Inhibition of orthotopic growth and metastasis of androgen-sensitive human prostate tumors in mice by bioactive soybean components

Abstract

Background: Systematic analysis of the influence of diet on the initiation and progression of prostate cancer is often difficult in human populations, for which dietary variables overlap a diversity of genetic backgrounds and social behaviors. Animal models that emulate human prostate cancer allow experimental analysis of the mechanisms of action of nutritional agents that show anti-prostate cancer activity.

Methods: We have used an orthotopic implant model to characterize the in vivo response of androgen-sensitive LNCaP prostate tumors to three well-characterized soy dietary supplements: isoflavone depleted soy protein, soy phytochemical concentrate (SPC), and genistin.

Results: In male SCID mice orthotopically implanted with the androgen-sensitive human prostate cell line LNCaP, dietary supplements of soy protein, genistin, and SPC reduced primary tumor weight by 42% (P = 0.07), 57% (P < 0.05) and 70% (P < 0.005), respectively. All three soy supplements significantly increased tumor apoptosis and decrease microvessel density, with no significant change in tumor proliferation. Each supplement produced a distinct serum androgen response, with genistin producing the greatest decrease in total serum testosterone and dihydrotestosterone (DHT) (P < 0.05) and the greatest increase in testosterone to DHT ratio (P < 0.05) and soy protein the greatest decrease in bioactive androgen (P < 0.05). Only SPC significantly inhibited metastases to lymph nodes and lungs, and only SPC produced a significant increase in tumor p53 expression.

Conclusion: Taken together, these data suggest that the anti-prostate cancer activity of dietary soy protein, soy phytochemicals, and genistin use different molecular pathways. In addition, we have demonstrated that this animal model can be used in the design of dietary strategies for prostate cancer prevention and therapy.

Fig 1

Effects of soy components on final tumor weight (A), serum levels of prostate specific antigen (PSA;B), lymph node metastases (C), and lung micrometastases (D) in SCID mice. Mice were treated with soy components containing experimental diets for 2 weeks, implanted intraprostatically with LNCaP cells, and continued experimental diets for10 more weeks. Dietary supplements of soy protein, genistin, and soy phytochemical concentrate (SPC) all reduced final tumor weights and final serum PSA. SPC was the only soy supplement SPC that significantly inhibited metastases to the lymph nodes and lungs. Values represent mean ± SEM. Asterisks indicate P at least <0.05 compared with the control.

Fig 2

Effects of soy components on apoptosis index (A), proliferation index (B), and the expression of p53 (C,D) in primary prostate tumors in mice. Primary tumors were determined for tumor cell apoptosis by the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) assay and proliferation by proliferating cell nuclear antigen staining. The expression ofp53 was determined by immunohistochemistry and quantified by image analysis. Dietary supplements of soy protein, genistin, and soy phytochemical concentrate (SPC) significantly increased tumor cell apoptosis but nonsignificantly reduced tumor cell proliferation index. Dietary supplement of SPC significantly up-regulated the expression of p53.In SPC-treated mice, p53-positive tumor cells had increased staining intensity in the nuclei. Values represent mean ± SEM. Asterisks indicate P at least <0.05 compared with the control.

Fig 3

Effects of soy components on tumor microvessel density (MVD) (A) and the expression of angiogenic factor basic fibroblast growth factor; (bFGF; B) in tumor cells in vivo. Primary tumors were determined for MVD as a parameter of angiogenesis by immunohistochemical staining for factor VIII. Dietary supplements of soy protein, genistin, and soy phytochemical concentrate (SPC) significantly reduced tumor MVD associated with reduced expression of bFGF in primary tumors. Values represent mean ± SEM. Asterisks indicate P at least <0.05 compared with the control.

Fig 4

Effects of soy components on serum levels of testosterone (A), dihydrotestosterone ; (DHT;B), testosterone to DHT ratio (C), and AR bioassay activity (D). Serum levels of total testosterone and DHT were measured by enzyme immunoassays. The AR bioassay activity was measured by yeast-based AR transactivation assay. Dietary supplement of genistin significantly reduced serum levels of DHT and increased total testosterone to DHT ratio, compared with the controls. Dietary soy protein significantly decreased bioactive androgen in vivo. Values represent mean ± SEM. Asterisks indicate P at least <0.05 compared with the control.