Depot-specific effects of the PPARgamma agonist rosiglitazone on adipose tissue glucose uptake and metabolism

PPAR-γ as a therapeutic target in cardiovascular disease: evidence and uncertainty

Peroxisome proliferator-activated receptor γ (PPAR-γ) is a key regulator of fatty acid metabolism, promoting its storage in adipose tissue and reducing circulating concentrations of free fatty acids. Activation of PPAR-γ has favorable effects on measures of adipocyte function, insulin sensitivity, lipoprotein metabolism, and vascular structure and function. Despite these effects, clinical trials of thiazolidinedione PPAR-γ activators have not provided conclusive evidence that they reduce cardiovascular morbidity and mortality. The apparent disparity between effects on laboratory measurements and clinical outcomes may be related to limitations of clinical trials, adverse effects of PPAR-γ activation, or off-target effects of thiazolidinedione agents. This review addresses these issues from a clinician's perspective and highlights several ongoing clinical trials that may help to clarify the therapeutic role of PPAR-γ activators in cardiovascular disease.

The role of liver fructose-1,6-bisphosphatase in regulating appetite and adiposity

Liver fructose-1,6-bisphosphatase (FBPase) is a regulatory enzyme in gluconeogenesis that is elevated by obesity and dietary fat intake. Whether FBPase functions only to regulate glucose or has other metabolic consequences is not clear; therefore, the aim of this study was to determine the importance of liver FBPase in body weight regulation. To this end we performed comprehensive physiologic and biochemical assessments of energy balance in liver-specific transgenic FBPase mice and negative control littermates of both sexes. In addition, hepatic branch vagotomies and pharmacologic inhibition studies were performed to confirm the role of FBPase. Compared with negative littermates, liver-specific FBPase transgenic mice had 50% less adiposity and ate 15% less food but did not have altered energy expenditure. The reduced food consumption was associated with increased circulating leptin and cholecystokinin, elevated fatty acid oxidation, and 3-β-hydroxybutyrate ketone levels, and reduced appetite-stimulating neuropeptides, neuropeptide Y and Agouti-related peptide. Hepatic branch vagotomy and direct pharmacologic inhibition of FBPase in transgenic mice both returned food intake and body weight to the negative littermates. This is the first study to identify liver FBPase as a previously unknown regulator of appetite and adiposity and describes a novel process by which the liver participates in body weight regulation.

Hedgehog-mediated regulation of PPARγ controls metabolic patterns in neural precursors and shh-driven medulloblastoma

Sonic hedgehog (Shh) signaling is critical during development and its aberration is common across the spectrum of human malignancies. In the cerebellum, excessive activity of the Shh signaling pathway is associated with the devastating pediatric brain tumor medulloblastoma. We previously demonstrated that exaggerated de novo lipid synthesis is a hallmark of Shh-driven medulloblastoma and that hedgehog signaling inactivates the Rb/E2F tumor suppressor complex to promote lipogenesis. Indeed, such Shh-mediated metabolic reprogramming fuels tumor progression, in an E2F1- and FASN-dependent manner. Here, we show that the nutrient sensor PPARγ is a key component of the Shh metabolic network, particularly its regulation of glycolysis. Our data show that in primary cerebellar granule neural precursors (CGNPs), proposed medulloblastoma cells-of-origin, Shh stimulation elicits a marked induction of PPARγ alongside major glycolytic markers. This is also documented in the actively proliferating Shh-responsive CGNPs in the developing cerebellum, and PPARγ expression is strikingly elevated in Shh-driven medulloblastoma in vivo. Importantly, pharmacological blockade of PPARγ and/or Rb inactivation inhibits CGNP proliferation, drives medulloblastoma cell death and extends survival of medulloblastoma-bearing animals in vivo. This coupling of mitogenic Shh signaling to a major nutrient sensor and metabolic transcriptional regulator define a novel mechanism through which Shh signaling engages the nutrient sensing machinery in brain cancer, controls the cell cycle, and regulates the glycolytic index. This also reveals a dominant role of Shh in the etiology of glucose metabolism in medulloblastoma and underscores the function of the Shh → E2F1 → PPARγ axis in altering substrate utilization patterns in brain cancers in favor of tumor growth. These findings emphasize the value of PPARγ downstream of Shh as a global therapeutic target in hedgehog-dependent and/or Rb-inactivated tumors.

