δ-tocopherol is more active than α - or γ -tocopherol in inhibiting lung tumorigenesis in vivo

Abstract

In contrast to strong epidemiologic, preclinical, and secondary clinical evidence for vitamin E (tocopherols) in reducing cancer risk, large-scale clinical cancer-prevention trials of α-tocopherol have been negative. This vexing contrast helped spur substantial preclinical efforts to better understand and improve the antineoplastic activity of tocopherol through, for example, the study of different tocopherol forms. We previously showed that the γ-tocopherol-rich mixture (γ-TmT) effectively inhibited colon and lung carcinogenesis and the growth of transplanted lung-cancer cells in mice. We designed this study to determine the relative activities of different forms of tocopherol in a xenograft model, comparing the anticancer activities of δ-tocopherol with those of α- and γ-tocopherols. We subcutaneously injected human lung cancer H1299 cells into NCr nu/nu mice, which then received α-, γ-, or δ-tocopherol or γ-TmT in the diet (each at 0.17% and 0.3%) for 49 days. δ-Tocopherol inhibited tumor growth most strongly. γ-Tocopherol and γ-TmT (at 0.3%) also inhibited growth significantly, but α-tocopherol did not. δ-Tocopherol also effectively decreased oxidative DNA damage and nitrotyrosine formation and enhanced apoptosis in tumor cells; again, γ-tocopherol also was active in these regards but less so, and α-tocopherol was not. Each supplemented diet increased serum levels of its tocopherol - up to 45 μmol/L for α-tocopherol, 9.7 μmol/L for γ-tocopherol, and 1.2 μmol/L for δ-tocopherol; dietary γ- or δ-tocopherol, however, decreased serum α-tocopherol levels, and dietary α-tocopherol decreased serum levels of γ-tocopherol. Each dietary tocopherol also increased its corresponding side-chain-degradation metabolites, with concentrations of δ-tocopherol metabolites greater than γ-tocopherol and far greater than α-tocopherol metabolites in serum and tumors. This study is the first in vivo assessment of δ-tocopherol in tumorigenesis and shows that δ-tocopherol is more active than α- or γ-tocopherol in inhibiting tumor growth, possibly through trapping reactive oxygen and nitrogen species and inducing apoptosis; δ-tocopherol metabolites could contribute significantly to these results.

Fig 1

Structures of tocopherols.

Fig 2

Effects of dietary tocopherol supplementation on xenograft tumor growth and tumor weight. Experimental conditions are described in Materials and Methods. In the growth curve (A), the growth of the groups on 0.17% of different tocopherols are not shown to avoid overcrowding the figure. The group with 0.17% δ-T overlaps the group with 0.3% γ-T. The curves for groups with 0.17% γ-TmT, γ-T and α-T are superimposed and they showed less inhibitory activity than 0.17% δ-T for the measurements on Days 46 and 49. For tumor weight (B), the values are mean ± S.E. (n=10). * (p < 0.05) and † (p < 0.01) in ANOVA-Dunnett’s test; ^a (p < 0.05) in two-tailed t-test when compared with the control.

Fig 3

Effects of tocopherol supplementation on 8-OHdG positive cells (A), γ-H2AX positive cell (B), nitrotyrosine positive cells (C), and apoptosis (D) in xenograft tumors. Arrows indicate immunopositive stained cells. Designations for statistical analysis are the same as Fig 2, except n=5.

Fig 4

Effects of tocopherol supplementation on serum levels of tocopherols and their metabolites. (A) HPLC chromatogram and structures of CMBHCs and CEHCs; the α-, γ- and δ-designations are the same as in Figure 1. (B) Serum levels of α-, γ- and δ-tocopherols shown on different scales. (C) Serum levels of CMBHCs. (D) Serum levels of CEHCs. Designations for statistical analysis are the same as Fig 3.

Fig 5

Effects of tocopherol supplementation on the levels of tocopherols and their metabolites in tumors and lung tissues. (A), (B) and (C): levels of tocopherols, CEHCs and CMBHCs in tumors; (D), (E) and (F): levels of tocopherols, CEHCs and CMBHCs in lung tissues, respectively. Designations for statistical analysis are the same as Fig 3.

Fig 6

Effects of tocopherol supplementation on colonic and urinary levels of tocopherols and their metabolites. Levels of tocopherols (A), CMBHCs (B) and CEHCs (C) in colon tissues; levels of CMBHCs (D) and CEHCs (E) in urine samples. Designations for statistical analysis are the same as Fig 3.