ACCELERATED PAPER: Intermittent 50 Hz magnetic field and skin tumour promotion in SENCAR mice

A number of epidemiological studies have indicated associationbetween exposure to extremely low frequency electromagneticfields and a variety of cancers, including leukaemia and braintumours among residentially exposed children and among occupationallyexposed adults. In order to test if intermittent magnetic fields(MF) act as a tumour promoter, a long-term skin carcinogenicitystudy of 50 Hz sinusoidal MF with flux densities of 50 µTand 0.5 mT, continuous as well as with an intermittence of 15s on/off, was performed. Female SENCAR mice were divided intoeight groups of 50 animals in each and treated according toan initiation- promotion scheme. 7,12-dimethylbenz[a] anthracene(DMBA) in acetone was applied to the dorsal skin at a subcarcinogenicdose, as an initiator and exposure to MF was performed for 19–21h/day during 104 weeks starting 1 week after the initiator treatment.The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA)was used as a positive control for skin tumour promoting activity.Two animals from each group were assigned for skin hyperplasiaanalysis at 2, 6, 12,18 and 21 months. The animals were observeddaily. The appearance of skin lesions and neoplasms were carefullyfollowed and histopathological diagnosis was made for all neoplasmspresent at death. The experiment was terminated after 105 weeks.DMBA-treatment alone yielded altogether two skin tumours intwo tumour-bearing animals and the animals exposed to acetonealone had one skin tumour. The animals exposed to continuousfields showed no skin tumour. Five animals exposed to 0.5 µTon/off had a total of 13 skin tumours and in the group exposedto 50 µT on/off four animals had a total of four skintumours. The on/off exposed groups differed significantly fromthe continuously exposed groups (P = 0.014) but the differencebetween the on/off exposure groups and the DMBA group was notstatistically significant when tumour-bearing animals and cumulatedskin tumours were compared. There was a statistically significantdose trend (P = 0.045) with flux density and Tesla-h for intermittentMF exposure for cumulated skin tumours per tumour-bearing animals.The epithelial thickness of DMBA + MF-treated animals was ofthe same magnitude as for DMBA-treated animals indicating that,in the case of a promoting effect being present, another mechanismthan one involving sustained hyperplasia may be involved.     

Sodium o-phenylphenate (OPP-Na) promotes skin carcinogenesis in CD-1 female mice initiated with 7, 12-dethylbenz[a]anthracene

Sodium o-phenylphenate (OPP-Na) was applied at a dose levelof 5.0 mg/animal dermally to the fur-clipped dorsal area offemale CD-I mice twice weekly for 47 weeks after applicationsof 10 µg of 7, 12-dimethylbenz[a]anthracene (DMBA) asan initiator twice weekly for 5 weeks. A total of 25 skin tumors(21 papillomas and four carcinomas) developed in 15 out of 20mice (75%). The incidence and yield of skin tumors in this DMBA/OPP-Nagroup were significantly higher than in the DMBA/acetone-treatedgroup where only six tumors (five papillomas and one carcinoma)were observed in five out of 20 mice (25%). Only one papillomadeveloped in OPP-Na/12-o-tetradecanoylphorbol 13-acetate (TPA)treated animals (initiation testing group), and no tumors wereobserved in mice receiving OPP-Na/acetone. OPP-Na elevated 5-bromodeoxyuridine(BrdU)incorporation in basal cells of mouse epidermis dose-relatedlyand the thickness of the skin in the treated group was alsoincreased to ? 2- or 3-fold that of controls. In addition, highdose application (20 mg/mouse) caused ukeration. O-Phenylphenol(OPP) administration was associated with extensive corrosiveeffects. The data suggest that OPP-Na is an ulcerogenk agentwhich induces epidermal proliferation and which can act as apromoter, but not as an initiator or a complete carcinogen,in two-stage mouse skin carcinogenesis.     

