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

Abstract

Developmental exposure to environmental factors has been linked to obesity risk later in life. Nuclear receptors are molecular sensors that play critical roles during development and, as such, are prime candidates to explain the developmental programming of disease risk by environmental chemicals. We have previously characterized the obesogen tributyltin (TBT), which activates the nuclear receptors peroxisome proliferator-activated receptor γ (PPARγ) and retinoid X receptor (RXR) to increase adiposity in mice exposed in utero. Mesenchymal stem cells (MSCs) from these mice are biased toward the adipose lineage at the expense of the osteoblast lineage, and MSCs exposed to TBT in vitro are shunted toward the adipose fate in a PPARγ-dependent fashion. To address where in the adipogenic cascade TBT acts, we developed an in vitro commitment assay that permitted us to distinguish early commitment to the adipose lineage from subsequent differentiation. TBT and RXR activators (rexinoids) had potent effects in committing MSCs to the adipose lineage, whereas the strong PPARγ activator rosiglitazone was inactive. We show that activation of RXR is sufficient for adipogenic commitment and that rexinoids act through RXR to alter the transcriptome in a manner favoring adipogenic commitment. RXR activation alters expression of enhancer of zeste homolog 2 (EZH2) and modifies genome-wide histone 3 lysine 27 trimethylation (H3K27me3) in promoting adipose commitment and programming subsequent differentiation. These data offer insights into the roles of RXR and EZH2 in MSC lineage specification and shed light on how endocrine-disrupting chemicals such as TBT can reprogram stem cell fate.

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

“…Enhanced adipogenesis as an adverse effect of exposure to MDCs is often accompanied by changes in epigenetic gene regulation as previously found [14][15][16][17][18][19][20]. For example, prenatal exposure to the MDC tributyltin (TBT) increases the number of adipocytes in mice offspring [21].…”

“…Additionally, prenatal exposure to TBT leads to a change in adipogenesis in the F4 generation, which is linked to changes in sperm chromatin structures. Of importance, activation of the retinoid X receptor (RXR) by TBT or a RXR-selective agonist (IRX4204) showed a reduced expression of EZH2 [20]. As a consequence, redistribution and an overall decrease of H3K27me3 were observed, especially close to genes involved in adipogenesis [20].…”

“…Of importance, activation of the retinoid X receptor (RXR) by TBT or a RXR-selective agonist (IRX4204) showed a reduced expression of EZH2 [20]. As a consequence, redistribution and an overall decrease of H3K27me3 were observed, especially close to genes involved in adipogenesis [20]. From the above it becomes evident that histone PTMs via EZH2 play a role in adipogenesis, but other PTMs or DNA methylation might play an equal important role in shaping the epigenetic landscape.…”

Transgenerational effects of obesogens

“…It was recently shown that TBT and other chemicals that activate RXR commit MSCs to the adipose lineage by activating RXR, but not PPARγ. TBT and pharmacological RXR agonists derepress genes important for adipogenic commitment by decreasing deposition of the repressive chromatin mark, histone 3 lysine 27 trimethyl (H3K27 me3 ) in the promoters and regulatory regions of these genes, leading to increased expression of their mRNAs . Therefore, it is likely that other RXR activators (these are collectively called “rexinoids”) such as the fungicide fludioxonil or the surfactant SPAN‐80 will produce similar effects.…”

“…TBT and pharmacological RXR agonists derepress genes important for adipogenic commitment by decreasing deposition of the repressive chromatin mark, histone 3 lysine 27 trimethyl (H3K27 me3 ) in the promoters and regulatory regions of these genes, leading to increased expression of their mRNAs. 92 Therefore, it is likely that other RXR activators (these are collectively called "rexinoids") such as the fungicide fludioxonil or the surfactant SPAN-80 will produce similar effects. Intriguingly, a variety of other fungicides with widely divergent structures, including flusilazole, zoxamide and quinoxyfen, are candidate obesogens, in vitro.…”