Current Cancer Drug Targets - Volume 7, Issue 5, 2007

Malignant cells are characterized by alterations in multiple signaling pathways that promote proliferation, inhibit apoptosis, promote angiogenesis in the case of solid tumors, and enable cancer cells to invade and migrate through tissues. A variety of foods and their bioactive dietary constituents appear to have merit in reducing cancer risk and modifying tumor behavior. All of the major signaling pathways, which are deregulated in cancer, and which serve as potential targets for cancer prevention, have been reported to respond to one or more dietary components. Herein, we provide a brief overview of the importance of diet as a modifier of carcinogen metabolism, DNA repair, cell proliferation, apoptosis, inflammation, immunity, differentiation, angiogenesis, hormonal regulation and cellular energetics. This special issue of Current Cancer Drug Targets provides a collection of articles from researchers who are actively involved in examining the role of dietary components in cancer prevention and therapy. The remaining articles in this series provide more details about the specifics about the importance of these processes during carcinogenesis and proof-of-principal about the modifying capabilities of food patterns, specific foods and individual bioactive food components.

Prevention is one of the most important and promising strategies to control cancer. Many dietary bioactive compounds, mostly phytochemicals, have been found to decrease the risk of carcinogenesis. Modulating the metabolism and disposition pathways of carcinogens represents one of the major mechanisms by which dietary compounds prevent carcinogenesis. In the present review, the specific molecular targets of dietary compounds within carcinogen metabolism, including various enzymes and transporters and their regulatory signaling pathways, are briefly reviewed. The expression of phase I enzymes, which presumably mostly activate carcinogens, is mainly regulated by xenobiotics sensing nuclear receptors such as AhR, CAR, PXR, and RXR. On the other hand, phase II enzymes catalyze the conjugations of carcinogens and generally are transcriptionally controlled by the Nrf2/ARE signaling pathways. The Nrf2/ARE signaling pathway, which regulates the expression of many detoxifying enzymes, is a major target of dietary compounds. The final excretion of carcinogens and their metabolites is mediated by phase III transporters, which share many regulatory mechanisms with phase I/II enzymes. Indeed, the expression of metabolizing enzymes and transporters is often coordinately regulated. Besides transcriptional regulation, the activities of phase I/II enzymes and phase III transporters could be directly activated or inhibited by dietary compounds. Furthermore, genetic polymorphisms have profound effects on the individual response to dietary compounds. Finally, the effects of cancer prevention and the risk of carcinogenesis are determined by the network composed of known/unknown molecular targets and signaling pathways and its interaction with various xenobiotics, including carcinogens, drugs, and diet. With the rapid advances in the post genomic sciences, it could be possible to decipher this network and better predict the clinical outcomes of cancer prevention by dietary bioactive compounds.

At its most fundamental, cancer is a genetic disease in the sense that the primary events in tumorigenesis involve damage to the genome. The genome is subject to damage continuously from both exogenous agents and endogenous processes but this becomes functionally important only if the damage is not detected and resolved in a timely and effective manner. In mammals there are 5 DNA repair pathways, encoded by approximately 150 genes, which appear to have arisen early in evolution and which are highly conserved. Given the substantial epidemiological and experimental evidence that variation in dietary intake accounts for a significant proportion of the variance in cancer prevalence, an a priori case can be made that dietary factors may influence the effectiveness of DNA repair. A review of the literature has identified 4 observational and 8 intervention studies in human subjects where DNA repair (or a component thereof) has been measured in relation to nutrition. This rather limited evidence base precludes drawing definitive conclusions, but the fact that there were significant effects of dietary supplements in 5 out of the 8 intervention studies suggests that food components and/or nutritional status may influence DNA repair. This review considers possible molecular mechanisms through which such factors could modulate repair.

Specific bioactive dietary components, such as the steroid receptor superfamily ligands vitamins A and D, have been studied extensively as potential cancer preventive and therapeutic agents due to their ability to regulate key processes in a variety of cell types which are dysregulated in neoplastic transformation namely, proliferation and differentiation. Alteration of one or more factors that regulate cell cycle control has been described as a predisposing event for early tumor development. In addition to tumor cell proliferation, the viability, growth and metastasis of solid tumors are also dependent on the vascularization of the tumor and establishment of blood flow. Both vitamins A and D exhibit anti-angiogenic properties which further strengthen their role as potential targets for the prevention and treatment of cancer. This review focuses on the role of vitamins A and D in preventing early tumor initiation and progression via control of the cell cycle in both tumor and vascular endothelial cells.

