冨永 薫

ABSTRACT While venetoclax‐based combinations have shown promising results in acute myeloid leukemia (AML), the remission duration is generally short, warranting strategies to further improve efficacy and overcome resistance. Here, we show that the natural quassinoid brusatol induces cell‐cycle arrest and apoptosis in multiple AML cell lines while enhancing venetoclax efficacy irrespective of inherent or acquired resistance. Mechanistically, brusatol increased p53 protein expression, leading to upregulation of its target genes/proteins, including CDKN1A (p21) and BBC3 (PUMA). Genetic deletion of TP53 attenuated brusatol‐induced apoptosis and its synergy with venetoclax, supporting p53 activation as a central mechanism underlying the anti‐leukemia response. Furthermore, the combination synergistically decreased mitochondrial membrane potential and respiratory activity, causing accumulation of reactive oxygen species in AML cells. Although brusatol and venetoclax exhibited limited effects individually, their combination markedly reduced leukemia burden and significantly prolonged survival in three independent cell line‐derived xenograft models, including venetoclax‐resistant and ‐refractory models. Notably, brusatol increased normal leukocyte and platelet counts while reducing leukemic infiltration in both bone marrow and extramedullary sites. These findings provide mechanistic insight into the synergistic effects of the brusatol‐venetoclax combination, supporting further evaluation of this therapeutic strategy in myeloid leukemias.

Ras gene mutations are frequently observed in many types of cancers. However, there are currently no effective anticancer drugs against Ras-induced cancers. Therefore, identifying the downstream effectors of the Ras signaling pathway can facilitate the development of promising novel therapeutic approaches. We previously showed that oncogenic Ras induces the expression of the receptor tyrosine kinase c-Mer proto-oncogene tyrosine kinase (MerTK) in an interleukin-1 family member NF-HEV/IL-33-dependent manner and that IL-33 and MerTK contribute to oncogenic Ras-induced cell migration. In the present study, we purified the MerTK complex from NIH-3T3 cells transformed by the expression of oncogenic Ras, H-Ras (G12V). Mass spectrometric analysis identified STK38 (also known as NDR1) as a candidate binding partner for MerTK. STK38 is a serine/threonine protein kinase that plays diverse roles in normal and cancerous cells. In addition to MerTK knockdown, STK38 knockdown effectively attenuated the H-Ras (G12V)-induced migration of NIH-3T3 cells. STK38 kinase activity is required for oncogenic Ras-induced cell migration and MerTK tyrosine phosphorylation. Furthermore, MerTK or STK38 knockdown attenuated the activation of Rac1 and Cdc42. Taken together, these results revealed a novel role for STK38 in oncogenic Ras-induced enhanced cell migration, which may be useful for developing novel therapeutic strategies targeting Ras-mutated cells.

Abstract Leukemia cells are consistently subjected to higher oxidative stress than normal cells. To mitigate reactive oxygen species (ROS) overload, which can trigger various forms of cell death, leukemia cells employ a robust antioxidant defense system and maintain redox homeostasis. Recent evidence suggests that dimethyl fumarate (DMF), a derivative of fumarate, inactivates the antioxidant glutathione (GSH), thereby inducing oxidative stress and metabolic dysfunction, eventually leading to cell death in cancer cells. In this study, we observed that DMF decreases the GSH/oxidated GSH ratio and increases intracellular ROS levels, the extent of which is closely correlated with cell death, in acute myeloid leukemia (AML) cell lines. DMF reduced the mitochondrial membrane potential and oxidative phosphorylation (OXPHOS), effects that were almost fully restored by the antioxidant N-acetylcysteine, suggesting that these responses are ROS-dependent. Electron microscopy and inhibition assays revealed that apoptosis, rather than necroptosis or ferroptosis, is the predominant form of cell death of AML cells following DMF treatment. Notably, the combination of DMF and the BCL-2 selective BH3-mimetic venetoclax induced marked cell death in AML cells, including venetoclax-refractory BCL-2 low expressing U937 and acquired venetoclax-resistant MOLM-14 cells. This combination also caused greater mitochondrial depolarization and a more profound reduction in OXPHOS activity than either agent alone. Collectively, our findings indicate that DMF exerts potent anti-leukemia activity in AML cells and sensitizes cells to venetoclax treatment by synergistically disrupting mitochondrial integrity through ROS accumulation.