Biological production of epicoccamide-aglycone and its cytotoxicity

Epicoccamide (EPC) is an O-d-mannosylated acyltetramic acid of Epicoccum origin and is a bolaamphiphilic fungal polyketide. EPC displays weak toxicity against Staphylococcus aureus and HeLa cell lines. The EPC biosynthetic gene cluster was previously identified in Epicoccum nigrum and knockout of the glycosyltransferase gene (epcB) abolished EPC production. EPC-aglycone was expected in the epcB knockout but was not found. This study demonstrates that extractive culture using the hydrophobic resin Diaion HP-20 resulted in the production of EPC-aglycone, which was isolated using chromatographic separation techniques, and its structural identity was substantiated by chemical analyses. EPC-aglycone displayed strong antibacterial activity against Staphylococcus aureus, with the minimal inhibitory concentration of 1 μg/mL (64 μg/mL for EPC). EPC-aglycone displayed higher levels of growth inhibition against HeLa cell line (the half inhibitory concentration, 19 μM) and WI-38 (15 μM) cell line than EPC (76 μM and 38 μM vs. HeLa and WI-38, respectively). The dose-response curve fit of growth inhibition indicated that EPC-aglycone adopted a shallow curve (low slope factor), which was different from that of EPC, suggesting that their cellular targets are distinct from each other. This study substantiates that the d-mannose attachment is the final step in EPC biosynthesis, showcasing a glycosylation-mediated modulation of the biological activity of simple acyltetramic acid. This study also highlights the usefulness of extractive cultures in mining cryptic microbial natural products.