Regulation of rat and bovine adrenal metabolism of polycyclic aromatic hydrocarbons by adrenocorticotropin and 2,3,7,8-tetrachlorodibenzo-p-dioxin

Developmental expression and regulation of adrenocortical cytochrome P4501B1 in the rat

A 57-kDa protein whose expression in rat adrenocortical microsomes is increased after weaning has been identified as cytochrome P4501B1 (CYP1B1). Levels of CYP1B1 protein were moderately expressed in late gestation fetuses and on postnatal day 1 (pdl), but were nearly undetectable on pd6 and pd1O. CYP1B1 expression initially increased in the late preweaning period (pd17-19) and again immediately postweaning (pd21-24). The temporal coincidence of CYP1B1 expression and weaning was not due to transition from suckling to solid food, as neonates that were prematurely weaned showed no increase in adrenal CYP1B1 compared with normally weaned littermates. The pattern of CYP1B1 expression paralleled changes in microsomal metabolism of 7,12-dimethylbenz[a]anthracene (DMBA), a marker of CYP1B1 activity. Twice daily injections of ACTH to rat pups (pd3-10) failed to significantly increase the expression of CYP1B1 in pd 10 adrenals, although the injections weakly stimulated steroidogenesis. Adrenocortical cells from pd17 neonates and adult cells, when cultured for 3 days, responded similarly to ACTH induction, although neonates showed more than 4-fold less basal activity. It is concluded that rat adrenal CYP1B1 may be developmentally suppressed, and its expression is independent of diet or the presence of a dam. This suppression is retained in cell culture, but is not due to deficient ACTH signaling. These results may explain the reported resistance of neonatal rat adrenals to the toxic effects of polycyclic aromatic hydrocarbons, which are metabolized by CYP1B1 into mutagenic by-products.

Cellular localization of cytochrome(s) P-450 metabolizing polycyclic aromatic hydrocarbons in the rat adrenal cortex

Cells were dispersed from the capsular, as well as the inner portion of female rat adrenal glands and subsequently separated on discontinuous Percoll gradients. The adrenal cells were distributed within a density interval ranging from 1.016 to 1.075 g/cm3 and different subpopulations showed distinct morphological appearances in suspension, as well as in culture. The total cells from the inner portion of the adrenals metabolized [14C]7,12-dimethylbenz(a)anthracene at a rate of 4.04 pmol/min 10(6) cells and synthesized corticosterone in response to ACTH stimulation at a rate of 1.07 micrograms/hr/10(6) cells. These activities were 4- and 2.5-fold higher, respectively, than the corresponding activities in cells isolated from the capsular portion. 7,12-Dimethylbenz(a)anthracene monoxygenase activity and ACTH-stimulated steroidogenesis were enriched in two subpopulations of cells obtained on the Percoll gradient and were estimated to be 13.1 pmol/min/10(6) cells and 3.21 micrograms/hr/10(6) cells, respectively, in the most active fraction (at the 1.034/1.040 g/cm3 interface). On the basis of cellular morphology, density and steroidogenic properties, it was concluded that adrenal 7,12-dimethylbenz(a)anthracene monoxygenase activity is localized mainly in the cells of the zona fasciculata.

Metabolism and toxicity of xenobiotics in the adrenal cortex, with particular reference to 7,12-dimethylbenz(a)anthracene

The adrenal cortex contains high amounts of detoxifying enzymes, as well as generators and protectors of reactive oxygen species. The high content of cytochrome P-450 enzymes in the adrenal cortex together with its remarkable tendency to accumulate hydrophobic substances probably contributes to the extraordinary vulnerability of the gland to a number of xenobiotics. The best studied adrenocorticolytic compounds are the potent carcinogen 7,12-dimethylbenz(a)anthracene (DMBA) and its liver metabolite 7-hydroxymethyl-12-methylbenz(a)anthracene (7-OHM-12-MBA). Adrenocorticolysis generated by these agents in vivo as well as in vitro demonstrates high regioselective requirements and is strongly influenced by the presence of ACTH, steroids, cytochrome P-450 inhibitors and antioxidants. Furthermore, 7-OHM-12-MBA has been demonstrated to uniquely generate selective and massive oxidation of mitochondrial glutathione in cultured rat adrenal cells. The DMBA-induced adrenocorticolysis is thoroughly discussed in this review with particular emphasis on the metabolism of DMBA and the influence of various effectors. A working hypothesis involving a possible peroxidative mechanism is also presented.

Steroid and xenobiotic effects on the adrenal cortex: mediation by oxidative and other mechanisms

Because steroids reach high concentrations within the adrenal cortex, effects of the direct interaction of steroids and cytochrome P450 enzymes are possible and may involve oxidative damage. Steroid pseudosubstrate effects studied in cultured adrenocortical cells show that these effects are probably not mediated by steroid receptors. Release of oxidants during pseudosubstrate interaction with cytochrome P450s may be responsible for loss of enzymatic activity observed; enzyme activity can be protected by cytochrome P450 inhibitors, antioxidants, and lowered oxygen concentration. There may be pathological effects of pseudosubstrates in the adrenal cortex. Cytochrome P450/pseudosubstrate effects could be involved in the aging and death of adrenocortical cells in vivo, and necrosis of the adrenal cortex due to excessive ACTH stimulation or due to the action of adrenolytic chemicals could result from damage by oxygen radicals originating from cytochrome P450s. The possible mechanism of damage to the adrenal cortex by the xenobiotics dimethylbenzanthracene, TCDD, 3-methylcholanthrene, and o', p'-DDD are reviewed.

