Retinoid X Receptor Activation Alters the Chromatin Landscape To Commit Mesenchymal Stem Cells to the Adipose Lineage

Dysregulation of adipogenesis and disrupted lipid metabolism by the antidepressants citalopram and sertraline

Selective Serotonin Reuptake Inhibitors (SSRIs) are widely used medications for the treatment of major depressive disorder. However, long-term SSRI use has been associated with weight gain and altered lipid profiles. These findings suggest that SSRIs may have negative effects on metabolism. Exposure to certain chemicals called 'obesogens' is known to promote lipid accumulation and obesity by modulating adipogenesis. Here, we investigated whether citalopram (CIT) and sertraline (SER) interfere with the process of adipogenesis, using human mesenchymal stem cells (MSCs) in a 2D and a 3D model. Assessment of intracellular lipid accumulation by fluorescence staining was used as a measure for enhanced adipogenesis. To explore possible mechanisms behind SSRIs' effects, receptor mediated activity was studied using responsive cell lines for various nuclear receptors. Furthermore, RNA sequencing was performed in the 3D model, followed by differential gene expression and pathway analysis. A dose dependent increase in lipid accumulation was observed in both models with CIT and SER. For the 3D model, the effect was seen in a range close to reported steady-state plasma concentrations (0.065-0.65 μM for SER and 0.12-0.92 μM for CIT). Pathway analysis revealed unexpected results of downregulation in adipogenesis-related pathways and upregulation in phospholipids and lysosomal pathways. This was confirmed by an observed increase in lysosomes in the 2D model. Our findings suggest lysosomal dysfunction and disrupted lipid metabolism in mature adipocytes, leading to excessive phospholipid synthesis. Moreover, important adipogenic processes are inhibited, potentially leading to dysfunctional adipocytes, which might have implications in the maintenance of a healthy metabolic balance.

Assessing the contribution of plastic-associated obesogenic compounds to cardiometabolic diseases

PURPOSE OF REVIEW

To present recent evidence that strengthens the concept that exogenous pollutants contribute to adipose dysfunction and increased rates of disease and to highlight the ineffective regulation of this risk as industry switches to related but similarly toxic variants.

RECENT FINDINGS

Substitutes for common phthalates and the highly regulated bisphenol A (BPA) show similar deleterious effects on adipocytes. The well tolerated limit for BPA exposure has been reduced in Europe to below the level detected in recent population studies. Additionally, the role for BPA-induced inflammation mediated by interleukin 17a has been described in animal and human studies.

SUMMARY

Despite experimental and associative evidence that supports plastics and plastic associated chemicals deleteriously influencing adipose homeostatasis and contributing to metabolic diseases, structurally related alternate chemicals are being substituted by manufacturers to circumvent trailing regulatory actions.

Epistasis between mutator alleles contributes to germline mutation spectrum variability in laboratory mice

Maintaining germline genome integrity is essential and enormously complex. Although many proteins are involved in DNA replication, proofreading, and repair, mutator alleles have largely eluded detection in mammals. DNA replication and repair proteins often recognize sequence motifs or excise lesions at specific nucleotides. Thus, we might expect that the spectrum of de novo mutations - the frequencies of C>T, A>G, etc. - will differ between genomes that harbor either a mutator or wild-type allele. Previously, we used quantitative trait locus mapping to discover candidate mutator alleles in the DNA repair gene Mutyh that increased the C>A germline mutation rate in a family of inbred mice known as the BXDs (Sasani et al., 2022, Ashbrook et al., 2021). In this study we developed a new method to detect alleles associated with mutation spectrum variation and applied it to mutation data from the BXDs. We discovered an additional C>A mutator locus on chromosome 6 that overlaps Ogg1, a DNA glycosylase involved in the same base-excision repair network as Mutyh (David et al., 2007). Its effect depends on the presence of a mutator allele near Mutyh, and BXDs with mutator alleles at both loci have greater numbers of C>A mutations than those with mutator alleles at either locus alone. Our new methods for analyzing mutation spectra reveal evidence of epistasis between germline mutator alleles and may be applicable to mutation data from humans and other model organisms.

Heritable changes in chromatin contacts linked to transgenerational obesity

Burgeoning evidence demonstrates that effects of environmental exposures can be transmitted to subsequent generations through the germline without DNA mutations1,2. This phenomenon remains controversial because underlying mechanisms have not been identified. Therefore, understanding how effects of environmental exposures are transmitted to unexposed generations without DNA mutations is a fundamental unanswered question in biology. Here, we used an established murine model of male-specific transgenerational obesity to show that exposure to the obesogen tributyltin (TBT) elicited heritable changes in chromatin interactions (CIs) in primordial germ cells (PGCs). New CIs were formed within the Ide gene encoding Insulin Degrading Enzyme in the directly exposed PGCs, then stably maintained in PGCs of the subsequent (unexposed) two generations. Concomitantly, Ide mRNA expression was decreased in livers of male descendants from the exposed dams. These males were hyperinsulinemic and hyperglycemic, phenocopying Ide-deficient mice that are predisposed to adult-onset, diet-induced obesity. Creation of new CIs in PGCs, suppression of hepatic Ide mRNA, increased fat mass, hyperinsulinemia and hyperglycemia were male-specific. Our results provide a plausible molecular mechanism underlying transmission of the transgenerational predisposition to obesity caused by gestational exposure to an environmental obesogen. They also provide an entry point for future studies aimed at understanding how environmental exposures alter chromatin structure to influence physiology across multiple generations in mammals.

Organotin mixtures reveal interactions that modulate adipogenic differentiation in 3T3-L1 preadipocytes

Organotin chemicals (butyltins and phenyltins) are the most widely used organometallic chemicals worldwide and are used in industrial applications, such as biocides and anti-fouling paints. Tributyltin (TBT) and more recently, dibutyltin (DBT) and triphenyltin (TPT) have been reported to stimulate adipogenic differentiation. Although these chemicals co-exist in the environment, their effect in combination remains unknown. We first investigated the adipogenic effect of eight organotin chemicals (monobutyltin (MBT), DBT, TBT, tetrabutyltin (TeBT), monophenyltin (MPT), diphenyltin (DPT), TPT, and tin chloride (SnCl4)) in the 3T3-L1 preadipocyte cell line in single exposures at two doses (10 and 50 ng/ml). Only three out of the eight organotins induced adipogenic differentiation with TBT eliciting the strongest adipogenic differentiation (in a dose-dependent manner) followed by TPT and DBT, as demonstrated by lipid accumulation and gene expression. We then hypothesized that, in combination (TBT, DBT, and TPT), adipogenic effects will be exacerbated compared to single exposures. However, at the higher dose (50 ng/ml), TBT-induced differentiation was reduced by TPT and DBT when in dual or triple combination. We tested whether TPT or DBT would interfere with adipogenic differentiation stimulated by a peroxisome proliferator-activated receptor (PPARγ) agonist (rosiglitazone) or a glucocorticoid receptor agonist (dexamethasone). Both DBT50 and TPT50 reduced rosiglitazone-, but not dexamethasone-stimulated adipogenic differentiation. In conclusion, DBT and TPT interfere with TBT's adipogenic differentiation possibly via PPARγ signaling. These findings highlight the antagonistic effects among organotins and the need to understand the effects and mechanism of action of complex organotin mixtures on adipogenic outcomes.