Genetics of chemical carcinogenesis--II. Papilloma induction and malignant conversion in susceptible (Car-S) and resistant (Car-R) lines of mice produced by bidirectional selective breeding and in their (Car-SxCar-R) F1 hybrids

Susceptible (Car-S) and resistant (Car-R) lines of mice separatedby 10 consecutive generations of bidirectional selective breedingpresent a very large difference in responsiveness to two-stageskin carcinogenesis. Car-S mice initiated wIth 0.5 µg9, 10-dimethyl-1, 2-benzanthracene (DMBA) and promoted with0.25 µg 12-O-tetradecanoyl-phorbol-13-acetate (TPA) for77 days showed a papilloma incidence of 88% and a tumour multiplicityof 3.2 ± 0.4 (mean ± SE), with a tumour inductionrate of 0.415. Car-R mice initiated with larger DMBA and TPAdoses (50 µg and 20 µg respectively) and promotedfor 111 days gave a comparable papilloma response: incidence65%, tumour multiplicity 3.2 ± 0.6 and tumour inductionrate 0.288. The difference in papilloma response between thetwo lines is due to the interaction of genetic and environmentalfactors. In order to overcome the genetic effect with environmentalfactors and induce in Car-R a papilloma response comparableto that of Car-S, the DMBA dose had to be increased up to 100times, that of TPA 40 times and the promotion time augmentedby 44%. Papilloma to carcinoma conversion 112 days after theend of promotion depends on the DMBA and TPA doses applied.The number of carcinomas induced in Car-S mice and in (Car-SxCar-R)F1 hybrids was larger than that Induced in Car-R mice, but theratio of carcinoma conversion was lower, therefore a largerproportion of the small number of papillomas induced in theCar-R mice progressed to malignancy. The dominance effect measuredin (Car-SxCar-R) F1 hybrids demonstrated that the susceptibilityto papilloma induction was an incomplete dominant conversionthe resistance was incompletely(d/a = 0.38), whereas for carcinomaconversion the resistance was incompletely dominant (d/a = –0.49).     

Exposure of DMBA-treated female rats in a 50-Hz, 50 {micro}Tesla magnetic field: effects on mammary tumor growth, melatonin levels, and T lymphocyte activation

There is growing public concern about the possible health risks,particularly increased cancer risks of exposure to magneticfields (MF) associated with power distribution systems. Recently,we have started a series of animal studies to investigate thisissue, using the DMBA (7, 12-dimethylbenz[a]anthracene) modelof breast cancer in female rats. In the present study, femalerats were chronically exposed to a 50-Hz, 50 µTesla (µT)MF with or without DMBA treatment Because alterations in circulatinglevels of the pineal hormone melatonin and impaired immune systemfunctions have been involved in breast cancer growth, and bothmelatonin and immune system are thought to be targets for MF-effects,serum melatonin and the proliferative capacity of splenic lymphocyteswere determined in MF-exposed and sham-exposed rats. For thispurpose, 216 female Sprague-Dawley rats were divided into fourgroups. Two of the groups (with 99 animals each) received oralapplications of DMBA and were either sham-exposed or exposedin a 50-Hz, 50 µT MF for 24 h/day 7 days/week for a periodof 91 days. The other two groups (9 animals each) were eithersham-exposed or MF-exposed without DMBA treatment The exposurechambers and all other environmental factors were identicalfor MF-exposed and sham-exposed animals. The DMBA-treated animalswere palpated once weekly to assess the development of mammarytumors. At the end of the three-month period of MF exposure,the number and size of mammary tumors was determined by autopsy.In controls, DMBA induced tumors in     

Tumor initiating activity of 9- and 10-fluoro-7,12-dimethylbenz[a]-anthracene (DMBA) and the effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin on tumor initiation by monofluoro derivatives of DMBA in SENCAR mice

