Cleavage of the Hippo kinases and programmed cell death in murine macrophages exposed to sterile stimuli and bacterial pathogens
Mammalian STE20-like kinases MST1 and MST2 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 play a critical role in host immunity, 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 determine MST1/2 cleavage in macrophages under various inflammatory conditions and challenges with pathogenic bacteria. The sterile molecules ATP and nigericin induce MST1/2 cleavage and apoptosis when the NLRP3 inflammasome and GSDMD-mediated pyroptosis are activated. Remarkably, in conditions without NLRP3 or GSDMD activation, MST1/2 are still cleaved by caspases to promote cell death in macrophages treated with these sterile molecules. During infection, wildtype macrophages trigger MST1/2 cleavage and apoptosis against L. pneumophila and Yersinia pseudotuberculosis but preferentially activate GSDMD-mediated pyroptosis against Pseudomonas aeruginosa and Salmonella enterica Typhimurium. Interestingly, GSDMD knockout macrophages opt to cleave MST1/2 and undergo apoptosis in response to P. aeruginosa and S. enterica, 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, which illustrates the broad implications of the host Hippo kinases in infections and sterile inflammation.