TCDD administration after the pro-adipogenic differentiation stimulus inhibits PPARgamma through a MEK-dependent process but less effectively suppresses adipogenesis

Obesity II: Establishing causal links between chemical exposures and obesity

Obesity is a multifactorial disease with both genetic and environmental components. The prevailing view is that obesity results from an imbalance between energy intake and expenditure caused by overeating and insufficient exercise. We describe another environmental element that can alter the balance between energy intake and energy expenditure: obesogens. Obesogens are a subset of environmental chemicals that act as endocrine disruptors affecting metabolic endpoints. The obesogen hypothesis posits that exposure to endocrine disruptors and other chemicals can alter the development and function of the adipose tissue, liver, pancreas, gastrointestinal tract, and brain, thus changing the set point for control of metabolism. Obesogens can determine how much food is needed to maintain homeostasis and thereby increase the susceptibility to obesity. The most sensitive time for obesogen action is in utero and early childhood, in part via epigenetic programming that can be transmitted to future generations. This review explores the evidence supporting the obesogen hypothesis and highlights knowledge gaps that have prevented widespread acceptance as a contributor to the obesity pandemic. Critically, the obesogen hypothesis changes the narrative from curing obesity to preventing obesity.

Aryl Hydrocarbon Receptor: From Homeostasis to Tumor Progression

Transcription factor aryl hydrocarbon receptor (AHR) has emerged as one of the main regulators involved both in different homeostatic cell functions and tumor progression. Being a member of the family of basic-helix-loop-helix (bHLH) transcriptional regulators, this intracellular receptor has become a key member in differentiation, pluripotency, chromatin dynamics and cell reprogramming processes, with plenty of new targets identified in the last decade. Besides this role in tissue homeostasis, one enthralling feature of AHR is its capacity of acting as an oncogene or tumor suppressor depending on the specific organ, tissue and cell type. Together with its well-known modulation of cell adhesion and migration in a cell-type specific manner in epithelial-mesenchymal transition (EMT), this duality has also contributed to the arise of its clinical interest, highlighting a new potential as therapeutic tool, diagnosis and prognosis marker. Therefore, a deregulation of AHR-controlled pathways may have a causal role in contributing to physiological and homeostatic failures, tumor progression and dissemination. With that firmly in mind, this review will address the remarkable capability of AHR to exert a different function influenced by the phenotype of the target cell and its potential consequences.

Cytochrome P4501B1 in bone marrow is co-expressed with key markers of mesenchymal stem cells. BMS2 cell line models PAH disruption of bone marrow niche development functions

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous pollutants that are metabolized to carcinogenic dihydrodiol epoxides (PAHDE) by cytochrome P450 1B1 (CYP1B1). This metabolism occurs in bone marrow (BM) mesenchymal stem cells (MSC), which sustain hematopoietic stem and progenitor cells (HSPC). In BM, CYP1B1-mediated metabolism of 7, 12-dimethylbenz[a]anthracene (DMBA) suppresses HSPC colony formation within 6 h, whereas benzo(a)pyrene (BP) generates protective cytokines. MSC, enriched from adherent BM cells, yielded the bone marrow stromal, BMS2, cell line. These cells express elevated basal CYP1B1 that scarcely responds to Ah receptor (AhR) inducers. BMS2 cells exhibit extensive transcriptome overlap with leptin receptor positive mesenchymal stem cells (Lepr+ MSC) that control the hematopoietic niche. The overlap includes CYP1B1 and the expression of HSPC regulatory factors (Ebf3, Cxcl12, Kitl, Csf1 and Gas6). MSC are large, adherent fibroblasts that sequester small HSPC and macrophage in the BM niche (Graphic abstract). High basal CYP1B1 expression in BMS2 cells derives from interactions between the Ah-receptor enhancer and proximal promoter SP1 complexes, boosted by autocrine signaling. PAH effects on BMS2 cells model Lepr+MSC niche activity. CYP1B1 metabolizes DMBA to PAHDE, producing p53-mediated mRNA increases, long after the in vivo HSPC suppression. Faster, direct p53 effects, favored by stem cells, remain possible PAHDE targets. However, HSPC regulatory factors remained unresponsive. BP is less toxic in BMS2 cells, but, in BM, CYP1A1 metabolism stimulates macrophage cytokines (Il1b > Tnfa> Ifng) within 6 h. Although absent from BMS2 and Lepr+MSC, their receptors are highly expressed. The impact of this cytokine signaling in MSC remains to be determined.

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BMS2 and Lepr+MSC cells co-express CYP1B1 and 12 functional niche activity markers.

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CYP1B1 mRNA in BMS2 cells depends on activation of SP1 coupled to an AhR enhancer unit.

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DMBA metabolism by CYP1B1 activates p53 gene targets in BMS2 cells far more than BP.

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HSPC suppression by CYP1B1 generation of PAHDE requires rapid, non-genomic targets.

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BMS2 and Lepr+MSC share receptors activated by BP stimulation of macrophage cytokines.

