IN-SITU AROMATIZATION ENHANCES BREAST-TUMOR ESTRADIOL LEVELS AND CELLULAR PROLIFERATION

The high concentrations of estradiol (E-2) found in breast tumors of p ostmenopausal women could be the result of enhanced uptake from plasma or lit situ aromatization of androgens to estrogens, To test the rela tive importance of these two mechanisms, a model system allowing preci se 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 confo unding effect of peripheral aromatization was first excluded experimen tally, A- cells mere inoculated into OVX mice as homoimplants (A- cell s on both flanks) or heteroimplants (A- cells on one flank and A+ cell s on the other), and growth of A-cells in response to exogenous aromat ase substrate, androstenedione (Delta(4)A), was evaluated, A- cells di d not grow in either group during the 8 weeks of observation, indicati ng the lack of peripheral aromatization in OVX mice, The biological ef fects of ill situ aromatization were then directly examined, We found that A+ cells in the heteroimplant group grew rapidly, and that the av erage weight of A+ tumor was 7.6-fold Larger and tissue E-2 concentrat ion was 3-4-fold higher than A- tumors grown in the same animals, Thes e results demonstrate that in situ aromatization rather than uptake ca n be a determinant of tumor E-2 content and growth stimulation, An add itional experiment was then designed to evaluate the relative importan ce of iii situ synthesis versus uptake under conditions reflecting pos tmenopausal physiology, Groups of OVX mice bearing A+ cells received E -2 Silastic implants to clamp plasma levels at 5, 7, 10, and 20 pg/ml or Delta(4)A by injection, The highest tumor E-2 concentration and gro wth rate were found in the group receiving Delta(4)A. E-2 delivered by Silastic implants always produced lower tissue E-2 levels and tumor g rowth rates than resulted from ill situ synthesis, These data provide direct evidence that under physiological conditions reflecting those i n postmenopausal women, in situ aromatization in breast tumor makes a major contribution to tissue E-2 content, As further validation that o ur experimental paradigm models the postmenopausal state, we studied O VX animals not given Delta(4)A as substrate, A+ cells also grew under these conditions, and the aromatase inhibitor 4-hydroxyandrostenedione reduced both tumor E-2 level and growth rate, providing additional ev idence of the importance of in situ synthesis, These studies provide t he first direct evidence that in situ synthesis of E-2 in breast tumor s, as opposed to peripheral aromatization and uptake from plasma, can enhance tissue E-2 levels and stimulate tumor growth.