Maaike H. Oosterveer

Fenofibrate Simultaneously Induces Hepatic Fatty Acid Oxidation, Synthesis, and Elongation in Mice

A growing body of evidence indicates that peroxisome proliferator-activated receptor α (PPARα) not merely serves as a transcriptional regulator of fatty acid catabolism but also exerts a much broader role in hepatic lipid metabolism. We determined adaptations in hepatic lipid metabolism and related aspects of carbohydrate metabolism upon treatment of C57Bl/6 mice with the PPARα agonist fenofibrate. Stable isotope procedures were applied to assess hepatic fatty acid synthesis, fatty acid elongation, and carbohydrate metabolism. Fenofibrate treatment strongly induced hepatic de novo lipogenesis and chain elongation (±300, 150, and 600% for C16:0, C18:0, and C18:1 synthesis, respectively) in parallel with an increased expression of lipogenic genes. The lipogenic induction in fenofibrate-treated mice was found to depend on sterol regulatory element-binding protein 1c (SREBP-1c) but not carbohydrate response element-binding protein (ChREBP). Fenofibrate treatment resulted in a reduced contribution of glycolysis to acetyl-CoA production, whereas the cycling of glucose 6-phosphate through the pentose phosphate pathway presumably was enhanced. Altogether, our data indicate that β-oxidation and lipogenesis are induced simultaneously upon fenofibrate treatment. These observations may reflect a physiological mechanism by which PPARα and SREBP-1c collectively ensure proper handling of fatty acids to protect the liver against cytotoxic damage.

Peroxisome Proliferator–Activated Receptor α Improves Pancreatic Adaptation to Insulin Resistance in Obese Mice and Reduces Lipotoxicity in Human Islets

Peroxisome proliferator–activated receptor (PPAR) α is a transcription factor controlling lipid and glucose homeostasis. PPARα-deficient (−/−) mice are protected from high-fat diet–induced insulin resistance. However, the impact of PPARα in the pathophysiological setting of obesity-related insulin resistance is unknown. Therefore, PPARα−/− mice in an obese (ob/ob) background were generated. PPARα deficiency did not influence the growth curves of the obese mice but surprisingly resulted in a severe, age-dependent hyperglycemia. PPARα deficiency did not aggravate peripheral insulin resistance. By contrast, PPARα−/− ob/ob mice developed pancreatic β-cell dysfunction characterized by reduced mean islet area and decreased insulin secretion in response to glucose in vitro and in vivo. In primary human pancreatic islets, PPARα agonist treatment prevented fatty acid–induced impairment of glucose-stimulated insulin secretion, apoptosis, and triglyceride accumulation. These results indicate that PPARα improves the adaptative response of the pancreatic β-cell to pathological conditions. PPARα could thus represent a promising target in the prevention of type 2 diabetes.

The liver X receptor : Control of cellular lipid homeostasis and beyond Implications for drug design

Liver X receptor (LXR) α and β are nuclear receptors that control cellular metabolism. LXRs modulate the expression of genes involved in cholesterol and lipid metabolism in response to changes in cellular cholesterol status. Because of their involvement in cholesterol homeostasis, LXRs have emerged as promising drug targets for anti-atherosclerotic therapies. In rodents, synthetic LXR agonists promote cellular cholesterol efflux, transport and excretion. As a result, the progression of atherosclerosis is halted. However, pharmacological LXR activation also induces hepatic steatosis and promotes the secretion of atherogenic triacylglycerol-rich VLDL particles by the liver, complicating the clinical application of LXR agonists. The more recently emerged roles of LXRs in fat tissue, pituitary and brain may have implications for treatment of obesity and Alzheimer disease. In addition to the improvements in atherosclerosis, LXR activation exerts beneficial effects on glucose control in mouse models of type 2 diabetes. Future therapeutic strategies aiming to exert beneficial effects on cholesterol and glucose homeostasis, while circumventing the undesired effects on hepatic lipid metabolism, should target specific LXR-mediated processes. Therefore, tissue and/or isotype-specific effects of LXR action need to be established. The consequences of combinatorial drug approaches and the identification of the co-regulatory networks involved in the LXR-mediated control of particular genes may contribute to development of novel LXR agonists. Finally, pathway analyses of LXR actions provide tools to evaluate and optimize the effectiveness of novel therapeutic strategies to prevent and/or treat metabolic diseases.

An Increased Flux through the Glucose 6-Phosphate Pool in Enterocytes Delays Glucose Absorption in Fxr(-/-) Mice

The farnesoid X receptor (FXR) is involved in regulation of bile acid and lipid metabolism. Recently, a role for FXR in control of glucose metabolism became evident. Because FXR is expressed along the length of the small intestine, we evaluated the potential role of FXR in glucose absorption and processing. During intravenous infusion of a trace amount of d-[6,6-2H2]glucose, a d-[U-13C]glucose-enriched oral glucose bolus was given, and glucose kinetics were determined in wild-type and Fxr–/– mice. Compared with wild-type mice, Fxr–/– mice showed a delayed plasma appearance of orally administered glucose. Multicompartmental kinetic modeling revealed that this delay was caused by an increased flux through the glucose 6-phosphate pool in enterocytes. Thus, our results show involvement of FXR in intestinal glucose absorption, representing a novel physiological function for this nuclear receptor.

