Structurally-diverse, PPARγ-activating environmental toxicants induce adipogenesis and suppress osteogenesis in bone marrow mesenchymal stromal cells

Abstract

Environmental obesogens are a newly recognized category of endocrine disrupting chemicals that have been implicated in contributing to the rising rates of obesity in the United States. While obesity is typically regarded as an increase in visceral fat, adipocyte accumulation in the bone has been linked to increased fracture risk, lower bone density, and osteoporosis. Exposure to environmental toxicants that activate peroxisome proliferator activated receptor γ (PPARγ), a critical regulator of the balance of differentiation between adipogenesis and osteogenesis, may contribute to the increasing prevalence of osteoporosis. However, induction of adipogenesis and suppression of osteogenesis are separable activities of PPARγ, and ligands may selectively alter these activities. It currently is unknown whether suppression of osteogenesis is a common toxic endpoint of environmental PPARγ ligands. Using a primary mouse bone marrow culture model, we tested the hypothesis that environmental toxicants acting as PPARγ agonists divert the differentiation pathway of bone marrow-derived multipotent mesenchymal stromal cells towards adipogenesis and away from osteogenesis. The toxicants tested included the organotins tributyltin and triphenyltin, a ubiquitous phthalate metabolite (mono-(2-ethylhexyl) phthalate, MEHP), and two brominated flame retardants (tetrabromobisphenol-a, TBBPA, and mono-(2-ethylhexyl) tetrabromophthalate, METBP). All of the compounds activated PPARγ1 and 2. All compounds increased adipogenesis (lipid accumulation, Fabp4 expression) and suppressed osteogenesis (alkaline phosphatase activity, Osx expression) in mouse primary bone marrow cultures, but with different potencies and efficacies. Despite structural dissimilarities, there was a strong negative correlation between efficacies to induce adipogenesis and suppress osteogenesis, with the organotins being distinct in their exceptional ability to suppress osteogenesis. As human exposure to a mixture of toxicants is likely, albeit at low doses, the fact that multiple toxicants are capable of suppressing bone formation supports the hypothesis that environmental PPARγ ligands represent an emerging threat to human bone health.

Tributyltin Engages Multiple Nuclear Receptor Pathways and Suppresses Osteogenesis in Bone Marrow Multipotent Stromal Cells

“…Human and mouse PPARγ activation by TBT occurs at similar concentrations; we measured an EC 50 of ~10 nM in reporter assays of transactivation of either human or mouse PPARγ’s and an EC 50 of 10 nM for stimulation of adipogenesis in a mouse-derived BMS2 cells. 25, 33 In addition, binding affinities of rodent and human PPARγ’s for rosiglitazone are similar. 52 In contrast, both PPARγ and RXR agonists had anti-osteogenic capabilities.…”

Editor’s Highlight: Screening ToxCast Prioritized Chemicals forPPARGFunction in a Human Adipose-Derived Stem Cell Model of Adipogenesis

“…Diclofop-methyl, another diphenyl ether with specificity for PPARA in transient reporter assays, increases hepatic Cyp4A, a PPARA target gene, in mice (Takeuchi et al , 2006). MEHP activates both PPARA and PPARG in reporter assays (Maloney and Waxman, 1999), causes peroxisome proliferation and liver tumors in rodents, and induces adipogenesis (Feige et al , 2007; Hao et al , 2012; Pereira-Fernandes et al , 2013a; Watt and Schlezinger, 2015). As a structurally related member of the PPAR nuclear receptor super family, PPARA is predominantly expressed in liver, heart, and kidney, with lower expression in muscle and brown adipose tissues.…”

The Effects of Endocrine Disruptors on Adipogenesis and Osteogenesis in Mesenchymal Stem Cells: A Review

“…Thus, PPARγ and SREBF-1 are key transcriptional factors in adipogenesis. The expression of C/EBPα and expression of factors promoted by PPARγ, such as lipoprotein lipase and fatty acid binding protein 4/adipocyte protein 2 (aP2), have been shown to be increased in response to DDT, DDE, 4-NP, BPA, PCB-77, DEHP, MEHP, and BBP (37, 39, 42, 45, 46, 66, 68, 69, 73, 75, 76, 78, 82–84, 86–89). p , p ′-DDT has also been shown to increase binding of C/EBPα to its DNA response element, demonstrating that the promotion of adipogenesis may be occurring through both increased expression and activation of targeted receptors (66).…”

“…Similar to adipogenesis, osteogenic differentiation relies on a mechanical stimulus with the appropriate growth surface stiffness, topography, tension, cytoskeletal organization, and soluble medium factors (95–99). A general principle is that stimulation of adipogenesis results in suppression of osteogenesis and vice-versa (76, 100–102). Thus, given the induction of adipogenesis by many EDCs, it is logical that EDCs have been shown to reduce the expression of genes and activity of transcription factors involved in MSCs undergoing osteogenic differentiation.…”

“…Increased acetylcholine has been shown to reduce ALP activity by nicotinic acetylcholine receptors and muscarinic acetylcholine receptors in preosteoblasts and osteoblasts, reducing osteogenic differentiation (104). MEHP has been shown to significantly suppress ALP activity, Runx2 expression, and osterix expression in MSCs (76). DEHP results in reduction of ALP expression, Runx2 protein levels, and mineralization in osteoblasts (105).…”