Makoto Asamoto

Chemopreventive Effects of β‐Carotene, α‐Tocopherol and Five Naturally Occurring Antioxidants on Initiation of Hepatocarcinogenesis by 2‐Amino‐3‐methylimidazo[4,5‐f] qumoline in the Rat

Inhibitory effects of naturally occurring antioxidants on the initiation stage of hepatocarcinogenesis were studied. Group 1 rats were given a diet containing β‐carotene (β8‐CT, 0.02%), α‐tocopherol (α‐TP, 1.5%), glutathione (GLT, 5%), vanillin (VNL, 1%), quercetin (QCT, 1%) or ellagic acid (ELA, 1%), or 3 doses of diallyl sulfide (DAS, 200 mg/kg, i.g) over an 8‐day period. On day 7, the animals received a single dose of 2‐amino‐3‐methylimidazo[4,5‐f]jumoline (IQ, 100 mg/kg, i.g.), 12 h after two‐thirds partial hepatectomy for initiation and 2 weeks thereafter, were placed on promotion regimen comprising phenobarbital (0.05% in diet) and a single dose of D‐galactosamine (100 mg/kg, i.p.). Groups 2 and 3 were treated as described for Group 1, but without test material or IQ, respectively. Survivors were killed at week 11 and antioxidant influence was assessed by comparing values for preneoplastic glutathione S‐transferase placental form‐positive (GST‐P+) foci between Groups 1 and 2. All lesions larger than 70 βm in diameter consisting of approximately 5 cells in cross section were counted. Numbers of GST‐P+ foci/cm2 in Group 1 were: β ‐CT, 7.99; α‐TP, 8.21; GLT, 9.71; DAS, 10.37; VNL, 10.57; QCT, 11.1; ELA, 12.5 (n = 11‐15). All, except ELA, showed a significant decrease as compared with the Group 2 value of 14.54 (n=15). Only β ‐CT showed a significant decrease for the area value. This is the first report to show that β ‐CT, α‐TP, GLT, DAS, VNL, QCT exert inhibitory effects on initiation of hepatocarcinogenesis by the food carcinogen IQ, suggesting that these antioxidants might find application as chemopreventive agents. Furthermore, the current protocol proved practical for the assessment of chemopreventive agents within 11 weeks, a relatively Short period.

Inhibitory effects of docosahexaenoic acid on colon carcinoma 26 metastasis to the lung

Unsaturated fatty acids, including n-3 polyunsaturated fatty acids (PUFAs) such as docosahexaenoic acid (C22:6, DHA) and eicosapentaenoic acid (C20:5, EPA), and a series of n-6 PUFAs were investigated for their anti-tumour and antimetastatic effects in a subcutaneous (s.c.) implanted highly metastatic colon carcinoma 26 (Co 26Lu) model. EPA and DHA exerted significant inhibitory effects on tumour growth at the implantation site and significantly decreased the numbers of lung metastatic nodules. Oleic acid also significantly inhibited lung metastatic nodules. Treatment with arachidonic acid showed a tendency for reduction in colonization. However, treatment with high doses of fatty acids, especially linoleic acid, increased the numbers of lung metastatic nodules. DHA and EPA only inhibited lung colonizations when administered together with the tumour cells, suggesting that their incorporation is necessary for an influence to be exerted. Chromatography confirmed that contents of fatty acids in both tumour tissues and plasma were indeed affected by the treatments. Tumour cells pretreated with fatty acids in vivo, in particular DHA, also showed a low potential for lung colony formation when transferred to new hosts. Thus, DHA treatment exerted marked antimetastatic activity associated with pronounced change in the fatty acid component of tumour cells. The results indicate that uptake of DHA into tumour cells results in altered tumour cell membrane characteristics and a decreased ability to metastasize.

