Manganese inhibition of sarcoma induction by benzo[a]pyrene in rats

The effects of Mn and Cr dusts upon carcinogenesis by polycyclic aromatic hydrocarbons were tested in male Fischer 344 rats. In Experiment I, rats were given i.m. injections of benzo[a]pyrene (BP, 1.2 mg), Mn dust (4.4 mg), and Cr dust (4.4 mg), alone, and in various combinations. By 100 weeks, sarcomas occurred at the injection site in 17/20 rats that received BP alone, versus 10/19 rats that received BP plus Mn dust (P < 0.05). The sarcoma incidences were 0/20 in 3 control groups that received the vehicle, Mn dust alone, or Cr dust alone, and 20/20 in rats that received BP plus Cr dust. In Experiment II, rats were given i.m. injections of BP (0.6 mg), 7,12-dimethylbenz[a]anthracene (DMBA, 0.6 mg), Mn dust (4.4 mg), and Cr dust (4.4 mg), alone, and in various combinations. Local sarcomas occurred in 17/20 rats that received BP alone, versus 5/17 rats that received BP plus Mn dust (P < 0.001). Sarcoma induction was not inhibited when BP was injected in the right thigh and Mn dust in the left thigh (sarcoma incidence = 16/20). The sarcoma incidences were 0/19 in a vehicle control group; 0/18 in 2 control groups that received Mn dust or Cr dust alone; 18/20 in rats that received BP plus Cr dust; 14/20 in rats that received DMBA alone; 17/19 in rats that received DMBA plus Mn dust; and 13/18 in rats that received DMBA plus Cr dust. These experiments show that Mn dust inhibits the carcinogenicity of BP when the Mn dust and BP are administered to rats by a single i.m. injection. Under identical conditions, Mn dust does not affect the carcinogenicity of DMBA; Cr dust does not affect the carcinogenicity of BP or DMBA.     

Metabolism of [14C]benzo[a]pyrene in vivo in the rat

[¹⁴C]Benzo[a]pyrene ([¹⁴C]B[a]P) injected intraperitoneally into rats appeared rapidly in liver, lung and kidney, and remained detectable in these tissues for at least 7 days. A large proportion (7–13%) of the ¹⁴C became covalently bound to tissue macromolecules, probably primarily proteins. Subcellular organelles of the liver were all found to bind the carcinogen, the microsomes most rapidly and the light mitochondrial fraction taking up ¹⁴C later. Nuclear bound ¹⁴C was detected in both liver and lung.Purification of the cytosolic ¹⁴C from liver revealed specific binding to the same cytosolic proteins purified from the in vitro reaction of [¹⁴C]B[a]P.     

Mammary carcinogenicity in female CD rats of fjord region diol epoxides of benzo[c]phenanthrene, benzo[g]chrysene and dibenzo[a,l]pyrene

We compared the mammary carcinogenicity in female CD rats ofthree fjord region diol epoxides to test our hypothesis thatsuch sterically hindered molecules would be potent carcinogens.The diol epoxides tested were racemic anti-3,4-dihydroxy-l,2-epoxy-l,2,3,4-tetrahydrobenzo[c]phenanthrene(BcPDE), anti-11,12-dihydroxy-13,14-epoxy-11,12,13,14-tetrahydrobenzo[g]chry-sene(BgCDE) and anti-11,12-dihydroxy-13,14-epoxy-11,12,13,14-tetrahydrodibenzo[a,l]pyrene(DB[a,l]PDE). Each diol epoxide was dissolved in dimethylsulfoxide(DMSO) and injected under the six nipples on the left side ofthe rat, with DMSO only being injected under the nipples onthe right side. The total dose of each diol epoxide was 1.2µmol/rat and there were 20 rats/group. The experimentwas terminated 41 weeks after treatment. All three diol epoxideswere potent mammary carcinogens, with activity greater thanpreviously observed for a bay region diol epoxide, anti-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene(BPDE). DB[a,l]PDE induced tumors most rapidly, followed byBcPDE and BgCDE. However, different types of tumors were induced.For induction of adenomas and adenocarcinomas, BcPDE and BgCDEhad comparable potency; both were more active than DB[a,l]PDE.In contrast, for induction of sarcomas, DB[a,l]PDE was significantlymore active than BcPDE and BgCDE. The results of this studysupport our hypothesis that sterically hindered fjord regiondiol epoxides are potent mammary carcinogens in the rat.     

