Differential effects of adrenocorticotropin in vivo on cytochromes P4502D16 and P450c17 in the guinea pig adrenal cortex

Functional Zonation of the Adult Mammalian Adrenal Cortex

The standard model of adrenocortical zonation holds that the three main zones, glomerulosa, fasciculata, and reticularis each have a distinct function, producing mineralocorticoids (in fact just aldosterone), glucocorticoids, and androgens respectively. Moreover, each zone has its specific mechanism of regulation, though ACTH has actions throughout. Finally, the cells of the cortex originate from a stem cell population in the outer cortex or capsule, and migrate centripetally, changing their phenotype as they progress through the zones. Recent progress in understanding the development of the gland and the distribution of steroidogenic enzymes, trophic hormone receptors, and other factors suggests that this model needs refinement. Firstly, proliferation can take place throughout the gland, and although the stem cells are certainly located in the periphery, zonal replenishment can take place within zones. Perhaps more importantly, neither the distribution of enzymes nor receptors suggest that the individual zones are necessarily autonomous in their production of steroid. This is particularly true of the glomerulosa, which does not seem to have the full suite of enzymes required for aldosterone biosynthesis. Nor, in the rat anyway, does it express MC2R to account for the response of aldosterone to ACTH. It is known that in development, recruitment of stem cells is stimulated by signals from within the glomerulosa. Furthermore, throughout the cortex local regulatory factors, including cytokines, catecholamines and the tissue renin-angiotensin system, modify and refine the effects of the systemic trophic factors. In these and other ways it more and more appears that the functions of the gland should be viewed as an integrated whole, greater than the sum of its component parts.

Adrenocortical zonation and ACTH

The clear morphological distinction between the cells of the different adrenocortical zones has attracted speculation and experiment to interpret their functions and the ways in which they are regulated. Considerable data have been produced in recent years that has benefited a fuller understanding of the processes of steroidogenesis and of cell proliferation at the molecular level. This now enables the reexamination of earlier concepts. It is evident that there is considerable species variation, and this article, dealing mainly with the rat, reaches conclusions that do not necessarily apply to other mammals. In the rat adrenal, however, the evidence suggests that the greatest differences between the functions of the zones are between the glomerulosa and the fasciculata. Here the sometimes all-or-nothing demarcation in their complement of components associated with steroidogenesis or with cell proliferation suggests a stark division of labor. In this model the fasciculata is the main engine of steroid hormone output and the glomerulosa is the site of cell proliferation, recruitment, and differentiation. Regulating these functions are angiotensin II and other paracrine components that modulate and maintain the glomerulosa, and ACTH, that maintains the fasciculata, and recruits new fasciculata cells by transformation of proliferating glomerulosa cells. Grafted onto this mostly vegetative function of the glomerulosa is CYP11B2, limited to just a fraction of the outer glomerulosa in rats on a normal laboratory diet and generating aldosterone (and 18-hydroxycorticosterone) from precursors whose origin is not, from the evidence summarized here, very clear, but may include the fasciculata, directly or indirectly. The biosynthesis of aldosterone in the rat certainly requires reinterpretation.

Effects of ACTH administration on zonation of the guinea pig adrenal cortex

Experiments were done to determine the actions of ACTH on the morphologic and functional characteristics of the zona fasciculata (ZF) and zona reticularis (ZR) in the guinea pig adrenal cortex. In control guinea pigs, a number of morphologic differences distinguished the ZF from the ZR, including the presence of far more lipid in the ZF than in the ZR. Treatment with ACTH decreased the lipid droplet content of the ZF cells, equalizing the amount of lipid in the two zones. Other morphologic differences between the ZF and ZR were also diminished by ACTH treatment. Immunohistochemical analyses indicated that CYP17 protein was found in both the ZF and ZR in control animals, but with greater immunostaining intensity in the ZF. The enzyme protein distribution corresponded with higher 17alpha-hydroxylase activity in the ZF than in the ZR. After ACTH treatment, the intensity of staining and enzyme activities in the two zones were similar, attributable largely to increases in the ZR. In situ hybridization-and immunohistochemistry showed that in control animals CYPD216 was highly expressed in the ZR but not in the ZF. ACTH treatment dramatically reduced the intensity of CYP2D16 mRNA and protein staining in the ZR. Bufuralol 1'-hydroxylase activity, a marker for CYP2D subfamily members, was also decreased significantly in the ZR by ACTH treatment. The data indicate that administration of ACTH to guinea pigs has opposite effects on the expression of CYP17 and CYP2D16 in the ZR, and diminishes or eliminates some of the structural and functional differences between the ZF and ZR. The results suggest a role for ACTH in establishing and maintaining adrenocortical zonation.

Cloning and characterization of the CYP2D1-binding protein, retinol dehydrogenase

A CYP2D1-binding protein, 29 k-protein (p29), has been isolated and its N-terminal amino acid sequence has been reported (Ohishi et al. (1993) Biochim. Biophys. Acta 1158, 227-236). In this study, p29 cDNA was isolated by PCR with oligonucleotide probes designed from the N-terminal amino acid sequence and p29 was found to be a microsomal retinol dehydrogenase, a member of the short-chain alcohol dehydrogenase family which metabolize hydroxysteroids and prostaglandins. CYP2D1 and p29 were expressed in Saccharomyces cerevisiae to characterize these proteins. CYP2D1 had an absorption maximum at 448 nm in a CO-reduced form. Expressed p29 in yeast cells was detected with anti-p29 antibody. Solubilized CYP2D1 and p29 from yeast microsomes were mixed and applied to an anti-CYP2D1 antibody-binding column. Both proteins were retained in the column and eluted with glycine buffer (pH 2.8). However, when applied alone, p29 was not retained in the column. The findings indicated that CYP2D1 bound tightly with p29. Catalytic activities of p29 expressed in yeast were investigated. p29 had retinal reductase activity in the presence of NADPH. Addition of CYP2D1 and NADPH-P450 reductase increased the retinal reductase activity of p29. These findings suggest that the complex of CYP2D1, p29, and NADPH-P450 reductase has an important role in the metabolism of retinoids.