Major involvement of mTOR in the PPARγ-induced stimulation of adipose tissue lipid uptake and fat accretion

Evidence points to a role of the mammalian target of rapamycin (mTOR) signaling pathway as a regulator of adiposity, yet its involvement as a mediator of the positive actions of peroxisome proliferator-activated receptor (PPAR)γ agonism on lipemia, fat accretion, lipid uptake, and its major determinant lipoprotein lipase (LPL) remains to be elucidated. Herein we evaluated the plasma lipid profile, triacylglycerol (TAG) secretion rates, and adipose tissue LPL-dependent lipid uptake, LPL expression/activity, and expression profile of other lipid metabolism genes in rats treated with the PPARγ agonist rosiglitazone (15 mg/kg/day) in combination or not with the mTOR inhibitor rapamycin (2 mg/kg/day) for 15 days. Rosiglitazone stimulated adipose tissue mTOR complex 1 and AMPK and induced TAG-derived lipid uptake (136%), LPL mRNA/activity (2- to 6-fold), and fat accretion in subcutaneous (but not visceral) white adipose tissue (WAT; 50%) and in brown adipose tissue (BAT; 266%). Chronic mTOR inhibition attenuated the upregulation of lipid uptake, LPL expression/activity, and fat accretion induced by PPARγ activation in both subcutaneous WAT and BAT, which resulted in hyperlipidemia. In contrast, rapamycin did not affect most of the other WAT lipogenic genes upregulated by rosiglitazone. Together these findings demonstrate that mTOR is a major regulator of adipose tissue LPL-mediated lipid uptake and a critical mediator of the hypolipidemic and lipogenic actions of PPARγ activation.

Olanzapine, but not aripiprazole, weight-independently elevates serum triglycerides and activates lipogenic gene expression in female rats

Metabolic adverse effects such as weight gain and dyslipidaemia represent a major concern in treatment with several antipsychotic drugs, including olanzapine. It remains unclear whether such metabolic side-effects fully depend on appetite-stimulating actions, or whether some dysmetabolic features induced by antipsychotics may arise through direct perturbation of metabolic pathways in relevant peripheral tissues. Recent clinical and preclinical studies indicate that dyslipidaemia could occur independently of weight gain. Using a rat model, we showed that subchronic treatment with olanzapine induces weight gain and increases adipose tissue mass in rats with free access to food. This effect was also observed for aripiprazole, considered metabolically neutral in the clinical setting. In pair-fed rats with limited food access, neither olanzapine nor aripiprazole induced weight gain. Interestingly, olanzapine, but not aripiprazole, induced weight-independent elevation of serum triglycerides, accompanied by up-regulation of several genes involved in lipid biosynthesis, both in liver and in adipose tissues. Our findings support the existence of tissue-specific, weight-independent direct effects of olanzapine on lipid metabolism.

Prophylactic treatment with telmisartan induces tissue-specific gene modulation favoring normal glucose homeostasis in Cohen-Rosenthal diabetic hypertensive rats

The objectives were to assess the potential of long-term prophylactic administration of telmisartan, an angiotensin II receptor antagonist and a partial peroxisome proliferator activator receptor (PPAR)γ agonist, in preventing the development of hypertension and hyperglycemia and to demonstrate the alteration in gene expression associated with the development of hyperglycemia and insulin resistance in Cohen-Rosenthal diabetic hypertensive rat, a unique model of hypertension and type 2 diabetes mellitus comorbidity. Cohen-Rosenthal diabetic hypertensive rats were continuously treated with telmisartan (3 mg/[kg d]) starting at age 6 to 8 weeks before developing hypertension or diabetes. Weight changes, blood pressure, blood insulin, adiponectin, glucose tolerance, and insulin sensitivity were monitored. Fat, liver, and muscle messenger RNAs were examined for the expression of genes potentially involved in the onset of insulin resistance. In addition to the expected antihypertensive effect of prophylactic telmisartan, diabetes was blunted, evidenced at the end of the study by a significantly lower glucose level. This was accompanied by improved glucose tolerance, increased sensitivity to insulin, reduction in fasting insulin levels and homeostasis model assessment index, as well as an increase in serum adiponectin. Telmisartan also prevented the increase in serum triglycerides and the associated appearance of lipid droplets in the liver. Diabetes induced tissue-specific changes in messenger RNAs expression of the following selected genes, which were restored by telmisartan treatment: PPARγ, PPARδ, PPARγ coactivator 1α, adiponectin, adiponectin receptor 1, adiponectin receptor 2, phosphotyrosine binding domain and a pleckstrin homology domain-containing adaptor protein, adenosine monophosphate kinase, and glucose translocator 4. Telmisartan blunted the development of hypertension, insulin resistance, and diabetes in prediabetic Cohen-Rosenthal diabetic hypertensive rats through pleiotropic activity, involving specific gene regulation of target organs.

