Fungal pathogens exploit diverse mechanisms to survive exposure to antifungal drugs. This poses concern given the limited number of clinically useful antifungals and the growing population of immunocompromised individuals vulnerable to life-threatening
fungal infection. To identify molecules that abrogate resistance to the most widely deployed class of antifungals, the
azoles, we conducted a screen of 1,280 pharmacologically active compounds. Three out of seven hits that abolished
azole resistance of a resistant mutant of the model yeast Saccharomyces cerevisiae and a clinical isolate of the leading human fungal pathogen Candida albicans were inhibitors of
protein kinase C (PKC), which regulates cell wall integrity during growth, morphogenesis, and response to cell wall stress. Pharmacological or genetic impairment of Pkc1 conferred
hypersensitivity to multiple drugs that target synthesis of the key cell membrane
sterol ergosterol, including
azoles, allylamines, and
morpholines. Pkc1 enabled survival of cell membrane stress at least in part via the
mitogen activated protein kinase (MAPK) cascade in both species, though through distinct downstream effectors. Strikingly, inhibition of Pkc1 phenocopied inhibition of the
molecular chaperone Hsp90 or its client
protein calcineurin. PKC signaling was required for
calcineurin activation in response to
drug exposure in S. cerevisiae. In contrast, Pkc1 and
calcineurin independently regulate drug resistance via a common target in C. albicans. We identified an additional level of regulatory control in the C. albicans circuitry linking PKC signaling, Hsp90, and
calcineurin as genetic reduction of Hsp90 led to depletion of the terminal MAPK, Mkc1. Deletion of C. albicans PKC1 rendered fungistatic
ergosterol biosynthesis inhibitors fungicidal and attenuated virulence in a murine model of
systemic candidiasis. This work establishes a new role for PKC signaling in drug resistance, novel circuitry through which Hsp90 regulates drug resistance, and that targeting stress response signaling provides a promising strategy for treating life-threatening
fungal infections.