Induction of apoptosis in cultured human proximal tubule cells by fumonisins and fumonisin metabolites.

Fumonisin B1 (FB1) causes apoptosis in a variety of cell types and tissues but the apoptotic potential of other fumonisins and fumonisin metabolites has not been determined and the underlying mechanisms are not completely understood. In our studies we exposed human proximal tubule-derived cells (IHKE cells) to FB1, fumonisin B2 (FB2), fumonisin B3 (FB3), hydrolyzed fumonisin B1 (HFB1) and N-palmitoyl-hydrolyzed fumonisin B1 (N-Pal-HFB1) and investigated caspase-3 activation, chromatin condensation and DNA fragmentation. Exposure to 10 micromol/L FB1 for 24 h led to a significant increase in caspase-3 activity, chromatin condensation and to DNA fragmentation. All other tested compounds did not show any significant activation of caspase-3 activity nor chromatin condensation and DNA-fragmentation. Furthermore, we examined if a sphinganine accumulation is correlated with an induction of apoptosis in IHKE cells. Therefore we used a liquid chromatography/electrospray ionization-mass spectrometry(LC/ESI-MS)-method using phytosphingosine as an internal standard to determine sphinganine and sphingosine concentrations in IHKE cells. Whereas a significant increase of sphinganine (up to 7000% compared to control cells) was observed with all fumonisin-derivates, sphingosine levels nearly remained unchanged indicating that all substrates inhibited ceramide synthase effectively. These results demonstrate that all compounds let to increased sphinganine levels in IHKE cells but only FB1 was able to induce apoptosis. We conclude that the inhibition of the ceramide synthase is not per se a predictor whether or not fumonisins induce apoptosis.