We have determined the skin tumor initiating activity in SENCARmice of 6 monofluoro derivatives of 7,12-dimethylbenz[a]anthracene(DMBA). 9-Fluoro-DMBA (9-F-DMBA) was approximately as active,and 10-F-DMBA was more active than the parent hydrocarbon, DMBA.The difference between DMBA and 10-F-DMBA was most dramaticat the highest initiating doses of 10-F-DMBA tested. 4-F-DMBA,which was only weakly active as an initiator, was also testedas a complete carcinogen on mouse skin; after 30 weeks of treatment,50- and 100-nmol weekly doses failed to elicit papillomas orcarcinomas. Animals treated with 50 nmol of DMBA weekly exhibiteda 100% papiloma incidence and a 42% carcinoma incidence. Pretreatmentwith 2,3,7,8-tetra-chlorodibenzo-p-dioxin (TCDD) effectivelyinhibited tumor initiation with all of the monofluoro derivativesof DMBA tested. The ED₅₀ (dose of TCDD producing half-maximalinhibition) for the inhibition of DMBA initiation in SENCARmice was determined to be 1.8 x 10^–3 µg/mouse (5.6pmol). The results indicate that the introduction of a fluorineatom in ring D of DMBA has no effect (positions 9 and 11) orenhances (position 10) tumor initiating activity. We believe10-F-DMBA to be the first example of a hydrocarbon with a fluorosubstituent giving rise to increased tumor initiating activity.The results also indicate that structural modifications thatalter tumor initiating activity do not alter the ability ofTCDD to inhibit tumorigenesis by DMBA.     

SHORT COMMUNICATION: Lack of initiating capacity of the genotoxic air pollutant 2-nitrofluorene in the mouse skin two-stage carcinogenesis system

2-Nitrofluorene (NF) is a potent genotoxic substance found inenvironments where incomplete combustion takes place. NF isa mutagen and a carcinogen in animal models. NF or 7, 12-dimethylbenz[a]anthracene(DMBA) was administered once topically to female SENCAR micefollowed by promotion by 12-O-tetradecanoylphorbol-13-acetate(TPA) in two different studies. TPA (2–5 µg) wasadministered twice a week for 13 or 19 weeks after initiationof DMBA (5–10 µg) or NF (50–1500 µg).DMBA administration (positive control) resulted in the formationof many papillomas, which were seen from week 8–9 afterinitiation. The negative controls administered acetone onlywere free of tumours. NF administration did not result in papillomaformation, even at high initiating doses of NF combined witha dose of TPA causing a systemic toxic effect in terms of asignificant reduced body weight. The mechanism behind the absenceof papilloma formation after administration of genotoxic andcarcinogenic NF remains to be investigated.     

Promotion of skin tumours by TPA in the progeny of mice exposed pre-natally to DMBA

Pregnant SHR mice were treated once with 7,12-dimethyl-benz[a]anthracene(DMBA) on day 17–19 of gestation, and F₁ and F₂ descendantsreceived multiple skin applications of 12-O-tetradecanoylphorbol-13-acetate(TPA) twice a week for 24 weeks beginning at 12 weeks of age.Post-natal promoter treatment resulted in a high incidence ofskin twmours in F₁ and F₂ mice (37.3 and 19.7%, respectively),whereas only 6.6% of control animals treated with TPA only developedskin tumours. DMBA was shown previously to be capable of initiating skin carcinogenesis transplacentally; however, ourresults on the second generation provide suggestive evidenceof hereditary transmission of at least part of the initiatingaction of this carcinogen.     

FVB/N mice: an inbred strain sensitive to the chemical induction of squamous cell carcinomas in the skin

