PCa is the most frequently diagnosed malignancy in elderly American men (35
). Several epidemiological studies indicate that the PCa incidence and associated death rate are lower in Asian countries as compared to Western countries (3
). This has been attributed to the difference in dietary pattern which is recognized as one of the major etiologic factors responsible for a variation in PCa incidence and mortality between Asian and Western male population (37
). The dietary composition in the industrialized Western countries includes highly processed foods rich in meat, dairy products, and refined carbohydrates; however, in Asian countries, diets are rich in fiber content, whole grain cereals, legumes, vegetables, and fruits (36
). Research groups, worldwide, have directed considerable efforts towards the identification of dietary or non-dietary naturally occurring chemical agents for both prevention and intervention of PCa (40
). One such dietary agent is IP6, which has shown anticancer efficacy against various in vitro
and in vivo
cancer models, including PCa (3
). IP6 is abundantly present in high fiber content diets, most cereals, legumes, nuts and soybean (1
). The consumption of these dietary agents have been associated with reduced risk, incidence of, and mortality due to PCa in Asian countries (1
We have previously studied the in vivo
anticancer efficacy of oral IP6 against human prostate carcinoma DU145 xenograft growth, in which IP6 suppressed the tumor growth without any toxicity (27
). IP6 has also been found effective in animal tumorigenesis models of other cancer types without any toxicity (3
). The effect of IP6 on prostate tumor progression has not been studied till now in any preclinical animal model. In this regard, the most relevant available animal model is TRAMP which closely mimics the progression of prostate cancer as it occurs in humans (31
). Therefore, our present study of chemopreventive efficacy of IP6 in TRAMP model could have potential clinical significance. The most important and novel findings in the present study are that oral IP6 feeding for 24 weeks starting from the 4th
week of age inhibits prostate tumor growth and progression in TRAMP mice. This anti-tumor progression effect of IP6 is accompanied by the arrest of tumor progression at PIN stage with a concomitant reduction in the incidence of adenocarcinoma. There were more mice with LGPIN and HGPIN stages (75%) and fewer with well differentiated and poorly differentiated adenocarcinoma stages (25%) in IP6-fed group. Whereas in the positive control group, no mouse was found with LGPIN but there were some with HGPIN (39%) and more with adenocarcinoma stages (61%). IP6 also significantly inhibited the progression through the different stages of adenocarcinoma, and overall decreased the severity of the lesions as observed by the mean peak histological score analysis. Additionally, no apparent toxic or adverse effect was observed in mice having IP6-supplemented drinking water, as monitored by the general health, water and diet consumption, body weight gain, and gross pathological examination during necropsy. These observations indicate for the clinical potential of IP6 in suppressing the PCa growth and progression.
Cell proliferation and apoptosis are well established biomarkers to study the anti-tumor effect of a given agent (43
). Many naturally occurring and synthetic agents have been found to inhibit cell proliferation and induce apoptosis in cancer cells (41
). In this regard, IP6 has been found to inhibit cell proliferation as well as induce apoptosis in human and mouse PCa cells in culture and in DU145 tumor xenograft in nude mice (reviewed in ref. 3
). In the present study, to examine whether inhibition of prostate tumor growth and progression by IP6 is associated with its effect on cell proliferation and survival, prostate tissues were also immunohistologically analyzed for PCNA and TUNEL staining. IP6 significantly inhibited cell proliferation and induced apoptotic cell population in prostate tissues. These observations suggest the role of the anti-proliferative and pro-apoptotic effects of IP6 in suppression of PCa growth and progression.
Our further concern was to address whether the observed anti-PCa effect of IP6 is due to its effect of Tag expression that drives neoplastic transformation in prostate epithelial cells and subsequently prostate tumorigenesis in TRAMP model or by other mechanisms. The immunohistochemical analysis of prostate tissue did not show any considerable change in Tag expression at different stages of tumor development. This observation suggests that anti-PCa growth and progression effects of IP6 are not related to suppression of Tag expression but to the direct suppression of tumorigenesis. In this regard, it is likely that IP6 may inhibit cell cycle progression to suppress tumor progression by targeting CDK (cyclin-dependent kinase)-CDKI (CDK inhibitor)-cyclin axis and RB (retinoblastoma) family proteins and E2F cell cycle regulators as we have observed in PCa cell culture studies with IP6 (10
). Furthermore, other potential mechanisms of IP6 could be the inhibition of EGFR (epidermal growth factor receptor), PI-3K (phosphatidyl inositol-3 kinase)-Akt and NF-κB (nuclear factor-kappaB) signaling and induction of mitochondrial as well as caspase-independent apoptosis which have been reported in PCa cells (24
). IP6 may also target IGF-1 (insulin-like growth factor-1)-IGFBP-3 (IGF binding protein-3) axis for its antiproliferative and proapoptotic effects as has been observed in DU145 tumor xenograft study (27
). However, additional studies are needed in future to examine the molecular mechanisms involved in the anti-PCa efficacy by oral IP6 feeding in this pre-clinical mouse model.
In humans, prostate tumorigenesis takes considerable time from the onset of the disease and progression to a detectable tumor and then to a hormone-refractory stage. Therefore, a considerable window of time could be available to employ various intervention strategies, including dietary chemoprevention (40
). In this regard, the findings in the present study are both novel and highly significant in establishing that IP6 feeding causes suppression of prostate tumor progression at the neoplastic stage thereby reducing the incidence of the advanced forms of the disease, the various progressive stages of adenocarcinoma. Further, this pre-clinical study advocates for a potential clinical trial of IP6 in PCa patients which may improve the morbidity and survival time in cancer patients.