dox-triggered cytotoxicity in ABC-DLBCL cells is supported by a direct correlation between the dose response to Dox and H2O2 among ABC-DLBCL cell lines and by the ability of N-acetyl cysteine (NAC) to significantly impede Dox-triggered cell death. Interestingly, STAT3 inhibition by either siRNA-mediated gene silencing or the small molecule inhibitor, CPA-7, led to significant increase in MitoSox staining. STAT3 siRNA also enhanced Dox-triggered superoxide production and cell death. These findings implicate a ROS-detoxification role of STAT3 in ABC-DLBCLs which could be explained, at least in part, by its ability to promote the expression of the superoxide dismutase 2 (SOD2) gene. Finally, our results demonstrate that the STAT3 inhibitor, CPA-7, can markedly sensitize ABC-DLBCL cells to killing by Dox both in cell culture and in a xenograft model of ABC-DLBCL. Our study not only uncovers a novel mechanism of action for Dox in ABC-DLBCL, it also implicates STAT3 in cellular redox control and provides the proof-of-concept data for development of STAT3-directed lymphoma therapy for ABC-DLBCLs. Citation Format: J. Jessica Yu, Yun Mai, Enguang Bi, Hongshan Chen, B. Belinda Ding, Bhaskar Das, Samir Parekh, Yiyu Zou, B. Hilda Ye. STAT3 inhibition overcomes CHOP-resistance in the activated B-cell subtype of DLBCL by a ROS-dependent mechanism. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2913. doi:10.1158/1538-7445.AM2013-2913