Selenoprotein deficiency accelerates prostate carcinogenesis in a transgenic model

Abstract

Considerable animal and human data have indicated that selenium is effective in reducing the incidence of several different types of cancer, including that of the prostate. However, the mechanism by which selenium inhibits carcinogenesis remains unknown. One possibility is that dietary selenium influences the levels of selenium-containing proteins, or selenoproteins. Selenoproteins contain selenium in the form of selenocysteine and perform a variety of cellular functions, including antioxidant defense. To determine whether the levels of selenoproteins can influence carcinogenesis independent of selenium intake, a unique mouse model was developed by breeding two transgenic animals: mice with reduced selenoprotein levels because of the expression of an altered selenocysteine-tRNA (i6A-) and mice that develop prostate cancer because of the targeted expression of the SV40 large T and small t oncogenes to that organ [C3(1)/Tag]. The resulting bigenic animals (i6A-/Tag) and control WT/Tag mice were assessed for the presence, degree, and progression of prostatic epithelial hyperplasia and nuclear atypia. The selenoprotein-deficient mice exhibited accelerated development of lesions associated with prostate cancer progression, implicating selenoproteins in cancer risk and development and raising the possibility that selenium prevents cancer by modulating the levels of these selenoproteins.

Fig. 1.

GPx activity (nmol of NADPH oxidized per mg of protein per min) in ventral prostate tissues from 32-week-old WT/Tag and i⁶A⁻/Tag mice. Tissues were homogenized in 0.1M Na₂HPO₄ buffer (pH 7.5) and enzyme activity measured by using a standard coupled spectrophotometric method. Values are expressed as mean ± SEM. As compared with control WT/Tag mice, GPx-1 activity was 5-fold lower in ventral prostates and 7-fold lower in the dorsal prostates (data not shown) of i⁶A⁻/Tag mice (P < 0.05; two-tailed Student's t test). (Inset) A representative Western blot of GPx-1 protein levels in 20-week prostate tissues. A significant reduction in GPx-1 expression was observed in prostate tissues from i⁶A⁻/Tag mice relative to the WT/Tag animals, confirming the selenoprotein-deficient genotype of the i⁶A⁻/Tag animals.

Fig. 2.

Representative prostate histopathology observed in the ventral lobes of WT/Tag and i⁶A⁻/Tag mice across time. (A) Normal ventral prostate epithelium (WNL) from a week-12 WT/Tag animal. (B) Epithelial hyperplasia without evidence of nuclear atypia (left acini) in a week-20 WT/Tag specimen. (C) Low-grade prostatic intraepithelial neoplasia (LGPIN) characterized by slight nuclear enlargement (arrow) and cellular crowding in a week-20 WT/Tag prostate. (D and E) HGPIN in week-20 i⁶A⁻/Tag prostate, as characterized by markedly enlarged or elongated nuclei, irregular nuclear membranes, increased nuclear/cytoplasmic ratios, hyperchromasia, and loss of polarity. (F) Microinvasion in week-32 i⁶A⁻/Tag prostate showing basement membrane breakdown and invasion of epithelium into the acinar wall (arrows) of associated HGPIN epithelium. (G) Focal carcinoma in week-42 i⁶A⁻/Tag prostate. Low-power image shows excessive intralumenal proliferation, PIN, basement membrane breakdown, and locally invasive carcinoma (boxed region). Higher magnification reveals discreet small glands (arrows). (H) Carcinoma in 42-week i⁶A⁻/Tag prostate with extensive invasion into stromal and loose periprostatic connective tissue and small-gland formation of variable size and shape with moderately differentiated epithelium. The magnification is indicated by the bar in each image, which is 50 μm.

Fig. 3.

Incidence of PIN lesions and local invasive carcinoma in the ventral prostate lobes of WT/Tag (Left) and i⁶A⁻/Tag (Right) mice at 12, 20, 32, and 42 weeks of age. Prostate specimens were examined histologically for prostatic lesions in a double-blinded manner and scored for the highest grade PIN or carcinoma lesion observed across multiple sections. For all Tag mice, there was a decreasing incidence of WNL and an increasing incidence of a lesion-associated event (LGPIN plus HGPIN plus microinvasion) over time, indicating appropriate progression in this transgenic model.