ApoB100-LDL Acts as a Metabolic Signal from Liver to Peripheral Fat Causing Inhibition of Lipolysis in Adipocytes

Background

Free fatty acids released from adipose tissue affect the synthesis of apolipoprotein B-containing lipoproteins and glucose metabolism in the liver. Whether there also exists a reciprocal metabolic arm affecting energy metabolism in white adipose tissue is unknown.

Methods and Findings

We investigated the effects of apoB-containing lipoproteins on catecholamine-induced lipolysis in adipocytes from subcutaneous fat cells of obese but otherwise healthy men, fat pads from mice with plasma lipoproteins containing high or intermediate levels of apoB100 or no apoB100, primary cultured adipocytes, and 3T3-L1 cells. In subcutaneous fat cells, the rate of lipolysis was inversely related to plasma apoB levels. In human primary adipocytes, LDL inhibited lipolysis in a concentration-dependent fashion. In contrast, VLDL had no effect. Lipolysis was increased in fat pads from mice lacking plasma apoB100, reduced in apoB100-only mice, and intermediate in wild-type mice. Mice lacking apoB100 also had higher oxygen consumption and lipid oxidation. In 3T3-L1 cells, apoB100-containing lipoproteins inhibited lipolysis in a dose-dependent fashion, but lipoproteins containing apoB48 had no effect. ApoB100-LDL mediated inhibition of lipolysis was abolished in fat pads of mice deficient in the LDL receptor (Ldlr^−/−Apob ^100/100).

Conclusions

Our results show that the binding of apoB100-LDL to adipocytes via the LDL receptor inhibits intracellular noradrenaline-induced lipolysis in adipocytes. Thus, apoB100-LDL is a novel signaling molecule from the liver to peripheral fat deposits that may be an important link between atherogenic dyslipidemias and facets of the metabolic syndrome.     

Soy hydrolysate enhances the isoproterenol-stimulated lipolytic pathway through an increase in β-adrenergic receptor expression in adipocytes

ABSTRACT

Activation of the adipose lipolytic pathway during lipid metabolism is mediated by protein kinase A (PKA), which responds to β-adrenergic stimulation, leading to increased lipolysis. Soy is well known as a functional food and it is able to affect lipolysis in adipocytes. However, the mechanism by which soy components contribute to the lipolytic pathway remains to be fully elucidated. Here, we show that hydrolyzed soy enhances isoproterenol-stimulated lipolysis and activation of PKA in 3T3-L1 adipocytes. We also found that the expression of β-adrenergic receptors, which coordinate the activation of PKA, is elevated in adipocytes differentiated in the presence of soy hydrolysate. The activity of the soy hydrolysate towards β-adrenergic receptor expression was detected in its hydrophilic fraction. Our results suggest that the soy hydrolysate enhances the PKA pathway through the upregulation of β-adrenergic receptor expression and thereby, increase lipolysis in adipocytes.     

Extracellular vesicles-released parathyroid hormone-related protein from Lewis lung carcinoma induces lipolysis and adipose tissue browning in cancer cachexia

Cancer cachexia is a metabolic disorder characterized by skeletal muscle wasting and white adipose tissue browning. Specific functions of several hormones, growth factors, and cytokines derived from tumors can trigger cachexia. Moreover, adipose tissue lipolysis might explain weight loss that occurs owing to cachexia. Extracellular vesicles (EVs) are involved in intercellular communication. However, whether EVs participate in lipolysis induced by cancer cachexia has not been thoroughly investigated. Using Lewis lung carcinoma (LLC) cell culture, we tested whether LLC cell-derived EVs can induce lipolysis in 3T3-L1 adipocytes. EVs derived from LLC cells were isolated and characterized biochemically and biophysically. Western blotting and glycerol assay were used to study lipolysis. LLC cell-derived EVs induced lipolysis in vivo and vitro. EVs fused directly with target 3T3-L1 adipocytes and transferred parathyroid hormone-related protein (PTHrP), activating the PKA signaling pathway in 3T3-L1 adipocytes. Blocking PTHrP activity in LLC-EVs using a neutralizing antibody and by knocking down PTHR expression prevented lipolysis in adipocytes. Inhibiting the PKA signaling pathway also prevents the lipolytic effects of EVs. In vivo, suppression of LLC-EVs release by knocking down Rab27A alleviated white adipose tissue browning and lipolysis. Our data showed that LLC cell-derived EVs induced adipocyte lipolysis via the extracellular PTHrP-mediated PKA pathway. Our data demonstrate that LLC-EVs induce lipolysis in vitro and vivo by delivering PTHrP, which interacts with PTHR. The lipolytic effect of LLC-EVs was abrogated by PTHR knockdown and treatment with a neutralizing anti-PTHrP antibody. Together, these data show that LLC-EV-induced lipolysis is mediated by extracellular PTHrP. These findings suggest a novel mechanism of lipid droplet loss and identify a potential therapeutic strategy for cancer cachexia.Subject terms: Oncogenes, Mechanisms of disease     

