Immunohistochemical Demonstration of Intestinal-type Alkaline Phosphatase in Stomach Tumors Induced by N-Methyl-N'-nitro-N-nitrosoguanidine in Rats

A potyclonal antibody against rat intestinal-type alkaline phosphatase (I-ALP) was generated and proven to be applicable immunohistochemically to paraffin-embedded sections. Expression of I-ALP in normal tissues, intestinal metaplasia and stomach tumors induced by N-methyl-N'-nitro-N-nitroso-guanidine (MNNG) was then investigated in five different strains of rats. Male SD (Crj:CD), Lewis (LEW/Crj), WKY (WKY/NCrj), Wistar (CrjrWistar) and F344 (F344/DuCrj) animals were given drinking water containing 100/μ/ml of MNNG for 30 weeks and were killed at week 50. Among the 5 strains, stomach adenocarcinomas were found most frequently in the SD case. The susceptibility of rats to induction of stomach carcinoma did not correlate with the development of intestinal metaplasias in each strain. Histochemical staining for mucin demonstrated all stomach tumors (adeno-matous hyperplasias and well-differentiated adenocarcinomas) to consist mainly of gastric type cells (pyloric gland cell and surface mucous cell types), with intestinal-type tumor cells (goblet cell and intestinal absorptive cell types) being only occasional findings. Immunohistochemically, I-ALP was strongly positive on the striated cell borders of small intestinal absorptive cells of the villus and on brush borders of epithelial cells of kidney proximal tubules. I-ALP was also detected in the normal stomach, limited to the striated cell borders of absorptive cells of the upper one-fourth of intestinal metaplastic glands. I-ALP may thus be a useful marker for stomach tumor cells of intestinal absorptive cell type, indicative of maturation and differentiation. No stomach tumors consisting mainly of intestinal-type cells were found, and therefore there was no suggestion of any derivation from intestinal metaplasias.     

Para-methoxyphenol strongly stimulates cell proliferation in the rat forestomach but is not a promoter of rat forestomach carcinogenesis

The modifying effects of para-methoxyphenol (PMP) second stagetreatment on N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-initiatedrat forestomach carcinogenesis were investigated. Groups of15 6 week old male F344 rats were given a single intragastricadministration of 150 mg/kg body wt MNNG and starting 1 weeklater were administered powdered diet containing 2.0, 1.0, 0.5,0.25 or 0% PMP until they were killed at week 52. PMP causedepithelial damage and hyperplasia in a dose-dependent mannerin the fore-stomach epithelium, but nevertheless was not associatedwith any increase in the incidences of either papillomas orsquamous cell carcinomas. The results thus clearly indicatedthat stimulation of cell proliferation does not necessarilycorrelate with promotion in the second stage of two-stage forestomachcarcinogenesis.     

Chemopreventive activity of oltipraz against induction of glandular stomach carcinogenesis in rats by N-methyl-N'-nitro-N-nitrosoguanidine [published erratum appears in Carcinogenesis 1998 May;19(5):955]

The modifying effects of oltipraz on induction of glandular stomach carcinogenesis by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were investigated in a total of 120 male 6-week-old Wistar rats, divided into six groups. Groups 1-3 (30 animals each) were given 100 p.p.m. MNNG in their drinking water for 10 weeks as an initiation treatment for gastric cancer induction and respectively fed diets supplemented with 0.04%, 0.02% and 0% oltipraz for 12 weeks, starting 1 week before and finishing 1 week after the carcinogen exposure. Groups 4-6 (10 animals each) were similarly treated without the application of MNNG. At the end of the 80th experimental week, all surviving animals were autopsied and examined histopathologically for the existence of gastric proliferative lesions. The incidence and multiplicity of adenocarcinomas were significantly (P < 0.01) lower in group 1 than in group 3. In addition, the multiplicity of atypical hyperplasias in the pyloric region was significantly (P < 0.05) decreased in group 1 as compared with the group 3 value. No gastric proliferative lesions were found in groups 4-6. In an additional short-term experiment, oltipraz significantly reduced cell proliferative activity (P < 0.01) and elevated glutathione levels (P < 0.05) in the glandular stomach mucosa of rats treated with MNNG. Thus our results clearly indicate that oltipraz can inhibit induction of proliferative glandular stomach lesions by MNNG in the rat.      