Pdgfrα-Cre mediated knockout of the aryl hydrocarbon receptor protects mice from high-fat diet induced obesity and hepatic steatosis

Aryl hydrocarbon receptor (AHR) agonists such as dioxin have been associated with obesity and the development of diabetes. Whole-body Ahr knockout mice on high-fat diet (HFD) have been shown to resist obesity and hepatic steatosis. Tissue-specific knockout of Ahr in mature adipocytes via adiponectin-Cre exacerbates obesity while knockout in liver increases steatosis without having significant effects on obesity. Our previous studies demonstrated that treatment of subcutaneous preadipocytes with exogenous or endogenous AHR agonists disrupts maturation into functional adipocytes in vitro. Here, we used platelet-derived growth factor receptor alpha (Pdgfrα)-Cre mice, a Cre model previously established to knock out genes in preadipocyte lineages and other cell types, but not liver cells, to further define AHR's role in obesity. We demonstrate that Pdgfrα-Cre Ahr-floxed (Ahrfl/fl) knockout mice are protected from HFD-induced obesity compared to non-knockout Ahrfl/fl mice (control mice). The Pdgfrα-Cre Ahrfl/fl knockout mice were also protected from increased adiposity, enlargement of adipocyte size, and liver steatosis while on the HFD compared to control mice. On a regular control diet, knockout and non-knockout mice showed no differences in weight gain, indicating the protective phenotype arises only when animals are challenged by a HFD. At the cellular level, cultured cells from brown adipose tissue (BAT) of Pdgfrα-Cre Ahrfl/fl mice were more responsive than cells from controls to transcriptional activation of the thermogenic uncoupling protein 1 (Ucp1) gene by norepinephrine, suggesting an ability to burn more energy under certain conditions. Collectively, our results show that knockout of Ahr mediated by Pdgfrα-Cre is protective against diet-induced obesity and suggest a mechanism by which enhanced UCP1 activity within BAT might confer these effects.

The chemical exposome of type 2 diabetes mellitus: Opportunities and challenges in the omics era

Type 2 diabetes mellitus (T2DM) is a global silent killer, with > 450 million affected adults worldwide. A diverse array of non-modifiable risk factors such as family history, age (> 45 yrs), race/ethnicity, genetics, and history of gestational diabetes and modifiable risk factors such as physical inactivity, high body fat, body weight, high blood pressure, and high cholesterol for progression of prediabetes to T2DM. Given, that the modern world human population is constantly exposed to multiple stressors in the form of physical (i.e., sound, weather etc.) and chemical environment (i.e., diet, pollutants etc.), industrialization, and modernization has led to form a basis for exposomal correlation with T2DM incidence. Over the past decade, there have been emerging reports on association of levels of persistent organic pollutants (POPs), phthalates, antibiotics, drugs, air pollution, pesticides, and heavy metals with T2DM. In this review, we discuss the well known chemical exposome that has been associated with T2DM; the tools and approaches to capture this chemical exposome, and future opportunities and challenges in this exciting area of research. We further provide a window of thoughts, whether omics technologies can help fill in the gaps to help provide high throughput exposomics datasets in an unbiased manner to help understand T2DM pathophysiology in the context of industrialization, drastic lifestyle changes, urbanization, and pollution. We also discuss and provide guidelines/call to action for future exposomics studies investigating the association of T2DM with exposomes in the context of both epidemiological and experimental approaches.

In utero dioxin exposure and cardiometabolic risk in the Seveso Second Generation Study

BACKGROUND/OBJECTIVES

In utero exposure to endocrine-disrupting compounds such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) may alter risk of obesity and related metabolic disease later in life. We examined the relationship of prenatal exposure to TCDD with obesity and metabolic syndrome (MetS) in children born to a unique cohort of TCDD-exposed women resulting from a 1976 explosion in Seveso, Italy.

SUBJECTS/METHODS

In 2014, nearly 40 years after the explosion, we enrolled 611 post-explosion offspring, 2 to 39 years of age, in the Seveso Second Generation Study. In utero TCDD exposure was defined primarily as TCDD concentration measured in maternal serum collected soon after the explosion and alternately as TCDD estimated at pregnancy. We measured height, weight, waist circumference, body fat, blood pressure, and fasting blood levels of lipids and glucose, which were combined to assess body mass index (BMI) and MetS.

RESULTS

Children (314 female, 297 male) averaged 23.6 (±6.0) years of age. Among the 431 children ≥18 years, a 10-fold increase in initial maternal TCDD concentration was inversely associated with BMI in daughters (adj-β = -0.99 kg/m2; 95% CI -1.86, -0.12), but not sons (adj-β = 0.41 kg/m2; 95% CI -0.35, 1.18) (p-int = 0.02). A similar relationship was found in the younger children (2-17 years); a 10-fold increase in initial maternal TCDD was inversely associated with BMI z-score (adj-β = -0.59 kg/m2; 95% CI -1.12, -0.06) among daughters, but not sons (adj-β = 0.04 kg/m2; 95% CI -0.34, 0.41) (p-int = 0.03). In contrast, in sons only, initial maternal TCDD was associated with increased risk for MetS (adj-RR = 2.09, 95% CI 1.09, 4.02). Results for TCDD estimated at pregnancy were comparable.

CONCLUSIONS

These results suggest prenatal TCDD exposure alters cardiometabolic endpoints in a sex-specific manner. In daughters, in utero TCDD is inversely associated with adiposity measures. In sons, in utero TCDD is associated with increased risk for MetS.

