Recent studies have established that vitamin D plays multiple biological roles beyond calcium metabolism; however, whether vitamin D is involved in energy metabolism is unknown. To address this question, we characterized the metabolic phenotypes of vitamin D receptor (VDR)-null mutant mice. Under a normocalcemic condition, VDR-null mice displayed less body fat mass and lower plasma triglyceride and cholesterol levels compared with wild-type (WT) mice; when placed on a high-fat diet, VDR-null mice showed a slower growth rate and accumulated less fat mass globally than WT mice, even though their food intake and intestinal lipid transport capacity were the same as WT mice. Consistent with the lower adipose mass, plasma leptin levels were lower and white adipocytes were histologically smaller in VDR-null mice than WT mice. The rate of fatty acid beta-oxidation in the white adipose tissue was higher, and the expression of uncoupling protein (UCP) 1, UCP2 and UCP3 was markedly upregulated in VDR-null mice, suggesting a higher energy expenditure in the mutant mice. Experiments using primary brown fat culture confirmed that 1,25-dihydroxyvitamin D3 directly suppressed the expression of the UCPs. Consistently, the energy expenditure, oxygen consumption, and CO2 production in VDR-null mice were markedly higher than in WT mice. These data indicate that vitamin D is involved in energy metabolism and adipocyte biology in vivo in part through regulation of beta-oxidation and UCP expression.

g BNip3 localizes to the outer mitochondrial membrane, where it functions in mitophagy and mitochondrial dynamics. While the BNip3 protein is constitutively expressed in adult liver from fed mice, we have shown that its expression is superinduced by fasting of mice, consistent with a role in responses to nutrient deprivation. Loss of BNip3 resulted in increased lipid synthesis in the liver that was associated with elevated ATP levels, reduced AMP-regulated kinase (AMPK) activity, and increased expression of lipogenic enzymes. Conversely, there was reduced ␤-oxidation of fatty acids in BNip3 null liver and also defective glucose output under fasting conditions. These metabolic defects in BNip3 null liver were linked to increased mitochondrial mass and increased hepatocellular respiration in the presence of glucose. However, despite elevated mitochondrial mass, an increased proportion of mitochondria exhibited loss of mitochondrial membrane potential, abnormal structure, and reduced oxygen consumption. Elevated reactive oxygen species, inflammation, and features of steatohepatitis were also observed in the livers of BNip3 null mice. These results identify a role for BNip3 in limiting mitochondrial mass and maintaining mitochondrial integrity in the liver that has consequences for lipid metabolism and disease. Modulation of mitochondrial mass is emerging as a major adaptive response to changes in energy balance arising from deficiencies in oxygen or glucose availability, among other nutrient stresses. For example, nutrient-sensitive changes in PGC-1␣ activity alter expression of genes required for mitochondrial biogenesis, in addition to genes required for fatty acid metabolism (17, 38). While mitochondrial biogenesis increases mitochondrial mass, this is countered by the role of mitophagy in targeting dysfunctional mitochondria for degradation at the autophagosome, resulting in reduced mitochondrial mass (28,29,70). Defects in autophagy have been linked to liver cancer (25,44,65) and have also been shown to promote hepatic insulin resistance (19, 67). However, this cannot be attributed to defective mitochondrial function, since autophagy-deficient liver also exhibits increased endoplasmic reticulum (ER) stress (67), protein aggregation (31), and defective lipidophagy (59). To date, a specific role for mitophagy in preventing hepatic steatosis or other liver pathologies has not been identified.Hypoxia modulates mitochondrial mass through both decreasing mitochondrial biogenesis (74) and increasing mitophagy (3,64,73). These effects are mediated by hypoxia-inducible factor (HIF) transcription factors, acting on the one hand to inhibit Myc-induced expression of PGC-1␤ (74) and on the other to induce expression of the mitochondrial proteins BNIP3 and NIX (3,4,64,73). Initial functional characterization of BNIP3 and NIX indicated that these proteins were loosely conserved members of the BH3-only subgroup of the Bcl-2 family of cell death regulators (7,8,52,68), and indeed, evidence from ischemia-reperfusion injury expe...

