Induction of chromosomal damage in exfoliated buccal and nasal cells of road markers

Road markers are exposed to various chemicals and particles. The aim of this study was to determine whether road worker exposure induceschromosomal damage which is indicative for increased cancer risks. Micronucleus (MN) cytome assays were thus conducted with exfoliated nasal and buccal cells collected from 42 workers and 42 matched controls. The frequencies of MN (reflecting chromosomal aberrations), nuclear buds (NBuds; reflecting gene amplifications) and binucleated cells (BN; reflecting disturbed mitosis) were scored. Further, the rates of nuclear anomalies indicative of acute cytotoxicity (condensed chromatin, karyorrhexis, karyolysis, pyknosis) were evaluated. Data demonstrated marked induction of MN, NBuds, and BN by 1.34-fold, 1.24-fold and 1.14-fold in buccal cells. In nasal cells, only MN frequencies were elevated, 1.23-fold. These effects were paralleled by increased rates of condensed chromatin, karyorrhexis and karyolysis in both cell types. The effects were more pronounced in individuals who had worked for more than 10 years while smoking did not produce synergistic responses. This is the first investigation concerning the induction of genetic damage in road markers and the results are suggestive for enhanced cancer risks. It is conceivable that exposure to silica dust (known to induce cancer and genetic damage) and/or benzoyl peroxide which forms reactive radicals may be associated with the observed genetic damage in road workers. Further investigations of the cancer risks of these workers are warranted.

Effects of flour bleaching agent on mice liver antioxidant status and ATPases

Benzoyl peroxide (BPO) is a strong oxidizing agent and widely used as flour bleaching agent. However their potential risk of liver damage is unknown. The aim of this study was to investigate the effects of BPO on mice liver antioxidant status and ATPases according to the actual amount of BPO in flour from Jinan, China. The results showed that the maximum concentration of BPO reached up to 284.6 mg/kg and content of BPO mainly ranged from 0 to 240 mg/kg. Therefore, four groups of mice were gavaged daily with BPO at doses of 0, 50, 100, 200mg/kg b.w./d for 42 days, respectively. In liver tissue, superoxide dismutase (SOD) activity was significantly decreased, while the content of malondialdehyde (MDA) significantly increased following BPO exposure at 200mg/kg b.w. BPO significantly decreased the Mg(2+)-ATPase and Ca(2+)-ATPase activities of the liver at 200mg/kg b.w. BPO, at all of the doses assayed, produced non-significant effects on glutathione peroxidase (GSH-Px) and Na(+)K(+)-ATPase activities. Experimental results suggested that BPO had certain adverse effects on antioxidant status and the activities of Mg(2+)-ATPase and Ca(2+)-ATPase of liver tissue.

Genotoxicity of dental resin polymerization initiators in vitro

The polymerization initiators for resins cured using visible light usually consist of a photosensitizer, primarily camphorquinone (CQ), and a reducing agent, which is often a tertiary amine (DMPT, DMAEMA), while the initiator used for self-curing resins consists of benzoyl peroxide (BPO) and a tertiary amine (DMPT). The genotoxicities of camphorquinone (CQ), benzoyl peroxide (BPO), dimethyl-para-toluidine (DMPT), 2-dimethylamino-ethyl-methacrylate (DMAEMA), and 1-allyl-2-thiourea (ATU) were examined using the bioluminescent bacterial genotoxicity test. 4-Nitroquinoline-N-oxide (4NQO) was prepared for comparison with these chemicals. Acetone solutions of the five polymerization initiators and 4NQO were prepared. Benzoyl peroxide (BPO), dimethyl-para-toluidine (DMPT), and 1-allyl-2-thiourea (ATU) showed significant genotoxic activity at 24 h in the bioluminescent bacterial genotoxicity test, at concentrations of approximately 5 microM, 4 mM, and 1 mM, respectively. 2-Dimethyloamino-ethyl-methacrylate (DMAEMA) did not have genotoxic activity and CQ had questionable genotoxic activity. In comparison, 4NQO had strong genotoxicity, at 4 microM, roughly the same as that of BPO. Therefore, BPO should be used carefully in clinical dentistry.