Protection by muscimol against gastric carcinogenesis induced by N-methyl-N'-nitro-N-nitrosoguanidine in spontaneously hypertensive rats.

The effects of prolonged administration of the gamma-aminobutyric acid receptor agonist muscimol on enhanced induction of gastric carcinogenesis by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in spontaneously hypertensive rats (SHR), and on the norepinephrine concentration in the gastric wall and the labeling index of gastric mucosa were investigated. SHR and normotensive Wistar Kyoto (WKY) rats as controls were given a solution of MNNG (25 micrograms/ml) for 25 weeks and then i.p. injections of 0.5 mg/kg body weight of muscimol every other day. In control WKY rats, gastric cancers were found in 1 (7%) of 14 rats examined at week 52. In SHR treated with NaCl solution only, the incidence of gastric cancers was significantly increased to 50% compared with that in control WKY rats. However, treatment of SHR with muscimol significantly increased its incidence to 12% compared with the value in SHR treated with NaCl solution only. The norepinephrine concentration in the gastric wall and the labeling index of the gastric mucosa were significantly greater in SHR than in WKY rats. Prolonged administration of muscimol to SHR significantly reduced the norepinephrine concentration in the antral portion of the gastric wall or the labeling index of the antral epithelial cells. These findings indicate that long-term treatment of SHR with muscimol attenuated the enhancement of gastric carcinogenesis in SHR.     

Antagonistic effect of diethylmaleate on the promotion of forestomach carcinogenesis by butylated hydroxyanisole (BHA) in rats pretreated withN-methyl-N'-nitro-N-nitrosoguanidine

The effects of diethyhaleate (DEM), previously demonstratedto inhibit butylated hydroxyanisole (BHA)-induced forestomachhyperplasia, on BHA promotion of forestomach carcinogenesisin rats pretreated with N-methyl-N'-nitro-N- nitrosoguanidine(MNNG) were examined. Groups of male 6-week-old F344 animalswere given a single i.g. administration of 150 mg/kg body weightMNNG and starting 1 week later administered powdered diet containing1% BHA plus 0.2% DEM, 1% BHA, 0.2% DEM or basal diet alone for51 weeks. Further groups of rats were treated with 1% BHA plus0.2% DEM, 1% BHA, 0.2% DEM or basal diet alone without MNNGpretreatment. Histopathological assessment of lesions at week52 revealed enhancement of MNNG-initiated papilloma (100 versus50%) and squamous cell carcinoma (100 versus 0%) developmentby BHA as compared to controls. Additional treatment with DEM,however, significantly reduced the relative incidences of carcinomain situ (0 versus 35.7%) and squamous cell carcinoma (35.7 versus100%), as well as BHA-induced forestomach hyperplasia with orwithout prior MNNG treatment. The results thus clearly demonstratethat DEM acts as a potent antagonist to BHA-promotion of ratforestomach carcinogenesis.     

Inhibition by 6-hydroxydopamine of enhanced gastric carcinogenesis induced by N-methyl-N'-nitro-N-nitrosoguanidine in spontaneously hypertensive rats.

The effects of chemical sympathectomy induced by 6-hydroxydopamine (6-OHDA) on the enhanced induction of gastric carcinogenesis by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in spontaneously hypertensive rats (SHR), and the norepinephrine (NE) concentrations in their gastric wall and the labeling index of gastric epithelial cells were investigated. SHR rats and normotensive Wistar Kyoto rats (WKY) as controls were given MNNG (25 micrograms/ml) in their drinking water for 25 weeks and then i.p. injections of 6-OHDA (42 mg/kg twice within 24 hr, and then 105 mg/kg every 2 weeks from 1 week later). In control group (WKY rat + NaCl), gastric cancers were found in 2 (11%) of 18 rats examined in week 52. In SHR rats treated with NaCl solution only, the incidence of gastric cancers significantly increased, to 53% compared with that in control WKY rats. Treatment of SHR rats with 6-OHDA significantly decreased its incidence to 12% compared with the value in SHR rats treated with NaCl solution only. Prolonged administration of 6-OHDA to SHR rats significantly reduced the NE concentration in the antral portion of the gastric wall and the labeling index of antral epithelial cells. These findings indicate that prolonged i.p. treatment with 6-OHDA attenuated the normally higher incidence of MNNG-induced gastric cancer in SHR rats.     

