Instruct: Given a scientific claim, retrieve relevant passages that support or refute the claim.
Query: Increased lipolysis leads to higher P38 phosphorylation in adipose tissue.
Cigarette smoking promotes body weight reduction in humans while paradoxically also promoting insulin resistance (IR) and hyperinsulinemia. However, the mechanisms behind these effects are unclear. Here we show that nicotine, a major constituent of cigarette smoke, selectively activates AMP-activated protein kinase α2 (AMPKα2) in adipocytes, which in turn phosphorylates MAP kinase phosphatase-1 (MKP1) at serine 334, initiating its proteasome-dependent degradation. The nicotine-dependent reduction of MKP1 induces the aberrant activation of both p38 mitogen-activated protein kinase and c-Jun N-terminal kinase, leading to increased phosphorylation of insulin receptor substrate 1 (IRS1) at serine 307. Phosphorylation of IRS1 leads to its degradation, protein kinase B inhibition, and the loss of insulin-mediated inhibition of lipolysis. Consequently, nicotine increases lipolysis, which results in body weight reduction, but this increase also elevates the levels of circulating free fatty acids and thus causes IR in insulin-sensitive tissues. These results establish AMPKα2 as an essential mediator of nicotine-induced whole-body IR in spite of reductions in adiposity.
Although mast cell functions have classically been related to allergic responses, recent studies indicate that these cells contribute to other common diseases such as multiple sclerosis, rheumatoid arthritis, atherosclerosis, aortic aneurysm and cancer. This study presents evidence that mast cells also contribute to diet-induced obesity and diabetes. For example, white adipose tissue (WAT) from obese humans and mice contain more mast cells than WAT from their lean counterparts. Furthermore, in the context of mice on a Western diet, genetically induced deficiency of mast cells, or their pharmacological stabilization, reduces body weight gain and levels of inflammatory cytokines, chemokines and proteases in serum and WAT, in concert with improved glucose homeostasis and energy expenditure. Mechanistic studies reveal that mast cells contribute to WAT and muscle angiogenesis and associated cell apoptosis and cathepsin activity. Adoptive transfer experiments of cytokine-deficient mast cells show that these cells, by producing interleukin-6 (IL-6) and interferon-gamma (IFN-gamma), contribute to mouse adipose tissue cysteine protease cathepsin expression, apoptosis and angiogenesis, thereby promoting diet-induced obesity and glucose intolerance. Our results showing reduced obesity and diabetes in mice treated with clinically available mast cell-stabilizing agents suggest the potential of developing new therapies for these common human metabolic disorders.
Adipocytes are part of hematopoietic microenvironment, even though up to now in humans, their role in hematopoiesis is still questioned. We have previously shown that accumulation of fat cells in femoral bone marrow (BM) coincides with increased expression of neuropilin-1 (NP-1), while it is weakly expressed in hematopoietic iliac crest BM. Starting from this observation, we postulated that adipocytes might exert a negative effect on hematopoiesis mediated through NP-1. To test this hypothesis, we set up BM adipocytes differentiated into fibroblast-like fat cells (FLFC), which share the major characteristics of primitive unilocular fat cells, as an experimental model. As expected, FLFCs constitutively produced macrophage colony stimulating factor and induced CD34(+) differentiation into macrophages independently of cell-to-cell contact. By contrast, granulopoiesis was hampered by cell-to-cell contact but could be restored in transwell culture conditions, together with granulocyte colony stimulating factor production. Both functions were also recovered when FLFCs cultured in contact with CD34(+) cells were treated with an antibody neutralizing NP-1, which proved its critical implication in contact inhibition. An inflammatory cytokine such as interleukin-1 beta or dexamethasone modulates FLFC properties to restore granulopoiesis. Our data provide the first evidence that primary adipocytes exert regulatory functions during hematopoiesis that might be implicated in some pathological processes. Disclosure of potential conflicts of interest is found at the end of this article.
