Intestinal tumorigenesis in multiple intestinal neoplasia mice induced by the food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine: perinatal susceptibility, regional variation, and correlation with DNA adducts

In our previous experiments, multiple injections with the food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) were used to induce intestinal tumors in C57BL/6J-multiple intestinal neoplasia (Min)/+ mice. To define the period of highest susceptibility to PhIP perinatally, we first determined the effect of a single s.c. injection. Ten or 50 mg/kg PhIP increased the number and diameter of small intestinal tumors dose-dependently in 3-day-old Min/+ mice. In the colon, only 50 mg/kg PhIP increased the incidence and number of tumors. The number of dysplastic aberrant crypt foci decreased from weeks 7 to 11. In the same period, an increase in the number of tumors was seen, indicating that over time the dysplastic aberrant crypt foci develop into tumors. Min/+ mice were then exposed in utero through their dams being given one s.c. injection of 50 mg/kg PhIP 3 days before giving birth or were exposed directly to the same dose on day 3, 12, or 36 after birth. Remarkably, the most susceptible period for tumorigenesis in the small intestine was between days 3-12 after birth, whereas in the colon it was from day 3 before to day 3 after birth. Furthermore, we examined whether the formation of DNA adducts determined after 24 h could explain the observed time-dependent and regional susceptibility to PhIP. A higher level of PhIP-DNA adducts was found after exposure on day 12 after birth, compared with day 36 after birth, in all parts of the small intestine but not in the colon, which was in close accordance with the numbers of tumors present. The levels of PhIP-DNA adducts along the intestines were highest in the middle and distal parts of the small intestine, where tumor numbers were also the highest. In conclusion, Min/+ mice are most susceptible to intestinal tumor induction by PhIP from day 3 before to day 12 after birth, and this susceptibility could at least partly be explained by the formation of PhIP-DNA adducts.

Qualitative and quantitative relationship between dysplastic aberrant crypt foci and tumorigenesis in the Min/+ mouse colon

The multiple intestinal neoplasia (Min)/+ mouse, which harbors only one functional allele of the Apc gene, is susceptible to environmental factors that disrupt this gene and subsequently trigger Apc-driven tumorigenesis in the colon. Aberrant crypt foci (ACF) are assumed to be preneoplastic lesions in colon carcinogenesis. Recently, we reported the absence of "classical" ACF in the colon of untreated Min/+ mice. Instead we identified flat dysplastic lesions, which we denoted ACF(Min) (J. E. Paulsen et al., Scand. J. Gastroenterol., 35: 534-539, 2000). In contrast to the classical type, ACF(Min) are not elevated above the surrounding mucosa, and their detection is totally dependent on methylene blue staining and transillumination. In the present study, we treated Min/+ mice with 5 mg/kg body weight azoxymethane (AOM) at weeks 1 and 2 and demonstrated induction of both types of lesions. However, only ACF(Min) appeared to be associated with the development of adenomas. Monocryptal ACF(Min), large ACF(Min), and adenomas showed a uniform histopathological picture of dysplasia and cytoplasmic overexpression of beta-catenin, indicating a qualitative relationship between these lesions. Also a quantitative relationship was suggested because the dramatic decrease in ACF(Min) number from week 7 to 11 was paralleled by a reciprocal increase in tumor number, indicating fast-crypt multiplication of ACF(Min). In AOM-treated +/+ (wild-type) littermates, a low number of ACF(Min) and tumors with the same characteristics as in Min/+ mice was seen. In contrast to ACF(Min), histopathological and immunohistochemical examination of classical ACF showed normal or hyperplastic crypts with normal levels of beta-catenin expression. In AOM-treated Min/+ mice, the number of classical ACF was virtually constant from week 7 to 11, and only a modest increase of crypt multiplicity was observed. The number of AOM-induced classical ACF at week 11 was not different in Min/+ mice and +/+ mice. In conclusion, we identified two distinct populations of altered crypts in the colon of Min/+ mice after AOM treatment. The ACF(Min), which resemble the dysplastic ACF described previously, clearly showed a continuous development from the monocryptal stage to adenoma, and they were characterized by fast-growing crypts with altered control of beta-catenin. In contrast, the classical ACF, which resemble the hyperplastic ACF described previously, were characterized by slow-growing crypts with normal beta-catenin expression, and they were probably not related to tumorigenesis.

