Omega‐3 fatty acids in colorectal cancer prevention

Colon cancer, which is the fourth most common cancer in the world, is one of the leading causes of cancer death in both men and women in the United States, Canada, Northern and Western Europe, Australia and New Zealand.1, 2 It is markedly less frequent in Asia, Africa and South America.1, 2 Therefore, it is a major public health problem. Migrant and temporal trend studies suggest that colon cancer is determined largely by environmental exposures, especially nutritional habits.1 Marked international differences in the incidence and mortality of colon cancer and increase of risk in populations migrating from low- to high-risk areas such as from Japan, China and the Philippines to the United States within 1 or 2 generations suggest that environmental factors, specifically dietary habits, rather than the genetic factors play an important role in the etiology of this cancer. This upward trend in incidences of colon cancer among Japanese immigrants in Hawaii and California compared to Japanese in Japan stimulated epidemiologists to investigate the reasons for this increase. Although the relationship between nutrition and cancer is complex and sometimes perplexing to nutritionists and to those who visualize carcinogenesis in terms of a specific carcinogen, it should be recognized that nutritional factors and diet may relate to cancer risk in several ways; first, food additives, contaminants, a particular dietary component, or products formed during food preparation may act as carcinogens, cocarcinogens and/or promoters; second, nutrient deficiencies and excesses may lead to biochemical/molecular alterations that may promote neoplastic processes; third, changes in the intake of selected macronutrients may induce metabolic, biochemical and molecular abnormalities that enhance cancer risk; and fourth, certain dietary constituents act as anticarcinogens or chemopreventives. During the last 3 decades, substantial progress has been made in understanding the relationship between dietary constituents and colon cancer risk. Fish oils are unique because they contain high levels of polyunsaturated omega-3 fatty acids (n-3 PUFAs) that are not present in vegetable oils or in saturated fats. Omega-3 fatty acids that are present in fish oil include docosahexaenoic acid (DHA; C22:6), eicosapentaenoic acid (EPA; C20:5) and docosapentaenoic acid (DPA; C22:5). Vegetable oils including corn oil and safflower oil contain high levels of linoleic acid (LA; C18:2). LA has the terminal double-bond 6 carbon atoms from the terminal (omega) methyl group of fatty acid, whereas DHA has the terminal double-bond 3 carbon atoms from the terminal (omega) methyl group of fatty acid (Fig. 1). Chemical structures of omega-3 and omega-6 fatty acids. Nutritional epidemiologic studies have provided evidence that dietary factors are important determinants of colorectal cancer in different populations worldwide. Cancer statistics in Japan for 2001 published by the Foundation for Promotion of Cancer Research indicate that there is an upward trend in age-adjusted mortality rates for colon cancer from 1955 to 1999.3 According to this report, the death rates due to colon cancer in Japanese men and women in 1955 were 2.9 and 3.0, respectively, whereas they increased to 14.7 and 9.8 in 1999. This upward trend in death rates due to colon cancer is mainly attributable to Westernization of Japanese food habits.3, 4 In addition, the report by the Foundation for Promotion of Cancer Research provided the data on the time trends in food consumption, which show increased dietary intakes of animal fat and meat and decreased consumption of whole grains from 1960 to 1999. For example, animal fat consumption in 1960 was about 25 g/day (per capita), whereas in 1999 it increased to about 58 g/day. Meat intake was increased from 19 to 78 g/day (per capita), whereas grain consumption decreased from 453 to 245 g/day during these years.3 The importance of types of dietary fat differing in fatty acid composition rather than total fat cannot be discounted because several preclinical studies using well-established colon cancer models strongly supported the notion that the colon tumor-promoting effect of dietary fat or lack of such effect depends on its fatty acid composition.5 A recent report by an expert panel assembled by the American Institute for Cancer Research/World Cancer Research Fund came to the scientific consensus that evidence for an association between the intake of saturated fat and/or animal fat and colon cancer risk is very strong.6 Continuing population studies revealed that diets particularly high in total fat, especially animal fat, are generally associated with increased risk of developing colon cancer, whereas high dietary fish oil or fish reduces this risk.7, 8 A recent ecologic study suggests that mortality data for