Lipid droplets are ubiquitous triglyceride and sterol ester storage organelles required for energy storage homeostasis and biosynthesis. Although little is known about lipid droplet formation and regulation, it is clear that members of the PAT (perilipin, adipocyte differentiation related protein, tail interacting protein of 47 kDa) protein family coat the droplet surface and mediate interactions with lipases that remobilize the stored lipids. We identified key Drosophila candidate genes for lipid droplet regulation by RNA interference (RNAi) screening with an image segmentation-based optical read-out system, and show that these regulatory functions are conserved in the mouse. Those include the vesicle-mediated Coat Protein Complex I (COPI) transport complex, which is required for limiting lipid storage. We found that COPI components regulate the PAT protein composition at the lipid droplet surface, and promote the association of adipocyte triglyceride lipase (ATGL) with the lipid droplet surface to mediate lipolysis. Two compounds known to inhibit COPI function, Exo1 and Brefeldin A, phenocopy COPI knockdowns. Furthermore, RNAi inhibition of ATGL and simultaneous drug treatment indicate that COPI and ATGL function in the same pathway. These data indicate that the COPI complex is an evolutionarily conserved regulator of lipid homeostasis, and highlight an interaction between vesicle transport systems and lipid droplets.
CONTEXT The mechanisms that drive progression from fatty liver to steatohepatitis and cirrhosis are unknown. In animal models, obese mice with fatty livers are vulnerable to liver adenosine triphosphate (ATP) depletion and necrosis, suggesting that altered hepatic energy homeostasis may be involved. OBJECTIVE To determine if patients with fatty liver disease exhibit impaired recovery from hepatic ATP depletion. DESIGN Laboratory analysis of liver ATP stores monitored by nuclear magnetic resonance spectroscopy before and after transient hepatic ATP depletion was induced by fructose injection. The study was conducted between July 15 and August 30, 1998. SETTING University hospital. PATIENTS Eight consecutive adults with biopsy-proven nonalcoholic steatohepatitis and 7 healthy age- and sex-matched controls. MAIN OUTCOME MEASURE Level of ATP 1 hour after fructose infusion in patients vs controls. RESULTS In patients, serum aminotransferase levels were increased (P = .02 vs controls); albumin and bilirubin values were normal and clinical evidence of portal hypertension was absent in both groups. However, 2 patients had moderate fibrosis and 1 had cirrhosis on liver biopsy. Mean serum glucose, cholesterol, and triglyceride levels were similar between groups but patients weighed significantly more than controls (P = .02). Liver ATP levels were similar in the 2 groups before fructose infusion and decreased similarly in both after fructose infusion (P = .01 vs initial ATP levels). However, controls replenished their hepatic ATP stores during the 1-hour follow-up period (P<.02 vs minimum ATP) but patients did not. Hence, patients' hepatic ATP levels were lower than those of controls at the end of the study (P = .04). Body mass index (BMI) correlated inversely with ATP recovery, even in controls (R = -0.768; P = .07). Although BMI was greater in patients than controls (P = .02) and correlated strongly with fatty liver and serum aminotransferase elevations, neither of the latter 2 parameters nor the histologic severity of fibrosis strongly predicted hepatic ATP recovery. CONCLUSIONS These data suggest that recovery from hepatic ATP depletion becomes progressively less efficient as body mass increases in healthy controls and is severely impaired in patients with obesity-related nonalcoholic steatohepatitis.
Pancreatic β-cell dysfunction and death are central in the pathogenesis of type 2 diabetes (T2D). Saturated fatty acids cause β-cell failure and contribute to diabetes development in genetically predisposed individuals. Here we used RNA sequencing to map transcripts expressed in five palmitate-treated human islet preparations, observing 1,325 modified genes. Palmitate induced fatty acid metabolism and endoplasmic reticulum (ER) stress. Functional studies identified novel mediators of adaptive ER stress signaling. Palmitate modified genes regulating ubiquitin and proteasome function, autophagy, and apoptosis. Inhibition of autophagic flux and lysosome function contributed to lipotoxicity. Palmitate inhibited transcription factors controlling β-cell phenotype, including PAX4 and GATA6. Fifty-nine T2D candidate genes were expressed in human islets, and 11 were modified by palmitate. Palmitate modified expression of 17 splicing factors and shifted alternative splicing of 3,525 transcripts. Ingenuity Pathway Analysis of modified transcripts and genes confirmed that top changed functions related to cell death. Database for Annotation, Visualization and Integrated Discovery (DAVID) analysis of transcription factor binding sites in palmitate-modified transcripts revealed a role for PAX4, GATA, and the ER stress response regulators XBP1 and ATF6. This human islet transcriptome study identified novel mechanisms of palmitate-induced β-cell dysfunction and death. The data point to cross talk between metabolic stress and candidate genes at the β-cell level.