Heterogeneity among white adipose tissue depots in male C57BL/6J mice

The widespread prevalence of obesity has lead to extensive research on white adipose tissue (WAT), which frequently uses the C57BL/6J mouse strain as a model. In many studies, results obtained in one WAT depot are often extrapolated to all WAT. However, functional differences among WAT depots are now becoming apparent. Thus, to identify the molecular mechanisms responsible for WAT depot-specific differences under "normal" conditions, four C57BL/6J mouse WAT depots (inguinal, mesenteric, epididymal, and retroperitoneal) were analyzed. Depot proteomic profiles, along with weights, protein contents, adipocyte sizes and oxidative stress were determined. Mesenteric WAT had almost twice the protein content of the other depots analyzed. Mean adipocyte size was highest in epididymal and lowest in mesenteric and inguinal depots. The proteome of inguinal WAT displayed low levels of enzymes involved in ATP generation, glucose and lipid metabolism, and antioxidant proteins. Higher levels of these proteins were observed in mesenteric and epididymal WAT, with variable levels in the retroperitoneal depot. Some of these proteins showed depot-specific correlations with plasma levels of insulin, leptin, and adiponectin. In agreement with the proteomic data, levels of the antioxidant protein heat shock protein β1 (HSPβ1) also were lower in inguinal WAT when analyzed by western blotting and immunohistochemistry. Also, lipid peroxidation products showed similar trends. Our results are consistent with lower triglyceride turnover and lower oxidative stress in inguinal than mesenteric and epididymal WAT. The observed WAT depot-specific differences provide clues as to the mechanisms leading to these depots' respective diverse functions.

Green tea extract suppresses NFκB activation and inflammatory responses in diet-induced obese rats with nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis (NASH) is characterized by oxidative stress and inflammatory responses that exacerbate liver injury. The objective of this study was to determine whether the antioxidant and antiinflammatory activities of green tea extract (GTE) would protect against NASH in a model of diet-induced obesity. Adult Wistar rats were fed a low-fat (LF) diet or high-fat (HF) diet containing no GTE or GTE at 1% or 2% (HF+2GTE) for 8 wk. The HF group had greater (P ≤ 0.05) serum alanine (ALT) and aspartate aminotransferases and hepatic lipids than the LF group. Both GTE groups had lower ALT and hepatic lipid than the HF group. In liver and epididymal adipose, the HF group had lower glutathione as well as greater mRNA and protein expression of TNFα and monocyte chemoattractant protein-1 (MCP-1) and NFκB binding activity than the LF group. Compared to the HF group, the HF+2GTE group had greater glutathione and lower protein and mRNA levels of inflammatory cytokines in both tissues. NFκB binding activities at liver and adipose were also lower, likely by inhibiting the phosphorylation of inhibitor of NFκB. NFκB binding activities in liver and adipose (P ≤ 0.05; r = 0.62 and 0.46, respectively) were correlated with ALT, and hepatic NFκB binding activity was inversely related to liver glutathione (r = -0.35). These results suggest that GTE-mediated improvements in glutathione status are associated with the inhibition of hepatic and adipose inflammatory responses mediated by NFκB, thereby protecting against NASH.

Role of PPARg2 transcription factor in thiazolidinedione-induced insulin sensitization

OBJECTIVES

Adipose tissue is the key regulator of energy balance, playing an active role in lipid storage and metabolism and may be a dynamic buffer to control fatty acid flux. Peroxisome proliferator-activated receptor gamma isoform-2 (PPARg2), an isoform of the nuclear hormone receptor superfamily, has been implicated in almost all aspects of human metabolic alterations such as obesity, insulin resistance, type-2 diabetes and dyslipidaemia. The PPARg2 isoform is highly present in adipose tissue where it functions as a thrifty phenotype, which promotes adipocyte differentiation and triglyceride storage. Thiazolidinediones, antidiabetic drugs, induce insulin sensitivity by controlling adipokines. The thiazolidinediones bind with PPARg2 in adipocytes and exert an agonist effect by enhancing adipogenesis and fatty acid uptake. Thiazolidinediones stimulate PPARg2, by which they down-regulate tumour necrosis factor-α, leptin, interleukin-6 and plasminogen and also enhance insulin sensitivity. The aim of this work is to define role of PPARg2 transcription factor in thiazolidinedione-induced insulin sensitization.

