The peroxisome proliferator-activated receptor-gamma (PPARgamma) is a transcription factor that has a pivotal role in adipocyte differentiation and expression of adipocyte-specific genes. The PPARgamma1 and gamma2 isoforms result from alternative splicing and have ligand-dependent and -independent activation domains. PPARgamma2 has an additional 28 amino acids at its amino terminus that renders its ligand-independent activation domain 5-10-fold more effective than that of PPARgamma1. Insulin stimulates the ligand-independent activation of PPARgamma1 and gamma2 (ref. 5), however, obesity and nutritional factors only influence the expression of PPARgamma2 in human adipocytes. Here, we report that a relatively common Pro12Ala substitution in PPARgamma2 is associated with lower body mass index (BMI; P=0.027; 0.015) and improved insulin sensitivity among middle-aged and elderly Finns. A significant odds ratio (4.35, P=0.028) for the association of the Pro/Pro genotype with type 2 diabetes was observed among Japanese Americans. The PPARgamma2 Ala allele showed decreased binding affinity to the cognate promoter element and reduced ability to transactivate responsive promoters. These findings suggest that the PPARgamma2 Pro12Ala variant may contribute to the observed variability in BMI and insulin sensitivity in the general population.

PPAR␥ is a member of the PPAR subfamily of nuclear receptors. In this work, the structure of the human PPAR␥ cDNA and gene was determined, and its promoters and tissue-specific expression were functionally characterized. Similar to the mouse, two PPAR isoforms, PPAR␥1 and PPAR␥2, were detected in man. The relative expression of human PPAR␥ was studied by a newly developed and sensitive reverse transcriptasecompetitive polymerase chain reaction method, which allowed us to distinguish between PPAR␥1 and ␥2 mRNA. In all tissues analyzed, PPAR␥2 was much less abundant than PPAR␥1. Adipose tissue and large intestine have the highest levels of PPAR␥ mRNA; kidney, liver, and small intestine have intermediate levels; whereas PPAR␥ is barely detectable in muscle. This high level expression of PPAR␥ in colon warrants further study in view of the well established role of fatty acid and arachidonic acid derivatives in colonic disease. Similarly as mouse PPAR␥s, the human PPAR␥s are activated by thiazolidinediones and prostaglandin J and bind with high affinity to a PPRE. The human PPAR␥ gene has nine exons and extends over more than 100 kilobases of genomic DNA. Alternate transcription start sites and alternate splicing generate the PPAR␥1 and PPAR␥2 mRNAs, which differ at their 5 -ends. PPAR␥1 is encoded by eight exons, and PPAR␥2 is encoded by seven exons. The 5 -untranslated sequence of PPAR␥1 is comprised of exons A1 and A2, whereas that of PPAR␥2 plus the additional PPAR␥2-specific N-terminal amino acids are encoded by exon B, located between exons A2 and A1. The remaining six exons, termed 1 to 6, are common to the PPAR␥1 and ␥2. Knowledge of the gene structure will allow screening for PPAR␥ mutations in humans with metabolic disorders, whereas knowledge of its expression pattern and factors regulating its expression could be of major importance in understanding its biology.White adipose tissue is composed of adipocytes, which play a central role in lipid homeostasis and the maintenance of energy balance in vertebrates. These cells store energy in the form of triglycerides during periods of nutritional affluence and release it in the form of free fatty acids at times of nutritional deprivation. Excess of white adipose tissue leads to obesity (1-3), whereas its absence is associated with lipodystrophic syndromes (4). In contrast to the development of brown adipose tissue, which mainly takes place before birth, the development of white adipose tissue is the result of a continuous differentiation/development process throughout life (2, 5). During development, cells that are pluripotent become increasingly restricted to specific differentiation pathways. Adipocyte differentiation results from coordinate changes in the expression of several proteins, which are mostly involved in lipid storage and metabolism, that give rise to the characteristic adipocyte phenotype. The changes in expression of these specialized proteins are mainly the result of alterations in the transcription rates of their genes.Several transcription fac...

The peroxisome proliferator-activated receptor ␥ (PPAR␥) is a ligand-dependent transcription factor that has been demonstrated to regulate fat cell development and glucose homeostasis. PPAR␥ is also expressed in a subset of macrophages and negatively regulates the expression of several proinf lammatory genes in response to natural and synthetic ligands. We here demonstrate that PPAR␥ is expressed in macrophage foam cells of human atherosclerotic lesions, in a pattern that is highly correlated with that of oxidationspecific epitopes. Oxidized low density lipoprotein (oxLDL) and macrophage colony-stimulating factor, which are known to be present in atherosclerotic lesions, stimulated PPAR␥ expression in primary macrophages and monocytic cell lines. PPAR␥ mRNA expression was also induced in primary macrophages and THP-1 monocytic leukemia cells by the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA). Inhibition of protein kinase C blocked the induction of PPAR␥ expression by TPA, but not by oxLDL, suggesting that more than one signaling pathway regulates PPAR␥ expression in macrophages. TPA induced the expression of PPAR␥ in RAW 264.7 macrophages by increasing transcription from the PPAR␥1 and PPAR␥3 promoters. In concert, these observations provide insights into the regulation of PPAR␥ expression in activated macrophages and raise the possibility that PPAR␥ ligands may inf luence the progression of atherosclerosis.

