Characterization of 5α-reductase activity and isoenzymes in human abdominal adipose tissues

• The SRD5A3 isoenzyme is the most highly expressed 5α-reductase in human abdominal adipose tissues.
• Little influence of adipocyte differentiation is seen on SRD5A1 and SRD5A3 mRNA expression.
• 4-dione is the main local source for DHT formation in human preadipocyte cultures.
• 5α-reductase isoenzymes may play a role in the regulation of adipogenesis.

IntroductionThe substrate for the generation of 5α-dihydrotestosterone (DHT) is either androstenedione (4-dione) which is first converted to androstanedione and then to DHT through 17-oxoreductase activity, or testosterone, which is directly converted to DHT. Three 5α-reductase isoenzymes have been characterized and designated as types 1, 2 and 3 (SRD5A1, 2 and 3).ObjectiveTo define the predominant source of local DHT production in human adipose tissues, identify 5α-reductase isoenzymes and test their impact on preadipocyte differentiation.MethodsCultures of omental (OM) and subcutaneous (SC) preadipocytes were treated for 0, 6 or 24 h with 30 nM 14C-4-dione or 14C-testosterone, with and without 500 nM 5α-reductase inhibitors 17-N,N-diethylcarbamoyl-4-methyl-4-aza-5-androstan-3-one (4-MA) or finasteride. Protein level and mRNA abundance of 5α-reductase isoenzymes/transcripts were examined in whole SC and OM adipose tissue. HEK-293 cells stably transfected with 5α-reductase type 1, 2 or 3 were used to test 5α-reductase inhibitors. We also assessed the impact of 5α-reductase inhibitors on preadipocyte differentiation.ResultsOver 24 h, DHT formation from 4-dione increased gradually (p < 0.05) and was significantly higher compared to that generated from testosterone (p < 0.001). DHT formation from both 4-dione and testosterone was blocked by both 5α-reductase inhibitors. In whole adipose tissue from both fat compartments, SRD5A3 was the most highly expressed isoenzyme followed by SRD5A1 (p < 0.001). SRD5A2 was not expressed. In HEK-293 cells, 4-MA and finasteride inhibited activity of 5α-reductases types 2 and 3 but not type 1. In preadipocyte cultures where differentiation was inhibited by 4-dione (p < 0.05, n = 7) or testosterone (p < 0.05, n = 5), the inhibitors 4-MA and finasteride abolished these effects.ConclusionAlthough 4-dione is the main source of DHT in human preadipocytes, production of this steroid by 5α-reductase isoenzymes mediates the inhibitory effect of both 4-dione and testosterone on preadipocyte differentiation.