Reciprocal Effect of Environmental Stimuli to Regulate the Adipogenesis and Osteogenesis Fate Decision in Bone Marrow-Derived Mesenchymal Stem Cells (BM-MSCs)

Mesenchymal stem cells derived from bone marrow (BM-MSCs) can differentiate into adipocytes and osteoblasts. Various external stimuli, including environmental contaminants, heavy metals, dietary, and physical factors, are shown to influence the fate decision of BM-MSCs toward adipogenesis or osteogenesis. The balance of osteogenesis and adipogenesis is critical for the maintenance of bone homeostasis, and the interruption of BM-MSCs lineage commitment is associated with human health issues, such as fracture, osteoporosis, osteopenia, and osteonecrosis. This review focuses on how external stimuli shift the fate of BM-MSCs towards adipogenesis or osteogenesis. Future studies are needed to understand the impact of these external stimuli on bone health and elucidate the underlying mechanisms of BM-MSCs differentiation. This knowledge will inform efforts to prevent bone-related diseases and develop therapeutic approaches to treat bone disorders associated with various pathological conditions.

miR-223 Plays a Key Role in Obesogen-Enhanced Adipogenesis in Mesenchymal Stem Cells and in Transgenerational Obesity

Exposure of pregnant F0 mouse dams to the obesogen tributyltin (TBT) predisposes unexposed male descendants to obesity and diverts mesenchymal stem cells (MSCs) toward the adipocytic lineage. TBT promotes adipogenic commitment and differentiation of MSCs in vitro. To identify TBT-induced factors predisposing MSCs toward the adipocytic fate, we exposed mouse MSCs to TBT, the peroxisome proliferator activated receptor gamma (PPARγ)-selective agonist rosiglitazone, or the retinoid X receptor (RXR)-selective agonist LG-100268. Then we determined their transcriptomal profiles to determine candidate microRNAs (miR) regulating adipogenic commitment and differentiation. Of the top 10 candidate microRNAs predicted by Ingenuity Pathway Analysis, miR-21, miR-33, and miR-223 were expressed consistent with an ability to differentially regulate target genes during adipogenesis. We found that 24-hour exposure to 50nM TBT caused miR-223 levels in MSCs to increase; expression of its target genes ZEB1, NFIB, and FOXP1 was decreased. Rosiglitazone and TBT increased miR-223 levels. This induction was inhibited by the PPARγ antagonist T0070907 but not by the RXR antagonists HX531 or UVI3003, placing miR-223 downstream of PPARγ. Chromatin immunoprecipitation confirmed TBT-induced binding of PPARγ to regulatory elements in the miR-223 promoter. miR-223 levels were elevated in white adipose tissue of F2 and F3 male descendants of pregnant F0 mouse dams exposed to 50nM TBT throughout gestation. miR-223 levels were potentiated in males fed an increased fat diet. We infer that TBT induced miR-223 expression and increased adipogenesis in MSCs through the PPARγ pathway and that transgenerationally increased expression of miR-223 plays an important role in the development of obesity caused by TBT exposure.

Regulation of adipogenesis through retinoid X receptor and/or peroxisome proliferator-activated receptor by designed lignans based on natural products in 3T3-L1 cells

We previously synthesized two retinoid X receptor (RXR) agonists, 4'-hydroxy-3'-propyl-[1,1'-biphenyl]-3-propanoic acid ethyl ester (4'OHE) and 6-hydroxy-3'-propyl-[1,1'-biphenyl]-3-propanoic acid ethyl ester (6OHE), based on the structure of magnaldehyde B, a natural product obtained from Magnolia obovata. 4'OHE and 6OHE exhibited different selectivities for peroxisome proliferator-activated receptor (PPAR)/RXR heterodimers. To examine the regulatory effects of these compounds in adipogenesis, 3T3-L1 mouse preadipocytes were treated with a differentiation cocktail with or without test compounds to induce differentiation, and subsequently treated with test compounds in insulin-containing medium every alternate day. Lipid droplets were stained with Oil Red O to examine lipid accumulation. In addition, adipogenesis-related gene expression was measured using RT-qPCR and immunoblotting. The results showed that a PPARγ agonist, 4'OHE, which exerts agonistic effects on PPARγ and RXRα, enhanced adipogenesis similar to rosiglitazone. However, unlike GW501516, a PPARδ agonist, 6OHE and its hydrolysis product (6OHA), which exert agonistic effects on PPARδ and RXRα, suppressed adipogenesis. In a manner similar to 6OHE and 6OHA, bexarotene, an RXR agonist, suppressed adipocyte differentiation, and its anti-adipogenic effect was reversed by an RXR antagonist. Furthermore, 6OHA and bexarotene inhibited the increase in Pparγ2 and Cebpa mRNA levels 2 days after the induction of differentiation. We demonstrated the adipogenic effect of 4'OHE and anti-adipogenic effects of 6OHE and 6OHA in 3T3-L1 cells. Previously, RXR agonists have been reported to positively regulate the differentiation of mesenchymal stem cells into adipocytes, but our current data showed that they inhibited the differentiation of preadipocytes, at least 3T3-L1 cells, into adipocytes.

Cetyl Alcohol Polyethoxylates Disrupt Metabolic Health in Developmentally Exposed Zebrafish

Alcohol polyethoxylates (AEOs), such as cetyl alcohol ethoxylates (CetAEOs), are high-production-volume surfactants used in laundry detergents, hard-surface cleaners, pesticide formulations, textile production, oils, paints, and other products. AEOs have been suggested as lower toxicity replacements for alkylphenol polyethoxylates (APEOs), such as the nonylphenol and octylphenol polyethoxylates. We previously demonstrated that nonylphenol polyethoxylates induced triglyceride accumulation in several in vitro adipogenesis models and promoted adiposity and increased body weights in developmentally exposed zebrafish. We also demonstrated that diverse APEOs and AEOs were able to increase triglyceride accumulation and/or pre-adipocyte proliferation in a murine pre-adipocyte model. As such, the goals of this study were to assess the potential of CetAEOs to promote adiposity and alter growth and/or development (toxicity, length, weight, behavior, energy expenditure) of developmentally exposed zebrafish (Danio rerio). We also sought to expand our understanding of ethoxylate chain-length dependent effects through interrogation of varying chain-length CetAEOs. We demonstrated consistent adipogenic effects in two separate human bone-marrow-derived mesenchymal stem cell models as well as murine pre-adipocytes. Immediately following chemical exposures in zebrafish, we reported disrupted neurodevelopment and aberrant behavior in light/dark activity testing, with medium chain-length CetAEO-exposed fish exhibiting hyperactivity across both light and dark phases. By day 30, we demonstrated that cetyl alcohol and CetAEOs disrupted adipose deposition in developmentally exposed zebrafish, despite no apparent impacts on standard length or gross body weight. This research suggests metabolic health concerns for these common environmental contaminants, suggesting further need to assess molecular mechanisms and better characterize environmental concentrations for human health risk assessments.

2,4-Di-tert-butylphenol Induces Adipogenesis in Human Mesenchymal Stem Cells by Activating Retinoid X Receptors

2,4-Di-tert-butylphenol (2,4-DTBP) is an important commercial antioxidant and a toxic natural secondary metabolite that has been detected in humans. However, there is scant information regarding its toxicological effects. We asked whether 2,4-DTBP is a potential obesogen. Using a human mesenchymal stem cell adipogenesis assay, we found that exposure to 2,4-DTBP led to increased lipid accumulation and expression of adipogenic marker genes. Antagonist assays revealed that 2,4-DTBP increased lipid accumulation by activating the peroxisome proliferator-activated receptor (PPAR) γ-retinoid X receptor (RXR) heterodimer. 2,4-DTBP likely activated the PPARγ/RXRα heterodimer by activating RXRα but not directly binding to PPARγ. We confirmed that 2,4-DTBP directly bound to RXRα by solving the crystal structure of this complex, then predicted and demonstrated that related compounds could also activate RXRα. Our study demonstrated that 2,4-DTBP and related chemicals could act as obesogens and endocrine disruptors via RXRs. These data showed that 2,4-DTBP belongs to a family of compounds whose endocrine-disrupting and obesogenic effects can be strongly modulated by their chemical composition. Structure-activity studies such as the present one could help guide the rational development of safer antioxidants that do not interact with important nuclear receptors having broad effects on human development and physiology.

