New Insights in the Functional Zonation of the Canine Adrenal Cortex

Background Current understanding of adrenal steroidogenesis is that the production of aldosterone or cortisol depends on the expression of aldosterone synthase (CYP11B2) and 11β‐hydroxylase cytochrome P450 (CYP11B1), respectively. However, this has never been studied in dogs, and in some species, a single CYP11B catalyzes both cortisol and aldosterone formation. Analysis of the canine genome provides data of a single CYP11B gene which is called CYP11B2, and a large sequence gap exists near the so‐called CYP11B2 gene. Objectives To investigate the zonal expression of steroidogenic enzymes in the canine adrenal cortex and to determine whether dogs have 1 or multiple CYP11B genes. Animals Normal adrenal glands from 10 healthy dogs. Methods Zona fasciculata (zF) and zona glomerulosa (zG) tissue was isolated by laser microdissection. The mRNA expression of steroidogenic enzymes and their major regulators was studied with RT‐qPCR. Southern blot was performed to determine whether the sequence gap contains a CYP11B gene copy. Immunohistochemistry (IHC) was performed for 17α‐hydroxylase/17,20‐lyase (CYP17). Results Equal expression (P = .62) of the so‐called CYP11B2 gene was found in the zG and zF. Southern blot revealed a single gene. CYP17 expression (P = .05) was significantly higher in the zF compared with the zG, which was confirmed with IHC. Conclusions and Clinical Importance We conclude that there is only 1 CYP11B gene in canine adrenals. The zone‐specific production of aldosterone and cortisol is probably due to zone‐specific CYP17 expression, which makes it an attractive target for selective inhibition of cortisol synthesis without affecting mineralocorticoid production in the zG.