Intestinal tumours induced by the food carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine in multiple intestinal neoplasia mice have truncation mutations as well as loss of the wild-type Apc(+) allele

C57BL/6J-Min/+ (multiple intestinal neoplasia) is a murine model for familial adenomatous polyposis (FAP), where the mice are heterozygous for a nonsense Apc(Min) (adenomatous polyposis coli) mutation, and therefore develop numerous spontaneous adenomas in the small intestine and colon. Neonatal exposure of Min/+ mice to the food carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) (eight subcutaneous injections of 25 or 50 mg/kg PhIP to pups or 50 mg/kg PhIP to lactating dams) markedly increased (2--9-fold) the number of intestinal tumours, especially in the small intestine. We examined whether the Apc gene was affected in small intestinal and colonic tumours induced by PhIP. In spontaneous tumours formed in these mice, the main mechanism for tumour induction is loss of the wild-type Apc(+) allele, i.e. loss of heterozygosity (LOH). Also in the PhIP-induced tumours, this is a major mechanism, since large fractions of PhIP-induced tumours had LOH in APC: However, mechanisms other than LOH must also prevail, since a lower frequency of LOH was found in the small intestinal tumours from male mice exposed to PhIP either via breast milk (65%) or by direct injection (68%), compared with the untreated controls (92%). Tumours that had retained the wild-type Apc(+) allele were further analysed for presence of truncated Apc proteins with in vitro synthesized protein (IVSP) assay. Truncated Apc proteins, indicating truncation mutations in exon 15 of the Apc gene, were detected in 20% (8 of 40) of the tumours not showing LOH from the small intestine after PhIP exposure, all in segment 2 (codons 686--1217). Seventeen percent (2 of 12) of the colonic tumours had a truncated Apc protein in segment 3 (codons 1099--1693). Importantly, no truncated proteins were detected in tumours from unexposed mice with apparently retained wild-type Apc(+) allele. These results show that PhIP induces intestinal tumours in the Min/+ mice both by causing LOH and truncation mutations in the wild-type Apc(+) allele.

DNA adduct levels and intestinal lesions in congenic rapid and slow acetylator syrian hamsters admi food mutagens 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) or 2-amino-3-methylimidazo[4,5-f]quinoline (IQ)

Epidemiological studies indicate that rapid acetylators with a high intake of well-done red meat have an increased risk of colorectal cancer. Arylamine N-acetyltransferase enzymes (E.C. 2.3.1.5) activate carcinogenic heterocyclic amines found in the crust of fried meat via O-acetylation of their N-hydroxylamines to reactive intermediates that bind covalently to DNA and produce mutations. Syrian hamsters as well as humans express two N-acetyltransferase isozymes (NAT1 and NAT2) which differ in substrate specificity and genetic control. Nucleic acid substitutions in the NAT2 gene segregate individuals into rapid, intermediate and slow acetylator phenotypes. In the present paper, we examined the role of the polymorphic NAT2 acetylator genotype in carcinogenesis induced by the food mutagens 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) or 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) by comparing Syrian hamster lines congenic at the NAT2 locus. No differences were found between rapid and slow acetylator congenic hamsters in levels of intestinal PhIP-DNA adducts. In contrast to previous studies in rats, no carcinogen-related induction of the preneoplastic lesions aberrant crypt foci or tumors was found in the intestines of rapid and slow acetylator congenic Syrian hamsters administered PhIP or IQ.

Identification and quantification of aberrant crypt foci in the colon of Min mice--a murine model of familial adenomatous polyposis

Min mice are heterozygous for a nonsense mutation in the murine adenomatous polyposis coli (APC) gene and spontaneously develop multiple intestinal neoplasms similar to the familial adenomatous polyposis (FAP) syndrome in humans. Aberrant crypt foci (ACF) are assumed to be preneoplastic lesions in both murine and human colon carcinogenesis and have been observed in FAP patients. Light microscopic examination of the colonic mucosa of 42 Min mice did not show even a single 'classical' ACF on the basis of previously defined criteria, specifying that they are elevated above the surrounding mucosa. However, in Min mice we discovered aberrant crypt foci of a different type, which we denoted ACF(Min). In contrast to the classical type, ACF(Min) were not elevated above the surrounding mucosa, their detection was totally dependent on methylene blue staining and transillumination, and they could not be identified with scanning electron microscopy. Histopathologic examination of ACF(Min) showed dysplastic crypts, similar to those found in larger lesions--that is, microadenomas in the Min mouse. The number of ACF(Min) increased up to the age of 6 weeks and then seemed to remain at a constant level of approximately four per colon. In conclusion, by transillumination of whole-mount preparations stained with methylene blue, we have identified and quantified small microscopic lesions that may be precursors of colonic adenomas in Min mice.

