Germ cells: a new source of estrogens in the male gonad

The cytochrome P450 aromatase (P450arom) is a key enzyme responsible for the formation of estrogens from androgens and is present in the endoplasmic reticulum of various tissues. P450arom has been immunolocalized in Leydig cells of numerous species as well as in germ cells of mouse, bank vole and brown bear. Aromatase activity has been measured in vitro in immature and mature rat Leydig cells and Sertoli cells, whereas in pig, ram and humans the enzyme activity is only present in Leydig cells. In the mature rat testis we have used complementary approaches to demonstrate that not only somatic cells but also germ cells represent a new source of estrogens. In pachytene spermatocytes and Leydig cells, the amount of P450arom mRNA measured by a quantitative competitive RT-PCR method is 10-fold higher than in Sertoli cells. According to the stage of the germ cell maturation, the amount of aromatase transcripts decreases, being more elevated in younger than in mature rat germ cells. By contrast, the aromatase activity in the microsomal fractions is two- to four-fold greater in spermatozoa when compared to the two other enriched germ cell preparations used. We have immunolocalized the P450arom in elongated spermatids and spermatozoa. Moreover, we described the existence of alternative splicing events of P450arom mRNA in pachytene spermatocytes and round spermatids that are not likely to encode functional aromatase molecules. Therefore, the aromatase gene expression and its transduction in a fully active protein in rat germ cells evidences an additional site for estrogen production within the testis of some mammals. Taking into account the large distribution of estrogen receptors in the testicular cells, we begin to understand the physiological role of these female hormones in the male gonad.