Tumor-initiating activity and carcinogenicity of dibenzo[a,l]pyrene versus 7, 12-dimethylbenz[a]anthracene and benzo[a]pyrene at low doses in mouse skin

Dibenzo[a,l]pyrene (DB[a,l]P) is an extremely potent carcinogenthat may be present in environmental samples. Dose-responsestudies were conducted at low doses in mouse skin by initiation-promotionand repeated application to compare its activity to that of7,12-dimethylbenz[a]anthracene (DMBA), benzo[a]pyrene (B[a]P),DB[a,l]P-8,9-dihydrodiol and DB[a,l]P-11,12-dihydrodiol. FemaleSENCAR mice were initiated with 1 or 0.25 nmol of DB[a, l]P,DMBA, B[a]P or DB[a,l]P-11,12-dihydrodiol and promoted withphorbol ester acetate. At 1 nmol, DB[a, l]P induced 2.6 tumors/mouse,whereas DB[a,l]P-11,12-dihydrodiol and DMBA induced 0.17 and0.29 tumors/mouse respectively. At the low dose, DB[a,l]P induced0.79 tumors/mouse, but the other two compounds were virtuallyinactive. B[a]P, tested only at 1 nmol, was inactive. Thesethree compounds, as well as DB[a,l]P-8,9-dihydrodiol, were testedby repeated application twice weekly for 40 weeks at 1 and 4nmol per dose. In addition, DB[a,l]P, DMBA and B[a]P were alsotested at 8 nmol. At 8 and 4 nmol, DB[a,l]P induced malignanttumors in 91 and 70% of mice respectively. At 4 nmol DB[a, l]P-11,12-dihydrodiolelicited only benign tumors in 36% of mice. At 4 nmol DMBA inducedtwo carcinomas in one mouse and at 8 nmol it induced one papillomaand one sebaceous gland adenoma. B[a]P and DB[a,l]P-8,9-dihydrodiolwere inactive at all doses tested. These results demonstratethat DB[a, l]P is a much more potent carcinogen than DMBA, thearomatic hydrocarbon previously considered to be the most potent.Combination of these results with previous comparisons of DB[a,l]P,DB[a,l]P-11,12-dihydrodiol, DMBA and B[a]P at higher doses (E.L.Cavalieri et al. (1991) Carcinogenesis, 12, 1939–1944)shows clearly the interference of toxicity with the tumorigenicityof DB[a,l]P and its 11,12-dihydrodiol.     

Topical treatment of mice with benzo[a]pyrene or parenteral administration of benzo[a]pyrene diol epoxide--DNA to rats results in faecal excretion of a putative benzo[a]pyrene diol epoxide--deoxyguanosine adduct

The administration of [³H]BPDE-DNA, whether by i.p. or i.v.injection, to male Wistar rats resulted in the majority of theradioactivity being recovered in the faeces. Excretion was rapid:within 24 h post-injection, 45% of the applied dose was recoveredin the faeces. H.p.l.c. analysis of radioactive material extractedfrom the faeces by methanol showed that it contained a singlecomponent which co-chromatographed with [³H]BPDE-dGuo and whichwas not affected by treatment with alkaline phosphatase, arylsulphatase or ß-glucuronidase. To determine if thisphenomenon occurs after topical application of BP to a targettissue, such as mouse skin, animals were treated with [³H]BPand their faeces collected. After an extensive extraction procedureinvolving differential solubility in organic solvents, SephadexLH-20 chromatography and h.p.l.c, a product was isolated frommice faeces which had characteristics consistent with a [³H]BPDE-dGuoadduct. These findings are discussed in relation to detectionof BPDE adducts in human populations.     

