Loss of GPR75 protects against non-alcoholic fatty liver disease and body fat accumulation

Approximately 1 in 4 people worldwide have non-alcoholic fatty liver disease (NAFLD); however, there are currently no medications to treat this condition. This study investigated the role of adiposity-associated orphan G protein-coupled receptor 75 (GPR75) in liver lipid accumulation. We profiled Gpr75 expression and report that it is most abundant in the brain. Next, we generated the first single-cell-level analysis of Gpr75 and identified a subpopulation co-expressed with key appetite-regulating hypothalamic neurons. CRISPR-Cas9-deleted Gpr75 mice fed a palatable western diet high in fat adjusted caloric intake to remain in energy balance, thereby preventing NAFLD. Consistent with mouse results, analysis of whole-exome sequencing data from 428,719 individuals (UK Biobank) revealed that variants in GPR75 are associated with a reduced likelihood of hepatic steatosis. Here, we provide a significant advance in understanding of the expression and function of GPR75, demonstrating that it is a promising pharmaceutical target for NAFLD treatment.

GLP-1 receptor agonist improves metabolic disease in a pre-clinical model of lipodystrophy

Aims

Individuals with lipodystrophies typically suffer from metabolic disease linked to adipose tissue dysfunction including lipoatrophic diabetes. In the most severe forms of lipodystrophy, congenital generalised lipodystrophy, adipose tissue may be almost entirely absent. Better therapies for affected individuals are urgently needed. Here we performed the first detailed investigation of the effects of a glucagon like peptide-1 receptor (GLP-1R) agonist in lipoatrophic diabetes, using mice with generalised lipodystrophy.

Methods

Lipodystrophic insulin resistant and glucose intolerant seipin knockout mice were treated with the GLP-1R agonist liraglutide either acutely preceding analyses of insulin and glucose tolerance or chronically prior to metabolic phenotyping and ex vivo studies.

Results

Acute liraglutide treatment significantly improved insulin, glucose and pyruvate tolerance. Once daily injection of seipin knockout mice with liraglutide for 14 days led to significant improvements in hepatomegaly associated with steatosis and reduced markers of liver fibrosis. Moreover, liraglutide enhanced insulin secretion in response to glucose challenge with concomitantly improved glucose control.

Conclusions

GLP-1R agonist liraglutide significantly improved lipoatrophic diabetes and hepatic steatosis in mice with generalised lipodystrophy. This provides important insights regarding the benefits of GLP-1R agonists for treating lipodystrophy, informing more widespread use to improve the health of individuals with this condition.

Proceedings of the annual meeting of the European Consortium of Lipodystrophies (ECLip), Pisa, Italy, 28-29 September 2023

Lipodystrophy syndromes are rare diseases primarily affecting the development or maintenance of the adipose tissue but are also distressing indirectly multiple organs and tissues, often leading to reduced life expectancy and quality of life. Lipodystrophy syndromes are multifaceted disorders caused by genetic mutations or autoimmunity in the vast majority of cases. While many subtypes are now recognized and classified, the disease remains remarkably underdiagnosed. The European Consortium of Lipodystrophies (ECLip) was founded in 2014 as a non-profit network of European centers of excellence working in the field of lipodystrophies aiming at promoting international collaborations to increase basic scientific understanding and clinical management of these syndromes. The network has developed a European Patient Registry as a collaborative research platform for consortium members. ECLip and ECLip registry activities involve patient advocacy groups to increase public awareness and to seek advice on research activities relevant from the patients perspective. The annual ECLip congress provides updates on the research results of various network groups members.

When Adipose Tissue Lets You Down: Understanding the Functions of Genes Disrupted in Lipodystrophy

Lipodystrophy syndromes are conditions in which the adipose tissue mass of an individual is altered inappropriately. The change in adipose mass can range from a relatively modest and subtle redistribution in some individuals with partial lipodystrophy to a near-complete absence of adipose tissue in the most severe forms of generalized lipodystrophy. The common feature is a disconnection between the need of the individual for a safe, healthy lipid storage capacity and the available adipose mass to perform this critical role. The inability to partition lipids for storage in appropriately functioning adipocytes leads to lipid accumulation in other tissues, which typically results in conditions such as diabetes, dyslipidemia, fatty liver, and cardiovascular disease. Several genes have been identified whose disruption leads to inherited forms of lipodystrophy. There is a link between some of these genes and adipose dysfunction, so the molecular basis of disease pathophysiology appears clear. However, for other lipodystrophy genes, it is not evident why their disruption should affect adipose development or function or, in the case of partial lipodystrophy, why only some adipose depots should be affected. Elucidating the molecular functions of these genes and their cellular and physiological effects has the capacity to uncover fundamental new insights regarding the development and functions of adipose tissue. This information is also likely to inform better management of lipodystrophy and improved treatments for patients. In addition, the findings will often be relevant to other conditions featuring adipose tissue dysfunction, including the more common metabolic disease associated with obesity.

Phosphoprotein enriched in astrocytes (PEA)-15 is a novel regulator of adipose tissue expansion

Excessive expansion of adipose tissue in obesity typically leads to overflow and accumulation of lipids in other tissues, causing fatty liver disease and atherosclerosis. The intracellular protein, phosphoprotein enriched in astrocytes (PEA)-15 has been linked to metabolic disease but its role in lipid storage has not been examined. To delineate the role of PEA-15 in adipose tissue, we placed PEA-15^−/− mice on a high fat diet. These mice developed increased body weight and greater white adipose tissue expansion compared to high fat diet-fed wild type mice. This was due to increased adipocyte cell size in PEA-15^−/− mice consistent with greater lipid storage capacity. Surprisingly, PEA-15^−/− mice exhibited improvements in whole body insulin sensitivity, lower hepatic weight and decreased serum triglycerides indicating a protective phenotype. To determine effects on atherosclerosis, PEA-15^−/− mice were crossed with the ApoE^−/− mice on a high fat diet. Strikingly, these mice were protected from atherosclerosis and had less hepatic lipid accumulation despite increased adiposity. Therefore, we reveal for the first time that PEA-15 plays a novel role in regulating the expansion of adipose tissue. Decreasing PEA-15 expression increases the sequestering of lipids in adipose tissue, protecting other tissues in obesity, thereby improving metabolic health.

Guanidinoneomycin-maleimide molecular transporter: synthesis, chemistry and cellular uptake

Guanidinoglycosides are a class of non-cytotoxic molecular transporters capable of delivering high molecular weight bioactive cargos into cells at low nanomolar concentrations. Efficient bioconjugation with guanidinoglycosides has been previously demonstrated by utilizing a guanidinoneomycin decorated with a reactive but also unstable N-hydroxysuccinimmide ester-containing linker. Herein we report the synthesis, chemistry, and application of a new, stable guanidinoneomycin derivative armed with a highly specific maleimide moiety which allows for thiol-maleimide click chemistry, a highly popular bioconjugation strategy, widening the field of application of these intriguing and useful delivery vehicles.

