PKA inhibition kills l-asparaginase-resistant leukemic cells from relapsed acute lymphoblastic leukemia patients

Abstract

Abstract Despite the success in treating newly diagnosed pediatric acute lymphoblastic leukemia (aLL), the long-term cure rate for the 20% of children who relapse is poor, making relapsed aLL the primary cause of cancer death in children. By unbiased genome-wide retroviral RNAi screening and knockdown studies, we previously discovered opioid receptor mu 1 (OPRM1) as a new aLL cell resistance biomarker for the aLL chemotherapeutic drug, l-asparaginase, i.e., OPRM1 loss triggers l-asparaginase resistance. Indeed, aLL cell OPRM1 level is inversely proportional to l-asparaginase IC50: the lower the OPRM1 level, the higher the l-asparaginase IC50, indicating that aLL cells expressing reduced OPRM1 levels show resistance to l-asparaginase. In the current study, we utilized OPRM1-expressing and -knockdown aLL cells as well as relapsed patient aLL cells to identify candidate targeted therapy for l-asparaginase-resistant aLL. In OPRM1-expressing cells, l-asparaginase induces apoptosis via a cascade of events that include OPRM1-mediated decline in [cAMP]i, downregulation of PKA-mediated BAD S118 phosphorylation that can be reversed by 8-CPT-cAMP, cyt C release from the mitochondria, and subsequent caspase activation and PARP1 cleavage. The critical role of PKA inhibition due to a decrease in [cAMP]i in this apoptotic process is evident in the killing of OPRM1-knockdown and low OPRM1-expressing relapsed patient aLL cells by the PKA inhibitors, H89 and 14–22 amide. These findings demonstrate for the first time that PKA can be targeted to kill aLL cells resistant to l-asparaginase due to OPRM1 loss, and that H89 and 14–22 amide may be utilized to destroy l-asparaginase-resistant patient aLL cells.