The widespread use of FVB/N mice for the establishment of transgeniclines containing active oncogenes suggested the importance oftesting the parent FVB/N mice for sensitivity to experimentalcarcinogenesis. After initiation of mouse skin by a single treatmentwith 7, 12-dimethylbenz[a]anthracene (DMBA) and promotion by20 weekly applications of 12-O-tetradecanoylphorbol-13-acetate(TPA), the skin tumor incidence was compared in FVB/N mice,TPA-sensitive (SENCAR and CD-I) and TPA-resistant mice (BALB/cand C57BL/6). Initiation by 25 µg DMBA followed by promotionwith a low dose of TPA (2 µg/week) induced one or morepapillomas in only 25% of FVB/N mice, compared with 100% inSENCAR, 53% in CD-I, 17% in BALB/c and 0% in C57BL/6 mice. Ata more effective dose of TPA (5 /ig/week), FVB/N mice initiatedby 5, 25 or 100µg DMBA developed 3.4, 6.9 and 11.8 papillomasper mouse. In contrast, the incidence of squamous cell carcinomas(SCCs) (17–18/30 mice) did not increase with DMBA dose.TPA promotion of non-initiated mice induced only six papillomas,but three progressed to SCCs, a high rate of malignant conversion.Skin tumor induction by 20 weekly treatments with 10 µgDMBA produced few papillomas, but 50.0% of the papillomas progressedto carcinomas in FVB/N mice, compared with 9.15% in SENCAR,37.5% in CD-I, 23.1% in BALB/c and 15.0% in C57CL/6 mice. Thefirst carcinomas appeared after 14 weeks in FVB/N, 24 weeksin SENCAR, 26 weeks in CD-I and C57BL/6 and 34 weeks in BALB/cmice. Thus, FVB/N mice develop an unusually high incidence ofSCCs after treatment with repeated DMBA, DMBA initiation-TPApromotion and even TPA alone.     

Skin tumorigenesis and carcinogenesis studies with 7, 12-dimethylbenz[a]anthracene, ultraviolet light, benzoyl peroxide (Panoxyl gel 5%) and ointment gel

In a previous paper (1) it was demonstrated on hairless mouseskin that 5% benzoyl peroxide (BP) in a gel (Panoxyl), or gelalone, applied just before UV radiation had a protective effectagainst UV-induced tumorigenesis, but both enhanced 7, 12-dimethylbenz[a]anthracene(DMBA)-induced tumorigenesis. Groups of hairless(hr/hr) mice were therefore given ultraviolet (UV) Irradiationwith or without additional treatment with Panoxyl or gel inorder to see whether Panoxyl or the gel given long time before,or after, irradiation influenced UV-induced tumorigenesis. Consequently,in some animals Panoxyl or gel was applied in the evening andthe mice were irradiated the next day; in others, Panoxyl orgel was applied 5–30 min after UV irradiation. Enhancementof DMBA-iuduced carcinogenesis in hr/hr mice by the gel alone(assumed to be inert) was unexpected (1), and hence one groupof hr/hr mice was first given 51.2 µg DMBA in acetoneand thereafter treated twice a week with gel alone. All micewere tested and observed for skin tumors and other lesions for52 weeks. Neither Panoxyl nor gel influenced UV twmorigenesisor carcinogenesis under these experimental conditions. In hr/hrmice there was this time no enhancement of DMBA-induced tumorigenesisby the gel, and a slight reduction of carcinogenesis. In addition,several groups of SENCAR mice (which have been bred for highsensitivity to skin carcinogenesis) were also treated, withacetone alone, with a single application of DMBA alone, withPanoxyl alone, or with DMBA followed by treatment with the ointmentgel or with Panoxyl twice a week throughout the experiment.In SENCAR mice there was no difference between the results oftreatment with DMBA followed by Panoxyl, or DMBA followed bygel, and both substances tended to reduce the tumorigenicityof DMBA alone, and Panoxyl or gel showed no tumorigenicity oftheir own. The total dose of UV used in this study was lowerthan that used in the first study. This reduction in dose significantlyincreased the tumorigenic effect of UV.     

Effects of dose and duration of treatment with the tumor-promoting agent, 12-O-tetradecanoylphorbol-13-acetate on mouse skin carcinogenesis