MNNG-induced gastric carcinoma in ferrets infected with Helicobacter mustelae

N-Methyl-N-nitro-N'-nitrosoguanidine (MNNG) is a gastric carcinogenin several animal species and has been used in a number of systemsto dissect the co-carcinogenic potential of various compoundsin the induction of gastric adeno-carcinoma. Recent epidemiologicalevidence suggests that Helicobacter pylori may play a role asa co-carcinogen in the etiology of this tumor in humans andwe have been interested in developing an animal model to studythis possibility. A related organism, H.mustelae, naturallycolonizes the ferret stomach and causes persistent chronic gastritis.The pathology elicited by H.mustelae in ferrets has many similaritieswith the human disease including different stages of multifocalatrophic gastritis which underlie the gastric ulcer and gastriccarcinoma syndrome. There is little evidence, however, demonstratingthe susceptibility of ferrets toward chemical carcinogenesis.We have consequently undertaken a study to ascertain whether10 6-month-old female ferrets given a single oral dose of MNNG(50–100 mg/kg) would develop adeno-carcinoma of the stomach.Five age-matched unmanipulated control animals were includedfor comparative purposes. All 15 ferrets were infected withH.mustelae. Nine of 10 ferrets dosed with MNNG developed gastricadenocarcinoma (29–55 months after dosing), while noneof the five historical control ferrets examined an average of63 months after the initiation of the study developed gastrictumors. By comparison, we have not observed gastric adenocarcinoma,nor has it been reported, in >10 years of observation ofuntreated ferrets naturally infected with H.mustelae. The H.mustelae-infectedferret, with demonstrated susceptibility to a gastric carcinogen,plus the recent availability of specific pathogen-free ferrets,should now allow longitudinal studies in vivo to probe the roleof Helicobacter in the development of gastric cancer.     

DNA methylation in rat stomach and duodenum following chronic exposure to N-methyl-N'-nitro-N-nitrosoguanidine and the effect of dietary taurocholate

N-Methyl-N'-nitro-N-nitrosoguanidine (MNNG) induces a high incidenceof carcinomas in the glandular stomach of rats following chronicadministration in the drinking water. We determined the levelof 7-methylguanine and O⁶-mehtylguanine in gastric and duodenalDNA during chronic exposure to MNNG (80 p.p.m.). After considerablefluctuations during the initial 3 weeks, levels of methylpurinesreached a steady state which was approximately three times higherin the pylorus (i.e. the preferential site of tumor induction)thanin the fundus and duodenum, with 7-methylguanine and O⁶-methylguaninevalues in the rangeof 520 and 110 µmol/mol guanine, respectively.When rats were given MNNG in the drinking water at concentrationsranging from 10 to 80 p.p.m. for 3 weeks, levels of methylpurinesreached maximum values already at 10–20 p.p.m. At higherMNNG concentrations, there was no further increase in DNA alkylation.The reason for this lack of dose response remained unclear.Immunohistochemical analyses showed that DNA methylatlon byMNNG is restricted to epithelial cells bordering the luminalsurface. The possibility exists that in this target cell populationthe content of free thiols is a limiting factor for the decompositionof MNNG and its reaction with macromolecules in the gastricmucosa. Addition to the diet of sodium taurocholate, a bileacid previously shown to enhance MNNG-induced stomach carcinogenesis,did not influence the extent of DNA methylation, indicatingthat it acts as a promoter.     