colorectal cancer in 22 European countries, the United States and Canada correlate with the consumption of animal fat.7 That eating a diet rich in n-3 PUFAs may decrease the risk of colorectal cancer has been hypothesized in relation to fish and fish oil.7 Caygill and Hill et al.8 reported an inverse correlation between fish and fish oil consumption and colorectal cancer when expressed as a proportion of total or animal fat. This inverse relationship was significant for both male and female colorectal cancer, whether the intakes were in the current period or 10 years or 23 years before cancer mortality, It is noteworthy that these effects were only observed in countries with a high (> 85 g/caput/day) animal fat intake.8 Also, Mediterranean diet rich in olive oil and fish is associated with a low risk of colorectal cancer.9 On the basis of epidemiologic evidence, it is reasonable to suggest that diets high in saturated fats increase the risk of colorectal cancer, whereas diets high in fish and fish oil rich in n-3 PUFAs reduce the risk. Animal models have contributed significantly to understanding of the carcinogenesis process and to study the multiple environmental factors against the pathogenesis of colon cancer.10 Several studies have utilized these relevant animal models to investigate the modulation of colon carcinogenesis by nutritional and chemopreventive agents. These animal models include induction of colon tumors in rats by administration of aromatic amines such as 3,2′-dimethyl-4-aminobi-phenyl (DMBA); derivatives and analogs of cycacin, such as methylazoxymethanol (MAM), 1,2-dimethyl-hydrazine (DMH) and azoxymethane (AOM) in rats and mice of selected strains; direct-acting carcinogens of the type of alkylureas, such as methylnitrosourea (MNU) or N-methyl-N′-nitro-N-nitrosoguanidine (MNNG); and heterocyclic amines such as 2-amino-3-methylimidazo [4,5-f] quinoline (IQ) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). The spectrum of epithelial lesions induced in the colon by these agents is similar to various types of neoplastic lesions observed in the colorectum of humans. Azoxymethane (AOM), which is a potent inducer of carcinomas of the large intestine in various strains of male and female rats, has been used extensively by many investigators to induce colon tumors and to study the effects of nutritional factors and chemopreventive agents in colon carcinogenesis.10, 11, 12, 13, 14, 15, 16, 17 Colons of Fischer (F344) rats treated with AOM seem to have light and electron microscopic morphology as well as histochemical properties that are quite similar to that of humans and the biologic behavior of AOM-induced rat colon carcinomas is similar to that of human colon carcinomas.10, 18 Other characteristics of the human disease process reflected in the AOM rat model are the occurrence of both adenomas and adenocarcinomas.18 Also, aberrant crypt foci (ACF), which are recognized as early appearing preneoplastic lesions, develop in experimentally induced colon carcinogenesis in laboratory rodents as well as in the colonic mucosa of patients with colon cancer.19, 20 Recently, β-catenin-accumulated crypts were identified in the colonic mucosa at the early stages of AOM-induced colon carcinogenesis and are considered as early-appearing preneoplastic lesions.21 Therefore, ACF are now regarded as putative preneoplastic lesions for colon cancers and are used as biomarkers to evaluate potential chemopreventive agents against colon carcinogenesis.22 AOM treatment also induces oncogene mutations at codon 12 of K- and H-ras and increases in the expression of the ras family of protooncogenes have been causally associated with colon tumor development.23, 24 Enhanced ras oncogene expression has been observed in a variety of human colon tumors.25 AOM-induced colon tumors also demonstrate enhanced cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) expression similar to human colon tumors.13, 26 Mutations in the tumor suppressor gene, APC, are known to be early events in the colon cancer process in humans and have been identified in patients with familial adenomatous polyposis, who have germline mutation in one of the APC alleles, and in sporadic colorectal cancer.27, 28 Evidence in humans thus implicates the APC suppressor gene as causal in large bowel carcinogenesis. Recent studies indicating the presence of APC mutation in AOM-induced colon tumors in rats strengthens the concept that these models are appropriate for human colon cancer studies.29 It has been increasingly apparent that β-catenin signaling pathway is closely associated with the development of colon cancer.30 Also, frequent mutations in the β-catenin gene are confirmed in AOM-induced colon tumors in rodent models.30 it has been that the expression of β-catenin is with the of colon of of of adenomas and ACF and several molecular between human colon tumors