Inhibition of Sphingolipid Biosynthesis in Rat Primary Hepatocyte Cultures by Fumonisin B 1 and Other Structurally Related Compounds

Abstract

The fumonisins and toxins produced by Alternaria alternata f. sp. lycopersici (AAL toxins) are structurally related mycotoxins that disrupt sphingolipid biosynthesis by inhibiting the rate-limiting enzyme, ceramide synthase. Rat primary hepatocytes were exposed to fumonisin B 1 (FB 1), its N-acetyl analogue, FA 1, its fully hydrolysed analogue, AP 1 and the AAL toxins (TA and TB) at concentrations of 1 μ m for 40 hr in culture. The extent to which these compounds disrupt sphingolipid biosynthesis in hepatocytes in vitro was investigated by analysing the sphingosine (So) and sphinganine (Sa) levels by HPLC. The inhibition of ceramide synthase was irreversible as the Sa:So ratio was maximally increased by FB 1 after 24 hr of exposure and the subsequent removal of FB 1 had no effect on the ratio as compared with the 40-hr incubation period in the presence of FB 1. The Sa concentration was significantly ( P<0.01) increased in all the cultures treated with the different structurally related compounds, while only AP 1 increased the So concentration significantly ( P<0.05) above the control. As AP 1 was found to be less effective in disrupting sphingolipid biosynthesis it would appear that the tricarballylic (TCA) moiety is required for maximal inhibition of ceramide synthase. The presence of an amino group appears not to be a requisite for activity, since FA 1 increased the Sa:So ratio to the same extent as FB 1. The AAL toxins TA and TB increased the Sa concentration significantly ( P<0.01) above that of FB 1 and FA 1, while the Sa:So ratios were altered to the same extent. The structural requirements for the induction of cytotoxicity differ from those required for ceramide synthase inhibition as TA and TB were significantly ( P<0.05 to P<0.01) less toxic to primary hepatocytes than FB 1 at all the concentrations tested.