Differential effect of subcellular localization of communication impairing gap junction protein connexin43 on tumor cell growth in vivo

There is a large body of evidence suggesting the connexin gap junction proteins appear to act as tumor suppressors, and their tumor inhibitory effect is usually attributed to their main function of cell coupling through gap junctions. However, some cancer cells (e.g. the rat bladder carcinoma BC31 cell line) are cell–cell communication proficient. Using specific site-directed mutagenesis in the third membrane-spanning (3M) domain of connexin43 (Cx43), we abolished the intrinsic gap junction intercellular communication (GJIC) in BC31 cells either by closing the gap junctional channels or by disruption of the transport of connexin complexes to the lateral membrane. Clones of BC31 cells transfected with a dominant negative Cx43 mutant giving rise to gap junctional channels, permeable only for a small tracer (neurobiotin), displayed accelerated growth rate in vivo, showing the critical role of selective gap junctional permeability in the regulation of cell growth in vivo. The use of other dominant-negative mutants of Cx43 also suggested that the effect of impaired communication on the tumorigenicity of cancer cells depends on the subcellular location of connexin. Inhibition of intrinsic GJIC in BC31 cells by sequestering of Cx protein inside the cytoplasm, due to expression of dominant-negative transport-deficient Cx43 mutants, did not significantly enhance the growth of transfectants in nude mice, but occasionally slightly retarded it. In contrast, augmentation of GJIC in BC31 cells by forced expression of wild-type Cx43, or a communication-silent mutant, fully suppressed tumorigenicity of these cells. Overall, these results show that cell coupling is a strong, but not the sole, mechanism by which Cx suppresses growth of tumorigenic cells in vivo; a GJIC-independent activity of Cx proteins should be considered as another strong tumor-suppressive factor.

Inhibitory effects of oleic and docosahexaenoic acids on lung metastasis by colon-carcinoma-26 cells are associated with reduced matrix metalloproteinase-2 and -9 activities.

In order to determine the effects of single unsaturated fatty acids (UFAs) or combinations on establishment of lung metastatic colonies, UFAs were administered orally to CDF1 mice bearing s.c. implants of the highly metastatic colon carcinoma 26. Oleic acid (OA), docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) demonstrated significant inhibition. In the case of DHA, this inhibitory potential was markedly reduced by co‐administration of linoleic acid (LA) or EPA. Furthermore, while tumor cells treated with DHA showed a very low potential for lung colony formation when injected i.v., this again being partially reversed by co‐administration of EPA. UFAs were found to be well absorbed into tumor tissues after oral administration, causing marked changes in relative levels, the arachidonic acid (AA) content, in particular, being markedly decreased by treatment with DHA or EPA, but not with DHA plus EPA or with DHA plus LA. Investigation of the gelatinolytic activity of the 57‐kDa and 92‐kDa isoforms of type‐IV collagenase (MMP‐2 and MMP‐9, respectively) showed a clear reduction in the former by treatment with OA, while DHA, but not DHA plus LA or EPA, caused a decrease in the 92‐kDa isoform, which was well correlated with AA content in tumor tissues (r = 0.900, p < 0.001). These results suggest that inhibition of metastasis due to treatment with OA and DHA might be due to depressed type‐IV collagenase activity. Int. J. Cancer 73:607–612, 1997.

Chemoprevention by lycopene of mouse lung neoplasia after combined initiation treatment with DEN, MNU and DMH

An investigation was conducted to assess the chemopreventive potential of lycopene (LP), a naturally occurring hydrocarbon carotenoid found in tomatoes and their products, administered during the post-initiation stage in a multiorgan carcinogenesis model. One hundred eighteen B6C3F1 mice of both sexes were subjected to combined treatment with diethylnitrosamine (DEN), N-methyl-N-nitrosourea (MNU) and 1,2-dimethylhydrazine (DMH) from day 11 after birth to week 9 (DMD treatment) (groups 1 and 2) or their vehicles (group 3). Then group 1 received LP (25 or 50 ppm in drinking water) for 21 weeks from weeks 11 to 32. Group 2 served as a carcinogen alone control and group 3 was given only LP (25 or 50 ppm). All surviving animals were sacrificed at week 32 and the major organs, including the liver, lung, kidney and colon, were histologically examined. The incidences and multiplicities of lung adenomas plus carcinomas combined in male mice in group 1 receiving 50 ppm LP were significantly decreased as compared to the DMD alone or DMD and 25 ppm LP groups (75.0 versus 18.8%, P < 0.02; 0.94 +/- 0.17 versus 0.25 +/- 0.14, P < 0.001). No such effect was observed for females. Although hepatocellular carcinomas were lacking in the DMD and LP groups while two cases were found in the DMD alone group, this difference was not statistically significant. The values for aberrant crypt foci (ACF) and tumors in the colon and kidney did not show any significant variation among the carcinogen-treated subgroups. The results suggest that LP exerts a chemopreventive effect limited to male lung carcinogenesis when given in the post-initiation stage to B6C3F1 mice.