Co-expression analysis of lncRNA and mRNA identifies potential adipogenesis regulatory non-coding RNAs involved in the transgenerational effects of tributyltin

The obesity epidemic is considered a global public health crisis, with an increase in caloric intake, sedentary lifestyles and/or genetic predispositions as contributing factors. Although the positive energy balance is one of the most significant causes of obesity, recent research has linked early exposure to Endocrine-Disrupting Chemicals (EDCs) such as the obesogen tributyltin (TBT) to the disease epidemic. In addition to their actions on the hormonal profile, EDCs can induce long-term changes in gene expression, possibly due to changes in epigenetic patterns. Long non-coding RNAs (lncRNAs) are epigenetic mediators that play important regulatory roles in several biological processes, through regulation of gene transcription and/or translation. In this study, we explored the differential expression of lncRNAs in gonadal white adipose tissue samples from adult male C57BL/6J F4 generation, female C57BL/6J offspring exposed (F0 generation) to 50 nM TBT or 0.1% DMSO (control of vehicle) via drinking water provided during pregnancy and lactation, analyzing RNA-seq data from a publicly available dataset (GSE105051). A total of 74 lncRNAs were differentially expressed (DE), 22 were up-regulated and 52 were down-regulated in the group whose F4 ancestor was exposed in utero to 50nM TBT when compared to those exposed to 0.1% DMSO (control). Regulation of DE lncRNAs and their potential partner genes in gonadal white adipose tissue of mice ancestrally exposed to EDC TBT may be related to the control of adipogenesis, as pathway enrichment analyses showed that these gene partners are mainly involved in the metabolism of lipids and glucose and in insulin-related pathways, which are essential for obesity onset and control.

Retinoid-X receptor agonists increase thyroid hormone competence in lower jaw remodeling of pre-metamorphic Xenopus laevis tadpoles

Thyroid hormone (TH) signaling plays critical roles during vertebrate development, including regulation of skeletal and cartilage growth. TH acts through its receptors (TRs), nuclear hormone receptors (NRs) that heterodimerize with Retinoid-X receptors (RXRs), to regulate gene expression. A defining difference between NR signaling during development compared to in adult tissues, is competence, the ability of the organism to respond to an endocrine signal. Amphibian metamorphosis, especially in Xenopus laevis, the African clawed frog, is a well-established in vivo model for studying the mechanisms of TH action during development. Previously, we’ve used one-week post-fertilization X. laevis tadpoles, which are only partially competent to TH, to show that in the tail, which is naturally refractive to exogenous T3 at this stage, RXR agonists increase TH competence, and that RXR antagonism inhibits the TH response. Here, we focused on the jaw that undergoes dramatic TH-mediated remodeling during metamorphosis in order to support new feeding and breathing styles. We used a battery of approaches in one-week-old tadpoles, including quantitative morphology, differential gene expression and whole mount cell proliferation assays, to show that both pharmacologic (bexarotene) and environmental (tributyltin) RXR agonists potentiated TH-induced responses but were inactive in the absence of TH; and the RXR antagonist UVI 3003 inhibited TH action. Bex and TBT significantly potentiated cellular proliferation and the TH induction of runx2, a transcription factor critical for developing cartilage and bone. Prominent targets of RXR-mediated TH potentiation were members of the matrix metalloprotease family, suggesting that RXR potentiation may emphasize pathways responsible for rapid changes during development.

Obesity I: Overview and molecular and biochemical mechanisms

Obesity is a chronic, relapsing condition characterized by excess body fat. Its prevalence has increased globally since the 1970s, and the number of obese and overweight people is now greater than those underweight. Obesity is a multifactorial condition, and as such, many components contribute to its development and pathogenesis. This is the first of three companion reviews that consider obesity. This review focuses on the genetics, viruses, insulin resistance, inflammation, gut microbiome, and circadian rhythms that promote obesity, along with hormones, growth factors, and organs and tissues that control its development. It shows that the regulation of energy balance (intake vs. expenditure) relies on the interplay of a variety of hormones from adipose tissue, gastrointestinal tract, pancreas, liver, and brain. It details how integrating central neurotransmitters and peripheral metabolic signals (e.g., leptin, insulin, ghrelin, peptide YY_3-36) is essential for controlling energy homeostasis and feeding behavior. It describes the distinct types of adipocytes and how fat cell development is controlled by hormones and growth factors acting via a variety of receptors, including peroxisome proliferator-activated receptor-gamma, retinoid X, insulin, estrogen, androgen, glucocorticoid, thyroid hormone, liver X, constitutive androstane, pregnane X, farnesoid, and aryl hydrocarbon receptors. Finally, it demonstrates that obesity likely has origins in utero. Understanding these biochemical drivers of adiposity and metabolic dysfunction throughout the life cycle lends plausibility and credence to the "obesogen hypothesis" (i.e., the importance of environmental chemicals that disrupt these receptors to promote adiposity or alter metabolism), elucidated more fully in the two companion reviews.

Obesity III: Obesogen assays: Limitations, strengths, and new directions

There is increasing evidence of a role for environmental contaminants in disrupting metabolic health in both humans and animals. Despite a growing need for well-understood models for evaluating adipogenic and potential obesogenic contaminants, there has been a reliance on decades-old in vitro models that have not been appropriately managed by cell line providers. There has been a quick rise in available in vitro models in the last ten years, including commercial availability of human mesenchymal stem cell and preadipocyte models; these models require more comprehensive validation but demonstrate real promise in improved translation to human metabolic health. There is also progress in developing three-dimensional and co-culture techniques that allow for the interrogation of a more physiologically relevant state. While diverse rodent models exist for evaluating putative obesogenic and/or adipogenic chemicals in a physiologically relevant context, increasing capabilities have been identified for alternative model organisms such as Drosophila, C. elegans, zebrafish, and medaka in metabolic health testing. These models have several appreciable advantages, including most notably their size, rapid development, large brood sizes, and ease of high-resolution lipid accumulation imaging throughout the organisms. They are anticipated to expand the capabilities of metabolic health research, particularly when coupled with emerging obesogen evaluation techniques as described herein.

Polybrominated diphenyl ether congener 99 (PBDE 99) promotes adipocyte lineage commitment of C3H10T1/2 mesenchymal stem cells

Obesogens are defined as chemicals that trigger obesity partially by stimulating adipogenesis. Adipogenesis consists of two successive processes: the adipocyte lineage commitment of pluripotent stem cells and the differentiation of preadipocytes. Compared with the differentiation of preadipocytes, the effects of most environmental obesogens on adipocyte lineage commitment remain largely unknown. In this study, investigations are performed to explore the influences of PBDE 99 on the adipocyte lineage commitment based on C3H10T1/2, which has been widely used as a mesenchymal stem cell (MSC) model. Our results indicated that exposure to PBDE 99 during commitment stage resulted in significant up-regulation of subsequent adipogenesis in C3H10T1/2 MSCs. Interestingly, PBDE 99 did not affect the osteogenesis of C3H10T1/2 MSCs, although the adipogenesis and osteogenesis of MSCs are typically reciprocal. PBDE 99 was further demonstrated to significantly decrease the expression of Pref1, the marker of very early adipose mesenchymal precursor, and its downstream effector, Sox9. This result strongly suggested that PBDE 99 facilitated adipocyte commitment to exert adipogenic effect on C3H10T1/2 MSCs. Mechanistic studies revealed that PBDE 99 efficiently inhibited Hedgehog signaling transduction, a conserved negative regulator of the adipocyte lineage commitment. Furthermore, the effects of PBDE 99 on adipogenesis were abrogated by the co-treatment with SAG, a specific Hedgehog signaling activator, suggesting inhibition of Hedgehog signaling is responsible for the effect of PBDE 99 on adipocyte commitment. Taking together, these results strongly suggested enhanced adipocyte lineage commitment was involved in potential obesogenic effect of PBDE 99, presumably through repressing Hedgehog signalling during commitment stage. Moreover, the results of this study indicated that C3H10T1/2 can be used as a feasible MSCs cell model to evaluate the capabilities of potential obesogens on adipocyte commitment.

