GPR84-mediated signal transduction affects metabolic function by promoting brown adipocyte activity.

The G protein-coupled receptor 84 (GPR84), a medium-chain fatty acid receptor, has garnered attention because of its potential involvement in a range of metabolic conditions. However, the precise mechanisms underlying this effect remain elusive. Our study has shed light on the pivotal role of GPR84, revealing its robust expression and functional significance within brown adipose tissue (BAT). Mice lacking GPR84 exhibited increased lipid accumulation in BAT, rendering them more susceptible to cold exposure and displaying reduced BAT activity compared with their WT counterparts. Our in vitro experiments with primary brown adipocytes from GPR84-KO mice revealed diminished expression of thermogenic genes and reduced O2 consumption. Furthermore, the application of the GPR84 agonist 6-n-octylaminouracil (6-OAU) counteracted these effects, effectively reinstating the brown adipocyte activity. These compelling in vivo and in vitro findings converge to highlight mitochondrial dysfunction as the primary cause of BAT anomalies in GPR84-KO mice. The activation of GPR84 induced an increase in intracellular Ca2+ levels, which intricately influenced mitochondrial respiration. By modulating mitochondrial Ca2+ levels and respiration, GPR84 acts as a potent molecule involved in BAT activity. These findings suggest that GPR84 is a potential therapeutic target for invigorating BAT and ameliorating metabolic disorders.

Hepatic sialic acid synthesis modulates glucose homeostasis in both liver and skeletal muscle.

OBJECTIVE: Sialic acid is a terminal monosaccharide of glycans in glycoproteins and glycolipids, and its derivation from glucose is regulated by the rate-limiting enzyme UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE). Although the glycans on key endogenous hepatic proteins governing glucose metabolism are sialylated, how sialic acid synthesis and sialylation in the liver influence glucose homeostasis is unknown. Studies were designed to fill this knowledge gap. METHODS: To decrease the production of sialic acid and sialylation in hepatocytes, a hepatocyte-specific GNE knockdown mouse model was generated, and systemic glucose metabolism, hepatic insulin signaling and glucagon signaling were evaluated in vivo or in primary hepatocytes. Peripheral insulin sensitivity was also assessed. Furthermore, the mechanisms by which sialylation in the liver influences hepatic insulin signaling and glucagon signaling and peripheral insulin sensitivity were identified. RESULTS: Liver GNE deletion in mice caused an impairment of insulin suppression of hepatic glucose production. This was due to a decrease in the sialylation of hepatic insulin receptors (IR) and a decline in IR abundance due to exaggerated degradation through the Eph receptor B4. Hepatic GNE deficiency also caused a blunting of hepatic glucagon receptor (GCGR) function which was related to a decline in its sialylation and affinity for glucagon. An accompanying upregulation of hepatic FGF21 production caused an enhancement of skeletal muscle glucose disposal that led to an overall increase in glucose tolerance and insulin sensitivity. CONCLUSION: These collective observations reveal that hepatic sialic acid synthesis and sialylation modulate glucose homeostasis in both the liver and skeletal muscle. By interrogating how hepatic sialic acid synthesis influences glucose control mechanisms in the liver, a new metabolic cycle has been identified in which a key constituent of glycans generated from glucose modulates the systemic control of its precursor.

Adipose tissue macrophages exert systemic metabolic control by manipulating local iron concentrations.

Iron is essential to many fundamental biological processes, but its cellular compartmentalization and concentration must be tightly controlled. Although iron overload can contribute to obesity-associated metabolic deterioration, the subcellular localization and accumulation of iron in adipose tissue macrophages is largely unknown. Here, we show that macrophage mitochondrial iron levels control systemic metabolism in male mice by altering adipocyte iron concentrations. Using various transgenic mouse models to manipulate the macrophage mitochondrial matrix iron content in an inducible fashion, we demonstrate that lowering macrophage mitochondrial matrix iron increases numbers of M2-like macrophages in adipose tissue, lowers iron levels in adipocytes, attenuates inflammation and protects from high-fat-diet-induced metabolic deterioration. Conversely, elevating macrophage mitochondrial matrix iron increases M1-like macrophages and iron levels in adipocytes, exacerbates inflammation and worsens high-fat-diet-induced metabolic dysfunction. These phenotypes are robustly reproduced by transplantation of a small amount of fat from transgenic to wild-type mice. Taken together, we identify macrophage mitochondrial iron levels as a crucial determinant of systemic metabolic homeostasis in mice.