and AOM-induced colon it is that the AOM model and induced colon models to be appropriate colon cancer Recent progress in the of molecular carcinogenesis has identified multiple molecular for the and nutritional modulation of colon cancer. The multiple which a mutation codon of the APC gene from a to a is markedly different from that of patients with familial adenomatous in which adenomas are in the colon and whereas in adenomas are in the intestine and in the mice also develop at an early These are potential of and models for agents for potential chemopreventive properties against colon carcinogenesis. The development of for of colorectal cancer by dietary has been markedly by the of relevant laboratory animal including colon cancer the neoplastic process that in humans. and evidence from preclinical studies have provided evidence that not only the also types of dietary fat differing in fatty acid composition are important factors in effect of this nutrient in colon tumor in laboratory and those of have that diets high in and corn oil in the significantly increased induced colon carcinogenesis in and rats as compared to diets low in these studies in laboratory also demonstrate that male rats diets or corn oil rich in PUFAs were to colon carcinogenesis compared with those diets or corn et that dietary n-3 PUFAs methylazoxymethanol of areas of and colon whereas PUFAs enhance colon in In a recent et reported a effect of dietary fish oil against AOM-induced colon carcinogenesis in male dietary fish oil significantly colon tumors as compared to high corn oil In addition, colon tumor by fish oil diet was associated with levels of in the colon compared with corn oil These studies provided evidence in preclinical models that diets high of saturated fat of animal or PUFAs a colon effect than diets low in such fatty whereas diets high in n-3 PUFAs such effect studies in laboratory have the effects of high dietary corn oil and safflower oil rich in olive oil high in fatty acid acid, oil high in fatty acids such as acid and fish oil during the of AOM-induced colon carcinogenesis in male diets high corn oil or safflower oil a incidence of colon tumors than those diets low in fat diets high in olive oil or fish oil such colon The effects of different types of fat on colon carcinogenesis during suggest that fatty acid composition is one of the factors in colon tumor by a dietary fat and that the of types and of dietary fat is during the of In this it is to that in a 2 of patients with colonic dietary fish oil have in in the colonic progress has been made with to the relationship between dietary fat intake and colon cancer risk in that of the tumor-promoting effects of diets rich in PUFAs and saturated fatty acids and lack of such effects by n-3 it should be recognized that among the of dietary fat, animal fat with its fatty acid is by the most important to about to the Western dietary fat intake in the United States and Canada and Western countries, colon cancer rates are of a of and polyunsaturated A recent in mice that high dietary fat composition of the diet lesions in the colon of In of the of in colon cancer and because of potential properties of n-3 have a study to the effects of diets that contain rich in saturated fatty acids and to with the effects of fish oil during the different stages of colon carcinogenesis in male preneoplastic lesions, were in the diets for 23 and which are putative preneoplastic lesions in the were observed at high in the colonic mucosa of patients with colon and of rats and mice treated with colon ACF and are to be biomarkers of the effects of agents carcinogenesis in the ACF were observed in the of the diet a significantly of compared with those the corn oil or fish oil diet at time The incidence of aberrant foci was also in the diet group than in the or diet that administration of the diet significantly the and of preneoplastic lesions in the whereas the diet the of such Also, dietary significantly increased colon tumor incidence and when compared with the or rats the diet incidence of colonic compared with incidences of and in rats the and Also, the of was significantly in the diet as compared to those the the diet fat in the of fish induced tumors than diet the of total fat from This that both the type and the of fatty acids in the diet play a role in colon carcinogenesis. In evidence from preclinical studies is with the epidemiologic The of dietary n-3 PUFAs including DHA and against colon carcinogenesis has also been in rodent et reported that administration of of DHA a for 4 and 12 significantly AOM-induced ACF in the et also reported that administration of 1 of DHA a for significantly AOM-induced colon tumor specifically in the and colon in male et compared dietary at LA at or LA at against AOM-induced colon carcinogenesis in male The indicate that the rats a significantly colon tumor incidence