Novel therapeutic strategies are urgently needed to treat patients with B-NHL that are unresponsive to conventional therapies. TRAIL, a member of the TNF-\#945; family, has been shown to induce apoptosis in transformed cells with minimal toxicity to normal tissues; however, the majority of tumor cell lines and freshly derived tumor cells are resistant to TRAIL-induced apoptosis. Studies by us and others have reported that inhibition of overexpressed anti-apoptotic gene products, such as Bcl-2 and Bcl-XL, can reverse resistance to TRAIL apoptosis. The BH3 mimetic Obatoclax (GX15-070, Gemin X Pharmaceuticals, Malvern, PA) inhibits diverse anti-apoptotic molecules like Bcl-2, Bcl-XL and Mcl-1 through direct binding and can reverse tumor cells resistance to chemotherapeutic drugs. We hypothesized that treatment of B-NHL cell lines with Obatoclax will result in tumor cell sensitization to TRAIL apoptosis. Using the B-NHL Ramos cell line as model, cells were treated with different concentrations of Obatoclax (7-28 nM) and TRAIL (2.5-20 ng/ml, 24 h). Significant potentiating of apoptosis and synergy were achieved by the combination treatment. There was also depolarization of the mitochondrial membrane potential by the combination treatment observed 4 hours after treatment. We then examined the potential underlying mechanism of sensitization by Obatoclax. It has been reported that Mcl-1, an anti-apoptotic protein, confers resistance to TRAIL-mediated apoptosis due to the interaction between Mcl-1 and the truncated Bid protein (Tanai et. al., Can Res 64: 3517, 2004; Clohessy JG, JBC: 5750, 2006). We show here a disruption of the Mcl-1 binding to Bak and to Bid by immunoprecipitation. Further, we also show inhibition of the NF-\#954;B activity and downstream anti-apoptotic gene products regulated by NF-\#954;B (Bcl-XL, Mcl-1 and XIAP) as assessed by western. NF-\#954;B regulates the transcription of the DR5 transcription repressor Yin Yang 1 (YY1). Treatment of Ramos with Obatoclax significantly inhibited YY1 expression and concurrently, upregulated total and surface DR5 expression. In addition to its direct role in inhibiting the Bcl-2 family members, Obatoclax induces gene modification by a different mechanism than that reported by Song et. al. for the ABT-737 inhibitor (JBC, 283:25003, 2008). Present findings demonstrate that Obatoclax sensitizes TRAIL resistant B-NHL cells to TRAIL apoptosis. In addition to its known effect on inhibiting physically Bcl-2 family members, it also inhibits NF-\#954;B activity and downstream gene products that regulate both chemo-resistance and immune-resistance.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 3276.

100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO

American Association for Cancer Research
2009