Dietary fiber protects against chronic inflammatory diseases by dampening immune responses through short-chain fatty acids (SCFAs). Here we examined the effect of dietary fiber in viral infection, where the anti-inflammatory properties of SCFAs in principle could prevent protective immunity. Instead, we found that fermentable dietary fiber increased survival of influenza-infected mice through two complementary mechanisms. High-fiber diet (HFD)-fed mice exhibited altered bone marrow hematopoiesis, characterized by enhanced generation of Ly6c patrolling monocytes, which led to increased numbers of alternatively activated macrophages with a limited capacity to produce the chemokine CXCL1 in the airways. Blunted CXCL1 production reduced neutrophil recruitment to the airways, thus limiting tissue immunopathology during infection. In parallel, diet-derived SCFAs boosted CD8 T cell effector function by enhancing cellular metabolism. Hence, dietary fermentable fiber and SCFAs set an immune equilibrium, balancing innate and adaptive immunity so as to promote the resolution of influenza infection while preventing immune-associated pathology.

BackgroundThe incidence of the insulin resistance syndrome has increased at an alarming rate worldwide, creating a serious challenge to public health care in the 21st century. Recently, epidemiological studies have associated the prevalence of type 2 diabetes with elevated body burdens of persistent organic pollutants (POPs). However, experimental evidence demonstrating a causal link between POPs and the development of insulin resistance is lacking.ObjectiveWe investigated whether exposure to POPs contributes to insulin resistance and metabolic disorders.MethodsSprague-Dawley rats were exposed for 28 days to lipophilic POPs through the consumption of a high-fat diet containing either refined or crude fish oil obtained from farmed Atlantic salmon. In addition, differentiated adipocytes were exposed to several POP mixtures that mimicked the relative abundance of organic pollutants present in crude salmon oil. We measured body weight, whole-body insulin sensitivity, POP accumulation, lipid and glucose homeostasis, and gene expression and we performed microarray analysis.ResultsAdult male rats exposed to crude, but not refined, salmon oil developed insulin resistance, abdominal obesity, and hepatosteatosis. The contribution of POPs to insulin resistance was confirmed in cultured adipocytes where POPs, especially organochlorine pesticides, led to robust inhibition of insulin action. Moreover, POPs induced down-regulation of insulin-induced gene-1 (Insig-1) and Lpin1, two master regulators of lipid homeostasis.ConclusionOur findings provide evidence that exposure to POPs commonly present in food chains leads to insulin resistance and associated metabolic disorders.

When preadipocytes reenter the cell cycle, PPAR gamma expression is induced, coincident with an increase in DNA synthesis, suggesting the involvement of the E2F family of cell cycle regulators. We show here that E2F1 induces PPAR gamma transcription during clonal expansion, whereas E2F4 represses PPARg amma expression during terminal adipocyte differentiation. Using a combination of in vivo experiments with knockout and chimeric animals and in vitro experiments, we demonstrate that the absence of E2F1 impairs, whereas depletion of E2F4 stimulates, adipogenesis. E2Fs hence represent the link between proliferative signaling pathways, triggering clonal expansion, and terminal adipocyte differentiation through regulation of PPAR gamma expression. This underscores the complex role of the E2F protein family in the control of both cell proliferation and differentiation.

Members of the peroxisome proliferator-activated receptor (PPAR) family might be involved in pathologies with altered lipid metabolism. They participate in the control of the expression of genes involved in lipid metabolism and adipocyte differentiation. In addition, thiazolidinediones improve insulin resistance in vivo by activating PPAR gamma. However, little is known regarding their tissue distribution and relative expression in humans. Using a quantitative and sensitive reverse transcription (RT)-competitive polymerase chain reaction (PCR) assay, we determined the distribution and relative mRNA expression of the four PPARs (alpha,beta, gamma1, and gamma2) and liver X receptor-alpha (LXR alpha) in the main tissues implicated in lipid metabolism. PPAR alpha and LXR alpha were mainly expressed in liver, while PPAR gamma1 predominated in adipose tissue and large intestine. We found that PPAR gamma2 mRNA was a minor isoform, even in adipose tissue, thus causing question of its role in humans. PPAR beta mRNA was present in all the tissues tested at low levels. In addition, PPAR gamma mRNA was barely detectable in skeletal muscle, suggesting that improvement of insulin resistance with thiazolidinediones may not result from a direct effect of these agents on PPAR gamma in muscle. Obesity and NIDDM were not associated with change in PPARs and LXR alpha expression in adipose tissue. The mRNA levels of PPAR gamma1, the predominant form in adipocytes, did not correlate with BMI, leptin mRNA levels, or fasting insulinemia in 29 subjects with various degrees of obesity. These results indicated that obesity is not associated with alteration in PPAR gene expression in abdominal subcutaneous adipose tissue in humans.

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a nuclear receptor implicated in adipocyte differentiation and insulin sensitivity. We investigated whether PPARgamma expression is dependent on the activity of adipocyte differentiation and determination factor 1/sterol regulatory element binding protein 1 (ADD-1/SREBP-1), another transcription factor associated with both adipocyte differentiation and cholesterol homeostasis. Ectopic expression of ADD-1/SREBP-1 in 3T3-L1 and HepG2 cells induced endogenous PPARgamma mRNA levels. The related transcription factor SREBP-2 likewise induced PPARgamma expression. In addition, cholesterol depletion, a condition known to result in proteolytic activation of transcription factors of the SREBP family, induced PPARgamma expression and improved PPRE-driven transcription. The effect of the SREBPs on PPARgamma expression was mediated through the PPARgamma1 and -3 promoters. Both promoters contain a consensus E-box motif that mediates the regulation of the PPARgamma gene by ADD-1/SREBP-1 and SREBP-2. These results suggest that PPARgamma expression can be controlled by the SREBP family of transcription factors and demonstrate new interactions between transcription factors that can regulate different pathways of lipid metabolism.