4.2.1 Tumor Initiation, Promotion, and Progression
Human epidemiological and experimental laboratory studies have long indicated that a latent period (often decades in humans) exists between the exposure to a chemical and the appearance of cancer. This led to the formulation of a sequential model that divided the carcinogenic process into 3 stages termed tumor initiation, tumor promotion, and tumor progression.
Tumor initiation was regarded as involving the interaction of a reactive chemical species (often a procarcinogen metabolite, see Sec. 4.2.3) with DNA to produce damage which, if not repaired before the next cell division, would lead to erroneous DNA replication resulting in fixation of mutations within the genome of individual cells. Thus 3 cellular functions are important in determining the likelihood of tumor initiation: the rate of procarcinogen activation, the efficiency and fidelity of DNA repair (see Chap. 5), and the capacity for cell proliferation. If a mutation disrupts the function of a gene whose product plays a role in maintaining the terminally differentiated function of the cell, the cell may acquire an altered (usually less differentiated) phenotype. Although initiation is irreversible, not all initiated cells will go on to establish a tumor, as many of these cells may die by apoptosis (see Chap. 9, Sec. 9.4), and further proliferation-enhancing signals are required for initiated cells to progress along the pathway to autonomous (cancerous) growth.
Tumor promotion was viewed as the clonal expansion of an initiated cell as a consequence of events that alter gene expression, so as to provide the cell with a selective proliferative advantage. Although there is no single unifying mechanistic feature of tumor-promoting agents, they tend to be nongenotoxic and to cause, directly or indirectly, cells to divide but not to terminally differentiate or die, resulting in the survival and proliferation of preneoplastic cells and the formation of benign lesions such as papillomas, nodules, or polyps. Many of these lesions may regress spontaneously, but a few cells may acquire additional mutations that allow them to progress to a malignant neoplasm. Figure 4–2 shows the structures of 3 established tumor promoters: tetradecanoyl phorbol acetate (TPA), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and phenobarbital. Early studies of mouse skin carcinogenesis illustrated the tumor-promoting activity of croton oil, which contains TPA, when applied following low doses of the tumor-initiator benzo[a]pyrene. Administration of benzo[a]pyrene led to skin tumors only when followed by repeated dosing with croton oil, even though croton oil alone was not carcinogenic.
Structures of some established tumor promoters.
Tumor progression described the stage whereby benign lesions acquire the ability to further grow, to invade adjacent tissues, and to establish distant metastases.
Although this simple 3-stage model can be a useful conceptual framework for understanding carcinogenesis, we now recognize that the process does not so neatly compartmentalize within such stages: multiple sequential mutations in combination with epigenetic changes and prolonged alterations in the cellular microenvironment are required to convert a normal cell into a malignant tumor.
4.2.2 Genetic Instability and the Hallmarks of Cancer
Increased genetic instability and alterations in karyotype are often observed in tumor cells (see Chap. 5, Sec. 5.2). Inherited or acquired mutations in genes such as p53, retinoblastoma (Rb) or DNA mismatch repair genes can create a "mutator phenotype" of enhanced random mutation that accelerates the accumulation of further spontaneous or chemical-induced DNA damage that may be required for the development of cancer. This concept underlines the potential importance of DNA-damaging chemicals not only at the initiation stage but also at later stages of the carcinogenic process. The carcinogenic process requires that cells acquire Hanahan and Weinberg's (2000) "6 hallmarks" of cancer: self-sufficiency in growth signals; insensitivity to growth inhibitory signals; evasion of apoptosis; limitless replicative potential; sustained angiogenesis; and potential for metastatic/tissue invasion. Acquisition of each of these properties may be driven by both genetic and epigenetic changes, and they relate to 5 overlapping models of carcinogenesis: mutational; genomic instability; nongenotoxic clonal expansion; cell selection; and microenvironment. Key to the focus of the current chapter is the recognition that exogenous chemicals may contribute to the genetic, epigenetic, and microenvironment alterations that are required for the acquisition of these features, and thus for tumor growth to proceed.
