Fadrozole and finasteride exposures modulate sex steroid- and thyroid hormone-related gene expression in Silurana (Xenopus) tropicalis early larval development

Steroidogenic enzymes and their steroid products play critical roles during gonadal differentiation in amphibians; however their roles during embryogenesis remain unclear. The objective of this study was to investigate the expression and activity of aromatase (cyp19; estrogen synthase) and 5β-reductase (srd5beta; 5β-dihydrotestosterone synthase) during amphibian embryogenesis. Expression and activity profiles of cyp19 and srd5beta were first established during Silurana (Xenopus) tropicalis embryogenesis from Nieuwkoop–Faber (NF) stage 2 (2-cell stage; 1 h post-fertilization) to NF stage 46 (beginning of feeding; 72 h post-fertilization). Exposures to fadrozole (an aromatase inhibitor; 0.5, 1.0 and 2.0 μM) and finasteride (a putative 5-reductase inhibitor; 25, 50 and 100 μM) were designed to assess the consequences of inhibiting these enzymes on gene expression in early amphibian larval development. Exposed embryos showed changes in both enzyme activities and sex steroid- and thyroid hormone-related gene expression. Fadrozole treatment inhibited cyp19 activity and increased androgen receptor and thyroid hormone receptor (α and β) mRNAs. Finasteride treatment inhibited srd5beta (activity and mRNA), decreased cyp19 mRNA and activity levels and increased estrogen receptor α mRNA. Both treatments altered the expression of deiodinases (thyroid hormone metabolizing enzymes). We conclude that cyp19 and srd5beta are active in early embryogenesis and larval development in Silurana tropicalis and their inhibition affected transcription of genes associated with the thyroid and reproductive axes.