The SET-domain protein CgSet4 negatively regulates antifungal drug resistance via the ergosterol biosynthesis transcriptional regulator CgUpc2a.

Invasive fungal infections, that pose a serious threat to human health, are increasingly associated with high a mortality rate and elevated healthcare costs, owing to rising resistance to current antifungals and emergence of multidrug-resistant fungal species. Candida glabrata is the second to fourth common cause of Candida bloodstream infections. Its high propensity to acquire resistance towards two mainstream drugs, azoles (inhibit ergosterol biosynthesis) and echinocandins (target cell wall), in clinical settings, and inherent low azole susceptibility render antifungal therapy unsuccessful in many cases. Here, we demonstrate a pivotal role for the SET-domain containing protein, CgSet4, in azole and echinocandin resistance via negative regulation of the multidrug transporter-encoding and ergosterol biosynthesis (ERG) genes through the master transcriptional factors CgPdr1 and CgUpc2A, respectively. RNA-Sequencing analysis revealed that C. glabrata responds to caspofungin (echinocandin antifungal) stress by downregulation and upregulation of ERG and cell wall organization genes, respectively. Further, we show for the first time that CgSet4 acts as a repressor of the ergosterol biosynthesis pathway via CgUPC2A transcriptional downregulation, with CgSET4 deletion resulting in elevated ergosterol content. Our genetic and transcriptional analyses revealed that the caspofungin-induced ERG gene repression is not dependent on CgSet4, with CgSet4 showing diminished abundance on the CgUPC2A promoter in caspofungin-treated cells. Additionally, we show a role for the last three enzymes of the ergosterol biosynthesis pathway, CgErg3, CgErg5 and CgErg4, in antifungal susceptibility and virulence in C. glabrata. Altogether, our results unveil the link between ergosterol biosynthesis and echinocandin resistance, and have implications for combination antifungal therapy.