Lxrα Deficiency Hampers the Hepatic Adaptive Response to Fasting in Mice

Besides its well established role in control of cellular cholesterol homeostasis, the liver X receptor (LXR) has been implicated in the regulation of hepatic gluconeogenesis. We investigated the role of the major hepatic LXR isoform in hepatic glucose metabolism during the feeding-to-fasting transition in vivo. In addition, we explored hepatic glucose sensing by LXR during carbohydrate refeeding. Lxrα-/- mice and their wild-type littermates were subjected to a fasting-refeeding protocol and hepatic carbohydrate fluxes as well as whole body insulin sensitivity were determined in vivo by stable isotope procedures. Lxrα-/- mice showed an impaired response to fasting in terms of hepatic glycogen depletion and triglyceride accumulation. Hepatic glucose 6-phosphate turnover was reduced in 9-h fasted Lxrα-/- mice as compared with controls. Although hepatic gluconeogenic gene expression was increased in 9-h fasted Lxrα-/- mice compared with wild-type controls, the actual gluconeogenic flux was not affected by Lxrα deficiency. Hepatic and peripheral insulin sensitivity were similar in Lxrα-/- and wild-type mice. Compared with wild-type controls, the induction of hepatic lipogenic gene expression was blunted in carbohydrate-refed Lxrα-/- mice, which was associated with lower plasma triglyceride concentrations. Yet, expression of “classic” LXR target genes Abca1, Abcg5, and Abcg8 was not affected by Lxrα deficiency in carbohydrate-refed mice. In summary, these studies identify LXRα as a physiologically relevant mediator of the hepatic response to fasting. However, the data do not support a role for LXR in hepatic glucose sensing.

LRH-1-dependent programming of mitochondrial glutamine processing drives liver cancer

Various tumors develop addiction to glutamine to support uncontrolled cell proliferation. Here we identify the nuclear receptor liver receptor homolog 1 (LRH-1) as a key regulator in the process of hepatic tumorigenesis through the coordination of a noncanonical glutamine pathway that is reliant on the mitochondrial and cytosolic transaminases glutamate pyruvate transaminase 2 (GPT2) and glutamate oxaloacetate transaminase 1 (GOT1), which fuel anabolic metabolism. In particular, we show that gain and loss of function of hepatic LRH-1 modulate the expression and activity of mitochondrial glutaminase 2 (GLS2), the first and rate-limiting step of this pathway. Acute and chronic deletion of hepatic LRH-1 blunts the deamination of glutamine and reduces glutamine-dependent anaplerosis. The robust reduction in glutaminolysis and the limiting availability of α-ketoglutarate in turn inhibit mTORC1 signaling to eventually block cell growth and proliferation. Collectively, these studies highlight the importance of LRH-1 in coordinating glutamine-induced metabolism and signaling to promote hepatocellular carcinogenesis.

A systems biology approach reveals the physiological origin of hepatic steatosis induced by liver X receptor activation

Liver X receptor (LXR) agonists exert potent antiatherosclerotic actions but simultaneously induce excessive triglyceride (TG) accumulation in the liver. To obtain a detailed insight into the underlying mechanism of hepatic TG accumulation, we used a novel computational modeling approach called analysis of dynamic adaptations in parameter trajectories (ADAPT). We revealed that both input and output fluxes to hepatic TG content are considerably induced on LXR activation and that in the early phase of LXR agonism, hepatic steatosis results from only a minor imbalance between the two. It is generally believed that LXR‐induced hepatic steatosis results from increased de novo lipogenesis (DNL). In contrast, ADAPT predicted that the hepatic influx of free fatty acids is the major contributor to hepatic TG accumulation in the early phase of LXR activation. Qualitative validation of this prediction showed a 5‐fold increase in the contribution of plasma palmitate to hepatic monounsaturated fatty acids on acute LXR activation, whereas DNL was not yet significantly increased. This study illustrates that complex effects of pharmacological intervention can be translated into distinct patterns of metabolic regulation through state‐of‐the‐art mathematical modeling.—Hijmans, B. S., Tiemann, C. A., Grefhorst, A., Boesjes, M., van Dijk, T. H., Tietge, U. J. F., Kuipers, F., van Riel, N. A. W., Groen, A. K., Oosterveer, M. H. A systems biology approach reveals the physiological origin of hepatic steatosis induced by liver X receptor activation. FASEB J. 29, 1153‐1164 (2015). www.fasebj.org