Summation effect of uracil on the two-stage and multistage models of urinary bladder carcinogenesis in F344 rats initiated by N-butyl-N-(4-hydroxybutyl)nitrosamine

Uracil is known to cause reversible urolithiasis and to inducepapillomatosis in the urinary bladder of F344 rats. We examinedwhether the marked urothelial cell proliferation caused by uracil,given in the middle of the post-initiation stages, enhancesthe promoting activity of a promoter in the two-stage modelor the promoting and/or carcinogenic activity of a low-dosecarcinogen in the multistage model of urinary bladder carcinogenesis.Rats were initiated with 0.05% N-butyl-N-(4-hydroxybutyl)nitrosamine(BBN) for 4 weeks, and then 2% butylated hydroxyanisole (BHA)or 0.002% N-ethyl-N-(4-hydroxybutyl)nitrosamine (EHBN) weregiven during experimental weeks 4–9 and weeks 12–20.Uracil was given during weeks 9–12 at a level of 3% ofthe diet. Rats in the control group were treated with BBN anduracil. Rats were killed at weeks 16 and 20. At week 16, higheroccurrences of papillary or nodular (PN) hyperplasia and papillomawere observed in uracil—BHA-treated rats than in the controls.At week 20, significantly higher incidences of PN hyperplasiaand papilloma were observed in both uracil-EHBN-and uracil-BHA-treatedgroups, and a summation effects of uracil was observed. Theseresults indicate that uracil given in the middle of the post-initiationstage enhanced the promoting activity of the compound throughmarked proliferation of the bladder epithelium.     

Protective effect by potassium chloride against gastric carcinogenesis induced by N-methyl-N'-nitro-N-nitrosoguanidine in spontaneously hypertensive rats

The effects of oral potassium supplementation on the enhanced induction of gastric carcinogenesis by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in spontaneously hypertensive rats (SHR), and the norepinephrine concentration in their gastric wall were investigated. The SHR and normotensive Wistar Kyoto rats (WKY) as controls were given a solution of the carcinogen for 25 weeks and then 1% KCl solution or tap water to drink. In Week 52, the incidence of gastric cancers and their number per rat and the norepinephrine concentration in the gastric wall were significantly greater in SHR than in WKY. Prolonged oral treatment of SHR with potassium significantly reduced the incidence of gastric cancers and their number per rat, as well as the blood pressure and the norepinephrine concentration in the antral portion of the gastric wall. These findings indicate that prolonged treatment with KCl attenuated the enhancement of gastric carcinogenesis by MNNG in SHR.     

N-Methyl-N'-nitro-N-nitrosoguanidine-resistant HeLa S3 cells still have little O6-methylguanine-DNA methyltransferase activity and are hypermutable by alkylating agents

To clarify the involvement of O⁶-methylguanine (O⁶-MeG) in mutagenesis,we isolated N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-resistantcells, MR10-1 from HeLa S3 mer^– cells. MR10-1 cells were40 times more resistant to MNNG than the parental cells. MR10-1cells were also significantly more resistant to N-methyl-N-nitrosoureaand slightly more resistant to methyl methanesulfonate and dimethylsulfate than parental cells. However, we found that MR10-1 cellshad still little O⁶-MeG-DNA methyltransferase activity and weresensitive to 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosoureahydrochloride, like HeLa mer^– cells, thereby showing thatMR10-1 cells are still mer^–. When induced 6-thioguanine(6TG)-resistant colonies were plotted as a function of the correspondingpercentage survival, the resistant colonies of MR10-1 cellswere induced much more frequently than in the case of HeLa mer^–cells. However, induction of 6TG-resistant cells in the bothcell lines did not differ significantly in terms of mutant cellsper 0.1 µM MNNG. On the contrary, MR10-1 cells (mer^–)and two HeLa S3 mer⁺ cells lines differed in the induction ofmutation as a function of MNNG concentration. The HeLa mer⁺cell lines were not mutable, while MR10-1 cells were highlymutable. These above results clearly show that the HeLa mer^–cell has at least two defects in the repair of the alkylatedadducts which are related to cell killing and mutation, andalso suggest that O⁶-MeG is involved in the induction of mutation.     

