Targeting Apoptotic Pathways To Overcome Drug Resistance In Acute Myeloid Leukemia

Abstract

Evasion of apoptosis is integral to tumorigenesis and drug resistance. BCL-2 and p53 proteins represent two focal nodes in convergent apoptosis signaling. Upregulation of anti-apoptotic BCL-2 family members and inactivation of p53 functions are two canonical approaches exploited by cancer cells to escape apoptosis. In the current study, we find that BCL-2 protein is highly expressed in acute myeloid leukemia (AML) cells. BCL-2–specific inhibitor ABT-199 potently induces mitochondrial apoptosis in AML cells and effectively kills AML stem/progenitor cells. Our biomarker studies demonstrate that both BH3 profiling and the expression profiling of BCL-2 proteins may serve as predictive biomarkers for the efficacy of ABT-199. Despite the initial success, we find that MCL-1 protein renders cancer cells resistant to ABT-199. Next we investigated whether p53 activation could overcome MCL-1–mediated resistance to BCL-2 inhibitor ABT-199. We demonstrate that p53 activation by MDM2 inhibitor RG7388 effectively overcomes the acquired or inherent resistance to ABT-199 both in vitro and in vivo. Mechanistic studies indicate that ABT-199 induces stabilizing phosphorylation of MCL-1. Surprisingly, p53 activation mediates the conversion from stabilized MCL-1 to destabilized MCL-1. Further mechanistic studies demonstrate that p53 activation inhibits N-Ras/Raf/MEK/ERK signaling and releases GSK3 activity to modulate MCL-1 phosphorylation and promote Mcl-1 degradation. Additionally, p53 activation also induces pro-apoptotic proteins such as PUMA and BAX to counteract MCL-1, thus overcoming Mcl-1 mediated resistance at multiple levels. During the course of this study, we find that AML cells resistant to BCL-2 inhibition may also be resistant to p53 activation. We demonstrate that p53-induced p21Cip1 protein confer tumor resistance by promoting reversible G1 arrest. Importantly, BCL-2 inhibition switches the outcomes of p53 activation from pro-survival G1 arrest to apoptosis, thus overcoming AML resistance to p53 activation. The combination of BCL-2 inhibition and p53 induction markedly prolongs survival in three resistant mouse models of AML. These results collectively indicate that p53 activation and BCL-2 inhibition reciprocally overcome apoptosis resistance to either strategy alone. Our study provides novel mechanistic insights into the mechanisms of apoptosis resistance and forms the concept for an international phase II trial in AML