Regulation of aromatase activity within the breast.

Local oestrogen biosynthesis within the breast can be highly variable, in vitro aromatase activity both in breast cancers and mammary adipose tissue displaying over a 40-fold range between the highest and lowest levels. Evidence is presented to show that: (i) transcriptional activity may influence oestrogen biosynthesis within breast cancers in that both aromatase mRNA and STAT nuclear binding are correlated positively to in vitro aromatase activity; (ii) the local presence of cancer may enhance aromatase activity in particulate fractions and primary fibroblast cultures from mammary adipose tissue; (iii) tumour extracts and breast cyst fluids may induce aromatase in cultured fibroblasts, the active principles responsible for these effects being incompletely defined (although the combination of interleukin (IL)-6 and its soluble receptor dramatically enhances aromatase activity, it is unclear whether this particular cytokine system can account for the stimulatory effects of breast extracts and fluids); (iv) the aromatase activities in both breast cancer and adipose tissues are susceptible to classical aromatase inhibitors such as aminoglutethimide and 4-hydroxyandrostenedione (and to newer inhibitors such as CGS16949 and CGS20267 at low nanomolar concentrations) but reduced sensitivity to 4-hydroxyandrostenedione may be observed in certain breast cancers. These findings may have important implications for the development and progression of hormone-dependent cancers within the breast.