Constitutive expression of the steroid sulfatase gene supports the growth of MCF-7 human breast cancer cells in vitro and in vivo

Many human breast tumors are driven by high intratumor concentrations of 17 beta -estradiol that appear to be locally synthesized. The role of aromata se is well established, but the possible contribution of the steroid sulfat ase (STS), which liberates estrogens from their biologically inactive sulfa tes, has been inadequately assessed and remains unclear. To evaluate the ro le of STS further, we transduced estrogen-dependent MCF-7 human breast canc er cells with a retroviral vector directing the constitutive expression of the human STS gene. Gene integration was confirmed by Southern hybridizatio n, production of the appropriately sized messenger RNA by Northern hybridiz ation, and expression of functional protein by metabolism of [H-3]estrone s ulfate to [H-3]estrone. Maximum velocity estimates of estrone formation are 64.2 pmol estrone/mg protein(.)h in STS-transduced cells (STS Clone 20), l evels comparable to those seen in some human breast tumors. Lower levels of endogenous activity are seen in MCF-7 cells (13.0 pmol estrone/mg protein h) and in cells transduced with vector lacking the STS gene (Vector 3 cells ; 12.0 pmol estrone/mg protein h). 17 beta -Estradiol sulfate induces expression of the progesterone receptor messenger RNA only in STS Clone 20 cells, whereas estrone sulfate produces the greatest stimulation of anchorage-independent growth in these cells. ST S Clone 20 cells retain responsiveness to antiestrogens, which block the ab ility of estrogen sulfate to increase the proportion of cells in both the S and G(2)/M phases of the cell cycle. Consistent with these in vitro observ ations, only STS Clone 20 cells exhibit a significant increase in the propo rtion of proliferating tumors in nude ovariectomized mice supplemented with 17 beta -estradiol sulfate. The primary activity in vivo appears to be fro m intratumor STS, rather than hepatic STS. Surprisingly, 17 beta -estradiol sulfate appears more effective than 17 beta -estradiol when both are admin istered at comparable concentrations. This effect, which is seen only in ST S Clone 20 cells, may reflect differences in the cellular pharmacology of e xogenous estrogens compared with those released by the activity of intracel lular STS. These studies directly demonstrate that intratumor STS activity can support estrogen-dependent tumorigenicity in an experimental model and may contribute to the promotion of human breast tumors.