Effect of salt-induced mucosal damage and healing on penetration of N-methyl-N'-nitro-N-nitrosoguanidine to proliferative cells in the gastric mucosa of rats

We have studied the effect of gastric exposure to 4.5 M NaClon penetration of a carcinogen, N-[³H]methyl-N'-nitro-N-nitrosoguanidine(³H-MNNG) from the gastric lumen to proliferative cells in thegastric mucosa of Wistar rats at different time intervals aftersalt exposure. Cells in S-phase were labeled by incorporationof bromodeoxyuridine. Cells in S-phase labeled with ³H-MNNG(double-labeled cells) are the cell population at risk of N-methyl-N'-nitro-N-nitrosoguanidine(MNNG)-induced gastric carcinogenesis. Ten minutes after saltdamage the average percentage S-phase cells labeled with ³Hin pylorus was significantly decreased compared to control (1.2± 0.6 and 9.5 ± 0.7). Ten minutes after salt exposurea marked increase in gastric mucosal blood flow and leakageof fluid from the mucosa into the gastric lumen were observed,and the damaged gastric mucosa was covered by a thick mucoidlayer. These factors may contribute to the reduced ³H penetrationinto mucosa immediately after damage. Two hours after salt exposurethe number of double-labeled cells (8.6 ± 3.7/mm) andpercentage S-phase cells labeled with ³H-MNNG (10.4 ±3.1) in pylorus did not differ from control (6.1 ± 0.9/mmand 9.5 ± 0.7). Twelve and 24 h after salt exposure thenumber of double-labeled cells (79.6 ± 13.4/mm and 32.4± 2.4/mm) and the percentage S-phase cells labeled with³H-MNNG (29.7 ± 2.8 and 18.9 ± 1.3) in pyloruswere significantly increased compared to control. Increasednumber of S-phase cells, a higher location of the proliferativezone in the glandular layer were observed 12–24 h aftersalt exposure and increased permeability of the mucosa to carcinogenwas observed 12 h after salt exposure. These factors explainthe increased number of double-labeled cells and the increasedpenetration of carcinogens to the proliferative cells, and maycontribute to explain the previously described cocarcinogeniceffect of salt on gastric carcinogenesis.     

DNA mutilation of the pepsin 1 gene during rat glandular stomach carcinogensis induced by N-methyl-N'-nitro-N-nitrosoguanidine or catechol

The methylation patterns of the rat pepsinogen 1 (Pg1) genein preneoplastic and neoplastic stomach lesions induced by genotoxicN-methyl-N'-nitro-N-nitrosoguanidine (MNNG) or the non-genotoxiccarcinogen catechol were investigated. Male WKY/Ncrj rats weregiven MNNG in their drinking water (50 mg/l) for 30 weeks or0.8% catechol throughout the experiment (60 weeks). MNNG inducedPg1 altered pyloric glands (PAPG), adenomatous hyperplasiasand well-differentiated adenocarcinomas. Catechol also inducedPAPG and adenomatous hyperplasias although cancers did not develop.Adenomatous hyperplasias and adenocarcinomas all consideredof gastric type cells resembling surface mucous cells or pyloricgland cells with little or no Pg1 expression. In MNNG-inducedstomach cancers generally lacking Pg1, altered Pg1 gene methylationwas observed with both CCGG and GCGC sites being methylatedmore than normal pyloric mucosa. MNNG or catechol-induced adenomatoushyperplasias also demonstrated essentially the same methylationchanges in the CCGG, but not in the GCGC sites. In the mucosacontaining PAPG in groups treated with MNNG or catechol themethylation patterns of the Pg1 gene were quite similar to thoseof normal pyloric mucosa, although the CCGG sites tended todemonstrate slightly increased methylation. The results suggestthat the altered methylation of the Pg1 gene observed in stomachcancers is acquired early in the carcinogenic process and progressivemethylation changes occur with tumor development.     