Neurochemical Characterization of Brainstem Pro-Opiomelanocortin Cells

Genetic research has revealed pro-opiomelanocortin (POMC) to be a fundamental regulator of energy balance and body weight in mammals. Within the brain, POMC is primarily expressed in the arcuate nucleus of the hypothalamus (ARC), while a smaller population exists in the brainstem nucleus of the solitary tract (POMCNTS). We performed a neurochemical characterization of this understudied population of POMC cells using transgenic mice expressing green fluorescent protein (eGFP) under the control of a POMC promoter/enhancer (PomceGFP). Expression of endogenous Pomc mRNA in the nucleus of the solitary tract (NTS) PomceGFP cells was confirmed using fluorescence-activating cell sorting (FACS) followed by quantitative PCR. In situ hybridization histochemistry of endogenous Pomc mRNA and immunohistochemical analysis of eGFP revealed that POMC is primarily localized within the caudal NTS. Neurochemical analysis indicated that POMCNTS is not co-expressed with tyrosine hydroxylase (TH), glucagon-like peptide 1 (GLP-1), cholecystokinin (CCK), brain-derived neurotrophic factor (BDNF), nesfatin, nitric oxide synthase 1 (nNOS), seipin, or choline acetyltransferase (ChAT) cells, whereas 100% of POMCNTS is co-expressed with transcription factor paired-like homeobox2b (Phox2b). We observed that 20% of POMCNTS cells express receptors for adipocyte hormone leptin (LepRbs) using a PomceGFP:LepRbCre:tdTOM double-reporter line. Elevations in endogenous or exogenous leptin levels increased the in vivo activity (c-FOS) of a small subset of POMCNTS cells. Using ex vivo slice electrophysiology, we observed that this effect of leptin on POMCNTS cell activity is postsynaptic. These findings reveal that a subset of POMCNTS cells are responsive to both changes in energy status and the adipocyte hormone leptin, findings of relevance to the neurobiology of obesity.

Oligomers of the lipodystrophy protein seipin may co-ordinate GPAT3 and AGPAT2 enzymes to facilitate adipocyte differentiation

Seipin deficiency causes severe congenital generalized lipodystrophy (CGL) and metabolic disease. However, how seipin regulates adipocyte development and function remains incompletely understood. We previously showed that seipin acts as a scaffold protein for AGPAT2, whose disruption also causes CGL. More recently, seipin has been reported to promote adipogenesis by directly inhibiting GPAT3, leading to the suggestion that GPAT inhibitors could offer novel treatments for CGL. Here we investigated the interactions between seipin, GPAT3 and AGPAT2. We reveal that seipin and GPAT3 associate via direct interaction and that seipin can simultaneously bind GPAT3 and AGPAT2. Inhibiting the expression of seipin, AGPAT2 or GPAT3 led to impaired induction of early markers of adipocyte differentiation in cultured cells. However, consistent with normal adipose mass in GPAT3-null mice, GPAT3 inhibition did not prevent the formation of mature adipocytes. Nonetheless, loss of GPAT3 in seipin-deficient preadipocytes exacerbated the failure of adipogenesis in these cells. Thus, our data indicate that GPAT3 plays a modest positive role in adipogenesis and argue against the potential of GPAT inhibitors to rescue white adipose tissue mass in CGL2. Overall, our study reveals novel mechanistic insights regarding the molecular pathogenesis of severe lipodystrophy caused by mutations in either seipin or AGPAT2.

Ablation of Bscl2/seipin in hepatocytes does not cause metabolic dysfunction in congenital generalised lipodystrophy

Mutations affecting the BSCL2 gene cause the most severe form of congenital generalised lipodystrophy (CGL). Affected individuals develop severe metabolic complications including diabetes and hepatic steatosis. Bscl2-deficient mice almost entirely reproduce the CGL phenotype. Adipose tissue-specific loss of Bscl2 is also sufficient to cause early-onset generalised lipodystrophy in mice. However, these mice do not show severe metabolic dysfunction, even when challenged with a high-fat diet. Germline Bscl2 loss in mice and BSCL2 disruption in humans causes severe hepatic steatosis, and the encoded protein, seipin, has acknowledged roles in lipid accumulation. Given the critical role of the liver in glucose regulation, we speculated that intact hepatic Bscl2 expression may protect adipose tissue-specific Bscl2-deficient mice from metabolic disease. To investigate this, we generated a novel mouse model in which Bscl2 has been deleted in both adipose tissue and hepatocytes simultaneously using an adeno-associated viral vector. Despite achieving efficient disruption of Bscl2 in the liver, hepatic lipid accumulation and metabolic homeostasis was unaffected in mice fed a high-fat diet for 4 weeks. We also investigated the consequences of BSCL2 ablation in the human hepatocyte HepG2 cell line using CRISPR/Cas9 genome editing. No significant increases in lipid accumulation were observed in BSCL2 knockout cell lines. Overall, we reveal that Bscl2/BSCL2 does not appear to play a cell-autonomous role in the regulation of lipid accumulation in the liver. Loss of hepatic BSCL2 is therefore unlikely to contribute significantly to the development of hepatic steatosis or metabolic dysfunction in this form of CGL.

Editor's choice: Hepatic Bscl2 ablation in adipose tissue-specific Bscl2 knockout mice does not cause metabolic dysfunction. CRISPR/Cas9 genome editing reveals that seipin does not play a cell-autonomous role in regulating hepatocyte lipid accumulation.

The burden of metabolic syndrome on osteoarthritic joints

Background

The prevalence of osteoarthritis (OA) increases with obesity, with up to two thirds of the elderly obese population affected by OA of the knee. The metabolic syndrome (MetS), frequently associated with central obesity and characterised by elevated waist circumference, raised fasting plasma glucose concentration, raised triglycerides, reduced high-density lipoproteins, and/or hypertension, is implicated in the pathogenesis of OA. This narrative review discusses the mechanisms involved in the influence of MetS on OA, with a focus on the effects on macrophages and chondrocytes.

Main text

A skewing of macrophages towards a pro-inflammatory M1 phenotype within synovial and adipose tissues is thought to play a role in OA pathogenesis. The metabolic perturbations typical of MetS are important drivers of pro-inflammatory macrophage polarisation and activity. This is mediated via alterations in the levels and activities of the cellular nutrient sensors 5′ adenosine monophosphate-activated protein kinase (AMPK) and mammalian target of rapamycin complex 1 (mTORC1), intracellular accumulation of metabolic intermediates such as succinate and citrate, and increases in free fatty acids (FFAs) and hyperglycaemia-induced advanced glycation end-products (AGEs) that bind to receptors on the macrophage surface. Altered levels of adipokines, including leptin and adiponectin, further influence macrophage polarisation. The metabolic alterations in MetS also affect the cartilage through direct effects on chondrocytes by stimulating the production of pro-inflammatory and catabolic factors and possibly by suppressing autophagy and promoting cellular senescence.

Conclusions

The influence of MetS on OA pathogenesis involves a wide range of metabolic alterations that directly affect macrophages and chondrocytes. The relative burden of intra-articular versus systemic adipose tissue in the MetS-associated OA remains to be clarified. Understanding how altered metabolism interacts with joints affected by OA is crucial for the development of further strategies for treating this debilitating condition, such as supplementing existing therapies with metformin and utilising ω-3 fatty acid derivatives to restore imbalances in ω-3 and ω-6 fatty acids.

Promethin Is a Conserved Seipin Partner Protein

Seipin (BSCL2/SPG17) is a key factor in lipid droplet (LD) biology, and its dysfunction results in severe pathologies, including the fat storage disease Berardinelli-Seip congenital lipodystrophy type 2, as well as several neurological seipinopathies. Despite its importance for human health, the molecular role of seipin is still enigmatic. Seipin is evolutionarily conserved from yeast to humans. In yeast, seipin was recently found to cooperate with the lipid droplet organization (LDO) proteins, Ldo16 and Ldo45, two structurally-related proteins involved in LD function and identity that display remote homology to the human protein promethin/TMEM159. In this study, we show that promethin is indeed an LD-associated protein that forms a complex with seipin, and its localization to the LD surface can be modulated by seipin expression levels. We thus identify promethin as a novel seipin partner protein.