GPR92 activation in islet macrophages controls ß cell function in a diet-induced obesity model.

The molecular mechanisms underlying obesity-induced increases in ß cell mass and the resulting ß cell dysfunction need to be elucidated further. Our study revealed that GPR92, expressed in islet macrophages, is modulated by dietary interventions in metabolic tissues. Therefore, we aimed to define the role of GPR92 in islet inflammation by using a high-fat diet-induced (HFD-induced) obese mouse model. GPR92-KO mice exhibited glucose intolerance and reduced insulin levels - despite the enlarged pancreatic islets - as well as increased islet macrophage content and inflammation level compared with WT mice. These results indicate that the lack of GPR92 in islet macrophages can cause ß cell dysfunction, leading to disrupted glucose homeostasis. Alternatively, stimulation with the GPR92 agonist farnesyl pyrophosphate results in the inhibition of HFD-induced islet inflammation and increased insulin secretion in WT mice, but not in GPR92-KO mice. Thus, our study suggests that GPR92 can be a potential target to alleviate ß cell dysfunction via the inhibition of islet inflammation associated with the progression of diabetes.

Mitochondrial metabolism is a key regulator of the fibro-inflammatory and adipogenic stromal subpopulations in white adipose tissue.

The adipose tissue stroma is a rich source of molecularly distinct stem and progenitor cell populations with diverse functions in metabolic regulation, adipogenesis, and inflammation. The ontology of these populations and the mechanisms that govern their behaviors in response to stimuli, such as overfeeding, however, are unclear. Here, we show that the developmental fates and functional properties of adipose platelet-derived growth factor receptor beta (PDGFRß)+ progenitor subpopulations are tightly regulated by mitochondrial metabolism. Reducing the mitochondrial ß-oxidative capacity of PDGFRß+ cells via inducible expression of MitoNEET drives a pro-inflammatory phenotype in adipose progenitors and alters lineage commitment. Furthermore, disrupting mitochondrial function in PDGFRß+ cells rapidly induces alterations in immune cell composition in lean mice and impacts expansion of adipose tissue in diet-induced obesity. The adverse effects on adipose tissue remodeling can be reversed by restoring mitochondrial activity in progenitors, suggesting therapeutic potential for targeting energy metabolism in these cells.

Perivascular mesenchymal cells control adipose-tissue macrophage accrual in obesity.

Chronic low-grade white adipose tissue (WAT) inflammation is a hallmark of metabolic syndrome in obesity. Here, we demonstrate that a subpopulation of mouse WAT perivascular (PDGFRß+) cells, termed fibro-inflammatory progenitors (FIPs), activate proinflammatory signalling cascades shortly after the onset of high-fat diet feeding and regulate proinflammatory macrophage accumulation in WAT in a TLR4-dependent manner. FIPs activation in obesity is mediated by the downregulation of zinc-finger protein 423 (ZFP423), identified here as a transcriptional corepressor of NF-κB. ZFP423 suppresses the DNA-binding capacity of the p65 subunit of NF-κB by inducing a p300-to-NuRD coregulator switch. Doxycycline-inducible expression of Zfp423 in PDGFRß+ cells suppresses inflammatory signalling in FIPs and attenuates metabolic inflammation of visceral WAT in obesity. Inducible inactivation of Zfp423 in PDGFRß+ cells increases FIP activity, exacerbates adipose macrophage accrual and promotes WAT dysfunction. These studies implicate perivascular mesenchymal cells as important regulators of chronic adipose-tissue inflammation in obesity and identify ZFP423 as a transcriptional break on NF-κB signalling.

Revisiting PPARγ as a new friend of GPR120 in the treatment of metabolic disorders.