Binding of benzo[a]pyrene and intracellular transport of a bound electrophilic benzo[a]pyrene metabolite by lipoproteins

Human serum lipoproteins were isolated by means of size exclusionh.p.l.c. Non-covalent uptake of [³H]benzo[a]pyrene was quantitatedfor fractions collected from the effluent of a liquid chromatographicseparation of human serum, and was found to directly correlatewith the lipoprotein concentration. An electrophilic benzo[a]pyrenemetabolite, [³H]trans 7,8-dihydrodiol-9,10-epoxybenzo[a]pyrene,non-covalently associated with low density lipoproteins wastransferred to human lymphocytes in vitro and bound acid-precipitablenucleic acids of the lymphocytes as a function of time. Benzo[a]-pyrenemetabolite binding to lymphocyte DNA was demonstrated by meansof CaCl density gradient analysis. Non-mitogen-stimulated lymphocytesexposed to very low concentrations of carcinogen in the presenceof low density lipo-protein demonstrated [³H]thymidine incorporation;without the concomitant addition of low density lipoproteinthe low concentrations of carcinogen did not stimulate [³H]thymidineincorporation.     

Synthesis of a novel fluorinated benzo[a]pyrene: 4,5-difluorobenzo[a]pyrene

The synthesis of 4,5-difluorobenzo[a]pyrene, as a fluorinated probe to investigate the involvement of the K-region in the further metabolic activation of benzo[a]pyrene metabolites, is described. Benzo[a]pyrene-4,5-dione obtained from 2,3-dichloro-5,6-dicyano-1,4-benzoquinone oxidation of cis-4,5-dihydro-4,5-dihydroxybenzo[a]pyrene was fluorinated with dimethylaminosulfur trifluoride to give 4H,5H,4,4,5,5,-tetra-fluorobenzo[a]pyrene. Defluorination using lithium aluminum hydride in tetrahydrofuran gave 4,5,-difluorobenzo[a]pyrene.     

Metabolism of benzo[a]pyrene and benzo[a]pyrene-7,8-diol by human cytochrome P450 1B1

Benzo[a]pyrene (B[a]P), a ubiquitous environmental, tobacco and dietary carcinogen, has been implicated in human cancer etiology. The role of human cytochrome P450 1B1 in the metabolism of B[a]P is poorly understood. Using microsomal preparations of human P450 1A1, 1A2 and 1B1 expressed in baculovirus-infected insect cells, as well as human and rat P450 1B1 expressed in yeast, we have determined the metabolism of B[a]P, with and without the addition of exogenous epoxide hydrolase, and B[a]P-7,8-dihydrodiol (7,8-diol), each substrate at a concentration of 10 microM. HPLC analysis detected eight major metabolites of B[a]P and four metabolites of the 7,8-diol. The results of these studies indicate that cytochrome P450 1B1 carries out metabolism of B[a]P along the pathway to the postulated ultimate carcinogen, the diol epoxide 2, at rates much higher than P450 1A2 but less than P450 1A1. The rates of formation of the 7,8-diol metabolite in incubations with epoxide hydrolase are 0.17 and 0.38 nmol/min/nmol P450 for human P450 1B1 and 1A1, respectively, and undetectable for 1A2. The rates of total tetrol metabolite formation from the 7,8-diol, which are indicative of diol epoxide formation, are 0.60, 0.43 and 2.58 nmol/min/nmol P450 for 1B1, 1A2 and 1A1 respectively. In agreement with other reports of rat P450 1B1 activity, our data show this rat enzyme to be very active for B[a]P and 7,8-diol, with rates higher than human P450 1B1. In addition to the established role of P450 1A1 in B[a]P metabolism, P450 1B1 may significantly contribute to B[a]P and 7,8-diol metabolism and carcinogenesis in rodent tumor models and in humans.      