Metformin is a widely used drug for treatment of type 2 diabetes with no defined cellular mechanism of action. Its glucose-lowering effect results from decreased hepatic glucose production and increased glucose utilization. Metformin's beneficial effects on circulating lipids have been linked to reduced fatty liver. AMP-activated protein kinase (AMPK) is a major cellular regulator of lipid and glucose metabolism. Here we report that metformin activates AMPK in hepatocytes; as a result, acetyl-CoA carboxylase (ACC) activity is reduced, fatty acid oxidation is induced, and expression of lipogenic enzymes is suppressed. Activation of AMPK by metformin or an adenosine analogue suppresses expression of SREBP-1, a key lipogenic transcription factor. In metformin-treated rats, hepatic expression of SREBP-1 (and other lipogenic) mRNAs and protein is reduced; activity of the AMPK target, ACC, is also reduced. Using a novel AMPK inhibitor, we find that AMPK activation is required for metformin's inhibitory effect on glucose production by hepatocytes. In isolated rat skeletal muscles, metformin stimulates glucose uptake coincident with AMPK activation. Activation of AMPK provides a unified explanation for the pleiotropic beneficial effects of this drug; these results also suggest that alternative means of modulating AMPK should be useful for the treatment of metabolic disorders.
The object of this study was to assess the role of brown adipose tissue (BAT) and the sympathetic nervous system in the rise in heat production associated with endotoxin-induced fever. Oxygen consumption (VO2) was found to be significantly increased (28%) over a 4-h period after two doses of endotoxin (Escherichia coli lipopolysaccharide, 0.3 mg/100 g body wt) given 24 h apart. Injection of a mixed beta-adrenoceptor antagonist (propranolol) reduced VO2 by 14% in endotoxin-treated rats, whereas the selective beta 1- (atenolol) or beta 2- (ICI 118551) antagonists suppressed VO2 by 10%. These drugs did not affect VO2 in control animals. BAT thermogenic activity assessed from measurements of in vitro mitochondrial guanosine 5'-diphosphate (GDP) binding was elevated by 54% in interscapular BAT and by 171% in other BAT depots. Surgical denervation of one lobe of the interscapular depot prevented these responses. Endotoxin failed to stimulate GDP binding in rats fed protein-deficient diets. This may have been because BAT thermogenic activity was already elevated in control rats fed these diets or because endotoxin caused a marked suppression of food intake in the protein-deficient animals. The results indicate that sympathetic activation of BAT is involved in the thermogenic responses to endotoxin and that these can be modified by dietary manipulation.
Fibrosis is a pathological result of a dysfunctional repair response to tissue injury and occurs in a number of organs, including the lungs1. Cellular metabolism regulates tissue repair and remodelling responses to injury2-4. AMPK is a critical sensor of cellular bioenergetics and controls the switch from anabolic to catabolic metabolism5. However, the role of AMPK in fibrosis is not well understood. Here, we demonstrate that in humans with idiopathic pulmonary fibrosis (IPF) and in an experimental mouse model of lung fibrosis, AMPK activity is lower in fibrotic regions associated with metabolically active and apoptosis-resistant myofibroblasts. Pharmacological activation of AMPK in myofibroblasts from lungs of humans with IPF display lower fibrotic activity, along with enhanced mitochondrial biogenesis and normalization of sensitivity to apoptosis. In a bleomycin model of lung fibrosis in mice, metformin therapeutically accelerates the resolution of well-established fibrosis in an AMPK-dependent manner. These studies implicate deficient AMPK activation in non-resolving, pathologic fibrotic processes, and support a role for metformin (or other AMPK activators) to reverse established fibrosis by facilitating deactivation and apoptosis of myofibroblasts.