G Protein-coupled receptor 120 (GPR120; fatty acid receptor 4, FFAR4) and PPARγ agonists both lead to anti-inflammatory and insulin sensitizing effects despite signalling through distinct pathways. We recently reported the overarching idea that these two pathways are interactive. Specifically, treatment of obese mice with the PPARγ agonist rosiglitazone (a thiazolidinedione, TZD) in combination with the GPR120 agonist compound A synergistically improves glucose tolerance and insulin sensitivity. We have deconvoluted the mechanisms underlying this feed-forward effect in the study. Taken together, our study shows that low dose TZD administration, in combination with GPR120 agonists, produces additive beneficial effects on glucose tolerance and insulin sensitivity without the undesirable adverse effects of TZD. Our study suggests potential value of combination PPARγ and GPR120 agonists to treat metabolic disease.

Positive Reinforcing Mechanisms between GPR120 and PPARγ Modulate Insulin Sensitivity.

G protein-coupled receptor 120 (GPR120) and PPARγ agonists each have insulin sensitizing effects. But whether these two pathways functionally interact and can be leveraged together to markedly improve insulin resistance has not been explored. Here, we show that treatment with the PPARγ agonist rosiglitazone (Rosi) plus the GPR120 agonist Compound A leads to additive effects to improve glucose tolerance and insulin sensitivity, but at lower doses of Rosi, thus avoiding its known side effects. Mechanistically, we show that GPR120 is a PPARγ target gene in adipocytes, while GPR120 augments PPARγ activity by inducing the endogenous ligand 15d-PGJ2 and by blocking ERK-mediated inhibition of PPARγ. Further, we used macrophage- (MKO) or adipocyte-specific GPR120 KO (AKO) mice to show that GRP120 has anti-inflammatory effects via macrophages while working with PPARγ in adipocytes to increase insulin sensitivity. These results raise the prospect of a safer way to increase insulin sensitization in the clinic.

SREBP-regulated adipocyte lipogenesis is dependent on substrate availability and redox modulation of mTORC1.

The synthesis of lipid and sterol species through de novo lipogenesis (DNL) is regulated by two functionally overlapping but distinct transcription factors: the sterol regulatory element-binding proteins (SREBPs) and carbohydrate response element binding protein (ChREBP). ChREBP is considered to be the dominant regulator of DNL in adipose tissue (AT); however, the SREBPs are highly expressed and robustly regulated in adipocytes, suggesting that the model of AT DNL may be incomplete. Here we describe a new mouse model of inducible, adipocyte-specific overexpression of the insulin-induced gene 1 (Insig1), a negative regulator of SREBP transcriptional activity. Contrary to convention, Insig1 overexpression did block AT lipogenic gene expression. However, this was immediately met with a compensatory mechanism triggered by redox activation of mTORC1 to restore SREBP1 DNL gene expression. Thus, we demonstrate that SREBP1 activity sustains adipocyte lipogenesis, a conclusion that has been elusive due to the constitutive nature of current mouse models.

Constitutive Activation of the Nutrient Sensor mTORC1 in Myeloid Cells Induced by Tsc1 Deletion Protects Mice from Diet-Induced Obesity.

SCOPE: To test whether myeloid cells Tsc1 deletion and therefore constitutive activation of the nutrient sensor mTORC1 protects from high-fat diet (HFD)-induced obesity, glucose intolerance, and adipose tissue inflammation. METHODS AND RESULTS: Mice with Tsc1 deletion in myeloid cells (MTsc1KO) and littermate controls (MTsc1WT) were fed with HFD for 8 weeks and evaluated for body weight, glucose homeostasis, and adipose tissue inflammation. MTsc1KO mice were protected from HFD-induced obesity and glucose intolerance. MTsc1KO, however, displayed, independently of the diet, abnormal behavior, episodes of intense movement, and muscle spasms followed by temporary paralysis. To investigate whether obesity protection was due to myeloid cells Tsc1 deletion, bone marrow was transplanted from MTsc1WT and MTsc1KO into irradiated C57BL6/J mice. Mice transplanted with MTsc1KO bone marrow displayed reduced body weight gain, adiposity, and inflammation, and enhanced energy expenditure, glucose tolerance and adipose tissue M2 macrophage content upon HFD feeding, in the absence of abnormal behavior. In vitro, Tsc1 deletion increased in a mTORC1-dependent manner macrophage polarization to M2 profile and mRNA levels of fatty acid binding protein 4 and PPARγ. CONCLUSION: Constitutive mTORC1 activation in myeloid cells protects mice from HFD-induced obesity, adipose tissue inflammation, and glucose intolerance by promoting macrophage polarization to M2 pro-resolution profile and increasing energy expenditure.