Discovering metabolic disease gene interactions by correlated effects on cellular morphology

Objective

Impaired expansion of peripheral fat contributes to the pathogenesis of insulin resistance and Type 2 Diabetes (T2D). We aimed to identify novel disease–gene interactions during adipocyte differentiation.

Methods

Genes in disease-associated loci for T2D, adiposity and insulin resistance were ranked according to expression in human adipocytes. The top 125 genes were ablated in human pre-adipocytes via CRISPR/CAS9 and the resulting cellular phenotypes quantified during adipocyte differentiation with high-content microscopy and automated image analysis. Morphometric measurements were extracted from all images and used to construct morphologic profiles for each gene.

Results

Over 10⁷ morphometric measurements were obtained. Clustering of the morphologic profiles accross all genes revealed a group of 14 genes characterized by decreased lipid accumulation, and enriched for known lipodystrophy genes. For two lipodystrophy genes, BSCL2 and AGPAT2, sub-clusters with PLIN1 and CEBPA identifed by morphological similarity were validated by independent experiments as novel protein–protein and gene regulatory interactions.

Conclusions

A morphometric approach in adipocytes can resolve multiple cellular mechanisms for metabolic disease loci; this approach enables mechanistic interrogation of the hundreds of metabolic disease loci whose function still remains unknown.

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Loss-of-function genetic screen in human adipocytes for 125 genes selected from metabolic disease-associated loci.

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Genetic screen read out by cellular morphometry— 77,000 images taken with 400 morphological features extracted per image.

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Pairwise mechanistic interactions between genes identified by correlations of cellular morphometry—two interactions validated.

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Novel interaction between BSCL2 and PLIN1 from biophysical association of proteins at lipid droplet surface.

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Novel interaction between CEBPA and AGPAT2 from CEBPA dependent transcription of AGPAT2.

Nucleus of the Solitary Tract Serotonin 5-HT2C Receptors Modulate Food Intake

To meet the challenge to human health posed by obesity, a better understanding of the regulation of feeding is essential. Medications targeting 5-hydroxytryptamine (5-HT; serotonin) 2C receptors (htr2c; 5-HT_2CR) improve obesity. Here we probed the functional significance of 5-HT_2CRs specifically within the brainstem nucleus of the solitary tract (5-HT_2CR^NTS) in feeding behavior. Selective activation of 5-HT_2CR^NTS decreased feeding and was sufficient to mediate acute food intake reductions elicited by the 5-HT_2CR agonist obesity medication lorcaserin. Similar to pro-opiomelanocortin neurons expressed within the hypothalamic arcuate nucleus (POMC^ARC), a subset of POMC^NTS neurons co-expressed 5-HT_2CRs and were activated by 5-HT_2CR agonists. Knockdown of POMC^NTS prevented the acute appetite-suppressive effect of lorcaserin, whereas POMC^ARC knockdown prevented the full anorectic effect. These data identify 5-HT_2CR^NTS as a sufficient subpopulation of 5-HT_2CRs in reducing food intake when activated and reveal that 5-HT_2CR agonist obesity medications require POMC within the NTS and ARC to reduce food intake.

Adipose specific disruption of seipin causes early-onset generalised lipodystrophy and altered fuel utilisation without severe metabolic disease

Objective

Mutations to the BSCL2 gene disrupt the protein seipin and cause the most severe form of congenital generalised lipodystrophy (CGL). Affected individuals exhibit a near complete loss of white adipose tissue (WAT) and suffer from metabolic disease. Seipin is critical for adipocyte development in culture and mice with germline disruption to Bscl2 recapitulate the effects of BSCL2 disruption in humans. Here we examined whether loss of Bscl2 specifically in developing adipocytes in vivo is sufficient to prevent adipose tissue development and cause all features observed with congenital BSCL2 disruption.

Methods

We generated and characterised a novel mouse model of Bscl2 deficiency in developing adipocytes (Ad-B2^(−/−)) using the adipose-specific Adiponectin-Cre line.

Results

We demonstrate that Ad-B2^(−/−) mice display early onset lipodystrophy, in common with congenital Bscl2 null mice and CGL2 patients. However, glucose intolerance, insulin resistance, and severe hepatic steatosis are not apparent. Food intake and energy expenditure are unchanged, but Ad-B2^(−/−) mice exhibit significantly altered substrate utilisation. We also find differential effects of seipin loss between specific adipose depots revealing new insights regarding their varied characteristics. When fed a high-fat diet, Ad-B2^(−/−) mice entirely fail to expand adipose mass but remain glucose tolerant.

Conclusions

Our findings demonstrate that disruption of Bscl2 specifically in developing adipocytes is sufficient to cause the early-onset generalised lipodystrophy observed in patients with mutations in BSCL2. However, this significant reduction in adipose mass does not cause the overt metabolic dysfunction seen in Bscl2 knockout mice, even following a high-fat diet challenge.

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Seipin loss only in developing adipocytes drives severe early-onset lipodystrophy.

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This leads to significantly altered use of metabolic substrates.

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We uncover developmental differences between poorly characterised adipose depots.

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Despite severely reduced adipose mass mice do not show overt metabolic disease.

Lorcaserin improves glycemic control via a melanocortin neurocircuit

Objective

The increasing prevalence of type 2 diabetes (T2D) and associated morbidity and mortality emphasizes the need for a more complete understanding of the mechanisms mediating glucose homeostasis to accelerate the identification of new medications. Recent reports indicate that the obesity medication lorcaserin, a 5-hydroxytryptamine (5-HT, serotonin) 2C receptor (5-HT_2CR) agonist, improves glycemic control in association with weight loss in obese patients with T2D. Here we evaluate whether lorcaserin has an effect on glycemia without body weight loss and how this effect is achieved.

Methods

Murine models of common and genetic T2D were utilized to probe the direct effect of lorcaserin on glycemic control.

Results

Lorcaserin dose-dependently improves glycemic control in mouse models of T2D in the absence of reductions in food intake or body weight. Examining the mechanism of this effect, we reveal a necessary and sufficient neurochemical mediator of lorcaserin's glucoregulatory effects, brain pro-opiomelanocortin (POMC) peptides. To clarify further lorcaserin's therapeutic brain circuit, we examined the receptor target of POMC peptides. We demonstrate that lorcaserin requires functional melanocortin4 receptors on cholinergic preganglionic neurons (MC4R^ChAT) to exert its effects on glucose homeostasis. In contrast, MC4R^ChAT signaling did not impact lorcaserin's effects on feeding, indicating a divergence in the neurocircuitry underpinning lorcaserin's therapeutic glycemic and anorectic effects. Hyperinsulinemic-euglycemic clamp studies reveal that lorcaserin reduces hepatic glucose production, increases glucose disposal and improves insulin sensitivity.

Conclusions

These data suggest that lorcaserin's action within the brain represents a mechanistically novel treatment for T2D: findings of significance to a prevalent global disease.

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Obesity medication lorcaserin directly improves glycemic control without altering energy balance or body weight.

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Unlike current frontline type 2 diabetes medications, lorcaserin acts within the brain to improve glycemic control.