Expression and function of three cytochrome P-450 isozymes in rat extrahepatic tissues

Western blots using a polyclonal and a monoclonal antibody raised against rat liver cytochrome P-450b indicate tissue-specific expression of low levels of cytochrome P-450's b and e. P-450b and P-450e were expressed very selectively in, respectively, lung and adrenal microsomes of untreated rats but neither isozyme was detected in the corresponding kidney or small intestine microsomes. The regioselectivity of microsomal metabolism of 7,12-dimethylbenz[a]anthracene (DMBA) as well as the sensitivity to inhibition by anti P-450b/e IgG established that low levels of "b-like" P-450's are functional in lung and adrenal microsomes from uninduced rats, but not in microsomes from the kidney or small intestine. Functional P-450c was also detected at low levels in liver, lung, kidney, and adrenals of untreated rats. Among the extrahepatic tissues examined, DMBA metabolism was the highest in rat adrenal microsomes. However, only 30% of this activity was due to P-450's b, e, or c. Phenobarbital (PB) treatment of rats increased microsomal DMBA metabolism in all extrahepatic tissues examined. The selectivity of this increase for 12-methyl hydroxylation of DMBA and the near complete inhibition by anti-P-450b/e are consistent with induction of P-450e even though P-450b was preferentially induced in each of the extrahepatic tissues examined. The levels of expression of P-450b were increased by PB in all sets of adrenal, lung, and intestinal microsomes and in three out of six sets of kidney microsomes. The levels of P-450e were also increased by PB in all sets of adrenal microsomes. Following PB treatment, P-450e became immunoquantifiable (greater than 2 pmol/mg protein) in three of six sets of lung and kidney microsomes but remained below detection in all sets of intestinal microsomes. Based on the activity of purified P-450e, undetectable levels (less than 1 pmol/mg protein) could account for increased DMBA metabolism in this tissue. The high constitutive level of P-450b in the lung (approximately 40 pmol/mg), was remarkably inactive in DMBA metabolism and was only slightly increased by PB treatment (50%). In contrast, PB treatment caused a 2.5- to 10-fold increase in 12-methyl hydroxylation of DMBA that was highly sensitive to anti-P-450b/e. A protein comigrating with P-450e was well above detection (6-7 pmol/mg) in two of six preparations of lung microsomes that showed highest induction of this activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Altered regulation of adrenal steroidogenesis in 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated rats

A single treatment of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (50 micrograms/kg) produced two distinct effects on adrenal steroidogenesis in rats 13 days post-treatment. In unstressed rats, the very low corticosterone levels early in the light phase (AM) increased 4-fold relative to ad libitum-fed control (ALC) rats, but the peak level of corticosterone that is seen late in the light phase (PM) decreased up to 40% relative to ALC rats. The AM stimulation was also observed in rats pair-fed to compensate for the diminished feed intake of TCDD-treated animals, indicating that the change results from nutritional deprivation. The PM suppression, however, was not observed in pair-fed rats. In rats given a lower dose of TCDD (15 micrograms/kg), there was no AM stimulation, whereas the suppression of the PM diurnal peak of corticosterone was retained. Plasma adrenocorticotropin (ACTH) levels and adrenal size were not changed by these treatments, indicating that TCDD affects adrenal responsiveness. TCDD did not, however, have a significant effect on corticosterone secretion in rats receiving high doses of ACTH. In control animals, the availability of cholesterol to cytochrome P-450scc limits the rate of steroidogenesis. While the specific content of the cytochrome was unaffected by TCDD, cholesterol turnover by this enzyme appeared to be affected following TCDD treatment, as evidenced by small increases in the mitochondrial levels of free cholesterol, reactive cholesterol, and in the proportion of P-450scc complexed with cholesterol relative to both ad libitum- and pair-fed controls. This accumulation of mitochondrial cholesterol following TCDD treatment is consistent with an inhibition of cholesterol metabolism at cytochrome P-450scc in vivo that is removed upon isolation of the mitochondria. These TCDD-induced increases were enhanced substantially in ACTH-stimulated rats, probably because ACTH enhances cholesterol influx into the mitochondria. Normally, substrate availability is rate limiting in cholesterol side-chain cleavage, and the AM stimulation of steroidogenesis by TCDD may result from such increased cholesterol transfer. The inhibition of cholesterol side-chain cleavage resulting from TCDD treatment may, however, only become rate limiting for corticosterone synthesis when cholesterol transfer is more substantially activated, as for peak PM secretion.