Adipocyte mTORC1 deficiency promotes adipose tissue inflammation and NLRP3 inflammasome activation via oxidative stress and de novo ceramide synthesis.

Mechanistic target of rapamycin complex (mTORC)1 activity is increased in adipose tissue of obese insulin-resistant mice, but its role in the regulation of tissue inflammation is unknown. Herein, we investigated the effects of adipocyte mTORC1 deficiency on adipose tissue inflammation and glucose homeostasis. For this, mice with adipocyte raptor deletion and controls fed a chow or a high-fat diet were evaluated for body mass, adiposity, glucose homeostasis, and adipose tissue inflammation. Despite reducing adiposity, adipocyte mTORC1 deficiency promoted hepatic steatosis, insulin resistance, and adipose tissue inflammation (increased infiltration of macrophages, neutrophils, and B lymphocytes; crown-like structure density; TNF-α, interleukin (IL)-6, and monocyte chemoattractant protein 1 expression; IL-1ß protein content; lipid peroxidation; and de novo ceramide synthesis). The anti-oxidant, N-acetylcysteine, partially attenuated, whereas treatment with de novo ceramide synthesis inhibitor, myriocin, completely blocked adipose tissue inflammation and nucleotide oligomerization domain-like receptor pyrin domain-containing 3 (NLRP3)-inflammasome activation, but not hepatic steatosis and insulin resistance induced by adipocyte raptor deletion. Rosiglitazone treatment, however, completely abrogated insulin resistance induced by adipocyte raptor deletion. In conclusion, adipocyte mTORC1 deficiency induces adipose tissue inflammation and NLRP3-inflammasome activation by promoting oxidative stress and de novo ceramide synthesis. Such adipose tissue inflammation, however, is not an underlying cause of the insulin resistance displayed by these mice.

mTORC1 inhibition with rapamycin exacerbates adipose tissue inflammation in obese mice and dissociates macrophage phenotype from function.

Genetic- and diet-induced obesity and insulin resistance are associated with an increase in mechanistic target of rapamycin complex (mTORC) 1 activity in adipose tissue. We investigated herein the effects of pharmacological mTORC1 inhibition in the development of adipose tissue inflammation induced by high-fat diet (HFD) feeding, as well as in the polarization, metabolism and function of bone marrow-derived macrophages (BMDM). For this, C57BL/6J mice fed with a standard chow diet or a HFD (60% of calories from fat) and treated with either vehicle (0.1% Me2SO, 0.2% methylcellulose) or rapamycin (2mg/kg/ day, gavage) during 30days were evaluated for body weight, adiposity, glucose tolerance and adipose tissue inflammation. Although rapamycin did not affect the increase in body weight and adiposity, it exacerbated the glucose intolerance and adipose tissue inflammation induced by HFD feeding, as evidenced by the increased adipose tissue percentage of M1 macrophages, naive and activated cytotoxic T lymphocytes, and mRNA levels of proinflammatory molecules, such as TNF-α, IL-6 and MCP-1. In BMDM in vitro, pharmacological mTORC1 inhibition induced phosphorylation of NFκB p65 and spontaneous polarization of macrophages to a proinflammatory M1 profile, while it impaired M2 polarization induced by IL-4+IL-13, glycolysis and phagocytosis. Altogether, these findings indicate that mTORC1 activity is an important determinant of adipose tissue inflammatory profile and macrophage plasticity, metabolism and function.

