2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alters the endogenous metabolism of all-trans-retinoic acid in the rat

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to influence vitamin A homeostasis. In order to investigate the mechanism behind this retinoid disruption, male Sprague-Dawley rats were exposed to TCDD at doses ranging from 0.1 to 100 mug/kg body weight, and were killed 3 days after exposure. Additional groups of rats were killed 1 and 28 days after a single oral dose of 10 mug TCDD/kg body weight. Serum, kidney, and liver were investigated for retinoid levels, as well as gene expression and enzyme activities relevant for retinoid metabolism. Besides the well known effects of TCDD on apolar retinoids, i.e. decreased hepatic and increased renal retinyl ester (RE) levels, we have found dose-dependent elevation of all-trans-retinoic acid (all-trans-RA) levels in all investigated tissues. In the liver, 9-cis-4-oxo-13,14-dihydro-RA was drastically decreased by TCDD in a dose-dependent manner. In serum, cis-isomers of all-trans-RA, including 9,13-di-cis-RA, were significantly reduced already at the lowest dose level. Protein and mRNA levels of cellular retinol binding protein I (CRBP-I) in liver or kidneys were not significantly altered by TCDD exposure at doses at which retinoid levels were affected, making CRBP-I an unlikely candidate to account for the alterations in retinoid metabolism caused by TCDD. The expression and activities of relevant cytochrome P450 (CYP) enzymes with potential roles in all-trans-RA synthesis and/or degradation (CYP1A1, 1A2, and 2B1/2) were also monitored. A possible role of CYP1A1 in TCDD-induced all-trans-RA synthesis is suggested from the time-course relationship between CYP1A1 activity and all-trans-RA levels in liver and kidney. The significant alteration of the all-trans-RA metabolism has the potential to contribute significantly to the toxicity of TCDD.