Abstract 799: Diminishing Mcl-1 protein leads to apoptosis of acute myeloid leukemia cells responding to all trans retinoic acid differentiation
Authors: Wang Rui , Xia Lijuan , Gabrilove Janice , Waxman Samuel , Jing Yongkui 
Published: May 2015

Abstract
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