Cyclin degradation for cancer therapy and chemoprevention

Cancer is characterized by uncontrolled cell division resulting from multiple mutagenic events. Cancer chemoprevention strategies aim to inhibit or reverse these events using natural or synthetic pharmacologic agents. Ideally, this restores normal growth control mechanisms. Diverse classes of compounds have been identified with chemopreventive activity. What unites many of them is an ability to inhibit the cell cycle by specifically modulating key components. This delays division long enough for cells to respond to mutagenic damage. In some cases, damage is repaired and in others cellular damage is sufficient to trigger apoptosis. It is now known that pathways responsible for targeting G1 cyclins for proteasomal degradation can be engaged pharmacologically. Emergence of induced cyclin degradation as a target for cancer therapy and chemoprevention in pre-clinical models is discussed in this article. Evidence for cyclin D1 as a molecular pharmacologic target and biological marker for clinical response is based on experience of proof of principle trials.     

Green tea and cancer chemoprevention.

Worldwide interest in green tea as a cancer preventive agent for humans has increased, because it is non-toxic and it is effective in a wide range of organs. (-)-Epigallocatechin gallate (EGCG) is the main constituent of green tea; the others are (-)-epicatechin gallate, (-)-epigallocatechin and (-)-epicatechin (EC). This paper reports the results of our latest pharmacological and biochemical studies with 3H-EGCG, along with studies on human subjects. The study on bioavailability of 3H-EGCG in mice revealed the wide distribution of radioactivity in multiple organs. Specifically, radioactivity was found in all reported target organs of EGCG and green tea extract (digestive tract, liver, lung, pancreas, mammary gland and skin) as well as other organs (brain, kidney, uterus and ovary or testes) in mice. Recently, we demonstrated that EC enhanced incorporation of 3H-EGCG into human lung cancer cell line PC-9 cells. EC along with another cancer preventive agent sulindac also synergistically enhanced apoptosis in PC-9 cells induced by EGCG. Moreover, a case-control study on breast cancer patients revealed that high daily consumption of green tea was associated with a lower recurrence rate among Stages I and II patients. All the results suggest that consumption of green tea is a practical and effective cancer preventive both before cancer onset and after cancer treatment.     

Nitric oxide in cancer and chemoprevention

Nitric oxide (NO) is a key molecule involved in many physiological functions. However, evidence is accumulating that sustained high levels of NO over extended periods of time contribute to carcinogenesis. This article reviews recent data and outlines a dual role of NO in animal carcinogenesis. Following an inhibition of NO production, some studies find a protection, while others find an exacerbation of tumorigenesis. These studies reflect the importance of (i). choosing the appropriate compound for NO inhibition; and (ii). genetic background, target tissue, levels of NO, and surrounding free radicals in the overall affects of NO on the tumor growth. These findings highlight the importance of further study of the use of NO inhibitors to inhibit human carcinogenesis.     

Cyclooxygenase-2 and chemoprevention of breast cancer

This article discusses the role of cycclooxygenase-2 (COX-2) in the aetiology and progression of breast cancer. Renewed interest in chemoprevention using non-steroidal antiinflammatory drugs (NSAIDS) has come from observations that regular NSAID use is associated with a reduced incidence of some cancers including that of the breast. There is an increasing body of evidence supporting a role for COX-2 in breast cancer development and progression via effects on angiogenesis and apoptosis as well as via effects on intratumoural aromatase. New selective inhibitors of COX-2 are currently licensed for use in the treatment of arthritis and more recently in the chemoprevention of familial adenomatous polyposis (FAP). Large clinical chemoprevention studies with COX-2 inhibitors are already underway in colorectal cancer. Their role in breast cancer prevention and treatment has yet to be fully characterised, but merits further investigation.     