Our previous studies demonstrated a high fat diet-resistant lean phenotype of vitamin D receptor (VDR)-null mutant mice mainly due to increased energy expenditure, suggesting an involvement of the VDR in energy metabolism. Here, we took a transgenic approach to further define the role of VDR in adipocyte biology. We used the aP2 gene promoter to target the expression of the human (h) VDR in adipocytes in mice. In contrast to the VDR-null mice, the aP2-hVDR Tg mice developed obesity compared with the wild-type counterparts without changes in food intake. The increase in fat mass was mainly due to markedly reduced energy expenditure, which was correlated with decreased locomotive activity and reduced fatty acid ␤-oxidation and lipolysis in the adipose tissue in the transgenic mice. Consistently, the expression of genes involved in the regulation of fatty acid transport, thermogenesis, and lipolysis were suppressed in the transgenic mice. Taken together, these data confirm an important role of the VDR in the regulation of energy metabolism.The maintenance of body weight depends on the balance between energy intake and energy utilization. Obesity results when energy consumed exceeds energy utilized. Energy is acquired through diets and can be stored in adipose tissue or utilized by the body to maintain basic cellular functions and physical activities. Energy can also be used for adaptive thermogenesis in response to a cold environment (1). The adipose tissue is unique in that it represents both arms of energy balance. The white adipose tissue (WAT) 2 has the ability to sense the energy state of the body. When energy availability is high, the WAT stores the excess energy as triglyceride in lipid droplets.When energy is needed, triglyceride is broken down to free fatty acids to release into the circulation. This process, known as lipolysis, is regulated by two enzymes, adipose triglyceride lipase (ATGL), and hormone-sensitive lipase (HSL). ATGL initiates the first rate-limiting step of lipolysis by hydrolyzing triglyceride to diacylglyceride (2, 3), which is further broken down to monoglyceride by HSL (4). Monoglyceride lipase cleaves the final ester bond of monoglyceride to release glycerol, and this step is not rate-limiting (5). Lipolysis is activated by catacholamines through the cAMP signaling pathway, leading to protein kinase A activation. Protein kinase A phosphorylates HSL, which promotes HSL translocation to the lipid droplet and access to triglyceride stores (6). HSL and ATGL activity is suppressed by insulin during feeding, as insulin increases the amount of perilipin around the lipid droplets to prevent their access to triglycerides (7).The principal role of the brown adipose tissue (BAT) is to regulate adaptive thermogenesis through the expression of uncoupling proteins (UCPs). UCP1 separates oxidative phosphorylation from ATP production to release energy as heat (8 -10). Studies have shown that increased expression of UCP1 in the BAT or its ectopic expression in the WAT results in increased metabolism...

Dioxins and dioxin-like compounds encompass a group of structurally related heterocyclic compounds that bind to and activate the aryl hydrocarbon receptor (AhR). The prototypical dioxin is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a highly toxic industrial byproduct that incites numerous adverse physiological effects. Global commercial production of the structurally similar polychlorinated biphenyls (PCBs), however, commenced early in the 20th century and continued for decades; dioxin-like PCBs therefore contribute significantly to total dioxin-associated toxicity. In this study, PCB 126, the most potent dioxin-like PCB, was evaluated with respect to its direct effects on hepatic glucose metabolism using primary mouse hepatocytes. Overnight treatment with PCB 126 reduced hepatic glycogen stores in a dose-dependent manner. Additionally, PCB 126 suppressed forskolin-stimulated gluconeogenesis from lactate. These effects were independent of acute toxicity, as PCB 126 did not increase lactate dehydrogenase release nor affect lipid metabolism or total intracellular ATP. Interestingly, provision of cells with glycerol instead of lactate as the carbon source completely restored hepatic glucose production, indicating specific impairment in the distal arm of gluconeogenesis. In concordance with this finding, PCB 126 blunted the forskolin-stimulated increase in phosphoenolpyruvate carboxykinase (PEPCK) mRNA levels without affecting glucose-6-phosphatase expression. Myricetin, a putative competitive AhR antagonist, reversed the suppression of PEPCK induction by PCB 126. Furthermore, other dioxin-like PCBs demonstrated similar effects on PEPCK expression in parallel with their ability to activate AhR. It therefore appears that AhR activation mediates the suppression of PEPCK expression by dioxin-like PCBs, suggesting a role for these pollutants as disruptors of energy metabolism.

High glucose (HG) has been shown to induce insulin resistance in both type 1 and type 2 diabetes. However, the molecular mechanism behind this phenomenon is unknown. Insulin receptor substrate (IRS) proteins are the key signaling molecules that mediate insulin's intracellular actions. Genetic and biological studies have shown that reductions in IRS1 and/or IRS2 protein levels are associated with insulin resistance. In this study we have shown that proteasome degradation of IRS1, but not of IRS2, is involved in HG-induced insulin resistance in Chinese hamster ovary (CHO) cells as well as in primary hepatocytes. To further investigate the molecular mechanism by which HG induces insulin resistance, we examined various molecular candidates with respect to their involvement in the reduction in IRS1 protein levels. In contrast to the insulin-induced degradation of IRS1, HG-induced degradation of IRS1 did not require IR signaling or phosphatidylinositol 3-kinase/Akt activity. We have identified glycogen synthase kinase 3b (GSK3b or GSK3B as listed in the MGI Database) as a kinase required for HG-induced serine 332 phosphorylation, ubiquitination, and degradation of IRS1. Overexpression of IRS1 with mutation of serine 332 to alanine partially prevents HG-induced IRS1 degradation. Furthermore, overexpression of constitutively active GSK3b was sufficient to induce IRS1 degradation. Our data reveal the molecular mechanism of HG-induced insulin resistance, and support the notion that activation of GSK3b contributes to the induction of insulin resistance via phosphorylation of IRS1, triggering the ubiquitination and degradation of IRS1.