Thyromimetic actions of tetrabromobisphenol A (TBBPA) in steatotic FaO rat hepatoma cells

doi:10.1016/j.chemosphere.2014.03.114

Chemosphere

Volume 112, October 2014, Pages 511-518

https://doi.org/10.1016/j.chemosphere.2014.03.114 Get rights and content

Highlights

•
TBBPA shares structural similarities with thyroid hormones (THs).
•
THs modulate lipid metabolism through TR-dependent and independent pathways.
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The effects of TBBPA and T₃ in steatotic FaO hepatoma cells were compared.
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TBBPA decreased lipid accumulation through stimulation of oxidative pathways.
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Novel, receptor independent thyromimetic actions of TBBPA were identified.

Abstract

Tetrabromobisphenol A (2,2-bis(3,5-dibromo-4-hydroxyphenyl propane-TBBPA) is the most produced brominated flame retardant, detected in the environment and in biological samples. TBBPA shares structural similarities with thyroid hormones (THs), and it has been shown to interfere with different aspects of TH physiology, this raising concern on its possible effects as an endocrine disruptor in humans and wildlife. THs play a major role in lipid metabolism, with the liver representing one of their main target tissues. At the cellular level, THs act through interactions with TH receptors (TRs), as well as through TR-independent mechanisms.

Rat hepatoma FaO cells (a liver cell line defective for functional TRs) overloaded with lipids have been utilized as a model to investigate the anti-steatotic effects of THs in the hepatocyte. In this work, the possible effects of TBBPA in steatotic FaO cells were investigated. Exposure to TBBPA for 24 h reduced triglyceride (TAG) content and the size of lipid droplets (LDs); similar effects were obtained with equimolar doses (10⁻⁶ M) of T₃ (3,3′,5-L-triiodothyronine). TBBPA and T₃ showed common effects on transcription of genes involved in lipid homeostasis. In particular, TBBPA mainly up-regulated mRNA levels for LD-associated oxidative tissue-enriched PAT protein (OXPAT), peroxisome proliferator-activated receptor (PPAR) isoform β/δ, and the mitochondrial uncoupling protein 2 (UCP2). The results demonstrate that TBBPA can decrease lipid accumulation in steatotic cells through stimulation of oxidative pathways. These data identify novel thyromimetic actions of TBBPA at the cellular level.

Introduction

Brominated flame retardants (BFRs) are used in a variety of commercial products including textiles, paper, plastics, electronic equipment, mobile devices and building materials. BFRs are persistent and lipophilic compounds that may bioaccumulate and are thus regarded as a potential environmental health problem (Darnerud, 2003, de Wit et al., 2010). Tetrabromobisphenol A (2,2-bis(3,5-dibromo-4-hydroxyphenyl) propane, TBBPA) is the most-produced BFR (>200 000 tons year⁻¹) (de Wit et al., 2010). First employed to replace highly persistent BFRs, due to its estimated relatively short half-life in mammals and consequently lower risk of human exposure and toxicity (Kuester et al., 2007), TBBPA has been repeatedly detected in the environment and in human samples (Law et al., 2006, EFSA, 2011, Shaw et al., 2013). This finding is raising particular concern, given the potentially adverse effects of this compound as an endocrine disruptor (Fini et al., 2012).

TBBPA shares structural similarities with Thyroid Hormones (THs) T₃ (3,3′,5-L-triiodothyronine) and T₄ (3,3′,5,5′-L-tetraiodothyronine) (Kitamura et al., 2005) that are essential for the normal development, growth, differentiation and metabolism of all vertebrates (Zoeller et al., 2002). At the cellular level, THs act through interaction with nuclear Thyroid Receptors (TRs), as well as through TR-independent mechanisms, including mitochondrial uncoupling and activation of rapid cytosolic signal transduction pathways (Senese et al., 2014). There is increasing evidence that TBBPA has the potential to interfere with different aspects of TH physiology: it has been shown to bind transthyretin (Meerts et al., 2000), to have thyroidogenic as well as TH antagonist activity (Kitamura et al., 2005, Terasaki et al., 2011, Fini et al., 2012), to modulate the activity of deiodinases (Butt et al., 2011), to induce mitochondrial uncoupling (Nakagawa et al., 2007), to interfere with complex gene regulation systems involving interaction of corepressor and coactivator proteins with TRs (Levy-Bimbot et al., 2012).

THs play an important role in lipid homeostasis (synthesis, mobilization and degradation), with the liver representing one of their main target tissues (Lanni et al., 2005, Zhu and Cheng, 2010). The direct anti-steatotic effects of THs on the hepatocyte have been investigated in rat hepatoma FaO cells overloaded with lipids by exposure to excess free fatty acids-FFAs (Grasselli et al., 2011a, Grasselli et al., 2014). In this well differentiated liver cell line, defective for functional TRs, both T₃ and its derivative T₂ (3′,5′-diiodo-L-thyronine) were able to exert lipid lowering effects though TR-independent mechanisms of action, including modulation of transcription of genes involved in lipid metabolism, in particular Peroxisome Proliferator-Activated Receptors (PPARs), and mitochondrial uncoupling (Grasselli et al., 2011a, Grasselli et al., 2014).

In this work, the possible receptor-independent effects of TBBPA on lipid homeostasis were investigated in lipid-loaded (steatotic) FaO cells. Intracellular lipid accumulation was evaluated by determination of triglyceride (TAG) content and lipid droplet (LD) size. Transcription of PPAR isoforms α, β/δ, and γ and of some of their target genes was determined by quantitative (q-PCR). The effects of TBBPA were compared with those of equimolar doses of T₃.

