Overexpressing STAMP2 attenuates adipose tissue angiogenesis and insulin resistance in diabetic ApoE−/−/LDLR−/− mouse via a PPARγ/CD36 pathway

The aim of this study was to investigate whether overexpression of STAMP2 improves insulin resistance by regulating angiogenesis in adipose tissues. The characteristics of diabetic mice were measured by serial metabolite and pathology tests. Samples were obtained from epididymal, subcutaneous and brown adipose tissues. Histological and morphological analysis demonstrated that STAMP2 gene overexpression reduced adipocyte size, angiogenesis in epididymal and brown adipose tissues. On aortic ring assay, microvessels sprouting from aortas were significantly inhibited after STAMP2 gene overexpression. The cellular effect of STAMP2 on angiogenesis was explored in human umbilical vein endothelial cells (HUVECs) model. Correlation of STAMP2 and angiogenesis was validated by Ad‐STAMP2 transfection and STAMP2 siRNA inhibition. In vitro, overexpression of STAMP2 significantly inhibited endothelial cell migration, tube formation. The effects of Ad‐STAMP2 transfection on HUVECs were abolished by treatment with PPARγ antagonist GW9662 (2.5 μM), and the roles of STAMP2 siRNA on HUVECs were also reversed by treatment with PPARγ agonist rosiglitazone (RSG) (0.1 mM). RT‐PCR indicated that STAMP2 could regulate levels of adhesion molecules, vascular endothelial growth factor A and CD36. The expression of PPARγ and CD36 was decreased when STAMP2 was inhibited by siRNA, while PPARγ and CD36 were highly expressed after overexpression of STAMP2. Our results suggested that STAMP2 gene overexpression may improve insulin resistance via attenuating angiogenesis in epididymal and brown adipose tissues through the PPARγ/CD36 signalling pathway.     

Direct action of capsaicin in brown adipogenesis and activation of brown adipocytes

The ingestion of capsaicin, the principle pungent component of red and chili peppers, induces thermogenesis, in part, through the activation of brown adipocytes expressing genes related to mitochondrial biogenesis and uncoupling such as peroxisome proliferator‐activated receptor (Ppar) γ coactivator‐1α (Pgc‐1α) and uncoupling protein 1 (Ucp1). Capsaicin has been suggested to induce the activation of brown adipocytes, which is mediated by the stimulation of sympathetic nerves. However, capsaicin may directly affect the differentiation of brown preadipocytes, brown adipocyte function, or both, through its significant absorption. We herein demonstrated that Trpv1, a capsaicin receptor, is expressed in brown adipose tissue, and that its expression level is increased during the differentiation of HB2 brown preadipocytes. Furthermore, capsaicin induced calcium influx in brown preadipocytes. A treatment with capsaicin in the early stage of brown adipogenesis did not affect lipid accumulation or the expression levels of Fabp4 (a gene expressed in mature adipocytes), Pparγ2 (a master regulator of adipogenesis) or brown adipocyte‐selective genes. In contrast, a treatment with capsaicin in the late stage of brown adipogenesis slightly increased the expression levels of Fabp4, Pparγ2 and Pgc‐1α. Although capsaicin did not affect the basal expression level of Ucp1, Ucp1 induction by forskolin was partially inhibited by capsaicin, irrespective of the dose of capsaicin. The results of the present study suggest the direct effects of capsaicin on brown adipocytes or in the late stage of brown adipogenesis.     

Minireview: PPARγ as the target of obesogens

The peroxisome proliferator-activated receptor gamma (PPARγ) is a key regulator of adipogenesis and is medically important for its connections to obesity and the treatment of type II diabetes. Activation of this receptor by certain natural or xenobiotic compounds has been shown to stimulate adipogenesis in vitro and in vivo. Obesogens are chemicals that ultimately increase obesity through a variety of potential mechanisms, including activation of PPARγ. The first obesogen for which a definitive mechanism of action has been elucidated is the PPARγ and RXR activator tributyltin; however, not all chemicals that activate PPARγ are adipogenic or correlated with obesity in humans. There are multiple mechanisms through which obesogens can target PPARγ that may not involve direct activation of the receptor. Ligand-independent mechanisms could act through obesogen-mediated post-translational modification of PPARγ which cause receptor de-repression or activation. PPARγ is active in multipotent stem cells committing to the adipocyte fate during fat cell development. By modifying chromatin structure early in development, obesogens have the opportunity to influence the promoter activity of PPARγ, or the ability of PPARγ to bind to its target genes, ultimately biasing the progenitor pool towards the fat lineage. Obesogens that act by directly or indirectly activating PPARγ, by increasing the levels of PPARγ protein, or enhancing its recruitment to promoters of key genes in the adipogenic pathway may ultimately play an important role in adipogenesis and obesity.     