Constitutive adipocyte mTORC1 activation enhances mitochondrial activity and reduces visceral adiposity in mice.

Mechanistic target of rapamycin complex 1 (mTORC1) loss of function reduces adiposity whereas partial mTORC1 inhibition enhances fat deposition. Herein we evaluated how constitutive mTORC1 activation in adipocytes modulates adiposity in vivo. Mice with constitutive mTORC1 activation in adipocytes induced by tuberous sclerosis complex (Tsc)1 deletion and littermate controls were evaluated for body mass, energy expenditure, glucose and fatty acid metabolism, mitochondrial function, mRNA and protein contents. Adipocyte-specific Tsc1 deletion reduced visceral, but not subcutaneous, fat mass, as well as adipocyte number and diameter, phenotypes that were associated with increased lipolysis, UCP-1 content (browning) and mRNA levels of pro-browning transcriptional factors C/EBPß and ERRα. Adipocyte Tsc1 deletion enhanced mitochondrial oxidative activity, fatty acid oxidation and the expression of PGC-1α and PPARα in both visceral and subcutaneous fat. In brown adipocytes, however, Tsc1 deletion did not affect UCP-1 content and basal respiration. Adipocyte Tsc1 deletion also reduced visceral adiposity and enhanced glucose tolerance, liver and muscle insulin signaling and adiponectin secretion in mice fed with purified low- or high-fat diet. In conclusion, adipocyte-specific Tsc1 deletion enhances mitochondrial activity, induces browning and reduces visceral adiposity in mice.

Omega-3 fatty acids protect from diet-induced obesity, glucose intolerance, and adipose tissue inflammation through PPARγ-dependent and PPARγ-independent actions.

SCOPE: We tested herein the hypothesis that peroxisome proliferator activated receptor γ (PPARγ) is a major mediator of omega-3 (n-3) protective actions against high-fat diet (HFD) induced obesity, glucose intolerance, and adipose tissue inflammation. METHODS AND RESULTS: C57BL6 wild-type and fat-1 transgenic (fat-1) mice were fed a low-fat diet (LFD) or HFD, treated or not with PPARγ antagonist, and evaluated for energy balance, adiposity, glucose tolerance, and adipose tissue inflammation. Fat-1 mice were protected from obesity, fasting hyperglycemia, glucose intolerance, and adipose tissue inflammation. PPARγ inhibition completely abolished fat-1 protection against HFD-induced glucose intolerance, but not obesity or adipose tissue inflammation. To investigate the role of myeloid cell as mediator of n-3 beneficial metabolic actions, mice with deletion (LyzM-PPARγ(KO)) or nondeletion (LyzM-PPARγ(WT)) of PPARγ in myeloid cells were fed either LFD or HFD (lard) or an HFD rich in n-3 (fish oil). Our findings indicate that myeloid cell associated PPARγ is not involved in the attenuation of HFD-induced glucose intolerance and adipose tissue inflammation induced by n-3. CONCLUSION: High endogenous n-3 fatty acid levels protect from HFD obesity, glucose intolerance, and adipose tissue inflammation. Among these, only protection against glucose intolerance is mediated by non-myeloid cell PPARγ.

PPARγ activation attenuates glucose intolerance induced by mTOR inhibition with rapamycin in rats.