Calyculin and okadaic acid promote perilipin phosphorylation and increase lipolysis in primary rat adipocytes

Lipolysis is primarily regulated by protein kinase A (PKA), which phosphorylates perilipin and hormone-sensitive lipase (HSL), and causes translocation of HSL from cytosol to lipid droplets in adipocytes. Perilipin coats lipid droplet surface and assumes to prevent lipase access to triacylglycerols, thus inhibiting basal lipolysis; phosphorylated perilipin facilitates lipolysis on PKA activation. Here, we induced lipolysis in primary rat adipocytes by inhibiting protein serine/threonine phosphatase with specific inhibitors, okadaic acid and calyculin. The incubation with calyculin promotes incorporation of 32Pi into perilipins, thus, confirming that perilipin is hyperphosphorylated. The lipolysis response to calyculin is gradually accompanied by increased accumulation of phosphorylated perilipin A in a concentration- and time-responsive manner. When perilipin phosphorylation is abrogated by the addition of N-ethylmaleimide, lipolysis ceases. Different from a considerable translocation of HSL upon PKA activation with isoproterenol, calyculin does not alter HSL redistribution in primary or differentiated adipocytes, as confirmed by both immunostaining and immunoblotting. Thus, we suggest that inhibition of the phosphatase by calyculin activates lipolysis via promoting perilipin phosphorylation rather than eliciting HSL translocation in adipocytes. Further, we show that when the endogenous phosphatase is inhibited by calyculin, simultaneous PKA activation with isoproterenol converts most of the perilipin to the hyperphosphorylated species, and induces enhanced lipolysis. Apparently, as PKA phosphorylates perilipin and stimulates lipolysis, the phosphatase acts to dephosphorylate perilipin and attenuate lipolysis. This suggests a two-step strategy governed by a kinase and a phosphatase to modulate the steady state of perilipin phosphorylation and hence the lipolysis response to hormonal stimulation.     

NAT8L (N-Acetyltransferase 8-Like) Accelerates Lipid Turnover and Increases Energy Expenditure in Brown Adipocytes

NAT8L (N-acetyltransferase 8-like) catalyzes the formation of N-acetylaspartate (NAA) from acetyl-CoA and aspartate. In the brain, NAA delivers the acetate moiety for synthesis of acetyl-CoA that is further used for fatty acid generation. However, its function in other tissues remained elusive. Here, we show for the first time that Nat8l is highly expressed in adipose tissues and murine and human adipogenic cell lines and is localized in the mitochondria of brown adipocytes. Stable overexpression of Nat8l in immortalized brown adipogenic cells strongly increases glucose incorporation into neutral lipids, accompanied by increased lipolysis, indicating an accelerated lipid turnover. Additionally, mitochondrial mass and number as well as oxygen consumption are elevated upon Nat8l overexpression. Concordantly, expression levels of brown marker genes, such as Prdm16, Cidea, Pgc1α, Pparα, and particularly UCP1, are markedly elevated in these cells. Treatment with a PPARα antagonist indicates that the increase in UCP1 expression and oxygen consumption is PPARα-dependent. Nat8l knockdown in brown adipocytes has no impact on cellular triglyceride content, lipogenesis, or oxygen consumption, but lipolysis and brown marker gene expression are increased; the latter is also observed in BAT of Nat8l-KO mice. Interestingly, the expression of ATP-citrate lyase is increased in Nat8l-silenced adipocytes and BAT of Nat8l-KO mice, indicating a compensatory mechanism to sustain the acetyl-CoA pool once Nat8l levels are reduced. Taken together, our data show that Nat8l impacts on the brown adipogenic phenotype and suggests the existence of the NAT8L-driven NAA metabolism as a novel pathway to provide cytosolic acetyl-CoA for lipid synthesis in adipocytes.     