A delayed proinflammatory response of human preadipocytes to PCB126 is dependent on the aryl hydrocarbon receptor

Inflammation in adipose tissue is recognized as a causative factor in the development of type II diabetes. Adipocyte hypertrophy as well as bacterial and environmental factors have been implicated in causing inflammation in mature adipocytes. Exposure to persistent organic pollutants such as polychlorinated biphenyls (PCBs) has been associated with the development of type II diabetes. We show here that PCB126, a dioxin-like PCB, activates a robust proinflammatory state in fat cell precursors (preadipocytes). The response was found to be dependent on aryl hydrocarbon receptor (AhR) activation, although induction of the response was delayed compared to upregulation of CYP1A1, a classic AhR-responsive gene. Treatment of preadipocytes with a nuclear factor kappa-light-chain-enhancer of activated B cell (NF-κB) inhibitor partially attenuated the PCB126-induced inflammatory response and partly, but not completely, ameliorated disruption of adipogenesis caused by PCB126. Our results indicate a role for PCB126 in mediating an inflammatory response through AhR in preadipocytes that interferes with adipogenesis.

Adipose Tissue as a Site of Toxin Accumulation

We examine the role of adipose tissue, typically considered an energy storage site, as a potential site of toxicant accumulation. Although the production of most persistent organic pollutants (POPs) was banned years ago, these toxicants persist in the environment due to their resistance to biodegradation and widespread distribution in various environmental forms (e.g., vapor, sediment, and water). As a result, human exposure to these toxicants is inevitable. Largely due to their lipophilicity, POPs bioaccumulate in adipose tissue, resulting in greater body burdens of these environmental toxicants with obesity. POPs of major concern include polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and furans (PCDDs/PCDFs), and polybrominated biphenyls and diphenyl ethers (PBBs/PBDEs), among other organic compounds. In this review, we (i) highlight the physical characteristics of toxicants that enable them to partition into and remain stored in adipose tissue, (ii) discuss the specific mechanisms of action by which these toxicants act to influence adipocyte function, and (iii) review associations between POP exposures and the development of obesity and diabetes. An area of controversy relates to the relative potential beneficial versus hazardous health effects of toxicant sequestration in adipose tissue. © 2017 American Physiological Society. Compr Physiol 7:1085-1135, 2017.

2,3,7,8-Tetrachlorodibenzo-p-Dioxin Alters Lipid Metabolism and Depletes Immune Cell Populations in the Jejunum of C57BL/6 Mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent aryl hydrocarbon receptor agonist that elicits dose-dependent hepatic fat accumulation and inflammation that can progress to steatohepatitis. To investigate intestine-liver interactions that contribute to TCDD-elicited steatohepatitis, we examined the dose-dependent effects of TCDD (0.01, 0.03, 0.1, 0.3, 1, 3, 10, or 30 µg/kg) on jejunal epithelial gene expression in C57BL/6 mice orally gavaged every 4 days for 28 days. Agilent 4x44K whole-genome microarray analysis of the jejunal epithelium identified 439 differentially expressed genes (|fold change| ≥ 1.5, P1(t) ≥ 0.999) across 1 or more doses, many related to lipid metabolism and immune system processes. TCDD-elicited differentially expressed genes were associated with lipolysis, fatty acid/cholesterol absorption and transport, the Kennedy pathway, and retinol metabolism, consistent with increased hepatic fat accumulation. Moreover, several major histocompatibility complex (MHC) class II genes (H2-Aa, H2-Ab1, H2-DMb1, Cd74) were repressed, coincident with decreased macrophage and dendritic cell levels in the lamina propria, suggesting migration of antigen-presenting cells out of the intestine. In contrast, hepatic RNA-Seq analysis identified increased expression of MHC class II genes, as well as chemokines and chemokine receptors involved in macrophage recruitment (Ccr1, Ccr5, Ccl5, Cx3cr1), consistent with hepatic F4/80 labeling and macrophage infiltration into the liver. Collectively, these results suggest TCDD elicits changes that support hepatic lipid accumulation, macrophage migration, and the progression of hepatic steatosis to steatohepatitis.

PCB126 inhibits adipogenesis of human preadipocytes

Emerging evidence indicates that persistent organic pollutants (POPs), including polychlorinated biphenyls (PCBs), are involved in the development of diabetes. Dysfunctional adipocytes play a significant role in initiating insulin resistance. Preadipocytes make up a large portion of adipose tissue and are necessary for the generation of functional mature adipocytes through adipogenesis. PCB126 is a dioxin-like PCB and a potent aryl hydrocarbon receptor (AhR) agonist. We hypothesized that PCB126 may be involved in the development of diabetes through disruption of adipogenesis. Using a newly developed human preadipocyte cell line called NPAD (Normal PreADipocytes), we found that exposure of preadipocytes to PCB126 resulted in significant reduction in their subsequent ability to fully differentiate into adipocytes, more so than when the cells were exposed to PCB126 during differentiation. Reduction in differentiation by PCB126 was associated with downregulation of transcript levels of a key adipocyte transcription factor, PPARγ, and late adipocyte differentiation genes. An AhR antagonist, CH223191, blocked this effect. These studies indicate that preadipocytes are particularly sensitive to the effects of PCB126 and suggest that AhR activation inhibits PPARγ transcription and subsequent adipogenesis. Our results validate the NPAD cell line as a useful model for studying the effects of POPs on adipogenesis.