Chemoprevention of colonic aberrant crypt foci in Fischer rats by sulforaphane and phenethyl isothiocyanate

Epidemiological studies have linked consumption of broccolito a reduced risk of colon cancer in individuals with the glutathioneS-transferase M1 (GSTM1) null genotype. GSTs are involved inexcretion and elimination of isothiocyanates (ITCs), which aremajor constituents of broccoli and other cruciferous vegetablesand have cancer chemopreventive potential, so it is speculatedthat ITCs may play a role in protection against human coloncancer. However, there is a lack of data from animal studiesto support this. We carried out a bioassay to examine whethersulforaphane (SFN) and phenethyl isothiocyanate (PEITC), majorITCs in broccoli and watercress, respectively, and their correspondingN-acetylcysteine (NAC) conjugates, show any chemopreventiveactivity towards azoxymethane (AOM)-induced colonic aberrantcrypt foci (ACF) in F344 rats. Groups of six male F344 ratswere treated with AOM subcutaneously (15 mg/kg body wt) onceweekly for 2 weeks. SFN and PEITC and their NAC conjugates wereadministered by gavage either three times weekly for 8 weeks(5 and 20 µmol, respectively) after AOM dosing (post-initiationstage) or once daily for 3 days (20 and 50 µmol, respectively)before AOM treatment (initiation stage). The bioassay was terminatedon week 10 after the second AOM dosing and ACF were quantified.SFN, SFN-NAC, PEITC and PEITC-NAC all significantly reducedthe formation of total ACF from 153 to 100–116 (P <0.01) and multicrypt foci from 52 to 27–38 (more thanfour crypts/focus; P < 0.05) during the post-initiation treatment.However, only SFN and PEITC were effective during the initiationphase, reducing the total ACF from 153 to 109–115 (P <0.01) and multicrypt foci from 52 to 35 (more than four crypts/focus;P < 0.05). The NAC conjugates were inactive as anti-initiatorsagainst AOM-induced ACF. These findings provide important laboratoryevidence for a potential role of SFN and PEITC in the protectionagainst colon cancer.     

Mechanism for ammonia-induced promotion of gastric carcinogenesis in rats

Although an association is suggested between gastric cancerand prior infection with Helicobacter pylori (HP), the roleof HP in gastric carcinogenesis remains obscure. HP has potenturease activity and produces ammonia, a factor causing HP-relatedgastroduodenal mucosal lesions. In this study, rats were examinedin an effort to determine effects of ammonia on gastric carcinogenesisinduced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Afterpretreatment with MNNG (83 mg/l) for 24 weeks, a solution ofeither 0.01% ammonia or plain tap water was administered tothe animals as drinking water for an additional 24 weeks. Theadministration of the 0.01% ammonia solution significantly increasedthe incidence and number of cancers in the glandular stomach.The numbers of cases in which these cancers penetrated the musclelayer or deeper and of low-grade differentiated adenocarcinomaswere significantly higher in rats receiving the ammonia solution.Continuing administration of ammonia accelerated cell proliferationin the gastric mucosa, but had no effect on the serum gastrinlevel. Therefore, gastric ammonia, which stimulates mucosalcell proliferation, appears to be an important promoter in carcinogenesisin rats and possibly in the HP-related gastric carcinogenesisin humans.     

DNA contents and chromosomes of clonal lines of transformed rat liver epithelial cells and of cells from their derived tumors

Clonal lines of transformed rat liver epithelial cells, derivedfrom a single population of cloned diploid rat liver epithelial(stem-like) cell line (WB-F344) by exposure in vitro to N methyl-N'-nitro-N-nitrosoguanidine(MNNG), produce hepatocellular carcinomas, hepatoblastomas andadeno carcinomas in syngeneic rats (Tsao and Grisham, Am. J.Pathol., 127, 168–181, 1987). In this study we show thatthese clonal lines demonstrate near-diploid (GN clones) or near-triploid(GP clones) aneuploidy and the universal occurrence of non-randomchromosomal abnormalities. Marker chromo somes that involvedfour autosomes-a non-reciprocal translocation involving chromosomes1 and 7 (t1q43;7q34), and addition of DNA of unknown originto the pericentro meric regions of chromosomes 4 and 10—occurredin all of the cells of all transformed clones and in the cellsof tumors that grew from them. New marker chromosomes involvingthe same regions of chromosomes 4 and 7 were found in severalcell lines established from independent tumors. The preservationof marker chromosomes in tumor cells in the face of random lossand gain of other chromosomes suggests that these non-randomaberrations were necessary for tumor formation. The presenceof marker chromosomes was associated with increased expressionof the c-myc gene (located at q34 on chromosome 7), the c-H-rasgene (located at q41–43 on chromosome 1) and the c-K-rasand TGF     

