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% i ncrease in [H-3]serine labelling of PtdSer. While fumonisin (20 mu M) caused sustained inhibition of sphingomyelin synthesis, PtdEtn labelli ng peaked at 7-fold above controls at 12 h and declined to 4-fold by 2 4 h. Fumonisin treatment for 12 h increased the in vitro activity of P tdSer synthase by 62% and inhibited PtdSer decarboxylase by 35%, sugge sting that increased PtdEtn labelling by [H-3]serine is not by this pa thway. An ethanolamine 'trap' experiment was performed to assess the c ontribution of phosphoethanolamine from sphinganine degradation for Pt dEtn labelling. Stimulation of [H-3]serine incorporation into PtdEtn b y fumonisin could be reduced by 60% with the inclusion of 50 mu M unla belled ethanolamine in the culture medium. The ethanolamine-mediated r eduction 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]pho sphoethanolamine to accumulate. Consistent with elevated phosphoethano lamine production in fumonisin-treated cells, [H-3]ethanolamine incorp oration into PtdEtn was inhibited by 75% after 12 h. The degradation o f endogenous long-chain bases to phosphoethanolamine and entry into th e CDP-ethanolamine pathway appears to be a major pathway for PtdEtn sy nthesis in fumonisin-treated CHO-K1 cells.