Effect of plasma triglyceride metabolism on lipid storage in adipose tissue: Studies using genetically engineered mouse models

The obesity epidemic is associated with an increased incidence of type 2 diabetes, cardiovascular morbidity and various types of cancer. A better insight into the molecular mechanisms that underlie adipogenesis and obesity may result in novel therapeutic handles to fight obesity and these associated diseases. Adipogenesis is determined by the balance between uptake of fatty acids (FA) from plasma into adipocytes, intracellular FA oxidation versus esterification of FA into triglycerides (TG), lipolysis of TG by intracellular lipases, and secretion of FA from adipocytes. Here, we review the mechanisms that are specifically involved in the entry of FA into adipose tissue. In plasma, these originating FA are either present as TG within apoB-containing lipoproteins (i.e. chylomicrons and VLDL) or as free FA bound to albumin. Kinetic studies, however, have revealed that TG are the major source of FA entering adipose tissue, both in the fed and fasted condition. In fact, studies with genetically engineered mice have revealed that the activity of lipoprotein lipase (LPL) is a major determinant for the development of obesity. As a general rule, high fat diet-induced adipogenesis is aggravated by stimulated LPL activity (e.g. by adipose tissue-specific overexpression of LPL or deficiency for apoCIII), and attenuated by inhibited LPL activity (e.g. by adipose-specific deficiency for LPL, overexpression of apoCI or angptl4, or by deficiency for apoE or the VLDL receptor). In addition, we describe that the trans-membrane transport of FA and cytoplasmic binding of FA in adipocytes can also dramatically affect adipogenesis. The relevance of these findings for human pathophysiology is discussed.     

Obesity and weight loss result in increased adipose tissue ABCG1 expression in db/db mice

The prevalence of obesity has reached epidemic proportions and is associated with several co-morbid conditions including diabetes, dyslipidemia, cancer, atherosclerosis and gallstones. Obesity is associated with low systemic inflammation and an accumulation of adipose tissue macrophages (ATMs) that are thought to modulate insulin resistance. ATMs may also modulate adipocyte metabolism and take up lipids released during adipocyte lipolysis and cell death. We suggest that high levels of free cholesterol residing in adipocytes are released during these processes and contribute to ATM activation and accumulation during obesity and caloric restriction. Db/db mice were studied for extent of adipose tissue inflammation under feeding conditions of ad libitum (AL) and caloric restriction (CR). The major finding was a marked elevation in epididymal adipose ABCG1 mRNA levels with obesity and CR (6-fold and 16-fold, respectively) over that seen for lean wild-type mice. ABCG1 protein was also elevated for CR as compared to AL adipose tissue. ABCG1 is likely produced by cholesterol loaded ATMs since this gene is not highly expressed in adipocytes and ABCG1 expression is sterol mediated. Our data supports the concept that metabolic changes in adipocytes due to demand lipolysis and cell death lead to cholesterol loading of ATMs. Based on finding cholesterol-loaded peritoneal leukocytes with elevated levels of ABCG1 in CR as compared to AL mice, we suggest that pathways for cholesterol trafficking out of adipose tissue involve ATM egress as well as ABCG1 mediated cholesterol efflux. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).     

Response of white adipocyte of mouse and rabbit to catecholamines and ACTH

Summary Hormone stimulated lipolysis of mouse and rabbit adipocytes as measured by both free fatty acid and glycerol release, is proportionally elevated with increase in the adipocyte cAMP level up to 1 nmole/g. The correlation coefficients are 0.94 and 0.97 for FFA/cAMP and glycerol/cAMP respectively. Increments in cAMP greater than 1 nmole/g show no correlation with increase in lipolysis. The release of lipolytic products, glycerol and free fatty acids, from white adipocytes in response to ACTH, epinephrine or morepinephrine was measured using radiochemical assays in short term incubation systems, with cAMP levels measured at the same time and from the same cell sample. Under the conditions studied, epinephrine is a more effective lipolytic hormone than ACTH in mouse adipocyte, and ACTH is more effective than epinephrine in rabbit adipocyte. The effect of catecholamines on the rabbit adipocyte is not modified by phentolamine (10 μM), but it is potentiated by 1-methyl-3-isobutyl xanthine (0.1 mM). The results suggest that cAMP mediates the action of these lipolytic hormones in white adipocytes of mouse and rabbit.     

