Candida glabrata, second only to C. albicans as an infectious pathogen in candidiasis, contributes to an average of 11% of candidal infections, varying from 7% to 20% depending on geographic location . In the survey providing data for the present study of immunocompromised patients, C. glabrata accounted for about 15% of candidal infections. Twenty-five percent of the isolates were resistant to fluconazole (Zaini et al., unpublished data). Although several genes may be implicated in azole resistance, the molecular pathways involved are not completely understood.
We report, for the first time using cDNA-AFLP, AKR transcripts upregulated in resistant clinical isolates. The AKR gene is not the first suggested to be involved in azole resistance. Upregulation of CgCDR1 and CgCDR2 genes associated with increased expression of the ABC transporter has been well documented [19, 20]. The PDR1 gene is important in acquired azole resistance , and so-called gain-of-function mutations of the CgPDR1 gene have been shown to play an essential role in azole resistance by C. glabrata[22–25]. These mutations indicate that many genes are differentially regulated in azole resistant isolates as compared to the wild type. Multiple genes (from 27–235) show increased expression, and aldo-keto-reductase (CAGL0C04543g: XM_445372.1) is upregulated (1.5 to 2-fold) . The results of the present study also demonstrated that AKR mRNA expression in azole resistance reaches levels of about twice that found in sensitive strains. The molecular mechanisms of azole resistance in C. albicans are connected with overexpression of ATP-binding cassette (ABC) transporters or major facilitator superfamily, and upregulation of these genes can also be brought about by exposure to benomyl and fluphenazine. In the presence of benomyl, some genes belonging to the aldo-keto reductase family, such as IFD genes, IPF5987, and GRP2, can be activated with the oxido-reductase function .
The aldo-keto-reductase superfamily (AKR) comprises several proteins with similar kinetic and structural properties and has been found in a wide range of phyla, including both prokaryotes and eukaryotes . AKRs catalyze the reduction of aldehydes and ketones to their corresponding alcohol products by reducing nicotinamide adenine dinucleotide phosphate (NADPH) cofactor . Several drugs and pharmaceuticals are reactive carbonyls and aldehydes or are converted to carbonyls during in vivo metabolism. An important role of AKRs is in preventing carbonyl toxicity . The physiological roles of this superfamily have been studied in the yeast Saccharomyces cerevisiae, a simple eukaryote containing various AKR genes that encode proteins similar in structure and function to mammalian AKRs, including those of humans. The physiological activity of yeast AKRs is largely unclear. Prior studies have identified six open reading frames (YHR104W, YOR120W, YDR368W, YBR149W, YJR096W, YDL124W) in the S. cerevisiae genome that encode proteins with activity overlapping human aldose reductase . YHR104W and YDR368W are stress response proteins . The product of YOR120W is a galactose-inducible crystalline-like yeast protein, and YBR149W encodes a dehydrogenase that plays a role in the direction of arabinose oxidation rather than reduction . Numerous studies have demonstrated that aldo-keto-reductases are active in stress conditions. Another important role of the yeast AKR genes is to protect against heat shock stress . In addition, AKRs potentially play roles in oxidative defense and transcriptional regulation . AKR function has also been reported in drug metabolism and detoxification of pharmaceuticals, drugs, and xenobiotics in humans , and it seems that the physiological roles of the AKR gene in yeast are similar to those in humans. Our results suggest upregulation of AKR gene is involved in the molecular mechanism of drug resistance in C. glabrata.