TNFα is a potent inducer of platelet-activating factor synthesis in adipocytes but not in preadipocytes. Differential regulation by PI3K
Introduction
White Adipose Tissue (WAT),1 besides its traditional role in energy storage, is now well documented as a secretory and endocrine organ, which actively participates in the regulation of various physiological and pathological processes including immunity and inflammation [1]. Apart from adipocytes, which comprise the main cell type, WAT also contains stromovascular and immune cells, as well as connective tissue matrix and nerve tissue. WAT secretes biologically active proteins, collectively known as adipokines, derived from both adipocytes and the non-adipocyte fraction of the adipose tissue [2]. Several of these adipokines are known to be associated with inflammation as well as with obesity or metabolic syndrome [1], [3], [4]. Among them, tumour necrosis factor α (TNFα), which is oversecreted from WAT of obese individuals [5], has been suggested to be responsible for metabolic disturbances associated with obesity, mainly with insulin resistance [6].
TNFα is a proinflammatory cytokine implicated in a number of cell functions such as differentiation, proliferation and apoptosis [7]. Many cell types, including adipocytes, express and secrete TNFα, which acts in a paracrine, autocrine and even endocrine manner via two distinct receptors: TNFR1 and TNFR2 [8], [9], [10]. The downstream targets of TNFα, however, are not fully elucidated, although TNFα has been shown to activate phosphoinositide 3-kinase (PI3K) [11], [12], [13] as well as members of the mitogen-activated protein kinase (MAPK) family [14], [15], [16]. In addition, the implication of lipid mediators, such as platelet-activating factor (PAF), has been reported [17], [18], [19]. PAF, which is also a mediator of inflammation, is synthesized in many cell types by de novo or remodeling pathways [20]. Upon stimulation, PAF is synthesized mainly through the remodeling pathway which involves phospholipase A2 (PLA2) action on membrane glyceryl ethers and the consequent acetylation of the lyso-product at the sn-2 position by PAF-acetyltransferase [21]. Interestingly, in this pathway, extracellular signal-regulated kinase (ERK) is implicated; it has been shown that, in human neutrophils, ERK participates in PLA2 activation which is the initial step of PAF synthesis [22]. PAF mediates a number of biological activities of TNFα, including TNFα-induced lethal hepatitis and the impairment of skeletal muscle contractility [17], [18].
In adipocytes, TNFα has multiple effects, such as regulation of gene expression [12], [23], downregulation of adipocyte specific proteins [24], regulation of adipocyte number and size [8], [25], induction of insulin resistance [26] and stimulation of lipolysis [14], [27], [28]. There is evidence that PI3K as well as ERK1/2 are involved in some of these effects [11], [12], [14], [28].
Our research group has already shown the ability of mature adipocytes to synthesize PAF under stimulation and the presence of PAF-acetylhydrolase (PAF-AH) activity in these cells. This enzyme catalyzes the hydrolysis of the acetyl ester at the sn-2 position of PAF and converts it into its inactive metabolite, lyso-PAF, thus regulating PAF levels in a number of cells [29]. In the present study we investigate the possibility that TNFα stimulates PAF biosynthesis in adipocytes. We are also interested in whether a similar effect exists in preadipocytes. This would suggest that PAF, a potent inducer of inflammation, is recruited by TNFα to exert its inflammatory effects in adipose tissue. Our interest is also focused on the possible implication of PI3K and ERK1/2 in this process.
Section snippets
Materials
Cell culture media and reagents were purchased from Biochrom. Collagenase (from Clostridium histolyticum, type VII), TNFα (rat recombinant) and wortmannin were purchased from Sigma. 1-O-Hexadecyl-2-[3H]acetyl-sn-glycero-3-phosphocholine ([3H]acetyl-PAF, specific radioactivity 6 Ci/mmol) and 1-O-hexadecyl-2-sn-glycero-3-phosphocholine ([3H]lyso-PAF, specific radioactivity 58.5 Ci/mmol) were purchased from DuPont NEN. Solid phase extraction (SPE) columns were from IST. Tris–HCl was from Bio-Rad.