In the 20th century industrialized nations have become afflicted with an unprecedented pandemic of increased adiposity. In the United States, the epicenter of the epidemic, over 2/3 of the population, is overweight and 1 of every 6 Americans carries the diagnosis of metabolic syndrome. Although genes determine susceptibility to environmental factors, the epidemic is clearly due to increased consumption of calorie-dense, highly lipogenic foods, coupled with a marked decrease in physical exertion resulting from modern technologies. If this lifestyle continues, morbid consequences are virtually inevitable. They include type II diabetes and a cluster of disorders known as "the metabolic syndrome" usually appearing in middle age. The morbid consequences of the chronic caloric surplus are buffered before middle age by the partitioning of these calories as fat in the adipocyte compartment which is specifically designed to store triglycerides. Leptin has been proposed as the major hormonal regulator of the partitioning of surplus calories. However, multiple factors can determine the storage capacity of the fat tissue and when it is exceeded ectopic lipid deposition begins. The organs affected in metabolic syndrome include skeletal muscle, liver, heart and pancreas, which are now known to contain abnormal levels of triglycerides. While neutral fat is probably harmless, it is an index of ectopic lipid overload. Fatty acid derivatives can interfere with the function of the cell and ultimately lead to its demise through lipoapoptosis, the consequences of which are gradual organ failure.
1. The changes in the metabolite content in freeze-clamped livers of fed rats occurring on perfusion with 10mm-d-fructose have been examined. 2. The most striking effects of fructose were an accumulation of fructose 1-phosphate, as already known, up to 8.7mumol/g of liver within 10min, a loss of total adenine nucleotides (up to 35% after 40min) with a decrease in the ATP content to 23% within 10min, a sevenfold rise in the concentration of IMP to 1.1mumol/g and an eightfold rise of alpha-glycerophosphate to 1.1mumol/g. 3. There was a transient decrease in P(i) from 4.2 to 1.7mumol/g. Within 40min the P(i) content recovered to the normal value, probably because of an uptake of P(i) from the perfusion medium. 4. The degradation of the adenine nucleotides beyond the stage of AMP can be accounted for by the decrease of ATP and P(i). As ATP inhibits 5-nucleotidase, and as P(i) inhibits AMP deaminase any AMP arising in the tissue is liable to undergo dephosphorylation or deamination under the conditions occurring after fructose loading. 5. The content of lactate increased to 4.3mumol/g at 80min; pyruvate also increased and the [lactate]/[pyruvate] ratio remained within physiological limits. 6. The concentration of free fructose within the liver remained much below that in the perfusion medium, indicating that the rate of penetration of fructose into the tissue was lower than the rate of utilization. 7. The fission of fructose 1-phosphate by liver aldolase is inhibited by several phosphorylated intermediates, especially by IMP. This inhibition is competitive with a K(i) of 0.1mm. 8. The maximal rates of the enzymes synthesizing and splitting fructose 1-phosphate are about equal. The accumulation of fructose 1-phosphate on fructose loading is due to the inhibition of the fission of fructose 1-phosphate by the IMP arising from the degradation of the adenine nucleotides.
Increased lipid supply causes beta cell death, which may contribute to reduced beta cell mass in type 2 diabetes. We investigated whether endoplasmic reticulum (ER) stress is necessary for lipid-induced apoptosis in beta cells and also whether ER stress is present in islets of an animal model of diabetes and of humans with type 2 diabetes. Expression of genes involved in ER stress was evaluated in insulin-secreting MIN6 cells exposed to elevated lipids, in islets isolated from db/db mice and in pancreas sections of humans with type 2 diabetes. Overproduction of the ER chaperone heat shock 70 kDa protein 5 (HSPA5, previously known as immunoglobulin heavy chain binding protein [BIP]) was performed to assess whether attenuation of ER stress affected lipid-induced apoptosis. We demonstrated that the pro-apoptotic fatty acid palmitate triggers a comprehensive ER stress response in MIN6 cells, which was virtually absent using non-apoptotic fatty acid oleate. Time-dependent increases in mRNA levels for activating transcription factor 4 (Atf4), DNA-damage inducible transcript 3 (Ddit3, previously known as C/EBP homologous protein [Chop]) and DnaJ homologue (HSP40) C3 (Dnajc3, previously known as p58) correlated with increased apoptosis in palmitate- but not in oleate-treated MIN6 cells. Attenuation of ER stress by overproduction of HSPA5 in MIN6 cells significantly protected against lipid-induced apoptosis. In islets of db/db mice, a variety of marker genes of ER stress were also upregulated. Increased processing (activation) of X-box binding protein 1 (Xbp1) mRNA was also observed, confirming the existence of ER stress. Finally, we observed increased islet protein production of HSPA5, DDIT3, DNAJC3 and BCL2-associated X protein in human pancreas sections of type 2 diabetes subjects. Our results provide evidence that ER stress occurs in type 2 diabetes and is required for aspects of the underlying beta cell failure.