The drinking water chlorination by-products 3,4-dichloro-5-hydroxy-2[5H]-furanone(mucochloric acid) and 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone do not induce preneoplastic or neoplastic intestinal lesions in F344 rats, balb/ca mice or C57bl/6J-min mice

Epidemiological studies indicate an association between exposure to chlorinated drinking water and risk of intestinal cancer. In order to study this experimentally, we have examined the effects of 3,4-dichloro-5-hydroxy-2[5H]-furanone (mucochloric acid, MCA) and 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX), mutagenic and genotoxic compounds in drinking water, on aberrant crypt foci and tumours in the intestines of male F344 rats and Balb/cA mice, and C57BL/6J-Min (multiple intestinal neoplasia)/+ mice of both sexes, in six independent experiments. In some experiments the effects of MCA and MX on aberrant crypt foci induced by the colon carcinogens 1,2-dimethylhydrazine or its metabolite azoxymethane were also studied. Neither MCA nor MX alone induced aberrant crypt foci or intestinal tumours when given in drinking water. With this route of exposure neither MCA nor MX, when given in combination with 1,2-dimethylhydrazine or azoxymethane, had any effect on the induction or growth of the aberrant crypt foci. Drinking water exposure of MX did not affect the number or growth of aberrant crypt foci or intestinal tumours in the Minl+ mice. When administered intrarectally MCA had a weak inducing effect on aberrant crypt foci in the colons of Balb/cA mice. Exposure to MCA and MX intrarectally apparently promoted the growth of aberrant crypt foci both in rats and mice, increasing the crypt multiplicity, aberrant crypts/aberrant crypt foci. Based on an overall evaluation of these experiments, the intestinal tract, at least in rats and mice, seems not to be a main target organ for effects of MCA or MX on preneoplastic or neoplastic development.

Neonatal exposure to the food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine via breast milk or directly induces intestinal tumors in multiple intestinal neoplasia mice

We examined whether the food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) could increase intestinal tumorigenesis in neonatal C57BL/6J-Min/+ mice, a murine model for familial adenomatous polyposis. Min/+ mice are heterozygous for a nonsense mutation in the adenomatous polyposis coli gene and spontaneously develop multiple intestinal adenomas, primarily in the small intestine. Neonatal Min/+ mice (3-6 days old) were exposed to PhIP via breast milk from lactating dams given 8 s.c. injections of 50 mg/kg PhIP three times a week or to 8 s.c. injections of 25 or 50 mg/kg PhIP directly, over the same period. At the age of 11 weeks, the number, diameter and location of the intestinal tumors were scored. Remarkably, a 2- to 4-fold increase in the number of small intestinal tumors was seen in Min/+ mice exposed to PhIP via breast milk (P < 0.001). To our knowledge, this is the first time PhIP has been reported to induce tumors following exposure via breast milk from PhIP-exposed dams. Upon direct exposure to 50 mg/kg PhIP, a 6- to 9-fold increase in the number of small intestinal tumors was observed (P < 0.001). The diameter of the PhIP-induced small intestinal tumors was slightly increased (P < 0.001). In the colon, a 3- to 4-fold increase in the number of tumors was seen in Min/+ mice exposed to PhIP via breast milk (P = 0. 004). Direct exposure to 50 mg/kg PhIP caused a 2- to 6-fold increase in the number of colonic tumors (P = 0.014). The PhIP-induced colonic tumors were located more distally and displayed a smaller diameter than the tumors from the controls (P < 0.05). In contrast to a previous study, where PhIP showed only a moderate tumorigenic effect in adult Min/+ mice, the present study demonstrates a strong tumorigenic effect of PhIP in neonatally exposed Min/+ mice, even after exposure via breast milk from PhIP-exposed dams.