An overview of alopecias

Hair loss is a topic of enormous public interest and understanding the pathophysiology and treatment of various alopecias will likely make a large impact on patients' lives. The investigation of alopecias also provides important insight in the basic sciences; for instance, the abundance of stem cell populations and regenerative cycles that characterize a hair follicle render it an excellent model for the study of stem cell biology. This review seeks to provide a concise summary of the major alopecias with regard to presentation and management, and correlate these to recent advances in relevant research on pathogenesis.

Obesity and metabolic comorbidities: environmental diseases?

Obesity and metabolic comorbidities represent increasing health problems. Endocrine disrupting compounds (EDCs) are exogenous agents that change endocrine function and cause adverse health effects. Most EDCs are synthetic chemicals; some are natural food components as phytoestrogens. People are exposed to complex mixtures of chemicals throughout their lives. EDCs impact hormone-dependent metabolic systems and brain function. Laboratory and human studies provide compelling evidence that human chemical contamination can play a role in obesity epidemic. Chemical exposures may increase the risk of obesity by altering the differentiation of adipocytes. EDCs can alter methylation patterns and normal epigenetic programming in cells. Oxidative stress may be induced by many of these chemicals, and accumulating evidence indicates that it plays important roles in the etiology of chronic diseases. The individual sensitivity to chemicals is variable, depending on environment and ability to metabolize hazardous chemicals. A number of genes, especially those representing antioxidant and detoxification pathways, have potential application as biomarkers of risk assessment. The potential health effects of combined exposures make the risk assessment process more complex compared to the assessment of single chemicals. Techniques and methods need to be further developed to fill data gaps and increase the knowledge on harmful exposure combinations.

TCDD-elicited effects on liver, serum, and adipose lipid composition in C57BL/6 mice

The aryl hydrocarbon receptor (AhR) mediates alterations in hepatic lipid composition elicited by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In order to further investigate the effects of TCDD, liver, serum, and gonadal white adipose tissue (gWAT) fatty acid methyl esters (FAMEs) and lipids were examined in fasted 4-week-old female mice orally gavaged with 30 µg/kg TCDD at 24, 72, and 168 h postdose. Mean hepatic FAME levels increased (236.7 µmol/g in controls compared with 392.2 µmol/g in TCDD treated) with minimal changes in gWAT and serum. In the liver, TCDD decreased saturated fatty acids (SFAs 16:0, 18:0, 20:0, and 22:0) and increased monounsaturated fatty acids (MUFAs 16:1n7, 18:1n9, and 20:1n9). Hepatic polyunsaturated fatty acids (PUFAs) 20:2n6, 20:3n6, 18:3n3, and 22:5n3 also increased, whereas 20:4n6 and 22:6n3 levels decreased. gWAT PUFAs 20:2n6 and 20:3n6 exhibited modest increases, whereas serum 18:0 decreased and 18:1n9 increased. Serum analyses also identified a ~25% decrease in total cholesterol (CHOL), low-density lipoprotein (LDL), and high-density lipoprotein following TCDD treatment. The decrease in serum CHOL was consistent with the induction of hepatic reverse CHOL transport genes Lcat (2.0-fold), Apoa1 (1.7-fold), and Ldlr (3.6-fold), and the repression of CHOL biosynthesis genes Hmgcs1 (-2.1-fold) and Hmgcr (-2.3-fold). In addition, TCDD decreased serum Apob100 (4.4-fold) and Apob48 (2.2-fold) protein levels, suggesting serum lipid clearance and decreased hepatic efflux. Collectively, the TCDD-elicited decreases in serum lipid levels are consistent with AhR-mediated enhancement of dietary fat distribution to the liver.

Comparative metabolomic and genomic analyses of TCDD-elicited metabolic disruption in mouse and rat liver

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) elicits a broad spectrum of species-specific effects that have not yet been fully characterized. This study compares the temporal effects of TCDD on hepatic aqueous and lipid metabolite extracts from immature ovariectomized C57BL/6 mice and Sprague-Dawley rats using gas chromatography-mass spectrometry and nuclear magnetic resonance-based metabolomic approaches and integrates published gene expression data to identify species-specific pathways affected by treatment. TCDD elicited metabolite and gene expression changes associated with lipid metabolism and transport, choline metabolism, bile acid metabolism, glycolysis, and glycerophospholipid metabolism. Lipid metabolism is altered in mice resulting in increased hepatic triacylglycerol as well as mono- and polyunsaturated fatty acid (FA) levels. Mouse-specific changes included the induction of CD36 and other cell surface receptors as well as lipases- and FA-binding proteins consistent with hepatic triglyceride and FA accumulation. In contrast, there was minimal hepatic fat accumulation in rats and decreased CD36 expression. However, choline metabolism was altered in rats, as indicated by decreases in betaine and increases in phosphocholine with the concomitant induction of betaine-homocysteine methyltransferase and choline kinase gene expression. Results from these studies show that aryl hydrocarbon receptor-mediated differential gene expression could be linked to metabolite changes and species-specific alterations of biochemical pathways.