Mitogen-regulated protein/proliferin mRNA induction following single applications of tumor promoters to murine skin

Mitogen-regulated protein/proliferin (mrp/plf) gene family transcripts rise in abundance as a response to diverse chemical and physical agents that promote morphological transformation in the murine C3H/10T1/2 cultured cell model of multi-step carcinogenesis. To determine if proliferin genes respond to tumor promoters in vivo, RNA was extracted from the whole skin of SENCAR mice after single applications of 2 or 20 microg 12-O-tetradecanoylphorbol-13-acetate (TPA); 3.2 or 32 nmole), 20 or 40 mg benzoyl peroxide (BPO; 83, 165 micromole), or acetone vehicle alone (2.72 mmole). RNA samples were prepared from treated skin areas, 2-48 h after painting. Mrp/plf-mRNA was not detected in Northern blot hybridizations, but large increases in mRNAs for ornithine decarboxylase gene and mRNA (odc), v-jun oncogene-related transcription factor gene and mRNA (junB), egr1 (early growth response protein gene and mRNA) were measured relative to beta 2 microglobulin gene and mRNA (b2m) mRNA in response to TPA. BPO induced small relative changes in these mRNAs. Reverse transcriptase (RT)-polymerase chain reactions (PCR) detected fully-processed MRP/plf-mRNA 16-48 h after TPA treatments in five of six animals, and in three of six BPO-treated animals. The MRP/plf-mRNA species expressed in the skin were predominantly plf1 and mrp3 as determined by gene-specific restriction enzyme sites within the RT-PCR products. Expression was either undetectable or found at low levels in acetone-painted controls and was not detected during the anagen phase of the normal hair growth cycle in unpainted animals. These results demonstrate that mrp/plf-mRNA is differentially expressed in murine skin in response to mechanistically distinct tumor promoters and has potential utility as a short-term biomarker for tumor promoting effects in chemical carcinogenesis.

Effect of Hibiscus rosa sinensis Extract on Hyperproliferation and Oxidative Damage Caused by Benzoyl Peroxide and Ultraviolet Radiations in Mouse Skin

The present study was conducted to investigate the ameliorative potential of Hibiscus rosa sinensis extract in mice skin. Combination of a single topical application of benzoyl peroxide (20 mg/0.2 ml/animal) followed by ultraviolet radiations (0.420 J/m2/s) was used to induce hyperproliferation and oxidative stress. Single benzoyl peroxide application prior to ultraviolet B radiations exposure caused significant depletion in the detoxification and antioxidant enzymes, while malondialdehyde formation, hydrogen peroxide content, ornithine decarboxylase activity and DNA synthesis were raised significantly. However, pretreatment of H. rosa sinensis extract (3.5 mg and 7 mg/ kg b.wt.) partly restored the levels of cellular protective enzymes (P<0.05). Besides, malondialdehyde formation and hydrogen peroxide content (P<0.05) were statistically significantly reduced at both doses. The ornithine decarboxylase activity and thymidine incorporation in DNA were also reduced dose dependently (P<0.05) by the plant extract. Therefore, we propose that H. rosa sinensis extract exerts a protective effect against the tumour promotion stage of cancer development.

Modulatory effect of gentisic acid on the augmentation of biochemical events of tumor promotion stage by benzoyl peroxide and ultraviolet radiation in Swiss albino mice

The present study was carried out to study the effect of gentisic acid (2,5-dihydroxybenzoic acid (2,5-DHBA)) on the tumor promotion related events of carcinogenesis in murine skin. Benzoyl peroxide (BPO) (20 mg/0.2 ml/animal) and ultraviolet radiations (UVR) (0.420 J/m2/s) were used to induce tumor promotion response and oxidative stress and caused significant depletion in the detoxification and antioxidant enzyme armory with concomitant elevation in malondialdehyde (MDA) formation, hydrogen peroxide (H2O2) generation, ornithine decarboxylase (ODC) activity and unscheduled DNA synthesis. However, gentisic acid pretreatment at two different doses restored the levels of the above said parameters (P < 0.05) in a dose-dependent manner except in the case of ODC activity. Therefore, we propose that it might suppress the promotion stage via inhibition of oxidative stress but may not affect the polyamine biosynthetic pathway.