Cinnamaldehydes inhibit thioredoxin reductase and induce Nrf2: potential candidates for cancer therapy and chemoprevention

Trans-cinnamaldehyde (CA) and its analogs 2-hydroxycinnamaldehyde and 2-benzoyloxycinnamaldehyde have been reported to possess antitumor activity. CA is also a known Nrf2 activator. In this study, a series of ortho-substituted cinnamaldehyde analogs was synthesized and screened for antiproliferative and thioredoxin reductase (TrxR)-inhibitory activities. Whereas CA was weakly cytotoxic and TrxR inhibiting, hydroxy and benzoyloxy substitutions resulted in analogs with enhanced antiproliferative activity paralleling increased potency in TrxR inactivation. A novel analog, 5-fluoro-2-hydroxycinnamaldehyde, was identified as exhibiting the strongest antitumor effect (GI₅₀ 1.6 μM in HCT 116 cells) and TrxR inhibition (IC₅₀ 7 μM, 1 h incubation with recombinant TrxR). CA and its 2-hydroxy- and 2-benzoyloxy-substituted analogs possessed dual TrxR-inhibitory and Nrf2-inducing effects, both attributed to an active Michael acceptor pharmacophore. At lethal concentrations, TrxR-inhibitory potencies correlated with the compounds' antiproliferative activities. The penultimate C-terminal selenocysteine residue was shown to be a possible target. Conversely, at sublethal concentrations, these agents induced an adaptive antioxidant response through Nrf2-mediated upregulation of phase II enzymes, including TrxR induction. We conclude from the results obtained that TrxR inactivation contributes at least partly to cinnamaldehyde cytotoxicity. These Michael acceptor molecules can potentially be exploited for use in different concentrations in chemotherapeutic and chemopreventive strategies.     

The analysis of cancer chemoprevention experiments

This paper is concerned with the analysis of certain cancer chemoprevention experiments that involve Type I censoring. In experiments of this nature, two common response variables are the number of induced cancers and the rate at which they develop. In this study we assume that the number of induced tumors and their times to detection are described by the Poisson and gamma distributions, respectively. Using the method of maximum likelihood, we discuss a procedure for estimating the parameters characterizing these two distributions. We apply standard techniques in order to construct a confidence region and conduct a hypothesis test concerning the parameters of interest. We discuss a method for comparing the effects of two different treatments using the likelihood ratio principle. A technique for isolating group differences in terms of the mean number of promoted tumors and the mean time to detection is described. Using the techniques developed in this paper, we reanalyze an existing data set in the cancer chemoprevention literature and obtain contrasting results.     

On the natural chemoprevention of cancer

Cancer is a complex disease to treat and the treatments have not progressed significantly in the last few years. Alternative strategies such as chemoprevention are being investigated. Proof of concept of chemoprevention has been shown with the non-steroidal anti-inflammatory drugs (NSAIDs); however there is significantly more interest in plant and naturally available compounds for chemoprevention. A number of different naturally occurring chemical compounds are reviewed here for their potential benefits and the pathways which they may target, in particular the polyamine pathway.     

Mechanisms of cancer chemoprevention by curcumin

Curcumin is a major component of the Curcuma species, which is commonly used as a yellow coloring and flavoring agent in foods. Curcumin has shown anti-carcinogenic activity in animals as indicated by its ability to block colon tumor initiation by azoxymethane and skin tumor promotion induced by phorbol ester TPA. Recently, curcumin has been considered by oncologists as a potential third generation cancer chemopreventive agent, and clinical trials using it have been carried out in several laboratories. Curcumin possesses anti-inflammatory activity and is a potent inhibitor of reactive oxygen-generating enzymes, such as lipoxygenase/cyclooxygenase, xanthine dehydrogenase/oxidase and inducible nitric oxide synthase. Curcumin is also a potent inhibitor of protein kinase C, EGF-receptor tyrosine kinase and IkappaB kinase. In addition, curcumin inhibits the activation of NFkappaB and the expression of c-jun, c-fos, c-myc and iNOS. It is proposed that curcumin may suppress tumor promotion by blocking signal transduction pathways in the target cells. Curcumin was first biotransformed to dihydrocurcumin and tetrahydrocurcumin, and these compounds were subsequently convened into monoglucuronide conjugates. The experimental results suggest that curcumin-glucuronide, dihydrocurcumin-glucuronide, tetrahydrocurcumin-glucuronide and tetrahydrocurcumin are major metabolites of curcumin in mice.     