Epigenetic Transgenerational Inheritance of the Effects of Obesogen Exposure

Obesity and metabolic disorders have become a worldwide pandemic affecting millions of people. Although obesity is a multifaceted disease, there is growing evidence supporting the obesogen hypothesis, which proposes that exposure to a subset of endocrine disrupting chemicals (EDCs), known as obesogens, promotes obesity. While these effects can be observed in vitro using cell models, in vivo and human epidemiological studies have strengthened this hypothesis. Evidence from animal models showed that the effects of obesogen exposure can be inherited transgenerationally through at least the F4 generation. Transgenerational effects of EDC exposure predispose future generations to undesirable phenotypic traits and diseases, including obesity and related metabolic disorders. The exact mechanisms through which phenotypic traits are passed from an exposed organism to their offspring, without altering the primary DNA sequence, remain largely unknown. Recent research has provided strong evidence suggesting that a variety of epigenetic mechanisms may underlie transgenerational inheritance. These include differential DNA methylation, histone methylation, histone retention, the expression and/or deposition of non-coding RNAs and large-scale alterations in chromatin structure and organization. This review highlights the most recent advances in the field of epigenetics with respect to the transgenerational effects of environmental obesogens. We highlight throughout the paper the strengths and weaknesses of the evidence for proposed mechanisms underlying transgenerational inheritance and why none of these is sufficient to fully explain the phenomenon. We propose that changes in higher order chromatin organization and structure may be a plausible explanation for how some disease predispositions are heritable through multiple generations, including those that were not exposed. A solid understanding of these possible mechanisms is essential to fully understanding how environmental exposures can lead to inherited susceptibility to diseases such as obesity.

PFAS and Potential Adverse Effects on Bone and Adipose Tissue Through Interactions With PPARγ

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are a widely dispersed, broad class of synthetic chemicals with diverse biological effects, including effects on adipose and bone differentiation. PFAS most commonly occur as mixtures and only rarely, if ever, as single environmental contaminants. This poses significant regulatory questions and a pronounced need for chemical risk assessments, analytical methods, and technological solutions to reduce the risk to public and environmental health. The effects of PFAS on biological systems may be complex. Each may have several molecular targets initiating multiple biochemical events leading to a number of different adverse outcomes. An exposure to mixtures or coexposures of PFAS complicates the picture further. This review illustrates how PFAS target peroxisome proliferator-activated receptors. Additionally, we describe how such activation leads to changes in cell differentiation and bone development that contributes to metabolic disorder and bone weakness. This discussion sheds light on the importance of seemingly modest outcomes observed in test animals and highlights why the most sensitive end points identified in some chemical risk assessments are significant from a public health perspective.

Transgenerational metabolomic fingerprints in mice ancestrally exposed to the obesogen TBT

BACKGROUND

Endocrine disrupting chemicals (EDCs) contribute to the etiology of metabolic disorders such as obesity, insulin resistance and hepatic dysfunction. Concern is growing about the consequences of perinatal EDC exposure on disease predisposition later in life. Metabolomics are promising approaches for studying long-term consequences of early life EDC exposure. These approaches allow for the identification and characterization of biomarkers of direct or ancestral exposures that could be diagnostic for individual susceptibility to disease and help to understand mechanisms through which EDCs act.

OBJECTIVES

We sought to identify metabolomic fingerprints in mice ancestrally exposed to the model obesogen tributyltin (TBT), to assess whether metabolomics could discriminate potential trans-generational susceptibility to obesity and recognize metabolic pathways modulated by ancestral TBT exposure.

METHODS

We used non-targeted ¹H NMR metabolomic analyses of plasma and liver samples collected from male and female mice ancestrally exposed to TBT in two independent transgenerational experiments in which F3 and F4 males became obese when challenged with increased dietary fat.

RESULTS

Metabolomics confirmed transgenerational obesogenic effects of environmentally relevant doses of TBT in F3 and F4 males, in two independent studies. Although females never became obese, their specific metabolomic fingerprint evidenced distinct transgenerational effects of TBT in female mice consistent with impaired capacity for liver biotransformation.

DISCUSSION

This study is the first application of metabolomics to unveil the transgenerational effects of EDC exposure. Very early, significant changes in the plasma metabolome were observed in animals ancestrally exposed to TBT. These changes preceded the onset of obesogenic effects elicited by increased dietary fat in the TBT groups, and which ultimately resulted in significant changes in the liver metabolome. Development of metabolomic fingerprints could facilitate the identification of individuals carrying the signature of ancestral obesogen exposure that might increase their susceptibility to other risk factor such as increased dietary fat.

The Impact of Endocrine-Disrupting Chemicals in Male Fertility: Focus on the Action of Obesogens

The current scenario of male infertility is not yet fully elucidated; however, there is increasing evidence that it is associated with the widespread exposure to endocrine-disrupting chemicals (EDCs), and in particular to obesogens. These compounds interfere with hormones involved in the regulation of metabolism and are associated with weight gain, being also able to change the functioning of the male reproductive axis and, consequently, the testicular physiology and metabolism that are pivotal for spermatogenesis. The disruption of these tightly regulated metabolic pathways leads to adverse reproductive outcomes. The permanent exposure to obesogens has raised serious health concerns. Evidence suggests that obesogens are one of the leading causes of the marked decline of male fertility and key players in shaping the future health outcomes not only for those who are directly exposed but also for upcoming generations. In addition to the changes that lead to inefficient functioning of the male gametes, obesogens induce alterations that are “imprinted” on the genes of the male gametes, establishing a link between generations and contributing to the transmission of defects. Unveiling the molecular mechanisms by which obesogens induce toxicity that may end-up in epigenetic modifications is imperative. This review describes and discusses the suggested molecular targets and potential mechanisms for obesogenic–disrupting chemicals and the subsequent effects on male reproductive health.

The new kids on the block: Emerging obesogens

The current obesity epidemic is calling for action in the determination of contributing factors. Although social and life-style factors have been traditionally associated with metabolic disruption, a subset of endocrine-disrupting chemicals (EDCs), called obesogens are garnering increasing attention for their ability to promote adipose tissue differentiation and accumulation. For some chemicals, such as tributyltin, there is conclusive evidence regarding their ability to promote adipogenesis and their mechanism of action. In recent years, the list of chemicals that exert obesogenic potential is increasing. In this chapter, we review current knowledge of the most recent developments in the field of emerging obesogens with a specific focus on food additives, surfactants, and sunscreens, for which the mechanism of action remains unclear. We also review new evidence relative to the obesogenic potential of environmentally relevant chemical mixtures and point to potential therapeutic approaches to minimize the detrimental effects of obesogens. We conclude by discussing the available tools to investigate new obesogenic chemicals, strategies to maximize reproducibility in adipogenic studies, and future directions that will help propel the field forward.

The endocrine disruptor cadmium: a new player in the pathophysiology of metabolic diseases

Cadmium (Cd), a highly toxic heavy metal, is found in soil, environment and contaminated water and food. Moreover, Cd is used in various industrial activities, such as electroplating, batteries production, fertilizers, while an important non-occupational source is represented by cigarette smoking, as Cd deposits in tobacco leaves. Since many years it is clear a strong correlation between Cd body accumulation and incidence of many diseases. Indeed, acute exposure to Cd can cause inflammation and affect many organs such as kidneys and liver. Furthermore, the attention has focused on its activity as environmental pollutant and endocrine disruptor able to interfere with metabolic and energy balance of living beings. Both in vitro and in vivo experiments have demonstrated that the Cd-exposure is related to metabolic diseases such as obesity, diabetes and osteoporosis even if human studies are still controversial. Recent data show that Cd-exposure is associated with atherosclerosis, hypertension and endothelial damage that are responsible for cardiovascular diseases. Due to the large environmental diffusion of Cd, in this review, we summarize the current knowledge concerning the role of Cd in the incidence of metabolic and cardiovascular diseases.

