Azole resistance in Candida glabrata: coordinate upregulation of multidrug transporters and evidence for a Pdr1-like transcription factor.

Candida glabrata has emerged as a common cause of fungal infection. This yeast has intrinsically low susceptibility to azole antifungals such as fluconazole, and mutation to frank azole resistance during treatment has been documented. Potential resistance mechanisms include changes in expression or sequence of ERG11 encoding the azole target. Alternatively, resistance could result from upregulated expression of multidrug transporter genes; in C. glabrata these include CDR1 and PDH1. By RNA hybridization, 10 of 12 azole-resistant clinical isolates showed 6- to 15-fold upregulation of CDR1 compared to susceptible strains. In 4 of these 10 isolates PDH1 was similarly upregulated, and in the remainder it was upregulated three- to fivefold, while ERG11 expression was minimally changed. Laboratory mutants were selected on fluconazole-containing medium with glycerol as carbon source (to eliminate mitochondrial mutants). Similar to the clinical isolates, six of seven laboratory mutants showed unchanged ERG11 expression but coordinate CDR1-PDH1 upregulation ranging from 2- to 20-fold. Effects of antifungal treatment on gene expression in susceptible C. glabrata strains were also studied: azole exposure induced CDR1-PDH1 expression 4- to 12-fold. These findings suggest that these transporter genes are regulated by a common mechanism. In support of this, a mutation associated with laboratory resistance was identified in the C. glabrata homolog of PDR1 which encodes a regulator of multidrug transporter genes in Saccharomyces cerevisiae. The mutation falls within a putative activation domain and was associated with PDR1 autoupregulation. Additional regulatory factors remain to be identified, as indicated by the lack of PDR1 mutation in a clinical isolate with coordinately upregulated CDR1-PDH1.