Differential metabolism of benzo[a]pyrene and benzo[a]pyrene-7,8-dihydrodiol by human CYP1A1 variants

Cytochrome P450 1A1 (CYP1A1) plays a key role in the metabolism of carcinogens, such as benzo[a]pyrene (B[a]P) and metabolites to ultimate carcinogens. Three human allelic variants, namely wild-type (CYP1A1.1), CYP1A1.2 (I462V) and CYP1A1.4 (T461N), were coexpressed by coinfection of baculovirus-infected insect cells with human NADPH-P450 reductase. These recombinant enzymes (in microsomal membranes) were used to analyze whether CYP1A1 polymorphisms affect catalytic activities towards B[a]P and B[a]P-7,8-dihydrodiol. The complete spectrum of phase I metabolites, including the tetrahydrotetrols resulting from hydrolysis of the ultimate carcinogen, B[a]P-7,8-dihydrodiol-9,10-epoxide, was examined by HPLC. Wild-type enzyme showed the highest total metabolism of B[a]P, CYP1A1.2 was approximately 50%, and CYP1A1.4 approximately 70%. Km values for all metabolites with CYP1A1.2 were generally significantly lower than with wild-type enzyme (e.g. B[a]P-7,8-diol formation: 13.8 microM for wild-type, 3.5 microM for CYP1A1.2 and 7.7 microM for CYP1A1.4). Addition of epoxide hydrolase markedly increases the relative diol-to-phenol activities by all three variants. However, CYP1A1.4 exhibits the greatest efficiency to produce diol species. Each variant produced the diol epoxides from B[a]P-7,8-dihydrodiol. CYP1A1.1 exhibited with 10.4 pmol/min/pmol CYP1A1 the greatest total rate for 7,8-diol metabolites followed by CYP1A1.2 (7.2 pmol/min/pmol CYP1A1) and CYP1A1.4 (5.5 pmol/min/pmol CYP1A1). All enzyme variants produced about three times more diol epoxide 2-derived metabolites than diol epoxide 1-derived ones, whereby both rare allelic variants exhibited statistically significantly increased formation of diol epoxide 2. This study showed that the three CYP1A1 variants had different enzyme kinetics properties to produce both the diol metabolites from B[a]P and the ultimate mutagenic species diol epoxide 2 from B[a]P-7,8-dihydrodiol, which must be considered in the evaluation of individual susceptibility to cancer.     

Metabolism of benzo[a]pyrene and benzo[a]pyrene-7,8-dihydrodiol in human mammary epithelial cells: feedback inhibition by 7-hydroxybenzo[a]pyrene

The metabolism of benzo[a]pyrene (B[a]P) and (-)-transbenzo[a]pyrene-7,8-dihydrodiol (B[a]P-diol) was compared in human mammary epithelial cells (HMEC) grown in serum-free medium, MCDB-170. Conversion of B[a]P-diol to the carcinogen (+)-benzo[a]pyrene-7,8-dihydroxy-9,10-epoxide (BPDE), as measured by analysis of their tetraol hydrolysis products, occurred much more efficiently in cultures incubated with [3H]B[a]P-diol than in cultures incubated with [3H]B[a]P. In cultures pretreated with unlabeled B[a]P (24 h, 400 nM), the conversion of [3H]-B[a]P-diol to [3H]tetraols is inhibited 49%, while the conversion of [3H]B[a]P to [3H]B[a]P-diol- is not affected. These observations led to the identification of a major B[a]P-derived metabolite as 7-hydroxybenzo[a]pyrene (B[a]P-7-ol), which was found to be an extremely potent and selective inhibitor of the conversion of B[a]P-diol to BPDE, with a KI estimated at 3-12 nM. Thus B[a]P activation in HMEC appears to be significantly limited by a feedback inhibition pathway induced by B[a]P-7-ol. The potency and selectivity of the B[a]P-7-ol-induced inhibition suggests that the diol to diolepoxide conversion is affected by a selective oxygenase in HMEC, rather than a non-enzymatic, peroxy radical-induced mechanism. B[a]P-7-ol should prove to be a valuable tool in the study of B[a]P carcinogenesis.     

