Identifying molecular targets of lifestyle modifications in colon cancer prevention

Abstract

One in four deaths in the United States is cancer-related, and colorectal cancer (CRC) is the second leading cause of cancer-associated deaths. Screening strategies are utilized but have not reduced disease incidence or mortality. In this regard, there is an interest in cancer preventive strategies focusing on lifestyle intervention, where specific etiologic factors involved in cancer initiation, promotion, and progression could be targeted. For example, exposure to dietary carcinogens, such as nitrosamines and polycyclic aromatic hydrocarbons influences colon carcinogenesis. Furthermore, dietary deficiencies could alter sensitivity to genetic damage and influence carcinogen metabolism contributing to CRC. High alcohol consumption increases the risk of mutations including the fact that acetaldehyde, an ethanol metabolite, is classified as a group 1 carcinogen. Tobacco smoke exposure is also a risk factor for cancer development; approximately 20% of CRCs are associated with smoking. Additionally, obese patients have a higher risk of cancer development, which is further supported by the fact that physical activity decreases CRC risk by 55%. Similarly, chronic inflammatory conditions also increase the risk of CRC development. Moreover, the circadian clock alters digestion and regulates other biochemical, physiological, and behavioral processes that could influence CRC. Taken together, colon carcinogenesis involves a number of etiological factors, and therefore, to create effective preventive strategies, molecular targets need to be identified and beleaguered prior to disease progression. With this in mind, the following is a comprehensive review identifying downstream target proteins of the above lifestyle risk factors, which are modulated during colon carcinogenesis and could be targeted for CRC prevention by novel agents including phytochemicals.

Keywords: colorectal cancer; grape seed extract; lifestyle modification; molecular targets; phytochemicals; prevention; silibinin.

Figure 1

Effect of red meat consumption on signaling pathways involved in colon carcinogenesis. Consumption of red meat has been shown to induce epigenetic changes in host DNA. These changes occur specifically through altering the levels of histone deacetylase-2 (HDAC-2). Red and processed meat further contains iron, heme, and nitrosyl heme, all of which at high levels may increase the risk of CRC development. Both heme and nitrosyl heme undergo catalysis resulting in the formation of N-Nitroso compounds (NOC); these NOCs can either result in DNA damage or DNA-adduct formation. Red meat consumption specifically leads to mutations in p53 and KRAS genes, further leading to the initiation and progression of colon carcinogenesis. Alternatively, heme catalysis can also lead to generation of lipid peroxidation end products, such as malondialdehyde (MDA), 4-hydroxynonenal (4-HNE), oxysterols, and aldehydes. MDA exposure can result in DNA-adduct formation, leading to DNA mutations and aberrant proliferation, further contributing to the initiation of CRC. 4-HNE is cytotoxic and genotoxic compound, that targets colon cells that carry a wild-type APC gene; this selective toxicity results in enhancement of colon cells that carry a mutated APC gene, resulting in CRC promotion and progression. Additionally lipid peroxidation results in the formation of oxysterols and aldehydes, which further alter hormone signaling, specifically TGF-β, ultimately resulting in uncontrolled proliferation that contributes to the promotion and progression of CRC. Another major component of red meat is iron, ferric iron (Fe³⁺) binds to transferrin, resulting in receptor activation and endocytosis. Ferric iron is further converted to ferrous iron (Fe²⁺) via divalent metal transporter 1 (DMT1), which then contributes to the cell’s overall iron pool. Iron has been linked to the production of reactive oxygen species (ROS), specifically H₂O₂; these reactive species can then up regulate inflammatory mediators, such as NFκB, IL-6, IL-8, IL-1β, and TNF-α, leading to the promotion and progression of CRC. Furthermore IL-1β signaling up regulates NFκB, which then activates DMT1 iron transporters, resulting in increased levels of ferrous iron within the cell, representing a feed-back loop in iron regulation. The illustration also denotes the modulatory role of lifestyle interventions at various levels on these aberrant signaling pathways. The photographs of lifestyle interventions were sourced and adapted from http://www.freedigitalphotos.net/.

