Differential activity and expression of human 5ß-reductase (AKR1D1) splice variants.

Steroid hormones, including glucocorticoids and androgens, exert a wide variety of effects in the body across almost all tissues. The steroid A-ring 5ß-reductase (AKR1D1) is expressed in human liver and testes, and three splice variants have been identified (AKR1D1-001, AKR1D1-002, AKR1D1-006). Amongst these, AKR1D1-002 is the best described; it modulates steroid hormone availability and catalyses an important step in bile acid biosynthesis. However, specific activity and expression of AKR1D1-001 and AKR1D1-006 are unknown. Expression of AKR1D1 variants were measured in human liver biopsies and hepatoma cell lines by qPCR. Their three-dimensional (3D) structures were predicted using in silico approaches. AKR1D1 variants were overexpressed in HEK293 cells, and successful overexpression confirmed by qPCR and Western blotting. Cells were treated with either cortisol, dexamethasone, prednisolone, testosterone or androstenedione, and steroid hormone clearance was measured by mass spectrometry. Glucocorticoid and androgen receptor activation were determined by luciferase reporter assays. AKR1D1-002 and AKR1D1-001 are expressed in human liver, and only AKR1D1-006 is expressed in human testes. Following overexpression, AKR1D1-001 and AKR1D1-006 protein levels were lower than AKR1D1-002, but significantly increased following treatment with the proteasomal inhibitor, MG-132. AKR1D1-002 efficiently metabolised glucocorticoids and androgens and decreased receptor activation. AKR1D1-001 and AKR1D1-006 poorly metabolised dexamethasone, but neither protein metabolised cortisol, prednisolone, testosterone or androstenedione. We have demonstrated the differential expression and role of AKR1D1 variants in steroid hormone clearance and receptor activation in vitro. AKR1D1-002 is the predominant functional protein in steroidogenic and metabolic tissues. In addition, AKR1D1-001 and AKR1D1-006 may have a limited, steroid-specific role in the regulation of dexamethasone action.

The A-ring reduction of 11-ketotestosterone is efficiently catalysed by AKR1D1 and SRD5A2 but not SRD5A1.

Testosterone and its 5α-reduced form, 5α-dihydrotestosterone, were previously thought to represent the only active androgens in humans. However, recent studies have shown that the potent androgen, 11-ketotestosterone, derived from the adrenal androgen precursor, 11ß-hydroxyandrostenedione, may in fact serve as the primary androgen in healthy women. Yet, despite recent renewed interest in these steroids, their downstream metabolism has remained undetermined. We therefore set out to investigate the metabolism of 11-ketotestosterone by characterising the 5α- or 5ß-reduction commitment step. We show that inactivation of 11-ketotestosterone is predominantly driven by AKR1D1, which efficiently catalyses the 5ß-reduction of 11-ketotestosterone, committing it to a metabolic pathway that terminates in 11-ketoetiocholanolone. We demonstrate that 5α-reduction of 11-ketotestosterone is catalysed by SRD5A2, but not SRD5A1, and terminates in 11-ketoandrosterone, but is only responsible for a minority of 11-ketotestosterone inactivation. However, as 11-ketoetiocholanolone is also generated by the metabolism of the glucocorticoid cortisone, 11-ketoandrosterone should be considered a more specific urinary marker of 11-ketotestosterone production.