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Brain Pro-opiomelanocortin (POMC) peptides are a neurochemical mediator of lorcaserin's glucoregulatory effects.

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Lorcaserin increases insulin sensitivity, reduces hepatic glucose production and increases glucose disposal.

Sex difference in physical activity, energy expenditure and obesity driven by a subpopulation of hypothalamic POMC neurons

OBJECTIVE

Obesity is one of the primary healthcare challenges of the 21st century. Signals relaying information regarding energy needs are integrated within the brain to influence body weight. Central among these integration nodes are the brain pro-opiomelanocortin (POMC) peptides, perturbations of which disrupt energy balance and promote severe obesity. However, POMC neurons are neurochemically diverse and the crucial source of POMC peptides that regulate energy homeostasis and body weight remains to be fully clarified.

METHODS

Given that a 5-hydroxytryptamine 2c receptor (5-HT2CR) agonist is a current obesity medication and 5-HT2CR agonist's effects on appetite are primarily mediated via POMC neurons, we hypothesized that a critical source of POMC regulating food intake and body weight is specifically synthesized in cells containing 5-HT2CRs. To exclusively manipulate Pomc synthesis only within 5-HT2CR containing cells, we generated a novel 5-HT 2C R (CRE) mouse line and intercrossed it with Cre recombinase-dependent and hypothalamic specific reactivatable Pomc (NEO) mice to restrict Pomc synthesis to the subset of hypothalamic cells containing 5-HT2CRs. This provided a means to clarify the specific contribution of a defined subgroup of POMC peptides in energy balance and body weight.

RESULTS

Here we transform genetically programed obese and hyperinsulinemic male mice lacking hypothalamic Pomc with increased appetite, reduced physical activity and compromised brown adipose tissue (BAT) into lean, healthy mice via targeted restoration of Pomc function only within 5-HT2CR expressing cells. Remarkably, the same metabolic transformation does not occur in females, who despite corrected feeding behavior and normalized insulin levels remain physically inactive, have lower energy expenditure, compromised BAT and develop obesity.

CONCLUSIONS

These data provide support for the functional heterogeneity of hypothalamic POMC neurons, revealing that Pomc expression within 5-HT2CR expressing neurons is sufficient to regulate energy intake and insulin sensitivity in male and female mice. However, an unexpected sex difference in the function of this subset of POMC neurons was identified with regard to energy expenditure. We reveal that a large sex difference in physical activity, energy expenditure and the development of obesity is driven by this subpopulation, which constitutes approximately 40% of all POMC neurons in the hypothalamic arcuate nucleus. This may have broad implications for strategies utilized to combat obesity, which at present largely ignore the sex of the obese individual.

Seipin oligomers can interact directly with AGPAT2 and lipin 1, physically scaffolding critical regulators of adipogenesis

OBJECTIVE

Disruption of the genes encoding either seipin or 1-acylglycerol-3-phosphate O-acyltransferase 2 (AGPAT2) causes severe congenital generalized lipodystrophy (CGL) in humans. However, the function of seipin in adipogenesis remains poorly defined. We demonstrated recently that seipin can bind the key adipogenic phosphatidic acid (PA) phosphatase lipin 1 and that seipin forms stable dodecamers. As AGPAT2 generates PA, the substrate for lipin 1, we investigated whether seipin might bind both enzymes of this lipid biosynthetic pathway, which is required for adipogenesis to occur.

METHODS

We employed co-immunoprecipitation and immunofluorescence methods to determine whether seipin can interact with AGPAT2 and the consequences of this in developing adipocytes. Atomic force microscopy was used to determine whether these interactions involved direct association of the proteins and to define the molecular architecture of these complexes.

RESULTS

Our data reveal that seipin can bind AGPAT2 during adipogenesis and that stabilizing this interaction during adipogenesis can increase the nuclear accumulation of PPARγ. Both AGPAT2 and lipin 1 can directly associate with seipin dodecamers, and a single seipin complex can simultaneously bind both AGPAT2 and lipin with a defined orientation.

CONCLUSIONS

Our study provides the first direct molecular link between seipin and AGPAT2, two proteins whose disruption causes CGL. Moreover, it provides the first example of an interaction between seipin and another protein that causally influences a key aspect of adipogenesis. Together our data suggest that the critical role of seipin in adipogenesis may involve its capacity to juxtapose important regulators of this process in a multi-protein complex.

Analyzing the functions and structure of the human lipodystrophy protein seipin

Disruption of the gene BSCL2, which encodes the protein seipin, causes severe generalized lipodystrophy in humans with a near complete absence of adipose tissue. Moreover, cell culture studies have demonstrated that seipin plays a critical cell-autonomous role in adipocyte differentiation. These observations reveal seipin as a critical regulator of human adipose tissue development; however, until recently very little has been known about the potential molecular functions of this intriguing protein. Despite significant recent interest in the function of seipin, our understanding of its molecular role(s) remains limited. The topology of seipin and lack of evidence for any enzymatic domains or activity indicate that it may act principally as a scaffold for other proteins or play a structural role in altering membrane curvature and/or budding. Work in this area has been hampered by several factors, including the lack of homology that might imply testable functions, the poor availability of antibodies to the endogenous protein and the observation that this hydrophobic ER membrane-resident protein is difficult to analyze by standard Western blotting techniques. Here we summarize some of the techniques we have applied to investigate the association of seipin with a recently identified binding partner, lipin 1. In addition, we describe the use of atomic force microscopy (AFM) to image oligomers of the seipin protein. We believe that AFM will offer a valuable tool to examine the association of candidate binding proteins with the seipin oligomer.

Analysis of naturally occurring mutations in the human lipodystrophy protein seipin reveals multiple potential pathogenic mechanisms

AIMS/HYPOTHESIS

In humans, disruption of the gene BSCL2, encoding the protein seipin, causes congenital generalised lipodystrophy (CGL) with severe insulin resistance and dyslipidaemia. While the causative gene has been known for over a decade, the molecular functions of seipin are only now being uncovered. Most pathogenic mutations in BSCL2 represent substantial disruptions including significant deletions and frameshifts. However, several more subtle mutations have been reported that cause premature stop codons or single amino acid substitutions. Here we have examined these mutant forms of seipin to gain insight into how they may cause CGL.

METHODS

We generated constructs expressing mutant seipin proteins and determined their expression and localisation. We also assessed their capacity to recruit the key adipogenic phosphatidic acid phosphatase lipin 1, a recently identified molecular role of seipin in developing adipocytes. Finally, we used atomic force microscopy to define the oligomeric structure of seipin and to determine whether this is affected by the mutations.

RESULTS

We show that the R275X mutant of seipin is not expressed in pre-adipocytes. While the other premature stop mutant forms fail to bind lipin 1 appropriately, the point mutants T78A, L91P and A212P all retain this capacity. We demonstrate that wild-type human seipin forms oligomers of 12 subunits in a circular configuration but that the L91P and A212P mutants of seipin do not.

CONCLUSIONS/INTERPRETATION

Our study represents the most comprehensive analysis so far of mutants of seipin causing lipodystrophy and reveals several different molecular mechanisms by which these mutations may cause disease.

