THE ROLE OF METABOLISM IN MAMMARY EPITHELIAL-CELL GROWTH-INHIBITION BY THE ISOFLAVONES GENISTEIN AND BIOCHANIN-A

The basis for the differential sensitivity of cultured normal human ma mmary epithelial (HME) cells and a transformed human breast cancer MCF -7 cell line to growth inhibition by the isoflavone genistein and its 4'-methyl ether derivative, biochanin A, was examined, In HME cells ge nistein is 5-fold more potent as a growth inhibitor than biochanin A, whereas in MCF-7 cells biochanin A and genistein are equally potent as growth inhibitors, Based on its properties as an in vitro protein tyr osine kinase (PTK) inhibitor, biochanin A would be expected to be a le ss potent growth inhibitor than genistein, To determine whether isofla vone metabolism could account for the observed differences in growth i nhibition, metabolism experiments were conducted with HME and MCF-7 ce lls using [4-C-14]genistein and [4-C-14]biochanin A. MCF-7 cells exten sively metabolized both isoflavones, producing two genistein metabolit es with molecular weights of 350 and 380 and three biochanin A metabol ites with molecular weights of 270, 350 and 380, In contrast, signific ant genistein or biochanin A metabolism was not observed in HME cells, Using mass spectrometry and nuclear magnetic resonance analysis, meta bolite 350 from genistein and biochanin A experiments was identified a s genistein 7-sulfate; biochanin A metabolite 270 was identified as ge nistein, Metabolite 380 was not unequivocally identified, but appeared to be a hydroxylated and methylated form of genistein sulfate, In MCF -7 cells, genistein 7-sulfate and metabolite 380 were detected primari ly in the cell media fraction, suggesting that once formed these polar metabolites were excreted from the cells, These data show that isofla vone metabolism by transformed breast epithelial cells modulates the g rowth inhibitory effects of genistein and biochanin A, In MCF-7 cells, genistein metabolism was correlated with a decrease in growth inhibit ion, whereas biochanin A metabolism was associated with an increase in growth inhibition.