The SAGA components Gcn5 and Ada2 influence the activity of fluconazole against Candida glabrata independent of the Pdr1 transcriptional pathway

Purpose: Candida glabrata exhibits reduced susceptibility to azole antifungals.  In an effort to identify strategies to improve the utility of the azoles against this fungal pathogen, we screened a library of C. glabrata transcription factor disruption mutants to identify transcriptional pathways that when inactivated impart fungicidal activity to the fungistatic agent fluconazole.

Methods: Fluconazole minimum inhibitory concentrations (MICs) and minimum fungicidal concentrations (MFCs) were determined according to CLSI standard methods. Expression of PDR1 and CDR1 with and without fluconazole treatment was measured using relative qPCR.

Results: Of the 217 mutants of C. glabrata tested, four consistently showed increased susceptibility to fluconazole. Strains disrupted for PDR1, GAL11A, GCN5 and ADA2 demonstrated decreased MIC at 24 and 48 hours in both RPMI and YPD media.   Interestingly, strains disrupted for GCN5 and ADA2 also showed decreased MFC.  Fluconazole treatment induced expression of both CDR1 (an ABC transporter involved in azole resistance) and PDR1 (a transcription factor that controls expression of ABC transporters) in the wild type strain.  In strains disrupted for GCN5 and ADA2 this response was only modestly attenuated.  Fluconazole was unable to induce CDR1 expression in the strain disrupted for GAL11A.

Conclusions:  The four genes identified in this screen are associated with azole resistance in other yeasts.  Pdr1 is a transcription factor that controls expression of drug efflux pumps including Cdr1.  Gal11, part of the Mediator complex, is involved in transcriptional regulation of many pathways and is required for induction of PDR1 expression by fluconazole and other xenobiotics.  Gcn5 and Ada2 are both part of the SAGA complex which is responsible for histone post-translational modification.  The ability of fluconazole to exhibit fungicidal activity against the GCN5 and ADA2 disruptant strains suggest the SAGA complex or its target genes may represent potential targets for enhancing azole activity against C. glabrata.