[Chlorination of drinking water--possible cancer risk from a by-product]

In a recent study, 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX; CAS no 77439-76-0) which is formed during chlorination of water containing organic substances, was found to be carcinogenic in the rat, at multiple sites in both sexes. MX is known as a potent bacterial mutagen. Epidemiological studies have suggested an association between chlorinated water consumption and a moderate increase in the risk of cancer. Although MX is a strong mutagen in prokaryotes, its genotoxic effects in mammalian cells are not so large, and more variable results are obtained. Very low concentrations of MX are found in drinking water (ng/L), whereas the genotoxic and carcinogenic effects in experimental animals of this compound are detectable at relatively high doses (mg/kg body weight). Relative to the risk for infectious diseases from the consumption of contaminated drinking water, the possible cancer risk associated with MX exposure appears to be low. Even so, efforts should be made to reduce disinfection byproduct formation by removing organic matter before chlorination.

[Heterocyclic amines in cooked meat]

The age adjusted incidence of cancer has increased on average 1% annually since the beginning of this century, and cancer is now one of the most prevalent causes of death. Diet is suggested to be responsible for about 30-70% of all cancer cases. The heterocyclic amines (HCA) produced during processing of meats and fish at temperatures above 150 degrees C are candidate dietary causes. Amounts in food range from less than 1 ng/g in cooked meat or fish up to over 300 ng/g in well done flame grilled chicken breast meat. The most important parameters determining HCA amounts are cooking temperature and cooking time. 20 different HCAs are identified from cooked or grilled meats and fish. HCAs are causing cancer in various organs in mice, rats and cynomolgus monkeys. It is of interest to note that in rats, PhIP, the most abundant heterocyclic amine in cooked food, causes colon, prostate and mammary cancer, which are the most prevalent cancers in humans. Epidemiological studies show a correlation between intake of red meat and colon, mammary and prostate cancer. Based on the adverse effects of HCA, a reduced intake is recommended and practical advice on how this can be done is given.

A fish oil derived concentrate enriched in eicosapentaenoic and docosahexaenoic acid as ethyl ester suppresses the formation and growth of intestinal polyps in the Min mouse

We examined whether the n-3 polyunsaturated fatty acid ethyl ester enriched fish oil K85 (54.4% of eicosapentaenoic acid and 30.3% of docosahexaenoic acid as ethyl esters) could inhibit the intestinal tumorigenesis in Min mice, a murine model of familial adenomatous polyposis (FAP). Min mice that are heterozygous for a nonsense mutation in the Apc gene, develop spontaneously multiple intestinal neoplasms, primarily in the small intestine. K85 was dissolved in corn oil (vehicle) and mixed into the AIN-76A diet. The total oil content (K85 + corn oil) was 12% in all diets. The various experimental diets contained 0 (vehicle control), 0.4, 1.25 or 2.5% of K85. In the small intestine, the mean number of tumors/mouse was 105 +/- 18 (SEM) in control males and 70 +/- 11 in control females. Dietary K85 treatment reduced the number of small intestinal tumors: in males, the maximum reduction was 66% (P = 0.002) with 0.4% of K85; and in females, the maximum reduction was 48% (P = 0.043) with 2.5% of K85, but the inhibition was only slightly increased from 0.4% to 2.5% of K85. The mean tumor diameter was 1.33 +/- 0.08 mm in control males and 1.06 +/- 0.08 in control females, and the diameter ranged from <0.1 mm (monocryptal adenomas) to 4 mm. The small intestinal tumor diameter was reduced by K85 in a dose-dependent manner: in males, with a maximum reduction of 26% (r = -0.64, P = 0.004); and in females, with a maximum reduction of 38% (r = -0.61, P < 0.004). In the large intestine, the mean number of tumors/mouse was 1.0 +/- 0.5 in males and 0.8 +/- 0.2 in females. Although K85 treatment tended to reduce the number and diameter of the large intestinal tumors, these effects did not reach statistical significance. Aberrant crypt foci not elevated from the flat mucosa (ACF(Min)) occurring in the colon of Min mice were also scored. The mean number of ACF(Min)/colon (3.8 +/- 0.9) and the crypt multiplicity (1.49 +/- 0.28) in females were reduced by 73% (P = 0.03) and 60% (P = 0.048) with 2.5% of K85, respectively, whereas no significant effect could be observed in the males.