Connexins as targets for cancer chemoprevention and chemotherapy

Cells within a tissue continuously interact to coordinate normal tissue functions and maintain homeostasis. Gap junctional communication (GJC), mediated by the connexin protein family, allows this type of intercellular crosstalk resulting in synchronized and cooperative tissue behavior such as cardiac contraction. In cancer, loss of these types of cell:cell interactions has been shown to facilitate tumorigenesis and enable the autonomous cell behavior associated with transformed cells. Indeed, many human tumor lines demonstrate deficient or aberrant GJC and/or loss of connexin expression. Restoration of exogenous connexin expression/GJC function is correlated with increased cell growth control both in vitro and in vivo. In support of this growth regulatory hypothesis, decreased connexin expression has been observed in situ in early human neoplasia of various organs. Additionally, genetically engineered mice lacking particular connexins (Connexins 32 or 43) exhibit increased susceptibility to radiation and chemically-induced liver and/or lung tumorigenesis. These studies strongly suggest that connexins and GJC serve a tumor suppressor role. Consistent with this proposed role, in a model cell culture system, retinoids and carotenoids up-regulate Connexin43 (Cx43) expression in direct proportion to their ability to suppress carcinogen-induced neoplastic transformation. Here, we discuss the important role of connexins and GJC in tumorigenesis and suggest the possibility of connexins as potential anti-oncogenic targets for chemoprevention and/or chemotherapy.     

Mechanistic approaches to chemoprevention of mutation and cancer

This is the eighth special issue of 'Mutation Research' to focus on antimutagenesis and anticarcinogenesis. It covers a wide range of mechanisms from prevention of cancer initiation by antimutagens through to inhibition of tumour angiogenesis and selective estrogen receptor modulators. New screening methods and new biomarkers are also elucidated. There is increasing reason to believe that the long-term use of a combination of anticarcinogens, over an extended time span, may provide a realistic prospect of reducing the current burden of human cancers.     

COX-2 and iNOS, good targets for chemoprevention of colon cancer

Cyclooxygenase (COX)-2 has been suggested to play an important role in colon carcinogenesis. We found that the COX-2 selective inhibitor, nimesulide, reduces azoxymethane (AOM)-induced aberrant crypt foci (ACF) in rats and colon carcinogenesis in mice, as well as formation of intestinal polyps in Min mice. Thus, selective inhibitors of COX-2, which catalyzes the synthesis of prostanoids, could be good candidates as chemopreventive agents against colon cancer. Examination of the effect of prostanoid receptor deficiency and a selective antagonist of prostanoid receptor on the development of AOM-induced ACF in mice revealed the involvement of the EP1 receptor. Moreover, a selective EP1 antagonist reduced the number of intestinal polyps in Min mice. These results suggest that PGE2 contributes to colon carcinogenesis through binding to the EP1 receptor. Nitric oxide synthase (NOS) is known to be overexpressed in colon cancers of humans and rats, and a NOS inhibitor, L-NG-nitroarginine methyl ester, was found to inhibit the development of AOM-induced ACF in rats. Thus, NOS including iNOS could also be a good target for chemoprevention of colon cancer, as in the COX-2 case.     

Nitric oxide signaling in colon cancer chemoprevention

Nitric oxide (NO) is a pleiotrophic regulator, pivotal to numerous biological processes, including vasodilation, neurotransmission, and macrophage-mediated immunity. The highly reactive free radicals, produced by NO synthases (NOS) have been implicated in the modulation of carcinogenesis. Over-expression of inducible NOS (iNOS), a common phenomenon during chronic inflammatory conditions, generates sustainable amounts of NO, that its reactive intermediates are mutagenic, causing DNA damage or impairment of DNA repair, has been well established in carcinogenesis. Recent studies also implicate NO as having a key signaling molecule that regulates processes of tumorigenesis. Increased expression of iNOS has been observed in tumors of the colon, lung, oropharynx, reproductive organs, breast, and central nervous system besides its occurrence in chronic inflammatory diseases. Progression of a large majority of human and experimental colon tumors appears to progress by NO resulting from stimulation of proinflammatory cytokines, and inactivation (nitrosylation) of p53 mediated caspase activities in the tumors, whereas in some cases it associated with induction of apoptosis and tumor regression. This dichotomy is largely explained by the complexity of signaling pathways in tumor cells, that respond to NO very differently depending on its concentration. p53 mutation, functional loss, activation, and inactivation of apoptotic proteins all have been linked with NO resistance and dependence. Evidence from both in vitro and in vivo experiments support that NO and its reactive metabolite peroxynitrite stimulate COX-2 activity leading generation of tumor growth enhancing prostaglandins. Thus, NO mediated signaling can augment the tumor growth and metastasis by promoting invasive and angiogenic properties of tumor cells, which includes triggering and activation of COX-2. Thus, developing selective inhibitors of iNOS and NO-releasing agents may lead to important strategies for chemoprevention of colon cancer. Chemoprevention studies at preclinical level with several selective inhibitors of iNOS in both chemically and transgenic models of colon cancer are encouraging.     