γ-synuclein is a novel player in the control of body lipid metabolism

Synucleins are a family of homologous, predominantly neuronal proteins known for their involvement in synaptic transmission and neurodegeneration. γ-synuclein is predominantly localized in axons and presynaptic terminals of selected populations of peripheral and central neurons but is also highly expressed in human white adipose tissue (WAT) and increased in obesity. We have recently shown that γ-synuclein is nutritionally regulated in murine adipocytes while its loss protects mice from high fat diet (HFD)-induced obesity and associated metabolic complications. This protection was coupled with increased adipocyte lipolysis, lipid oxidation, and energy expenditure in HFD-fed γ-synuclein-null mutant compared with wild-type mice. Cellular studies suggest that relocalization of ATGL to the lipid droplet in γ-synuclein-deficient adipocytes may contribute to increased lipolysis in these cells. Loss of γ-synuclein in adipocytes also attenuates the assembly of SNARE complexes, an important component of lipid droplet fusion machinery, possibly due to reduced chaperoning of SNAP-23 to the assembling SNARE complex by γ-synuclein. Together our data suggests that not only is γ-synuclein a novel regulator of lipid handling in adipocytes but also that the deficiency of this protein has a significant effect on whole body energy expenditure.

Increased lipolysis and altered lipid homeostasis protect γ-synuclein-null mutant mice from diet-induced obesity

Synucleins are a family of homologous proteins principally known for their involvement in neurodegeneration. γ-Synuclein is highly expressed in human white adipose tissue and increased in obesity. Here we show that γ-synuclein is nutritionally regulated in white adipose tissue whereas its loss partially protects mice from high-fat diet (HFD)-induced obesity and ameliorates some of the associated metabolic complications. Compared with HFD-fed WT mice, HFD-fed γ-synuclein-null mutant mice display increased lipolysis, lipid oxidation, and energy expenditure, and reduced adipocyte hypertrophy. Knockdown of γ-synuclein in adipocytes causes redistribution of the key lipolytic enzyme ATGL to lipid droplets and increases lipolysis. γ-Synuclein-deficient adipocytes also contain fewer SNARE complexes of a type involved in lipid droplet fusion. We hypothesize that γ-synuclein may deliver SNAP-23 to the SNARE complexes under lipogenic conditions. Via these independent but complementary roles, γ-synuclein may coordinately modulate lipid storage by influencing lipolysis and lipid droplet formation. Our data reveal γ-synuclein as a regulator of lipid handling in adipocytes, the function of which is particularly important in conditions of nutrient excess.

Identification and Characterisation of a Novel Pathogenic Mutation in the Human Lipodystrophy Gene AGPAT2 : C48R: A Novel Mutation in AGPAT2

Loss-of-function mutations in AGPAT2, encoding 1-acylglycerol-3-phosphate-O-acyltransferase 2 (AGPAT2), produce congenital generalised lipodystrophy (CGL). We screened the AGPAT2 gene in two siblings who presented with pseudoacromegaly, diabetes and severe dyslipidaemia and identified a novel mutation in AGPAT2 causing a single amino acid substitution, p.Cys48Arg. We subsequently investigated the molecular pathogenic mechanism linking both this mutation and the previously reported p.Leu228Pro mutation to clinical disease. Wild-type and mutant AGPAT2 were expressed in control and AGPAT2-deficient preadipocyte cell lines. mRNA and protein expression was determined, and the ability of each AGPAT2 species to rescue adipocyte differentiation in AGPAT2-deficient cells was assessed. Protein levels of both p.Cys48Arg and p.Leu228Pro AGPAT2 were significantly reduced compared with that of wild-type AGPAT2 despite equivalent mRNA levels. Stable expression of wild-type AGPAT2 partially rescued adipogenesis in AGPAT2 deficient preadipocytes, whereas stable expression of p.Cys48Arg or p.Leu228Pro AGPAT2 did not. In conclusion, unusually severe dyslipidaemia and pseudoacromegaloid overgrowth in patients with diabetes should alert physicians to the possibility of lipodystrophy. Both the previously unreported pathogenic p.Cys48Arg mutation in AGPAT2, and the known p.Leu228Pro mutation result in decreased AGPAT2 protein expression in developing adipocytes. It is most likely that the CGL seen in homozygous carriers of these mutations is largely accounted for by loss of protein expression.

Neuroanatomical characterisation of the expression of the lipodystrophy and motor-neuropathy gene Bscl2 in adult mouse brain

The endoplasmic reticulum localised protein seipin, encoded by the gene Berardinelli-Seip congenital lipodystrophy type 2 (BSCL2), serves a critical but poorly defined function in the physiology of both adipose and neural tissue. In humans, BSCL2 loss-of-function mutations cause a severe form of lipodystrophy, whilst a distinct set of gain-of-toxic-function mutations are associated with a heterogeneous group of neuropathies. However, despite the importance of seipin dysfunction to the pathophysiology of these conditions, little is known about its physiological role in adipocytes or neurons. BSCL2 mRNA has previously been identified in human and mouse brain, yet no definitive assessment of its expression has been undertaken. Here we comprehensively characterised the neuroanatomical distribution of mouse Bscl2 using complementary in situ hybridisation histochemistry and immunohistochemistry techniques. Whilst Bscl2 was broadly expressed throughout the rostral-caudal extent of the mouse brain, it exhibited a discrete neuroanatomical profile. Bscl2 was most abundantly expressed in the hypothalamus and in particular regions associated with the regulation of energy balance including, the paraventricular, ventromedial, arcuate and dorsomedial nuclei. Bscl2 expression was also identified within the brainstem dorsal vagal complex, which together with the paraventricular nucleus of the hypothalamus represented the site of highest expression. Further neurochemical profiling of these two nuclei revealed Bscl2/seipin expression within energy balance related neuronal populations. Specifically, seipin was detected in oxytocin neurons of the paraventricular nucleus of the hypothalamus and in catecholamine neurons of the dorsal vagal complex. These data raise the possibility that in addition to its role in adipose tissue development, seipin may also be involved in the central regulation of energy balance.

Investigating the involvement of the ATF6α pathway of the unfolded protein response in adipogenesis

The unfolded protein response (UPR) is activated by endoplasmic reticulum stress resulting from an accumulation of unfolded or mis-folded proteins. The UPR is divided into three arms, involving the activation of ATF-6, PERK and IRE-1, that together act to restrict new protein synthesis and increase the production of chaperones. Recent studies have implicated the PERK and IRE-1 components of the UPR in adipocyte differentiation. In this study, we investigate the importance of ATF6α during adipogenesis using stable knockdown of this protein in the model adipogenic cell line, C3H10T1/2. Reduction of ATF6α expression by >70% resulted in impaired expression of key adipogenic genes and reduced lipid accumulation following the induction of adipogenesis. In contrast, loss of ATF6α did not impair the ability of cells to undergo osteogenic differentiation. Overall, our data indicate that all three arms of the UPR, including ATF6α, must be intact to permit adipogenesis to occur.

Setting the tone: reactive oxygen species and the control of appetitive melanocortin neurons

The brain melanocortin system is a primary gateway through which energy balance is controlled. Diano and colleagues report a novel cellular mechanism mediated via reactive oxygen species (ROS) that regulates the activity of these melanocortin neurons in response to energy status, thereby modulating appetitive behavior (Diano et al., 2011).