[Genetic and environmental factors in colorectal cancer. Mutations in the familial adenomatous polyposis gene]

The incidence of colon cancer has increased during the last 30 years in Norway and is now the second most common newly diagnosed type of cancer in women and the third in men. Familial adenomatous polyposis, hereditary colorectal cancer, is caused primarily by inactivation of the tumour suppressor gene adenomatous polyposis coli (APC). The protein coded for by this gene has a possible role in cell-cell signalling or adhesion by binding to catenins which bind to the cell adhesion molecule E-cadherin, or in anchoring the cytoskeleton. Both germ-line and somatic APC gene mutations result in a truncated protein, due to introduction of a stop codon. The positions of the germ-line mutations seem to correlate with the seriousness of polyposis. The food mutagen PhIP causes specific mutations in the Apc gene in rats, and is a possible environmental mutagen also in humans. The Min mouse with mutated Apc-gene is a good model for studies of both induction and prevention of inherited and sporadic intestinal cancer.

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine increases the numbers of tumors, cystic crypts and aberrant crypt foci in multiple intestinal neoplasia mice

The multiple intestinal neoplasia (Min) mice have a mutation in the murine adenomatous polyposis coli (Apc) gene rendering them highly susceptible to spontaneous intestinal adenoma formation, similar to the familial adenomatous polyposis (FAP) syndrome in humans. We studied whether the most abundant mutagenic heterocyclic amine isolated from cooked food, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), could influence early intestinal neoplasia in C57BL/6J-Min/+ and C57BL/6J- +/+ (wild-type) mice of both sexes. PhIP was given in 4 weekly i.p. injections of 50 mg/kg. Ten weeks after the start of the experiment, PhIP had significantly increased the numbers of small tumors and cystic crypts in the proximal section of the small intestine in male Min/+ mice, and the numbers of aberrant crypt foci (ACF) in the large intestines of both males and females. The effects of PhIP were more pronounced in male than in female Min/+ mice. In +/+ mice, no tumors or cystic crypts in the small intestine, and no tumors and only a very few ACF in the large intestine, were induced by PhIP. These results show that a substance frequently present in the human diet is able to enhance the neoplastic process induced by a genetic lesion, which is also commonly found both in inherited and sporadic colon carcinomas in humans.

Effect of acetylator genotype on 3,2'-dimethyl-4-aminobiphenyl induced aberrant crypt foci in the colon of hamsters

Aberrant crypt foci (ACF) are assumed to be preneoplastic lesions in both rodent and human carcinogenesis. The colon carcinogen 3,2'-dimethyl-4-aminobiphenyl (DMAB), like other arylamines, undergoes N-acetylation and O-acetylation by polymorphic acetyltransferase (NAT2). In the present study we characterized ACF in hamster colon for the first time and compared the ability of DMAB to induce ACF in homozygous rapid and slow acetylator congenic Syrian hamsters (Bio 1.5/H-NAT2r and Bio 1.5/H-NAT2s, respectively), differing only at the NAT2 gene locus and other closely linked loci. The animals received DMAB (75 mg/kg body weight s.c.) or vehicle (PBS/DMSO 1:1) as a control, twice weekly for 2 weeks, then once a week for 4 weeks. Ten weeks after the first injection ACF were observed in the DMAB treated hamsters, but not in the controls. However, the number of ACF was three times higher (P=0.016) in the colons of the NAT2r hamsters compared with the colons of the NAT2s hamsters. In the two congenic hamster lines we also studied the induction of ACF with 1,2-dimethylhydrazine (DMH) treatment, a colon carcinogen not metabolized by NAT2. Hamsters given DMH (25 mg/kg body weight s.c.), once a week for 3 weeks, showed ACF induction in the colon after 10 weeks, but there was no difference between the NAT2r and NAT2s hamsters. Further scanning electron microscopic and histological examination of ACF observed with the light microscope, revealed the same gross morphology and therefore confirmed the basis for the scoring of ACF. The ACF in hamster colons were principle similar to the lesions observed in other species.

Unexpected antagonistic action of 3,2'-dimethyl-4-aminobiphenyl on aberrant crypt induction by 1,2-dimethylhydrazine or azoxymethane in rat colon

