We have previously reported that ingestion of mepanipyrim induces fatty liver in rats due to the inhibitory effect on the synthesis or secretion of hepatocytic very low density lipoproteins (VLDL). To clarify the mechanism by which mepanipyrim induces fatty liver, morphological and biochemical effects of mepanipyrim on the movement of VLDL in rat liver and in the primary culture of rat hepatocytes were investigated. In in vivo experiments, rats were fed for 4 days a diet containing mepanipyrim at 4,000 ppm. VLDL accumulation in the Golgi apparatus of the liver, especially in the secretory vacuoles, was observed in the treated rats and in the hepatocytes treated for 2 hr with 25 micrograms/ml mepanipyrim. Using 6-[N-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl)amino]caproyl-sphingosine (C6-NBD-ceramide), a selective staining agent for the Golgi apparatus, it was found that mepanipyrim inhibited C6-NBD-ceramide transport from the Golgi to the cell surface of cultured hepatocytes. The density of the VLDL-loaded secretory vacuoles isolated from the Golgi fractions was greater in mepanipyrim-treated rat livers compared with that in the control. Immunofluorescence micrograph of rat hepatocytes stained with anti-alpha-tubulin monoclonal antibody demonstrated that mepanipyrim neither affected microtubule network nor changed the intracellular ATP level. These results together suggested that fatty liver induced by mepanipyrim results mainly from the inhibition of the transport of hepatic VLDL from the Golgi to the cell surface. The inhibition of the transport of hepatic VLDL appears to result from qualitative changes in VLDL such as alteration of the apoprotein composition and/or insufficient lipidation of VLDL.
Copyright 1999 Academic Press.