The PI3K signaling pathway regulates cell growth and movement and is heavily mutated in cancer. Class I PI3Ks synthesize the lipid messenger PI(3,4,5)P3. PI(3,4,5)P3 can be dephosphorylated by 3- or 5-phosphatases, the latter producing PI(3,4)P2. The PTEN tumor suppressor is thought to function primarily as a PI(3,4,5)P3 3-phosphatase, limiting activation of this pathway. Here we show that PTEN also functions as a PI(3,4)P2 3-phosphatase, both in vitro and in vivo. PTEN is a major PI(3,4)P2 phosphatase in Mcf10a cytosol, and loss of PTEN and INPP4B, a known PI(3,4)P2 4-phosphatase, leads to synergistic accumulation of PI(3,4)P2, which correlated with increased invadopodia in epidermal growth factor (EGF)-stimulated cells. PTEN deletion increased PI(3,4)P2 levels in a mouse model of prostate cancer, and it inversely correlated with PI(3,4)P2 levels across several EGF-stimulated prostate and breast cancer lines. These results point to a role for PI(3,4)P2 in the phenotype caused by loss-of-function mutations or deletions in PTEN.
Hypoglycemia is among the most injurious metabolic disorders caused by endotoxemia. In experimental endotoxemia with lipopolysaccharide (LPS) in animals, a marked glucose consumption is observed in macrophage-rich organs. However, the direct effect of LPS on the uptake of glucose by macrophages has not been fully understood, and the present study was undertaken to shed light on this point. The consumption and uptake of glucose, as measured with 2-deoxy-D-[3H]glucose, by murine peritoneal exudate macrophages in culture were accelerated two- to threefold by stimulation with 3 ng of LPS per ml. The rate of glucose uptake reached a plateau after 20 min of stimulation and remained at the maximum as long as LPS was present. Northern (RNA) blot analysis with cDNA probes for five known isoforms of glucose transporter (GLUT) revealed that the expression of GLUT by macrophages was restricted to the GLUT1 isoform during LPS stimulation and the amount of GLUT1 mRNA was increased by the stimulation. These results suggest that macrophage responses to LPS are supported by a rapid and sustained glucose influx via GLUT1 and that this is a participating factor in the development of systemic hypoglycemia when endotoxemia is prolonged.
Uncoupling protein 1 (Ucp1), which is localized in the mitochondrial inner membrane of mammalian brown adipose tissue (BAT), generates heat by uncoupling oxidative phosphorylation. Upon cold exposure or nutritional abundance, sympathetic neurons stimulate BAT to express Ucp1 to induce energy dissipation and thermogenesis. Accordingly, increased Ucp1 expression reduces obesity in mice and is correlated with leanness in humans. Despite this significance, there is currently a limited understanding of how Ucp1 expression is physiologically regulated at the molecular level. Here, we describe the involvement of Sestrin2 and reactive oxygen species (ROS) in regulation of Ucp1 expression. Transgenic overexpression of Sestrin2 in adipose tissues inhibited both basal and cold-induced Ucp1 expression in interscapular BAT, culminating in decreased thermogenesis and increased fat accumulation. Endogenous Sestrin2 is also important for suppressing Ucp1 expression because BAT from Sestrin2(-/-) mice exhibited a highly elevated level of Ucp1 expression. The redox-inactive mutant of Sestrin2 was incapable of regulating Ucp1 expression, suggesting that Sestrin2 inhibits Ucp1 expression primarily through reducing ROS accumulation. Consistently, ROS-suppressing antioxidant chemicals, such as butylated hydroxyanisole and N-acetylcysteine, inhibited cold- or cAMP-induced Ucp1 expression as well. p38 MAPK, a signaling mediator required for cAMP-induced Ucp1 expression, was inhibited by either Sestrin2 overexpression or antioxidant treatments. Taken together, these results suggest that Sestrin2 and antioxidants inhibit Ucp1 expression through suppressing ROS-mediated p38 MAPK activation, implying a critical role of ROS in proper BAT metabolism.
