diarachidonyl-diglyceride and Choline-Deficiency

Rats fed a choline-deficient diet develop foci of enzyme-altered hepatocytes with subsequent formation of hepatic tumors. They also develop fatty livers, because choline is needed for hepatic secretion of lipoproteins. We have previously reported that 1,2-sn-diradylglycerol accumulates in the livers of rats fed a choline-deficient diet for 1-27 weeks, and that protein kinase C activity in the hepatic plasma membrane is elevated during that time (da Costa et al., J. Biol. Chem., 268, 2100-2105, 1993). In the present study, we examined the changes that occur in rat liver at 52 weeks of choline deficiency and determined whether these changes were reversible when choline was returned to the diet of the deficient animals for 1 or 16 weeks. At 52 weeks, non-tumor liver samples from the experimental animals had increased 1,2-sn-diradylglycerol concentrations in the lipid droplets compared with control animals. Plasma membrane 1,2-sn-diradylglycerol levels in the liver did not differ between the two groups, but an age-related increase in membrane 1,2-sn-diradylglycerol concentrations was observed. Unsaturated free fatty acids, another activator of protein kinase C, accumulated in the deficient livers. Protein kinase C activity associated with the plasma membrane remained significantly elevated at 52 weeks in deficient livers. Hepatic foci expressing gamma-glutamyltranspeptidase were detected only in the deficient rats (0.83% of liver volume) and 15% of these rats had hepatocellular carcinoma at 1 year on the diet. At 53 weeks (1 week after choline was returned to the deficient group), 1,2-sn-diradylglycerol concentrations in the lipid droplets and hepatic free fatty acids had dropped to control levels. By 68 weeks (16 weeks of re-feeding choline), the membrane protein kinase C activity had returned to normal. At this time, 14% of the experimental animals had hepatocellular carcinoma. We suggest that choline deficiency altered the protein kinase C-mediated signal transduction within liver and this contributed to hepatic carcinogenesis in these animals.

Topics: Aging; Animals; Cell Membrane; Choline; Choline Deficiency; Diglycerides; Fatty Acids; gamma-Glutamyltransferase; Liver; Liver Neoplasms, Experimental; Male; Phosphorylcholine; Protein Kinase C; Rats; Rats, Inbred F344; Time Factors

Choline deficiency, via deprivation of labile methyl groups, is associated with a greatly increased incidence of hepatocarcinoma in experimental animals. This dietary deficiency also causes fatty liver, because choline is needed for hepatic secretion of lipoproteins. We hypothesized that fatty liver might be associated with the accumulation of 1,2-sn-diradylglycerol and subsequent activation of protein kinase C. Several lines of evidence indicate that cancers might develop secondary to abnormalities in protein kinase C-mediated signal transduction. We observed that rats fed a choline-deficient diet for 1, 6, or 27 weeks had increased hepatic concentrations of 1,2-diradylglycerol. At 1 and 6 weeks, hepatic plasma membrane from choline-deficient rats had increased concentrations of 1,2-sn-diacylglycerol and 1-alkyl, 2-acylglycerol, with the latter accounting for 20-26% of membrane 1,2-sn-diradylglycerol (as compared with only 2-5% in controls). Protein kinase C activity was increased in hepatic plasma membrane at 1 week of choline deficiency. By Western blotting there was an increase in the amount of protein kinase C zeta and a decrease in the amount of protein kinase C delta in liver at 1 week. By 6 weeks of choline deficiency, hepatic plasma membrane and cytosolic protein kinase C (PKC) activities were increased significantly, with increased amounts of hepatic plasma membrane protein kinase C alpha, and delta detected by Western blotting. Glycogen synthase activity in liver was diminished after 1 week of choline deficiency; this enzyme is inhibited by PKC-mediated phosphorylation. We suggest that choline deficiency perturbed PKC-mediated transmembrane signaling within liver and that this contributed to the development of hepatic cancer in these animals.

Topics: Animals; Blotting, Western; Cell Membrane; Choline; Choline Deficiency; Diet; Diglycerides; Fatty Liver; Glycogen Synthase; Liver; Liver Neoplasms, Experimental; Male; Phosphorylation; Protein Kinase C; Rats; Rats, Inbred F344; Signal Transduction; Triglycerides