Plac8 is required for White Adipocyte Differentiation in vitro and Cell Number Control in vivo

Plac8 belongs to an evolutionary conserved family of proteins, mostly abundant in plants where they control fruit weight through regulation of cell number. In mice, Plac8 is expressed both in white and brown adipose tissues and we previously showed that Plac8^−/− mice develop late-onset obesity, with abnormal brown fat differentiation and reduced thermogenic capacity. We also showed that in brown adipocytes, Plac8 is an upstream regulator of C/EBPβ expression. Here, we first assessed the role of Plac8 in white adipogenesis in vitro. We show that Plac8 is induced early after induction of 3T3-L1 adipocytes differentiation, a process that is prevented by Plac8 knockdown; similarly, embryonic fibroblasts obtained from Plac8 knockout mice failed to form adipocytes upon stimulation of differentiation. Knockdown of Plac8 in 3T3-L1 was associated with reduced expression of C/EBPβ, Krox20, and Klf4, early regulators of the white adipogenic program, and we show that Plac8 could transactivate the C/EBPβ promoter. In vivo, we show that absence of Plac8 led to increased white fat mass with enlarged adipocytes but reduced total number of adipocytes. Finally, even though Plac8^−/− mice showed impaired thermogenesis due to brown fat dysfunction, this was not associated with changes in glycemia or plasma free fatty acid and triglyceride levels. Collectively, these data indicate that Plac8 is an upstream regulator of C/EBPβ required for adipogenesis in vitro. However, in vivo, Plac8 is dispensable for the differentiation of white adipocytes with preserved fat storage capacity but is required for normal fat cell number regulation.     

Perilipin overexpression in white adipose tissue induces a brown fat-like phenotype

Background

Perilipin A (PeriA) exclusively locates on adipocyte lipid droplets and is essential for lipid storage and lipolysis. Previously, we reported that adipocyte specific overexpression of PeriA caused resistance to diet-induced obesity and resulted in improved insulin sensitivity. In order to better understand the biological basis for this observed phenotype, we performed additional studies in this transgenic mouse model.

Methodology and Principal Findings

When compared to control animals, whole body energy expenditure was increased in the transgenic mice. Subsequently, we performed DNA microarray analysis and real-time PCR on white adipose tissue. Consistent with the metabolic chamber data, we observed increased expression of genes associated with fatty acid β-oxidation and heat production, and a decrease in the genes associated with lipid synthesis. Gene expression of Pgc1a, a regulator of fatty acid oxidation and Ucp1, a brown adipocyte specific protein, was increased in the white adipose tissue of the transgenic mice. This observation was subsequently verified by both Western blotting and histological examination. Expression of RIP140, a regulator of white adipocyte differentiation, and the lipid droplet protein FSP27 was decreased in the transgenic mice. Importantly, FSP27 has been shown to control gene expression of these crucial metabolic regulators. Overexpression of PeriA in 3T3-L1 adipocytes also reduced FSP27 expression and diminished lipid droplet size.

Conclusions

These findings demonstrate that overexpression of PeriA in white adipocytes reduces lipid droplet size by decreasing FSP27 expression and thereby inducing a brown adipose tissue-like phenotype. Our data suggest that modulation of lipid droplet proteins in white adipocytes is a potential therapeutic strategy for the treatment of obesity and its related disorders.     

