The Roles of α‐Vitamin E and Its Analogues in Prostate Cancer

Combination Effect of Bowman-Birk Inhibitor and α-Tocopheryl Succinate on Prostate Cancer Stem-Like Cells

The reoccurrence of androgen-dependent prostate cancer after anti-androgen therapy mainly depends on prostate cancer stem-like cells. To reduce the risk, it is important to delete the cancer stem-like cells. Furthermore, to induce differentiation of cancer stem-like cells is critical to abrogate stemness of the cells. Therefore, we tried to investigate a possibility on the establishment of a new effective therapy to eradicate the cancer stem-like cells via the induction of differentiation in this study. Prostate cancer stem-like cells from an androgen-dependent prostate cancer cell line (LNCaP cell) had severe resistance against an anti-androgen therapeutic agent. We selected Bowman-Birk inhibitor (BBI) from soybeans reported as a chemopreventive agent in prostate cancer to differentiate the caner stem-like cells and α-tocopheryl succinate (TOS) known as a mitocan to induce effectively cytotoxic effect against the cancer stem-like cells. In fact, only TOS treatment had cytotoxic effect against the cancer stem-like cells, but the addition of BBI treatment to the cells treated with TOS reinforced TOS-mediated cytotoxicity in the cancer stem-like cells. This reinforcement coincided with the combination-enhanced apoptosis in the stem-like cells. Also, we confirmed caspase9-caspase3 cascade mainly contributed to the enhancement of the cytotoxicity in the stem-like cells caused by the combination, indicating that the reinforcement of BBI on TOS-mediated apoptosis via mitochondria related to the enhancing cytotoxic effect of the combination on the prostate cancer stem-like cells. Overall, it seems that the combination is an effective new approach to reduce the reoccurrence of prostate cancer targeting prostate cancer stem cells.

The antioxidants vitamins A and E and selenium do not reduce the incidence of asbestos-induced disease in a mouse model of mesothelioma

Epidemiological evidence indicates that supplementation with some dietary factors is associated with a lower incidence of cancer. An effective cancer prevention strategy for the millions of people worldwide who have been exposed to asbestos could have enormous benefit. We tested whether dietary supplementation of the antioxidants vitamin A, E, and selenium could alter the pattern of disease in the MexTAg transgenic mouse model, in which mice uniformly develop mesothelioma after asbestos exposure. We focused on antioxidants because one of the most widely accepted hypotheses for the mechanism by which asbestos fibers cause cancer proposes the involvement of reactive oxygen and nitrogen species. We compared the survival of MexTAg mice that had been inoculated with asbestos fed on diets supplemented with 250,000 IU/kg vitamin A (retinoic acid), or 1,000 mg/kg vitamin E (α-tocopherol acetate) or 3 mg/kg selenium, or both vitamin E and selenium concurrently and, additionally, diets deficient in each antioxidant. We found that neither the time to develop symptoms of disease nor overall survival times were altered by any of the diets. We conclude that the data do not support the notion that dietary antioxidants will moderate the rate of mesothelioma in asbestos-exposed populations.

Diet and prostate cancer: mechanisms of action and implications for chemoprevention

As one of the most prevalent cancers, prostate cancer has enormous public health significance and prevention strategies would attenuate its economic, emotional, physical and social impact. Until recently, however, we have had only modest information about risk factors for this disease, apart from the well-established characteristics of age, family history and place of birth. The large worldwide variation in the incidence of prostate cancer and the increased risk in migrants who move from low-risk to high-risk countries provide strong support for modifiable environmental factors, particularly diet, in its etiology. Thus, dietary agents have gained considerable attention as chemopreventive agents against prostate cancer. Dietary fat, red and processed meat, vitamin E, selenium, tomatoes, cruciforms and green tea have all been linked with the development and aggressiveness of prostate cancer, through a range of molecular mechanisms. The direction of future clinical trials lies in clarifying the effects of these agents and exploring the biological mechanisms responsible for the prevention of prostate cancer. However, owing to the short time period between diagnosis and treatment, conventional dietary intervention techniques are not always realistic. Until large randomized trials confirm the benefit of chemopreventive and dietary modifications, patients can be advised to pursue a diet and lifestyle that enhances overall health.

Selenomethionine and alpha-tocopherol do not inhibit prostate carcinogenesis in the testosterone plus estradiol-treated NBL rat model

Previous studies with selenium and/or vitamin E in prostate carcinogenesis animal models have been negative, but these models may not involve oxidative stress mechanisms. In this study, we examined the potential of selenomethionine and alpha-tocopherol to modulate prostate cancer development in the testosterone plus estradiol-treated NBL rat, a model that does involve sex hormone-induced oxidative stress mechanisms and prostatic inflammation. One week following the implantation with hormone-filled Silastic implants, rats were fed diets containing l-selenomethionine (1.5 or 3.0 mg/kg), DL-alpha-tocopherol acetate (2,000 or 4,000 mg/kg), or a natural ingredient control diet (NIH-07). The development of prostate carcinomas was not affected by dietary treatment with either agent. Food intake, body weight, and mortality were also not affected. The high dose of selenomethionine reduced the severity of epithelial dysplasia in the lateral prostate that was not associated with inflammation, and alpha-tocopherol reduced in a dose-related fashion the incidence of marked inflammation and marked epithelial dysplasia in the lateral prostate, regardless of whether these lesions were associated with inflammation. alpha-Tocopherol significantly increased the incidence of adenocarcinomas of the mammary glands at both dietary concentrations. Collectively, our findings suggest that selenomethionine and alpha-tocopherol supplementation does not prevent prostate cancer in rats fed diets with nutritionally adequate levels of selenium and vitamin E. Importantly, the results of the current animal studies and those reported previously were fully predictive of the outcome of the Selenium and Vitamin E Cancer Prevention Trial.

