Aromatase expression in male germ cells.

The cytochrome P450 aromatase (P450arom) is the terminal enzyme responsible for the formation of estrogens from androgens. According to the age, aromatase activity has been measured in immature and mature rat Leydig cells, as well as in Sertoli cells whereas in pig, ram and human the aromatase is mainly present in Leydig cells. In the rat testis, we have immunolocalised the P450arom not only in Leydig cells but also in germ cells and especially in elongated spermatids. Related to the stage of germ cell maturation, we have shown that the level of P450arom mRNA transcripts decreases, it is much more abundant in younger than in mature germ cells whereas the aromatase activity is two- to four-fold greater in spermatozoa when compared to the two other enriched-germ cell preparations. Moreover, we have reported the existence of alternative splicing events of P450arom mRNA in pachytene spermatocytes and round spermatids giving rise to two isoforms lacking the last coding exon which, therefore, cannot encode functional aromatase molecules. In rat germ cells, the aromatase gene expression is not only under androgen control but also subjected to cytokine (TNFalpha) and growth factor (TGFbeta) regulation. In the bank-vole testis, we have evidenced a synchronisation between a fully developed spermatogenesis and a strong positive immunoreactivity for both P450arom and estrogen receptor (ERbeta) in spermatids. Therefore, the aromatase gene expression and its translation in a fully active protein in rodent germ cells evidence an additional site for estrogen production within the testis. Our recent data showing that human ejaculated spermatozoa expressed specific transcripts for P450arom reinforced the observations reported in germ cells of other mammalian species. Together with the widespread distribution of ERs in testicular cells these data bring enlightenments on the hormonal regulation of male reproductive function. Indeed these female hormones (or the ratio androgens/estrogens) do play a physiological role (either directly on germ cells or via testicular somatic cells) in the maintenance of male gonadal functions and obviously, several steps are concerned particularly the spermatid production and the epididymal sperm maturation.