DIFFERENTIAL EFFECT OF GENISTEIN ON THE STIMULATION OF CHOLESTEROL PRODUCTION BY BASIC-PROTEIN-II IN NORMAL AND HYPERAPOB FIBROBLASTS

We studied further the basis for the abnormal effect of human serum ba sic protein II (BP II) on cholesterol production in hyperapobetalipopr oteinemia (hyperapoB) fibroblasts and whether this effect involves pro tein tyrosine kinase phosphorylation (TKP). Genistein, a specific inhi bitor of TKP was used as a probe. Compared with normal cells, BP II st imulated significantly the cellular mass of total cholesterol (6.4-fol d), unesterified cholesterol (3.6-fold), and esterified cholesterol (6 .7-fold) in hyperapoB fibroblasts. The addition of genistein to BP II in hyperapoB cells markedly inhibited these abnormal stimulatory effec ts of BP II on cell. sterol mass. In normal cells, the addition of gen istein to BP Il produced an opposite effect: a marked stimulation in t he mass of total (5.5-fold) and esterified (18.3-foId) cholesterol and a decrease in unesterified cholesterol (3.4-fold). These effects of g enistein on the formation of cellular cholesterol by BP II were both t ime and concentration dependent. The inhibition of the stimulatory eff ect of BP II on cholesterol production by genistein in hyperapoB cells may be mediated through 3-hydroxy 3-methylglutaryl coenzyme A reducta se, the rate-limiting enzyme of cholesterol biosynthesis, since the ra te of incorporation of [C-14]acetate, but not [H-3]mevalonate, into un esterified cholesterol was decreased by genistein in the hyperapoB cel ls. When the mass of cell total cholesterol in the cells treated with BP II was subtracted from those treated with BP II plus genistein, a n egative number was produced in each of the six hyperapoB cell lines, w hile each of the normal cell lines retained a positive number. The mea n difference for the mass of total cholesterol between the hyperapoB a nd normal fibroblasts under these conditions was 128.2 nmol/mg cell pr otein, a difference that was separated by >3 SD. This study supports f urther the tenet that there is a defect in the response of hyperapoB c ells to BP II and that this defect results in an abnormality in choles terol metabolism that appears mediated through a protein TKP-mediated process.