Macrophages activated by the Gram-negative bacterial product lipopolysaccharide switch their core metabolism from oxidative phosphorylation to glycolysis. Here we show that inhibition of glycolysis with 2-deoxyglucose suppresses lipopolysaccharide-induced interleukin-1β but not tumour-necrosis factor-α in mouse macrophages. A comprehensive metabolic map of lipopolysaccharide-activated macrophages shows upregulation of glycolytic and downregulation of mitochondrial genes, which correlates directly with the expression profiles of altered metabolites. Lipopolysaccharide strongly increases the levels of the tricarboxylic-acid cycle intermediate succinate. Glutamine-dependent anerplerosis is the principal source of succinate, although the ‘GABA (γ-aminobutyric acid) shunt’ pathway also has a role. Lipopolysaccharide-induced succinate stabilizes hypoxia-inducible factor-1α, an effect that is inhibited by 2-deoxyglucose, with interleukin-1β as an important target. Lipopolysaccharide also increases succinylation of several proteins. We therefore identify succinate as a metabolite in innate immune signalling, which enhances interleukin-1β production during inflammation.
The proteasome is a multicatalytic enzyme complex responsible for the degradation of both normal and damaged proteins. An age-related decline in proteasomal activity has been implicated in various age-related pathologies. The relevance of decreased proteasomal activity to aging and age-related diseases remains unclear, however, because suitable animal models are not available. In the present study, we established a transgenic (Tg) mouse model with decreased proteasomal chymotrypsin-like activity. Tg mice exhibited a shortened life span and developed age-related phenotypes. In Tg mice, polyubiquitinated and oxidized proteins accumulated, and the expression levels of cellular proteins such as Bcl-xL and RNase L were altered. When Tg mice were fed a high-fat diet, they developed more pronounced obesity and hepatic steatosis than did wild-type mice. Consistent with its role in lipid droplet formation, the expression of adipose differentiation-related protein (ADRP) was elevated in the livers of Tg mice. Of note, obesity and hepatic steatosis induced by a high-fat diet were more pronounced in aged than in young wild-type mice, and aged wild-type mice had elevated levels of ADRP, suggesting that the metabolic abnormalities present in Tg mice mimic those in aged mice. Our results provide the first in vivo evidence that decreased proteasomal chymotrypsin-like activity affects longevity and aggravates age-related metabolic disorders, such as obesity and hepatic steatosis.
Accumulation of adipocytes and collagen type-I-producing cells (fibrosis) is observed in muscular dystrophies. The origin of these cells had been largely unknown, but recently we identified mesenchymal progenitors positive for platelet-derived growth factor receptor alpha (PDGFRα) as the origin of adipocytes in skeletal muscle. However, the origin of muscle fibrosis remains largely unknown. In this study, clonal analyses show that PDGFRα(+) cells also differentiate into collagen type-I-producing cells. In fact, PDGFRα(+) cells accumulated in fibrotic areas of the diaphragm in the mdx mouse, a model of Duchenne muscular dystrophy. Furthermore, mRNA of fibrosis markers was expressed exclusively in the PDGFRα(+) cell fraction in the mdx diaphragm. Importantly, TGF-β isoforms, known as potent profibrotic cytokines, induced expression of markers of fibrosis in PDGFRα(+) cells but not in myogenic cells. Transplantation studies revealed that fibrogenic PDGFRα(+) cells mainly derived from pre-existing PDGFRα(+) cells and that the contribution of PDGFRα(-) cells and circulating cells was limited. These results indicate that mesenchymal progenitors are the main origin of not only fat accumulation but also fibrosis in skeletal muscle.
Severe quantitative and qualitative brown adipocyte defects are common in obesity. To investigate whether aberrant expression of tumor necrosis factor alpha (TNF-alpha) in obesity is involved in functional brown fat atrophy, we have studied genetically obese (ob/ob) mice with targeted null mutations in the genes encoding the two TNF receptors. The absence of both TNF receptors or p55 receptor alone resulted in a significant reduction in brown adipocyte apoptosis and an increase in beta(3)-adrenoreceptor and uncoupling protein-1 expression in obese mice. Increased numbers of multilocular functionally active brown adipocytes, and improved thermoregulation was also observed in obese animals lacking TNF-alpha function. These results indicate that TNF-alpha plays an important role in multiple aspects of brown adipose tissue biology and mediates the abnormalities that occur at this site in obesity.