Protective Effect by Potassium Chloride against Gastric Carcinogenesis Induced by N-Methyl-N'-nitro-N-nitrosoguanidine in Spontaneously Hypertensive Rats

The effects of oral potassium supplementation on the enhanced induction of gastric carcinogenesis by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in spontaneously hypertensive rats (SHR), and the norepinephrine concentration in their gastric wall were investigated. The SHR and normotensive Wistar Kyoto rats (WKY) as controls were given a solution of the carcinogen for 25 weeks and then 1% KCl solution or tap water to drink. In Week 52, the incidence of gastric cancers and their number per rat and the norepinephrine concentration in the gastric wall were significantly greater in SHR than in WKY. Prolonged oral treatment of SHR with potassium significantly reduced the incidence of gastric cancers and their number per rat, as well as the blood pressure and the norepinephrine concentration in the antral portion of the gastric wall. These findings indicate that prolonged treatment with KCl attenuated the enhancement of gastric carcinogenesis by MNNG in SHR.     

Relationship between gastric tumorigenesis and intestinal metaplasia in rats given x-radiation and/or N-methyl-N'-nitro-N-nitrosoguanidine

The influence of x-radiation and N-methyl-N'-nitro-N-nitrosoguanidine [(MNNG) CAS: 70-25-7] on intestinal metaplasia and gastric tumorigenesis was examined in 5-week-old male Crj:CD(SD) rats. The animals were treated either with two 10-Gy fractions of x-rays separated by 3 days for a total of 20 Gy to the gastric region and/or with MNNG orally for 4 months. Simultaneous treatment with x-rays and MNNG (group II) and MNNG only (group IV) induced gastric tumors in the majority of the animals. Sequential treatment with x-radiation and MNNG, either x-ray 2 months prior to MNNG (group I) or MNNG 2 months prior to x-ray (group III), resulted in a lower incidence of gastric tumors as compared with the incidence after treatment with MNNG alone. The frequencies of intestinal metaplasia in the x-irradiated groups (groups I and V) were significantly higher than those in group II, III, or IV. The incidence of intestinal metaplasia and of gastric tumor was inversely proportional. These results indicate that intestinal metaplasia does not play a role in the induction of gastric tumors by MNNG.     

MNNG-induced partial phenotypic reversion of Mer- cells

The effect of pretreatment with N-methyl-N'-nitro-N-nitrosoguanidine(MNNG) on MINNG sensitivity of the surviving population wascompared in two HeLa lines, one of the Mer⁺ phenotype (HeLaS3) and one of the Mer^– phenotype (HeLa MR). Whereas MNNGpretreatment of HeLa Mer⁺ cells had no effect on the MNNG sensitivityof surviving cells, Mer^– cells surviving a first exposureto MNNG became much more resistant to MNNG Comparison of thesensitivity of individual HeLa MR clones with their MNNG-pretreatedpopulation and analysis of the composition of the pretreatedpopulation showed that the majority of cells surviving the MNNG-pretreatmentnow displayed the Mer⁺ phenotype in respect to sensitivity toMINNG. One MNNG-resistant clone derived from a pretreated HeLaMR population (Cl 4) was characterized further. It had a similarsensitivity to the Mer⁺ line to all monofunctional alkylatingagents, but was as sensitive as the Mer^– line to the crosslinkingagent chloroethylntrosourea. Cl 4 cells, like the Mer^–cells, did not repair O⁶-methylguanine (O6MeG). The resultssuggest that the two characteristics which are usually coupledwith the Mer^– phenotype - lack of O⁶MeG repair and hypersensitivityto MNNG - can be separated.