The Hippo kinases MST1/2 integrate sterile and infectious signals to regulate macrophage cell death

Mammalian STE20-like kinases MST1 and MST2 (mammalian STE20-like protein kinase-1/2) are the conserved Hippo kinases known for their importance in organ development and tumor suppression. Notably, humans and mice lacking these kinases have increased susceptibility to infection, indicating a role of MST1/2 in immunity. In macrophages that are critical immune cells in host defense, MST1/2 are proteolytically cleaved to coordinate different forms of programmed cell death, including apoptosis and pyroptosis. This cleavage event occurs when the innate immune sensors, inflammasomes, are activated by the bacterial pathogen, Legionella pneumophila, or damage-associated molecular patterns. In this report, we examine MST1/2 cleavage in macrophages under various inflammatory conditions and challenges with pathogenic bacteria. ATP and nigericin induce MST1/2 cleavage and apoptosis, while the NLRP3 inflammasome and gasdermin D (GSDMD)-mediated pyroptosis are activated. Remarkably, in conditions that do not support activation of NLRP3 or GSDMD, MST1/2 are still cleaved by caspases to promote cell death in macrophages treated with these molecules. During infection, WT macrophages trigger MST1/2 cleavage and apoptosis against L. pneumophila and Yersinia pseudotuberculosis but preferentially activate GSDMD-mediated pyroptosis against Pseudomonas aeruginosa. Interestingly, GSDMD KO macrophages opt to cleave MST1/2 and undergo apoptosis in response to P. aeruginosa, suggesting an interplay between GSDMD and MST1/2. Together, macrophages funnel apoptotic death signals through MST1/2 cleavage upon stimulation of the inflammatory molecules and pathogens, illustrating the broad implications of the host Hippo kinases in infections and inflammation.