Diet and prostate cancer

The importance of dietary factors for prostate carcinogenesis has been proven by epidemiological studies of immigrants from Asia into the USA. Intake of foodstuffs rich in fat, including meat, is suggested to be a risk factor. Experimentally, while some studies demonstrated high fat intake to promote rat prostate carcinogenesis, others did not. Charcoal-cooked red meat and fish have been demonstrated to contain heterocyclic amines that are carcinogenic in rodents and non-primates. Among them, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) has been shown to induce cancers in the mammary glands, colon and prostate of rats. Although there are epidemiological data showing that PhIP could contribute to the development of breast cancer, equivalent evidence for prostate cancer is lacking. However, as protective dietary factors, micronutrients such as selenium, zinc, isoflavones, carotenoids and lycopenes and vitamins E and D have been listed. Animal experimentation on prostate cancer has consistently supported preventive potential for carotenoids and isoflavones, in contrast to the inconsistent results with high fat diets. Although the diet has apparently an important influence on prostate carcinogenesis in man, further research is necessary for clarification of specific factors in man.

Early detection of carcinogenic substances and modifiers in rats

Over the past 20 years, we have been developing in vivo medium-term bioassay systems in rats for detecting carcinogenic and modifying effects of test compounds. The systems are based on the two-step hypothesis of carcinogenesis. In a liver model, male F344 rats are initially given a single dose of diethylnitrosamine (DEN, 200 mg/kg, i.p.) and starting 2 weeks later are treated with test compounds for 6 weeks and then killed, all rats being subjected to two-thirds partial hepatectomy at week 3. Carcinogenic potential is scored by comparing the numbers and areas per cm(2) of induced glutathione S-transferase placental form (GST-P) positive foci in the livers of groups of about 15 rats with those of corresponding control groups given DEN alone. A positive response is defined as a significant increase in the quantitative values of GST-P-positive foci, such a negative response as no change or a decrease. The results obtained have been compared with reported Salmonella/microsome and long-term carcinogenicity test findings for the same compounds. Of the liver carcinogens, 30 out of 31 (97%) mutagenic and 29 out of 33 (88%) non-mutagenic compounds gave positive results. Carcinogens other than hepatocarcinogens gave a lower proportion of positive results (9 out of 42, 21%). This bioassay also provides information concerning inhibitory potential. The practical utility and benefits of a multi-organ medium-term experimental protocol for early detection of carcinogenic agents and modifiers acting at sites other than the liver are also discussed.

Protective effects of citrus nobiletin and auraptene in transgenic rats developing adenocarcinoma of the prostate (TRAP) and human prostate carcinoma cells

