OBJECTIVE:To characterize the developmental changes in adipose and muscle lipoprotein lipase (LPL) activity in the atherosclerosis-prone JCR:LA-corpulent rat, and to test the hypothesis that tissue-specific abnormalities in LPL activity precede the establishment of obesity. DESIGN: Lean ( þ =?) and obese cp=cp male JCR:LA rats were studied at 4, 5 and 8 weeks of age, that is at the onset of obesity, and at a time when obesity is well established. Assessment was made of plasma variables related to glucose and lipid metabolism and of LPL activity in several adipose depots, skeletal muscles and the heart. RESULTS: At week 4, body weights were identical in both genotypes and began to diverge at week 5. Eight-week-old cp=cp rats weighed 35% more than their lean counterparts. Perirenal and epididymal adipose depot weights were also identical in both genotypes at week 4 and began to increase in cp=cp rats at week 5, whereas the subcutaneous depot of 4-week-old cp=cp rats was slightly enlarged. At week 4, the cp=cp rats were hyperinsulinemic (5-fold), hyperleptinemic (30-fold) and hypertriglyceridemic (3-fold) compared to their lean counterparts, and their liver contained twice as much triglyceride. The 4-week-old cp=cp rats displayed 2 -7-fold higher LPL specific activity in the various adipose depots compared to lean rats, and enzyme activity remained higher in obese than in lean rats at all subsequent ages. In contrast, LPL activity in the vastus lateralis, gastrocnemius and heart muscles of 4-week-old obese rats was approximately half that observed in lean animals. CONCLUSION: Profound, persistent alterations in the tissue-specific modulation of LPL activity are established in the JCR:LA cp=cp rat prior to the development of frank obesity. The increase in adipose tissue LPL activity and its decrease in muscle tissues are likely to be related to the concomitant alterations in insulinemia and triglyceridemia, respectively. The pre-obesity, tissuespecific alterations in LPL activity may be considered as an integrated adaptation to increased lipid flux aimed at driving lipids toward storage sites and limiting their uptake by triglyceride-laden muscles.

OBJECTIVE:To investigate mechanisms for the regulation of glucose incorporation into triacylgycerols in adipocytes by ceramides, which mediate some actions of tumour necrosis factor-alpha (TNFa). DESIGN: The effects of C 2 -and C 6 -ceramides (N-acetyl-and N-hexanoyl-sphingosines, respectively) on glucose uptake and incorporation into triacylglycerols and pathways of signal tansduction were measured in 3T3-L1 adipocytes. RESUTLS: C 6 -ceramide increased basal 2-deooxyglucose uptake but decreased insulin-stimulated uptake without changing the EC 50 for insulin. Incubating 3T3-L1 adipocytes from 2 to 24 h with C 2 -ceramide progressively increased glucose incorporation into the fatty acid and especially the glycerol moieties of triacylglycerol. These effects were accompanied by increased GLUT1 synthesis resulting from ceramide-induced activation phosphatidylinositol 3-kinase, ribosomal S6 kinase and mitogen-activated protein kinase. C 2 -ceramide also increased p21-activated kinase and protein kinase B activities. However, C 2 -ceramide decreased the insulin-stimulated component of these signalling pathways and also glucose incorporation into triacylglycerol after 2 h. CONCLUSIONS: Cell-permeable ceramides can mimic some effects of TNFa in producing insulin resistance. However, ceramides also mediate long-term effects that enable 3T3 L1 adipocytes to take up glucose and store triacylglycerols in the absence of insulin. These observations help to explain part of the nature and consequence of TNFa-induced insulin resistance and the control of fat accumulation in adipocytes in insulin resistance and obesity.

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Hepatic Mg2 + -dependent phosphatidate phosphohydrolase (PAP-I) is involved in the de now synthesis of triacylglycerols, phosphatidylcholine and phosphatidylethanolamine. The biosynthesis of phospholipids takes precedence over that of triacylglycerols when the rate of synthesis of diacylglycerol is low. This ensures that membrane turnover and bile secretion are maintained. PAP-1 is located in the cytosol but for metabolic function it translocates to the endoplasmic reticulum. Such translocation is stimulated by high intrecellular fatty acid concentrations and this is believed to be a major point of regulation of the synthesis of hiacylglycerol and phospholipids Ill.Considerable interest has been centred on the hamster as a model for studying lipoprotein metabolism. This is because cholesterol metabolism in this species resembles that of humans in a number of respects, including the respoase to dietary cholesterol and fat 121. Far less interest has focused on Eriacylglycerol metabolism. although Ontko et al I31 have reported an apparent hypertriglyceridaemia in hamsters, relative to rats. This isassociated w i t h iacreased incorporation of nonesterified fatty acids into triacylglycerol and increased hepatic production of VLDL. In the present study we have looked at the effect of dietary cholesterol on plasma VLDL concentrations and the activity of PAP-1.24 Male Golden Syrian hamsters, 3-4 months old, were fed diets containing O,O.M,O. 12 or 0.24% (wlw) cholesterol, for 28 days. At the end of the trial, fasting blood was collected by cardiac plncture and livers were removed and frozen prior to measuring PAP-1 activity. Lipoproteins were separated from serum by preparative ultracentrifugation and VLDL cholesterol and triacylglycerol was measured enzymatically. Serum nonesterified fatty acids were also measured by an enzymatic method. PAP-I activity was measured by the method of Martin et a1 141 and is defmed as n-ethylmaleimide sensitive, Mg2+ -Stimulated PAP activity. Table 1 summarizes our findings. A strong correlation was found between VLDL triacylglycerol concentrations and dietary cholesterol (r = 0.981). Further correlations were also seen between dietary cholesterol and serum non-esterified fatty acids (r = 0.999) and hepatic PAP-1 (r = 0.971). VLDL triacylglycerol and PAP-I were also highly correlated (r = 0.861).Finally, a strong correlation between VLDL cholesterol and triacylglycerol was also found (r = 0.977).These results suggest that cholesterol feeding induces hypertriglyceridemia in hamsters. This i?mease in serum VLDL triacylglycerol is associated with increases in serum nonesterified fatty acids and hepatic PAP-I activity. The reason for the increase in non-esterified fatty acids is unclear. It may be that cholesterol feeding induces an insulin resistant state and that this results in increased lipolytic activity in adipose tissue. It seems likely that PAP-I activity is increased in response to the elevated fatty acid levels and this results in increased synthesis and secretion of triacylglycerol...