Effects of purpurin on proton-pumping ATPase and morphological transition in Candida albicans

Abstract

Opportunistic human fungal pathogen Candida albicans poses a serious threat to human health. The unicellular microbe exists as part of the normal microbiota on the skin and mucosal surfaces of oral cavity, digestive tract, and urogenital system, but can become invasive and cause local and/or disseminated diseases (candidiasis) in immunocompromised patients with high morbidity and mortality rates (40–60%). Clinical usefulness of the current limited arsenal of antifungal agents has been hampered by toxic side effects, poor pharmacokinetics, and emergence of drug-resistant isolates, indicating a dire need of new antifungal agents. In our earlier study, we have first reported the potent in vitro anti-Candidal activity of purpurin (an anthraquinone pigment found in madder root) against six pathogenic Candida species. One striking virulence trait of C. albicans is its ability to grow and switch between budded yeast and filamentous forms (hyphae), and this yeast-tohypha transition is closely linked with external pH. It is thus conceivable that perturbation of pH homeostasis can be attractive in the management of candidiasis through indirect modulation of morphogenesis. To this end, we extended the investigation and demonstrated the inhibitory actions of purpurin on pH homeostasis and hyphal growth in C. albicans SC5314. At sub-MIC levels (￡0.5 lg ml-1), purpurin suppressed glucosemediated proton pumping ATPase activity in a concentration-dependent manner, and partially inhibited yeast-to-hypha transition and biofilms. Physiological disturbance of cellular metabolism could be excluded as C. albicans growth was not affected. Safe concern and high selectivity of purpurin for C. albicans were justified by its non-toxic nature to primary human gingival fibroblasts (2•MIC; viability = 94%) and keratinocytes (1•MIC; viability = 95%). Therefore, purpurin may represent a potential candidate that deserves further investigations in the development of antifungal strategies against candidiasis - for example, combinational use of purpurin with antifungal agents possessing different modes of action may reduce the likelihood of acquired drug resistance.