TET1 promotes RXRα expression and adipogenesis through DNA demethylation

Adipose tissue is important for systemic metabolic homeostasis in response to environmental changes, and adipogenesis involves dynamic transcriptional regulation. Ten-eleven translocation (TET) enzymes (TET1, 2 and 3) oxidize the 5-methylcytosine (5mC) in DNA to 5-hydroxylmethylcytosine (5hmC), which associates with transcriptional activation. Step by step, 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC) are further generated by TETs and the cytosine can be restored through base-excision repair. It is still unclear how DNA demethylation is involved in adipogenesis. Through a phenotypic screen, we found TET inhibition decreased adipocyte differentiation from mesenchymal stem cells (MSCs). Comparing with the undifferentiated MSCs, the differentiated adipocytes exhibited much higher levels of 5hmC and slightly increased 5fC and 5caC. Higher 5hmC was associated with better differentiation at single-cell level by image analysis. TET1 is upregulated in differentiation and depletion of it significantly impaired the gain of 5hmC. Furthermore, Tet1 depletion significantly hampered the adipocyte differentiation. Using RNA-seq, 5mC and 5hmC-DNA immunoprecipitation, we found that Tet1 knockout led to lower expression of genes associated with lipid metabolism and fat cell differentiation. Genes with loss of 5mC or gain of 5hmC in adipocytes include Lipe, Bmp4 and Rxra, etc. RXRα agonist partially rescued the inhibitory effect of Tet1 knockout for adipogenesis. So, Rxra is one of the critical TET1 modulated genes. Together, TET1-mediated active DNA demethylation plays an important role in adipogenesis.

Epigenetic Regulation of Adipogenesis in Development of Metabolic Syndrome

Obesity is one of the biggest public health concerns identified by an increase in adipose tissue mass as a result of adipocyte hypertrophy and hyperplasia. Pertaining to the importance of adipose tissue in various biological processes, any alteration in its function results in impaired metabolic health. In this review, we discuss how adipose tissue maintains the metabolic health through secretion of various adipokines and inflammatory mediators and how its dysfunction leads to the development of severe metabolic disorders and influences cancer progression. Impairment in the adipocyte function occurs due to individuals' genetics and/or environmental factor(s) that largely affect the epigenetic profile leading to altered gene expression and onset of obesity in adults. Moreover, several crucial aspects of adipose biology, including the regulation of different transcription factors, are controlled by epigenetic events. Therefore, understanding the intricacies of adipogenesis is crucial for recognizing its relevance in underlying disease conditions and identifying the therapeutic interventions for obesity and metabolic syndrome.

Reiterative infusions of MSCs improve pediatric osteogenesis imperfecta eliciting a pro-osteogenic paracrine response: TERCELOI clinical trial

BACKGROUND

Osteogenesis imperfecta (OI) is a rare genetic disease characterized by bone fragility, with a wide range in the severity of clinical manifestations. The majority of cases are due to mutations in the COL1A1 or COL1A2 genes, which encode type I collagen. Mesenchymal stem cells (MSCs), as the progenitors of the osteoblasts, the main type I collagen secreting cell type in the bone, have been proposed and tested as an innovative therapy for OI with promising but transient outcomes.

METHODS

To overcome the short-term effect of MSCs therapy, we performed a phase I clinical trial based on reiterative infusions of histocompatible MSCs, administered in a 2.5-year period, in two pediatric patients affected by severe and moderate OI. The aim of this study was to assess the safety and effectiveness of this cell therapy in nonimmunosuppressed OI patients. The host response to MSCs was studied by analyzing the sera from OI patients, collected before, during, and after the cell therapy.

RESULTS

We first demonstrated that the sequential administration of MSCs was safe and improved the bone parameters and quality of life of OI patients along the cell treatment plus 2-year follow-up period. Moreover, the study of the mechanism of action indicated that MSCs therapy elicited a pro-osteogenic paracrine response in patients, especially noticeable in the patient affected by severe OI.

CONCLUSIONS

Our results demonstrate the feasibility and potential of reiterative MSCs infusion for two pediatric OI and highlight the paracrine response shown by patients as a consequence of MSCs treatment.

Obesogens: How They Are Identified and Molecular Mechanisms Underlying Their Action

Adult and childhood obesity have reached pandemic level proportions. The idea that caloric excess and insufficient levels of physical activity leads to obesity is a commonly accepted answer for unwanted weight gain. This paradigm offers an inconclusive explanation as the world continually moves towards an unhealthier and heavier existence irrespective of energy balance. Endocrine disrupting chemicals (EDCs) are chemicals that resemble natural hormones and disrupt endocrine function by interfering with the body's endogenous hormones. A subset of EDCs called obesogens have been found to cause metabolic disruptions such as increased fat storage, in vivo. Obesogens act on the metabolic system through multiple avenues and have been found to affect the homeostasis of a variety of systems such as the gut microbiome and adipose tissue functioning. Obesogenic compounds have been shown to cause metabolic disturbances later in life that can even pass into multiple future generations, post exposure. The rising rates of obesity and related metabolic disease are demanding increasing attention on chemical screening efforts and worldwide preventative strategies to keep the public and future generations safe. This review addresses the most current findings on known obesogens and their effects on the metabolic system, the mechanisms of action through which they act upon, and the screening efforts through which they were identified with. The interplay between obesogens, brown adipose tissue, and the gut microbiome are major topics that will be covered.

Adipose Tissue and Endocrine-Disrupting Chemicals: Does Sex Matter?

Obesity and metabolic-related diseases, among which diabetes, are prominent public health challenges of the 21st century. It is now well acknowledged that pollutants are a part of the equation, especially endocrine-disrupting chemicals (EDCs) that interfere with the hormonal aspect. The aim of the review is to focus on adipose tissue, a central regulator of energy balance and metabolic homeostasis, and to highlight the significant differences in the endocrine and metabolic aspects of adipose tissue between males and females which likely underlie the differences of the response to exposure to EDCs between the sexes. Moreover, the study also presents an overview of several mechanisms of action by which pollutants could cause adipose tissue dysfunction. Indeed, a better understanding of the mechanism by which environmental chemicals target adipose tissue and cause metabolic disturbances, and how these mechanisms interact and sex specificities are essential for developing mitigating and sex-specific strategies against metabolic diseases of chemical origin. In particular, considering that a scenario without pollutant exposure is not a realistic option in our current societies, attenuating the deleterious effects of exposure to pollutants by acting on the gut-adipose tissue axis may constitute a new direction of research.

Differential susceptibility to endocrine disruptor-induced epimutagenesis

There is now considerable evidence indicating the potential for endocrine disrupting chemicals to alter the epigenome and for subsets of these epigenomic changes or "epimutations" to be heritably transmitted to offspring in subsequent generations. While there have been many studies indicating how exposure to endocrine disrupting chemicals can disrupt various organs associated with the body's endocrine systems, there is relatively limited information regarding the relative susceptibility of different specific organs, tissues, or cell types to endocrine disrupting chemical-induced epimutagenesis. Here we review available information about different organs, tissues, cell types, and/or cell lines which have been shown to be susceptible to specific endocrine disrupting chemical-induced epimutations. In addition, we discuss possible mechanisms that may be involved, or impacted by this tissue- or cell type-specific, differential susceptibility to different endocrine disrupting chemicals. Finally, we summarize available information indicating that certain periods of development display elevated susceptibility to endocrine disrupting chemical exposure and we describe how this may affect the extent to which germline epimutations can be transmitted inter- or transgenerationally. We conclude that cell type-specific differential susceptibility to endocrine disrupting chemical-induced epimutagenesis is likely to directly impact the extent to, or manner in, which endocrine disrupting chemical exposure initially induces epigenetic changes to DNA methylation and/or histone modifications, and how these endocrine disrupting chemical-induced epimutations can then subsequently impact gene expression, potentially leading to the development of heritable disease states.

Control of Adipose Cell Browning and Its Therapeutic Potential

Adipose tissue is the largest endocrine organ in humans and has an important influence on many physiological processes throughout life. An increasing number of studies have described the different phenotypic characteristics of fat cells in adults. Perhaps one of the most important properties of fat cells is their ability to adapt to different environmental and nutritional conditions. Hypothalamic neural circuits receive peripheral signals from temperature, physical activity or nutrients and stimulate the metabolism of white fat cells. During this process, changes in lipid inclusion occur, and the number of mitochondria increases, giving these cells functional properties similar to those of brown fat cells. Recently, beige fat cells have been studied for their potential role in the regulation of obesity and insulin resistance. In this context, it is important to understand the embryonic origin of beige adipocytes, the response of adipocyte to environmental changes or modifications within the body and their ability to transdifferentiate to elucidate the roles of these cells for their potential use in therapeutic strategies for obesity and metabolic diseases. In this review, we discuss the origins of the different fat cells and the possible therapeutic properties of beige fat cells.