The effects of dose and duration of treatment with the potenttumor-promoting agent 12-O-tetra-decanoylphorbol-13-acetate(TPA) on the formation of skin tumors in Charles River CD-1mice was studied. Mice were initiated with a single applicationof 0.2 µmol of 7, 12-dimethylbenz[a]anthracene (DMBA)in 0.2 ml acetone. Beginning two weeks after initiation, micewere treated twice weekly with various doses (0.01 – 20nmol) of TPA in 0.2ml acetone. Application of either 0.01 or0.1 nmol of TPA did not elcity tumors during the 50 weeks durationof treatment. A dose-dependent increase in the number of papillomaswas observed through the range of 1 to 10 nmol of TPA. Twiceweekly applications of 20nmol of TPA did not further enhancethe papilloma incidence. A good correlation was observed betweenthe induction of ornithine decarboxylase (ODC) activity andthe formation of skin tumors by various doses of TPA. To determine the effect of promotion duration on the incidenceof papillomas and carcinomas, mice were treated with 10 nmolof TPA for various durations (6,12,18,24,30, or 36 weeks) beginning2 weeks after initiation with 0.2 µmol of DMBA. Mice promotedfor only 6 weeks developed papilllomas and carcinomas afterpromotion had been discontinued. There was an intermediate incidenceof tumors in the group treated for 12 weeks. Promotion for 18,24, 30, or 36 weeks elicited virtually identical yields of papillomas.The incidence of carcinomas was proportional to promotion durationtimes of 6, 12, and 18 weeks, but carcinoma incidence was lessthan maximal in mice promoted for 24 weeks or longer. The results indicate that a) the incidence of papillomas servesas a rapid (18 weeks) index for subsequent appearance of carcinomas,b) twice weekly applications of 10 nmol of TPA for 18 weeksfollowing initiation of female CD-1 mice with 0.2 µmnolof DMBA is an appropriate protocol for maximum tumor yield ininitiation-promotion experiments, and c) ODC induction may bean important component of the mechanism of skin tumor promotionby TPA.     

Failure of genotoxic carcinogens to produce tumors in human skin xenografts transplanted to SCID mice

Chemical carcinogenesis of human skin was investigated usinghuman skin xenografts (16 full thickness and 48 split thicknessskin grafts) transplanted to CB-17-scid (SCID) mice. Topicalapplication of a carcinogen, i.e. 7, 12-dimethyl-benz[a]anthracene(DMBA), benzo[a]pyrene, methylchol-anthrene or N-methyl-N'-nitro-N-nitrosoguanidine,to the human skin xenografts once a week for 25–30 weeksfailed to produce skin tumors. Both DMBA application plus UV-Birradiation and alternate applications of the above four carcinogensin combination with UV-B irradiation also failed to producetumors. All of these treatments induced skin papillomas in skinsof host SCID mice. DMBA induced skin papillomas in allogenicCD-1 mouse skin grafts transplanted to SCID mice. These resultsindicate that susceptibility of human skin to these carcinogenicstimuli is much lower than that of mouse skin.     

Characterization of an inbred strain of the SENCAR mouse that is highly sensitive to phorbol esters

An inbred strain of SENCAR mice was developed that is more sensitiveto two-stage skin carcinogenesis protocols than the outbredparental stock. These mice, registered as SSIN/UTSP, were comparedto the SENCAR in initiation-promotion pro-tocols using the tumorpromoter 12-O-tetradecanoylphorbol-13-acetate (TPA) twice weeklyfor 22 weeks at dose levels of 0.5, 1.0 and 2.0 µg. With0.5 µg of TPA, the number of papillomas in the SSLN was3-fold higher than in the SEN CAR; the tumor incidence was 100versus 60%, respectively. Similar, although less dramatic, resultswere found with 1.0 and 2.0 µg of TPA. In two-step promotionprotocols using TPA twice weekly for 2 weeks and 2 µgmezerein twice weekly for 15 weeks the SSIN produced –50%more tumors than the SENCAR at 0.5 µg of TPA. At 2 µgof TPA the tumor response between the two mice was not significantlydifferent. Epidermal hyperplasia was considerably greater inthe SSIN at 0.5 µg of TPA as was ornithine decarboxylaseactivity. These TPA-sensitive inbred mice should be useful ininvestiga tions on the biochemical and genetic factors involvedin skin tumor promotion.     

