An acquired mechanism of antifungal drug resistance simultaneously enables <i>Candida albicans</i> to escape from intrinsic host defenses

<div><p>The opportunistic fungal pathogen <i>Candida albicans</i> frequently produces genetically altered variants to adapt to environmental changes and new host niches in the course of its life-long association with the human host. Gain-of-function mutations in zinc cluster transcription factors, which result in the constitutive upregulation of their target genes, are a common cause of acquired resistance to the widely used antifungal drug fluconazole, especially during long-term therapy of oropharyngeal candidiasis. In this study, we investigated if <i>C</i>. <i>albicans</i> also can develop resistance to the antimicrobial peptide histatin 5, which is secreted in the saliva of humans to protect the oral mucosa from pathogenic microbes. As histatin 5 has been shown to be transported out of <i>C</i>. <i>albicans</i> cells by the Flu1 efflux pump, we screened a library of <i>C</i>. <i>albicans</i> strains that contain artificially activated forms of all zinc cluster transcription factors of this fungus for increased <i>FLU1</i> expression. We found that a hyperactive Mrr1, which confers fluconazole resistance by upregulating the multidrug efflux pump <i>MDR1</i> and other genes, also causes <i>FLU1</i> overexpression. Similarly to the artificially activated Mrr1, naturally occurring gain-of-function mutations in this transcription factor also caused <i>FLU1</i> upregulation and increased histatin 5 resistance. Surprisingly, however, Mrr1-mediated histatin 5 resistance was mainly caused by the upregulation of <i>MDR1</i> instead of <i>FLU1</i>, revealing a previously unrecognized function of the Mdr1 efflux pump. Fluconazole-resistant clinical <i>C</i>. <i>albicans</i> isolates with different Mrr1 gain-of-function mutations were less efficiently killed by histatin 5, and this phenotype was reverted when <i>MRR1</i> was deleted. Therefore, antimycotic therapy can promote the evolution of strains that, as a consequence of drug resistance mutations, simultaneously have acquired increased resistance against an innate host defense mechanism and are thereby better adapted to certain host niches.</p>