4.2.3 Genotoxic Carcinogens, Metabolic Activation, and DNA-Damaging Species
Chemicals can contribute to the initiation, promotion, and progression stages of the carcinogenic process either through their ability to damage DNA and produce somatic mutations in cells, or as a consequence of their ability to establish a cellular microenvironment that provides initiated cells with a growth advantage via metabolic changes, local vascular alterations, and/or the ability to evade apoptosis, terminal differentiation and contact inhibition. The distinction between genotoxic carcinogens as tumor initiators and nongenotoxic carcinogens as tumor promoters is now considered overly simplistic as DNA damage in key genes may occur—and for most cancers is likely required—also at later stages of tumor development.
Genotoxic carcinogens have a wide diversity of chemical structures (see Fig. 4–1) but they share the property of either being directly electrophilic (electron-seeking) or being capable of conversion to electrophiles. These reactive electrophiles interact with nucleophilic (electron-rich) groups on intracellular molecules such as DNA and proteins, forming either covalent adducts or oxidative damage. These types of damage to DNA, if not repaired before the next cycle of DNA replication, may lead to errors in replication and hence to fixation of the damage as nucleotide substitutions. If the reactive electrophile damages key cellular proteins the result may be cytotoxicity and necrotic cell death, which can eliminate cells that also have damaged DNA, but which may also trigger the development of an inflammatory microenvironment that promotes the proliferation of any surviving initiated cells (Sec. 4.2.7).
Some genotoxic carcinogens, including carbamic acids, nitrosamides, epoxides, lactones, imines, and mustards (see Fig. 4–1), are "direct-acting" because they are either already electrophilic or are spontaneously hydrolyzed into electrophiles. However, the majority of genotoxic carcinogens require enzymatic bioactivation to electrophilic or electrophile-generating metabolites in order to damage DNA. These reactions are catalyzed largely by drug-metabolizing enzymes whose normal physiological role is protective, converting lipophilic chemicals into water-soluble metabolites that can be more readily eliminated from the body via the urine or bile. Drug-metabolizing enzymes have evolved both multiplicity and catalytic promiscuity to ensure that most potentially harmful environmental chemicals will undergo biotransformation, inactivation, and elimination. Chemicals that are carcinogenic, however, are those whose structures lead to their inadvertent biotransformation into reactive electrophiles with the potential to damage DNA. Because biotransformation can produce many metabolites from a single chemical via multiple cooperating and competing pathways, the net effect of exposure to a carcinogen in a particular individual will depend on the balance of activating versus detoxifying pathways, which may, in turn, be influenced by both genetic variation and by environmental factors, such as chemically-mediated enzyme induction or inhibition.
Among the many classes of drug-metabolizing enzymes that have been implicated in metabolic activation of carcinogens, members of the cytochrome P450 (CYP) mixed-function monooxygenase superfamily have been studied most intensively. These Phase I enzymes catalyze the hydroxylation of carbon, nitrogen and sulfur atoms on chemical molecules to produce metabolites that are more polar and either stable and excreted, reactive (ie, epoxides, nitroso compounds), or possess structures that make them suitable substrates for further metabolism by Phase II conjugating enzymes (see below). Oxidative procarcinogen bioactivation may also be catalyzed by other non-CYP Phase I enzymes such as nicotinamide adenine dinucleotide phosphate (NAD(P)H) quinone oxidoreductase, aldo-keto reductase, and various peroxidases (Shimada, 2006).
Phase II conjugating enzymes such as the uridine diphosphate (UDP)-glucuronosyltransferases (UGTs), sulfotransferases (SULTs), arylamine N-acetyltransferases (NATs), and glutathione S-transferases (GSTs) can also produce either stable conjugated metabolites or unstable conjugates that spontaneously decompose to reactive electrophiles. The likelihood of either protection against, or enhancement of DNA-damaging potential by the activity of Phase I and Phase II enzymes depends on a combination of the structure of the particular chemical in question, the chemical reactivity of the metabolites produced, and the relative levels of expression of the various enzymes that may compete or collaborate in the activation and detoxification processes in a given tissue. For example, potential carcinogen-bioactivating oxidases such as CYP1A2 show liver-selective expression, while others, such as CYP1A1 and cyclooxygenase, are present at high levels in extrahepatic target tissues, such as the lung and bladder, respectively.
