PXR activation was achieved by intraperitoneal injections of pregnenolone 16 a-carbonitrile, a mouse specific PXR activator. In AKR/J mice we demonstrate that PXR activation is capable of regulating lipid metabolism and energy expenditure, and consequently, preventing the development of high-fat diet-induced MK-4827 obesity and insulin resistance. They are also more insulin resistant and therefore, are commonly used as a model for research on diet-induced obesity and obesity-related insulin resistance. As shown in Figure 1A, PCN treatment did not affect the growth rate of mice on regular chow. However, for animals fed with high-fat diet, PCN treatment resulted in a significant decrease in growth rate as compared to those treated with DMSO. Results shown in Figures 4 and 6 demonstrate that PCNmediated PXR activation significantly prevented the lipid storage in adipose tissue and the liver. Except for inhibiting lipogenesis, a more important characteristic of PXR in diet-induced obesity was reduction of lipid uptake in liver and adipose, confirmed by inhibiting up-regulation of Cd36 expression by high-fat diet. Cd36 codes for a scavenger receptor with broad ligand specificity. Activation of Cd36 facilitates free fatty acid uptake from circulation and also contributes to obesity, hepatic steatosis and type-2 diabetes. Previous studies show that Cd36 is a shared transcriptional target of LXR, PXR and PPARc in their regulation of lipid homeostasis. In the PXR-transgenic mice, Cd36 was up-regulated when fed with regular chow, which is different from our result obtained from the high-fat diet-fed mice. In our opinion, as in drug metabolism, PXR may serve as a “sensor” for maintaining energy homeostasis. Unlike the effect of PCN in the liver and WAT, PCN treatment increased Cd36 transcript level by approximately 2-fold compared to the DMSO-treated group in BAT. The main function of BAT is to generate heat for thermogenesis. Bartelt et al. reported that increased BAT activity enhanced triglyceride-rich lipoprotein metabolism in mice. Exposure to low temperatures drastically accelerated plasma clearance of triglycerides as a result of increased uptake into BAT, which was mediated by an increase of transmembrane receptor CD36. Although it remains to be demonstrated for increased energy expenditure, mice with PCN treatment had a much slower decrease in body temperature than the control group, suggesting that the activation of PXR enhanced the thermogenesis activity in BAT. At the same time, data from real-time PCR showed a significant increase in the transcription of genes involved in fatty acid b-oxidization and thermogenesis, indicating increased energy expenditure in BAT, which contributed to cleaning more lipids in BAT. In summary, our study demonstrates that PCN-mediated PXR activation prevented diet-induced obesity in AKR/J mice, decreased lipid accumulation and maintained insulin sensitivity. Additional work is needed to illustrate the precise mechanisms through which PXR modulates energy metabolism and lipid homeostasis.