Abstract 799: Diminishing Mcl-1 protein leads to apoptosis of acute myeloid leukemia cells responding to all trans retinoic acid differentiation

All trans retinoic acid (ATRA) is successfully used for the treatment of acute promyelocytic leukemia (APL) through inducing terminal differentiation and death receptor-mediated apoptosis. However, ATRA has limited clinic efficacy in non-APL acute myeloid leukemia (AML) patients. To extend ATRA therapy to AML (non-APL), cell death and differentiation induction were tested and compared in a panel of AML cell lines after treatment with ATRA for 2, 4 and 6 days based on the staining of CD11b and annexin V, respectively. ATRA treatment in APL NB4 cells induces differentiation but not apoptosis at 2 days. ATRA differentiation peaks at 4 days and apoptosis becomes measurable at 4 days and peaks at 6 days. Western blot analysis revealed that both TRAIL and DR5 were induced by ATRA at 4 days, supporting that a death-receptor-mediated apoptosis program is initiated by ATRA. The mitochondrial apoptotic program was determined by measuring the levels of Bcl-2, Mcl-1 and the cleaved caspase-9. Although Bcl-2 protein was depleted, Mcl-1 protein was significantly increased after treatment with ATRA at 2 days. As the treatment continued to 4 and 6 days, the Mcl-1 protein levels declined and cleaved caspase 9 was detected. Similar phenomena were observed in non-APL HL-60 cells responding to ATRA-induced differentiation, but not in ATRA differentiation resistant NB4 and HL-60 subclones. Silencing of Mcl-1 with siRNA sensitized ATRA-induced apoptosis with decreased differentiation in NB4 cells treated with ATRA for 2 days. HL-60 cells with overexpression of Mcl-1 increased ATRA-induced differentiation with diminished apoptosis after treatment with ATRA for 6 days. Mechanistic studies of Mcl-1 induction by ATRA revealed that both Akt/mTOR and MEK/ERK signaling were activated associated with increased phosphorylation of glycogen synthase kinase 3β(GSK3β) and ribosomal S6 protein. Since inactivation of GSK3β by phosphorylation stabilizes Mcl-1 protein and S6 phosphorylation increases Mcl-1 protein translation, ATRA increases Mcl-1 levels through increasing translation and stability. Akt inhibitor Ly294002 and MEK inhibitor U0126, but not mTOR inhibitor rapamycin, blocked Mcl-1 induction and converted ATRA into an apoptosis inducer in HL-60 cells. More intriguingly sorafenib, a multi-kinase inhibitor, antagonized ATRA induction of Mcl-1, p-GSK3β and p-S6, but not Bcl-2 reduction, and synergistically induced apoptosis with ATRA in a panel of non-APL cell lines and primary AML cells. Our data suggest that Mcl-1 protein levels contribute to balance between cell death and differentiation of AML cells treated with ATRA and that ATRA therapy could be extended to non-APL cells by augmented cell death induction by diminishing Mcl-1 protein. Note: This abstract was not presented at the meeting. Citation Format: Rui Wang, Lijuan Xia, Janice Gabrilove, Samuel Waxman, Yongkui Jing. Diminishing Mcl-1 protein leads to apoptosis of acute myeloid leukemia cells responding to all trans retinoic acid differentiation. [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 799. doi:10.1158/1538-7445.AM2014-799