Abstract C45: Inhibition of antiapoptotic BCL2 members facilitates interferon regulatory factor-1 (IRF1) mediated prodeath signaling in breast cancer cells

Abstract

Abstract Antiestrogens (AEs) have long been a standard treatment for estrogen receptor-positive (ER+) breast cancer patients. However, antiestrogen resistance, either de novo or acquired, is a major hurdle for effective therapy in breast cancer patients. To address this, the underlying mechanisms of antiestrogen resistance must be understood. Antiestrogens are known to induce apoptosis in sensitive breast cancer cells. The tumor suppressor gene interferon regulatory factor-1 (IRF1) plays a key role in signaling to proapoptotic BH3 only BCL2 (B-cell lymphoma 2) family members (i.e., BIK, PUMA, and BMF). In an attempt to understand IRF1-mediated signaling in AE sensitivity, AE-sensitive and -resistant breast cancer cells were made to overexpress IRF1 or a dominant negative IRF1 (dnIRF1; lacks the transactivation domain of IRF1 but retains the DNA-binding domain). We found that IRF1 regulates the transcription of proapoptotic BIK, BMF, and PUMA, which are necessary to induce apoptosis in AE-sensitive cells. Conversely, AE-resistant breast cancer cells have low IRF1 expression and increased levels of antiapoptotic BCL2 and BCLW. Previous work by our group showed that the inflammatory cytokine IFNγ can be used to induce IRF1 expression in AE-resistant cells, and restores the responsiveness of these cells to fulvestrant (Faslodex; ICI 182,780) through BH3-mediated apoptosis, suggesting a key role of IRF1 in endocrine-resistant breast cancer. We also discovered that the pan BCL2 inhibitor GX15-070 (obatoclax) significantly reduces cell proliferation and, in combination with fulvestrant or tamoxifen, results in a synergistic inhibition of cell growth in AE-resistant breast cancer cells. Treatment with obatoclax in AE-resistant cells results in a reduction in Beclin1 (BECN1) protein levels with a corresponding increase in cleaved caspase-7, indicating a possible role of autophagy and apoptosis in GX15-070-induced cell death. Moreover, overexpression of BECN1 in AE-resistant cells enhances obatoclax-mediated cell death. Collectively, our data demonstrate that a combination of obatoclax and an antiestrogen increases the levels of free BH3 domain proteins, thus facilitating mitochondrial dysfunction, and rendering breast cancer cells more susceptible to tumor cell death by IRF1-mediated signaling. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the Second AACR International Conference on Frontiers in Basic Cancer Research; 2011 Sep 14-18; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2011;71(18 Suppl):Abstract nr C45.