Retinoids as chemopreventive agents

Retinoids are promising agents for cancer chemoprevention. The myriad effects of retinoids on biological processes including development, differentiation, homeostasis, carcinogenesis and apoptosis are mediated through their molecular targets, the retinoid and rexinoid receptors. Tissue specific expression patterns, ligand specificities, receptor numbers, their distinct functions and functional redundancy make retinoid signaling highly complex. The cross-talks of these receptors with cell surface receptors signaling pathways, as well as their interactions with multiple co-activators and co-repressors further add to the complexity of the pleiotropic effects of retinoids. Elucidation of retinoid signaling pathways and indepth understanding of the mechanisms that underlie the anti-proliferative and apoptotic action of retinoids has paved the way for designing synthetic retinoids for effective chemoprevention and therapy of cancer. Development of receptor selective synthetic retinoids is a major focus of molecular retinoid development. Other new avenues encompass identification of novel retinoid regulated genes, orphan-receptor ligands/functions, novel retinoid mechanisms involving receptor-independent apoptosis inducing activity and synergistic combinations with other agents for cancer prevention and therapy. This review focuses on recent advances in the understanding of molecular mechanisms underlying the action of retinoids and retinoid molecular targeting studies designed primarily to develop retinoids with reduced toxicity, while maintaining or enhancing activity in context of chemoprevention. The clinical efficacy of retinoid based chemoprevention trials is discussed.     

Mitochondria: A novel target for the chemoprevention of cancer

The mitochondria have emerged as a novel target for anticancer chemotherapy. This tenet is based on the observations that several conventional and experimental chemotherapeutic agents promote the permeabilization of mitochondrial membranes in cancerous cells to initiate the release of apoptogenic mitochondrial proteins. This ability to engage mitochondrial-mediated apoptosis directly using chemotherapy may be responsible for overcoming aberrant apoptosis regulatory mechanisms commonly encountered in cancerous cells. Interestingly, several putative cancer chemopreventive agents also possess the ability to trigger apoptosis in transformed, premalignant, or malignant cells in vitro via mitochondrial membrane permeabilization. This process may occur through the regulation of Bcl-2 family members, or by the induction of the mitochondrial permeability transition. Thus, by exploiting endogenous mitochondrial-mediated apoptosis-inducing mechanisms, certain chemopreventive agents may be able to block the progression of premalignant cells to malignant cells or the dissemination of malignant cells to distant organ sites as means of modulating carcinogenesis in vivo. This review will examine cancer chemoprevention with respect to apoptosis, carcinogenesis, and the proapoptotic activity of various chemopreventive agents observed in vitro. In doing so, I will construct a paradigm supporting the notion that the mitochondria are a novel target for the chemoprevention of cancer.     

Targeting signal transduction pathways by chemopreventive agents

Cancer is a dynamic process that involves many complex factors, which may explain why a "magic bullet" cure for cancer has not been found. Death rates are still rising for many types of cancers, which possibly contributes to the increased interest in chemoprevention as an alternative approach to the control of cancer. This strategy for cancer control is based on the presumption that because cancer develops through a multi-step process, each step may be a prospective target for reversing or suppressing the process. Thus, the design and development of chemopreventive agents that act on specific and/or multiple molecular and cellular targets is gaining support as a rational approach to control cancer. Nutritional or dietary factors have attracted a great deal of interest because of their perceived ability to act as highly effective chemopreventive agents. They are professed as being generally safe and may have efficacy as chemopreventive agents by preventing or reversing premalignant lesions and/or reducing second primary tumor incidence. Many of these dietary compounds appear to act on multiple target signaling pathways. Some of the most interesting and well documented are resveratrol and components of tea, including EGCG, theaflavins and caffeine. This review will focus on recent work regarding three well-accepted cellular/molecular mechanisms that may at least partially explain the effectiveness of selected food factors, including those indicated above, as chemopreventive anti-promotion agents. These food compounds may act by: (1) inducing apoptosis in cancer cells; (2) inhibiting neoplastic transformation through the inhibition of AP-1 and/or NF-kappaB activation; and/or (3) suppressing COX-2 overexpression in cancer cells.     

