Oral infections with the pathogenic yeast Candida albicans are one of the most frequent and earliest opportunistic infections in human immunodeficiency virus-infected patients. The widespread use of azole antifungal drugs has led to the development of drug-resistant isolates. Several molecular mechanisms that contribute to drug resistance have been identified, including increased mRNA levels for two types of efflux pump genes: the ATP binding cassette transporter CDRs (CDR1 and CDR2) and the major facilitator MDR1. Using Northern blot analyses, the expression patterns of these genes have been determined during logarithmic and stationary phases of cell growth and during growth in different carbon sources in a set of matched susceptible and fluconazole-resistant isolates that have been characterized previously. MDR1, CDR1, and CDR2 are expressed early during logarithmic growth, CDR4 is expressed late during logarithmic growth, and CDR1 is preferentially expressed in stationary-phase cells. There is a small decrease in expression of these genes when the cells are grown in carbon sources other than glucose. While increased mRNA levels of efflux pump genes are commonly associated with azole resistance, the causes of increased mRNA levels have not yet been resolved. Southern blot analysis demonstrates that the increased mRNA levels in these isolates are not the result of gene amplification. Nuclear run-on assays show that MDR1 and CDR mRNAs are transcriptionally overexpressed in the resistant isolate, suggesting that the antifungal drug resistance in this series is associated with the promoter and trans-acting factors of the CDR1, CDR2, and MDR1 genes.Candida albicans is a pathogenic yeast that causes oral, vaginal, and systemic infections (reviewed in reference 28). These infections are usually treated with antifungal drugs, including the polyene amphotericin B and the azoles, such as fluconazole. Azole-resistant strains of C. albicans are an increasing problem in human immunodeficiency virus-infected patients and other immunosuppressed individuals (37).

The sterol pathway in Candida albicans is the target for several classes of antifungal drugs. Intermediates in the sterol pathway are involved in ergosterol synthesis, prenylation and dolichol synthesis. This study examines gene expression of the sterol pathway in response to lovastatin, an inhibitor of HMG-CoA reductase (Hmg1p), and fluconazole, an inhibitor of 14 alpha-lanosterol demethylase (Erg11p). Minimum inhibitory concentration (MIC) studies indicated that lovastatin acts synergistically with fluconazole in vitro. Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) results indicated that genes in the early part of the sterol pathway, such as HMG1 and ERG20, did not alter expression in the presence of both lovastatin and fluconazole, whereas genes in the later part of the sterol pathway, such as ERG9 and ERG11, had increased expression in response to these drugs in mid-logarithmic growth. Genes involved in prenylation, such as RAM1 and RAM2, also respond to these drugs in mid-logarithmic growth, although another prenylation gene, CDC43, was not affected. After 24 h of growth, the relative expression of ERG20, ERG9, and ERG11 remained unchanged or increased in the presence of both drugs, while all other genes decreased in expression under all drug treatments.

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