Obesity and type 2 diabetes are related metabolic disorders of high prevalence. The constitutive androstane receptor (CAR) was initially characterized as a xenobiotic receptor regulating the responses of mammals to xenotoxicants. In this study, we have uncovered an unexpected role of CAR in preventing obesity and alleviating type 2 diabetes. Using a high fat diet (HFD)-induced obesity model, we showed that treatment of wild type mice with the CAR agonist 1,4-bis[2-(3,5 dichloropyridyloxy)] benzene (TCPOBOP) efficiently prevented obesity from happening or reversed preinduced obesity. Treatment with TCPOBOP improved insulin sensitivity in both the HFD-induced type 2 diabetic model and the ob/ob mice. In contrast, CAR null mice maintained on a chow diet showed spontaneous insulin insensitivity, which cannot be relieved by TOPOBOP treatment. The hepatic steatosis in HFD-treated mice and ob/ob mice was markedly reduced by the TCPOBOP treatment. The metabolic benefits of CAR activation may have resulted from the combined effect of inhibition of lipogenesis, very low density lipoprotein secretion and export of triglycerides, and gluconeogenesis as well as increases in brown adipose tissue energy expenditure and peripheral fat mobilization. Moreover, the skeletal muscle of CAR-activated mice showed a decreased incomplete oxidation, despite having a lower expression level of peroxisome proliferator-activated receptor ␣ and its target genes involved in fatty acid oxidation. In summary, our results have revealed an important metabolic function of CAR and may establish this "xenobiotic receptor" as a novel therapeutic target for the prevention and treatment of obesity and type 2 diabetes.

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MicroRNAs comprise a group of non-coding small RNAs (17-25 nt) involved in post-transcriptional regulation that have been identified in various plants and animals. Studies have demonstrated that miRNAs are associated with stem cell self-renewal and differentiation and play a key role in controlling stem cell activities. However, the identification of specific miRNAs and their regulatory roles in the differentiation of multipotent mesenchymal stromal cells (MSCs) have so far been poorly defined. We isolated and cultured human MSCs and osteo-differentiated MSCs from four individual donors. miRNA expression in MSCs and osteo-differentiated MSCs was investigated using miRNA microarrays. miRNAs that were commonly expressed in all three MSC preparations and miRNAs that were differentially expressed between MSCs and osteo-differentiated MSCs were identified. Four underexpressed (hsa-miR-31, hsa-miR-106a, hsa-miR-148a, and hsa-miR-424) and three novel overexpressed miRNAs (hsa-miR-30c, hsa-miR-15b, and hsa-miR-130b) in osteo-differentiated MSCs were selected and their expression were verified in samples from the fourth individual donors. The putative targets of the miRNAs were predicted using bioinformatic analysis. The four miRNAs that were underexpressed in osteo-differentiated MSCs were predicted to target RUNX2, CBFB, and BMPs, which are involved in bone formation; while putative targets for miRNAs overexpressed in osteo-differentiated MSCs were MSC maker (e.g., CD44, ITGB1, and FLT1), stemness-maintaining factor (e.g., FGF2 and CXCL12), and genes related to cell differentiation (e.g., BMPER, CAMTA1, and GDF6). Finally, hsa-miR-31 was selected for target verification and function analysis. The results of this study provide an experimental basis for further research on miRNA functions during osteogenic differentiation of human MSCs.

The pregnane X receptor (PXR), along with its sister receptor constitutive androstane receptor (CAR), was initially characterized as a xenobiotic receptor that regulates drug metabolism. In this study, we have uncovered an unexpected endobiotic role of PXR in obesity and type 2 diabetes. PXR ablation inhibited high-fat diet (HFD)–induced obesity, hepatic steatosis, and insulin resistance, which were accounted for by increased oxygen consumption, increased mitochondrial β-oxidation, inhibition of hepatic lipogenesis and inflammation, and sensitization of insulin signaling. In an independent model, introducing the PXR−/− allele into the ob/ob background also improved body composition and relieved the diabetic phenotype. The ob/ob mice deficient of PXR showed increased oxygen consumption and energy expenditure, as well as inhibition of gluconeogenesis and increased rate of glucose disposal during euglycemic clamp. Mechanistically, the metabolic benefits of PXR ablation were associated with the inhibition of c-Jun NH2-terminal kinase activation and downregulation of lipin-1, a novel PXR target gene. The metabolic benefit of PXR ablation was opposite to the reported prodiabetic effect of CAR ablation. Our results may help to establish PXR as a novel therapeutic target, and PXR antagonists may be used for the prevention and treatment of obesity and type 2 diabetes.

Glucocorticoids and estrogens are two classes of steroid hormones that have essential but distinct physiologic functions. Estrogens also represent a risk factor for breast cancer. It has been suggested that glucocorticoids can attenuate estrogen responses, but the mechanism by which glucocorticoids inhibit estrogenic activity is unknown. In this study, we show that activation of glucocorticoid receptor (GR) by dexamethasone (DEX) induced the expression and activity of estrogen sulfotransferase (SULT1E1 or EST), an enzyme important for the metabolic deactivation of estrogens, because sulfonated estrogens fail to activate the estrogen receptor. Treatment with DEX lowered circulating estrogens, compromised uterine estrogen responses, and inhibited estrogen-dependent breast cancer growth in vitro and in a xenograft model. We further showed that the mouse and human SULT1E1 genes are transcriptional targets of GR and deletion of Sult1e1/Est in mice abolished the DEX effect on estrogen responses. These findings have revealed a novel nuclear receptor-mediated and metabolism-based mechanism of estrogen deprivation, which may have implications in therapeutic development for breast cancers. Because glucocorticoids and estrogens are widely prescribed drugs, our results also urge caution in avoiding glucocorticoidestrogen interactions in patients. [Cancer Res 2008;68(18):7386-93]

The pregnane X receptor (PXR) and the constitutive androstane receptor (CAR) are two closely related and liver-enriched nuclear hormone receptors originally defined as xenobiotic receptors. Recently, an increasing body of evidence suggests that PXR and CAR also have endobiotic functions that impact glucose and lipid metabolism, as well as the pathogenesis of metabolic diseases. These new findings suggest that PXR and CAR not only regulate the transcription of drug-metabolizing enzymes and transporters, but also orchestrate energy metabolism and immune responses to accommodate stresses caused by xenobiotic exposures. The effectiveness of targeting PXR and CAR in the treatment of metabolic disorders, such as obesity, type 2 diabetes (T2D), dyslipidemia, and atherosclerosis, have been suggested in animal models. However, translation of these basic research results into clinical applications may require further investigation to determine the human relevance, and to obtain better understanding of the mechanisms through which PXR and CAR affect energy metabolism. Given a wide variety of natural or synthetic compounds that are PXR and CAR modulators, it is hoped that these two “xenobiotic receptors” can be harnessed for therapeutic potentials in managing metabolic diseases.

ABSTRACT:The pregnane X receptor (PXR) and the constitutive androstane receptor (CAR) are two closely related and liver-enriched nuclear hormone receptors originally defined as xenobiotic receptors. PXR and CAR regulate the transcription of drug-metabolizing enzymes and transporters, which are essential in protecting our bodies from the accumulation of harmful chemicals. An increasing body of evidence suggests that PXR and CAR also have an endobiotic function that impacts energy homeostasis through the regulation of glucose and lipids metabolism. Of note and in contrast, disruptions of energy homeostasis, such as those observed in obesity and diabetes, also have a major impact on drug metabolism. This review will focus on recent progress in our understanding of the integral role of PXR and CAR in drug metabolism and energy homeostasis.