Aromatase gene expression in the stallion

Adult stallion secretes very high estrogen levels in its testicular vein an d semen, and the responsible enzyme cytochrome P450 aromatase (P450 arom) i s known to be present mainly in Leydig cells. We studied in further details the distribution of equine aromatase in various adult tissues including th e brain (hypothalamic area), liver, kidney, small intestine, muscle, bulbou rethral gland and testes. The aromatase mRNA was essentially detected by RT -PCR in testis (169 +/- 14 amol of aromatase mRNA per mug of total RNA) and was barely detectable in brain, or below 0.1 amol/mug RNA in other tissues . This range of expression was confirmed by ELISA (50 +/- 7 pg/mug total pr otein) in the testis, and by immunoblot, evidencing a 53 kDA specific prote in band in testis and brain only. The corresponding aromatase activity was well detected, by (H2O)-H-3 release from 1 beta, 2 beta H-3-androstenedione . in testis and brain (200 +/- 23 and 25 +/- 6 pmol/min per mg, respectivel y) and below 3 pmol product formed;min per mg in other tissues. This study indicates that the testis, among the tissues analyzed, is the major source of aromatase in the adult stallion, and that the aromatase gene expression is specifically enhanced at this level, and is responsible for the high est rogen synthesis observed. Moreover, the study of aromatase in one colt test is has shown lower levels of transcripts, protein and enzyme activity, evid encing that aromatase is regulated during the development and may serve as a useful marker of testicular function. As the second organ where aromatase mRNA and activity are both well detected is brain, this study also underli nes the possible role of neurosteroids in stallion on behaviour, brain func tion or central endocrine control. (C) 2001 Elsevier Science Ireland Ltd. A ll rights reserved.