Three of the most extensively studied classes of chemical carcinogens that require metabolic activation are the PAHs, such as benzo[a]pyrene (B[a]P; Fig. 4–3), the aromatic amines, such as β-naphthylamine (βNF; Fig. 4–4), and the nitrosamines, such as diethylnitrosamine (DEN; Fig. 4–5). PAHs, aromatic amines, and nitrosamines together comprise a substantial fraction of the total number of chemicals that are either known or reasonably anticipated to be carcinogenic in humans according to the U.S. National Toxicology Program's (NTP) 12th Report on Carcinogens (2011). Although many of the chemicals on the NTP list are reagents or by-products of industrial processes, others are present in foodstuffs or in the natural environment. Notable examples of these include aflatoxin B1, a potent liver carcinogen produced by Aspergillus fungi that contaminate improperly stored grains and nuts, and heterocyclic amines such as 2-amino-3-methylimidazo[4,5-f ]quinoline (IQ), that form from the reaction of amino acids with creatine during high-temperature cooking of meat (Fig. 4–1). The single-step metabolic activation pathway of aflatoxin B1 is shown in Figure 4–6.
A proposed metabolic activation pathway for the polycyclic aromatic hydrocarbon Benzo[a]pyrene(B[a]P).
Proposed metabolic activation and detoxification pathways for the aromatic amine β-naphthylamine.
A proposed metabolic activation pathway for diethylnitrosamine.
A proposed metabolic activation pathway for aflatoxin B1.
The pathways shown in Figures 4–3, 4–4, 4–5, 4–6 illustrate the following principles of metabolic activation: (a) a central role is played by oxidative metabolism—often mediated by one or more CYP isozymes whose identities depend on the structure of the chemical—in initiating the activation process; (b) multiple enzymes may participate in producing the ultimate carcinogenic metabolite, either by acting sequentially on the chemical or by catalyzing the same reaction; and (c) the nonenzymatic, spontaneous chemical decomposition of unstable metabolites may contribute to activation. The pathways shown in Figures 4–3, 4–4, 4–5, 4–6 represent only a subset of those that contribute to metabolic activation, and do not include many of the competing enzyme pathways that can produce stable, excretable metabolites and are thus protective. For example, although the initial step of βNA activation likely requires N-oxidation of its primary amino functional group into a hydroxylamine (Fig. 4–4), a competing reaction at the amino group is N-acetylation by NATs, which produces a stable and noncarcinogenic N-acetamide. This example also illustrates the potential duality of enzyme effects: the same NAT enzymes that may be protective by catalyzing N-acetylation on the parent molecule can participate in metabolic activation by catalyzing the O-acetylation of the hydroxylamine metabolite to the unstable acetoxy ester. Furthermore, O-conjugation of the hydroxylamine by either sulfotransferases or UDP-glucuronosyltransferases can also produce unstable oxy esters that ultimately generate the same nitrenium ion DNA-binding species.
The structures of the ultimate reactive electrophilic species that are produced by metabolic activation of carcinogens may vary widely. DNA damage may arise from the covalent interaction of its nucleotide bases with at least 11 different types of carbon, nitrogen and sulfur electrophiles on carcinogen molecules (Klaunig and Kamendulis, 2008). Of these, key examples are epoxides produced from the PAHs (see Fig. 4–3) and aflatoxin B1 (see Fig. 4–6), nitrenium ions derived from aromatic amines (see Fig. 4–4), and carbonium ions derived from nitrosamines (see Fig. 4–5).
