It is well documented that human breast is
actively involved in the local formation of estrogens. To
determine the site(s) of action of enzymes involved in
synthesis and metabolism of the most potent estrogen
estradiol (E2), we have studied the expression of the
following enzymes: 3ß-hydroxysteroid dehydrogenase
(3-HSD), 17ß-HSD types 1, 2, 5, 7 and 12, aromatase,
steroid sulfatase (STS) and estrogen sulfotransferase
(EST) 1E1 at the cellular level in breast. Both in situ
hybridization and immunocytochemistry were used for
enzyme localization in normal breast tissues. For
immunocytochemistry, we used rabbit antibodies, while
in situ hybridization studies were performed using (35S)-
labeled cRNA probes. Similar results were obtained with
both approaches. All the enzymes (3ß-HSD; 17ß-HSD
types 1, 5, 7 and 12; aromatase) involved in the
conversion of circulating dehydroepiandrosterone
(DHEA) to E2 as well as STS which converts estradiol
sulfate (E2-S) to E2 have been found to be expressed in
epithelial cells of acini and/or ducts as well as the
stromal cells. Moreover, 17ß-HSD type 2 and EST1E1,
two enzymes which inactivate E2, have been also
localized in the same cell types. The present results
indicate the enzymes which play a role in the synthesis
and metabolism of E2 are expressed in both epithelial
and stromal cells in human breast