Effect of fumonisin B-1 on phosphatidylethanolamine biosynthesis in Chinese hamster ovary cells

Fumonisin B-1 has been shown to inhibit dihydroceramide synthesis and elevate cellular sphinganine levels in several cultured cell lines. In Chinese hamster ovary (CHO)-K1 cells, 20 mu M fumonisin B-1 inhibited sphingomyelin synthesis by 75% after 5 h, but stimulated [H-3]serine incorporation into PtdEtn by 5- to 7-fold. Fumonisin caused a 10-20% increase in [H-3]serine labelling of PtdSer. While fumonisin (20 mu M) caused sustained inhibition of sphingomyelin synthesis, PtdEtn labelling peaked at 7-fold above controls at 12 h and declined to 4-fold by 24 h. Fumonisin treatment for 12 h increased the in vitro activity of PtdSer synthase by 62% and inhibited PtdSer decarboxylase by 35%, suggesting that increased PtdEtn labelling by [H-3]serine is not by this pathway. An ethanolamine 'trap' experiment was performed to assess the contribution of phosphoethanolamine from sphinganine degradation for PtdEtn labelling. Stimulation of [H-3]serine incorporation into PtdEtn by fumonisin could be reduced by 60% with the inclusion of 50 mu M unlabelled ethanolamine in the culture medium. The ethanolamine-mediated reduction in [H-3]serine incorporation into PtdEtn was accompanied by 4-fold increase in cellular [H-3]phosphoethanolamine. In control cells labelled with [H-3]serine, 50 mu M ethanolamine did not cause [H-3]phosphoethanolamine to accumulate. Consistent with elevated phosphoethanolamine production in fumonisin-treated cells, [H-3]ethanolamine incorporation into PtdEtn was inhibited by 75% after 12 h. The degradation of endogenous long-chain bases to phosphoethanolamine and entry into the CDP-ethanolamine pathway appears to be a major pathway for PtdEtn synthesis in fumonisin-treated CHO-K1 cells.