Background & Aims:
Overweight, diabetes, and metabolic syndrome are the main risk factors for developing non alcoholic fatty liver disease (NAFLD). About one-third to one-half of type II diabetics suffer from NAFLD. Recent investigations revealed that the development of NAFLD is not only an accompanying phenomenon of type II diabetes. The increased accumulation of fats in the hepatocytes appears to be rather directly related to the development of insulin resistance and pathological glucose tolerance. NAFLD plays therefore a pathogenically relevant role in the development of type II diabetes. Underlying this observation the molecular mechanisms are poorly understood. The G protein-coupled thrombin receptor PAR4 is expressed on various cell types, e.g. thrombocytes, endothelial cells, smooth muscle cells, but also on adipocytes. Increased thrombin activity in adipose tissue has been associated with inflammatory macrophage recruitment and development of insulin resistance in mice. The contribution of PAR4 to metabolic and inflammatory changes in the course of diabetes and NAFLD development has not been reported to date.
Methods:
Eight week-old male PAR4-/- mice and wildtype controls (C57Bl/6J) were fed for up to 30 weeks with a high-fat diet (HFD) or standard chow. Body weight was recorded weekly. For glucose tolerance and insulin sensitivity tests mice were challenged p.o. with glucose (1 g/kg) or i.p. insulin (0.75 U/kg) after a 6h fast, followed by measurement of tail vein blood glucose. PBS-perfused liver was weighed and snap-frozen at -80 °C for RNA analysis by real time PCR or fixed in formaldehyde for histochemical analysis. The development of insulin resistance and NAFLD was monitored after 8, 12, 16, 20, 26 and 30 weeks.
Results:
High fat diet results in significant weight gain and impaired glucose tolerance and insulin sensitivity in mice within 30 weeks of feeding. PAR4-deficient mice showed delayed weight gain until 16 weeks of feeding. After this week the weight gain of these mice is higher than the weight gain of the PAR4-proficient mice and is still after 30 weeks of high-fat feeding. Improved glucose tolerance was observed after 6 and 12 weeks, which was no longer observed after 18 weeks of feeding. Insulin sensitivity was comparable. The PAR4-deficient animals were more sensitive to insulin administration after feeding for 7 and 14 weeks, later they showed similar insulin resistance after 30 weeks of feeding as the PAR4-competent animals fed a high-fat diet.
Conclusion:
The data suggest that PAR4 is involved in the development of insulin resistance and NAFLD. The influence does not seem to be consistent. Initially, PAR4 activation promotes weight gain. This promotion seems to be alleviated after 16 weeks of feeding, later PAR4 activation leads to decreased weight gain. These observations are reflected in glucose tolerance and insulin sensitivity data.