Neuronal growth regulator 1 promotes adipocyte lipid trafficking via interaction with CD36

Neuronal growth regulator 1 (NEGR1) is a glycosylphosphatidylinositol-anchored membrane protein associated with several human pathologies, including obesity, depression, and autism. Recently, significantly enlarged white adipose tissue, hepatic lipid accumulation, and decreased muscle capacity were reported in Negr1-deficient mice. However, the mechanism behind these phenotypes was not clear. In the present study, we found NEGR1 to interact with cluster of differentiation 36 (CD36), the major fatty acid translocase in the plasma membrane. Binding assays with a soluble form of NEGR1 and in situ proximal ligation assays indicated that NEGR1-CD36 interaction occurs at the outer leaflet of the cell membrane. Furthermore, we show that NEGR1 overexpression induced CD36 protein destabilization in vitro. Both mRNA and protein levels of CD36 were significantly elevated in the white adipose tissue and liver tissues of Negr1^?/? mice. Accordingly, fatty acid uptake rate increased in NEGR1-deficient primary adipocytes. Finally, we demonstrated that Negr1^?/? mouse embryonic fibroblasts showed elevated reactive oxygen species levels and decreased adenosine monophosphate-activated protein kinase activation compared with control mouse embryonic fibroblasts. Based on these results, we propose that NEGR1 regulates cellular fat content by controlling the expression of CD36.     

Somatic Cell Plasticity and Niemann-Pick Type C2 Protein: ADIPOCYTE DIFFERENTIATION AND FUNCTION*

The phenotypic stability of somatic cells is essential for the maintenance of both structural and functional organ integrity of the adult human body. Deregulated cell plasticity could result in the development of debilitating diseases such as cancer, fibrosis, atherosclerosis, obesity, and type 2 diabetes. We have previously demonstrated that a nonsense mutation in the NPC2 gene, which encodes ubiquitous, highly conserved, secretory protein with unknown function, leads to activation of human skin fibroblasts. The activated fibroblasts, also known as myofibroblasts, have the properties of mesenchymal stem cells and are able to differentiate along the mesodermal and endodermal lineages. Here we show that NPC2-null, but not the normal skin fibroblasts, possess characteristics of adipogenic progenitors as demonstrated by their specific gene expression pattern as well as the ability for efficient differentiation into white adipocytes. The presence of NPC2 in mature white adipocytes was also necessary for their maintenance because silencing NPC2 in differentiated cells by siRNA stimulated PPARG expression, which was followed by a shift toward a more favorable, brown adipocyte-like metabolic state characterized by up-regulated lipolysis and increased insulin sensitivity. It appears that NPC2 controls both the adipogenesis and the metabolic state of mature white adipocytes through a common mechanism that is linked to activation of FGFR2 that could be followed by induction of PPARG expression. Altogether, the current study highlights NPC2 as a novel intracrine/autocrine factor that controls adipocyte differentiation and function as well as potential therapeutic target for the treatment of type 2 diabetes and related metabolic disorders.     

Adipocytokine Orosomucoid Integrates Inflammatory and Metabolic Signals to Preserve Energy Homeostasis by Resolving Immoderate Inflammation

Orosomucoid (ORM), also called α-1 acid glycoprotein, is an abundant plasma protein that is an immunomodulator induced by stressful conditions such as infections. In this study, we reveal that Orm is induced selectively in the adipose tissue of obese mice to suppress excess inflammation that otherwise disturbs energy homeostasis. Adipose Orm levels were elevated by metabolic signals, including insulin, high glucose, and free fatty acid, as well as by the proinflammatory cytokine tumor necrosis factor-α, which is found in increased levels in the adipose tissue of morbid obese subjects. In both adipocytes and macrophages, ORM suppressed proinflammatory gene expression and pathways such as NF-κB and mitogen-activated protein kinase signalings and reactive oxygen species generation. Concomitantly, ORM relieved hyperglycemia-induced insulin resistance as well as tumor necrosis factor-α-mediated lipolysis in adipocytes. Accordingly, ORM improved glucose and insulin tolerance in obese and diabetic db/db mice. Taken together, our results suggest that ORM integrates inflammatory and metabolic signals to modulate immune responses to protect adipose tissue from excessive inflammation and thereby from metabolic dysfunction.     

