Effect of fatty acids on hyphal growth in the pathogenic yeast Candida albicans

The yeast Candida albicans is an inhabitant of the oral cavity, the gastrointestinal and genitourinary tracts of humans. Generally encountered as a commensal, it is also an opportunistic pathogen that causes a spectrum of infections, ranging from superficial mycoses (thrush, vulvovaginitis) to severe and life-threatening systemic infections. A striking feature of C. albicans is its ability to grow in different morphological forms, including budding yeasts, pseudohyphae, and hyphae. Environmental cues that mimic host conditions (elevated temperature, neutral or alkaline pH, and serum) induce the yeast-to-hypha transition. Morphogenesis is considered to be an attribute of pathogenesis, as mutants locked as yeasts or filamentous forms are avirulent. Given that the yeast-to-hypha transition is a virulence factor, it may also constitute a target for the development of antifungal drugs. Indeed, evidence has shown that impairing morphogenesis is a means to treat systemic candidiasis. Concurrently, a number of molecules have been reported to modulate morphogenesis in C. albicans. For instance, several fatty acids, including conjugated linoleic acid (CLA), inhibited the yeast-to-hypha transition. By interfering with an important attribute of C. albicans pathogenesis, CLA may harbor antifungal properties. However, before assessing its therapeutic potential in a clinical context, it is mandatory to address CLA’s mode of action.   The present study aims to further characterize the hypha-inhibiting properties of fatty acids and CLA and to elucidate the mechanism by which these molecules inhibit the yeast-to-hypha transition in C. albicans. Gene expression analyses were performed to gain insight into the transcriptional response of cells to CLA on a genome-wide scale and to probe the fatty acid’s mode of action. CLA downregulated the expression of hypha-specific genes and blocked the induction of genes encoding regulators of hyphal growth, including that of RAS1, which encodes the small GTPase Ras1p. A membrane-associated signaling protein, Ras1p plays a major role in morphogenesis. Quantitative PCR analyses showed that CLA prevented the increase in RAS1 mRNA levels which occurred at the onset of the yeast-to-hypha transition. Unexpectedly, CLA reduced the steady-state levels of Ras1p. Additionally, CLA caused the delocalization of GFP-Ras1p from the plasma membrane. These findings indicate that CLA treatment results in suboptimal Ras1p cellular concentrations and localization, which impedes Ras1p signaling and inhibits the yeast-to-hypha transition. CLA may indirectly affect Ras1p localization by altering the structure of the plasma membrane. These studies have provided the mechanism underlying CLA’s hypha-inhibiting properties and may serve as the rationale to examine CLA’s therapeutic potential in the context of a Candida infection. There is a general lack of clinical evidence demonstrating that impairing morphogenesis is a sound approach to treat candidiasis. To remedy this situation, the therapeutic potential of molecules that modulate morphogenesis, such as CLA, should be clinically assessed.

Keyphrases: Candida albicans, Ras1p, Candida albicans, Morphogenesis, Yeast-to-hypha transition, Hyphal growth, Fatty acids, Conjugated linoleic acid, Ras1p signaling