Lipodystrophy: metabolic insights from a rare disorder

Obesity, insulin resistance and their attendant complications are among the leading causes of morbidity and premature mortality today, yet we are only in the early stages of understanding the molecular pathogenesis of these aberrant phenotypes. A powerful approach has been the study of rare patients with monogenic syndromes that manifest as extreme phenotypes. For example, there are striking similarities between the biochemical and clinical profiles of individuals with excess fat (obesity) and those with an abnormal paucity of fat (lipodystrophy), including severe insulin resistance, dyslipidaemia, hepatic steatosis and features of hyperandrogenism. Rare lipodystrophy patients therefore provide a tractable genetically defined model for the study of a prevalent human disease phenotype. Indeed, as we review herein, detailed study of these syndromes is beginning to yield valuable insights into the molecular genetics underlying different forms of lipodystrophy, the essential components of normal adipose tissue development and the mechanisms by which disturbances in adipose tissue function can lead to almost all the features of the metabolic syndrome.

A truncation mutation in TBC1D4 in a family with acanthosis nigricans and postprandial hyperinsulinemia

Tre-2, BUB2, CDC16, 1 domain family member 4 (TBC1D4) (AS160) is a Rab-GTPase activating protein implicated in insulin-stimulated glucose transporter 4 (GLUT4) translocation in adipocytes and myotubes. To determine whether loss-of-function mutations in TBC1D4 might impair GLUT4 translocation and cause insulin resistance in humans, we screened the coding regions of this gene in 156 severely insulin-resistant patients. A female presenting at age 11 years with acanthosis nigricans and extreme postprandial hyperinsulinemia was heterozygous for a premature stop mutation (R363X) in TBC1D4. After demonstrating reduced expression of wild-type TBC1D4 protein and expression of the truncated protein in lymphocytes from the proband, we further characterized the biological effects of the truncated protein in 3T3L1 adipocytes. Prematurely truncated TBC1D4 protein tended to increase basal cell membrane GLUT4 levels (P = 0.053) and significantly reduced insulin-stimulated GLUT4 cell membrane translocation (P < 0.05). When coexpressed with wild-type TBC1D4, the truncated protein dimerized with full-length TBC1D4, suggesting that the heterozygous truncated variant might interfere with its wild-type counterpart in a dominant negative fashion. Two overweight family members with the mutation also manifested normal fasting glucose and insulin levels but disproportionately elevated insulin levels following an oral glucose challenge. This family provides unique genetic evidence of TBC1D4 involvement in human insulin action.

Temporal and spatial regulation of the phosphatidate phosphatases lipin 1 and 2

Lipins are the founding members of a novel family of Mg²⁺-dependent phosphatidate phosphatases (PAP1 enzymes) that play key roles in fat metabolism and lipid biosynthesis. Despite their importance, there is still little information on how their activity is regulated. Here we demonstrate that the functions of lipin 1 and 2 are evolutionarily conserved from unicellular eukaryotes to mammals. The two lipins display distinct intracellular localization in HeLa M cells, with a pool of lipin 2 exhibiting a tight membrane association. Small interfering RNA-mediated silencing of lipin 1 leads to a dramatic decrease of the cellular PAP1 activity in HeLa M cells, whereas silencing of lipin 2 leads to an increase of lipin 1 levels and PAP1 activity. Consistent with their distinct functions in HeLa M cells, lipin 1 and 2 exhibit reciprocal patterns of protein expression in differentiating 3T3-L1 adipocytes. Lipin 2 levels increase in lipin 1-depleted 3T3-L1 cells without rescuing the adipogenic defects, whereas depletion of lipin 2 does not inhibit adipogenesis. Finally, we show that the PAP1 activity of both lipins is inhibited by phosphorylation during mitosis, leading to a decrease in the cellular PAP1 activity during cell division. We propose that distinct and non-redundant functions of lipin 1 and 2 regulate lipid production during the cell cycle and adipocyte differentiation.

The human lipodystrophy gene BSCL2/seipin may be essential for normal adipocyte differentiation

OBJECTIVE

Berardinelli-Seip congenital lipodystrophy type 2 (BSCL2) is a recessive disorder featuring near complete absence of adipose tissue. Remarkably, although the causative gene, BSCL2, has been known for several years, its molecular function and its role in adipose tissue development have not been elucidated. Therefore, we examined whether BSCL2 is involved in the regulation of adipocyte differentiation and the mechanism whereby pathogenic mutations in BSCL2 cause lipodystrophy.

RESEARCH DESIGN AND METHODS

Following the characterization of BSCL2 expression in developing adipocytes, C3H10T1/2 mesenchymal stem cells were generated in which BSCL2 expression was knocked down using short hairpin RNA (shRNA). These cells were used to investigate whether BSCL2 is required for adipogenesis. BSCL2 constructs harboring pathogenic mutations known to cause lipodystrophy were also generated and characterized.

RESULTS

BSCL2 expression was strongly induced during adipocyte differentiation, and the induction of BSCL2 expression was essential for adipogenesis to occur. The initial induction of key adipogenic transcription factors, including peroxisome proliferator-activated receptor (PPAR)gamma and CAAT/enhancer-binding protein-alpha, was preserved in cells lacking BSCL2. However, the expression of these critical factors was not sustained, suggesting that the activity of PPARgamma was impaired. Moreover, expression of key genes mediating triglyceride synthesis, including AGPAT2, lipin 1, and DGAT2, was persistently reduced and lipid accumulation was inhibited. Analysis of pathogenic missense mutants of BSCL2 revealed that the amino acid substitution A212P causes aberrant targeting of BSCL2 within the cell, suggesting that subcellular localization of BSCL2 may be critical to its function.

CONCLUSIONS

This study demonstrates that BSCL2 is an essential, cell-autonomous regulator of adipogenesis.

Serotonin 5-HT2C receptor agonist promotes hypophagia via downstream activation of melanocortin 4 receptors

The neurotransmitter serotonin (5-hydroxytryptamine) is a well-established modulator of energy balance. Both pharmacological and genetic evidence implicate the serotonin 2C receptor (5-HT(2C)R) as a critical receptor mediator of serotonin's effects on ingestive behavior. Here we characterized the effect of the novel and selective 5-HT(2C)R agonist BVT.X on energy balance in obese and lean mice and report that BVT.X significantly reduces acute food intake without altering locomotor activity or oxygen consumption. In an effort to elucidate the mechanism of this effect, we examined the chemical phenotype of 5-HT(2C)R-expressing neurons in a critical brain region affecting feeding behavior, the arcuate nucleus of the hypothalamus. We show that 5-HT(2C)Rs are coexpressed with neurons containing proopiomelanocortin, known to potently affect appetite, in the arcuate nucleus of the hypothalamus of the mouse. We then demonstrate that prolonged infusion with BVT.X in obese mice significantly increases Pomc mRNA and reduces body weight, percent body fat, and initial food intake. To evaluate the functional importance of melanocortin circuitry in the effect of BVT.X on ingestive behavior, we assessed mice with disrupted melanocortin pathways. We report that mice lacking the melanocortin 4 receptor are not responsive to BVT.X-induced hypophagia, demonstrating that melanocortins acting on melanocortin 4 receptor are a requisite downstream pathway for 5-HT(2C)R agonists to exert effects on food intake. The data presented here not only indicate that the novel 5-HT(2C)R agonist BVT.X warrants further investigation as a treatment for obesity but also elucidate specific neuronal pathways potently affecting energy balance through which 5-HT(2C)R agonists regulate ingestive behavior.

