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ATPase/ATP synthase plays important roles in the regulation of carbohydrate, protein, and lipid metabolism through modulating energy homeostasis. The purpose of this study was to examine the effects of feeding soy proteins and isoflavones (ISF) on the enzymatic activity and protein modification of hepatic mitochondrial ATPase/ATP synthase. In Expt. 1, Sprague-Dawley rats aged 50 d were fed diets containing either 20% casein or 20% alcohol-washed soy protein isolate (SPI) with or without supplemental ISF (770.7 micromol/kg diet) for 70 d. In Expt. 2, weanling Sprague-Dawley rats were fed diets containing 20% casein with or without added ISF (154.1 micromol/kg diet) or 20% SPI for 90 d. Hepatic mitochondrial ATPase activity was significantly higher in the rats fed SPI than in those fed casein. Addition of ISF to SPI eliminated the action of SPI. ATPase/ATP synthase beta protein contents in the liver were unchanged; however, its patterns measured by 2-dimensional Western blot were different among dietary groups. The rats fed SPI or SPI plus ISF had 3 more major protein spots with the same molecular weights (80 kDa and 55 kDa) as those presented in the rats fed casein but with different isoelectric points. Pretreatment of hepatic mitochondrial proteins from the rats fed casein with alkaline phosphatase produced the same ATPase/ATP synthase beta patterns as observed in the SPI-fed rats and significantly elevated the ATPase activity. These results suggest that consumption of soy proteins increases hepatic ATPase activity, which might be a consequence of increased dephosphorylation or decreased phosphorylation of the mitochondrial ATPase/ATP synthase beta protein.

Retinoic acid receptors (RAR) belong to the same nuclear receptor superfamily as thyroid hormone receptors (TR) that were previously shown to be modulated by dietary soy protein isolate (SPI). This study has examined the effect of dietary SPI and isoflavones (ISF) on hepatic RAR gene expression and DNA binding activity. In Expt. 1, Sprague-Dawley rats were fed diets containing 20% casein or 20% alcohol-washed SPI in the absence or presence of increasing amounts of ISF (5-1250 mg/kg diet) for 70, 190, or 310 d. In Expt. 2, weanling Sprague-Dawley rats were fed diets containing 20% casein with or without supplemental ISF (50 mg/kg diet) or increasing amounts of alcohol-washed SPI (5, 10, and 20%) for 90 d. Intake of soy proteins significantly elevated hepatic RARbeta2 protein content dose-dependently compared with a casein diet, whereas supplemental ISF had no consistent effect. Neither RARbeta protein in the other tissues measured nor the other RAR (RARalpha and RARgamma) in the liver were affected by dietary SPI, indicating a tissue and isoform-specific effect of SPI. RARbeta2 mRNA abundances were not different between dietary groups except that its expression was markedly suppressed in male rats fed SPI for 310 d. DNA binding activity of nuclear RARbeta was significantly attenuated and the isoelectric points of RARbeta2 were shifted by dietary SPI. Overall, these results show for the first time, to our knowledge, that dietary soy proteins affect hepatic RARbeta2 protein content and RARbeta DNA binding activity, which may contribute to the suppression of retinoid-induced hypertriglyceridemia by SPI as reported.

Thyroid hormone receptors (TRs) are regulators of many genes involved in cholesterol and lipid metabolism. The purpose of this study was to examine the effect of soy protein isolate (SPI) and isoflavones on hepatic TRs in rats. In Expt. 1, Sprague-Dawley rats were fed diets containing either casein or alcohol-washed SPI with or without isoflavone supplementation (5-1250 mg/kg diet) for 70, 190, and 310 d. The offspring (F1) were fed the same diets as their parents (F0). In Expt. 2, Sprague-Dawley rats were fed diets containing casein or casein plus isoflavones (50-400 mg/kg diet) for 120 d. The mRNA and protein contents of the hepatic TRs were measured by semiquantitative RT-PCR and Western blot, respectively. TRalpha1, TRalpha2, and TRbeta2 contents were not affected by SPI. However, the content of the 52-kDa TRbeta1 protein, the major isoform present in the liver, was markedly increased by dietary SPI in both sexes of F0 and F1 compared with casein. The supplemental isoflavones had no effect on TRbeta1, whereas the high doses of isoflavones (250 and 1250 mg/kg diet) reduced the hepatic TRalpha1 protein content in F1 male rats on d 28. SPI had no effect on total T3 and T4 levels. However, higher dose of supplemental isoflavones markedly increased T4 level in female rats. Overall, this study demonstrates for the first time that SPI upregulates hepatic TRbeta1 expression, and that isoflavones reduce the hepatic TRalpha1 level in young male rats. The SPI-induced TRbeta1 may play a role in mediating the hypocholesterolemic and lipid-lowering actions of soy protein.