Prevention of prostate cancer through custom tailoring of chemopreventive regimen

One practical way to control cancer is through chemoprevention, which refers to the administration of synthetic or naturally occurring agents to block, reverse or delay the process of carcinogenesis. For a variety of reasons, the most important of which is human acceptance, for chemopreventive intervention naturally occurring diet-based agents are preferred over synthetic agents. For a long time, the prevailing mantra of cancer chemoprevention has been: "Find effective agents with acceptable or no toxicity and use them in preventing cancer in relatively healthy people or individuals at high risk for developing cancer". In pursuing this goal many naturally occurring phytochemicals capable of affording protection against carcinogenesis in preclinical settings in experimental animals have been described. However, clinical trials of single agents have yielded disappointing results. Since carcinogenesis is a multistage phenomenon in which many normal cellular pathways become aberrant, it is unlikely that one agent could prove effective in preventing cancer. This review underscores the need to build an armamentarium of naturally occurring chemopreventive substances that could prevent or slow down the development and progression of prostate cancer. Thus, the new effective approach for cancer prevention "building a customized mechanism-based chemoprevention cocktail of naturally occurring substances" is advocated.     

Induction of apoptosis in tumor cells by naturally occurring sulfur-containing compounds

Chemoprevention is regarded as one of the most promising and realistic approaches in the prevention of human cancer. Among naturally occurring products, sulfur-containing compounds (OSCs), especially garlic compounds (GCs) and isothiocyanates (ITCs), represent two important and promising chemopreventive families because of their potent chemopreventive effects in various in vivo and in vitro models. In recent years, numerous investigations have shown that sulfur-containing compounds induce apoptosis in multiple cell lines and experimental animals. In the course of apoptosis induction by GCs and ITCs, multiple signal-transduction pathways and apoptosis intermediates are modulated. In particular, modulation of MAPKs and production of reactive oxygen species (ROS) seem to play pivotal roles in apoptosis induction by most GCs and ITCs. However, the role of P53 is still controversial. Based on present knowledge, GCs and ITCs may target not only the metabolism of carcinogens but also apoptosis signaling molecules. The effects of ITCs and GCs at multiple points of cancer development make these compounds highly promising candidates in cancer chemoprevention. However, the mechanisms of their anticancer effects are not fully understood, and further studies are required, especially to elucidate the role of cell-death receptors (the extrinsic pathway) and whether these agents induce apoptotic effects in non-tumor cells.     

Carcinogenesis, cancer therapy and chemoprevention

Carcinogenesis and cancer therapy are two sides of the same coin, such that the same cytotoxic agent can cause cancer and be used to treat cancer. This review links carcinogenesis, chemoprevention and cancer therapy in one process driven by cytotoxic agents (carcinoagents) that select either for or against cells with oncogenic alterations. By unifying therapy and cancer promotion and by distinguishing nononcogenic and oncogenic mechanisms of resistance, I discuss anticancer- and chemopreventive agent-induced carcinogenesis and tumor progression and, vice versa, carcinogens as anticancer drugs, anticancer drugs as chemopreventive agents and exploiting oncogene-addiction and drug resistance for chemoprevention and cancer therapy.     

Mechanistic understanding of β-cryptoxanthin and lycopene in cancer prevention in animal models

To better understand the potential function of carotenoids in the chemoprevention of cancers, mechanistic understanding of carotenoid action on genetic and epigenetic signaling pathways is critically needed for human studies. The use of appropriate animal models is the most justifiable approach to resolve mechanistic issues regarding protective effects of carotenoids at specific organs and tissue sites. While the initial impetus for studying the benefits of carotenoids in cancer prevention was their antioxidant capacity and pro-vitamin A activity, significant advances have been made in the understanding of the action of carotenoids with regards to other mechanisms. This review will focus on two common carotenoids, provitamin A carotenoid β-cryptoxanthin and non-provitamin A carotenoid lycopene, as promising chemopreventive agents or chemotherapeutic compounds against cancer development and progression. We reviewed animal studies demonstrating that β-cryptoxanthin and lycopene effectively prevent the development or progression of various cancers and the potential mechanisms involved. We highlight recent research that the biological functions of β-cryptoxanthin and lycopene are mediated, partially via their oxidative metabolites, through their effects on key molecular targeting events, such as NF-κB signaling pathway, RAR/PPARs signaling, SIRT1 signaling pathway, and p53 tumor suppressor pathways. The molecular targets by β-cryptoxanthin and lycopene, offer new opportunities to further our understanding of common and distinct mechanisms that involve carotenoids in cancer prevention.This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.     

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.     