Apoptosis, a form of programmed cell death, is a pivotal defense against cancer and is essential in maintaining tissue homeostasis. Many diseases including cancer have been associated with aberrantly regulated apoptotic cell death, thus elucidation of events associated with both apoptosis and carcinogenesis provides the opportunity for dietary intervention with the plethora of bioactive components in the diet. Apoptosis occurs primarily through two well-recognized pathways in cells including the intrinsic, mitochondrialmediated pathway and the extrinsic, death receptor-mediated pathway. Dietary components can modulate apoptosis through effects on protein expression and function, mRNA expression, and on the human genome, either directly or indirectly, to modulate gene expression. Thus, apoptosis is an emerging target of dietary bioactive agents. However, apoptosis is a complex process, with numerous specific targets within each pathway that may or may not overlap. Furthermore, biological systems are also extremely complex and exhibit properties that extend far beyond observations associated with each independent cellular process. This is further complicated by the temporal nature of many of these effects. As a result, it is critical to evaluate the entire biological system from the nutrigenomics perspective to include critical evaluation of DNA polymorphisms or SNPs of a gene, expression of that specific gene, expression of specific processed mRNA (alternative splicing), protein production from that mRNA, post-translational modification of the resultant protein, and formation of respective metabolites. Evolution of the fields of nutrigenetics, epigenomics, transcriptomics, proteomics, and metabolomics has begun to permit this approach so that a comprehensive picture emerges from not only a single cell but tissues and whole organisms. Studies such as these can ultimately be used to study tumors to understand the molecular events that accompany carcinogenesis and perturbations that occur during cell death processes and how an individual's response to diet can impact these processes.

A causal association between inflammation and cancer has long been suspected. Multiple lines of compelling evidence from clinical, epidemiologic and laboratory studies support that inflammation plays a critical role in the promotion and progression stages of carcinogenesis. Recent progress in our understanding of the molecular biology of cancer highlights the intracellular signal transduction network, including that involved in mediating the inflammatory response, which often functions abnormally during carcinogenesis. One of the key players in inflammatory signaling is cyclooxygenase-2 (COX-2). Aberrant upregulation of COX-2 is frequently observed in various precancerous and malignant tissues. Pro-inflammatory stimuli trigger the activation of an intracellular signal transduction network comprising proline-directed serine/threonine kinases, and their downstream transcription factors, resulting in an inappropriate induction of COX-2. Therefore, the normalization of inappropriately overamplified signaling cascades implicated in chronic inflammation-associated carcinogenesis by use of COX-2 specific inhibitors has been recognized as a rational and pragmatic strategy in molecular target-based cancer prevention. This review highlights the cancer preventive effects of some anti-inflammatory phytochemicals derived from edible plants, and their underlying molecular mechanisms with a focus on representative transcription factors and upstream kinases responsible for COX-2 induction.

Dietary bioactive food components that interact with the immune response have considerable potential to reduce the risk of cancer. Reduction of chronic inflammation or its downstream consequences may represent a key mechanism that can be reduced through targeting signal transduction or through antioxidant effects. Major classes of macronutrients provide numerous examples, including amino acids such as glutamine or arginine, lipids such as the omega-3 polyunsaturated fatty acids, DHA or EPA, or novel carbohydrates such as various sources of beta-glucans. Vitamins such as C and E are commonly used as antioxidants, while zinc and selenium are minerals with a wide spectrum of impacts on the immune system. Some of the most potent immunomodulators are phytochemicals such as the polyphenols, EGCG or curcumin, or isothiocyanates such as PEITC. There is accumulating evidence for cancer prevention by probiotics and prebiotics, and these may also affect the immune response. Genomic approaches are becoming increasingly important in characterising potential mechanisms of cancer prevention, optimising the rational selection of dietary bioactive food components, or identifying humans with differing nutrient requirements for cancer protection.

Findings with experimental rodent models reveal that exposures to dietary factors during the in utero and pubertal periods when the mammary gland is undergoing extensive modeling and re-modeling, alter susceptibility to develop mammary tumors. Similar observations have been made in humans: childhood exposure to genistein in soy or to some other bioactive food components reduces later breast cancer risk, although they may have no effect if consumed during adulthood. Thus, food components may be more effective in affecting cancer risk in some periods of life than others. Many of these dietary exposures modify fetal and postnatal hormonal environment, including changing the concentrations of estrogens and leptin. The hormonal alterations then may induce persistent epigenetic changes by affecting gene promoter regions or by inducing histone modifications that affect chromatin transcription. The targets of epigenetic changes are likely to be the terminal end buds (TEBs), the structures where carcinogen-induced mammary tumors in rats and mice are initiated. More specifically, the site of these changes in TEBs may be the stem cells and their niche; this might explain how an exposure early in life affects the risk of breast cancer decades later. Similar structures in women, called terminal ductal lobular units, are the sites where most human breast cancers rise. According to this hypothetical model, cancer is initiated only when the epigenetically altered cells are exposed to carcinogens/radiation, etc. during adult life. In a “normal” stem cell or its niche, cancer initiating exposures do not necessarily cause cancer, because the cells can either repair the damage or undergo apoptosis. Thus, the most likely molecular targets of early life dietary exposures are genes that regulate DNA adduct formation, repair DNA damage or induce apoptosis, such as genes affecting cellular metabolism, tumor suppressor genes or genes promoting cell survival. It is possible that some of these epigenetic changes also explain why the number of TEBs generally, but not always, correlates with breast cancer risk. This hypothesis may imply that adult intake of some bioactive dietary components reduces cancer risk increased by early life dietary exposures or inhibits tumor growth by reversing epigenetic changes in various molecular targets.