Endocrine disruptors: from endocrine to metabolic disruption

Synthetic chemicals currently used in a variety of industrial and agricultural applications are leading to widespread contamination of the environment. Even though the intended uses of pesticides, plasticizers, antimicrobials, and flame retardants are beneficial, effects on human health are a global concern. These so-called endocrine-disrupting chemicals (EDCs) can disrupt hormonal balance and result in developmental and reproductive abnormalities. New in vitro, in vivo, and epidemiological studies link human EDC exposure with obesity, metabolic syndrome, and type 2 diabetes. Here we review the main chemical compounds that may contribute to metabolic disruption. We then present their demonstrated or suggested mechanisms of action with respect to nuclear receptor signaling. Finally, we discuss the difficulties of fairly assessing the risks linked to EDC exposure, including developmental exposure, problems of high- and low-dose exposure, and the complexity of current chemical environments.

An untargeted multi-technique metabolomics approach to studying intracellular metabolites of HepG2 cells exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Background

In vitro cell systems together with omics methods represent promising alternatives to conventional animal models for toxicity testing. Transcriptomic and proteomic approaches have been widely applied in vitro but relatively few studies have used metabolomics. Therefore, the goal of the present study was to develop an untargeted methodology for performing reproducible metabolomics on in vitro systems. The human liver cell line HepG2, and the well-known hepatotoxic and non-genotoxic carcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), were used as the in vitro model system and model toxicant, respectively.

Results

The study focused on the analysis of intracellular metabolites using NMR, LC-MS and GC-MS, with emphasis on the reproducibility and repeatability of the data. State of the art pre-processing and alignment tools and multivariate statistics were used to detect significantly altered levels of metabolites after exposing HepG2 cells to TCDD. Several metabolites identified using databases, literature and LC-nanomate-Orbitrap analysis were affected by the treatment. The observed changes in metabolite levels are discussed in relation to the reported effects of TCDD.

Conclusions

Untargeted profiling of the polar and apolar metabolites of in vitro cultured HepG2 cells is a valid approach to studying the effects of TCDD on the cell metabolome. The approach described in this research demonstrates that highly reproducible experiments and correct normalization of the datasets are essential for obtaining reliable results. The effects of TCDD on HepG2 cells reported herein are in agreement with previous studies and serve to validate the procedures used in the present work.

α3(V) collagen is critical for glucose homeostasis in mice due to effects in pancreatic islets and peripheral tissues

Collagen V, broadly expressed as α1(V)2 α2(V) heterotrimers that regulate collagen fibril geometry and strength, also occurs in some tissues, such as white adipose tissue (WAT), pancreatic islets, and skeletal muscle, as the poorly characterized α1(V) α2(V) α3(V) heterotrimer. Here, we investigate the role of α3(V) collagen chains by generating mice with a null allele of the α3(V) gene Col5a3 (Col5a3–/– mice). Female Col5a3–/– mice had reduced dermal fat and were resistant to high-fat diet–induced weight gain. Male and female mutant mice were glucose intolerant, insulin-resistant, and hyperglycemic, and these metabolic defects worsened with age. Col5a3–/– mice demonstrated decreased numbers of pancreatic islets, which were more susceptible to streptozotocin-induced apoptosis, and islets isolated from mutant mice displayed blunted glucose-stimulated insulin secretion. Moreover, Col5a3–/– WAT and skeletal muscle were defective in glucose uptake and mobilization of intracellular GLUT4 glucose transporter to the plasma membrane in response to insulin. Our results underscore the emerging view of the importance of ECM to the microenvironments that inform proper development/functioning of specialized cells, such as adipocytes, β cells, and skeletal muscle.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on adipogenic differentiation and insulin-induced glucose uptake in 3T3-L1 cells

Dioxin exposure has been positively associated with human type II diabetes. Because lipophilic dioxins accumulate mainly in adipose tissue, this study aimed to determine if dioxins induce metabolic dysfunction in fat cells. Using 3T3-L1 cells as an in vitro model, we analyzed the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a model dioxin, on adipogenic differentiation, glucose uptake, and lipolysis. TCDD inhibited adipogenic differentiation, as determined by using oil droplet formation and adipogenic marker gene expression, including PPARgamma (peroxisome proliferator-activated receptor gamma), C/EBPalpha (CCAAT/enhancer-binding protein alpha), and Glut4 (glucose transporter type 4). Effects of TCDD on glucose uptake were evaluated using fully differentiated 3T3-L1 adipocytes, revealing that TCDD significantly attenuated insulin-induced glucose uptake dose dependently. Inhibition of aryl hydrocarbon receptor (AhR) by alpha-naphthoflavone (alpha-NF), an AhR inhibitor, did not prevent the inhibitory effect of TCDD on glucose uptake, suggesting that TCDD attenuates insulin-induced glucose uptake in an AhR-independent manner. Effects of TCDD on lipolysis were determined using glycerol release assay. We found that TCDD had no marked effect on isoproterenol-induced glycerol release in fully differentiated 3T3-L1 adipocytes. These results provide in vitro evidence of TCDD's effects on fat cell metabolism, suggesting dioxin exposure in development of insulin resistance and type II diabetes.