The selenium analog of the chemopreventive compound S,S′-(1,4-phenylenebis[1,2-ethanediyl])bisisothiourea is a remarkable inducer of apoptosis and inhibitor of cell growth in human non-small cell lung cancer

Lung cancer continues to be the leading cause of cancer deaths throughout the world and conventional therapy remains largely unsuccessful. Although, chemoprevention is a plausible alternative approach to curb the lung cancer epidemic, clinically there are no effective chemopreventive agents. Thus, development of novel compounds that can target cellular and molecular pathways involved in the multistep carcinogenesis process is urgently needed. Previous studies have suggested that substitution of sulfur by selenium in established cancer chemopreventive agents may result in more effective analogs. Thus in the present study we selected the chemopreventive agent S,S′-(1,4-phenylenebis[1,2-ethanediyl])bisisothiourea (PBIT), also known to inhibit inducible nitric oxide synthase (iNOS), synthesized its selenium analog (Se-PBIT) and compared both compounds in preclinical model systems using non-small cell lung cancer (NSCLC) cell lines (NCI-H460 and A549); NSCLC is the most common histologic type of all lung cancer cases. Se-PBIT was found to be superior to PBIT as an inducer of apoptosis and inhibitor of cell growth. Se-PBIT arrested cell cycles at G1 and G2-M stage in both A549 and H460 cell lines. Although both compounds are weakly but equally effective inhibitors of iNOS protein expression and activity, only Se-PBIT significantly enhanced the levels of p53, p38, p27 and p21 protein expression, reduced levels of phospholipase A2 (PLA2) but had no effect on cyclooxygenase-2 (COX-2) protein levels; such molecular targets are involved in cell growth inhibition, induction of apoptosis and cell cycle regulation. The results indicate that Se-PBIT altered molecular targets that are involved in the development of human lung cancer. Although, the mechanisms that can fully account for these effects remain to be determined, the results are encouraging to further evaluate the chemopreventive efficacy of Se-PBIT against the development of NSCLC in a well-defined animal model.     

Preventive medicine: biomonitoring and chemoprevention

The aim of chemoprevention is to delay or prevent the development of pathological conditions, or to correct abnormal regulatory mechanisms and in some cases even reverse the process. For this intervention to succeed, biomarkers must be found that characterize impaired health status in a phase when impairment is still reversible. Genotoxicological parameters may function as biomarkers of this kind, such as inhibition or delay of mitosis, inhibition of apoptosis, increase in the number of chromosomal aberrations, decrease in the capacity of DNA repairing enzymes, or parameters characterizing immunological status (eg. decreased NK activity). With the help of early signs, impending negative changes or illness (which could not be prevented without effective chemoprevention) can be predicted in apparently healthy individuals. Thus, the first and most important step in chemoprevention is risk characterization. Risk characterization is a complex concept, which includes risk analysis, risk assessment and risk management. Chemoprevention is a part of the latter process. Biomarkers, which can be studied mainly by up-to-date molecular biological methods, are used in discovering risk factors and also in the assessment of caused biological effects. Besides avoiding risk factors, it is very important to strengthen the protective mechanisms, to promote the metabolism of toxic substances, and to repair damage (ward off denaturation of macromolecules caused by free oxygen radicals with antioxidants for example). Chemopreventive agents are therefore diverse in their targets. Most of them have antioxidant properties, such as plant-derived substances, like glycosides, flavonoids, various vitamins, carotinoids, and some trace elements such as selenium. Another group of chemopreventive agents inhibit cell proliferation, or induce programmed cell death (apoptosis). Another group influence terminal differentiation, or inhibit angiogenesis. Some chemopreventive agents affect the metabolism or detoxification of xenobiotics, or boost the functions of the immune system. In many cases these effects are mediated by the rate of methylation of DNA molecules.     

The role of human adult stem cells and cell-cell communication in cancer chemoprevention and chemotherapy strategies

Since carcinogenesis is a multi-stage, multi-mechanism process, involving mutagenic, cell death and epigenetic mechanisms, during the "initiation/promotion/and progression" phases, chemoprevention must be based on understanding the underlying mechanism(s) of each phase, In principle, prevention of each of these phases could reduce the risk to cancer. However, because reducing the mutagenic/initiation phase to a zero level is impossible, the most efficacious intervention would be at the promotion phase that requires a sustained exposure to promoting conditions/agents. In addition, assuming the "target" cells for carcinogenesis are the pluri-potent stem cells and their early progenitor or transit cells, chemoprevention strategies for inhibiting the promotion of these two types of pre-malignant "initiated" cells will require different kinds of agents. A hypothesis will be proposed that involves adult stem cells, which express Oct-4 gene and lack gap junctional intercellular communication (GJIC-) or the early progenitor cells which express GJIC+ and are partially-differentiated, if initiated, will be promoted by agents that either inhibit secreted negative growth regulators or by inhibitors of GJIC. Consequently, anti-tumor promoting chemopreventing agents to each of these two types of initiated cells must have different mechanisms of action and work on different target cells. Assuming stem cells are target cells for carcinogenesis, an alternative method of chemoprevention would be to reduce the stem cell pool. Many classes of anti-tumor promoter chemopreventive agents, such as green tea components, resveratrol, caffeic acid phenethylene ester, either up-regulate GJIC in stem cells or prevent the down regulation of GJIC by tumor promoters in early progenitor cells.     