The NR4A family of orphan nuclear receptors are not required for adipogenesis

Nuclear hormone receptors of the NR4A subfamily are rapidly induced during the early stages of adipogenesis, leading to the speculation that they may have important roles in this process. One of the three subfamily members, Nur77 has also been shown to play key roles in energy expenditure and lipolysis in skeletal muscle and in the control of hepatic gluconeogenesis. We, therefore, examined the role of NR4A factors in adipogenesis using the well-characterized 3T3-L1 preadipocyte model. Inhibition of Nur77 expression using siRNA did not affect induction of adipogenic genes, nor the accumulation of lipid. To inhibit the activity of all the three NR4A family members, we generated preadipocytes stably expressing a well-characterized dominant-negative Nur77 (DN-Nur77), known to block the function of the other NR4A factors, Nurr1 and Nor1, as well as Nur77. While the increased NR4A activity observed following adipogenic induction was completely abolished in these cells, DN-Nur77 expression did not affect the expression of genes characteristic of terminally differentiated adipocytes and had no impact on lipid accumulation in these cells. Thus, while members of the NR4A subfamily of nuclear receptors may have important metabolic roles in skeletal muscle and liver, we demonstrate that they are dispensable for normal adipocyte development.

Serotonin 2C receptor agonists improve type 2 diabetes via melanocortin-4 receptor signaling pathways

The burden of type 2 diabetes and its associated premature morbidity and mortality is rapidly growing, and the need for novel efficacious treatments is pressing. We report here that serotonin 2C receptor (5-HT(2C)R) agonists, typically investigated for their anorectic properties, significantly improve glucose tolerance and reduce plasma insulin in murine models of obesity and type 2 diabetes. Importantly, 5-HT(2C)R agonist-induced improvements in glucose homeostasis occurred at concentrations of agonist that had no effect on ingestive behavior, energy expenditure, locomotor activity, body weight, or fat mass. We determined that this primary effect on glucose homeostasis requires downstream activation of melanocortin-4 receptors (MC4Rs), but not MC3Rs. These findings suggest that pharmacological targeting of 5-HT(2C)Rs may enhance glucose tolerance independently of alterations in body weight and that this may prove an effective and mechanistically novel strategy in the treatment of type 2 diabetes.

Sequential regulation of diacylglycerol acyltransferase 2 expression by CAAT/enhancer-binding protein beta (C/EBPbeta) and C/EBPalpha during adipogenesis

Diacylglycerol acyltransferase 2 (DGAT2) catalyzes the final step of triacylglycerol (TG) synthesis. Despite the existence of an alternative acyltransferase (DGAT1), mice lacking DGAT2 have a severe deficiency of TG in adipose tissue, indicating a nonredundant role for this enzyme in adipocyte TG synthesis. We have studied the regulation of DGAT2 expression during adipogenesis. In both isolated murine preadipocytes and 3T3-L1 cells the temporal pattern of DGAT2 expression closely mimicked that of genes whose expression is regulated by CAAT/enhancer-binding protein beta (C/EBPbeta). Inhibition of C/EBPbeta expression in differentiating preadipocytes reduced DGAT2 expression, and electrophoretic mobility shift assay and chromatin immunoprecipitation experiments identified a promoter element in the DGAT2 gene that is likely to mediate this effect. The importance of C/EBPbeta in adipocyte expression of DGAT2 was confirmed by the finding of reduced DGAT2 expression in the adipose tissue of C/EBPbeta-null animals. However, DGAT2 expression is maintained at high levels during the later stages of adipogenesis, when C/EBPbeta levels decline. We show that, at these later stages of differentiation, C/EBPalpha is capable of substituting for C/EBPbeta at the same promoter element. These observations provide novel insight into the transcriptional regulation of DGAT2 expression. Moreover, they further refine the complex and serial roles of the C/EBP family of transcription factors in inducing and maintaining the metabolic properties of mature adipocytes.

ETO/MTG8 is an inhibitor of C/EBPbeta activity and a regulator of early adipogenesis

The putative transcriptional corepressor ETO/MTG8 has been extensively studied due to its involvement in a chromosomal translocation causing the t(8;21) form of acute myeloid leukemia. Despite this, the role of ETO in normal physiology has remained obscure. Here we show that ETO is highly expressed in preadipocytes and acts as an inhibitor of C/EBPbeta during early adipogenesis, contributing to its characteristically delayed activation. ETO prevents both the transcriptional activation of the C/EBPalpha promoter by C/EBPbeta and its concurrent accumulation in centromeric sites during early adipogenesis. ETO expression rapidly reduces after the initiation of adipogenesis, and this is essential to the normal induction of adipogenic gene expression. These findings define, for the first time, a molecular role for ETO in normal physiology as an inhibitor of C/EBPbeta and a novel regulator of early adipogenesis.

A family with severe insulin resistance and diabetes due to a mutation in AKT2

Inherited defects in signaling pathways downstream of the insulin receptor have long been suggested to contribute to human type 2 diabetes mellitus. Here we describe a mutation in the gene encoding the protein kinase AKT2/PKBbeta in a family that shows autosomal dominant inheritance of severe insulin resistance and diabetes mellitus. Expression of the mutant kinase in cultured cells disrupted insulin signaling to metabolic end points and inhibited the function of coexpressed, wild-type AKT. These findings demonstrate the central importance of AKT signaling to insulin sensitivity in humans.

Mice lacking pro-opiomelanocortin are sensitive to high-fat feeding but respond normally to the acute anorectic effects of peptide-YY(3-36)

Inactivating mutations of the pro-opiomelanocortin (POMC) gene in both mice and humans leads to hyperphagia and obesity. To further examine the mechanisms whereby POMC-deficiency leads to disordered energy homeostasis, we have generated mice lacking all POMC-derived peptides. Consistent with a previously reported model, Pomc(-/-) mice were obese and hyperphagic. They also showed reduced resting oxygen consumption associated with lowered serum levels of thyroxine. Hypothalami from Pomc(-/-) mice showed markedly increased expression of melanin-concentrating hormone mRNA in the lateral hypothalamus, but expression of neuropeptide Y mRNA in the arcuate nucleus was not altered. Provision of a 45% fat diet increased energy intake and body weight in both Pomc(-/-) and Pomc(+/-) mice. The effects of leptin on food intake and body weight were blunted in obese Pomc(-/-) mice whereas nonobese Pomc(-/-) mice were sensitive to leptin. Surprisingly, we found that Pomc(-/-) mice maintained their acute anorectic response to peptide-YY(3-36) (PYY(3-36)). However, 7 days of PYY(3-36) administration had no effect on cumulative food intake or body weight in wild-type or Pomc(-/-) mice. Thus, POMC peptides seem to be necessary for the normal response of energy balance to high-fat feeding, but not for the acute anorectic effect of PYY(3-36) or full effects of leptin on feeding. The finding that the loss of only one copy of the Pomc gene is sufficient to render mice susceptible to the effects of high fat feeding emphasizes the potential importance of this locus as a site for gene-environment interactions predisposing to obesity.

