Glucocorticoids are differentially synthesized along the murine and human respiratory tree

Background Synthetic glucocorticoids (GC) are effective in the treatment of inflammatory diseases of the lung. However, long‐term use leads to severe side effects. Endogenous GC can be synthesized locally, either de novo from cholesterol in a 11β‐hydroxylase (Cyp11b1)‐dependent manner, or by reactivation from 11‐dehydrocorticosterone/cortisone by 11β‐hydroxysteroid dehydrogenase 1 (Hsd11b1). We aimed to define the molecular pathways of endogenous GC synthesis along the respiratory tree to provide a basis for understanding how local GC synthesis contributes to tissue homeostasis. Methods Expression of steroidogenic enzymes in murine lung epithelium was analyzed by macroscopic and laser capture microdissection, followed by RT‐qPCR. Flow cytometry analysis was performed to identify the cellular source of steroidogenic enzymes. Additionally, the induction of steroidogenic enzyme expression in the lung was analyzed after lipopolysaccharide (LPS) injection. mRNA and protein expression of steroidogenic enzymes was confirmed in human lung tissue by RT‐qPCR and immunohistochemistry. Furthermore, GC synthesis was examined in ex vivo cultures of fresh tissue from mice and human lobectomy patients. Results We observed that the murine and human lung tissue differentially expresses synthesis pathway‐determining enzymes along the respiratory tree. We detected Hsd11b1 expression in bronchial, alveolar, club and basal epithelial cells, whereas Cyp11b1 expression was detectable only in tracheal epithelial cells of mice. Accordingly, de novo synthesis of bioactive GC occurred in the large conducting airways, whereas reactivation occurred everywhere along the respiratory tree. Strikingly, Cyp11b1 but not Hsd11b1 expression was enhanced in the trachea upon LPS injection in mice. Conclusion We report here the differential synthesis of bioactive GC along the murine and human respiratory tree. Thus, extra‐adrenal de novo GC synthesis and reactivation may differentially contribute to the regulation of immunological and inflammatory processes in the lung. The lung epithelium expresses steroidogenic enzymes and synthesizes GCs de novo from cholesterol as well as via reactivation from 11‐dehydrocorticosterone/cortisone. GC reactivation via 11β‐hydroxysteroid dehydrogenase 1 occurs mostly in the alveoli, whereas de novo GC synthesis via 11β‐hydroxylase predominantly in the large conducting airways. The concept of differential GC synthesis could be confirmed in murine as well as human