Dietary phytochemicals, including nobiletin and auraptene, have been shown to exert inhibiting effects in several chemically induced carcinogenesis models. We here investigated the influence of nobiletin and auraptene on prostate carcinogenesis using transgenic rats developing adenocarcinoma of the prostate (TRAP) bearing the SV40 T antigen transgene under control of the probasin promoter and human prostate cancer cells. Starting at 5 weeks of age, male TRAP rats received powder diet containing 500 p.p.m. nobiletin or auraptene, or the basal diet for 15 weeks and then were sacrificed for analysis of serum testosterone levels and histological changes. The body and relative prostate weights and serum testosterone levels did not differ among the groups. Since all animals developed prostate carcinomas, these were semiquantitatively measured and expressed as relative areas of prostate epithelial cells. Nobiletin caused significant reduction in the ventral (P < 0.01), lateral (P < 0.001) and dorsal (P < 0.05) prostate lobes, while decreasing high grade lesions (P < 0.05) in the ventral and lateral lobes. Feeding of auraptene also effectively reduced the epithelial component (P < 0.05) and high grade lesions (P < 0.05), in the lateral prostate. A further experiment demonstrated that growth of androgen sensitive LNCaP and androgen insensitive DU145 and PC3 human prostate cancer cells, was suppressed by both nobiletin and to a lesser extent auraptene in a dose‐dependent manner, with significant increase in apoptosis. In conclusion, these compounds, particularly nobiletin, may be valuable for prostate cancer prevention. (Cancer Sci 2007; 98: 471–477)

Inhibition of intrinsic gap‐junction intercellular communication and enhancement of tumorigenicity of the rat bladder carcinoma cell line BC31 by a dominant‐negative connexin 43 mutant

The tumor‐suppressive property of the connexin gap‐junction proteins was postulated from the fact that their function of cell coupling is impaired in most cancer cells. However, in conflict with this notion, certain cancer cells are able to communicate through gap junctions despite their malignancy. To explain this phenomenon, we studied by using a dominant‐negative strategy the effect on tumorigenicity of loss of intrinsic gap‐junction intercellular communication (GJIC) in the rat bladder carcinoma cell line BC31, which shows both expression of connexin 43 (Cx43) and intercellular communication. In cells transfected with a mutant Cx43 with seven residues deleted from the internal loop at positions 130–136 (Cx43Δ), transport of the resulting connexin protein to the plasma membrane occurred normally, but the GJIC of the cells was effectively abolished at the level of permeability of established gap junctions. Dominant‐negative inhibition of GJIC by Cx43Δ accelerated growth of BC31 cells in nude mice. In contrast, when GJIC in BC31 cells was artificially enforced by transfection of wild‐type Cx43, the cells lost the capacity to grow in vivo. Decreased phosphorylation of Cx43Δ suggested close interaction of the internal loop of connexin with its commonly phosphorylated domains in the C‐terminal tail and involvement of this interaction in gap‐junction permeability. Therefore, we conclude that the intrinsic GJIC observed in cancer cells should be considered a tumor‐suppressor factor and that its level may influence malignant growth capacity. Mol. Carcinog. 23:254–261, 1998.

Inhibition of Azoxymethane Initiated Colon Tumor and Aberrant Crypt Foci Development by Bovine Lactoferrin Administration in F344 Rats

The influence of bovine lactoferrin (bLf) on colon carcinogenesis was investigated in male F344 rats treated with azoxymethane (AOM). In experiment I, 2% and 0.2% bLf, and Bifidobacterium longum (B. longum) as a positive control at 3% were given in the diet for 4 weeks, along with two s.c. 15 mg/kg injections of AOM on days 1 and 8. The numbers of aberrant crypt foci (ACF) were decreased by both treatments. Similar results were obtained in experiment II of 13 weeks duration. In experiment III, animals were given three weekly injections of AOM and then received 2 or 0.2% bLf, 2% bLf-hydrolysate, or 0.1% bovine lactoferricin (bLfcin) for 36 weeks. No effects indicative of toxicity were noted, but significant reduction in both the incidence and number of adenocarcinomas of the large intestine was observed with almost all the treatments. Thus, the incidences of colon adenocarcinomas in the groups receiving 2 or 0.2% bLf, 2% bLf-hydrolysate, or 0.1% bLfcin were 15%, 25%, 26.3% and only 10%, respectively, in contrast to the 57.5% control value (p < 0.01). ACF values also exhibited reduced development. Investigation of beta-glucuronidase revealed decrease in the cecal contents of animals receiving bLf. In addition, demonstration of enhancement of NK activity by bLf indicated that its inhibitory effects could have been related to elevated immune cytotoxicity.