Parathyroid hormone (PTH), a major regulator of bone metabolism, activates the PTHR1 receptor on the osteoblast plasma membrane to initiate signaling and induce transcription of primary response genes. Subsequently, primary genes with transcriptional activity regulate expression of downstream PTH targets. We have identified the adenovirus E4 promoter-binding protein/nuclear factor regulated by IL-3 (E4bp4) as a PTH-induced primary gene in osteoblasts. E4BP4 is a basic leucine zipper (bZIP) transcription factor that represses or activates transcription in non-osteoblastic cells. We report here that PTH rapidly and transiently induced E4bp4 mRNA in osteoblastic cells and that this induction did not require protein synthesis. PTH also induced E4BP4 protein synthesis and E4BP4 binding to a consensus but not to a mutant E4BP4 response element (EBPRE). E4BP4 overexpression inhibited an EBPRE-containing promoter-reporter construct, whereas PTH treatment attenuated activity of the same construct in primary mouse osteoblasts. Finally, E4BP4 overexpression inhibited PTH-induced activity of a cyclooxygenase-2 promoter-reporter construct. Our data suggest a role for E4BP4 in attenuation of PTH target gene transcription in osteoblasts.
Experimental studies have suggested that Wingless-related integration site 5A (WNT5A) is a proinflammatory secreted protein that is associated with metabolic dysfunction in obesity. Impaired angiogenesis in fat depots has been implicated in the development of adipose tissue capillary rarefaction, hypoxia, inflammation, and metabolic dysfunction. We have recently demonstrated that impaired adipose tissue angiogenesis is associated with overexpression of antiangiogenic factor VEGF-A165b in human fat and the systemic circulation. In the present study, we postulated that upregulation of WNT5A is associated with angiogenic dysfunction and examined its role in regulating VEGF-A165b expression in human obesity. We biopsied subcutaneous and visceral adipose tissue from 38 obese individuals (body mass index: 44 ± 7 kg/m2, age: 37 ± 11 yr) during planned bariatric surgery and characterized depot-specific protein expression of VEGF-A165b and WNT5A using Western blot analysis. In both subcutaneous and visceral fat, VEGF-A165b expression correlated strongly with WNT5A protein (r = 0.9, P < 0.001). In subcutaneous adipose tissue where angiogenic capacity is greater than in the visceral depot, exogenous human recombinant WNT5A increased VEGF-A165b expression in both whole adipose tissue and isolated vascular endothelial cell fractions (P < 0.01 and P < 0.05, respectively). This was associated with markedly blunted angiogenic capillary sprout formation in human fat pad explants. Moreover, recombinant WNT5A increased secretion of soluble fms-like tyrosine kinase-1, a negative regulator of angiogenesis, in the sprout media (P < 0.01). Both VEGF-A165b-neutralizing antibody and secreted frizzled-related protein 5, which acts as a decoy receptor for WNT5A, significantly improved capillary sprout formation and reduced soluble fms-like tyrosine kinase-1 production (P < 0.05). We demonstrated a significant regulatory nexus between WNT5A and antiangiogenic VEGF-A165b in the adipose tissue of obese subjects that was linked to angiogenic dysfunction. Elevated WNT5A expression in obesity may function as a negative regulator of angiogenesis. NEW & NOTEWORTHY Wingless-related integration site 5a (WNT5A) negatively regulates adipose tissue angiogenesis via VEGF-A165b in human obesity.
The oxidative pentose phosphate pathway (PPP) contributes to tumour growth, but the precise contribution of 6-phosphogluconate dehydrogenase (6PGD), the third enzyme in this pathway, to tumorigenesis remains unclear. We found that suppression of 6PGD decreased lipogenesis and RNA biosynthesis and elevated ROS levels in cancer cells, attenuating cell proliferation and tumour growth. 6PGD-mediated production of ribulose-5-phosphate (Ru-5-P) inhibits AMPK activation by disrupting the active LKB1 complex, thereby activating acetyl-CoA carboxylase 1 and lipogenesis. Ru-5-P and NADPH are thought to be precursors in RNA biosynthesis and lipogenesis, respectively; thus, our findings provide an additional link between the oxidative PPP and lipogenesis through Ru-5-P-dependent inhibition of LKB1-AMPK signalling. Moreover, we identified and developed 6PGD inhibitors, physcion and its derivative S3, that effectively inhibited 6PGD, cancer cell proliferation and tumour growth in nude mice xenografts without obvious toxicity, suggesting that 6PGD could be an anticancer target.
