The siderophore transporter Sit1 is involved in the uptake of caspofungin by <i>Candida albicans</i>

Invasive infections caused by the yeast <i>Candida albicans</i> are a significant health concern due to their high rates of mortality and morbidity. Current treatment guidelines recommend echinocandin drugs as a primary therapeutic option. Despite the prevailing view that these drugs act extracellularly, evidence suggests that the echinocandin caspofungin can enter fungal cells via an as-yet-unidentified mechanism, with the implications of this internalization remaining unknown. Based on our previous findings demonstrating that caspofungin can bind iron and considering its structural resemblance to ferrichrome-type cyclic hexapeptide siderophores, we hypothesize that siderophore transporters could be involved in caspofungin uptake. In <i>C. albicans</i>, at the transcriptional level, caspofungin mimics a high iron response, which correlates with the increase in intracellular iron levels in drug-treated cells. This increase depends on the siderophore transporter Sit1, as a <i>C. albicans</i> mutant lacking Sit1 does not experience such an increase. Heterologous expression of Sit1 made <i>Saccharomyces cerevisiae</i> cells more sensitive to caspofungin, while <i>C. albicans</i> cells lacking Sit1 or a <i>S. cerevisiae</i> mutant of all siderophore transporters were more tolerant to the drug. Experiments using a newly synthesized fluorescent caspofungin molecule and mass spectrometry confirmed that caspofungin accumulation in yeast is partially mediated by Sit1. Overall, this work identifies siderophore transporters as important players in caspofungin uptake and tolerance in yeast.