Melatonin sensitizes leukemia cells to the MCL1 inhibitors S63845 and A-1210477 through multiple pathways

Several myeloid cell leukemia sequence 1 protein (MCL1) inhibitors including S64315 have undergone clinical testing for leukemia. Because of the toxicities after MCL1 inhibition, including hematopoietic, hepatic, and cardiac toxicities, there is substantial interest in finding agents that can sensitize leukemia cells to these MCL1 inhibitors. Melatonin is a chronobiotic that promotes chemo-induced cancer cell death while protecting normal cells from cytotoxic effects. In this study, we found melatonin sensitizes over 10 leukemia cell lines to the MCL1 inhibitors S63845 (S64315 analog) and A-1210477. Further studies demonstrate that melatonin sensitizes Jurkat cells to S63845 and A-1210477 independent of melatonin receptors MT1 and MT2, but through multiple mechanisms, including upregulating the death receptor pathway, increasing mitochondrial reactive oxygen species (ROS), inhibiting nuclear factor-κB (NF-κB) signaling, and causing cell cycle arrest. First, death receptor pathway inhibition only slightly diminishes the melatonin sensitization of S63845, while inhibiting mitochondrial ROS partially reduces the S63845/melatonin combination-induced apoptosis and depletion of the mitochondrial pathway totally abolishes it, indicating that both death receptor and mitochondrial apoptosis pathways are involved. Second, transcriptome sequencing analysis found that NF-κB signaling is downregulated by melatonin that inhibition of NF-κB signaling by parthenolide also dramatically sensitizes Jurkat cells to S63845. Third, melatonin induces G1 cell cycle arrest and upregulates NOXA while NOXA knockdown diminishes the sensitization to S63845 by melatonin. In addition, a xenograft model suggests that melatonin in combination with S63845 causes shrinkage of leukemic deposit while S63845 or melatonin monotherapy only has limited effects. Thus, our results demonstrate that melatonin efficiently sensitizes various leukemia to the MCL1 inhibitors, potentially allowing the usage of lower doses.