XMU-MP-1, Inhibitor of STE20-like MST1/2 Kinases of the Hippo Signaling Pathway, Suppresses the Cell Cycle, Activates Apoptosis and Autophagy, and Induces Death of Hematopoietic Tumor Cells

Background/Objectives: Currently, there is limited knowledge about the molecular mechanisms of the "non-canonical" Hippo signaling pathway in hematopoietic tumor cells. We have shown that targeting the MST1/2 kinases, which are key molecules in this signaling pathway, may be an effective approach to treating hematologic tumors. Methods: Cell growth assays, caspase assays, Western blot hybridizations, flow cytometry, and whole transcriptome analyses. These methods allowed us to better understand the molecular pathways at play. Results: Our results showed that XMU-MP-1, an inhibitor of MST1/2 kinase, specifically reduces the viability of hematopoietic cancer cells but not epithelial cells. It effectively inhibits the growth of B- and T-cell tumors by blocking cell cycle progression mainly during the G2/M phase, inducing apoptosis and autophagy. Under XMU-MP-1 treatment, caspase 3/7 activity increases and the level of cleaved PARP protein increases. The protein level of LC3-II also increases, while the level of p62 decreases. These changes are associated with apoptosis and autophagy, respectively. RNA-Seq analysis demonstrated that XMU-MP-1 suppresses the expression of cell cycle regulators, such as E2F and cell division cycle genes CDC6,7,20,25,45; cyclins A2,B1,B2; cyclin dependent kinases. At the same time, it increases the expression of genes involved in apoptosis, autophagy, and necroptosis. Additionally, XMU-MP-1 enhances the effects of doxorubicin, making it a potential candidate for combination therapy in cancer treatment. Conclusions: Combinations of growth assays, caspase assays, Western blotting, and RNA-seq have shown that the dramatic reduction in the number of hematopoietic tumor cells after treatment with XMU-MP-1 is due to both cytostatic and cytotoxic effects. The use of MST1/2 kinase inhibitors could be highly promising for complex therapies of hematological tumors.