Comparison of the tumor-initiating activity of 7,12-dimethylbenz[a]anthracene and benzo[a]pyrene in female SENCAR and CD-1 mice

The derivation of mice resistant and susceptible to skin tumorigenesisusing the initiation-promotion regimen is described. Dose-responserelationships for tumor-initiating activities of 7,12-dimethylbenz[a]-anthracene(DMBA) and benzo[a]pyrene (BP) in the susceptible line (SENCAR)are presented. A single topical dose of either 0.1, 1.0, 10or 100 nmol DMBA, followed one week later by twice weekly applicationsof 8.5 nmol 12–0-tetradecanoylphorbol-13-acetate (TPA)for 19 weeks, produced 0, 3.3, 4.9 and 23.1 papillomas per mouse,respectively. Single topical initiating doses of either 50,100 or 200 nmol BP produced 1.7, 3.8 or 7.8 papillomas per mouse,respectively, after 28 weeks of promotion with 8.5 nmol TPA.SENCAR mice were compared with CD-1 mice for the initiatingactivity of DMBA and BP. Initiating doses of 0.1, 1.0, 10 and100 nmol DMBA produced 0.6, 3.8, 7.0 and 24 papillomas per mouse,respectively, in SENCAR mice and in CD-1 mice produced 0, 0.2,3.0 and 5.6 papillomas per mouse, respectively, after 25 weeksof promotion with TPA. With BP as the initiator, 10, 50, 100and 200 nmol doses produced 0.9, 1.6, 3.8 and 8.3 papillomasper mouse, respectively, in SENCAR mice and in CD-1 mice produced0.1, 0.7,1.8 and 3.8 papillomas per mouse, respectively, after25 weeks of promotion with TPA. SENCAR mice were compared with CD-1 mice for possible differencesin the oxidative metabolism of DMBA using epidermal homogenatesas the enzyme source. Basal levels of monooxygenase activitytoward DMBA were similar in both mouse stocks. Epidermal monooxygenaseactivities following pre-treatment with inducers including DMBA,3-methylcholanthrene, dibenz[a,c]anthracene, Aroclor 1254 and2,3,7,8-tetrachlorodibenzo-p-dioxin, also were quite similarin both mouse stocks. High-pressure liquid chromatographic profilesof ethyl acetate/ acetone (2:1) extractable metabolites revealeda close similarity in the patterns as well as the rates of formationof specific metabolites. Metabolites of DMBA tentatively identifiedbased on cochromato-graphy with purified reference standardsincluded phenols, 12-hydroxymethyl-7-methylbenz[a]anthracene,7-hydroxymethyl-12-methylbenz[a]antnracene, 7,12-dihydroxymethylbenz[a]anthracene,(±)-trans-8,9-dihydro-8,9-dihydroxy-7,12-dimethylbenz[a]anthraceneand (±)-trans-5,6-dihydro-5,6-dihydroxy-7,12-dimethylbenz[a]anthracene.The results suggested that differences in oxidative metabolismof DMBA were not responsible for the differences in sensitivityto tumor-initiation between SENCAR and CD-1 mice.     

Tumor-producing and skin-irritating activity of dithranol (anthralin) and its 10-acyl analogues in SENCAR mice

The tumor-producing and skin-irritating activity of the anti-psoriaticdrug dithranol and its 10-acyl analogues butantrone (10-butyryldithranol), 10-isobutyryl dithranol and 10-valeryl dithranolwere studied in 650 SENCAR mice using a two-stage skin carcinogenesisassay. An initiation with 20 µg of 7, 12-dimethylbenz()anthracene(DMBA) was followed 2 weeks later by three applications perweek of the test compounds in 50 µl of acetone for 21weeks. In addition the compounds were studied without DMBA pre-treatmentusing application periods of 21 and 36 weeks. The concentrationof dithranol was the maximum tolerated, 3.5 mM. For the lessirritating 10-acyl analogues 30 mM solutions were used. Thefirst signs of skin irritation were observed after an applicationperiod of 1–2 weeks and the irritation continued to theend of the experiment in all groups except the acetone controls.Dithranol caused the most severe irritation although the differencesbetween the groups were not pronounced. On histopathology, themajority of animals had hyperplasia and other inflammatory changesof the skin. The first papillomas appeared 8–11 weeksafter initiation and the incidences of papillomas at the endof the experiment were 85% (dithranol 3.5 mM), 16% (butantrone30 mM), 36% (10-isobutyryl dithranol 30 mM) and 50% (10-valeryldithranol 30 mM). Without initiation the incidences were 6 and2% (dithranol), 2 and 2% (butantrone) and 2 and 0% (10-valeryldithranol) in the 21- and 36-week studies, respectively. Histologically,the papillomas were mostly squamous papillomas and only a fewkeratoacanthomas were found. It is concluded that the tumor-producingand skin-irritating activity of dithranol is clearly greaterthan that of butantrone, 10-isobutyryl dithranol and 10-valeryldithranol.     