Only a subset of 12-O-tetradecanoylphorbol-13-acetate-promoted mouse skin papillomas are promotable by benzoyl peroxide

The two-stage skin carcinogenesis model of initiation and promotion in Carcinogenesis-susceptible (Car-S) mice has been used to investigate the pathways of promotional activity of 12-O-tetradecanoylphorbol-13-acetate (TPA), a phorbol ester tumor promoter, and benzoyl peroxide (BzPo), a free radical-generating compound. To test whether distinct populations of 9,10-dimethyl-1,2-benzanthracene (DMBA)-initiated epidermal keratinocytes are responsive to the two promoters, tandem experiments were performed. DMBA-initiated Car-S mice were promoted twice weekly with maximal promoting doses of TPA or BzPo. When the number of papillomas/mouse reached a plateau, promotion in the TPA and BzPo groups was switched to BzPo or TPA, respectively, until achievement of a new plateau. Mice promoted with BzPo developed 11.0 +/- 1.3 papillomas/mouse and subsequent TPA promotion induced 13.8 additional papillomas, for a total of 24.8 +/- 2.1 papillomas/mouse. TPA-promoted mice developed 23.3 +/- 1.1 papillomas/mouse, and subsequent BzPo promotion for 91 days did not promote additional papillomas. Our results show a less than additive tumor response after sequential promotion with BzPo and TPA, or vice versa, indicating that the pathways of promotional activity of TPA and BzPo are interacting. While the final papilloma yield was similar at the end of the two tandem promotion experiments independently of promoter sequence, the percentage of mice developing carcinomas was significantly higher in mice that were promoted with BzPo in the first stage. No significant differences in the frequency and type of c-Ha-ras mutations were observed in TPA- and BzPo-promoted tumors, suggesting that promotion of DMBA-initiated cells by BzPo requires introduction of additional molecular alterations compared to TPA.

Effect of ?-tocopherol andN-acetylcysteine on benzoyl peroxide toxicity in human keratinocytes

Benzoyl peroxide is a free-radical generating compound widely used in the polymer industry and also in pharmaceuticals as antimicrobial agent to treat acne. However, benzoyl peroxide causes irritation and contact dermatitis in about 1% of patients. Concern over the use of this compound is motivated by the demonstration that it can also act as skin tumor promoter in mice. In addition, benzoyl peroxide induces DNA strand breaks in many cells, including keratinocytes. Benzoyl peroxide toxicity is presumably mediated by the formation of reactive free radicals and by the consumption of intracellular antioxidants. In this work we investigated the effect of both the lipophilic antioxidant alpha-tocopherol and the hydrophilic thiol donor N-acetylcysteine (NAC) in human keratinocyte line HaCaT exposed to benzoyl peroxide. A protective effect against benzoyl peroxide cytotoxicity was achieved when cells were grown on a alpha-tocopherol layer. On the contrary, the addition of alpha-tocopherol dissolved in ethanol had a pro-oxidant effect, leading to an enhancement of benzoyl peroxide toxicity. Cytotoxicity was also reduced adding NAC to the culture medium; the presence of both NAC and alpha-tocopherol exerts a synergistic cytoprotection.

Lupeol, a triterpene, prevents free radical mediated macromolecular damage and alleviates benzoyl peroxide induced biochemical alterations in murine skin

In our earlier communication we have shown that Lupeol inhibits early responses of tumour induction in murine skin. The free radical mediated damage to the cellular macromolecules such as DNA, proteins, lipids and alteration in the activities of quinone reductase and xanthine oxidase are important biochemical parameters of tumor development. The suppression of free radical mediated damage to cellular macromolecules and induction of quinone reductase along with depletion of xanthine oxidase are prominent characteristics of chemopreventive agents. In the present investigation, we have elucidated the mechanism of action of lupeol (Lup-20 (29)-en-3beta-ol), a triterpene found in moderate amount in many vegetables, fruits and anti-tumor herbs. In the present investigation, lupeol significantly reduced the free radical mediated DNA-sugar damage and microsomal lipid peroxidation in an iron/ascorbate free radical generating system in vitro. Benzoyl peroxide, a known free radical generating tumor promoter mediated oxidation of proteins and modulation in the activities of quinone reductase as well as xanthine oxidase was significantly prevented by lupeol when tested on murine skin in vivo. It was concluded from this study that lupeol acts as an effective chemopreventive agent against cutaneous toxicity.

Presenting an in vitro cell culture model to determine the cytotoxic effect of benzoyl peroxide vapours

Benzoyl peroxide (BP) is an initiator of polymerisation used in the synthesis of methyl methacrylate dental materials, and is a known allergen that causes allergic contact dermatitis during occupational exposure, especially in dentists and dental personnel. The eyes of dental technicians, dentists and even patients with an allergic history are the first level of exposure, and therefore complaints of allergic reactions of the eyes are usually noted. In this article the authors used a modification direct cell culture testing method to assess the cytotoxic potential of BP. L929 mouse fibroblasts as well as cells from human limbal eye rings were exposed to BP vapours in the experiments. The MTT and crystal violet assays were also employed to determine cell numbers and viability. Results indicated that there is an exponential decrease in cell viability. After exposure of five minutes, cell viability had already decreased by 20 and 40% for vapours derived from either a 10 or 20 microliters aliquot of BP, respectively. After a further five minutes, cell viability decreased by a further 10% with a statistical difference form the control of p < 0.01. Results indicated that BP vapours are cytotoxic to mouse L929 and human eye fibroblasts. Furthermore, the authors concluded that the permanent L929 mouse fibroblast cell line can provide valuable information regarding the cytotoxic effects of dental products and associated compounds that form toxic vapours.