4.2.4 Nature and Consequences of DNA Damage
Different types of DNA-damaging chemicals tend to produce distinctive patterns of damage on the individual bases of DNA. In general, damage can consist of either a carcinogen adduct covalently bound to DNA, or of oxidative DNA damage. The site and type of adduct depends on the strength (charge) of the electrophile, the availability of nucleophilic sites (the unpaired O: and N: atoms) on DNA bases or the phosphodiester backbone, and the structure of the DNA relative to the size of the adduct. Strong electrophiles are capable of binding to a wide range of nucleophilic targets, whereas weaker electrophiles are only capable of binding to strong nucleophiles. Thus distinct chemical-selective patterns of nucleotide damage and the resulting mutations (see below) may be observed. However, it is likely that the persistence of particular adducts in particular genes is important in predicting carcinogenic risk, and the identification of these genes is important in monitoring DNA mutation profiles as biomarkers of either carcinogen exposure or cancer risk.
DNA may also be oxidized by hydroxylation of the nucleotide bases. Although there are a large number of possible oxidized forms of each of the 4 bases, 8-oxo-deoxyguanosine is one of the most prevalent, and it has been used extensively as a sensitive marker of overall oxidative DNA damage. Oxidative DNA damage can result from increased intracellular levels of reactive oxygen species, including hydrogen peroxide, hydroxyl radical, hydroperoxyl radical, and superoxide anions. These can be produced both by exogenous chemicals, often as a byproduct of CYP metabolism, and by endogenous processes. The latter include oxidative phosphorylation and inflammatory cell activation (Sec. 4.2.7).
Once DNA damage has occurred there are 2 possible cellular outcomes. Most probable is repair of the damage by DNA repair enzymes present in the cell, which have evolved to recognize a variety of types of DNA damage and replace damaged bases with intact ones (see Chap. 5, Sec. 5.2). If DNA repair has not taken place before the next cycle of DNA replication prior to cell division, 3 types of mutational events may take place: (a) error-prone replication resulting in a nucleotide substitution, whereby DNA polymerase incorporates the wrong complementary base (often adenine) in the nascent daughter strand opposite an adducted, apurinic, or apyrimidinic site; (b) frame-shift mutations (most commonly single-base deletions) that tend to occur when a carcinogen adduct is bound to a nucleotide base; and (c) DNA strand breaks resulting from either incomplete excision repair or alkylation and cleavage of the phosphodiester backbone.
Many of the relevant gene targets for mutagenesis by carcinogens include those classified as protooncogenes and tumor-suppressor genes, and mutations at specific sites in these genes have been detected in tumors. In general, when protooncogenes are activated by mutational events, signals for cell growth are increased, whereas for tumor-suppressor genes, which downregulate cell growth, loss of function abolishes this negative regulation (see Chap. 7). For example, in chemically induced rodent tumors, mutations commonly activate the ras family of oncogenes. Rat mammary tumors induced by exposure to nitrosomethylurea contain H-ras genes that have been activated by a single point mutation in codon 12 of the gene, while in mouse skin papillomas induced by 7,12-dimethylbenz[a]anthracene an activating mutation occurs in codon 61 of the same gene. The reasons for such DNA site selectivity are unclear, but they may include a combination of localized DNA accessibility in the context of chromatin packing, the nucleophilicity of particular bases within the exposed DNA region, the structure of the bioactivated chemical relative to the topography of the exposed DNA region, and the catalytic efficiencies (both substrate affinity and turnover rates) of DNA repair enzymes expressed within a given target tissue.
It has been estimated that mutations in the tumor-suppressor gene p53 are present in more than 50% of human tumors. The sites of mutation are not random but occur at discrete hotspots (see Chap. 7, Sec. 7.6.1). Aside from the possible physical and biochemical mechanisms that may explain the occurrence of mutational hotspots, it is likely that selective growth and retention of function-altering mutations also occurs during tumor promotion and progression. Thus different carcinogens may leave distinct mutational signatures in p53 and other genes. Lung tumors that develop in nonsmokers contain a different spectrum of p53 mutations than those in smokers, while tumors in ex-smokers retain the smokers' pattern, indicating the persistence of molecular lesions that underlie the eventual manifestations of cancer (Hainaut and Pfeifer, 2001). Also, more than 50% of the liver tumors from aflatoxin B1-exposed populations in Africa and China have a G-to-T transition at codon 249 of the p53 gene, which is not present in tumors from patients with low aflatoxin B1 exposure (Shen and Ong, 1996). This mutation produces an amino acid change from Arg to Ser that alters the binding properties of the p53 protein to a hepatitis B viral antigen and confers a subtle growth advantage to initiated cells. Thus the codon 249 mutation in p53 is considered a molecular signature linking aflatoxin B1 exposure to the eventual development of hepatocellular carcinoma by providing this selective growth advantage to cells rather than by an inherently greater frequency of its occurrence in DNA.