Organochlorine exposure and incidence of diabetes in a cohort of Great Lakes sport fish consumers

BACKGROUND

Studies have demonstrated ubiquitous human exposure to persistent organic pollutants (POPs) such as p,p'-diphenyldichloroethene (DDE) and polychlorinated biphenyls (PCBs). Although there is considerable evidence that POP exposures are associated with prevalent diabetes, these studies do not establish causality because the cross-sectional study design does not allow for assessment of temporality of the exposure-disease association. Prospective studies, however, have been lacking.

OBJECTIVES

This study was designed to determine whether POP body burdens are related to incidence of diabetes in a cohort of Great Lakes sport fish consumers.

METHODS

The cohort was established in the early 1990s and followed through 2005. We tested serum for DDE and PCB congeners and assessed diabetes diagnosis, demographics, and fish consumption. Associations of diabetes with exposures were examined prospectively in participants without diabetes in 1994-1995, followed through 2005. Annual percent changes in DDE and PCB-132/153 from 1994 to 2005 were examined by diabetes status.

RESULTS

DDE exposure was associated with incident diabetes. Incident diabetes was not associated with mono-ortho PCB-118, total PCBs, or years of sport fish consumption. Annual percent change in DDE and PCB-132/153 did not differ significantly by diabetes status.

CONCLUSIONS

This study demonstrates an association between DDE exposure and incident diabetes. The findings of an association of DDE with incident diabetes and the lack of effect of diabetes on annual percent change in POPs do not support the hypothesis that associations of POPs with diabetes are attributable to reverse causality. Additional studies should address the biological pathways by which DDE could affect glucose homeostasis.

Hair follicle stem cell-specific PPARgamma deletion causes scarring alopecia

Primary cicatricial or scarring alopecias (CA) are a group of inflammatory hair disorders of unknown pathogenesis characterized by the permanent destruction of the hair follicle. The current treatment options are ineffective in controlling disease progression largely because the molecular basis for CA is not understood. Microarray analysis of the lymphocytic CA, Lichen planopilaris (LPP), compared to normal scalp biopsies identified decreased expression of genes required for lipid metabolism and peroxisome biogenesis. Immunohistochemical analysis showed progressive loss of peroxisomes, proinflammatory lipid accumulation, and infiltration of inflammatory cells followed by destruction of the pilosebaceous unit. The expression of peroxisome proliferator-activated receptor (PPAR) gamma, a transcription factor that regulates these processes, is significantly decreased in LPP. Specific agonists of PPARgamma are effective in inducing peroxisomal and lipid metabolic gene expression in human keratinocytes. Finally, targeted deletion of PPARgamma in follicular stem cells in mice causes a skin and hair phenotype that emulates scarring alopecia. These studies suggest that PPARgamma is crucial for healthy pilosebaceous units and it is the loss of this function that triggers the pathogenesis of LPP. We propose that PPARgamma-targeted therapy may represent a new strategy in the treatment of these disorders.

Prevalence of diabetes and body burdens of polychlorinated biphenyls, polybrominated diphenyl ethers, and p,p'-diphenyldichloroethene in Great Lakes sport fish consumers

Recent studies have demonstrated ubiquitous exposure to persistent organic pollutants (POPs) such as p,p'-diphenyldichloroethene (DDE), polychlorinated biphenyls (PCBs), and polybrominated diphenyl ethers (PBDEs). Although several studies have noted associations of dioxins, PCBs and DDE with diabetes, the results have not always been consistent, with few examining effects of PBDEs or simultaneous exposures. The purpose of this study was to determine whether POP body burdens are related to diabetes in a cross section of Great Lake sport fish consumers. The cohort, which was established in the early 1990s, was recontacted in 2004-2005. Serum was collected and tested for DDE, PCBs, PBDEs, hemoglobin A1c and lipids, and diabetes diagnosis, demographics and fish consumption were assessed by self report. Associations of diabetes with exposures were examined in cross sectional data from 2004 to 2005. DDE exposure and dioxin-like mono-ortho PCBs were associated with diabetes, but the association of dioxin-like mono-ortho PCBs with diabetes did not remain significant after control for DDE exposure. Neither the sum of PCBs or years of sport fish consumption were associated with prevalent diabetes. There was a non-significant association of PBDEs with diabetes only in those with hypothyroid disease. The current study confirms previous cross sectional associations of DDE exposure with diabetes. Future studies should address biologic pathways by which selective POPs affect glucose homeostasis.

The aryl hydrocarbon receptor at the crossroads of multiple signaling pathways

The aryl hydrocarbon receptor (AHR) has long been recognized as a ligand-activated transcription factor responsible for the induction of drug-metabolizing enzymes. Its role in the combinatorial matrix of cell functions was established long before the first report of an AHR cDNA sequence was published. It is only recently that other functions of this protein have begun to be recognized, and it is now clear that the AHR also functions in pathways outside of its well-characterized role in xenobiotic enzyme induction. Perturbation of these pathways by xenobiotic ligands may ultimately explain much of the toxicity of these compounds. This chapter focuses on the interactions of the AHR in pathways critical to cell cycle regulation, mitogen-activated protein kinase cascades, differentiation and apoptosis. Ultimately, the effect of a particular AHR ligand on the biology of the organism will depend on the milieu of critical pathways and proteins expressed in specific cells and tissues with which the AHR itself interacts.