Garcinia cambogia Extract Ameliorates Visceral Adiposity in C57BL/6J Mice Fed on a High-Fat Diet

The aim of present study is to evaluate the effects of Garcinia cambogia on the mRNA levels of the various genes involved in adipogenesis, as well as on body weight gain, visceral fat accumulation, and other biochemical markers of obesity in obesity-prone C57BL/6J mice. Consumption of the Garcinia cambogia extract effectively lowered the body weight gain, visceral fat accumulation, blood and hepatic lipid concentrations, and plasma insulin and leptin levels in a high-fat diet (HFD)-induced obesity mouse model. The Garcinia cambogia extract reversed the HFD-induced changes in the expression pattern of such epididymal adipose tissue genes as adipocyte protein aP2 (aP2), sterol regulatory element-binding factor 1c (SREBP1c), peroxisome proliferator-activated receptor γ2 (PPARγ2), and CCAT/enhancer-binding protein α (C/EBPα). These findings suggest that the Garcinia cambogia extract ameliorated HFD-induced obesity, probably by modulating multiple genes associated with adipogenesis, such as aP2, SREBP1c, PPARγ2, and C/EBPα in the visceral fat tissue of mice.     

HRASLS3 is a PPARgamma-selective target gene that promotes adipocyte differentiation

The prevalence of obesity and its associated metabolic diseases worldwide has focused attention on understanding the mechanisms underlying adipogenesis. The nuclear receptor PPARgamma has emerged as a central regulator of adipose tissue function and formation. Despite the identification of numerous PPARgamma targets involved in a range of processes, from lipid droplet formation to adipokine secretion, information is still lacking on targets downstream of PPARgamma that directly affect fat cell differentiation. Here we identify HRASLS3 as a novel PPARgamma regulated gene with a role in adipogenesis. HRASLS3 expression increases during the differentiation of preadipocyte cell lines and is highly expressed in white and brown adipose tissue in mice. HRASLS3 expression is induced by PPARgamma ligands in preadipocyte cell lines as well in adipose tissue in vivo. We demonstrate that the HRASLS3 promoter contains a functional PPAR response element and is a direct target for regulation by PPARgamma/RXR heterodimers. Finally, we show that overexpression of HRASLS3 augments PPARgamma-driven lipid accumulation and adipogenesis, whereas siRNA-mediated knockdown of HRASLS3 expression decreases differentiation. Together, these results identify HRASLS3 as one of the downstream effectors of PPARgamma action in adipogenesis.     

Roles for peroxisome proliferator-activated receptor γ (PPARγ) and PPARγ coactivators 1α and 1β in regulating response of white and brown adipocytes to hypoxia

Obese white adipose tissue is hypoxic but is incapable of inducing compensatory angiogenesis. Brown adipose tissue is highly vascularized, facilitating delivery of nutrients to brown adipocytes for heat production. In this study, we investigated the mechanisms by which white and brown adipocytes respond to hypoxia. Brown adipocytes produced lower amounts of hypoxia-inducible factor 1α (HIF-1α) than white adipocytes in response to low O(2) but induced higher levels of hypoxia-associated genes. The response of white adipocytes to hypoxia required HIF-1α, but its presence alone was incapable of inducing target gene expression under normoxic conditions. In addition to the HIF-1α targets, hypoxia also induced many inflammatory genes. Exposure of white adipocytes to a peroxisome proliferator-activated receptor γ (PPARγ) ligand (troglitazone) attenuated induction of these genes but enhanced expression of the HIF-1α targets. Knockdown of PPARγ in mature white adipocytes prevented the usual robust induction of HIF-1α targets in response to hypoxia. Similarly, knockdown of PPARγ coactivator (PGC) 1β in PGC-1α-deficient brown adipocytes eliminated their response to hypoxia. These data demonstrate that the response of white adipocytes requires HIF-1α but also depends on PPARγ in white cells and the PPARγ cofactors PGC-1α and PGC-1β in brown cells.     

Beta-arrestin-1 protein represses adipogenesis and inflammatory responses through its interaction with peroxisome proliferator-activated receptor-gamma (PPARgamma)

One of the master regulators of adipogenesis and macrophage function is peroxisome proliferator-activated receptor-γ (PPARγ). Here, we report that a deficiency of β-arrestin-1 expression affects PPARγ-mediated expression of lipid metabolic genes and inflammatory genes. Further mechanistic studies revealed that β-arrestin-1 interacts with PPARγ. β-Arrestin-1 suppressed the formation of a complex between PPARγ and 9-cis-retinoic acid receptor-α through its direct interaction with PPARγ. The interaction of β-arrestin-1 with PPARγ repressed PPARγ/9-cis-retinoic acid receptor-α function but promoted PPARγ/nuclear receptor corepressor function in PPARγ-mediated adipogenesis and inflammatory gene expression. Consistent with these results, a deficiency of β-arrestin-1 binding to PPARγ abolished its suppression of PPARγ-dependent adipogenesis and inflammatory responses. These results indicate that the regulation of PPARγ by β-arrestin-1 is critical. Furthermore, in vivo expression of β-arrestin-1 (but not the binding-deficient mutant) significantly repressed adipogenesis, macrophage infiltration, and diet-induced obesity and improved glucose tolerance and systemic insulin sensitivity. Therefore, our findings not only reveal a molecular mechanism for the modulation of obesity by β-arrestin-1 but also suggest a potential tactical approach against obesity and its associated metabolic disorders.     

