Franck Peiretti

Production of Plasminogen Activator Inhibitor 1 by Human Adipose Tissue: Possible Link Between Visceral Fat Accumulation and Vascular Disease

Plasminogen activator inhibitor type 1 (PAI-1) contributes to the pathogenesis of atherothrombosis. Its plasma level is strongly correlated with parameters that define the insulin resistance syndrome, in particular with BMI and visceral accumulation of body fat, suggesting that PAI-1 may be an adipose tissue–derived circulating peptide. The present study was designed to investigate PAI-1 expression by human adipose tissue and its different cellular fractions. Special interest has been paid to the amount of PAI-1 antigen produced by omental versus subcutaneous fat. PAI-1 protein detected by immunolocalization was present at the stromal and adipocyte levels. PAI-1 mRNA was detected in stromal vascular cells freshly isolated and under culture conditions. It was also detected in whole adipose tissue and adipocyte fraction under culture conditions. The mRNA signal from the adipocyte fraction was detected as early as 2 h of incubation. The increase in PAI-1 mRNA was followed by an increase in PAI-1 antigen in the conditioned medium that was suppressed by treatment with cycloheximide. Transforming growth factor-β1 significantly increased PAI-1 antigen production by the adipocyte fraction, whereas tumor necrosis factor-α did not have any effect. Interestingly, after 5 h of incubation, omental tissue explants produced significantly more PAI-1 antigen than did subcutaneous tissue from the same individual, whereas similar production of leptin by the two territories was observed. These results strongly suggest that human adipose tissue, in particular visceral tissue, can be an important contributor to the elevated plasma PAI-1 levels observed in central obesity.

Phosphorylation of eucaryotic translation initiation factor 4B Ser422 is modulated by S6 kinases.

The eucaryotic translation initiation factor 4B (eIF4B) stimulates the helicase activity of the DEAD box protein eIF4A to unwind inhibitory secondary structure in the 5′ untranslated region of eucaryotic mRNAs. Here, using phosphopeptide mapping and a phosphospecific antiserum, we identify a serum‐responsive eIF4B phosphorylation site, Ser422, located in an RNA‐binding region required for eIF4A helicase‐promoting activity. Ser422 phosphorylation appears to be regulated by the S6Ks: (a) Ser422 phosphorylation is sensitive to pharmacological inhibitors of phosphoinositide‐3 kinase and the mammalian target of rapamycin; (b) S6K1/S6K2 specifically phosphorylate Ser422 in vitro; and (c) rapamycin‐resistant S6Ks confer rapamycin resistance upon Ser422 phosphorylation in vivo. Substitution of Ser422 with Ala results in a loss of activity in an in vivo translation assay, indicating that phosphorylation of this site plays an important role in eIF4B function. We therefore propose that eIF4B may mediate some of the effects of the S6Ks on translation.

C3H/HeJ mice carrying a toll-like receptor 4 mutation are protected against the development of insulin resistance in white adipose tissue in response to a high-fat diet

Aims/hypothesisInflammation is associated with obesity and has been implicated in the development of diabetes and atherosclerosis. During gram-negative bacterial infection, lipopolysaccharide causes an inflammatory reaction via toll-like receptor 4 (TLR4), which has an essential function in the induction of innate and adaptative immunity. Our aim was to determine what role TLR4 plays in the development of metabolic phenotypes during high-fat feeding.Materials and methodsWe evaluated metabolic consequences of a high-fat diet in TLR4 mutant mice (C3H/HeJ) and their respective controls.ResultsTLR4 inactivation reduced food intake without significant modification of body weight, but with higher epididymal adipose tissue mass and adipocyte hypertrophy. It also attenuated the inflammatory response and increased glucose transport and the expression levels of adiponectin and lipogenic markers in white adipose tissue. In addition, TLR4 inactivation blunted insulin resistance induced by lipopolysaccharide in differentiated adipocytes. Increased feeding efficiency in TLR4 mutant mice was associated with lower mass and lower expression of uncoupling protein 1 gene in brown adipose tissue. Finally, TLR4 inactivation slowed the development of hepatic steatosis, reducing the liver triacylglycerol content and also expression levels of lipogenic and fibrosis markers.Conclusions/interpretationTLR4 influences white adipose tissue inflammation and insulin sensitivity, as well as liver fat storage, and is important in the regulation of metabolic phenotype during a fat-enriched diet.