Methods that resolve different contributions of clonal expansion to adipogenesis in 3T3-L1 and C3H10T1/2 cells

The mouse embryo fibroblast cell lines 3T3-L1 and C3H10T1/2 differentiate to adipocytes that exhibit similar insulin regulation of lipogenesis. These cell lines, however, differ appreciably in the processes that produce the major regulator PPAR gamma. Each line is stimulated by a mixture of insulin, dexamethasone, and methylisobutylxanthine (IDM). In the first 24h, IDM activates each cell type to produce similar regulatory changes and cell contraction. However, the increase in PPARy is delayed by 24h in typical 3T3-L1 cells compared with C3H10T1/2 cells. This delay is caused by the need for one or two rounds of cell division (clonal expansion) for PPAR gamma synthesis in 3T3-L1 cells. This expansion also occurs in C3H10T1/2 cells, but is not needed for PPAR gamma synthesis and differentiation. Other 3T3-L1 sublines have been described that follow the C3H10T1/2 pattern of differentiation. Culture conditions and inhibitors are described here that remove clonal expansion in C3H10T1/2 cells. With these constraints the cells retain full commitment to differentiation. This distinction is significant because many agents suppress differentiation in 3T3-L1 cells through inhibition of clonal expansion. Other effects on differentiation may be seen in C3H10T1/2 cells that are obscured in 3T3-L1 cells due to this inhibition of proliferation. Human preadipocytes do not need clonal expansion for adipogenesis, thus paralleling C3H10T1/2 cells.

Development of a human adipocyte model derived from human mesenchymal stem cells (hMSC) as a tool for toxicological studies on the action of TCDD

Abstract Efforts were made to develop a human adipocyte model that is useful for toxicological studies in vitro. For this purpose, a stem cell line derived from human bone marrow cells, originally from an adult, was induced to differentiate towards adipocytes by treating them with insulin, dexamethasone, indomethacin and 3-isobutyl-1-methylxanthine for 3 d, followed by additional incubation for 3 d in Dulbecco's modified Eagle's medium supplemented with insulin only. In most cases, thus differentiated cells through such one cycle of differentiation treatment were further subjected to the second cycle of differentiation. The resulting 2-cycle differentiated cells were found to exhibit many characteristics of typical adipocytes. Dioxin (TCDD), when added at the beginning of their treatment with differentiation-inducing hormone cocktail, clearly prevented them from becoming adipocytes, as in the case of TCDD-treated 3T3-L1 cells. Furthermore, TCDD, even when administered to previously differentiated human mesenchymal stem cells (hMSC) adipocytes, consistently induced the sign of inflammatory responses during the early period of TCDD action (24 h), which was followed by gradual loss of adipocyte-specific markers during the 5-d incubation period. In conclusion, hMSC-derived adipocytes appear to offer a promising human cell model suited for future toxicological studies.

PPARalpha ligands reduce PCB-induced endothelial activation: possible interactions in inflammation and atherosclerosis

Exposure to polychlorinated biphenyls (PCBs) can activate inflammatory responses in vascular endothelial cells. Activation of peroxisome proliferator-activated receptors (PPARs) by nutrients or synthetic agonists has been shown to block pro-inflammatory responses both in vitro and in vivo. Here we demonstrate that activation of PPARalpha by synthetic agonists can reduce 3,3'4,4'-tetrachlorobiphenyl (PCB77)-induced endothelial cell activation. Primary vascular endothelial cells were pretreated with the PPARalpha ligands fenofibrate or WY14643 followed by exposure to PCB77. PPARalpha activation protected endothelial cells against PCB77-induced expression of the pro-inflammatory proteins vascular cell adhesion molecule-1 (VCAM-1), cycloxygenase-2 (COX-2), and PCB77-induced expression and activity of the aryl hydrocarbon receptor (AHR) responsive cytochrome P450 1A1 (CYP1A1). Furthermore, basal AHR expression was downregulated by fenofibrate and WY14643. We also investigated the possible interactions between PCBs, and basal PPAR activity and protein expression. Treatment with PCB77 significantly reduced basal mRNA expression of PPARalpha and the PPAR responsive gene CYP4A1, as well as PPARalpha protein expression. Also, PCB77 exposure caused a significant decrease in basal PPAR-dependent reporter gene expression in MCF-7 cells. Overall, these findings suggest that PPARalpha agonists can reduce PCB77 induction of endothelial cell activation by inhibition of the AHR pathway, and that coplanar PCB induced pro-inflammatory effects could be mediated, in part, by inhibition of PPARalpha expression and function.

