Webb S J, Xiao G H, Geoghegan T E, Prough R A
Department of Biochemistry, School of Medicine, University of Louisville, Kentucky 40292, USA.
Mol Pharmacol. 1996 Feb;49(2):276-87.
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.
脱氢表雄酮（DHEA）在给予药理剂量时是一种过氧化物酶体增殖剂，但在基于细胞的试验中尚未显示其通过过氧化物酶体增殖物激活受体发挥作用。由于甲状腺激素/维生素A和D核受体亚家族的成员，包括PPAR，已知通过异源二聚化在基因表达中相互调节彼此的功能，我们试图确定DHEA和甲状腺激素是否相互作用以调节与过氧化物酶体增殖相关的几种肝和肾酶，即过氧化物酶体β-氧化、微粒体NADPH：细胞色素P450氧化还原酶和细胞色素P450 4A。在给予外源性T3以达到甲状腺功能亢进状态的大鼠中，DHEA对CYP4A的三种同工酶（4A1、4A2和4A3）的诱导在mRNA水平上被抑制>60-80%，其中CYP4A2 mRNA的诱导被完全抑制。核转录分析表明这种抑制作用在转录水平上受到调节。在任何测试条件下，DHEA或过氧化物酶体增殖剂萘芬诺酮对肝过氧化物酶体β-氧化的诱导在很大程度上不受T3处理动物的影响。微粒体NADPH：细胞色素P450氧化还原酶活性可被DHEA或T3诱导；联合处理导致相加诱导。当用较低剂量的外源性T3处理动物使其轻度甲状腺功能亢进时，与甲状腺功能正常的对照动物相比，仅DHEA或萘芬诺酮对肝CYP4A2 mRNA的诱导被显著抑制（>80%）。通过切除甲状腺使动物甲状腺功能减退，其肝脏中DHEA对CYP4A基因的诱导正常，这表明DHEA对其的诱导不依赖于甲状腺激素的存在。在DHEA存在的情况下，给甲状腺切除的大鼠给予外源性T3可在mRNA和蛋白质水平上有效抑制肝脏中所有三个基因的诱导。在培养的大鼠肝细胞实验中，生理浓度的T3可有效抑制萘芬诺酮对CYP4A2 mRNA水平的诱导，但对CYP4A1或4A3 mRNA的诱导影响很小。在较高的T3浓度下，CYP4A1/4A3 mRNA和蛋白质的诱导也受到抑制。这些结果表明，T3在生理相关浓度下在转录水平上调节CYP4A2的表达，但需要甲状腺功能亢进状态来抑制CYP4A1/4A3基因的表达。在甲状腺功能正常的啮齿动物肾脏中，在基础或DHEA诱导条件下仅表达CYP4A2，接近生理水平的T3可有效抑制DHEA或萘芬诺酮对过氧化物酶体增殖剂依赖性CYP4A2 mRNA水平的诱导。在甲状腺切除的大鼠中，CYP4A2 mRNA的基础表达相对于甲状腺功能正常的对照降低，但DHEA对该mRNA的诱导与在甲状腺功能正常的大鼠中一样有效。如在甲状腺功能正常的大鼠中所见，T3给药在基础或诱导条件下均有效抑制DHEA对CYP4A2 mRNA水平的诱导。尽管甲状腺功能减退动物的肝脏或肾脏中CYP4A的表达未被解除抑制，但我们的结果表明动物的甲状腺状态确实影响CYP4A2的基础表达，提示甲状腺激素或受甲状腺调节的其他一些因素参与其组成性表达。