Comparative dose-response study on the pulmonary carcinogenicity of 1, 6-dinitropyrene and benzo[a]pyrene in F344 rats

The dose dependencies of the lung carcinogenicity of 1, 6-dinitropyrene(1, 6-DNP) and benzo[a]pyrene (BaP) were examined by directinjections of these compounds into rat lungs. A total of 276male F344 rats were divided into 10 groups and given variousdoses of 1, 6-DNP or BaP, or no drug (control group). Both chemicalswere injected into the lung, as suspensions in beeswax-tricaprylinand the animals were then observed for 104 weeks. The incidencesof lung cancer were 0/39 (0%), 4/30 (13%), 13/31 (42%), 22/26(85%) and 6/9 (67%) in groups treated with 0.003, 0.01, 0.03,0.1 and 0.15 mg of 1, 6-DNP respectively, and 1/29 (3%), 7/30(23%) 22/29 (76%) and 9/13 (69%) in those treated with 0.03,0.1, 0.3 and 1.0 mg of BaP respectively. No lung cancer wasfound in control rats. Thus the incidences of lung cancer inducedby 1, 6-DNP and BaP showed significant dose dependence. At equaldoses, the incidence of lung cancer was much higher with 1,6-DNP than with BaP, and the induction of cancer by 1, 6-DNPwas higher even at one-third the dose of BaP. Histologically,most tumours induced by 1, 6-DNP were undifferentiated neoplasms,whereas most of those induced by BaP were well-differentiatedsquamous cell carcinomas.     

Reduction of respiratory tract binding of benzo[a]pyrene in mice by immunization.

Male inbred A/J mice immunized by combined ip and intranasal administration of a bovine serum albumin conjugate of 5-fluoro-12-methylbenzanthryl-7-acetic acid developed tracheal antibodies capable of binding the carcinogen benzo[a]pyrene (BP). Immunized mice administered 92 ng of [3H]BP intranasally exhibited a one-third reduction in BP content in respiratory tract tissues (nose and trachea) when compared with control mice 20 hours after BP administration.     

Dose-dependent inhibitory effect of melatonin on carcinogenesis induced by benzo[a]pyrene in mice

Three-month-old Swiss-derived SHR mice were subcutaneously injected with 2 mg of benzo[a]pyrene (BP) dissolved in 0.1 ml of olive oil. After the injections of the carcinogen two groups of mice were given melatonin with night drinking water at the doses of 2 mg/l or 20 mg/l and one group of mice was not treated with melatonin and served as a PB-control. At the 28th week after the carcinogen administration the experiment was stopped and animals were sacrificed. The results show that melatonin treatment inhibits BP-induced carcinogenesis, decreases the incidence of subcutaneous sarcomas, increases their latency and survival of mice. The malone dialdehyde (MDA) level in the serum of BP-induced tumor-bearing mice was increased by 2.6 times (p < 0.01) and in the tumors was increased by 11.1% (p < 0.01) as compared to intact control mice. Treatment with melatonin significantly decreased the MDA level both in the serum and tumor tissue. The activity of catalase in the serum of BP-induced tumor-bearing mice was increased by 12.1% as compared to the intact control mice (p < 0.01) and was unchanged in the tumor tissue. Treatment with melatonin at the dose of 2 mg/l significantly decreased activity of catalase in the serum (by 31.7%, p < 0.01) and in the tumor tissue (by 2.6 times, p < 0.01) as compared to the animals treated with BP alone. Thus, it wose shown for the first time an inhibitory effect of melatonina on malignancies of mesenchymal origin. Lower dose of melatonin appeared to be more effective in the inhibition of lipid peroxidation and tumorigenesis induced by chemical carcinogen than a higher one.     