Tailoring cancer chemoprevention regimens to the individual

The present article, which is a tribute to the memory of Dr. Edward Bresnick, emphasizes the importance of environmental and life-style factors for cancer causation in the human population and points out approaches to cancer prevention. These approaches include vaccinations for the prevention of cancers that are caused by infectious agents as well as the use of cancer chemopreventive agents. The use of tamoxifen and letrozole to prevent breast cancer, finasteride to prevent prostate cancer, sunscreens or topical applications of 5-fluorouracil to prevent sunlight-induced skin cancer, and aspirin or calcium to prevent colon cancer are a few examples of cancer chemoprevention in high risk individuals and in the general population. An underdeveloped area of cancer chemoprevention is the use of combinations of agents that work by different mechanisms. It was pointed out that animal studies indicate that many cancer chemopreventive agents inhibit carcinogenesis under one set of experimental conditions but enhance carcinogenesis under another set of experimental conditions. These observations suggest that tailoring the chemopreventive regimen to the individual or to groups of individuals living under different environmental conditions or with different mechanisms of carcinogenesis may be an important aspect of cancer chemoprevention in human populations. How to tailor cancer chemoprevention regimens to the individual is an important challenge for the future.     

A review of animal model studies of tomato carotenoids,lycopene, and cancer chemoprevention

There are relatively few reports on the cancer chemopreventive effects of lycopene or tomato carotenoids in animal models. The majority, but not all, of these studies indicate a protective effect. Inhibitory effects were reported in two studies using aberrant crypt foci, an intermediate lesion leading to colon cancer, as an end point and in two mammary tumor studies, one using the dimethylbenz(a)anthracene model, and the other the spontaneous mouse model. Inhibitory effects were also reported in mouse lung and rat hepatocarcinoma and bladder cancer models. However, a report from the author's laboratory found no effect in the N-nitrosomethylurea-induced mammary tumor model when crystalline lycopene or a lycopene-rich tomato carotenoid oleoresin was administered in the diet. Unfortunately, because of differences in routes of administration (gavage, intraperitoneal injection, intra-rectal instillation, drinking water, and diet supplementation), species and strain differences, form of lycopene (pure crystalline, beadlet, mixed carotenoid suspension), varying diets (grain-based, casein based) and dose ranges (0.5-500 ppm), no two studies are comparable. It is clear that the majority of ingested lycopene is excreted in the feces and that 1000-fold more lycopene is absorbed and stored in the liver than accumulates in other target organs. Nonetheless, physiologically significant (nanogram) levels of lycopene are assimilated by key organs such as breast, prostate, lung, and colon, and there is a rough dose-response relationship between lycopene intake and blood levels. Pure lycopene was absorbed less efficiently than the lycopene-rich tomato carotenoid oleoresin and blood levels of lycopene in rats fed a grain-based diet were consistently lower than those in rats fed lycopene in a casein-based diet. The latter suggests that the matrix in which lycopene is incorporated is an important determinant of lycopene uptake. A number of issues remain to be resolved before any definitive conclusions can be drawn concerning the anticancer effects of lycopene. These include the following: the optimal dose and form of lycopene, interactions among lycopene and other carotenoids and fat soluble vitamins such as vitamin E and D, the role of dietary fat in regulating lycopene uptake and disposition, organ and tissue specificity, and the problem of extrapolation from rodent models to human populations.     

Potential mechanisms of cancer chemoprevention by anthocyanins

Anthocyanins are the chemical components that give the intense color to many fruits and vegetables, such as blueberries, red cabbages and purple sweet potatoes. Epidemiological investigations have indicated that the moderate consumption of anthocyanin products such as red wine or bilberry extract is associated with a lower risk of cardiovascular disease and improvement of visual functions. Recently, there is increasing interesting in the pharmaceutical function of anthocyanins. This review summarizes current knowledge on the various molecular evidences of cancer chemoprevention by anthocyanins. These mechanisms can be subdivided into the following aspects: 1) the antioxidation; 2) the molecular mechanisms involved in anticarcinogenesis; 3) the molecular mechanisms involved in the apoptosis induction of tumor cells. Finally, the bioavailability and structure-activity relationship of anthocyanins are also summarized.