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

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.