Mechanism-based inhibition of ergosterol biosynthesis in Acanthamoeba castellanii and its therapeutic implications
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In this study we investigate the amoebacidal activity of 10 steroidal inhibitors which block sterol C24-methylation, and compare its efficacy to 5 medical azoles which block sterol C14-demethylation, against Acanthamoeba castellanii, the pathogenic protozoan responsible for corneal infections. Bioinformatic and chemical analyses reveal that Acanthamoeba operates a plant-based ergosterol biosynthesis pathway whereby trophozoites synthesize cycloartenol; the protosterol is converted to ergosterol (C28) and 28-ethyl ergosterol (C29), amphipathic compounds that act as membrane inserts. Alternatively, trophozoites contain primarily the C29- 7-dehydroporiferasterol and C28 – ergosterol. Cysts tend to continue the biosynthetic pathway to the ∆5- sterol products brassicasterol and poriferasterol. Surprisingly Acanthamoeba is capable of C29-ring B aromatic phenanthrenes derived from metabolism of the C28- and C29-∆5, 7- sterols but in these cells they tend to be dead. Treatment of trophozoites with medical azoles and steroidal inhibitors in a dose-dependent manner to inhibit proliferation. A small population of trophozoites treated with the test inhibitors transformed into nonviable cysts that possessed uncommon sterol profiles. Given that steroidal inhibitors supplied to cultured human embryonic kidney cells has no effect on growth or cholesterol biosynthesis to 40 µM- that thereby, affords a selective index (SI) for the inhibitor of 100 - or effected mice health fed the drug (Haubrich et al. J. Lipid Res. 56, 331-3410), reveals steroidal inhibitors are a highly potent and safe anti-amoebic agent. We propose steroidal inhibitors of Acanthamoeba ergosterol biosynthesis could provide new leads and a new treatment strategy (e.g., in combination with medical azoles) for chronic amebic keratitis.