4.2.5 Exogenous Versus Endogenous Chemical Carcinogens
It is important to place the DNA damage produced by foreign electrophiles in the context of substantial damage that occurs within cells in the absence of exogenous chemical exposure. It has been estimated that more than 10,000 DNA-damaging events occur in every cell each day. This damage is produced not only by ambient ionizing radiation but also by reactive oxygen and nitrogen species and products of lipid peroxidation that are generated in the course of normal endogenous oxidative metabolism. Much of this high "background" damage may also be caused by continual exposure to low levels of the hundreds of natural and synthetic toxic chemicals and food constituents that enter the human body. The massive scale of this damage emphasizes the important role of cellular DNA repair pathways that ensure high efficiency, redundancy, and fidelity of repair for a broad range of DNA damage (see Chap. 5, Sec. 5.2). DNA damage that is detectable as a result of exposure to exogenous chemicals must be of sufficient magnitude to produce a signal above this high level of endogenous damage and very efficient repair. As discussed further in Section 4.3.2, this has important implications for the interpretation of dose–response relationships and thresholds for chemical exposure, as many tests for carcinogen potency make the assumption that cancer risk at low doses may be predicted by a linear extrapolation from experimental administration of high doses. Also, it is known that DNA damage may be necessary but not sufficient for tumor formation, and that additional tumor-promoting effects of chemicals on the cellular homeostasis and microenvironment of DNA-damaged cells contribute strongly to tumor formation (Secs. 4.2.6 and 4.2.7).
4.2.6 Chemicals as Modifiers of Cell Proliferation, Senescence and Death
Relative to normal cells, cancerous cells have impaired ability to control cell division, to age, and to undergo apoptotic (programmed) cell death. Thus any chemical agent that triggers or contributes to the impairment of these processes could promote progression to malignancy, tumor growth and metastasis, especially with chronic exposure. The nongenotoxic tumor promoters shown in Figure 4–2 are thought to function in this manner. TPA acts as a proinflammatory agent and inducer of oxidative stress to provide a microenvironment favoring the proliferation of initiated cells. TCDD appears to function by inhibiting the apoptosis of initiated cells (Schrenk et al., 2004). Phenobarbital influences both cellular proliferation and apoptosis by altering patterns of DNA methylation, thus modifying epigenetic control of gene expression (see Chap. 2, Sec. 2.3) in cancer cells (Phillips and Goodman, 2009). Chemicals may also promote the growth of initiated cells indirectly; for example, by producing acute cytotoxicity with resultant necrotic cell death in nearby cells that triggers the establishment of a chronic tumor-promoting inflammatory environment (Sec. 4.2.7).
4.2.7 Chemicals as Modulators of Inflammation
Epidemiological and experimental evidence supports the concept that inflammatory cells and the innate immune response (see Chap. 21, Sec. 21.2) may play pivotal roles in carcinogenesis by facilitating both tumor-initiating and tumor-promoting events. Some of the most consistently observed associations between chronic inflammation and risk of human cancer include those between colitis and colon cancer, gastric acid reflux and esophageal cancer, hepatitis B or C infection and liver cancer, papillomavirus infection and cervical and head and neck cancer, and schistosomiasis infection and urinary bladder cancer. Recognition of the key importance of inflammation in cancer is reflected in a recent suggestion by Hanahan and Weinberg that the 6 hallmarks of cancer described in Section 4.2.2 may now be supplemented with 2 emerging hallmarks—modified energy metabolism and immune escape—and 2 enabling mechanisms—genomic instability and inflammation (Hanahan and Weinberg 2011).