Kinsenoside-mediated lipolysis through an AMPK-dependent pathway in C3H10T1/2 adipocytes: Roles of AMPK and PPARα in the lipolytic effect of kinsenoside

BackgroundCurrently, more than one-third of the global population is overweight or obese, which is a risk factor for major causes of death including cardiovascular disease, numerous cancers, and diabetes. Kinsenoside, a major active component of Anoectochilus formosanus exhibits antihyperglycemic, antihyperliposis, and hepatoprotective effects and can be used to prevent and manage obesity.PurposeThis study examined the catabolic effects of kinsenoside on lipolysis in adipocytes transformed from C3H10T1/2 cells.Study design/methodsThe lipolytic effect of kinsenoside in C3H10T1/2 adipocytes was evaluated by oil-red O staining and glycerol production. The underlying mechanisms were assessed by Western blots, chromatin immunoprecipitation (IP), Co-IP, EMSA and siRNAs verification.ResultsWe demonstrated that kinsenoside increased both adipose triglyceride lipase (ATGL)-mediated lipolysis, which was upregulated by AMP-activated protein kinase (AMPK) activation, and the hydrolysis of triglycerides to glycerol and fatty acids that require transportation into mitochondria for further β-oxidation. We also demonstrated that kinsenoside increased the phosphorylation of peroxisome proliferator-activated receptor alpha (PPARα) and CRE-binding protein (CREB), and the protein levels of silent information regulator T1 (SIRT1), peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α) and carnitine palmitoyltransferase I (CPT1) through an AMPK-dependent mechanism. SIRT1 deacetylated PGC-1α, facilitating AMPK-mediated PGC-1α phosphorylation and increasing the interaction of PPARα with its coactivator, PGC-1α. This interaction elevated the expression of CPT1, a shuttle for the mitochondrial transport of fatty acids, in kinsenoside-treated cells. In addition, AMPK-phosphorylation-mediated CREB activation caused kinsenoside-mediated PGC-1α upregulation.ConclusionAMPK activation not only elevated ATGL expression for lipolysis but also induced CPT1 expression for further mitochondrial translocation of fatty acids. The results suggested that the mechanism underlying the catabolic effects of kinsenoside on lipolysis and increased CPT1 induction was mediated through an AMPK-dependent pathway.     

Increased Adipogenesis of Human Adipose-Derived Stem Cells on Polycaprolactone Fiber Matrices

With accelerating rates of obesity and type 2 diabetes world-wide, interest in studying the adipocyte and adipose tissue is increasing. Human adipose derived stem cells - differentiated to adipocytes in vitro - are frequently used as a model system for white adipocytes, as most of their pathways and functions resemble mature adipocytes in vivo. However, these cells are not completely like in vivo mature adipocytes. Hosting the cells in a more physiologically relevant environment compared to conventional two-dimensional cell culturing on plastic surfaces, can produce spatial cues that drive the cells towards a more mature state. We investigated the adipogenesis of adipose derived stem cells on electro spun polycaprolactone matrices and compared functionality to conventional two-dimensional cultures as well as to human primary mature adipocytes. To assess the degree of adipogenesis we measured cellular glucose-uptake and lipolysis and used a range of different methods to evaluate lipid accumulation. We compared the averaged results from a whole population with the single cell characteristics – studied by coherent anti-Stokes Raman scattering microscopy - to gain a comprehensive picture of the cell phenotypes. In adipose derived stem cells differentiated on a polycaprolactone-fiber matrix; an increased sensitivity in insulin-stimulated glucose uptake was detected when cells were grown on either aligned or random matrices. Furthermore, comparing differentiation of adipose derived stem cells on aligned polycaprolactone-fiber matrixes, to those differentiated in two-dimensional cultures showed, an increase in the cellular lipid accumulation, and hormone sensitive lipase content. In conclusion, we propose an adipocyte cell model created by differentiation of adipose derived stem cells on aligned polycaprolactone-fiber matrices which demonstrates increased maturity, compared to 2D cultured cells.     

Adipocytes as a Link Between Gut Microbiota-Derived Flagellin and Hepatocyte Fat Accumulation