The Expression of Peroxisome Proliferator‐Activated Receptor γ in Pig Fetal Tissue and Primary Stromal‐Vascular Cultures

Objective: This study was designed to determine when peroxisome proliferator‐activated receptor γ (PPARγ) is expressed in developing fetal adipose tissue and stromal‐vascular adipose precursor cells derived from adipose tissue. In addition we examined developing tissue for CCAAT/enhancer‐binding protein β (C/EBPβ) expression to see if it was correlated with PPARγ expression. Pituitary function and hormones involved with differentiation (dexamethasone and retinoic acid) were also tested for their effects on PPARγ expression to determine if hormones known to affect differentiation also effect PPARγ expression in vivo and in cell culture. Research Methods and Procedures: Developing subcutaneous adipose tissues from the dorsal region of the fetal pig were collected at different gestation times and assayed using Western blot analysis to determine levels of PPARγ and C/EBPβ. Hypophysectomy was performed on 75‐day pig fetuses and tissue samples were then taken at 105 days for Western blot analysis. Adipose tissue was also taken from postnatal pigs to isolate stromal‐vascular (S‐V) cells. These adipose precursor cells were grown in culture and samples were taken for Western blot analysis to determine expression levels of PPARγ. Results: Our results indicate that PPARγ is expressed as early as 50 days of fetal development in adipose tissue and continues through 105 days. Expression of PPARγ was found to be significantly enhanced in adipose tissue from hypophysectomized fetuses at 105 days of fetal development (p < 0.05). C/EBPβ was not found in 50‐ or 75‐day fetal tissues and was found only at low levels in 105‐day tissues. C/EBPβ was not found in hypophysectomized (hypoxed) 105‐day tissue where PPARγ was elevated. S‐V cells freshly isolated from adipose tissue of 5‐ to 7‐day postnatal pigs showed the expression of PPARγ1. When S‐V cells were cultured, both PPARγ1 and 2 were expressed after the first day and continued as cells differentiated. High concentrations of retinoic acid decreased PPARγ expression in early S‐V cultures (p < 0.05). Discussion: Our data indicate that PPARγ is expressed in fetal adipose tissue very early before distinct fat cells are observed and can be expressed without the expression of C/EBPβ. The increase in PPARγ expression after hypophysectomy may explain the increase in fat cell size under these conditions. Adipose precursor cells (S‐V cells) from 5‐ to 7‐day postnatal pigs also express PPARγ in the tissue before being induced to differentiate in culture. Thus S‐V cells from newborn pig adipose tissue are probably more advanced in development than the 3T3‐L1 cell model. S‐V cells may be in a state where PPARγ and C/EBPα are expressed but new signals or vascularization are needed before cells are fully committed and lipid filling begins.     

Dietary alpha‐ketoglutarate promotes beige adipogenesis and prevents obesity in middle‐aged mice

Aging usually involves the progressive development of certain illnesses, including diabetes and obesity. Due to incapacity to form new white adipocytes, adipose expansion in aged mice primarily depends on adipocyte hypertrophy, which induces metabolic dysfunction. On the other hand, brown adipose tissue burns fatty acids, preventing ectopic lipid accumulation and metabolic diseases. However, the capacity of brown/beige adipogenesis declines inevitably during the aging process. Previously, we reported that DNA demethylation in the Prdm16 promoter is required for beige adipogenesis. DNA methylation is mediated by ten–eleven family proteins (TET) using alpha‐ketoglutarate (AKG) as a cofactor. Here, we demonstrated that the circulatory AKG concentration was reduced in middle‐aged mice (10‐month‐old) compared with young mice (2‐month‐old). Through AKG administration replenishing the AKG pool, aged mice were associated with the lower body weight gain and fat mass, and improved glucose tolerance after challenged with high‐fat diet (HFD). These metabolic changes are accompanied by increased expression of brown adipose genes and proteins in inguinal adipose tissue. Cold‐induced brown/beige adipogenesis was impeded in HFD mice, whereas AKG rescued the impairment of beige adipocyte functionality in middle‐aged mice. Besides, AKG administration up‐regulated Prdm16 expression, which was correlated with an increase of DNA demethylation in the Prdm16 promoter. In summary, AKG supplementation promotes beige adipogenesis and alleviates HFD‐induced obesity in middle‐aged mice, which is associated with enhanced DNA demethylation of the Prdm16 gene.