Agrochemicals and obesity

Obesity has become a very large concern worldwide, reaching pandemic proportions over the past several decades. Lifestyle factors, such as excess caloric intake and decreased physical activity, together with genetic predispositions, are well-known factors related to obesity. There is accumulating evidence suggesting that exposure to some environmental chemicals during critical windows of development may contribute to the rapid increase in the incidence of obesity. Agrochemicals are a class of chemicals extensively used in agriculture, which have been widely detected in human. There is now considerable evidence linking human exposure to agrochemicals with obesity. This review summarizes human epidemiological evidence and experimental animal studies supporting the association between agrochemical exposure and obesity and outlines possible mechanistic underpinnings for this link.

The GOLIATH Project: Towards an Internationally Harmonised Approach for Testing Metabolism Disrupting Compounds

The purpose of this project report is to introduce the European "GOLIATH" project, a new research project which addresses one of the most urgent regulatory needs in the testing of endocrine-disrupting chemicals (EDCs), namely the lack of methods for testing EDCs that disrupt metabolism and metabolic functions. These chemicals collectively referred to as "metabolism disrupting compounds" (MDCs) are natural and anthropogenic chemicals that can promote metabolic changes that can ultimately result in obesity, diabetes, and/or fatty liver in humans. This project report introduces the main approaches of the project and provides a focused review of the evidence of metabolic disruption for selected EDCs. GOLIATH will generate the world's first integrated approach to testing and assessment (IATA) specifically tailored to MDCs. GOLIATH will focus on the main cellular targets of metabolic disruption-hepatocytes, pancreatic endocrine cells, myocytes and adipocytes-and using an adverse outcome pathway (AOP) framework will provide key information on MDC-related mode of action by incorporating multi-omic analyses and translating results from in silico, in vitro, and in vivo models and assays to adverse metabolic health outcomes in humans at real-life exposures. Given the importance of international acceptance of the developed test methods for regulatory use, GOLIATH will link with ongoing initiatives of the Organisation for Economic Development (OECD) for test method (pre-)validation, IATA, and AOP development.

Mechanisms of action, chemical characteristics, and model systems of obesogens

There is increasing evidence for the role of environmental endocrine disrupting contaminants, coined obesogens, in exacerbating the rising obesity epidemic. Obesogens can be found in everyday items ranging from pesticides to food packaging. Although research shows that obesogens can have effects on adipocyte size, phenotype, metabolic activity, and hormone levels, much remains unknown about these chemicals. This review will discuss what is currently known about the mechanisms of obesogens, including expression of the PPARs, hormone interference, and inflammation. Strategies for identifying obesogenic chemicals and their mechanisms through chemical characteristics and model systems will also be discussed. Ultimately, research should focus on improving models to discern precise mechanisms of obesogenic action and to test therapeutics targeting these mechanisms.

Environmental Obesogens and Their Impact on Susceptibility to Obesity: New Mechanisms and Chemicals

The incidence of obesity has reached an all-time high, and this increase is observed worldwide. There is a growing need to understand all the factors that contribute to obesity to effectively treat and prevent it and associated comorbidities. The obesogen hypothesis proposes that there are chemicals in our environment termed obesogens that can affect individual susceptibility to obesity and thus help explain the recent large increases in obesity. This review discusses current advances in our understanding of how obesogens act to affect health and obesity susceptibility. Newly discovered obesogens and potential obesogens are discussed, together with future directions for research that may help to reduce the impact of these pervasive chemicals.

Inhibition of methyltransferase activity of enhancer of zeste 2 leads to enhanced lipid accumulation and altered chromatin status in zebrafish

Background

Recent studies indicate that exposure to environmental chemicals may increase susceptibility to developing metabolic diseases. This susceptibility may in part be caused by changes to the epigenetic landscape which consequently affect gene expression and lead to changes in lipid metabolism. The epigenetic modifier enhancer of zeste 2 (Ezh2) is a histone H3K27 methyltransferase implicated to play a role in lipid metabolism and adipogenesis. In this study, we used the zebrafish (Danio rerio) to investigate the role of Ezh2 on lipid metabolism and chromatin status following developmental exposure to the Ezh1/2 inhibitor PF-06726304 acetate. We used the environmental chemical tributyltin (TBT) as a positive control, as this chemical is known to act on lipid metabolism via EZH-mediated pathways in mammals.

Results

Zebrafish embryos (0–5 days post-fertilization, dpf) exposed to non-toxic concentrations of PF-06726304 acetate (5 μM) and TBT (1 nM) exhibited increased lipid accumulation. Changes in chromatin were analyzed by the assay for transposase-accessible chromatin sequencing (ATAC-seq) at 50% epiboly (5.5 hpf). We observed 349 altered chromatin regions, predominantly located at H3K27me3 loci and mostly more open chromatin in the exposed samples. Genes associated to these loci were linked to metabolic pathways. In addition, a selection of genes involved in lipid homeostasis, adipogenesis and genes specifically targeted by PF-06726304 acetate via altered chromatin accessibility were differentially expressed after TBT and PF-06726304 acetate exposure at 5 dpf, but not at 50% epiboly stage. One gene, cebpa, did not show a change in chromatin, but did show a change in gene expression at 5 dpf. Interestingly, underlying H3K27me3 marks were significantly decreased at this locus at 50% epiboly.

Conclusions

Here, we show for the first time the applicability of ATAC-seq as a tool to investigate toxicological responses in zebrafish. Our analysis indicates that Ezh2 inhibition leads to a partial primed state of chromatin linked to metabolic pathways which results in gene expression changes later in development, leading to enhanced lipid accumulation. Although ATAC-seq seems promising, our in-depth assessment of the cebpa locus indicates that we need to consider underlying epigenetic marks as well.

Phase 0 of the Xenobiotic Response: Nuclear Receptors and Other Transcription Factors as a First Step in Protection from Xenobiotics

This mini-review examines the crucial importance of transcription factors as a first line of defense in the detoxication of xenobiotics. Key transcription factors that recognize xenobiotics or xenobiotic-induced stress such as reactive oxygen species (ROS), include AhR, PXR, CAR, MTF, Nrf2, NF-κB, and AP-1. These transcription factors constitute a significant portion of the pathways induced by toxicants as they regulate phase I-III detoxication enzymes and transporters as well as other protective proteins such as heat shock proteins, chaperones, and anti-oxidants. Because they are often the first line of defense and induce phase I-III metabolism, could these transcription factors be considered the phase 0 of xenobiotic response?

Tributyltin chloride (TBT) induces RXRA down-regulation and lipid accumulation in human liver cells

A subset of environmental chemicals acts as "obesogens" as they increase adipose mass and lipid content in livers of treated rodents. One of the most studied class of obesogens are the tin-containing chemicals that have as a central moiety tributyltin (TBT), which bind and activate two nuclear hormone receptors, Peroxisome Proliferator Activated Receptor Gamma (PPARG) and Retinoid X Receptor Alpha (RXRA), at nanomolar concentrations. Here, we have tested whether TBT chloride at such concentrations may affect the neutral lipid level in two cell line models of human liver. Indeed, using high content image analysis (HCA), TBT significantly increased neutral lipid content in a time- and concentration-dependent manner. Consistent with the observed increased lipid accumulation, RNA fluorescence in situ hybridization (RNA FISH) and RT-qPCR experiments revealed that TBT enhanced the steady-state mRNA levels of two key genes for de novo lipogenesis, the transcription factor SREBF1 and its downstream enzymatic target, FASN. Importantly, pre-treatment of cells with 2-deoxy-D-glucose reduced TBT-mediated lipid accumulation, thereby suggesting a role for active glycolysis during the process of lipid accumulation. As other RXRA binding ligands can promote RXRA protein turnover via the 26S proteasome, TBT was tested for such an effect in the two liver cell lines. We found that TBT, in a time- and dose-dependent manner, significantly reduced steady-state RXRA levels in a proteasome-dependent manner. While TBT promotes both RXRA protein turnover and lipid accumulation, we found no correlation between these two events at the single cell level, thereby suggesting an additional mechanism may be involved in TBT promotion of lipid accumulation, such as glycolysis.