Acute inflammatory responses may be elicited by infections, metabolic stresses, generation of reactive oxygen species, hypoxia or tissue injury, which can act by producing necrotic or autophagic cell death as opposed to the generally noninflammatory apoptotic cell death. With the exception of infection, exogenous chemicals may trigger any of the events listed above. Contents released from dead cells, such as the chromatin-associated protein high mobility group box 1 (HMGB1) (Campana et al, 2008), can trigger the activation of nearby resident macrophages to release proinflammatory cytokines such as interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α), as well as oxidant-generating enzymes such as nicotinamide adenine dinucleotide phosphate (NADPH) oxidase in a transient "respiratory burst" that produces high levels of free radicals and other reactive oxygen and nitrogen species. Although designed to quickly kill invading pathogens, the respiratory burst can damage the DNA, RNA, proteins and lipids of neighboring cells (Ohshima et al, 2003), and activate signal transduction pathways involving pro-inflammatory transcription factors such as nuclear factor-κB (NF-κB), which drives the expression of genes whose products can result in chronic inflammatory conditions (Karin and Greten, 2005).
Some chemicals may increase cancer risk by more than one mechanism, whereas others may play chemopreventive roles. For example, DEN (see Figs. 4–1 and 4–5) is a potent liver carcinogen that produces not only liver DNA damage in mice, but also an acute hepatic necrosis associated with increased levels of reactive oxygen species (ROS) and release of intracellular damage-associated molecular pattern molecules (DAMPs); the net effect is an elevation in proinflammatory IL-6 and TNF-α levels, subsequent activation of NF-κB, and a compensatory cellular proliferation that promotes tumor growth. This suggests that DEN can act as a "complete carcinogen" because it serves both tumor-initiating and tumor-promoting functions (Maeda et al, 2005; Naugler et al, 2007). In contrast, administration of the antioxidant butylated hydroxyanisole to DEN-treated mice reduces tumorigenicity by preventing the accumulation of ROS that can both damage DNA and activate jun kinase-mediated cellular proliferation pathways (see Chap. 12, Sec. 12.3.4). These examples illustrate that some chemicals may result in the occurrence of tumors by multiple mechanisms, whereas others may be chemoprotective, as discussed in the next section.
4.2.8 Cancer Chemoprevention
The largest cancer chemoprevention trials to date have employed hormonal agents against breast cancer, such as tamoxifen and raloxifene, as discussed in further detail in Chapter 20, Section 20.4.1. There is also great interest in blocking carcinogenesis by other pharmacological means, especially in high-risk groups. Because the process of carcinogenesis is complex and multistage, chemopreventive agents could act by many different mechanisms. However, any chemical worthy of consideration to prevent cancer must pose a very low health risk itself, because it would require long-term administration. Candidates for chemopreventive therapy fall into 2 main categories: (a) the general population; and (b) those who may be at elevated risk because of genetic predisposition or heightened levels of carcinogen exposure. Epidemiological studies show consistently that diets rich in fruits and vegetables, which contain high levels of antioxidants, reduce risk for cancers at many sites. Also, increasing evidence suggests that obesity resembles a chronic inflammatory state, which is known to be tumor-promoting (Longo and Fontana, 2010). Thus for the general population, the most logical chemoprevention strategy is to encourage a healthy lifestyle that includes exercise and caloric moderation to maintain a healthy body weight, and the intake of diets that are rich in fruits and vegetables. More specific chemical interventions are being investigated for individuals at higher risk, and these may be aimed either at preventing DNA damage or reducing the likelihood that DNA-damaged cells will proliferate to form a malignancy.
As described in Section 4.2.3, many procarcinogens are activated to their DNA-binding metabolites by isozymes of the CYP superfamily. Chemicals that inhibit particular CYPs and/or induce the Phase II conjugating enzymes that facilitate excretion of CYP-produced oxidized metabolites could reduce metabolic activation of carcinogens. There is support for this concept from the dietary associations between intake of fruits and vegetables containing enzyme inducers and cancer risk: in experimental animals certain chemicals found in cruciferous vegetables can reduce the metabolism and covalent binding of carcinogens to DNA and subsequent tumorigenesis (Srinivasan et al, 2008; Takemura et al, 2010).