Effect of atorvastatin and fluvastatin on the expression of plasminogen activator inhibitor type-1 in cultured human endothelial cells

Inhibitors of HMG-CoA reductase, namely statins, improve endothelial function independently of their cholesterol-lowering effects. Plasminogen activator inhibitor type-1 (PAI-1) plays a critical role in vascular pathophysiology both at the intra- and extravascular levels. We therefore investigated the effects of atorvastatin (ATOR) and fluvastatin (FLU) on PAI-1 and also tissue-type plasminogen activator (t-PA) synthesis in 20% fetal calf serum-cultured human umbilical vein endothelial cells (HUVEC) stimulated or not by recombinant human pro-inflammatory cytokines, i.e. tumor necrosis factor alpha (TNFalpha) and interleukin 1alpha (IL-1alpha). In non-stimulated HUVEC, ATOR and FLU significantly diminished (-50% at 2.0 micromol/l) the constitutive production of PAI-1 (mRNA level and protein secretion). This effect was prevented by addition of mevalonate (100 micromol/l). In HUVEC cultivated in 20% fetal calf serum, the t-PA antigen accumulation was not significantly altered, whereas in low serum concentration (1%) a significant stimulatory effect of ATOR (+30%) and FLU (+76%) was observed. In TNFalpha-stimulated cells, ATOR and FLU had a modest down-modulating effect (-17 and -20%, respectively) on TNFalpha-induced increase in PAI-1 synthesis. No effect of statins was observed in IL-1alpha-stimulated HUVEC, suggesting that statins do not interfere with the up-regulation of PAI-1 synthesis by pro-inflammatory cytokines. However, ATOR and FLU inhibited the TNFalpha-induced decrease in t-PA release. In conclusion, these results show that statins favorably modulate the expression of fibrinolytic factors produced by human endothelial cells.

The inflammatory receptor CD40 is expressed on human adipocytes: contribution to crosstalk between lymphocytes and adipocytes.

Aims/hypothesisObesity is associated with adipose tissue inflammation. The CD40 molecule, TNF receptor superfamily member 5 (CD40)/CD40 ligand (CD40L) pathway plays a role in the onset and maintenance of the inflammatory reaction, but has not been studied in human adipose tissue. Our aim was to examine CD40 expression by human adipocytes and its participation in adipose tissue inflammation.MethodsCD40 expression was investigated in human whole adipose tissue and during adipocyte differentiation by real-time PCR, Western blot and immunohistochemistry. The CD40/CD40L pathway was studied using recombinant CD40L (rCD40L) in adipocyte culture and neutralising antibodies in lymphocyte/adipocyte co-culture.ResultsCD40 mRNA levels in subcutaneous adipose tissue were higher in the adipocyte than in the stromal–vascular fraction. CD40 expression was upregulated during adipocyte differentiation. Addition of rCD40L to adipocytes induced mitogen activated protein kinase (MAPK) activation, stimulated inflammatory adipocytokine production, and decreased insulin-induced glucose transport in parallel with a downregulation of IRS1 and GLUT4 (also known as SCL2A4). rCD40L decreased the expression of lipogenic genes and increased lipolysis. CD40 mRNA levels were significantly higher in subcutaneous adipose tissue than in visceral adipose tissue of obese patients and were positively correlated with BMI, and with IL6 and leptin mRNA levels. Lymphocyte/adipocyte co-culture led to an upregulation of proinflammatory adipocytokines and a downregulation of leptin and adiponectin. Physical separation of the two cell types attenuated these effects, suggesting the involvement of a cell–cell contact. Blocking the CD40/CD40L interaction with neutralising antibodies reduced IL-6 secretion from adipocytes.Conclusions/interpretationAdipocyte CD40 may contribute to obesity-related inflammation and insulin resistance. T lymphocytes regulate adipocytokine production through both the release of soluble factor(s) and heterotypic contact with adipocytes involving CD40.

Intracellular maturation and transport of tumor necrosis factor alpha converting enzyme.

The tumor necrosis factor alpha converting enzyme (TACE) activity is required for the shedding of a variety of biologically active membrane bound precursors. The activation of TACE necessitates the proteolytic cleavage of its prodomain, a process that was suggested to be catalyzed by the proprotein convertase furin. However, the involvement of furin in this activation process has never been experimentally demonstrated. We have shown that the furinlike cleavage site (R-V-K-R(214)) localized between the prodomain and the metalloprotease domain of TACE is the sole site that can be in vitro cleaved by furin. In Cos7 cells, the release of TACE-processed substrates was reduced by the overexpression of the furin-specific proprotein convertase inhibitor Portland alpha1-antitrypsin inhibitor, but the release of TACE-processed substrates was increased by overexpression of furin in LoVo cells (deficient in furin activity) in which a mature form of TACE was identified. The immature form of TACE was detected at the surface of LoVo cells and at the surface of Cos7 and HT29 cells upon proprotein convertase inhibition. These results suggest that furin is the major proprotein convertase involved in the maturation/activation of TACE which is not a prerequisite for its cell-surface expression.

Human CalDAG-GEFI gene (RASGRP2) mutation affects platelet function and causes severe bleeding.

First case of a human RASGRP2 mutation affecting Rap1 activation in platelets and causing severe bleeding.

Identification of SAP97 as an intracellular binding partner of TACE.

