Deletion of thrombin receptor PAR4 improves metabolic status and shows less severe NAFLD in mice fed high-fat diet

Question:

The main risk factors for developing non alcoholic fatty liver disease (NAFLD) are overweight, diabetes, and metabolic syndrome. About 30 – 50% of type II diabetics suffer from NAFLD. Findings suggest that the development of NAFLD is not only an accompanying phenomenon of type II diabetes. Rather, the increased accumulation of fats in the hepatocytes appears to be directly related to the development of insulin resistance and pathological glucose tolerance. Accordingly, recent findings suggest that the NAFLD plays a pathogenically relevant role in the development of type II diabetes. The molecular mechanisms underlying this observation are poorly understood, some studies suggest that certain messengers (adipokines) are of particular importance in this context. 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:

Six week-old male PAR4-/- mice and wildtype controls (C57Bl/6J) were fed for up to 8 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.

Results:

We reveal that HFD-fed wildtype mice showed 10-fold upregulated PAR4 expression, particularly in WAT, compared to chow-fed controls. In PAR4-deficient mice, HFD resulted in less weight gain, lower body fat mass and smaller WAT fat pads than in wildtype mice. Average adipocyte areas in both WAT and BAT was also reduced in PAR4-/- vs. wildtype mice fed HFD, as were CLS by 85%. HFD-induced impairment of glucose tolerance and insulin sensitivity was significantly worse in wildtype vs. PAR4-/- mice. These alterations coincided with a significant reduction of lipid accumulation in the liver in PAR4-/- mice. Both in liver and WAT expression of the proinflammatory cytokine MCP-1 is downregulated in PAR4-/- mice compared to wildtype controls.

Conclusion:

This preliminary study highlights PAR4 as a candidate regulator of adipose tissue inflammation and metabolic dysfunction in the development of diabetes and NAFLD.