Objective:Consumption of high-fat diet exerts adverse effects on learning and memory formation, which is linked to impaired hippocampal function. Activation of glucagon-like peptide-1 (GLP-1) signalling ameliorates detrimental effects of obesity-diabetes on cognitive function; however, mechanisms underlying these beneficial actions remain unclear. This study examined effects of daily subcutaneous treatment with GLP-1 mimetic, Liraglutide, on synaptic plasticity, hippocampal gene expression and metabolic control in adult obese diabetic (ob/ob) mice. Results:Long-term potentiation (LTP) induced by area CA1 was completely abolished in ob/ob mice compared with lean controls. Deleterious effects on LTP were rescued (P<0.001) with Liraglutide. Indeed, Liraglutide-treated mice exhibited superior LTP profile compared with lean controls (P<0.01). Expression of hippocampal brain-derived neurotropic factor and neurotrophic tyrosine kinase receptor-type 2 were not significantly different, but synaptophysin and Mash1 were decreased in ob/ob mice. Treatment with Liraglutide over 21 days increased expression of Mash1 in ob/ob mice (2.0-fold; P<0.01). These changes were associated with significantly reduced plasma glucose (21% reduction; P<0.05) and markedly improved plasma insulin concentrations (2.1- to 3.3-fold; P<0.05 to P<0.01). Liraglutide also significantly reduced the glycaemic excursion following an intraperitonal glucose load (area under curve (AUC) values: 22%; P<0.05) and markedly enhanced the insulin response to glucose (AUC values: 1.6-fold; P<0.05). O2 consumption, CO2 production, respiratory exchange ratio and energy expenditure were not altered by Liraglutide therapy. On day 21, accumulated food intake (32% reduction; P<0.05) and number of feeding bouts (32% reduction; P<0.05) were significantly reduced but simple energy restriction was not responsible for the beneficial actions of Liraglutide. Conclusion:Liraglutide elicits beneficial effects on metabolic control and synaptic plasticity in mice with severe obesity and insulin resistance mediated in part through increased expression of Mash1 believed to improve hippocampal neurogenesis and cell survival.
Studies of the identity and physiological function of mesenchymal stromal cells (MSCs) have been hampered by a lack of markers that permit both prospective identification and fate mapping in vivo. We found that Leptin Receptor (LepR) is a marker that highly enriches bone marrow MSCs. Approximately 0.3% of bone marrow cells were LepR(+) , 10% of which were CFU-Fs, accounting for 94% of bone marrow CFU-Fs. LepR(+) cells formed bone, cartilage, and adipocytes in culture and upon transplantation in vivo. LepR(+) cells were Scf-GFP(+), Cxcl12-DsRed(high), and Nestin-GFP(low), markers which also highly enriched CFU-Fs, but negative for Nestin-CreER and NG2-CreER, markers which were unlikely to be found in CFU-Fs. Fate-mapping showed that LepR(+) cells arose postnatally and gave rise to most bone and adipocytes formed in adult bone marrow, including bone regenerated after irradiation or fracture. LepR(+) cells were quiescent, but they proliferated after injury. Therefore, LepR(+) cells are the major source of bone and adipocytes in adult bone marrow.
Wnt7a signals through its receptor Fzd7 to activate the planar-cell-polarity pathway and drive the symmetric expansion of satellite stem cells resulting in enhanced repair of skeletal muscle. In differentiated myofibres, we observed that Wnt7a binding to Fzd7 directly activates the Akt/mTOR growth pathway, thereby inducing myofibre hypertrophy. Notably, the Fzd7 receptor complex was associated with Gα(s) and PI(3)K and these components were required for Wnt7a to activate the Akt/mTOR growth pathway in myotubes. Wnt7a-Fzd7 activation of this pathway was completely independent of IGF-receptor activation. Together, these experiments demonstrate that Wnt7a-Fzd7 activates distinct pathways at different developmental stages during myogenic lineage progression, and identify a non-canonical anabolic signalling pathway for Wnt7a and its receptor Fzd7 in skeletal muscle.