2,3,7,8-tetrachlorodibenzo-p-dioxin and epidermal growth factor cooperatively suppress peroxisome proliferator-activated receptor-gamma1 stimulation and restore focal adhesion complexes during adipogenesis: selective contributions of Src, Rho, and Erk distinguish these overlapping processes in C3H10T1/2 cells

Stimulation of PPARgamma1 and adipogenesis in multipotential C3H10T1/2 cells by the combination of dexamethasone and 3-isobutyl-1-methylxanthine (DM) is suppressed by 2,3,7,8 tetrachlorodibenzodioxin (TCDD) (10 nM). This suppression requires sustained activation of extracellular signal-regulated kinase (Erk)1/2. We show that it arises from an effect of TCDD on epidermal growth factor (EGF) signaling. DM initiates an early loss of cell adhesion that is reversed by this TCDD/EGF synergy. Src kinase activity was completely essential for adhesion restoration, sustained Erk activation, and suppression of peroxisome proliferator-activated receptor (PPAR)gamma1. MEK/Erk activity did not contribute, however, to TCDD-induced adhesion. Stimulation of adhesion may therefore precede elevation of Erk. Adhesion is produced by interaction of alphabeta integrins with extracellular matrix proteins and subsequent Src-mediated phosphorylation of focal adhesion kinase (FAK, Tyr576/577) and paxillin (Tyr118). TCDD enhanced the steady state Src-mediated phosphorylation of FAK but not of paxillin. Protein tyrosine phosphatase (PTPase) inhibition by orthovanadate (OVA) showed that this Src activity is highly restricted by PTPases. Partial inhibition of PTPases by OVA mimicked TCDD in producing EGF- and Src-dependent effects on cell adhesion and PPARgamma1 suppression. TCDD may therefore induce a protein that enhances Src effectiveness at adhesion sites. Rho kinase (ROCK) inhibition blocked TCDD/EGF stimulation of clustered focal adhesion complexes without affecting either sustained Erk activation or suppression of PPARgamma1. Thus, this ROCK-mediated clustering of integrin complexes is not needed for the effects of TCDD on Erk and PPARgamma1. A minimal cholesterol depletion with beta-methylcyclodextrin attenuated TCDD effects on PPARgamma1 and Erk activation. TCDD intervention is therefore linked to extracellular proteins. It indicates that TCDD-enhanced stimulation of EGF signaling to Erk may derive from the initial alphabeta integrin complexes.

Microarray analysis of early adipogenesis in C3H10T1/2 cells: cooperative inhibitory effects of growth factors and 2,3,7,8-tetrachlorodibenzo-p-dioxin

C3H10T1/2 mouse embryo fibroblasts differentiate into adipocytes when stimulated by a standard hormonal mixture (IDMB). 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), via the aryl hydrocarbon receptor (AhR), inhibits induction of the key adipogenic gene peroxisome proliferator-activated receptor gamma (PPARgamma) and subsequent adipogenesis. This TCDD-mediated inhibition requires activation of the extracellular signal-regulated kinase (ERK) pathway, which can be accomplished by serum, epidermal growth factor (EGF), or fibroblast growth factor (FGF). In the absence of serum or growth factors, IDMB induced adipogenesis without mitosis. Microarray analysis identified 200 genes that exhibited expression changes of at least twofold after 24 h of IDMB treatment. This time precedes most PPARgamma stimulation but follows the period of TCDD/ERK cooperation and periods of increased cell contraction and DNA synthesis. Functionally related gene clusters include genes associated with cell structure, triglyceride and cholesterol metabolism, oxidative regulation, and secreted proteins. In the absence of growth factors TCDD inhibited 30% of these IDMB responses without inhibiting the process of differentiation. A combination of EGF and TCDD that blocks differentiation cooperatively blocked a further 44 IDMB-responsive genes, most of which have functional links to differentiation, including PPARgamma. Cell cycle regulators that are stimulated by EGF were substantially inhibited by IDMB but these responses were unaffected by TCDD. By contrast, TCDD and EGF cooperatively reversed IDMB-induced changes in cell adhesion complexes immediately prior to increases in PPARgamma1 expression. Changes in adhesion-linked signaling may play a key role in TCDD affects on differentiation.

Differentiation of pluripotent C3H10T1/2 cells rapidly elevates CYP1B1 through a novel process that overcomes a loss of Ah Receptor

Stimulation of C3H10T1/2 cells by an adipogenic hormonal mixture (IDM) consisting of insulin (I), dexamethasone (D), and methylisobutylxanthine (M) substantially induces cytochrome P450 (CYP) 1B1 expression. This stimulation represents up to 40% of the level produced by maximum activation of the arylhydrocarbon receptor (AhR) with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Dexamethasone and methylisobutylxanthine in combination produced near maximum elevation of CYP1B1 along with a subsequent decline in AhR that paralleled the rise in peroxisome proliferator-activated receptorgamma1 (PPARgamma1). Inhibitors of AhR activity, which block TCDD induction, did not affect this increase of CYP1B1 expression, which was, therefore, independent of AhR activity. These responses were unaffected by inhibition of DNA synthesis, which was required for PPARgamma1 induction and terminal differentiation. Induction of CYP1B1 mRNA was paralleled by increased CYP1B1 promoter-luciferase reporter activity. The initial 0.8kb of promoter region, which was sufficient for 24h near maximum stimulation, did not contain either the key AhR-responsive elements that mediate the TCDD response or CREB and SF1 elements that mediate cAMP stimulation of rat CYP1B1 in steroidogenic cells. This reporter response to IDM stimulation, but not to TCDD, was maintained in AhR-null fibroblasts. CYP1B1 expression, unlike TCDD induction, was stimulated by IDM in only about half the cells. CYP1B1 expression partially overlapped with PPARgamma expression, which was also inversely related in clonal sub-lines. CYP1B1 expression may, therefore, represent an early stage of differentiation that requires factors associated with DNA synthesis to subsequently generate PPARgamma1.