Cell signaling pathways altered by natural chemopreventive agents

Epidemiological studies have indicated a significant difference in the incidence of cancers among ethnic groups, who have different lifestyles and have been exposed to different environmental factors. It has been estimated that more than two-thirds of human cancers, which are contributed by mutations in multiple genes, could be prevented by modification of lifestyle including dietary modification. The consumption of fruits, soybean and vegetables has been associated with reduced risk of several types of cancers. The in vitro and in vivo studies have demonstrated that some dietary components such as isoflavones, indole-3-carbinol (I3C), 3,3'-diindolylmethane (DIM), curcumin, (-)-epigallocatechin-3-gallate (EGCG), apigenin, etc., have shown inhibitory effects on human and animal cancers, suggesting that they may serve as chemopreventive agents. Experimental studies have also revealed that these components regulate the molecules in the cell signal transduction pathways including NF-kappaB, Akt, MAPK, p53, AR, and ER pathways. By modulating cell signaling pathways, these components, among other mechanisms, activate cell death signals and induce apoptosis in precancerous or cancer cells, resulting in the inhibition of cancer development and/or progression. This article reviews current studies regarding the effects of natural chemopreventive agents on cancer-related cell signaling pathways and provides comprehensive knowledge of the biological and molecular roles of chemopreventive agents in cancer cells.     

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.     

Cancer chemoprevention: a radical perspective

Cancer chemopreventive agents block the transformation of normal cells and/or suppress the promotion of premalignant cells to malignant cells. Certain agents may achieve these objectives by modulating xenobiotic biotransformation, protecting cellular elements from oxidative damage, or promoting a more differentiated phenotype in target cells. Conversely, various cancer chemopreventive agents can encourage apoptosis in premalignant and malignant cells in vivo and/or in vitro, which is conceivably another anticancer mechanism. Furthermore, it is evident that many of these apoptogenic agents function as prooxidants in vitro. The constitutive intracellular redox environment dictates a cell's response to an agent that alters this environment. Thus, it is highly probable that normal cells, through adaption, could acquire resistance to transformation via exposure to a chemopreventive agent that promotes oxidative stress or disrupts the normal redox tone of these cells. In contrast, transformed cells, which typically endure an oxidizing intracellular environment, would ultimately succumb to apoptosis due to an uncontrollable production of reactive oxygen species caused by the same agent. Here, we provide evidence to support the hypothesis that reactive oxygen species and cellular redox tone are exploitable targets in cancer chemoprevention via the stimulation of cytoprotection in normal cells and/or the induction of apoptosis in transformed cells.     

Resveratrol and black tea polyphenol combination synergistically suppress mouse skin tumors growth by inhibition of activated MAPKs and p53

Cancer chemoprevention by natural dietary agents has received considerable importance because of their cost-effectiveness and wide safety margin. However, single agent intervention has failed to bring the expected outcome in clinical trials; therefore, combinations of chemopreventive agents are gaining increasing popularity. The present study aims to evaluate the combinatorial chemopreventive effects of resveratrol and black tea polyphenol (BTP) in suppressing two-stage mouse skin carcinogenesis induced by DMBA and TPA. Resveratrol/BTP alone treatment decreased tumor incidence by ～67% and ～75%, while combination of both at low doses synergistically decreased tumor incidence even more significantly by ～89% (p<0.01). This combination also significantly regressed tumor volume and number (p<0.01). Mechanistic studies revealed that this combinatorial inhibition was associated with decreased expression of phosphorylated mitogen-activated protein kinase family proteins: extracellular signal-regulated kinase 1/2, c-Jun N-terminal kinase 1/2, p38 and increased in total p53 and phospho p53 (Ser 15) in skin tissue/tumor. Treatment with combinations of resveratrol and BTP also decreased expression of proliferating cell nuclear antigen in mouse skin tissues/tumors than their solitary treatments as determined by immunohistochemistry. In addition, histological and cell death analysis also confirmed that resveratrol and BTP treatment together inhibits cellular proliferation and markedly induces apoptosis. Taken together, our results for the first time lucidly illustrate that resveratrol and BTP in combination impart better suppressive activity than either of these agents alone and accentuate that development of novel combination therapies/chemoprevention using dietary agents will be more beneficial against cancer. This promising combination should be examined in therapeutic trials of skin and possibly other cancers.     

