Ethyl caffeate combined with fluconazole exhibits efficacy against azole-resistant oropharyngeal candidiasis via the EFGR/JNK/c-JUN signaling pathway

Ethyl caffeate (EC) is a phenylpropanoid compound derived from Elephantopus scaber. In our previous work, EC was investigated to have a strong synergistic antifungal effect against azole-resistant strains of Candida albicans when combined with fluconazole (FLU). However, the protective effect and mechanism of EC + FLU on oropharyngeal candidiasis (OPC) caused by drug-resistant strains of C. albicans have not been investigated. This study aimed to investigate the protective effect and mechanism of EC combined with FLU against C. albicans-resistant strains that lead to OPC. An OPC mouse model revealed that EC + FLU treatment reduced fungal load and massive hyphal invasion of tongue tissues, and ameliorated the integrity of the tongue mucosa. Periodic acid-Schiff staining results showed more structural integrity of the tongue tissues and reduced inflammatory cell infiltration after EC + FLU treatment. Phosphorylation of EGFR (epidermal growth factor receptor) and other proteins in the EFGR/JNK (c-Jun N-terminal kinase)/c-JUN (transcription factor Jun) signaling pathway was significantly downregulated by EC + FLU. EGFR and S100A9 mRNA expression were also reduced. The above results were verified in FaDu cells. ELISA results showed that the concentration of inflammatory factors in the cell supernatant was significantly reduced after EC combined with FLU treatment. Molecular docking revealed that EC exhibited high binding energy to EGFR. In conclusion, EC enhances the susceptibility of azole-resistant C. albicans to FLU, and the underlying mechanism is related to the inhibition of the EGFR/JNK/c-JUN signaling pathway. This result suggests that EC has potential to be developed as an antifungal sensitizer to treat OPC caused by azole-resistant C. albicans.