mTOR inhibition with rapamycin induces a diabetes-like syndrome characterized by severe glucose intolerance, hyperinsulinemia, and hypertriglyceridemia, which is due to increased hepatic glucose production as well as reduced skeletal muscle glucose uptake and adipose tissue PPARγ activity. Herein, we tested the hypothesis that pharmacological PPARγ activation attenuates the diabetes-like syndrome associated with chronic mTOR inhibition. Rats treated with the mTOR inhibitor rapamycin (2 mg·kg(-1)·day(-1)) in combination or not with the PPARγ ligand rosiglitazone (15 mg·kg(-1)·day(-1)) for 15 days were evaluated for insulin secretion, glucose, insulin, and pyruvate tolerance, skeletal muscle and adipose tissue glucose uptake, and insulin signaling. Rosiglitazone corrected fasting hyperglycemia, attenuated the glucose and insulin intolerances, and abolished the increase in fasting plasma insulin and C-peptide levels induced by rapamycin. Surprisingly, rosiglitazone markedly increased the plasma insulin and C-peptide responses to refeeding in rapamycin-treated rats. Furthermore, rosiglitazone partially attenuated rapamycin-induced gluconeogenesis, as evidenced by the improved pyruvate tolerance and reduced mRNA levels of phosphoenolpyruvate carboxykinase and glucose-6-phosphatase. Rosiglitazone also restored insulin's ability to stimulate glucose uptake and its incorporation into glycogen in skeletal muscle of rapamycin-treated rats, which was associated with normalization of Akt Ser(473) phosphorylation. However, the rapamycin-mediated impairments of adipose tissue glucose uptake and incorporation into triacylglycerol were unaffected by rosiglitazone. Our findings indicate that PPARγ activation ameliorates some of the disturbances in glucose homeostasis and insulin action associated with chronic rapamycin treatment by reducing gluconeogenesis and insulin secretion and restoring muscle insulin signaling and glucose uptake.

PPARγ activation attenuates cold-induced upregulation of thyroid status and brown adipose tissue PGC-1α and D2.

Here, we investigated whether pharmacological PPARγ activation modulates key early events in brown adipose tissue (BAT) recruitment induced by acute cold exposure with the aim of unraveling the interrelationships between sympathetic and PPARγ signaling. Sprague-Dawley rats treated or not with the PPARγ ligand rosiglitazone (15 mg·kg(-1)·day(-1), 7 days) were kept at 23°C or exposed to cold (5°C) for 24 h and evaluated for BAT gene expression, sympathetic activity, thyroid status, and adrenergic signaling. Rosiglitazone did not affect the reduction in body weight gain and the increase in feed efficiency, Vo(2), and BAT sympathetic activity induced by 24-h cold exposure. Rosiglitazone strongly attenuated the increase in serum total and free T4 and T3 levels and BAT iodothyronine deiodinase type 2 (D2) and PGC-1α mRNA levels and potentiated the reduction in BAT thyroid hormone receptor (THR) ß mRNA levels induced by cold. Administration of T3 to rosiglitazone-treated rats exacerbated the cold-induced increase in energy expenditure but did not restore a proper activation of D2 and PGC-1α, nor further increased uncoupling protein 1 expression. Regarding adrenergic signaling, rosiglitazone did not affect the changes in BAT cAMP content and PKA activity induced by cold. Rosiglitazone alone or in combination with cold increased CREB binding to DNA, but it markedly reduced the expression of one of its major coactivators, CREB binding protein. In conclusion, pharmacological PPARγ activation impairs short-term cold elicitation of BAT adrenergic and thyroid signaling, which may result in abnormal tissue recruitment and thermogenic activity.

Oral administration of oleic or linoleic acids modulates the production of inflammatory mediators by rat macrophages.

Oleic (OLA) and linoleic (LNA) acids are commonly consumed fatty acids and they can modulate the inflammatory response, in which macrophages play an important role. The aim of this study was to investigate the effects of these two fatty acids on the production of inflammatory mediators by macrophages. Rats received oral administration of water (control), OLA or LNA (0.22 g/kg body weight) daily for 10 days and peritoneal resident macrophages were then isolated. Subsequently, they were seeded in culture plates and the production of various inflammatory mediators was assessed. Oral administration with OLA decreased the production of IL-1ß, IL-6 and CINC-2αß by resident macrophages and LNA decreased the production of IL-1ß, IL-6 and VEGF in the absence of lipopolysaccharide (LPS), although it accelerated IL-1ß release and decreased IL-10 synthesis when cells were stimulated with LPS. Neither fatty acid affected the production of superoxide anion, hydrogen peroxide, nitrite, TNF-α, PGE(2), LTB(4) or 15(S)-HETE. Thus, OLA and LNA influence the production of several inflammatory mediators by macrophages.