Indiscriminate inhibition of CYP enzymes would not be a safe chemopreventive strategy because CYP enzymes constitute the major pathway for elimination of many potentially harmful chemicals, as well as therapeutically administered drugs. Moreover, the catalytic promiscuity of CYPs is such that even isoform-selective inhibition is likely to alter the disposition of many chemicals entering the body, including therapeutically useful drugs whose pharmacokinetics would be modified. Furthermore, a particular CYP isoform may play dual, competing roles in both carcinogen activation and carcinogen elimination that may not be apparent from in vitro investigations. For example, experimental evidence implicates the CYP isoform CYP1A2 as a key first step in the bioactivation of aromatic amines into DNA-binding electrophiles (see Fig. 4–4). This evidence includes cellular and molecular studies of CYP1A2-dependent production of reactive metabolites from aromatic amines such as the cigarette smoke component 4-aminobiphenyl, covalent DNA binding of these metabolites, production of DNA mutations, and transformation of cultured cells. However, in mice the absence of CYP1A2 (achieved by gene knockout) paradoxically does not protect against either DNA damage or the formation of liver tumors following 4-aminobiphenyl exposure. It is presumed that the protective effect resulting from CYP1A2's contribution to the efficient in vivo elimination of the chemical, hence reducing the overall exposure burden, outweighs its ability to produce DNA-damaging metabolites (Nebert et al, 2004).
A strategy of inducing expression of CYP enzymes so as to increase carcinogen elimination has similar challenges. High levels of Phase II drug-conjugating enzymes were thought to protect from chemical carcinogenesis. However, inhibition of a given Phase II enzyme may either be protective by preventing formation of an unstable metabolite (eg, acetylating, glucuronidating, and sulfonating enzymes can all produce very unstable oxyester metabolites of aromatic amines), or may be risk-enhancing if the enzyme's predominant function in vivo is to produce a stable and readily excretable metabolite.
Whether either Phase I or Phase II drug-metabolizing enzyme induction or inhibition is beneficial or harmful depends upon which chemical agent is the main threat. Unfortunately, exposure often occurs to mixtures of structurally unrelated chemicals (cigarette smoke contains several different PAHs, aromatic amines, and nitrosamines), and manipulations that protect from one class of carcinogen might increase the risk from another chemical class, or even between individual chemicals of the same class.
Chemicals that reduce either the levels of ROS, inflammatory mediators, or inflammation-inducing pathogens have the potential to protect against the development of tumors. For example, research is focusing on the identification of effective antioxidants, such as flavonoids, polyphenols, isothiocyanates and phytoalexins found in foods, that may reduce cellular oxidant burden and thus the levels of oxidative DNA and protein damage. Agents that inhibit inflammation, such as nonsteroidal antiinflammatory drugs that inhibit prostaglandin synthesis by blocking cyclooxygenase (COX) enzymes, have also been studied as potential cancer chemopreventive agents (Das et al, 2007; Lee et al, 2008). Although some randomized trials of selective COX-2 inhibitors have shown encouraging results, the possible cardiovascular side effects associated with large-scale use of these compounds limits their utility. Reducing the incidence of infection as a result of pathogens that cause chronic inflammatory conditions via immunization, such as papillomavirus vaccines for cervical cancer and hepatitis B/C virus vaccines for liver cancer, is effective in reducing cancer risk.
The relationships between exogenous chemicals and dietary/lifestyle factors to cancer risk and prevention are complex. For example, tumor production in rodents by potent genotoxic carcinogens such as aflatoxin B1 may be drastically reduced, even after initiation has occurred, by dietary manipulations such as reducing the percentage or even the source (ie, animal vs. plant) of dietary protein. It is likely that lifestyle-based cancer prevention—comprised of a combination of exposure avoidance and dietary measures—promises greater potential for overall population impact than targeted, chemical-based chemoprevention.