Human solid tumors remain latent in the absence of angiogenesis since it is a critical process for their further growth and progression. Experimental evidence suggests that targeting tumor angiogenesis may be a novel strategy to check tumor growth and metastases. Recent studies suggest that several bioactive food components can suppress tumor growth by inhibiting angiogenesis. This suppression occurs because of a direct effect on the tumor, as well as a direct effect on vascular endothelial cells. These food components can target epigenetic processes and thereby suppress the pro-angiogenic tumor microenvironment. One likely epigenetic target is inducible nitric oxide synthase (iNOS). iNOS is known to regulate vascular endothelial growth factor (VEGF) expression, and thereby tumor angiogenesis. The ability of food components to influence the inducible form of cyclooxygenase, COX-2 may also contribute to their impact on tumor growth and angiogenesis. This review focuses on recent developments related to the angiogenic role of the iNOSVEGF axis and how dietary components may target this axis. Overall, studies suggest that the anti-angiogenic potential of physiologically concentrations of relevant food components could be used as a practical approach for cancer prevention and intervention.

The prevalence of obesity, an established epidemiologic risk factor for many cancers, has risen steadily for the past several decades in the U.S. Particularly alarming are the increasing rates of obesity among children, portending continuing increases in the rates of obesity and obesity-related cancers for many years to come. Unfortunately, the mechanisms underlying the association between obesity and cancer are not well understood. In particular, the effects and mechanistic targets of interventions that modulate energy balance, such as reduced calorie diets and physical activity, on the carcinogenesis process have not been well characterized. The purpose of this review is to provide a strong foundation for future mechanistic-based research in this area by describing key animal and human studies of energy balance modulations involving diet, exercise, or pharmaceutical agents and by focusing on the interrelated pathways affected by alterations in energy balance. Particular attention in this review is placed on the components of the insulin/IGF-1/Akt pathway, which has emerged as a predominant target for disrupting the obesity-cancer link. Also discussed is the promise of global approaches, including genomics, proteomics, and metabolomics, for the elucidation of energy balance-responsive pathways. The ultimate goal of this work is to provide the missing mechanistic information necessary to identify targets for the prevention and control of cancers related to or caused by excess body weight.

Kit a type III receptor tyrosine kinase, along with its ligand the stem cell factor, play a critical role in normal cell growth, differentiation, development and survival. Ligand independent activation of kit (dysregulated kit function) has been found to be an important component of oncogenesis in a large number of neoplastic disorders such as systemic mastocytosis, gastro intestinal stromal tumors, germ cell tumors, acute myelogenous leukemia with the disruption of the core binding factor, amongst others. The identification of small molecule inhibitors with activity against Kit, has offered a wider and more effective range of therapeutic options in the treatment of these neoplastic processes. Novel tyrosine kinase inhibitors such as imatinib, nilotinib and dasatinib, have been found to be effective in the management of various subtypes of systemic mastocytosis and gastrointestinal stromal tumors. Non-tyrosine kinase inhibitors like rapamycin, 17-AAG and IMD- 0354 have been added to the therapeutic armamentarium, with the hope that combination therapy might have a synergistic effect, or prevent/delay the development of drug resistance.

The experimental cytotoxic drug cyclopentenyl cytosine (CPEC) is an analogue of cytidine. Besides its antiviral effect, its potential use in the treatment of cancer has become an important area of research. CPEC is activated by intracellular phosphorylation ultimately forming its metabolite CPEC-TP. CPEC-TP is a non competitive inhibitor of cytidine-5'-triphosphate synthetase (CTPsynthetase), an important enzyme in the formation of CTP. Studies have shown that cancer cells have a high CTP synthetase activity, thus making them interesting targets for chemotherapy. CPEC has been preclinically studied in different malignancy models. In vitro results on leukemia show activity in the nanomolar range on several cell lines. However in vivo results are conflicting and the findings vary from increase in life span over 100% to only limited effectiveness. Interesting results have been obtained in colorectal and neuroblastoma cells. In several neuroblastoma cell lines incubation with CPEC in combination with cytarabine or gemcitabine has resulted in increased cell death compared to incubation with with only one of the agents. CPEC has been studied in a phase I trial in patients with solid tumors. In five of 26 patients unexplained cardiotoxicity (extreme hypotension) occurred. The cardiotoxic effects could not be reproduced in animal models. However, precautions should be taken when using this drug in future clinical trials. Low dosage of CPEC seems necessary and intensive cardiac monitoring is advisable. In this manuscript, it is demonstrated that CPEC has an anti-cancer effect in several tumor models: CPEC might be a potentially useful drug in anticancer treatment.