Present trends in vitamin E research

Nearly after one century of research and thousands of publications, the physiological function(s) of vitamin E remain unclear. Available evidence suggests a role in cell homeostasis that occurs through the modulation of specific signaling pathways and genes involved in proliferative, metabolic, inflammatory, and antioxidant pathways. Vitamin E presence in the human body is under close metabolic control so that only alpha-tocopherol and, to a lower extent, gamma-tocopherol are retained and delivered to tissues. Other vitamin E forms that are not retained in the body in significant amounts, exhibit responses in vitro that are different form those of alpha-tocopherol and may include tumor cell specific toxicity and apoptosis. These responses provide a therapeutic potential for these minor forms, either as such or metabolically modified, to produce bioactive metabolites. These cellular effects go beyond the properties of lipophilic antioxidant attributed to alpha-tocopherol particularly investigated for its alleged protective role in atherosclerosis or other oxidative stress conditions. Understanding signaling and gene expression effects of vitamin E could help assign a physiological role to this vitamin, which will be discussed in this review. Besides vitamin E signaling, attention will be given to tocotrienols as one of the emerging topics in vitamin E research and a critical re-examination of the most recent clinical trials will be provided together with the potential use of vitamin E in disease prevention and therapy.

Anticancer actions of natural and synthetic vitamin E forms: RRR-alpha-tocopherol blocks the anticancer actions of gamma-tocopherol

Two naturally occurring dietary sources of vitamin E (i.e. RRR-alpha-tocopherol (alphaT) and RRR-gamma-tocopherol (gammaT)), the manufactured synthetic form of vitamin E, all-racemic-alpha-tocopherol (all-rac-alphaT), as well as a potent antitumor analog of vitamin E, RRR-alpha-tocopherol ether-linked acetic acid analog (alpha-TEA), were assessed for anticancer actions. Data showed that gammaT, all-rac-alphaT, and alpha-TEA but not alphaT or alphaT+gammaT significantly inhibited tumor burden of human MDA-MB-231 cells in nude mice. Immunohistochemical analyses of tumor tissue showed that all-rac-alphaT and alpha-TEA increased apoptosis and decreased proliferation in tumor cells while gammaT was associated with increased tumor cell apoptosis only. In vitro data showed alpha-TEA and gammaT but not all-rac-alphaT or alphaT to inhibit colony formation and induce apoptosis. Anticancer actions of alpha-TEA and gammaT involved death receptor 5 protein upregulation, Survivin protein downregulation and poly (ADP-ribose) polymerase cleavage, all of which were blocked by co-treatment with alphaT. In summary, both gammaT and alpha-TEA exhibited promising anticancer properties in vivo and in vitro, whereas all-rac-alphaT exhibited promising anticancer properties in vivo only. Importantly, alphaT not only failed to exhibit anticancer properties but it also reduced anticancer actions of gammaT in vivo and gammaT and alpha-TEA in vitro.

In vivo and in vitro studies of anticancer actions of alpha-TEA for human prostate cancer cells

BACKGROUND

Vitamin E analog, 2,5,7,8-tetramethyl-2R-(4R,8R, 12-trimethyltridecyl) chroman-6-yloxyacetic acid, referred to as alpha-TEA induces apoptosis in a variety of human cancer cells in cell culture and reduces tumor burden and metastases in preclinical animal models of breast and ovarian cancer. The goal of this study was to determine in vivo anticancer efficacy of alpha-TEA against human prostate cancer cells and identify mechanisms of action.

METHODS

A PC-3-GFP xenograft model was used to assess the effects of alpha-TEA formulated in liposomes and administered orally on tumor burden and metastases. Tumor tissue was examined by immunohistochemical staining for percentage of cells undergoing apoptosis by TUNEL or cell proliferation by Ki-67. In vitro analyses of mechanisms employed western immunoblotting to examine effects of alpha-TEA-treatments in LNCaP and PC-3-GFP cells on levels of pro-survival and pro-death factors. Functional significance was determined using ectopically expressed constitutively active forms, inhibitors, or siRNA.

RESULTS

alpha-TEA significantly reduced tumor burden and metastases, increased apoptosis and decreased proliferation of tumor cells (P < 0.05). alpha-TEA treatment of both LNCaP and PC-3-GFP cells in vitro reduced levels of pAkt1, pAkt2; FOXO1, c-FLIP(L) and survivin. Constitutively active Akt1, Akt2, c-FLIP or survivin reduced alpha-TEA-induced apoptosis. PI3K inhibitor enhanced apoptosis. Constitutively active FOXO1 enhanced alpha-TEA induced Fas ligand expression; whereas, FOXO1 siRNA reduced alpha-TEA induced Fas ligand expression.

CONCLUSIONS

alpha-TEA is an effective anticancer agent for human prostate cancer cells. Downregulation of pro-survival and upregulation of pro-death factors play roles in alpha-TEA-induced apoptosis.