Aberrant crypt foci (ACF) consisting of one more single aberrant crypts (AC) are putative preneoplastic lesions that have been proposed as intermediate biomarkers for colon cancer. Using ACF as the end-point we have studied the effects of two different classes of colon carcinogens, 1,2-dimethylhydrazine dihydrochloride (DMH; 10 or 20 mg/kg body wt/injection) or its metabolite azoxymethane (AOM; 5 mg/kg) and 3,2'dimethyl-4- aminobiphenyl hydrochloride (DMAB; 50 mg/kg) in F344 and Lewis rats. Each carcinogen was given alone or DMH/AOM and DMAB were given in combination in either alternating or successive order in multiple doses. Each compound given alone induced ACF in both rat strains and the effect was most pronounced in the F344 rats. DMAB, not previously tested for ability to induce ACF in rats, was clearly less potent than DMH or AOM. The highest number of ACF was found distally in the colon, independent of treatment or rat strain. Surprisingly, DMAB markedly decreased the carcinogenic effect of DMH, evaluated both as numbers of ACF and AC per colon, as well as number of ACF with four or more AC, when both classes of carcinogens were given alternately. A more pronounced reduction was found in F344 rats than in Lewis rats, being 75-77% and 64-68% respectively with the highest DMH dose. The same tendency was found with successive exposure to DMAB followed by DMH or AOM. These differences in timing of exposure and the different metabolic pathways used by the two classes of carcinogens make a metabolic interaction unlikely as the reason for the antagonistic effect of DMAB on DMH or AOM. The type of standard diet used was found to influence the induction of ACF by the colon carcinogen DMH.

Accumulation of metallothionein and its multiple forms by zinc, cadmium and dexamethasone in human peripheral T and B lymphocytes and monocytes

Metallothionein (MT) is produced at high rates in isolated monocytes, and T and B lymphocytes during induction in vitro. At optimal concentrations, 125 microM for Zn and 10 microM for Cd and dexamethasone (dex), MT was demonstrated after only 2 h in some cases, and in all cell types substantial levels were measured after 1 day of exposure to all three inductors. With Cd, lower amounts of MT were found, but maximum levels were reached faster than with Zn. The same result was found for dex compared to Zn. Zn and dex in combination showed the same accumulation rate as Zn alone. If the inductors were used in lower concentrations than optimal, reduced accumulation rates occurred, particularly during the first part of the exposure period. No MT was found for concentrations below 5 microM Zn, 1 microM Cd or 0.5 microM dex. The constitutive levels of MT (mean +/- S.E.M.) were 0.11 +/- 0.05, 0.54 +/- 0.3, 0.06 +/- 0.05 and 0.15 +/- 0.08 nmol Cd bound/5 x 10(6) unseparated mononuclear cells (MNC), monocytes, T lymphocytes and B lymphocytes, respectively. Monocytes accumulated 19 times and B lymphocytes 6 times more MT than T lymphocytes after 2 days of exposure to 125 microM Zn. Despite these differences in accumulated amounts of MT, the fold accumulation values were rather similar between the cell types, reflecting corresponding variations in background MT levels. After exposure of unseparated MNC to 125 microM Zn for 2 days, removal of the metal caused constitutive MT levels to be reestablished in 5 days. Five different MT forms, all capable of Cd complexation, were demonstrated in these cells. These forms had kinetically different behaviour during Zn exposure among the cell types, and the response to Cd was different from the Zn response. The results indicate metals to be closely controlled in MNC and emphasize a role for multiple MT forms in the process of regulation.

Cytotoxicity and accumulation of Hg, Ag, Cd, Cu, Pb and Zn in human peripheral T and B lymphocytes and monocytes in vitro

1. The cytotoxic effects of various heavy metals were assayed by trypan blue exclusion in vitro in human peripheral immune cells separated to high purity. T and B lymphocytes and monocytes were equally sensitive to metals. The individual metals could be ranked in the following decreasing order of cytotoxic potency, Hg approximately Ag > Cd approximately Cu > Pb approximately Zn, based on exposure time and concentration needed to give a particular percentage of dead cells. 2. The cytotoxic effects became irreversible after about 13 hr of metal exposure. 3. Examination by scanning electron microscopy showed that the heavy metals caused serious destruction of the cell membranes. 4. Solubility and uptake of metals into the cells were studied and discussed in relation to the cytotoxic effects. It was concluded that metal binding to cell surfaces or precipitate formation could inhibit ordinary uptake, thereby affecting cytotoxicity. For Pb in monocytes this appeared to lead to uptake of non-toxic complexes, probably by phagocytosis.

Scanning electron microscopy of aberrant crypt foci in rat colon

The surface of the colon mucosa of 1,2-dimethylhydrazine-treated F344 rats was examined with the scanning electron microscope. A detailed examination of the mucosal topography revealed foci with one to several aberrant crypts. These were seen as structures elevated from the background mucosa. The shape of the luminal openings of the aberrant crypts varied from elongated or tortuous to circular. However, we found no ultrastructural variations between the different aberrant crypt foci (ACF) or between the ACF and the background mucosa. There was no direct relationship between the size of ACF and the number of aberrant crypts per focus, which may be explained by the mechanism of crypt fission; in two aberrant crypts we discovered the formation of a transverse epithelial septum, dividing the large crypt into two smaller crypts. The gross morphology of the ACF observed by scanning electron microscopy and light microscopy was in principle the same.

