REGULATION OF SEX HORMONE-BINDING GLOBULIN PRODUCTION BY ISOFLAVONOIDS AND PATTERNS OF ISOFLAVONOID CONJUGATION IN HEPG2 CELL-CULTURES

The effect of the isoflavonoid phytoestrogens daidzein, equol, and gen istein on sex hormone-binding globulin (SHBG) levels, SHBG mRNA transc ript levels, and SHBG gene methylation was studied in HepG2 cell cultu res by fluoroimmunometric SHBG assay and Northern and Southern hybridi zations, respectively. The effect of 17 beta-estradiol on these parame ters was studied as a control. The metabolism of isoflavonoids in HepG 2 cells was determined by isotope dilution gas chromatography-mass spe ctrometry, after ion-exchange chromatography. Daidzein and equol incre ased SHBG levels in parallel intracellularly and extracellularly, wher eas genistein increased SHBG levels only within the cells, resembling thus the effect of 17 beta-estradiol. The difference may originate fro m the fact that genistein has move hydroxyl groups than daidzein and e quol. The regulation of SHBG production by phytoestrogens appears to o ccur at the post-transcriptional level. Firstly, daidzein, equol, or g enistein did not have a clear effect on the steady-state SHBG mRNA lev els. Secondly, no effect on SHBG gene methylation was observed by geni stein. The findings applied also to 17 beta-estradiol. However, as the SHBG gene was more methylated in SHBG-negative MCF-7 cells than in SH BG-positive HepG2 cells, DNA methylation may play a role in the tissue -specific activation of this gene. The metabolism of isoflavonoids in HepG2 cells yielded mainly unconjugated and sulfated compounds. Simila r metabolism in hepatocytes in vivo might retain their biological acti vity in tissues responsive to estrogens.