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. 2021 Feb 26;180(1):62-75.
doi: 10.1093/toxsci/kfaa180.

Effects of an Environmentally Relevant Mixture of Organophosphate Esters Derived From House Dust on Endochondral Ossification in Murine Limb Bud Cultures

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Effects of an Environmentally Relevant Mixture of Organophosphate Esters Derived From House Dust on Endochondral Ossification in Murine Limb Bud Cultures

Han Yan et al. Toxicol Sci. .

Abstract

Organophosphate esters (OPEs) are used widely as flame retardants and plasticizers but much remains unknown about their potential toxicity. Previously, we reported that 4 individual OPEs suppress endochondral ossification in murine limb bud cultures. However, real-life exposure is to complex OPE mixtures. In the present study, we tested the hypothesis that a Canadian household dust-based OPE mixture will affect endochondral ossification in gestation day 13 CD1 mouse embryo limb buds expressing fluorescent markers for the major cell populations involved in the process: collagen type II alpha 1-enhanced cyan fluorescent protein (proliferative chondrocytes), collagen type X alpha 1-mCherry (hypertrophic chondrocytes), and collagen type I alpha 1-yellow fluorescent protein (osteoblasts). Limbs were cultured for 6 days in the presence of vehicle or dilutions of the OPE mixture (1/1 000 000, 1/600 000, and 1/300 000). All 3 OPE mixture dilutions affected cartilage template development and the progression of endochondral ossification, as indicated by the fluorescent markers. The expression of Sox9, the master regulator of chondrogenesis, was unchanged, but the expression of Runx2 and Sp7, which drive chondrocyte hypertrophy and osteoblastogenesis, was dilution-dependently suppressed. RNA-seq revealed that exposure to the 1/300 000 dilution of the OPE mixture for 24 h downregulated 153 transcripts and upregulated 48 others by at least 1.5-fold. Downregulated transcripts were enriched for those related to the immune system and bone formation. In contrast, upregulated transcripts were enriched for those with stress response functions known to be regulated by ATF4 activation. Thus, exposure to the mixture of OPEs commonly found in house dust may have adverse effects on bone formation.

Keywords: endochondral ossification; flame retardants; house dust; limb bud culture; organophosphate esters; plasticizers.

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Figures

Figure 1.
Figure 1.
Representative fluorescent photographs of gestation day 13 forelimbs cultured in the presence of vehicle, 1/1 000 000, 1/600 000, or 1/300 000 dilutions of the organophosphate ester mixture on days 1, 3, and 6 of the 6-day culture period. Expression of collagen type II alpha 1-enhanced cyan fluorescent protein marks the developing cartilage template, collagen type X alpha 1-mCherry marks hypertrophic chondrocytes, and collagen type I alpha 1-yellow fluorescent protein marks osteoblasts (n = 8–9).
Figure 2.
Figure 2.
On (A) day 3 and (B) day 6 of culture, the CFP+ cartilage template of each limb was scored according to the extent of its morphological development. Scores were compared using Bonferroni-corrected Mann-Whitney U tests, ***p < .001 compared with control (n = 8–9).
Figure 3.
Figure 3.
Exposure of limb buds to different dilutions of the organophosphate ester (OPE) mixture had no effect on the mRNA expression of (A) Sox9. However, (B) Runx2, and (C) Sp7 levels were dilution-dependently decreased compared with control. Expression was normalized to Rpl8. #p < .01 control versus 1/1 000 000 OPE mixture, †p <.01 control versus 1/600 000 OPE mixture, ‡p < .01 control versus 1/300 000 OPE mixture (n = 5).
Figure 4.
Figure 4.
Volcano plot of data from the RNA-seq experiment showing gene expression changes induced by exposure to 1/300 000 dilution of the organophosphate ester mixture at 24 h, compared with control. Dashed gray lines indicate the cutoffs used (≥1.5-fold change and q-value < 0.05, corresponding to |log2[fold change]| ≥ 0.5849 and –log10[q-value] > 1.30, respectively). Based on these cutoffs, blue dots indicate downregulated transcripts and orange dots indicate upregulated transcripts (n =4).
Figure 5.
Figure 5.
Differential gene expression data from the comparison of limbs exposed to 1/300 000 OPE mixture and control limbs at 24 h were analyzed using IPA. The top 15 affected canonical pathways are shown. Each bar is labeled with its associated activation z-score: z-score ≤ −2 indicates pathway inhibition; z-score ≥ 2 indicates pathway activation; NP = no prediction (insufficient data on the activity pattern of said pathway in the Ingenuity Knowledge Base).
Figure 6.
Figure 6.
Differential gene expression data from the comparison of limbs exposed to 1/300 000 organophosphate ester mixture and control limbs at 24 h were analyzed using IPA. The upstream regulators predicted to have altered activation states (|activation z-score| ≥ 2) are shown.
Figure 7.
Figure 7.
The mRNA expression of Atf4 and 4 Atf4 target genes were assessed by quantitative reverse transcription-PCR at 24 h. Expression was normalized to Rpl8. #p < 0.01 control versus 1/1 000 000 organophosphate ester (OPE) mixture, †p < .01 control versus 1/600 000 OPE mixture, ‡p < .01 control versus 1/300 000 OPE mixture (n = 4).

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