CYP1A1 genotype-selective inhibition of benzo[a]pyrene activation by quercetin

Epidemiological studies suggest that food rich in quercetin and naringin may protect against certain types of lung cancer, and that genotype dependent inhibition of cytochrome P450 1A1 (CYP1A1)-mediated bioactivation of procarcinogens could be the underlying mechanism. We studied the inhibitory effects of quercetin and naringin on the terminal bioactivation step of benzo[a]pyrene (B[a]P), a member of the major class of lung carcinogens. This reaction (epoxidation of (+/-)-trans-7,8-dihydro-7,8-dihydroxy-B[a]P to the ultimate carcinogenic product, (+/-)-B[a]P-r-7,t-8-dihydrodiol-t-9,10-epoxide (diolepoxide 2)) was examined using three of the most common allelic variants of human CYP1A1, namely wild-type CYP1A1.1, CYP1A1.2, and CYP1A1.4. Quercetin potently inhibited diolepoxide 2 formation by all CYP1A1 types with IC(50) values between 1.6 and 7.0 microM. The differences between the wild-type enzyme and the variants were statistically highly significant (P < 0.01). Enzyme kinetics revealed quercetin as a mixed-type inhibitor of CYP1A1.1, CYP1A1.2, and CYP1A1.4 with K(i) values of 2.0, 6.4, and 9.3 microM, respectively. Naringin inhibited diolepoxide 2 formation only slightly. Our data support the hypothesis that quercetin may have a stronger chemopreventive effect in individuals carrying wild-type compared with variant CYP1A1 genes. Future studies should consider the influence of P450 polymorphisms on both procarcinogen activation and its inhibition to facilitate the development of genotype-specific chemoprevention regimes.     

Metabolism of benzo[a]pyrene by human melanocytes in culture

Benzo[a]pyrene (B[a]P) is a ubiquitous environmental pollutant and known skin carcinogen. In the present study, in vitro addition of [3H]B[a]P to normal human melanocytes in culture, isolated from adult human skin, resulted in the metabolism of [3H]B[a]P both intracellularly and extracellularly. HPLC analysis showed that [3H]B[a]P-9,10- and 7,8-diol were the major intracellular and extracellular metabolites followed by 3,6-quinone, 9-hydroxy and 3-hydroxy metabolites. Significant amounts of the [3H]B[a]P metabolites were found to be present in the sonicated cell suspension and culture medium as the glucuronide and sulfate conjugates. In total 37.3% of the [3H]B[a]P added in the culture medium was metabolized by melanocytes, of which 21.1% was quantified as the intracellular and 16.2% as the extracellular metabolites. Our data show that human melanocytes are capable of metabolizing B[a]P.     

Pulmonary carcinogenicity of 3,9- and 3,7-dinitrofluoranthene, 3-nitrofluoranthene and benzo[a]pyrene in F344 rats

3,9- and 3,7-Dinitrofluoranthene (3,9- and 3,7-DNF), 3-nitrofluoranthene (3-NF) and benzo[a]pyrene (B[a]P) were tested for pulmonary carcinogenicity by intrapulmonary implantation of the compounds into rat lung. These chemicals were given in various doses as suspensions in beeswax-trycaprylin and the animals were observed for 100 weeks. The control group received no drugs. The incidences of lung tumors were 19/21 (90.5%), 7/10 (70%) and 1/10 (10%) in rats treated with 200, 100 and 50 micrograms of 3,9-DNF, 4/9 (44.4%), 3/10 (30%) and 0/10 (0%) in rats treated with 200, 100 and 50 micrograms of B[a]P, 12/22 (54.5%) in rats treated with 200 micrograms of 3,7-DNF and 1/20 (5%) in rats treated with 1000 micrograms of 3-NF respectively. No lung tumors were found in control rats. The incidence of lung tumors induced by 3,9-DNF was twice as high as that induced by B[a]P, when the equivalent dose levels of the two compounds were compared. Histologically, most of the tumors induced by 3,9- and 3,7-DNF and B[a]P were squamous cell carcinomas.