Soya phytoestrogens, genistein and daidzein, decrease apolipoprotein B secretion from HepG2 cells through multiple mechanisms

Diets containing the soya-derived phytoestrogens, genistein and daidzein, decrease plasma cholesterol in humans and experimental animals. The mechanisms responsible for the hypocholesterolaemic effects of these isoflavones are unknown. The present study was conducted to determine if genistein and daidzein regulate hepatocyte cholesterol metabolism and apolipoprotein (apo) B secretion in cultured human hepatoma (HepG2) cells. ApoB secretion was decreased dose-dependently by up to 63% and 71% by genistein and daidzein (100 muM; P<0.0001) respectively. In contrast, no effect on apoAI secretion was observed. Cellular cholesterol synthesis was inhibited 41% by genistein (100 mu M; P<0.005) and 18% by daidzein (100 muM; P<0.05), which was associated with significant increases in 3-hydroxy-3-methylglutaryl-CoA reductase mRNA. Cellular cholesterol esterification was decreased 56% by genistein (100 mu M; P<0.04) and 29% by daidzein (100 muM; P<0.04); however, mRNA levels for acyl-CoA: cholesterol acyltransferase (ACAT) 1 and ACAT2 were unaffected. At 100 mu M, both isoflavones equally inhibited the activities of both forms of ACAT in cells transfected with either ACAT1 or ACAT2. Genistein (100 mu M) and daidzein (100 mu M) significantly decreased the activity of microsomal triacylglycerol transfer protein (MTP) by 30% and 24% respectively, and significantly decreased MTP mRNA levels by 35% and 55%. Both isoflavones increased low-density lipoprotein (LDL)-receptor mRNA levels by 3- to 6-fold (100 mu M; P<0.03) and significantly increased the binding, uptake and degradation of I-125-labelled LDL, suggesting that enhanced reuptake of newly secreted apoB-containing lipoproteins contributed to the net decrease in apoB secretion. These results indicate that genistein and daidzein inhibit hepatocyte apoB secretion, through several mechanisms, including inhibition of cholesterol synthesis and esterification, inhibition of MTP activity and expression and increased expression of the LDL-receptor.