Role of steroid sulfatase in local formation of estrogen in post-menopausal breast cancer patients.

More than two-thirds of breast cancers occur in post-menopausal women, and depend on the estrogens for their proliferation and survival. For the treatment of estrogen-dependent breast cancers, two major treatment options are now available. One is selective estrogen receptor modulator (SERM) such as Tamoxifen and another is aromatase inhibitor such as Anastrozole, Letrozole and Exemestane, which reduce local in situ formation of estrogens. Although these therapies are clinically active for advanced and early breast cancers, de novo and/or acquired resistance to SERM and/or aromatase inhibitors are also clinical problem. Recent studies suggest that local formation of estrogens in the breast tumors is more important than circulating estrogen in plasma for the growth and survival of estrogen-dependent breast cancer in post-menopausal women. The rationale for the importance of local formation of estrogens is based on the following evidences. Estradiol (E2) levels in breast tumors are equivalent to those of pre-menopausal patients, although plasma E2 levels are 50-fold lower after menopause. E2 concentrations in breast tumors of post-menopausal women are 10-40 times higher than serum level. Biosynthesis of estrogens in breast tumors tissues occurs via two major different routes, one is aromatase pathway and another is steroid-sulfatase (STS) pathway. Whereas many studies has been reported about aromatase inhibitor and its clinical trial results in breast cancer patients, limited information are available regarding to other estrogen regulating enzymes including STS, its role in breast tumors and STS inhibitors. STS is the enzyme that hydrolyses estrone 3-sulfate (E1S) and dehydroepiandrosterone-sulfate (DHEA-S) to their active un-sulfoconjugated forms, thereby stimulating the growth and survival of estrogen-dependent breast tumors. It has been well known that E1S level are much higher than E2 level both in plasma and tumor of post-menopausal patients. Recent reports show that more than 80% of breast tumors are stained with anti-STS antibody and the expression of STS is an independent prognostic factor in breast cancer. Taking these findings into consideration, local formation of estrogens could be partially synthesized from large amount of E1S by STS, which exist in breast cancer. On the other hand, aromatase localizes in stroma and adipocyte surrounding breast cancer. Furthermore, since estrogen formation from E1S and DHEA-S (STS pathway) cannot be blocked by aromatase inhibitors, STS is thought to be a new molecular target for the treatment of estrogen-dependent tumor post-SERM and/or aromatase inhibitors. In this symposium, these recent rationale for the importance of STS in post-menopausal breast cancer patients is reviewed as well as STS inhibitor.