Metabolites from Aspergillus sclerotiorum fungal growth medium are cytotoxic to and inhibit cell cycle progression in Drosophila melanogaster S2 cells in culture

Abstract

Aspergillus sclerotiorum has been identified as an entomopathogenic fungus with potential as a biological control agent, yet its cellular effects on insect hosts are not well characterized. To explore possible mechanisms, we tested cell-free fungal culture filtrates on Drosophila melanogaster S2 embryonic cells. Filtrates from older cultures were strongly cytotoxic and showed a reduced percentage of live S2 cells comparable with positive controls for cell death. Similar effects were seen in mouse fibroblast L929 cells.  At lower concentrations, the fungal medium did not cause overt cytotoxicity but did consistently reduce total S2 cell numbers. Few dead cells were present in these conditions, indicating the medium impaired replication rather than inducing cell death. This reduction was accompanied by a decreased expression of two cell cycle regulators. Proliferating cell nuclear antigen (PCNA) fell to less than 6% of controls, while Cyclin A dropped to below 30% of controls at the most dilute treatment. These results indicate that A. sclerotiorum secretes one or more metabolites that decrease cell viability at higher concentrations and inhibit proliferation at lower concentrations, potentially by interfering with cell cycle progression. ELISA analysis of ochratoxin A production in this strain showed only minimal levels of the toxin, suggesting other unidentified fungal factors may be the source of these observations. These findings advance understanding of fungal–insect interactions and suggest that soluble compounds produced by A. sclerotiorum may contribute to its pathogenicity in insect hosts.