Anti-Oxidants from Green Tea and Pomegranate for Chemoprevention of Prostate Cancer

Among males, prostate cancer has become the second leading cause of cancer-related deaths in North America, with similar trends in many Western and developing countries. One way to control prostate cancer is through chemoprevention, which refers to the administration of synthetic or naturally occurring agents to block, reverse, or delay the process of carcinogenesis. For a variety of reasons, the most important of which is human acceptance, for chemopreventive intervention, naturally occurring diet-based agents are preferred. Prostate cancer is an ideal candidate disease for chemopreventive intervention, because it grows very slowly, likely for decades, before symptoms arise and a diagnosis is finally established, it has a long latency period, and it is typically diagnosed in men >50 years of age. Most chemopreventive agents are antioxidant in nature. We have been defining the usefulness of dietary anti-oxidants for chemoprevention of prostate and other cancers. It is increasingly appreciated that some of these dietary anti-oxidants are nature’s gift molecules endowed with cancer preventive and therapeutic properties. This review will focus on prostate cancer chemopreventive effects of polyphenolic anti-oxidants derived from green tea and pomegranate. It is a challenge to custom-tailor these gift molecules as cocktails in concentrations that can easily be consumed by humans for delaying prostate and other cancers.     

Enhanced Anti-tumor Activity by the Combination of the Natural Compounds (?)-Epigallocatechin-3-gallate and Luteolin: POTENTIAL ROLE OF p53*

Natural dietary agents have drawn a great deal of attention toward cancer prevention because of their wide safety margin. However, single agent intervention has failed to bring the expected outcome in clinical trials; therefore, combinations of chemopreventive agents are gaining increasingly popularity. In the present study, we investigated a combinatorial approach using two natural dietary polyphenols, luteolin and EGCG, and found that their combination at low doses (at which single agents induce minimal apoptosis) synergistically increased apoptosis (3–5-fold more than the additive level of apoptosis) in both head and neck and lung cancer cell lines. This combination also significantly inhibited growth of xenografted tumors in nude mice. The in vivo findings also were supported by significant inhibition of Ki-67 expression and increase in TUNEL-positive cells in xenografted tissues. Mechanistic studies revealed that the combination induced mitochondria-dependent apoptosis in some cell lines and mitochondria-independent apoptosis in others. Moreover, we found more efficient stabilization and ATM-dependent Ser¹⁵ phosphorylation of p53 due to DNA damage by the combination, and ablation of p53 using shRNA strongly inhibited apoptosis as evidenced by decreased poly(ADP-ribose) polymerase and caspase-3 cleavage. In addition, we observed mitochondrial translocation of p53 after treatment with luteolin or the combination of EGCG and luteolin. Taken together, our results for the first time suggest that the combination of luteolin and EGCG has synergistic/additive growth inhibitory effects and provides an important rationale for future chemoprevention trials of head and neck and lung cancers.     

Effects of dietary flavonoids on apoptotic pathways related to cancer chemoprevention

Epidemiological studies have described the beneficial effects of dietary polyphenols (flavonoids) on the reduction of the risk of chronic diseases, including cancer. Moreover, it has been shown that flavonoids, such as quercetin in apples, epigallocatechin-3-gallate in green tea and genistein in soya, induce apoptosis. This programmed cell death plays a critical role in physiological functions, but there is underlying dysregulation of apoptosis in numerous pathological situations such as Parkinson's disease, Alzheimer's disease and cancer. At the molecular level, flavonoids have been reported to modulate a number of key elements in cellular signal transduction pathways linked to the apoptotic process (caspases and bcl-2 genes), but that regulation and induction of apoptosis are unclear. The aim of this review is to provide insights into the molecular basis of the potential chemopreventive activities of representative flavonoids, with emphasis on their ability to control intracellular signaling cascades responsible for regulating apoptosis, a relevant target in cancer-preventive approach.