Figure 2

Effect of chronic alcohol consumption on the growth and development of colon carcinogenesis. Chronic consumption of alcohol leads to deficiency of vitamins-A, B1, B2, B12, and folic acid. These deficiencies can further lead to alterations in epigenetic regulators; specifically, folate deficiency leads to altered S-adenosylmethionine (SAM) synthesis, resulting in altered p16 gene expression. These epigenetic modifications are involved in the initiation and promotion of CRC. Beer consumption can lead to increased reactive oxygen species (ROS). Specifically, maleic and succinylic acids that are found in beer can result in increase gastric secretion; this increase in acidic environment leads to alteration in the microflora present in the gut, ultimately resulting in increased ROS production. Consumed ethanol is metabolized via oxidation to acetaldehyde (AA); this metabolism is mediated via alcohol dehydrogenase (ADH), cytochrome P450 subenzyme 2E1 (CYP2E1), and catalase. Additional metabolic byproducts of ethanol metabolism include ROS, which are released during the CYP2E1 oxidation reaction; these reactive intermediates enhance lipid peroxidation leading to DNA-adduct formation. Adduct formation can then result in DNA mutations, specifically in the p53 gene, which contributes to the initiation and promotion of CRC. AA alone is a mutagenic compound known to form adducts with DNA and proteins, as well as, induce DNA mutations; furthermore protein adduct formation has been shown to induce inflammatory responses via TNF-α and IL-6. These inflammatory mediators further regulate the expression of matrix metalloproteinase MMP-2, -7, and -9, which are involved in the promotion and progression of CRC. Additionally AA can be oxidized by aldehydes dehydrogenase 2 (ALDH2) resulting in acetate and ROS formation; these ROS species have been indicated in the activation of NADPH oxidase. NADPH oxidase is an enzyme complex that further modulates downstream effectors proteins, such as ERK1/2 and HIF-1α signaling; this down stream protein modification results in alteration of proliferative and metastatic signaling pathways, which are involved in CRC promotion and progression. The illustration also denotes the modulatory role of lifestyle interventions at various levels on these aberrant signaling pathways. The photographs of lifestyle interventions were sourced and adapted from http://www.freedigitalphotos.net/.

Figure 3

Effect of cigarette smoke on the etiology of colon carcinogenesis. Cigarette smoke contains nicotine as well as numerous carcinogenic compounds that effect the initiation, promotion, and progression of colorectal cancer (CRC). These carcinogenic compounds include polycyclic aromatic hydrocarbons (PAHs), heterocyclic amines (HCAs), and aromatic amines. These compounds have the ability to induce epigenetic changes in KRAS and BRAF genes; both these genes are important in preventing the initiation of CRC. Furthermore, these carcinogenic compounds can be bioactivated via cytochrome P450 subenzyme (CYPs); this activation can result in DNA-adduct formation, potentially resulting in DNA mutations in KRAS, BRAF, and MYC if the DNA damage is not repaired. DNA repair pathways such as the base excision repair (BER) pathway can reverse the damage induced by these carcinogens through up regulation of various repair proteins such as, OGG1, APEX1/APE1, and XRCC1. Alternatively these carcinogenic compounds can be excreted from the body via glutathione S-transferases (GSTs) and/or UDP-glucuronosyltransferases (UGTs). Specific carcinogenic compounds, such as nicotine-derived nitrosamine ketone (NNK) have been shown to induce the production of ROS; these reactive intermediates can then activate numerous molecular pathways including MAPK and NFκB. The MAPK signaling cascade has numerous potential protein targets, including NFκB, AP-1, and C-myc; the activation of these pathways can further result in increased inflammatory markers such as COX-2, leading to the initiation and promotion of CRC. Additionally, NNK can activate nicotinic acetylcholine receptors (nAChRs), resulting in β-adrenaline upregulation; β-adrenaline can up regulate COX-2 levels, further leading to VEGF upregulation. Alternatively β-adrenaline can bind and activate the β-adrenaline receptor leading to cascade activation; receptor stimulation triggers ERK1/2 activation leading to up regulation of downstream targets such as, BCL-2, Bad, and AP-1. The downstream targets of the β-adrenaline receptor lead to the induction of apoptosis and further activation of metastatic proteins such as MMPs. Additionally, cigarette smoke contains nicotine which can activate multiple cell membrane receptors including, nAChRs, β-adrenaline, and EGFR. Activation of these membrane receptors leads to the up regulation of inflammatory, apoptotic, and metastatic proteins, such as COX-2, Bad, BCL-2, AP-1, and VEGF. The illustration also denotes the modulatory role of lifestyle interventions at various levels on these aberrant signaling pathways. The photographs of lifestyle interventions were sourced and adapted from http://www.freedigitalphotos.net/.