Transgenerational Self-Reconstruction of Disrupted Chromatin Organization After Exposure To An Environmental Stressor in Mice

Exposure to environmental stressors is known to increase disease susceptibility in unexposed descendants in the absence of detectable genetic mutations. The mechanisms mediating environmentally-induced transgenerational disease susceptibility are poorly understood. We showed that great-great-grandsons of female mice exposed to tributyltin (TBT) throughout pregnancy and lactation were predisposed to obesity due to altered chromatin organization that subsequently biased DNA methylation and gene expression. Here we analyzed DNA methylomes and transcriptomes from tissues of animals ancestrally exposed to TBT spanning generations, sexes, ontogeny, and cell differentiation state. We found that TBT elicited concerted alterations in the expression of “chromatin organization” genes and inferred that TBT-disrupted chromatin organization might be able to self-reconstruct transgenerationally. We also found that the location of “chromatin organization” and “metabolic” genes is biased similarly in mouse and human genomes, suggesting that exposure to environmental stressors in different species could elicit similar phenotypic effects via self-reconstruction of disrupted chromatin organization.

Transgenerational effects of obesogens

Obesity and associated disorders are now a global pandemic. The prevailing clinical model for obesity is overconsumption of calorie-dense food and diminished physical activity (the calories in-calories out model). However, this explanation does not account for numerous recent research findings demonstrating that a variety of environmental factors can be superimposed on diet and exercise to influence the development of obesity. The environmental obesogen model proposes that exposure to chemical obesogens during in utero and/or early life can strongly influence later predisposition to obesity. Obesogens are chemicals that inappropriately stimulate adipogenesis and fat storage, in vivo either directly or indirectly. Numerous obesogens have been identified in recent years and some of these elicit transgenerational effects on obesity as well as a variety of health end-points after exposure of pregnant F0 females. Prenatal exposure to environmental obesogens can produce lasting effects on the exposed animals and their offspring to at least the F4 generation. Recent results show that some of these transgenerational effects of obesogen exposure can be carried across the generations via alterations in chromatin structure and accessibility. That some chemicals can have permanent effects on the offspring of exposed animals suggests increased caution in the debate about whether and to what extent exposure to endocrine-disrupting chemicals and obesogens should be regulated.

The tributyltin leads to obesogenic mammary gland abnormalities in adult female rats

Tributyltin chloride (TBT) is an obesogen associated with several complications. However, few investigations have evaluated TBT effects on adult mammary glands (MG). In this investigation, we assessed whether TBT's obesogenic effects resulted in abnormal MG fat pad expansion and other irregularities. TBT was administered to female rats (100 ng/kg/day for 15 days via gavage), and their MG morphophysiological development was assessed. We further assessed the MG fat pad for PPARγ, ERα, and aromatase protein expression, as well as inflammation, oxidative stress (OS), apoptosis and fibrosis. Irregular MG morphological development such as lower TEB number, alveolar (AB), lobule and differentiation (DF) score were observed in TBT rats. TBT rats had abnormal MG fat accumulation as evidenced by increased numbers of hypertrophic adipocytes, triglyceride (TG) levels and PPARγ expression. A strong negative correlation between the MG obesogenic makers and TEB number, AB and DF score were observed in TBT rats. MG inflammation was observed in TBT rats. A positive correlation between the MG obesogenic markers and inflammation were observed. High ERα and aromatase expression were observed in MG of TBT rats. MG OS, apoptosis and fibrosis were present in the TBT rats. Additionally, a positive correlation between the MG obesogenic markers and OS were observed in TBT rats. Thus, these data suggest that obesogenic TBT effects led to MG irregularities in the adult female rats.

Endocrine-Mediated Mechanisms of Metabolic Disruption and New Approaches to Examine the Public Health Threat

Obesity and metabolic disorders are of great societal concern and generate substantial human health care costs globally. Interventions have resulted in only minimal impacts on disrupting this worsening health trend, increasing attention on putative environmental contributors. Exposure to numerous environmental contaminants have, over decades, been demonstrated to result in increased metabolic dysfunction and/or weight gain in cell and animal models, and in some cases, even in humans. There are numerous mechanisms through which environmental contaminants may contribute to metabolic dysfunction, though certain mechanisms, such as activation of the peroxisome proliferator activated receptor gamma or the retinoid x receptor, have received considerably more attention than less-studied mechanisms such as antagonism of the thyroid receptor, androgen receptor, or mitochondrial toxicity. As such, research on putative metabolic disruptors is growing rapidly, as is our understanding of molecular mechanisms underlying these effects. Concurrent with these advances, new research has evaluated current models of adipogenesis, and new models have been proposed. Only in the last several years have studies really begun to address complex mixtures of contaminants and how these mixtures may disrupt metabolic health in environmentally relevant exposure scenarios. Several studies have begun to assess environmental mixtures from various environments and study the mechanisms underlying their putative metabolic dysfunction; these studies hold real promise in highlighting crucial mechanisms driving observed organismal effects. In addition, high-throughput toxicity databases (ToxCast, etc.) may provide future benefits in prioritizing chemicals for in vivo testing, particularly once the causative molecular mechanisms promoting dysfunction are better understood and expert critiques are used to hone the databases. In this review, we will review the available literature linking metabolic disruption to endocrine-mediated molecular mechanisms, discuss the novel application of environmental mixtures and implications for in vivo metabolic health, and discuss the putative utility of applying high-throughput toxicity databases to answering complex organismal health outcome questions.

Current Research Approaches and Challenges in the Obesogen Field

Obesity is a worldwide pandemic that also contributes to the increased incidence of other diseases such as type 2 diabetes. Increased obesity is generally ascribed to positive energy balance. However, recent findings suggest that exposure to endocrine-disrupting chemicals such as obesogens during critical windows of development, may play an important role in the current obesity trends. Several experimental approaches, from in vitro cell cultures to transgenerational in vivo studies, are used to better understand the mechanisms of action of obesogens, each of which contributes to answer different questions. In this review, we discuss current knowledge in the obesogen field and the existing tools developed in research laboratories using tributyltin as a model obesogen. By understanding the advantages and limitations of each of these tools, we will better focus and design experimental approaches that will help expanding the obesogen field with the objective of finding potential therapeutic targets in human populations.

Obesogenic Endocrine Disrupting Chemicals: Identifying Knowledge Gaps

Endocrine disrupting chemicals (EDCs) are compounds that are part of everyday consumer products and industrial manufacturing processes. EDCs can interfere with the endocrine system, including the adipose tissue. Accumulating evidence from epidemiological, animal, and in vitro studies demonstrates that EDCs can alter body weight, adipose tissue expansion, circulating lipid profile, and adipogenesis, with some resulting in transgenerational effects. These outcomes appear to be mediated through multiple mechanisms, from nuclear receptor binding to epigenetic modifications. A better understanding of the signaling pathways via which these EDCs contribute to an obesogenic phenotype, the interaction amongst complex mixtures of obesogenic EDCs, and the risks they pose relative to the obesity epidemic are still needed for risk assessment and development of prevention strategies.

