Abstract 510: Inhibition of IRE1α-driven pro-survival pathways is a promising therapeutic application in acute myeloid leukemia

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA Survival of cancer cells rely on the unfolded protein response (UPR) to resist stress triggered by the accumulation of misfolded proteins within the endoplasmic reticulum (ER). IRE1α-XBP1 pathway, one of the most important branches of the UPR, is activated in different types of cancer. Upon IRE1α activation, the spliced form of XBP1 (XBP1s) is generated and acts as a transcription factor for many pro-survival genes to prevent cellular death and relieve cellular stress. Blockade of the IRE1α-XBP1 pathway can be considered as a potential anti-cancer strategy. Acute myeloid leukemia (AML) is the most common acute leukemia in adults. We find that XBP1 and XBP1s are up-regulated in AML cell lines and samples from patients. IRE1α RNase inhibitors including STF-083010, HNA and MKC-204 blocked XBP1 mRNA splicing and exhibited modest cytotoxicity in liquid culture against AML cell lines (HNA, mean IC50, 31 µM, n=8) and AML samples from patients (HNA, mean IC50, 35 µM, n=18). Notably, the IRE1α inhibitor HNA caused a significant inhibition (mean IC50, 6 µM, n=6) of clonogenic growth in soft agar of AML cells from patients of different leukemic subtypes. In contrast, HNA had very little toxicity against normal human marrow committed myeloid colony forming cells (mean IC50, 123 µM, n=4). IRE1α inhibition in AML cells induced caspase-dependent apoptosis and G1 cell cycle arrest which was associated with regulation of Bcl-2 family proteins, G1 phase controlling proteins (p21cip1, p27kip1 and Cyclin D1) and Chaperone proteins (CHOP, HERPUD1, DNAJC3, DNAJB9 and EMDM). Absence of Xbp1 (Cre-induced deletion of floxed XBP1) resulted in the myeloid cells becoming resistant to IRE1α inhibitors. Combination of HNA with either bortezomib or As2O3 synergistically inhibited growth of NB4 acute promyelocytic leukemia cells associated with enhanced levels of p-JNK and reduced expression of p-PI3K and p-MAPK. Also, in a dose- and time- dependent manner, inhibition of IRE1α RNase activity increased expression of many miRNAs (especially miR-17, -21, -34a, -96, -125b and -150) in AML cells. Furthermore, HNA treatment inhibited mRNA levels of several miR-34a targeted genes (c-Myc, Cyclin D1 and CDK4). The inhibition of miR-34a conferred cellular resistance to HNA, as measured by significant increase of cell viability. Addition of a miR-34a antagonist restored protein expression levels of c-Myc and Cyclin D1 that had been inhibited by HNA, suggesting a crucial role of miR-34a in the IRE1α-mediated stress response pathway. Taken together, our results strongly suggest that targeting the IRE1α driven pro-survival pathways is an exciting therapeutic approach for the treatment of AML. Note: This abstract was not presented at the meeting. Citation Format: Haibo Sun, Behzad Kharabi Masouleh, Sigal Gery, Qi Cao, Serhan Alkan, Takayuki Ikezoe, Chie Akiba, Ronald Paquette, Wenwen Chien, Carsten Muller-Tidow, Yang Jing, Kharabi Masouleh, Markus Muschen, H. Phillip Koeffler. Inhibition of IRE1α-driven pro-survival pathways is a promising therapeutic application in acute myeloid leukemia. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 510. doi:10.1158/1538-7445.AM2014-510