Paracetamol inhibits UV-induced DNA repair in resting human mononuclear blood cells in vitro

The effects of paracetamol on the repair of DNA damage in resting human peripheral mononuclear blood cells (MNC) in vitro were investigated by means of the alkaline elution technique. Low doses of UV light (254 nm, 3 J/m2) caused a transient increase in the amount of DNA single-strand breaks and alkali-labile sites (SSBs). Paracetamol (0.1-1.0 mM) present during post-irradiation incubation approximately doubled the maximum level of UV-induced (1-3 J/m2) SSBs and delayed the completion of repair. Although there were considerable variations between cells prepared from different donors, the level of UV-induced DNA SSBs was always higher with paracetamol. Hydroxyurea (0.3 mM), an inhibitor of ribonucleotide reductase, caused a similar increased accumulation and slow removal of SSBs, whereas cytosine-1-beta-D-arabinofuranoside (Ara C) (10 microM), an inhibitor of DNA polymerases, led to a steady accumulation of DNA SSBs. The increased levels of SSBs caused by paracetamol or hydroxyurea were both completely suppressed by concomitant addition of deoxyribonucleosides; this supports the notion that paracetamol as well as hydroxyurea inhibits ribonucleotide reductase. About the same rates of formation and removal of UV-induced SSBs were observed in T lymphocytes, B lymphocytes and monocytes. In both isolated T lymphocytes and B lymphocytes, paracetamol (0.3 mM) markedly increased the level of DNA SSBs induced by UV, whereas monocytes seemed to be less sensitive to the effect of paracetamol. It is concluded that the inhibition of DNA repair may contribute to the clastogenic effects of paracetamol.

Mitogenicity and metallothionein induction: two separate effects of zinc ions on human mononuclear blood cells

The mitogenic effect of elevated concentrations of zinc ions on human lymphocytes was found to be inhibited by the glucocorticoid hormone dexamethasone. The effect decreased progressively from complete block to partial inhibition when the culture period before the addition of dexamethasone was prolonged. In contrast, dexamethasone did not inhibit an induced formation of metallothionein in mononuclear cells. Furthermore, dexamethasone itself was found to induce small amounts of metallothionein. Apparently, the mitogenic effect and the induction of metallothionein by zinc ions in mononuclear cells occur by mutually independent mechanisms.

Effect of the metal in the reaction between metallothionein and antimetallothionein antibody

1. An antimetallothionein antibody, raised against Cd-carrying metallothionein, was applied in Western blotting of metallothionein. 2. Treatment of the electroblotted nitrocellulose sheets with metals belonging to the periodic system transition groups Ib and IIb, or with Pb, Ni or Cr, considerably enhanced binding of anti-metallothionein. A similar effect was found when the electroblotted sheets were treated with the strong alkylator N-ethylmaleimide. 3. It seems that the binding of metal to metallothionein modifies the configuration of the antibody binding sites by the formation of metal thiolate complexes. 4. Metal treatment of the nitrocellulose sheets after electroblotting, but before application of the primary antibody, offers a convenient method for use in Western blotting to significantly potentiate the reaction between metallothionein and the antimetallothionein antibody.

Induction of metallothionein production by zinc in human mononuclear cells

1. Low mol. wt proteins with high Cd-binding capacity were found to be induced by Zn in cultured monocytes and lymphocytes. 2. In T cell cultures one protein was found to be induced by 125 microM Zn for 6 days, while in monocytes and B enriched cells under these conditions two proteins were found, of which the one with higher mol. wt had similar electrophoretic mobility to the T cell protein on polyacrylamide gels. 3. Mol. wt criteria and crossreactivity towards anti-metallothionein (Mt) antibody identified these proteins to be Mts of about 23 and 27 kDa mol. wt. 4. Metal binding studies indicated that monocyte and lymphocyte Mts had a higher affinity to Zn compared to Cd than rat liver Mt and Mts from Cd-resistant substrains of a human epithelial and a murine fibroblast cell line. 5. The presence of the T cell mitogen phytohemagglutinin (PHA) was found not to be necessary for this Mt induction by Zn.