PAF synthesis from lyso-PAF in adipocytes and preadipocytes
In vivo PAF synthesis from lyso-PAF was studied using radioactive [3H]lyso-PAF and acetyl-CoA as substrates. In order to separate the synthesized PAF from the remaining non-esterified lyso-PAF a modified TLC method was used. The developing solvent consisted of chloroform–methanol–acetic acid–water (50:25:10:4). The efficiency of this separation method was first demonstrated by developing standard radioactive [3H]PAF and [3H]lyso-PAF on the same plate. After the development, the plate was
Discussion
Our results present evidence that TNFα is a potent inducer of PAF biosynthesis in the main cell type of white adipose tissue, namely adipocytes. TNFα also induces PAF synthesis in preadipocytes, but interestingly, the effect of TNFα on PAF production was 7-fold stronger in mature adipocytes. This difference reflects a maturation of TNFα–PAF interrelationship during the differentiation of preadipocytes into adipocytes, a process which is highly active during the development of obesity. Moreover,
Acknowledgments
This paper is part of the 03ED386 research project, implemented within the framework of the “Reinforcement Programme of Human Research Manpower” (PENED) and co-financed by National and Community Funds (25% from the Greek Ministry of Development-General Secretariat of Research and Technology and 75% from E.U.-European Social Fund.
References (55)
Adipose tissue, adipokines and inflammation
J Allergy Clin Immunol
(2005)- et al.
Fat as an endocrine organ: relationship to the metabolic syndrome
Am J Med Sci
(2005) - et al.
Tumor necrosis factors
Dev Comp Immunol
(2004) - et al.
Signaling pathways utilized by tumor necrosis factor receptor-1 in adipocytes to suppress differentiation
FEBS Lett
(2001) - et al.
Characterisation of receptor-specific TNFalpha functions in adipocyte cell lines lacking type 1 and 2 TNF receptors
FEBS Lett
(2000) - et al.
Tumor necrosis factor promotes phosphorylation and binding of insulin receptor substrate 1 to phosphatidylinositol 3-kinase in 3T3-L1 adipocytes
J Biol Chem
(1996) - et al.
TNF-α and leptin activate the α-isoform of class II phosphoinositide 3-kinase
Biochem Biophys Res Commun
(2003) - et al.
Involvement of a serine protease, but not of neutrophil elastase, in tumor necrosis factor-induced lethal hepatitis and induction of platelet-activating factor
J Hepatol
(2001) - et al.
Synthesis and release of Platelet-activating factor by human vascular endothelial cells treated with tumor necrosis factor or interleukin 1a
J Biol Chem
(1988) - et al.
Stress-induced platelet-activating factor synthesis in human neutrophils
Biochim Biophys Acta
(2005)
Insulin resistance in adipose tissue: direct and indirect effects of tumor necrosis factor-α
Cytokine Growth Factor Rev
The role of TNFα in adipocyte metabolism
Semin Cell Dev Biol
Plasma platelet activating factor-acetylhydrolase (PAF-AH)
Prog Lipid Res
Metabolism of isolated fat cells
J Biol Chem
Standard isolation of primary adipose cells from mouse epididymal fat pads induces inflammatory mediators and down-regulates adipocyte genes
J Biol Chem
A simple method for the separation and purification of total lipids from animal tissue
J Biol Chem
Characterization of a platelet activating factor acetylhydrolase from rat adipocyte
Life Sci
Protein measurement with the Folin phenol reagent
J Biol Chem
Expression of tumor necrosis factor alpha and its receptors during cellular differentiation
Cytokine
Les relations entre obésité, inflammation et insulinoresistance
C R Biol
Is obesity an inflammatory disease?
Surgery
Macrophage-secreted factors induce adipocyte inflammation and insulin resistance
Biochem Biophys Res Commun
Attenuation of folic acid-induced renal inflammatory injury in platelet-activating factor receptor-deficient mice
Am J Pathol
p110β is up-regulated during differentiation of 3T3-L1 cells and contributes to the highly insulin-responsive glucose transport activity
J Biol Chem
Regulation of proteins involved in insulin signaling pathways in differentiating human adipocytes
Biochem Biophys Res Commun
Adipose tissue as an endocrine organ
J Clin Endocrinol Metab
Role of adipokines in the obesity-inflammation relationship: the effect of fat removal
Plast Reconstr Surg
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