Abstract 1637: STAT3 as a new molecular target for treating ABC-like DLBCL

Persistent STAT3 signaling contributes to malignant progression in many diverse human tumors. STAT3 has been found to be constitutively active in activated B cell (ABC)-like diffuse large B cell lymphomas (DLBCL). We investigated the effect of STAT3 silencing in human DLBCL Ly3 cells both in vitro and in vivo. For this purpose, we established stably transduced STAT3 shRNA-expressing lentivirus Ly3 cells and control lentivirus Ly3 cells. The stable expression of STAT3 shRNA resulted in 40-50% reduction of STAT3 protein levels compared to the control cells. In vitro experiments revealed that STAT3 downregulation was associated with a higher percentage of dead cells in STAT3 shRNA cells compared with the control cells, and this percentage was even higher in serum starvation conditions. STAT3 silencing resulted in inhibition of IL-10-inducible upregulation of c-Myc associated with downregulation of IL-10-dependent STAT3 activation and inhibition of IL-10-inducible cell growth. STAT3 inhibition also reduced adhesion to the bone marrow stroma layer and migration toward SDF-1 alpha. Moreover, analysis of inflammatory cytokines and receptors indicated a substantial downregulation of CXCL6 mRNA levels in STAT3 shRNA cells compared to the control cells. Tumors in control Ly3-bearing mice grew progressively, whereas tumors in STAT3 shRNA Ly3-bearing mice regressed 4-5 days after injection. This tumor regression was associated with Caspase-3-dependent apoptosis and significant reduction of STAT3 target genes at the protein level such as Mcl-1, c-Myc and Survivin. Moreover, immunofluorescence analysis of tumor specimens showed that while granulocytes and endothelial cells infiltrated into the control cells-bearing tumors, tumor bearing STAT3 shRNA cells showed less or no infiltration of these cells. We also observed reduction of IL-10 production and the secretion of other cytokines was altered as well. Thus, our results show that even partial down-regulation of STAT3 could achieve complete suppression of the tumorigenesis of ABC-like DLBCL Ly3 cells by affecting also the tumor microenvironment. Furthermore, we tested two STAT3 inhibitors, STATTIC and S31-201, in this system. The viability of Ly3 cells grown in vitro was reduced by both compounds tested in a dose-dependent manner and the antiproliferative effect was associated with inhibition of STAT3 phosphorylation. The antilymphoma activity of STATTIC was also observed in vivo. This is the first demonstration in vivo that direct STAT3 inhibition in ABC-like DLBCL suppresses tumor growth. While developing a small-molecule STAT3 inhibitor as well as siRNA delivery for clinical trials is still a challenge, our studies validate STAT3 as a good target for therapy in DLBCL and also establish that STAT3 siRNA-based gene therapy is a feasible approach for DLBCL. In Sum, our data support continued development of new STAT3 inhibitors as well as STAT3 siRNA delivery strategies for treatment of DLBCL. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1637. doi:10.1158/1538-7445.AM2011-1637