In situ aromatization enhances breast tumor estradiol levels and cellular proliferation

The high concentrations of estradiol (E₂) found in breast tumors of postmenopausal women could be the result of enhanced uptake from plasma or in situ aromatization of androgens to estrogens. To test the relative importance of these two mechanisms, a model system allowing precise distinction between each is required. Such a model was established using aromatase (A+)- and sham (A-)-transfected MCF-7 cells inoculated into ovariectomized (OVX) nude mice. To validate the model, the confounding effect of peripheral aromatization was first excluded experimentally. A- cells were inoculated into OVX mice as homoimplants (Acells on both flanks) or heteroimplants (A- cells on one flank and A+ cells on the other), and growth of A- cells in response to exogenous aromatase substrate, androstenedione (Δ⁴A), was evaluated. A- cells did not grow in either group during the 8 weeks of observation, indicating the lack of peripheral aromatization in OVX mice. The biological effects of in situ aromatization were then directly examined. We found that A+ cells in the heteroimplant group grew rapidly, and that the average weight of A+ tumor was 7.6-fold larger and tissue E₂ concentration was 3-4-fold higher than A- tumors grown in the same animals. These results demonstrate that in situ aromatization rather than uptake can be a determinant of tumor E₂ content and growth stimulation. An additional experiment was then designed to evaluate the relative importance of in situ synthesis versus uptake under conditions reflecting postmenopausal physiology. Groups of OVX mice bearing A+ cells received E₂ Silastic implants to clamp plasma levels at 5, 7, 10, and 20 pg/ml or Δ⁴A by injection. The highest tumor E₂ concentration and growth rate were found in the group receiving Δ⁴A. E₂ delivered by Silastic implants always produced lower tissue E₂ levels and tumor growth rates than resulted from in situ synthesis. These data provide direct evidence that under physiological conditions reflecting those in postmenopausal women, in situ aromatization in breast tumor makes a major contribution to tissue E₂ content. As further validation that our experimental paradigm models the postmenopausal state, we studied OVX animals not given Δ⁴A as substrate. A+ cells also grew under these conditions, and the aromatase inhibitor 4-hydroxyandrostenedione reduced both tumor E₂ level and growth rate, providing additional evidence of the importance of in situ synthesis. These studies provide the first direct evidence that in situ synthesis of E₂ in breast tumors, as opposed to peripheral aromatization and uptake from plasma, can enhance tissue E₂ levels and stimulate tumor growth.