Regulation of CYP4A expression in rat by dehydroepiandrosterone and thyroid hormone

Dehydroepiandrosterone (DHEA) is a peroxisome proliferating agent when administered in pharmacological dosages, but it has not been shown to function through the peroxisome proliferator-activated receptor in cell-based assays. Because members of the thyroid hormone/vitamins A and D nuclear receptor subfamily, including PPAR, are known to modulate each other's function in gene expression by heterodimerization, we sought to establish whether DHEA and thyroid hormone interact to regulate several of the hepatic and renal enzymes associated with peroxisome proliferation, i.e., peroxisomal beta-oxidation and microsomal NADPH:cytochrome P450 oxidoreductase and the cytochromes P450 4A. In rats administered exogenous T3 to attain a hyperthyroid state, induction of the three isozymes of CYP4A (4A1, 4A2, and 4A3) by DHEA was suppressed > 60-80% at the mRNA level, with induction of CYP4A2 mRNA being completely inhibited. Nuclear run-on transcription assays indicated that this inhibitory effect was regulated at the level of transcription. Induction of hepatic peroxisomal beta-oxidation by DHEA or the peroxisome proliferator nafenopin was in large part unaffected by treatment of animals with T3 under any condition tested. Microsomal NADPH:cytochrome P450 oxidoreductase activity was induced by either DHEA or T3; cotreatment resulted in an additive induction. When animals were treated with a lower dose of exogenous T3 that rendered the animals slightly hyperthyroid, only induction of hepatic CYP4A2 mRNA by DHEA or nafenopin was significantly inhibited (> 80%) compared with euthyroid control animals. Animals that had been rendered hypothyroid through removal of the thyroid gland showed normal induction of CYP4A genes by DHEA in liver, suggesting that their induction by DHEA was not dependent on the presence of thyroid hormone. The administration of exogenous T3 to thyroidectomized rats in the presence of DHEA potently suppressed hepatic induction of all three genes at the mRNA and protein level. In experiments with cultured rat hepatocytes, physiological concentrations of T3 potently inhibited the induction of CYP4A2 mRNA levels by nafenopin but had little effect on induction of CYP4A1 or 4A3 mRNA. At higher T3 concentrations, the induction of CYP4A1/4A3 mRNA and protein was also inhibited. These results suggest that T3 modulates the expression of CYP4A2 at the level of transcription in physiologically relevant concentrations but that hyperthyroid conditions are required to suppress expression of CYP4A1/4A3 genes. In euthyroid rodent kidney, which only expresses CYP4A2 under either basal or DHEA-induced conditions, near-physiological levels of T3 caused potent suppression of peroxisome proliferator-dependent induction of CYP4A2 mRNA levels by either DHEA or nafenopin. In thyroidectomized rats, basal expression of CYP4A2 mRNA was decreased relative to euthyroid controls, but DHEA was as effective an inducer of this mRNA as it is in euthyroid rats. As seen in euthyroid rats, T3 administration potently suppressed DHEA induction of CYP4A2 mRNA levels under either basal or induced conditions. Although CYP4A expression was not derepressed in liver or kidneys of hypothyroid animals, our results indicated that the thyroid status of the animal did affect basal expression of CYP4A2, suggesting involvement of thyroid hormone or some other factor regulated by the thyroid gland on its constitutive expression.