Role of the POZ zinc finger transcription factor FBI-1 in human and murine adipogenesis

Poxvirus zinc finger (POZ) zinc finger domain transcription factors have been shown to play a role in the control of growth arrest and differentiation in several types of mesenchymal cells but not, as yet, adipocytes. We found that a POZ domain protein, factor that binds to inducer of short transcripts-1 (FBI-1), was induced during both murine and human preadipocyte differentiation with maximal expression levels seen at days 2-4. FBI-1 mRNA was expressed in human adipose tissue with the highest levels found in samples from morbidly obese subjects. Murine cell lines constitutively expressing FBI-1 showed evidence for accelerated adipogenesis with earlier induction of markers of differentiation and enhanced lipid accumulation, suggesting that FBI-1 may be an active participant in the differentiation process. Consistent with the properties of this family of proteins in other cell systems, 3T3L1 cells stably overexpressing FBI-1 showed reduced DNA synthesis and reduced expression of cyclin A, cyclin-dependent kinase 2, and p107, proteins known to be involved in the regulation of mitotic clonal expansion. In addition, FBI-1 reduced the transcriptional activity of the cyclin A promoter. Thus, FBI-1, a POZ zinc finger transcription factor, is induced during the early phases of human and murine preadipocyte differentiation where it may contribute to adipogenesis through influencing the switch from cellular proliferation to terminal differentiation.

Insulin and insulin-like growth factor-I induce vascular endothelial growth factor mRNA expression via different signaling pathways

In this study we have investigated the molecular mechanisms of insulin and insulin-like growth factor-I (IGF-I) action on vascular endothelial growth factor (VEGF) gene expression. Treatment with insulin or IGF-I for 4 h increased the abundance of VEGF mRNA in NIH3T3 fibroblasts expressing either the human insulin receptor (NIH-IR) or the human IGF-I receptor (NIH-IGFR) by 6- and 8-fold, respectively. The same elevated levels of mRNA were maintained after 24 h of stimulation with insulin, whereas IGF-I treatment further increased VEGF mRNA expression to 12-fold after 24 h. Pre-incubation with the phosphatidylinositol 3-kinase inhibitor wortmannin abolished the effect of insulin on VEGF mRNA expression in NIH-IR cells but did not modify the IGF-I-induced VEGF mRNA expression in NIH-IGFR cells. Blocking mitogen-activated protein kinase activation with the MEK inhibitor PD98059 abolished the effect of IGF-I on VEGF mRNA expression in NIH-IGFR cells but had no effect on insulin-induced VEGF mRNA expression in NIH-IR cells. Expression of a constitutively active PKB in NIH-IR cells induced the expression of VEGF mRNA, which was not further modified by insulin treatment. We conclude that VEGF induction by insulin and IGF-I occurs via different signaling pathways, the former involving phosphatidylinositol 3-kinase/protein kinase B and the latter involving MEK/mitogen-activated protein kinase.

Control of glycogen synthesis in cultured human muscle cells

The regulation of glycogen synthesis and associated enzymes was studied in human myoblasts and myotubes maintained in culture. Both epidermal growth factor (EGF) and insulin stimulated glycogen synthesis approximately 2-fold, this stimulation being accompanied by a rapid and stable activation of the controlling enzyme glycogen synthase (GS). EGF also caused inhibition of glycogen synthase kinase 3 (GSK-3) and activation of the alpha isoform of protein kinase B (PKB) with the time-course and magnitude of its effects being similar to those induced by insulin. An inhibitor of the mitogen-activated protein (MAP) kinase pathway did not prevent stimulation of GS by EGF, suggesting that this pathway is not essential for the effect. A partial decrease in the fold activation of GS was, however, observed when p70(S6k) activation was blocked with rapamycin, suggesting a contribution of this pathway to the control of GS by either hormone. Wortmannin, a selective inhibitor of phosphatidylinositol 3'-kinase (PI-3 kinase) completely blocked the effects of both EGF and insulin in these cells. These results demonstrate that EGF, like insulin, activates glycogen synthesis in muscle, acting principally via the PKB/GSK-3 pathway but with a contribution from a rapamycin-sensitive component that lies downstream of PI-3 kinase.

Signalling pathways involved in the stimulation of glycogen synthesis by insulin in rat hepatocytes

In hepatocytes glycogen storage is stimulated by insulin and this effect of insulin is counteracted by epidermal growth factor (EGF). The mechanism by which insulin stimulates glycogen synthesis in liver is unknown. We investigated the involvement of candidate protein kinases in insulin signalling in hepatocytes. Both insulin and EGF activated extracellular regulated kinase 2 (ERK-2), p70rsk and protein kinase B (PKB) and inactivated glycogen synthase kinase-3 (GSK-3). Whereas EGF caused a greater activation of ERK-2 than insulin, the converse was true for PKB. The stimulation by insulin of ERK-2 was blocked by a mitogen-activated protein (MEK) inhibitor (PD 98059) and of p70rsk by rapamycin. However, these inhibitors, separately or in combination, did not block the stimulation of glycogen synthesis by insulin, indicating that activation of these kinases is not essential for the stimulation of glycogen synthesis by insulin. Mono Q fractionation of hepatocyte extracts resolved a single myelin basic protein (MBP) kinase peak from extracts of EGF-treated cells (peak 1, eluting at 200 mmol/l NaCl) and two peaks from insulin-treated cells (peak 1 eluting at 200 mmol/l NaCl and peak 2 eluting at 400 mmol/l NaCl). In the combined presence of insulin and EGF, activation of peak 2 was abolished. In situ MBP kinase assays and immunoblotting established that peak 1 coincides with ERK-2 and peak 2 is not an activated form of ERK-1 or ERK-2. It is concluded that PKB, which is activated to a greater extent by insulin than EGF, and peak 2, which is activated by insulin and counteracted by EGF, are possible candidates in mediating the stimulation of glycogen synthesis by insulin.

Insulin action in cultured human myoblasts: contribution of different signalling pathways to regulation of glycogen synthesis

A key metabolic action of insulin is the stimulation of non-oxidative glucose utilization in skeletal muscle, by increasing both glucose uptake and glycogen synthesis. The molecular mechanism underlying this process has been investigated using a variety of experimental systems. We report here the use of cultured human myoblasts to study insulin control of glycogen synthesis in humans. In these cells insulin stimulates glycogen synthesis approx. 2.2-fold, associated with a similar activation of glycogen synthase (GS) which occurs within 5-10 min of the addition of insulin. Insulin also causes inactivation of glycogen synthase kinase-3 (GSK-3) and activation of protein kinase B, both processes being sufficiently rapid to account for the effects of insulin on GS. Activation by insulin of the protein kinases p70s6K, p90s6K and extracellular signal-regulated kinase 2 (ERK2) is observed, but is significantly slower than the activation of GS. Selective inhibitors of the p70s6K pathway (rapamycin), the ERK2/p90s6K pathway (PD98059) and phosphatidylinositol 3-kinase (wortmannin) have been used to probe the contribution of these components to insulin signalling in human muscle. Wortmannin blocks activation of both glycogen synthesis and GS and inactivation of GSK-3. PD98059 is without effect on these events, while rapamycin is without effect on inactivation of GSK-3 but partially blocks activation of glycogen synthesis and GS. Taken together, these findings suggest that protein kinase B is responsible for the inactivation of GSK-3, but that an additional rapamycin-sensitive mechanism may contribute to the activation of GS and stimulation of glycogen synthesis.

Inhibition of glycogen synthase kinase-3 by insulin in cultured human skeletal muscle myoblasts

The acute effects of insulin on the activity of glycogen synthase kinase 3 (GSK-3) have been investigated both in the rat L6 muscle cell line and in cultured human myoblasts. The alpha and beta isoforms of GSK-3 are present in both cell types, with the beta isoform being predominant in the human cells. Insulin causes a rapid inactivation of both isoforms in both cell lines, with 50% inactivation being observed in the human myoblasts.