Human exposure during pregnancy to persistent environmental pollutants, like 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related chemicals, results in decreased birth weights, neurological deficits, thyroid hormone alterations, lung auscultation, and hyperpigmentation. The mechanism by which TCDD mediates developmental toxicity has not been elucidated. The basic helix-loop-helix-PAS transcription factor, aryl hydrocarbon receptor (AhR), is required for TCDD-induced teratogenicity in mice and likely mediates teratogenicity in other species. TCDD toxicity may result from alterations in gene transcription by the AhR. One potential AhR gene target that could account for some of TCDD's teratogenic effects is the Na+/K+ ATPase alpha1. Putative dioxin response elements are conserved in the 5' enhancer region of the mammalian and avian Na+/K+ ATPase alpha1 gene. In the chick embryo, TCDD reduces myocardial Na+/K+ ATPase alpha1 protein expression, induces a dilated cardiomyopathy, and alters ECGs, all consistent with reduced Na+/K+ ATPase activity. In mice lacking the AhR, embryos develop a hypertrophic cardiomyopathy and cardiac fibrosis which worsens with age, consistent with the potential overexpression of Na+/K+ ATPase alpha1 and development of hypertension. I will use the chick embryo and AhR null mice to test the hypothesis that the AhR regulates myocardial expression of the Na+/K+ ATPase alpha1 gene, altering cardiovascular development.
The aims of this proposal are to (1) elucidate the regulation of myocardial Na+/K+ ATPase alpha1 gene in AhR null mice and in the developing chick embryo by TCDD using RT-PCR, and in vitro by promoter analysis of the avian gene; (2) determine the tissue significance of this regulation by quantitating myocardial ouabain binding sites and Na+/K+ ATPase enzyme activity in AhR null mice and TCDD-exposed chick embryos; (3) determine the functional significance by measuring blood pressure in AhR null mice and myocardial sensitivity to ouabain in TCDD-exposed chick embryos by ECG; and (4) analyze the expression of murine and avian homologues of candidates genes, whose expression is altered by TCDD in the rat embryo heart and lung as identified by Dr. Selmin, University of Arizona, by PCR-selected subtractive hybridization method and screening of gene microarrays.