and than those the LA of tumors that the rats on diet and than those on the LA Also, the of in the colon tumors of LA diet group was than that in the colon tumors of the diet These suggest that its effect the modulation of in colon Several potential have been for colon cancer of types of dietary fat. Several studies indicate that diets high in saturated fatty acids and and PUFAs oil or safflower increase the of colonic including acid and acid, whereas dietary fish oil high in n-3 PUFAs such epidemiologic studies that populations who are on Western diet and at high risk for colon cancer high levels of acids have been to in a similar to induce and a in and act as in colon these suggest that acids that are by types of dietary fat may be important for in relation to colon tumor in by et dietary PUFAs may and n-3 PUFAs may against colon carcinogenesis by and/or levels of AOM-induced were in fish rats as compared to those corn oil rich in et and et have also that fish oil an increase in in the colon compared with corn It is reasonable to that one of the by which n-3 PUFAs against colon carcinogenesis is in by the of and by the of colonic Also, of fish oil or n-3 fatty acid increased and DHA levels in the of at the of fatty It has been that of colon carcinogenesis by DHA is the of a of a large of n-3 PUFAs effects in the colon the of of the are factors that and are studies to indicate that inducible nitric oxide synthase which is at the is in human colon and in induced colon tumors of laboratory animal data also indicate that the of by is to carcinogenesis process and induces lesions, thus in and in the by family of also These data suggest a role for in tumor and in laboratory indicate that acid induces in with specific expression that one of the by which tumor including acids may an increase in expression of pathway that colon It is known that the fatty acid composition of is to in laboratory indicate that levels of dietary fish oil in rats increased the omega-3 fatty DHA and in the colonic at the of omega-6 PUFAs such as linoleic acid and acid, the that the DHA and of fish oil the of by acid and linoleic acid in the Therefore, the types of dietary fat the fatty acid composition of colonic It is well that acid and of its including play an important role in the signaling pathway associated with and gene increase promote and of which are in tumor The by which n-3 PUFAs colon carcinogenesis are in of an important role in colon and who have in the of of rat epithelial have that of lead to the of high intake of saturated fat and omega-6 PUFAs acid from and levels of have been observed in human colon tumors and induced colon tumors in rodent and human colon levels of that are by Northern Recent have a between the potential of APC mutations and by that of the gene reduces the of tumors in mice for an by than evidence a role for from which show a in colon tumors in rodents with Recent studies in laboratory have provided evidence that an diet AOM-induced expression of and from acid in colon tumors of rats, whereas the diet the levels of In this administration of the diet of in the colon significantly levels than the corn oil diet or the diet 28 indicating This suggests that of the modulation of may be important for the of n-3 PUFAs to colon Also, colon tumors of the diet a than was observed in the colon tumors of rats the The of these studies indicating that of in the tumors of the diet in to the diet and the tumor the that of lead to the of In colon the levels of may be to by and induction of and thus tumor A major that to be is which signaling are in of the These not only a between dietary fatty and of colon also molecular for colon cancer by which n-3 PUFAs colon carcinogenesis. The potential and molecular events by n-3 PUFAs against colon carcinogenesis. The of molecular events by n-3 PUFAs include including and specific and factors including that and dietary n-3 PUFAs by with the and of modulation of thus Recent studies from laboratory have that high dietary PUFAs enhance of including that have been or in colon tumor whereas diet n-3 PUFAs to the of these a gene family of that play in signaling events and are in and include several with unique to the from the et reported that chemopreventive of dietary fish oil is associated with the alterations in colonic a that is by PUFAs may the of the which a of It is that several have been to in The a of ras that is to the of in the of of are in the etiology of human colon It is also known that of from to is by a of closely including which is by It that of ras association of and neoplastic of in laboratory have provided data to indicate that high dietary PUFAs increases expression in colonic whereas high dietary n-3 PUFAs to by with and of the modulation of thus Several have against in both and laboratory animal in have the of in and of colon cancer. of is with the development of certain