Flavonoid Antifungal Effects on C. Albicans and Host Transduction Pathways

Objectives: Oral candidiasis is the most common fungal infection in humans and has shown increased resistance to traditional antifungal drugs. Extracellular proteinases, secreted aspartyl proteinase (SAP) and phospholipase B (PLB), are key determinants mediating pathogenicity and suitable targets in drug development. Flavonoids, myricetin and quercetin, show activity against Candida albicans (CA) and provide promising candidates for therapeutics.

These studies aimed to determine the antifungal activity of myricetin and quercetin and to elucidate their mechanisms by investigating the expression and secretion of CA virulence factors and host cell inflammatory cytokines.
Methods: Minimum inhibitory concentrations (MIC) were determined after 24h by adding serial dilutions of myricetin (0.7-70µM) and quercetin (4.5-45µM). Fluconazole and solutions with 1% DMSO served as positive and vehicle controls respectively. Co-culture of C. albicans (ATCC: MYA 2876) at 2.5x103 cells/mL and fibroblasts (ATCC: CRL2014) at 1.0x105 cells/mL were cultured in EMEM-medium, 10% FBS, treated with myricetin and quercetin (1xMIC) at 37^oC,5% CO₂ for 24h and harvested and evaluated for (n=9): A) Cell/Fungal viability; B) Extracellular host inflammatory cytokines (ELISA for IL-1a, IL-1b, IL-6, IL-8, IL-10, and GSM-CSF); c) host gene expression of inflammatory response; D) C. albicans gene expression for SAP and PLB; e) SAP and PLB activity and f) Architecture analyses by fluorescence microscopy.
Results: Myricetin and quercetin inhibit C. albicans growth and an MIC was established as 30µM and 25µM respectively. The compounds decreased secretion of pro-inflammatory cytokines (p<0.05), while increasing secretion of anti-inflammatory cytokines. In addition, the treatment decreased host expression of these cytokines and CA expression and activity of virulence factors, SAP1 and PLB-1 (p<0.05). The compounds also markedly altered the density and architecture of the fungal colonies.
Conclusions: Myricetin and quercetin demonstrate their therapeutic potential, not only through inhibition of fungal growth and virulence, but also in modulating the host inflammatory responses.