Upregulation of ERG genes in Candida species by azoles and other sterol biosynthesis inhibitors

Infections due to Candida albicans are usually treated with azole antifunga ls such as fluconazole, but treatment failure is not uncommon especially in immunocompromised individuals. Relatedly, in vitro studies demonstrate tha t azoles are nonfungicidal, with continued growth at strain-dependent rates even at high azole concentrations. We hypothesized that upregulation of ER G11, which encodes the azole target enzyme lanosterol demethylase, contribu tes to this azole tolerance in Candida species. RNA analysis revealed that ERG11 expression in C. albicans is maximal during logarithmic-phase growth and decreases as the cells approach stationary phase. Incubation with fluco nazole, however, resulted in a two- to fivefold increase in ERG11 RNA level s within 2 to 3 h, and this increase was followed by resumption of culture growth. ERG11 upregulation also occurred following treatment with other azo les (itraconazole, ketoconazole, clotrimazole, and miconazole) and was not dependent on the specific medium or pH. Within 1 h of drug removal ERG11 up regulation was reversed. Azole-dependent upregulation was not limited to ER G11: five of five ERG genes tested whose products function upstream and dow nstream of lanosterol demethylase in the sterol biosynthetic pathway were a lso upregulated. Similarly, ERG11 upregulation occurred following treatment of C. albicans cultures with terbinafine and fenpropimorph, which target o ther enzymes in the pathway. These data suggest a common mechanism for glob al ERG upregulation, e.g., in response to ergosterol depletion. Finally, az ole-dependent ERG11 upregulation was demonstrated in three additional Candi da species (C. tropicalis, C. glabrata, and C. krusei), indicating a conser ved response to sterol biosynthesis inhibitors in opportunistic yeasts.