Comparative Toxicogenomic Responses to the Flame Retardant mITP in Developing Zebrafish

Monosubstituted isopropylated triaryl
phosphate (mITP) is a major
component of Firemaster 550, an additive flame retardant mixture commonly
used in polyurethane foams. Developmental toxicity studies in zebrafish
established mITP as the most toxic component of FM 550, which causes
pericardial edema and heart looping failure. Mechanistic studies showed
that mITP is an aryl hydrocarbon receptor (AhR) ligand; however, the
cardiotoxic effects of mITP were independent of the AhR. We performed
comparative whole genome transcriptomics in wild-type and ahr2hu3335 zebrafish, which
lack functional ahr2, to identify transcriptional
signatures causally involved in the mechanism of mITP-induced cardiotoxicity.
Regardless of ahr2 status, mITP exposure resulted
in decreased expression of transcripts related to the synthesis of
all-trans-retinoic acid and a host of Hox genes.
Clustered gene ontology enrichment analysis showed unique enrichment
in biological processes related to xenobiotic metabolism and response
to external stimuli in wild-type samples. Transcript enrichments overlapping
both genotypes involved the retinoid metabolic process and sensory/visual
perception biological processes. Examination of the gene–gene
interaction network of the differentially expressed transcripts in
both genetic backgrounds demonstrated a strong AhR interaction network
specific to wild-type samples, with overlapping genes regulated by
retinoic acid receptors (RARs). A transcriptome analysis of control ahr2-null zebrafish identified potential cross-talk among
AhR, Nrf2, and Hif1α. Collectively, we confirmed that mITP is
an AhR ligand and present evidence in support of our hypothesis that
mITP’s developmental cardiotoxic effects are mediated by inhibition
at the RAR level.