Section snippets

Cell culture and treatments

Rat hepatoma cells (FaO) were supplied by European Collection of Cell Cultures (Sigma–Aldrich Corp. Milan, Italy). Cells were grown in Coon’s modified Ham’s F12 supplemented with 10% foetal bovine serum-FBS (Euroclone Milan, Italy), 100 Uml⁻¹ penicillin and 0.1 mg ml⁻¹ streptomycin and maintained at 37 °C in a humidified atmosphere of 95% air and 5% CO₂ and used at 70–80% confluence. For treatments cells were incubated in medium supplemented with 0.25% bovine serum albumin (BSA) without FBS. In

Effects of TBBPA on lipid accumulation in steatotic FaO cells

The effect of TBBPA on TAG accumulation in lipid loaded (steatotic) FaO cells was first evaluated (Fig. 1A). Cell exposure to a mixture of FFAs (oleate/palmitate) for 3 h resulted in significant TAG accumulation (+110%; P ⩽ 0.001) as previously demonstrated (Grasselli et al., 2011a, Grasselli et al., 2014). Subsequent exposure to TBBPA (10⁻⁸–10⁻⁵ M for 24 h) induced a dose-dependent decrease in intracellular TAG content, with maximal effect at both 10⁻⁶/10⁻⁵ M. Similar effects were observed with

Discussion

The results show that in steatotic FaO hepatoma cells exposure to TBBPA induced a dose-dependent decrease in intracellular TAG content. The maximal effect was observed at 10⁻⁶ M, and comparable with that induced by equimolar concentrations of T₃ (this work; Grasselli et al., 2014). In steatotic FaO cells, TAG secretion was increased in a likely attempt to eliminate excess lipids; this effect was reversed by both TBBPA and T₃, indicating that stimulation of lipid secretion is not involved in the

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Citation Excerpt :
Tetrabromobisphenol A (TBBPA) is one of the widely used brominated flame retardants (BFRs) in electrical equipments, electronics, plastics, textiles, transportation packaging, construction materials, and fire prevention materials (de Wit et al., 2010; Alaee et al., 2003; Covaci et al., 2009). TBBPA is a widespread environmental contaminant in wastewater, sewage sludge, soil, and biota (Hwang et al., 2012; Arnon et al., 2006; Sun et al., 2008; Gorga et al., 2013; Yang et al., 2013) and a persistent organic pollutant that can be bioaccumulated to cause neurotoxicity and endocrine disturbances in living organisms (Huang et al., 2013; Wang et al., 2014b; Grasselli et al., 2014; Knudsen et al., 2014). Several methods have been successfully used to degrade TBBPA, including microbial degradation (An et al., 2011; Peng et al., 2013; Uhnáková et al., 2011; Wang et al., 2013).
In present study, reductive graphene oxide and silver nanoparticles co-comodified TiO₂ nanotube arrays were prepared, and which was investigated to degrade tetrabromobisphenol A. The arrays co-modified with silver nanoparticles and reductive graphene oxide prepared by electrodeposition method exhibited good photoelectrocatalytic degradative activity for tetrabromobisphenol A, and the degradation efficiency reached 99.6% within 80 min. The synergistic effect of high photoresponse of Ag nanoparticles with their high capture ability for photogenerated electrons and the extended wavelength absorption range of reductive graphene oxide resulted in the highest degradation efficiencies. Degradation is postulated to follow a stepwise reductive debromination mechanism.
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Tetrabromobisphenol A (TBBPA) is a nonregulated brominated flame retardant with a high production volume, and it is applied in a wide variety of consumer products. TBBPA is ubiquitous in abiotic matrices, wildlife and humans around the world. This paper critically reviews the published scientific data concerning the disposition, metabolism or kinetics and toxicity of TBBPA in animals and humans. TBBPA is rapidly absorbed and widely distributed among tissues, and is excreted primarily in the feces. In rats, TBBPA and its metabolites have limited systemic bioavailability. TBBPA has been detected in human milk in the general population. It is available to both the developing fetus and the nursing pups following maternal exposure. It has been suggested that TBBPA causes acute toxicity, endocrine disruptor activity, immunotoxicity, neurotoxicity, nephrotoxicity, and hepatotoxicity in animals. Cell-based assays have shown that TBBPA can induce reactive oxygen species in a concentration-dependent manner, and it promotes the production of inflammatory factors such as TNF α, IL-6, and IL-8. Cells exposed to high levels of TBBPA exhibit seriously injured mitochondria and a dilated smooth endoplasmic reticulum. This review will enhance the understanding of the potential risks of TBBPA exposure to ecological and human health.
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Thyromimetic actions of tetrabromobisphenol A (TBBPA) in steatotic FaO rat hepatoma cells

Highlights

Abstract

Introduction

Section snippets

Cell culture and treatments

Effects of TBBPA on lipid accumulation in steatotic FaO cells

Discussion

Biochim. Biophys. Acta

Aquat. Toxicol.

Mol. Cell Endocrinol.

Environ. Int.

Sci. Total Environ.

Nat. Med.

Chemosphere

J. Hepatol.

Life Sci.

Chemosphere

Chemosphere

Biochim. Biophys. Acta.

Chemosphere

Chemosphere

Chemosphere

Postlipolytic insulin-dependent remodeling of micro lipid droplets in adipocytes

Mol. Biol. Cell

Peroxisome proliferator-activated receptor delta: a multifaceted metabolic player

Curr. Opin. Lipidol.

Halogenated phenolic contaminants inhibit the in vitro activity of the thyroid-regulating deiodinases in human liver

Toxicol. Sci.

Peroxisome proliferator-activated receptor delta: a conserved director of lipid homeostasis through regulation of the oxidative capacity of muscle

PPAR Res.