Tumor necrosis factor alpha converting enzyme (TACE) is the metalloprotease-disintegrin responsible for the ectodomain shedding of several proteins, including tumor necrosis factor α. Using the yeast two-hybrid system, we identified the scaffolding protein synapse associated protein 97 (SAP97) as a binding partner of the cytoplasmic domain of TACE. By deletions and site-directed mutagenesis, we demonstrated that this interaction involved the PDZ3 domain of SAP97 and the extreme C-terminal amino-acid sequence of TACE. This interaction as well as the identification of the specific domains involved was confirmed in vitro by affinity purification and in mammalian cells by co-immunoprecipitation and alteration of localization analyzed by immunofluorescence microscopy. In addition, confocal microscopy showed that endogenous TACE and SAP97 colocalized in some intracellular areas of COS-7 cells and CACO-2 cells. Furthermore, overexpression of SAP97, unlike that of a mutant form of SAP97 deleted for its PDZ3 domain, altered the ability of TACE to release its substrates. Altogether, these results demonstrate an interaction between TACE and SAP97, which may have a functional implication for the regulation of TACE shedding activity.

Upregulation of TNF-α-induced ICAM-1 surface expression by adenylate cyclase-dependent pathway in human endothelial cells

The level of adhesion molecules expressed at the endothelial cell surface is critical in the control of inflammation. Adenylate cyclase (AC) activity allowing cyclic adenosine monophosphate (cAMP) production can modulate the inflammatory process. We investigated the AC‐dependent modulation of ICAM‐1 surface expression in human umbilical venous endothelial cells (HUVEC). Pretreatment of HUVEC with forskolin significantly upregulated tumor necrosis factor alpha (TNF‐α)‐ and interleukin‐1 alpha (IL1‐α)‐induced ICAM‐1 surface expression exclusively after a prolonged time of incubation with forskolin (at least 7–8 h). A poorly metabolizable analog of cAMP, dibutyryl‐cAMP, mimicked forskolin effect on ICAM‐1 surface expression. Protein kinase A (PKA) inhibitor H89 prevented forskolin effect on ICAM‐1 surface expression. Upregulation of ICAM‐1 surface level occurred through the increase in its mRNA levels and also to a subsequent activation of ICAM‐1 intracellular trafficking towards cell surface. Stimulation by agonists of β‐adrenergic receptors did not alter the TNF‐α‐induced ICAM‐1 surface expression. Pretreatment of HUVEC with pertussis toxin, which is known to activate AC through Giα inhibition, upregulated mRNA levels and surface expression of ICAM‐1 induced by TNF‐α. This effect was serum‐dependent, since ICAM‐1 expression was no more upregulated by pertussis toxin in cells cultured in 1% instead of 20% serum‐enriched medium. However, forskolin treatment of HUVEC did not modify their overall adhesive properties. In conclusion, a persistent cAMP level elevation activating PKA is able to enhance ICAM‐1 expression at the cell surface of endothelial cells placed under pro‐inflammatory conditions. Combination of activation of gene transcription and membrane targeting may account for this augmentation.

Polymorphisms of the tumor necrosis factor-alpha (TNF) and the TNF-alpha converting enzyme (TACE/ADAM17) genes in relation to cardiovascular mortality: the AtheroGene study

Tumor necrosis factor (TNF) is a major cytokine involved in inflammatory reaction and a mortality predictor in patients with coronary artery disease (CAD). Plasma levels of soluble TNF (sTNF) depend on the rate of its synthesis but also on its shedding from cell surface, a mechanism mainly regulated by the TNF alpha converting enzyme (TACE or ADAM17). We investigated the relationship between ADAM17 and TNF polymorphisms, circulating levels of shed ADAM17 substrates (sTNF, sTNFR1 and sTNFR2), and cardiovascular risk in a prospective cohort of CAD patients. Five tag single-nucleotide polymorphisms (SNPs) of the ADAM17 gene as well as four previously described TNF SNPs were genotyped in the Atherogene Study composed of 1,400 CAD patients among which 136 died from a cardiovascular (CV) cause. sTNF, sTNFR1, and sTNFR2 concentrations were all significantly elevated in patients with future CV death, independently of other clinical/biological variables. While none of the studied TNF SNPs was associated with sTNF, sTNFR1, nor sTNFR2 levels, the ADAM17 −154A allele was found associated with a 14% increase of sTNF levels as compared to the −154C allele (p = 0.0066). Moreover, individuals carrying the 747Leu allele displayed a borderline increased risk of future cardiovascular death [odds ratio, 2.06 (1.05–4.04), p = 0.03]. These results suggest a role of ADAM17 in the regulation of sTNF plasma levels and identifies ADAM17 gene as a candidate for CAD. Tumor necrosis factor (TNF) is a major cytokine involved in inflammatory reaction and a mortality predictor in patients with coronary artery disease (CAD). We have studied the association of ADAM17 and TNF polymorphisms with circulating levels of shed ADAM17 substrates (sTNF, sTNFR1 and sTNFR2) and with cardiovascular risk in a large population of individuals with CAD (Atherogene Study, n = 1,400). Two newly identified polymorphisms, obtained by a systematic sequencing of the ADAM17 gene, C-154A and Ser747leu, slightly influence respectively sTNF plasma levels and the risk of cardiovascular death.