Alex Odermatt, David J. Morris
Historically, it was thought that primary steroids released from endocrine glands exert their hormonal effects through corresponding receptors in peripheral tissues, and that their metabolism then inactivates them, followed by excretion. However, the metabolism of primary steroids is not just a way of inactivating and excreting them, but generates a variety of metabolites with different biological properties. In this review, we outline how various active steroid metabolites were discovered, describe some of the ways they are generated, and how they can in a non-classical way act on receptors or alter the activity of steroid metabolizing enzymes, thereby indirectly affecting receptor activities. Examples include the 5α-reduced ring-A metabolites of 11-deoxycorticosterone (DOC) and progesterone that are formed in the brain, act as neurosteroids and exert effects through the GABA-A membrane receptor. Another example is 11-ketoprogesterone that potently activates mineralocorticoid receptors (MR), but not glucocorticoid receptors (GR), and is more potent than its 11β-hydroxylated form, in contrast to glucocorticoids. Moreover, we discuss the microbiome as important source of bioactive metabolites, exemplified by the 11β-hydroxylated 5α-reduced ring-A corticosteroid and progesterone metabolites that were shown as potent 11β-hydroxysteroid dehydrogenase 2 (11β-HSD2) inhibitors. 11β-HSD2 inhibition results in cortisol-induced MR activation, sodium retention and hypertension. Furthermore, microbial 17,20-desmolase activity can convert glucocorticoids to androgens, potentially influencing diseases and therapeutic outcomes. There are still many knowledge gaps regarding bioactive steroid metabolites. Identifying additional bioactive steroid metabolites and characterizing their genomic and non-genomic effects should help uncovering their cell-specific functions and contributions to the maintenance of homeostatic regulation.