Elucidating the mechanism of action of the novel marine macrolide iejimalide

Iejimalides are a novel family of marine macrolides shown to be active against cancer cells in vitro that inhibit vacuolar H+-ATPase (V-ATPase) activity in osteoclast cells. Iejimalide B (Iej B) arrests cells in G1- or S-phase of the cell cycle and induces apoptosis in a cell-line specific manner. Multiple genes in the p53 cell cycle arrest/cell death pathway are induced upon treatment by Iej B, suggesting that apoptosis is mediated by a p53-dependent mechanism. Iej A and B prevent acidification of lysosomes and depolarize mitochondria and induce reactive oxygen species production in a time-dependent manner. Mitochondrial depolarization is augmented by cyclosporin A treatment, suggesting an autophagic response to Iej B. Iej B is not an inhibitor of ATP synthase (F-ATPase) as measured by oxygen consumption of rat heart mitochondria in the presence of ADP. At least one target of ATM/ATR phosphorylation is de-phosphorylated in PC-3 prostate cancer cells in the presence of Iej B, suggesting a block in cell cycle progression at the intra-S-phase checkpoint. This arrest is irreversible in the aggressive PC-3 cell line. Iej A/B induce S-phase arrest in several cell lines regardless of p53 status, suggesting that this effect of Iej A/B is p53-independent. CHO-A8 cells, which re-duplicate their centrioles under conditions of prolonged S-phase arrest, maintain normal centriole numbers when arrested by Iej A/B. Iejimalides appear to have unique effects on cancer cells and should be further developed as anti-cancer agents.