of has also been to induce an effect that with and have that expression of was in colon tumors of rats than in colon Colon tumors from rats the diet high levels of PUFAs very low levels of whereas the tumors from the diet high of n-3 PUFAs not of These correlate with colon tumor incidences by dietary n-3 and It that modulation of a significant role in n-3 colon tumor and studies in laboratory have that DHA of colon cancer in and induces also the effects of DHA on the genetic of human colon cancer at the using in gene expression due to DHA treatment was observed to be in the multiple signaling in the of and of DHA on and induction of were by an increase in the of several of family of and of such as and of several of these and factors the of the chemopreventive of DHA and important n-3 PUFAs present in fish oil and thus colon cancer. Also, of these and factors provided several expressed biologic many of which suggest as molecular for by chemopreventive including nutritional is a major of and mortality in patients with cancer, including colorectal cancer. Several studies have provided evidence for effects of fish oil administration in cancer during Omega-3 fatty acids have been to have effects on in cancer in patients with cancer is to and is associated with a time and of A fish nutritional has the potential to be a and of a fish oil preparation of and and a preparation in patients with 85 The of of the of which in the of of cancer patients and death from attributable to of of in cancer by and this may be one of the for of tumor Also, fish oil at a of during In on the basis of epidemiologic evidence from ecologic and it is reasonable to suggest that diets high in saturated fats increase the risk of colorectal cancer, whereas diets high in n-3 PUFAs not increase its risk. The studies both epidemiologic and evidence for the effects of diets rich in n-3 in the of colorectal cancer. Also, recent demonstrate effects of fish oil administration in cancer and during and studies have provided evidence that colon tumor-promoting effect of dietary fat depends on its fatty acid that the composition of dietary fatty acids is to colon cancer risk than is the total of fat. studies also demonstrate that a diet high in including saturated fats of animal as well as high dietary PUFAs a potential to promote colon than of a diet on of fat n-3 Although the by which diets high in saturated fats as those in Western and PUFAs promote colon carcinogenesis are not the studies thus indicate that increased levels of colonic modulation of the on and the expression of by the types of dietary fat, especially may play a role in colon carcinogenesis. studies are to the role of n-3 PUFAs on the modulation of that are in colon and types of cancer. The of is to decrease the and mortality from colorectal cancer. and those of suggest that nutritional has the potential to be a major of colorectal cancer especially in the are and in n-3 PUFAs be before they are for cancer Although there are data to indicate and n-3 PUFAs should be for of colorectal cancer, levels of dietary n-3 PUFAs should be with the on epidemiologic studies of disease as by several These studies suggest that fish or as as g/day of fish the risk of Although a dietary for n-3 PUFAs not there is a that consumption of of fish in on the epidemiologic studies may also reduce the risk of colorectal cancer. et has that the of PUFAs to n-3 PUFAs may be important for The risk for and several types of cancer among Mediterranean and Western European populations may at in be on the basis of to n-3 of and 10 in on preclinical and epidemiologic studies on and et for a in the intake of linoleic acid and increase in the intake of n-3 PUFAs that a of 2 be for of and that this is in Western countries for of and and This may well be for the of colorectal cancer in the consumption of and is also for those in Western countries to reduce the risk of colorectal cancer. In the of colorectal cancer in omega-3 fatty acids are in and of be It should be recognized that with nutritional and/or diet may not be for of colorectal cancer in patients such as those with and sporadic colon This to colon cancer is of importance as have not been in the high incidence or low of colorectal cancer. by diet as to the with chemopreventive agents that or the development of those which with and progress to and is an for of colon cancer in these high-risk This is important when chemopreventive agents demonstrate significant may effects at It is certain that colon cancer be a significant of and in high-risk by molecular that or the process of carcinogenesis. there is a to in patients with sporadic colon using the n-3 diets in with a chemopreventive to the of events leading to The for preparation of the and studies on n-3 PUFAs in colon cancer are supported by the Cancer Institute and

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