High dietary retinoic acid prevents malignant conversion of skin papillomas induced by a two-stage carcinogenesis protocol in female SENCAR mice

We have previously reported that high dietary retinoic acid(RA; 30 µg/g diet) inhibits carcinoma formation in a twostageskin carcinogenesis protocol, using 7,12-dimethylbenz[a]anthracene(DMBA) as the initiator and 12-O-tetradecanoyl phorbol-13-acetate(TPA) as the tumorpromoter in female SENCAR mice. We next askedwhether switching the diets from high to control levels of RAand vice versa would influence carcinoma formation. Mice at3 weeks of age were initiated with DMBA (20     

On the persistence of tumour initiation and the acceleration of tumour progression in mouse skin tumorigenesis

Evidence has been obtained for the reversibility of initiation of carcinogenesis as evoked by 100 μ 7,12-dimethylbenz (a)anthracene (DMBA) applied to the skin of Swiss mice. Mice exposed to twice-weekly applications of a phorbol ester, TPA, for 15 weeks developed multiple papillomas when treatment was started 3 weeks after tumour initiation with DMBA, but very few when the interval was 50 weeks. This finding is not necessarily at variance with the postulate of the irreversibility of formation of “latent tumour cells” by subcarcinogenic doses of DMBA. Intraperitoneal injections of urethane increased the risk of development of malignant skin tumours by mice bearing multiple papillomas as a result of previous treatment with 100 μMg DMBA and TPA as compared with intraperitoneal injections of distilled water. This finding may allow a more clear-cut experimental definition of the stages in the process of tumour progression in mouse skin tumorigenesis.     

Chemopreventive action of mace (Myristica fragrans, Houtt) on DMBA-induced papillomagenesis in the skin of mice

The present paper reports the chemopreventive property of mace (aril covering the seed of Myristica fragrans) on DMBA-induced papillomagenesis in the skin of male Swiss albino mice. When a single topical application of DMBA (150 micrograms in 100 microliters of acetone) was followed, 2 weeks later, by repeated applications of croton oil (1% in acetone, three times/week) skin papillomas appeared in 100% animals and the average tumors per tumor-bearing animal was 5.67. On the other hand, when animals receiving similar treatments were put on a diet containing 1% mace during the periinitiational phase of tumorigenesis, the skin papilloma incidence was reduced to 50% and the average tumor per tumor-bearing mouse was only 1.75. This decline in papilloma was significant (P less than 0.05).     

NAD(P)H:quinone oxidoreductase 1 deficiency and increased susceptibility to 7,12-dimethylbenz[a]-anthracene-induced carcinogenesis in mouse skin

