Nonapoptotic Death of Saccharomyces cerevisiae Cells That Is Stimulated by Hsp90 and Inhibited by Calcineurin and Cmk2 in Response to Endoplasmic Reticulum Stresses

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Abstract
Endoplasmic reticulum (ER) stress can trigger apoptosis and necrosis in many types of mammalian cells. Previous studies in yeast found little or no cell death in response to the ER stressor tunicamycin, but a recent study suggested widespread apoptosis-like death. Here we show that wild-type laboratory Saccharomyces cerevisiae cells responding to tunicamycin die by nonapoptotic mechanisms in low-osmolyte culture media and survive for long periods of time in standard synthetic media. Survival requires calcineurin, a Ca 2+ /calmodulin-dependent protein phosphatase, but none of its known targets. The Ca 2+ /calmodulin-dependent protein kinase Cmk2 was identified as an indirect target of calcineurin that suppresses death of calcineurin-deficient cells. Death of Cmk2- and/or calcineurin-deficient S. cerevisiae cells was preceded by accumulation of reactive oxygen species but was not associated with hallmarks of apoptosis and was not dependent on Mca1, Aif1, Nuc1, or other factors implicated in apoptosis-like death. Cmk2 and calcineurin also independently suppressed the death of S. cerevisiae cells responding to dithiothreitol or miconazole, a common azole-class antifungal drug. Though inhibitors of Hsp90 have been shown to diminish calcineurin signaling in S. cerevisiae and to synergistically inhibit growth in combination with azoles, they did not stimulate death of S. cerevisiae cells in combination with miconazole or tunicamycin, and instead they prevented the death of calcineurin- and Cmk2-deficient cells. These findings reveal a novel prodeath role for Hsp90 and antideath roles for calcineurin and Cmk2 that extend the life span of S. cerevisiae cells responding to both natural and clinical antifungal compounds.