While the role of both elevated levels of circulating bacterial cell wall components and adipose tissue in hepatic fat accumulation has been recognized, it has not been considered that the bacterial components-recognizing adipose tissue receptors contribute to the hepatic fat content. In this study we found that the expression of adipose tissue bacterial flagellin (FLG)-recognizing Toll-like receptor (TLR) 5 associated with liver fat content (r = 0.699, p = 0.003) and insulin sensitivity (r = -0.529, p = 0.016) in humans (n = 23). No such associations were found for lipopolysaccharides (LPS)-recognizing TLR4. To study the underlying molecular mechanisms of these associations, human HepG2 hepatoma cells were exposed in vitro to the conditioned culture media derived from FLG or LPS-challenged human adipocytes. The adipocyte-mediated effects were also compared to the effects of direct HepG2 exposure to FLG and LPS. We found that the media derived from FLG-treated adipocytes stimulated fat accumulation in HepG2 cells, whereas either media derived from LPS-treated adipocytes or direct FLG or LPS exposure did not. This is likely due to that FLG-treatment of adipocytes increased lipolysis and secretion of glycerol, which is known to serve a substrate for triglyceride synthesis in hepatocytes. Similarly, only FLG-media significantly decreased insulin signaling-related Akt phosphorylation, IRS1 expression and mitochondrial respiratory chain ATP5A. In conclusion, our results suggest that the FLG-induced TLR5 activation in adipocytes increases glycerol secretion from adipocytes and decreases insulin signaling and mitochondrial functions, and increases fat accumulation in hepatocytes. These mechanisms could, at least partly, explain the adipose tissue TLR5 expression associated with liver fat content in humans.     

High lipolytic activity and dyslipidemia in a Spontaneous Hypertensive/NIH Corpulent (SHR/N-cp) rat: a genetic model of obesity and type 2 diabetes mellitus

In order to better understand the link between obesity and type 2 diabetes, lipolysis and its adrenergic regulation was investigated in various adipose depots of obese adult females SHR/N-cp rats. Serum insulin, glucose, free fatty acids (FFA), triglycerides (TG) and glycerol were measured. Adipocytes were isolated from subcutaneous (SC), parametrial (PM) and retroperitoneal (RP) fat pads. Total cell number and size, basal lipolysis or stimulated by norepinephrine (NE) and BRL 37344 were measured in each depot. Obese rats were hyperinsulinemic and hyperglycemic, suggesting high insulin resistance. They presented a marked dyslipidemia, attested by increased serum FFA and TG levels. High serum glycerol levels also suggest a strong lipolytic rate. Obese rats showed an excessive development of all fat pads although a more pronounced effect was observed in the SC one. The cellularity of this depot was increased 8 fold when compared to lean rats, but these fat cells were only 1.5 to 2-fold larger. SC adipocytes showed a marked increase in their basal lipolytic activity but a lack of change in responsiveness to NE or BRL 37344. The association between high basal lipolysis and increased cellularity yields to a marked adipose cell lipolytic rate, especially from the SC region. SHR/N-cp rats were characterized by a hyperplasic type of obesity with an excessive development of the SC depot. The dyslipidemia, attested by an altered serum lipid profile could be attributed to excessive lipolysis that contributes to increased FFA levels, and to early development of insulin resistance through a lipotoxicity effect.     

Unique biochemical properties of the protein tyrosine phosphatase activity of PTEN—Demonstration of different active site structural requirements for phosphopeptide and phospholipid phosphatase activities of PTEN

Missense PTEN mutations of the active site residues Asp-92, Cys-124 and Gly-129 contribute to Cowden syndrome. How their mutations affect phospholipid phosphatase activity and tumor suppressor function of PTEN has been defined. In this study, we investigated how their mutations affect the kinetics and catalytic mechanism of PTEN phosphoprotein phosphatase activity. Our data suggest that PTEN catalysis of phosphoprotein dephosphorylation follows a two-step mechanism with Cys-124 transiently phosphorylated to form the phosphoenzyme intermediate. In spite of this, we were unable to trap the genuine phosphoenzyme intermediate; instead, we unexpectedly discovered a novel phosphotransfer reaction in which the phosphate group is transferred from a tyrosyl phosphopeptide to PTEN to form a unique phosphorylated protein. Even though the finding is novel, the phosphotransfer reaction is likely an in vitro non-enzymatic reaction. Kinetic analysis revealed that mutation of Asp-92 has negligible impacts on phosphopeptide phosphatase activity of PTEN, suggesting that Asp-92 does not participate in the phosphopeptide dephosphorylation reaction. The results also imply that allosteric regulators facilitating the recruitment of Asp-92 to participate in catalysis will increase the activity of PTEN in dephosphorylating phosphoprotein and phosphopeptide substrates. Furthermore, kinetic analysis revealed that the G129E mutation has different effects on phospholipid and phosphoprotein phosphatase activities. Taken together, the data show that while the two phosphatase activities of PTEN follow a similar catalytic mechanism, they have notable differences in the requirements of the active site structure.