The Pivotal Role of Yin Yang 1 (YY1) Inhibition (and downstream Bcl-2/Bclxl) by Galiximab (anti-CD80 mAb) In the Reversal of Resistance of B-NHL Cells to Chemotherapy

Abstract 2887 Galiximab (anti-CD80 mAb) is a primatized mAb (human IgG1 constant region and Cynomogous macaque variable region) that binds CD80 on lymphoma cells. It has been shown in vitro that Galiximab inhibits tumor cell proliferation and mediates ADCC. Galiximab is currently in clinical trials for a variety of cancers. Our preliminary findings demonstrated that Galiximab treatment of B-NHL cell lines, like Raji, triggers the cells and inhibits the constitutively activated NF-κB pathway. We hypothesized that Galiximab-induced inhibition of NF-κB may result in the inhibition downstream of several anti-apoptotic gene products and sensitizes cells to drug-induced apoptosis. Raji cells were treated with Galiximab (20-100 μg/ml) for 18h and followed by treatment with the chemotherapeutic drug CDDP (5-10 μg/ml) for 24h and apoptosis was determined by flow for activation of caspase 3. The findings demonstrated that the cells treated with Galiximab were sensitized to CDDP-induced apoptosis. Analysis of the apoptotic pathway following treatment with Galiximab revealed the inhibition of anti-apoptotic gene products such as Bcl-2 and Bclxl. We have also found that Galiximab, like rituximab, inhibits the Fas and DR5 transcription repressor Yin Yang 1 (YY1) and the direct inhibition of YY1 resulted in tumor cell sensitization to both Fas-L and TRAIL. We examined whether inhibition of YY1 by Galiximab was also involved in the sensitization to CDDP apoptosis. Raji cells were treated with YY1 siRNA and, unlike control siRNA or non-treated siRNA cells, the tumor cells were sensitized to CDDP apoptosis. The inhibition of YY1 by siRNA correlated with the inhibition of Bcl-2 and Bclxl. The direct role of Bcl-2 and Bclxl in the regulation of resistance was corroborated by treatment of cells with the Bcl-2 family inhibitor, 2MMA3, and such cells mimicked Galiximab and were sensitive to CDDP-induced apoptosis. The mechanism by which treatment with YY1 siRNA resulted in the inhibition of Bcl-2 and Bclxl and the reversal of resistance is not clear. We suggest that YY1 inhibition, following Galiximab-induced inhibition of NF-κB, will result in the inhibition of Snail transcription (Palmer, MB et al., Mol cancer Res 7:221, 2009). Inhibition of the RKIP (Raf kinase inhibitor protein) repressor Snail will result in the induction of RKIP (Wu, K and Bonavida, B Crit Rev immu 29:241, 2009) and, in turn, RKIP will inhibit NF-κB and resulting downstream in the inhibition of Bcl-2 and Bclxl. In addition, it has been reported that YY1 negatively regulates p53 (Sui, G et al., Cell 117:889, 2004) and YY1 inhibition by Galiximab will upregulate p53 and which will result in the inhibition of Bcl-2 and Bclxl (see scheme below). The present findings demonstrate that Galiximab sensitizes drug-resistant B-NHL cells to drug-induced apoptosis via modulation of the NF-κB/YY1/Snail/RKIP/p53 loop. Current studies are validating the present findings with freshly-derived B-NHL cells and also examining the molecular mechanism by which YY1 regulates Bcl-2/Bclxl expression and the reversal of resistance. Disclosures: No relevant conflicts of interest to declare.