KEY FINDINGS

The PPARg2 alters the transcription of the target gene. This altered gene transcription results in the up-regulation of insulin-sensitizing factors and down-regulation of insulin-resistant factors. The variant Pro12Ala of the PPARg2 gene is an important modulator in metabolic control in the body. Thiazolidinediones stimulate PPARg2 transcription factor by which PPARg2 binds to responsive elements located in the promoter regions of many genes and modulates their transcriptive activity. There is a strong mutual relationship between receptor binding and agonism, which is evidence of the insulin-sensitizing target of thiazolidinediones in PPARg2. This evidently increases the biological potency of the glucose-lowering effect of thiazolidinediones in vivo as well as their antidiabetic activity.

CONCLUSIONS

PPARg2 transcription factor plays an important role in treatment of type-2 diabetes with thiazolidindiones. The variant Pro12Ala of the PPARg2 gene promotes the activity of thiazolidinediones in minimizing insulin resistance. Transcriptional activity of Pro12Ala variant improves the activity of insulin. Thus thiazolidinediones promote the phosphorylation of PPARg2 to induce insulin sensitivity.

Control of Brown Adipose Tissue Glucose and Lipid Metabolism by PPARγ

Brown adipose tissue (BAT) non-shivering thermogenesis impacts energy homeostasis in rodents and humans. Mitochondrial uncoupling protein 1 in brown fat cells produces heat by dissipating the energy generated by fatty acid and glucose oxidation. In addition to thermogenesis and despite its small relative size, sympathetically activated BAT constitutes an important glucose, fatty acid, and triacylglycerol-clearing organ, and such function could potentially be used to alleviate dyslipidemias, hyperglycemia, and insulin resistance. To date, chronic sympathetic innervation and peroxisome proliferator-activated receptor (PPAR) γ activation are the only recognized inducers of BAT recruitment. Here, we review the major differences between these two BAT inducers in the regulation of lipolysis, fatty acid oxidation, lipid uptake and triacylglycerol synthesis, glucose uptake, and de novo lipogenesis. Whereas BAT recruitment through sympathetic drive translates into functional thermogenic activity, PPARγ-mediated recruitment is associated with a reduction in sympathetic activity leading to increased lipid storage in brown adipocytes. The promising therapeutic role of BAT in the treatment of hypertriglyceridemic and hyperglycemic conditions is also discussed.

Lipins: multifunctional lipid metabolism proteins

The lipin proteins are evolutionarily conserved proteins with roles in lipid metabolism and disease. There are three lipin protein family members in mammals and one or two orthologs in plants, invertebrates, and single-celled eukaryotes. Studies in yeast and mouse led to the identification of two distinct molecular functions of lipin proteins. Lipin proteins have phosphatidate phosphatase activity and catalyze the formation of diacylglycerol in the glycerol-3-phosphate pathway, implicating them in the regulation of triglyceride and phospholipid biosynthesis. Mammalian lipin proteins also possess transcriptional coactivator activity and have been implicated in the regulation of metabolic gene expression. Here we review key findings in the field that demonstrate roles for lipin family members in metabolic homeostasis and in rare human diseases, and we examine evidence implicating genetic variations in lipin genes in common metabolic dysregulation such as obesity, hyperinsulinemia, hypertension, and type 2 diabetes.

Lpin1 in human visceral and subcutaneous adipose tissue: similar levels but different associations with lipogenic and lipolytic genes

LPIN1 is a gene with important effects on lipidic and metabolic homeostasis. Human subcutaneous LPIN1 expression levels in adipose tissue are related with a better metabolic profile, including insulin sensitivity markers. However, there are few data on the regulation of LPIN1 in visceral adipose tissue (VAT). Our aim was to perform a cross-sectional analysis of VAT compared with subcutaneous (SAT) LPIN1 expression in a well-characterized obese cohort, its relation with the expression of genes involved in lipid metabolism, and the in vitro response to lipogenic and lipolytic stimuli. A downregulation of total LPIN1 mRNA expression in subjects with obesity was found in VAT similarly to that in SAT. Despite similar total LPIN1 mRNA levels in SAT and VAT, a close relationship with clinical parameters and with many lipogenic and lipolytic genes was observed primarily in SAT depot. As shown in the in vitro analysis, the low-grade proinflammatory environment and the insulin resistance associated with obesity may contribute to downregulate LPIN1 in adipose tissue, leading to a worse metabolic profile.

