In the embryo-larval stages of fish, alkylphenanthrenes such as
retene (7-isopropyl-1-methylphenanthrene) produce a suite of developmental abnormalities typical of exposure to
2,3,7,8-tetrachlorodibenzo-p-dioxin (
TCDD), including pericardial and yolk sac
edema, cardiovascular dysfunction, and skeletal
deformities. To investigate the mechanism and target tissue of
retene toxicity, we used observational, histological, and
protein knockdown techniques in zebrafish (Danio rerio) embryos. The primary overt signs of toxicity are pericardial
edema and reduced blood flow, first observed at 36 h post-fertilization (hpf). The most pronounced effects at this stage are a reduced layer of cardiac jelly in the atrium and reduced diastolic filling. Conversely, an increased layer of cardiac jelly is observed at 72 hpf in
retene-exposed embryos. Induction of
cytochrome P4501A (CYP1A) is apparent in a subset of cardiomyocytes by 48 hpf suggesting that early cardiac effects may be due to AhR activation in the myocardium. Myocardial CYP1A induction is transient, with only endocardial induction observed at 72 hpf. Knockdown of cyp1a by
morpholino oligonucleotides does not affect
retene toxicity; however, ahr2 knockdown prevents toxicity. Thus, the mechanism of
retene cardiotoxicity is AhR2-mediated and CYP1A-independent, similar to
TCDD; however, the onset and proximate signs of
retene toxicity differ from those of
TCDD.
Retene cardiotoxicity also differs mechanistically from the cardiac effects of non-alkylated phenanthrane, illustrating that alkyl groups can alter
toxic action. These findings have implications for understanding the toxicity of
complex mixtures containing alkylated and non-alkylated
polycyclic aromatic hydrocarbons.