Retinoid X Receptor Activation During Adipogenesis of Female Mesenchymal Stem Cells Programs a Dysfunctional Adipocyte

Early life exposure to endocrine-disrupting chemicals (EDCs) is an emerging risk factor for the development of obesity and diabetes later in life. We previously showed that prenatal exposure to the EDC tributyltin (TBT) results in increased adiposity in the offspring. These effects linger into adulthood and are propagated through successive generations. TBT activates two nuclear receptors, the peroxisome proliferator-activated receptor (PPAR) γ and its heterodimeric partner retinoid X receptor (RXR), that promote adipogenesis in vivo and in vitro. We recently employed a mesenchymal stem cell (MSC) model to show that TBT promotes adipose lineage commitment by activating RXR, not PPARγ. This led us to consider the functional consequences of PPARγ vs RXR activation in developing adipocytes. We used a transcriptomal approach to characterize genome-wide differences in MSCs differentiated with the PPARγ agonist rosiglitazone (ROSI) or TBT. Pathway analysis suggested functional deficits in TBT-treated cells. We then compared adipocytes differentiated with ROSI, TBT, or a pure RXR agonist IRX4204 (4204). Our data show that RXR activators ("rexinoids," 4204 and TBT) attenuate glucose uptake, blunt expression of the antidiabetic hormone adiponectin, and fail to downregulate proinflammatory and profibrotic transcripts, as does ROSI. Finally, 4204 and TBT treatment results in an inability to induce markers of adipocyte browning, in part due to sustained interferon signaling. Taken together, these data implicate rexinoids in the development of dysfunctional white adipose tissue that could potentially exacerbate obesity and/or diabetes risk in vivo. These data warrant further screening and characterization of EDCs that activate RXR.

Environmental Obesogens: Mechanisms and Controversies

Obesity is a worldwide pandemic in adults as well as children and adds greatly to health care costs through its association with type 2 diabetes, metabolic syndrome, cardiovascular disease, and cancers. The prevailing medical view of obesity is that it results from a simple imbalance between caloric intake and energy expenditure. However, numerous other factors are important in the etiology of obesity. The obesogen hypothesis proposes that environmental chemicals termed obesogens promote obesity by acting to increase adipocyte commitment, differentiation, and size by altering metabolic set points or altering the hormonal regulation of appetite and satiety. Many obesogens are endocrine disrupting chemicals that interfere with normal endocrine regulation. Endocrine disrupting obesogens are abundant in our environment, used in everyday products from food packaging to fungicides. In this review, we explore the evidence supporting the obesogen hypothesis, as well as the gaps in our knowledge that are currently preventing a complete understanding of the extent to which obesogens contribute to the obesity pandemic.

miR‑330‑5p inhibits H2O2‑induced adipogenic differentiation of MSCs by regulating RXRγ

The elucidation of the underlying molecular mechanism of H2O2‑induced adipocyte differentiation in mesenchymal stem cells (MSCs) is important for the development of treatments for metabolic diseases. The aim of the present study was to identify microRNA (miR)‑330‑5p, which targets retinoid X receptor γ (RXRγ) and to determine the function of H2O2‑induced adipogenic differentiation of MSCs. During differentiation of MSCs into adipocytes induced by H2O2, miR‑330‑5p expression was decreased with a concomitant increase in RXRγ expression. A luciferase assay with RXRγ 3'‑untranslated region (UTR) reporter plasmid, including the miR‑330‑5p‑binding sequences, identified that the introduction of miR‑330‑5p decreases luciferase activity. However, it did not affect the activity of mutated RXRγ 3'‑UTR reporter. Enforced expression of miR‑330‑5p significantly inhibited adipocyte differentiation by decreasing RXRγ mRNA and protein levels. In contrast, inhibition of the endogenous miR‑330‑5p promoted the formation of lipid droplets by rescuing RXRγ expression. Furthermore, the effects of inhibition of RXRγ were similar to those of overexpression of miR‑330‑5p on H2O2‑induced adipogenic differentiation from MSCs. miR‑330‑5p inhibits H2O2‑induced adipogenic differentiation of MSCs, and this is dependent on RXRγ. Taken together, the results of the present study revealed that miR‑330‑5p acts as a critical regulator of RXRγ, and is able to determinate the fate of MSCs to differentiate into adipocytes. This suggests that miR‑330‑5p and RXRγ may be target molecules for controlling metabolic diseases.

RXR Ligands Modulate Thyroid Hormone Signaling Competence in Young Xenopus laevis Tadpoles

Appropriate thyroid hormone (TH) signaling through thyroid hormone receptors (TRs) is essential for vertebrate development. Amphibian metamorphosis is initiated and sustained through the action of TH on TRs, which are conserved across vertebrates. TRs heterodimerize with retinoid X receptors (RXRs) on thyroid hormone response elements (TREs) in the genome; however, in most cell line and adult animal studies, RXR ligands do not affect expression of TR target genes. We used a quantitative, precocious metamorphosis assay to interrogate the effects of the RXR agonist bexarotene (Bex) and the RXR antagonist UVI 3003 (UVI) on T3-induced resorption phenotypes in Xenopus laevis tadpoles 1 week postfertilization. Bex potentiated gill and tail resorption, and UVI abrogated T3 action. These results held in transgenic tadpoles bearing a TRE-driven luciferase reporter. Therefore, we used poly-A-primed RNA sequencing transcriptomic analysis to determine their effects on T3-induced gene expression. We also assayed the environmental pollutant tributyltin (TBT), which is an RXR agonist. We found that the proteases that carry out resorption were potentiated by Bex and TBT but were not significantly inhibited by UVI. However, several transcription factors from multiple families (sox4, fosl2, mxd1, mafb, nfib) were all inhibited by UVI and potentiated by Bex and TBT. All required T3 for induction. Time course analysis of gene expression showed that although the agonists could potentiate within 12 hours, the antagonist response lagged. These data indicate that the agonists and antagonist are not necessarily functioning through the same mechanism and suggest that RXR liganding may modulate TH competence in metamorphic signaling.

Effects of Perinatal Exposure to Dibutyltin Chloride on Fat and Glucose Metabolism in Mice, and Molecular Mechanisms, in Vitro

BACKGROUND

The organotin dibutyltin (DBT) is used in the manufacture of polyvinyl chloride (PVC) plastics, in construction materials, and in medical devices. Previous animal studies showed detrimental effects of DBT during in utero development at relatively high doses, but little was known about the effects of DBT exposure at environmentally relevant doses on endpoints such as obesity and metabolic disease.

OBJECTIVES

We tested the potential obesogenic effects of DBT using in vitro and in vivo models.

METHODS

We evaluated the effects of DBT on nuclear receptor activation and adipogenic potential using human and mouse multipotent mesenchymal stromal stem cells (MSCs). We also evaluated the effects of perinatal exposure to environmentally relevant doses of DBT in C57BL/6J mice.

RESULTS

DBT activated human and mouse PPARγ and RXRα in transient transfection assays, increased expression of adipogenic genes, promoted adipogenic differentiation and increased lipid accumulation in mouse and human MSCs, in vitro. DBT-induced adipogenic differentiation was abolished by the PPARγ antagonist T0070907, indicating that DBT was acting primarily through PPARγ. Perinatal exposure to low doses of DBT led to increased fat storage, decreased glucose tolerance, and increased circulating leptin levels in male, but not female, mice.

CONCLUSIONS

DBT acted as an obesogen by inducing lipid accumulation in human and mouse MSCs through a PPARγ-dependent pathway. In vivo exposure to biologically relevant doses of DBT during perinatal development led to increased fat storage, elevated leptin levels in plasma, and glucose intolerance in mice. Based on these findings, we posit that monitoring of DBT levels in human samples may aid in understanding and potentially preventing the rising rates of metabolic disorders in human populations. https://doi.org/10.1289/EHP3030.

Endocrine Disruptors and Developmental Origins of Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is a growing epidemic worldwide, particularly in countries that consume a Western diet, and can lead to life-threatening conditions such as cirrhosis and hepatocellular carcinoma. With increasing prevalence of NAFLD in both children and adults, an understanding of the factors that promote NAFLD development and progression is crucial. Environmental agents, including endocrine-disrupting chemicals (EDCs), which have been linked to other diseases, may play a role in NAFLD development. Increasing evidence supports a developmental origin of liver disease, and early-life exposure to EDCs could represent one risk factor for the development of NAFLD later in life. Rodent studies provide the strongest evidence for this link, but further studies are needed to define whether there is a causal link between early-life EDC exposure and NAFLD development in humans. Elucidating the molecular mechanisms underlying development of NAFLD in the context of developmental EDC exposures may identify biomarkers for people at risk, as well as potential intervention and/or therapeutic opportunities for the disease.