Functional characterization of the hexose transporter Hxt13p: An efflux pump that mediates resistance to miltefosine in yeast

Miltefosine (MI) has in vitro fungicidal activity against pathogenic fungi. However, mechanisms of resistance to MI have not been studied. By screening a genomic library of the model yeast, Saccharomyces cerevisiae, we identified HXT13 as a candidate genetic determinant of MI resistance. HXT13 belongs to the yeast hexose transporter family, which mediates hexose sugar uptake and is included in the major facilitator superfamily (MFS). We now report that overexpression of HXT13, but not of the closely-related genes, HXT15 and HXT17, and the more distantly related HXT14, resulted in a stable MI-resistant phenotype in S. cerevisiae. Resistance of the HXT13 overexpressing strain to MI correlated with higher cell viability following MI exposure as assessed by SYTOX® green staining compared with the control and overexpressing HXT14 strains. The mechanism of resistance in the HXT13 overexpressing strain was due to increased ATP-independent MI efflux. However, resistance to MI of the HXT13-overexpressing strain did not extend to other drugs including the echinocandins, amphotericin B, azoles, cycloheximide and sulfometuron methyl, ruling out the involvement of HXT13 in multidrug resistance. In summary, we have identified a new function of the hexose sugar transporter gene HXT13 when overexpressed in S. cerevisiae, namely, in efflux of MI and in mediating MI resistance.