Figure 4

Effect of physical activity and obesity on the growth and development of colon carcinogenesis. Vigorous physical activity had been shown to induce the production of reactive oxygen species (ROS) leading to DNA-adduct formation, potentially affecting the initiation of colorectal cancer (CRC). Alternatively, moderate physical exercise has been shown to induce an innate immune response through IL-6 receptor activation and downstream protein up regulation. This adaptive immune response protects the organisms against chronic inflammatory conditions, which are known to increase the risk of CRC development. Further, moderate physical activity has been shown to inhibit the expression of other inflammatory mediators, such as iNOS, which may play a role in chronic inflammatory conditions. Moderate physical activity has been also shown to alter specific hormones that are related to appetite and satiety signals; these hormones include YY, GLP-1, and PP, all of which are involved in glycogen secretion. Practicing moderate physical activity also results in the up regulation of superoxide dismutase (SOD); this enzyme further activates detoxification pathways via up regulation of the Nrf2 proteins and GSTs enzymes. Additionally, SOD inhibits PPAR-α leading to the reduced expression of VEGF, which is involved in the promotion and progression of CRC. Obesity is a result of an energy imbalance that results in aberrant activation of various receptor proteins involved in inflammation, proliferation, and hormone regulation. This abnormal activation creates an environment that is chronically inflamed and can result in the initiation, promotion and progression of CRC. One of the major inflammatory mediators is TNF-α, which is activated at the cell membrane, resulting in downstream protein cascade activation. TNF-α activation can result in induction of NFκB and COX-2 signaling which initiates an inflammatory response. Additionally, the TNF-α receptor pathway can trigger activation of the GSK3β enzyme; this enzyme regulates numerous proteins, including β-catenin and NFκB. β-catenin further regulates other proteins that are involved CRC development; these proteins include C-myc, cyclin D, and VEGF, which are involved in oncogene signaling, cell cycle regulation, and metastatic development. Adiponectin is another factor that is highly expressed in obese individuals, which has been shown to increase glucose uptake and fatty acid oxidation; once the adiponectin receptor is activated it results in PKA kinase upregulation, which further inhibits NFκB in adipocytes. This inhibition ultimately leads to down regulation of cell adhesion molecules that are important regulators of CRC promotion and progression. Insulin signaling is another important pathway that is chronically up regulated in obese individuals; binding of insulin to its receptor triggers a conformational change that results in the activation of downstream kinase targets such as, PI3K. PI3K has the ability to trigger PKB kinase activation, which ultimately results in increased glycogen secretion and decreased gluconeogenesis; alternatively PI3K can activate the JAK protein kinase, which plays a role in immune function and inflammatory responses. Another factor that has been shown to activate the JAK protein is leptin; this factor is secreted from adipose tissue and in addition to activating JAK it has also been shown to activate the MAPK signaling cascade. Furthermore, both JAK and MAPK signaling pathways are involved in the regulation of proliferative and metastatic associated molecules, such as PCNA and VEGF; both of which are important regulators of CRC promotion and progression. The illustration also denotes the modulatory role of lifestyle interventions at various levels on these aberrant signaling pathways. The photographs of lifestyle interventions were sourced and adapted from http://www.freedigitalphotos.net/.

Figure 5

Effect of circadian rhythm on growth and development of colon carcinogenesis. The mammalian circadian clock takes 24 h to complete and is a self-sustaining feedback loop of core clock genes. This group of clock genes regulate various cellular processes including: detoxification; DNA repair; proliferation; cell cycle regulation; apoptosis; metastatic signaling, and inflammation. Furthermore, these cellular processes are then regulated in a time specific manner; disruption of this circadian rhythm results in abnormalities in these processes, which contribute to colorectal cancer (CRC) development. The major genes regulating this 24 h timekeeping are PER1 and PER2; these genes modulate proliferative, apoptotic, inflammatory, and metastatic signaling. These genes have been shown to alter the expression of β-catenin; this protein is involved in proliferation and cell cycle regulation. Additionally, β-catenin can modulate oncogenic proteins such as C-myc, which have been shown to regulate iron metabolism; iron metabolism results in ROS formation, further triggering an inflammatory response and NFκB activation. Moreover, β-catenin can modify growth and metastatic signals, mediated via VEGF protein upregulation. Another protein that is under the control of PER1/PER2 genes is ATM, which is up regulated in response to cellular stress; this stress regulator protein can further modify downstream targets, such as p53. The p53 protein, known as the guardian of the genome, can up regulate p21 protein levels resulting in cell cycle arrest; the arrest allows the cell to repair the DNA damage that has occurred due to cellular stress. The PER1/PER2 gene complex further inhibits apoptotic signaling; specifically through Bim protein inhibition, which results in down regulation of cleaved-caspase-3 (cc3) and PARP. Alternatively, a group of binding proteins which are regulated in a circadian manner, including the proteins HLF and E4BP4 can further modulate detoxification of enzymes, such as cytochrome P450 subenzyme (CYPs). In addition HLF/E4BP4 binding proteins inhibit the apoptotic process through inhibition of the pro-apoptotic protein BCL-xL; BCL-xL inhibition results in down regulation of cleaved-caspase-9 (cc9). Additional genetic regulators of the circadian process are p21 and WEE1; these genes are involved in numerous cellular processes including proliferation, cell cycle regulation, and oncogene signaling. Both WEE1 and p21 regulate Cyclin D and Cyclin E expression; these cyclin proteins are involved in cell cycle regulation, which is abnormally regulated during CRC development. Additionally, p21 genetic regulation can lead to the induction of cellular apoptotic signals via cc3 induction; p21 can further regulate oncogenic proteins such as C-myc, which is an important protein in the development and progression of CRC. The illustration also denotes the modulatory role of lifestyle interventions at various levels on these aberrant signaling pathways. The photographs of lifestyle interventions were sourced and adapted from http://www.freedigitalphotos.net/.

Figure 6

Major molecular targets of dietary agents involved in their efficacy against colon carcinogenesis. Multiple signaling pathways, which are modified due to dietary intervention using probiotics, fiber, folic acid, grape seed extract (GSE), silibinin (Sb), curcumin (CUR), and Epigallocatechin-3-gallate (EGCG) are listed. The specific pathways involved are those related to, detoxification, oxidative stress, DNA repair, oncogenesis, epigenetic silencing, inflammation, hormone signaling, proliferation, apoptosis, cell cycle, and metastasis. The photographs of dietary agents were sourced and adapted from http://www.freedigitalphotos.net/.