Metabolic responses to long-term pharmacological inhibition of CB1-receptor activity in mice in relation to dietary fat composition

Background and objectives:The antiobesity effects of suppressed endocannabinoid signaling may rely, at least in part, on changes in lipid fluxes. As fatty acids exert specific effects depending on their level of saturation, we hypothesized that the dietary fatty acid composition would influence the outcome of treatment with a CB1-receptor antagonist (rimonabant).Methods:Mice were treated with rimonabant (10 mg kg−1 body weight per day) or vehicle while equicalorically fed either a low-fat diet (LF), a high-fat (HF) diet or an HF diet in which 10% of the saturated fatty acids (SFAs) were replaced by poly-unsaturated fatty acids (PUFA) from fish oil (FO). Food intake and body weight were registered daily. Indirect calorimetry was performed and feces were collected. After 3 weeks, mice were killed for blood and tissue collection.Results:Relative to the LF diet, the HF diet caused anticipated metabolic derangements, which were partly reversed by the HF/FO diet. The HF/FO diet, however, was most obesity-promoting despite inhibiting lipogenesis as indicated by low gene expression levels of lipogenic enzymes. On all three diets, rimonabant treatment improved metabolic derangements and led to significantly lower body weight gain than their respective controls. This latter effect appeared largest in the HF/FO group, but occurred without major changes in nutrient absorption and energy expenditure.Conclusion:The effects of chronic rimonabant treatment on body weight gain occurred irrespective of diet-induced changes in lipogenic activity, food intake and daily energy expenditure, and were, in fact, most pronounced in HF/FO mice. The effects of dietary PUFA replacement in an HF diet on expansion of adipose tissue might allow the favorable effects of dietary PUFA on dyslipidemia and hepatic steatosis. In light of other disadvantageous effects of weight gain, this might be a risky trade-off.

A Computational Model for the Analysis of Lipoprotein Distributions in the Mouse: Translating FPLC Profiles to Lipoprotein Metabolism

Disturbances of lipoprotein metabolism are recognized as indicators of cardiometabolic disease risk. Lipoprotein size and composition, measured in a lipoprotein profile, are considered to be disease risk markers. However, the measured profile is a collective result of complex metabolic interactions, which complicates the identification of changes in metabolism. In this study we aim to develop a method which quantitatively relates murine lipoprotein size, composition and concentration to the molecular mechanisms underlying lipoprotein metabolism. We introduce a computational framework which incorporates a novel kinetic model of murine lipoprotein metabolism. The model is applied to compute a distribution of plasma lipoproteins, which is then related to experimental lipoprotein profiles through the generation of an in silico lipoprotein profile. The model was first applied to profiles obtained from wild-type C57Bl/6J mice. The results provided insight into the interplay of lipoprotein production, remodelling and catabolism. Moreover, the concentration and metabolism of unmeasured lipoprotein components could be determined. The model was validated through the prediction of lipoprotein profiles of several transgenic mouse models commonly used in cardiovascular research. Finally, the framework was employed for longitudinal analysis of the profiles of C57Bl/6J mice following a pharmaceutical intervention with a liver X receptor (LXR) agonist. The multifaceted regulatory response to the administration of the compound is incompletely understood. The results explain the characteristic changes of the observed lipoprotein profile in terms of the underlying metabolic perturbation and resultant modifications of lipid fluxes in the body. The Murine Lipoprotein Profiler (MuLiP) presented here is thus a valuable tool to assess the metabolic origin of altered murine lipoprotein profiles and can be applied in preclinical research performed in mice for analysis of lipid fluxes and lipoprotein composition.

Impaired SUMOylation of nuclear receptor LRH-1 promotes nonalcoholic fatty liver disease

Hepatic steatosis is caused by metabolic imbalances that could be explained in part by an increase in de novo lipogenesis that results from increased sterol element binding protein 1 (SREBP-1) activity. The nuclear receptor liver receptor homolog 1 (LRH-1) is an important regulator of intermediary metabolism in the liver, but its role in regulating lipogenesis is not well understood. Here, we have assessed the contribution of LRH-1 SUMOylation to the development of nonalcoholic fatty liver disease (NAFLD). Mice expressing a SUMOylation-defective mutant of LRH-1 (LRH-1 K289R mice) developed NAFLD and early signs of nonalcoholic steatohepatitis (NASH) when challenged with a lipogenic, high-fat, high-sucrose diet. Moreover, we observed that the LRH-1 K289R mutation induced the expression of oxysterol binding protein-like 3 (OSBPL3), enhanced SREBP-1 processing, and promoted de novo lipogenesis. Mechanistically, we demonstrated that ectopic expression of OSBPL3 facilitates SREBP-1 processing in WT mice, while silencing hepatic Osbpl3 reverses the lipogenic phenotype of LRH-1 K289R mice. These findings suggest that compromised SUMOylation of LRH-1 promotes the development of NAFLD under lipogenic conditions through regulation of OSBPL3.