The Effect of Soy Protein on Colon Carcinogenesis in vitro and in vivo

Open Access
Brownschidle, Amy Laura
Graduate Program:
Food Science
Master of Science
Document Type:
Master Thesis
Date of Defense:
Committee Members:
  • Joshua D Lambert, Thesis Advisor
  • Ryan John Elias, Thesis Advisor
  • Soy protein
  • colon cancer
Previous studies have suggested a beneficial effect of dietary soy against inflammation and colon cancer. Much of this has been attributed to soy isoflavones; however, soy proteins and peptides have also shown bioactivity and antioxidant activity in some models. Enzymatic hydrolysis of proteins has also been shown to increase free radical scavenging by increasing solvent accessibility of active amino acids. We investigated the antioxidant capacity of an isoflavone-free soy protein concentrate (SPC) and an enzymatically-hydrolyzed SPC (SPH) using the oxygen radical absorbance capacity (ORAC) assay. Consistent with our expectations, SPH delayed the oxidation of fluorescein significantly longer than SPC (p<0.01), indicating its superior antioxidative capacity. The cytoprotective effects of SPC and SPH were also compared in the Caco-2 human intestinal cell model to evaluate their ability to prevent H2O2-induced oxidation. Cells were cotreated with 50 µM H2O2 and SPC or SPH (0-2 mg/ml) in PBS for 60 min or were pretreated with SPC or SPH (0-2 mg/ml) for 30 min and subsequently treated with 50 µM H2O2 for 60 min. Cell viability was assessed using the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) assay. Treatment with 0.5 mg/ml SPC increased cell viability from 42.8-50.0% in H2O2-only treated cells to 76.1 ± 9.0% and 60.8 ± 6.5% of the PBS control in the cotreatment and pretreatment trials, respectively. Higher concentrations of SPH were required to achieve similar cytoprotective effects. At a concentration of 2 mg/ml SPH, cell viability was increased to 85.3 ± 10.9% and 62.1 ± 3.2% in the cotreatment and pretreatment trials, respectively. Photomicrographs of treated cells showed that SPC prevented H2O2-induced changes in morphology and reduced intracellular reactive oxygen species (ROS). These effects were also observed for SPH treatment, but to a lesser extent. The effect of dietary supplementation with SPC on colon inflammation and carcinogenesis in male, CF-1 mice was investigated in one 10-week and one 20-week study. In both studies, following one week pretreatment with SPC or casein as the sole source of protein (19% kcal), mice were injected with the colon carcinogen azoxymethane (AOM, 10 mg/kg body wt). One week later, mice received dextran sodium sulfate (DSS, 1.5% w/v) as their sole drinking fluid for one week to induce colon-specific inflammation. On days 3 and 7, and weeks 7 and 20 (second study only) after DSS treatment, mice were euthanized and the colons were examined for markers of inflammation and carcinogenesis. All samples were scored using the inflammation index, which measures inflammation area, severity, ulceration and hyperplasia and dysplasia. In both studies, trends in these measures appeared to be highly dependent on sacrifice time-point. With the data from both studies combined, mean polyp multiplicity was significantly reduced in mice fed SPC at Week 7 and for the study overall (p<0.001 and p<0.01, respectively). Hyperplasia and dysplasia, inflammation area and total inflammation index score were significantly decreased in the SPC group compared to the control at Week 7 (p<0.10, p<0.10 and p<0.05, respectively). At 20 weeks, mice in both the control and SPC-treated groups showed colon tumors. A trend toward reduced tumor burden and multiplicity in mice fed the SPC diet was observed, but the effect was not statistically significant. Finally, the ORAC assay was used to compare the radical scavenging capacities of SPC and casein, to elucidate any effect this may have had in the mouse studies. In comparisons of the proteins alone and of the SPC and casein-containing mouse diets, casein was found to have significantly greater radical scavenging capacity (p<0.0001). In conclusion, while these studies suggest that soy may have antioxidant activity and colon cancer preventative effects independent of isoflavone content, further research is required to confirm this finding and the underlying mechanism.