Chronic rapamycin treatment causes glucose intolerance and hyperlipidemia by upregulating hepatic gluconeogenesis and impairing lipid deposition in adipose tissue

OBJECTIVE

The mammalian target of rapamycin (mTOR)/p70 S6 kinase 1 (S6K1) pathway is a critical signaling component in the development of obesity-linked insulin resistance and operates a nutrient-sensing negative feedback loop toward the phosphatidylinositol 3-kinase (PI 3-kinase)/Akt pathway. Whereas acute treatment of insulin target cells with the mTOR complex 1 (mTORC1) inhibitor rapamycin prevents nutrient-induced insulin resistance, the chronic effect of rapamycin on insulin sensitivity and glucose metabolism in vivo remains elusive.

RESEARCH DESIGN AND METHODS

To assess the metabolic effects of chronic inhibition of the mTORC1/S6K1 pathway, rats were treated with rapamycin (2 mg/kg/day) or vehicle for 15 days before metabolic phenotyping.

RESULTS

Chronic rapamycin treatment reduced adiposity and fat cell number, which was associated with a coordinated downregulation of genes involved in both lipid uptake and output. Rapamycin treatment also promoted insulin resistance, severe glucose intolerance, and increased gluconeogenesis. The latter was associated with elevated expression of hepatic gluconeogenic master genes, PEPCK and G6Pase, and increased expression of the transcriptional coactivator peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) as well as enhanced nuclear recruitment of FoxO1, CRTC2, and CREB. These changes were observed despite normal activation of the insulin receptor substrate/PI 3-kinase/Akt axis in liver of rapamycin-treated rats, as expected from the blockade of the mTORC1/S6K1 negative feedback loop.

CONCLUSIONS

These findings unravel a novel mechanism by which mTORC1/S6K1 controls gluconeogenesis through modulation of several key transcriptional factors. The robust induction of the gluconeogenic program in liver of rapamycin-treated rats underlies the development of severe glucose intolerance even in the face of preserved hepatic insulin signaling to Akt and despite a modest reduction in adiposity.

Lipin 1 represses NFATc4 transcriptional activity in adipocytes to inhibit secretion of inflammatory factors

Lipin 1 is a bifunctional protein that regulates gene transcription and, as a Mg(2+)-dependent phosphatidic acid phosphatase (PAP), is a key enzyme in the biosynthesis of phospholipids and triacylglycerol. We describe here the functional interaction between lipin 1 and the nuclear factor of activated T cells c4 (NFATc4). Lipin 1 represses NFATc4 transcriptional activity through protein-protein interaction, and lipin 1 is present at the promoters of NFATc4 transcriptional targets in vivo. Catalytically active and inactive lipin 1 can suppress NFATc4 transcriptional activity, and this suppression may involve recruitment of histone deacetylases to target promoters. In fat pads from mice deficient for lipin 1 (fld mice) and in 3T3-L1 adipocytes depleted of lipin 1 there is increased expression of several NFAT target genes including tumor necrosis factor alpha, resistin, FABP4, and PPARgamma. Finally, both lipin 1 protein and total PAP activity are decreased with increasing adiposity in the visceral, but not subcutaneous, fat pads of ob/ob mice. These observations place lipin 1 as a potentially important link between triacylglycerol synthesis and adipose tissue inflammation.

The lipin family: mutations and metabolism

PURPOSE OF REVIEW

The family of three lipin proteins act as phosphatidate phosphatase (PAP) enzymes required for glycerolipid biosynthesis, and also as transcriptional coactivators that regulate expression of lipid metabolism genes. The genes for lipin-1, lipin-2 and lipin-3 are expressed in key metabolic tissues, including adipose tissue, skeletal muscle and liver, but the physiological functions of each member of the family have not been fully elucidated. Here we examine the most recent studies that provide information about the roles of lipin proteins in metabolism and human disease.

RECENT FINDINGS

Recent studies have identified mutations that cause lipin-1 or lipin-2 deficiency in humans, leading to acute myoglobinuria in childhood or the inflammatory disorder Majeed syndrome, respectively. The effects of lipin-1 deficiency appear to include both the loss of glycerolipid building blocks and the accumulation of lipid intermediates that disrupt cellular function. Several studies have demonstrated that polymorphisms in the LPIN1 and LPIN2 genes are associated with metabolic disease traits, including insulin sensitivity, diabetes, blood pressure and response to thiazolidinedione drugs. Furthermore, lipin-1 expression levels in adipose tissue and/or liver are positively correlated with insulin sensitivity. Studies of lipin-1 in adipocytes have shed some light on its relationship with insulin sensitivity.

SUMMARY

Lipin-1 and lipin-2 are required for normal lipid homeostasis and have unique physiological roles. Future studies, for example using engineered mouse models, will be required to fully elucidate their specific roles in normal physiology and disease.