BACKGROUND: The phase II enzyme NAD(P)H :quinone oxidoreductase 1 (NQO1) catalyzes quinone detoxification, protecting cells from redox cycling, oxidative stress, mutagenicity, and cytotoxicity induced by quinones and its precursors. We have used NQO1(-/-) C57BL/6 mice to show that NQO1 protects them from skin cancer induced by the polycyclic aromatic hydrocarbon benzo[a]pyrene. Herein, we used NQO1(-/-) mice to investigate whether NQO1 also protects them against 7,12-dimethylbenz[a]anthracene (DMBA), where methyl substituents diminish primary quinone formation. METHODS: Dorsal skin of NQO1(-/-) or wild-type C57BL/6 mice was shaved. When tested as a complete carcinogen, DMBA (500 or 750 microg in 100 microL of acetone) alone was applied to the shaved area. When tested as a tumor initiator, DMBA (200 or 400 nmol in 100 microL of acetone) was applied to the shaved area; 1 week later, twice-weekly applications of phorbol 12-myristate 13-acetate (PMA)-10 microg dissolved in 200 microL of acetone-to the same area began and were continued for 20 weeks. Tumor development was monitored in all mice (12-15 per group). All statistical tests were two-sided. RESULTS: When DMBA (750 microg) was tested as a complete carcinogen, about 50% of the DMBA-treated NQO1(-/-) mice but no DMBA-treated wild-type mouse developed skin tumors. When DMBA (both concentrations) was used as a tumor initiator, NQO1(-/-) mice developed larger tumors at a greater frequency than their wild-type littermates. Twenty-three weeks after the first PMA treatment in the tumor initiator test, all 30 NQO1(-/-) mice given 400 nmol of DMBA had developed skin tumors, compared with 33% (10 of 30) of treated wild-type mice (P<.001). CONCLUSIONS: NQO1(-/-) mice are more susceptible to DMBA-induced skin cancer than are their wild-type littermates, suggesting that NQO1 may protect cells from DMBA carcinogenesis.     

Sandalwood oil prevent skin tumour development in CD1 mice.

Sandalwood oil (SW oil) has been used for the treatment of inflammatory and eruptive skin diseases. In the present study, the chemopreventive effects of SW oil on 7,12-dimethylbenz(a)-anthracene (DMBA)-initiated and 12-O-tetradecanoylphorbol-13-acetate(TPA)-promoted skin tumour development and TPA-induced ornithine decarboxylase (ODC) activity in CD1 mice were investigated. Female CD1 mice (5-6 weeks old) were divided in different groups, having 30 mice in each group. One week after topical application of DMBA (200 nmole in 100 microl acetone) alone or with SW oil at different concentrations (100 microl, 1.25, 2.5, 3.75, 5% in acetone), at different times (0.5, 1, 2 h) before DMBA, the mice were treated topically with TPA (5 nmole in 100 microl acetone) alone or with SW oil at different concentrations (100 microl, 1.25, 2.5, 3.75, 5% in acetone) at different times (0.5, 1, 2 h) before TPA applications twice a week for 20 weeks. The mice were weighed and papillomas counted weekly. The results indicate that SW oil pre-treatment decreased the papilloma incidence and multiplicity in a concentration and time-dependent manner. The pre-treatment with 5% SW oil (100 microl) 1 h before DMBA and TPA treatments provided a maximum of 67% and 96% decrease in papilloma incidence and multiplicity, respectively, after 20 weeks of promotion. The mice pre-treated with SW oil at all concentrations and time period before TPA had significantly lower ODC activity than the group treated with TPA alone. The data suggest that SW oil could be an effective chemopreventive agent against chemically-induced skin cancer.     

Promoting action of cashew nut shell oil in DMBA-initiated mouse skin tumour model system.

The commercially available oil derived from the shell of cashew nut (Anacardium occidentale) was tested for its potency in promoting the DMBA-initiated cells into papillomas in a murine two-stage skin tumorigenesis model system. Male Swiss albino mice (9-10-weeks-old) were assorted into different groups and treated topically with single sub-carcinogenic doses of DMBA (50 micrograms in 0.1 ml acetone) followed by application of 1% and 2% shell oil in acetone three times a week. Animals were sacrificed after 20 weeks from the commencement of the experiment. The results imply a weak tumour promoting effect of cashew nut shell oil as the mean tumour incidences were found to be 1.1 and 2.5 in 1% and 2% oil treatment groups, respectively, while the corresponding figure vas 6.6 in the positive control group (DMBA and 1% croton oil in acetone). Few speculative mechanisms for the observed effect of cashew nut shell oil on initiated skin are discussed.