We tested lycopene and vitamin E in the Dunning prostate cancer model to prove their efficacy in prostate cancer (PCA) prevention and to identify the mechanisms by which these nutrients reduce the risk of PCA, as observed in epidemiology.
30 Copenhagen rats were randomly assigned to five groups. The two control groups were not supplemented (control) or received placebo-supplemented diet (vehicle group). Treatment groups were fed diets enriched with 200 ppm lycopene, 540 ppm vitamin E, or both. After 4 wk of presupplementation, tumors were induced by injection of 105 MatLyLu prostate tumor cells into the ventral prostate lobe; supplementation was continued for an additional 18 days of tumor growth. Lycopene and vitamin E accumulation was followed by HPLC analysis. The biological activities of the nutrients in tumor tissue were evaluated by in vivo magnetic resonance (MR) imaging after 14 days of tumor growth as well as by GeneChip® analysis at trial termination 18 days after tumor induction. Gene regulations in key pathways were confirmed by TaqMan™ real-time RT-PCR.
1. Lycopene and vitamin E accumulate in tumor tissue
After 4 wk of presupplementation with vitamin E or vitamin E/lycopene cotreatment, plasma vitamin E levels increased to 47.98 and 46.44 μM, respectively. In the lycopene-supplemented groups, lycopene plasma levels of 1.02 and 0.92 μM were analyzed after the presupplementation period. Vitamin E and lycopene plasma levels both correspond to high physiological levels measured in humans.
Within 18 days of tumor growth, vitamin E and lycopene accumulated in the tumor tissue. Vitamin E levels were 75.46 and 47.60 μM in the vitamin E and the vitamin E/lycopene-treated group. Lycopene levels reached 0.38 and 0.42 μM in the two lycopene-treated groups.
2. MR imaging of tumor tissue
Fourteen days after tumor cell injection, tumors were examined in vivo by MR imaging. Vitamin E and lycopene single treatment significantly increased the necrotic area of the tumors to 36.37% and 35.97%, respectively, compared with 19.98% and 23.27% in the two control groups. The combination of vitamin E and lycopene nonsignificantly increased the necrosis rate to 27.47%.
3. GeneChip® analysis of tumor tissue
Changes in gene expression in treatment groups were analyzed relative to the expression in the vehicle group (Table 1⇓ ). The hallmark of both the lycopene and the vitamin E effect was suppression of genes involved in steroid metabolism and signaling. Lycopene reduced steroid 5-α-reductase 1 expression in the lycopene [fold induction (fold) 0.36] and in the cotreated group (fold 0.48). Consequently, a set of androgen target genes (cystatin-related protein 1 and 2, prostatic spermine binding protein, prostatic steroid binding protein C1, C2, and C3 chain, probasin) was consistently down-regulated in both lycopene-treated groups with a fold of up to 0.02. Although changes in steroid 5-α-reductase 1 expression were insignificant in the vitamin E group, the expression of the same set of androgen target genes was down-regulated as in the lycopene-treated groups. Furthermore, vitamin E alone or combined with lycopene reduced aromatase expression (fold 0.57 and 0.65). In the cotreated group, lycopene and vitamin E acted in an additive manner on androgen target gene repression. The strong repression of androgen target genes was confirmed by TaqMan™ RT-PCR, as shown for prostatic spermine binding protein, prostatic steroid binding protein C2, and cystatin-related protein 2 (Fig. 1⇓ ).