Phenotypic Resistance to Amphotericin B in Candida albicans: Relationship to Glucan Metabolism

Abstract

The phenotypic resistance to amphotericin methyl ester (AME) of stationary phase cultures of C. albicans was decreased by alkaline pH values and by treatment with 2-mercaptoethanol or glucanase preparations, and was increased by acid pH values, increased aeration, treatment with N-ethylmaleimide or the presence of inhibitors of protein synthesis such as trichodermin. The effects of such treatments on endogenous glucanase activity and on the incorporation of glucose residues into the glucan fraction of the organism were studied. The changes in the endogenous levels of lytic activities on laminarin [as a measure of the total (1 .fwdarw. 3)-.beta.-D-glucanase] and on p-nitrophenyl-.beta.-D-glucoside [reflecting the exo-(1 .fwdarw. 3)-.beta.-D-glucanase] were followed in C. albicans cells under a variety of conditions. Treatments which increased AME sensitivity stimulated both total and exo-(1 .fwdarw. 3)-.beta.-D-glucanase activities; treatments which promoted resistance decreased the levels of both (1 .fwdarw. 3)-.beta.-D-glucanases. Changes in the glucan fraction were followed by incubating suspensions of organisms in the presence of trace amounts of [U-14C]glucose. The rate of incorporation of radioactivity fell during the first 2-3 days of stationary phase culture and then rose to high values by 7-8 days; AME resistance increased throughout this period. The rate of incorporation was markedly stimulated by prior treatment of the organisms with 2-mercaptoethanol or glucanase and inhibited by trichodermin or treatment with N-ethylmaleimide. The addition in the concentration range 0.3-3 mM of the glucose analogs .beta.-D-allose, 3-O-methyl-D-glucose, 2-deoxy-D-glucose or 5-thio-D-glucose to cultures 24 h after inoculation prevented any further increase in AME resistance for the next 2-3 days and resulted in a decrease in the level of resistance established at the time of addition. Radioactivity from 14C or 3H-labeled analogs added, 24 h after inoculation, to stationary phase cultures was incorporated into the glucan fraction of the organisms. The incorporation of glucose residues into the glucan fraction is controlled by the activity of glucanases in producing glucose acceptor sites. The results confirm the correlation between glucan metabolism, glucanase activity and resistance to AME, in that any factor leading to increased glucanase action also results in decreased resistance and vice versa, while incorporation of certain glucose analogs into the glucan fraction delays the further increase in resistance.