Low iron state is associated with reduced tumor promotion in a two-stage mouse skin carcinogenesis model

The purpose of this study was to investigate the effect of low iron state in a two-stage mouse skin carcinogenesis model using 7,12-dimethylbenz[a]anthracene (DMBA)-initiated and benzoyl peroxide (BPO)-promoted cutaneous tumorigenesis. All mice were treated with DMBA. Low iron state was achieved by injection with phenylhydrazine hydrochloride and feeding low iron diet. A low iron state resulted in a decrease in tumor incidence (papillomas and carcinomas) and number of tumors/mouse. Also, the conversion of papillomas to carcinomas was lower in mice on a low iron state. BPO treatment enhanced epidermal lipid peroxidation (LPO) and was accompanied by a depletion in the level of epidermal reduced glutathione (GSH) and decrease in the activities of antioxidant enzymes. BPO treatment also increased ornithine decarboxylase (ODC) activity and [3H]thymidine incorporation into cutaneous DNA. Mice in a low iron state were less susceptible to the effects of BPO treatment, as was apparent from a partial recovery of GSH levels and the activities of antioxidant enzymes, as well as a lower induction in ODC activity, [3H]thymidine incorporation into cutaneous DNA and lesser epidermal LPO. As expected, cutaneous iron levels were lower in mice on a low iron state. Thus, our data show that the tumor-promoting potential of BPO is reduced by low iron state in a two-stage mouse skin carcinogenesis model.

Oral toxicity of topical preparations

This chapter covers the hazards that some topical pharmaceutical preparations pose to animals who consume them. Included are medications containing calcipotriene, vitamins A and D, zinc oxide, 5-fluorouracil, brimonidine, imidazoline decongestants, local anesthetics, corticosteroids, antibiotics, salicylates, and benzoyl peroxide.

Analysis of c-Ha-ras gene mutations in skin tumors induced in carcinogenesis-susceptible and carcinogenesis-resistant mice by different two-stage protocols or tumor promoter alone

In the present study we describe the molecular analysis of c-Ha-ras gene mutations in 47 papillomas and 17 carcinomas developed in two lines of mice, carcinogenesis-susceptible (Car-S) and carcinogenesis-resistant (Car-R), selectively bred for extreme susceptibility or resistance to chemical skin carcinogenesis initiated and promoted with different doses of 7,12-dimethylbenz[a]anthracene (DMBA) and 12-O-tetradecanoylphorbol-13-acetate (TPA). This study also presents the analysis of c-Ha-ras gene mutations in 22 papillomas and 22 carcinomas in Car-S mice initiated with DMBA and promoted with benzoyl peroxide (BzPo) and in seven papillomas and one carcinoma from a group of uniniated Car-S mice that received only BzPo treatment. The data showed that a A(182)-->T transversion in the c-Ha-ras gene was present in 100% and 81% of the skin tumors developed in Car-S and Car-R mice, respectively, after DMBA initiation and TPA promotion, suggesting that differences in genetic susceptibility can influence the frequency of c-Ha-ras mutations in the skin tumors produced. The same A(182)-->T mutation with an incidence of 68% was found in papillomas from DMBA-initiated and BzPo-promoted Car-S mice. The difference in the mutation frequency between DMBA/BzPo and DMBA/TPA papillomas suggested that the promotion step contributes to the final mutation pattern. The tumor induction experiment with BzPo alone showed that this compound can induce tumor development in 26% of Car-S mice, and the molecular analysis of the tumors showed a broad mutation spectrum, including mutations in codons 12, 13, and 61 of the c-Ha-ras gene. Mol. Carcinog. 30:111-118, 2001.

Attenuation of benzoyl peroxide-mediated cutaneous oxidative stress and hyperproliferative response by the prophylactic treatment of mice with spearmint (Mentha spicata)

The modulating effect of spearmint (Mentha spicata) on benzoyl peroxide-induced responses of tumor promotion in murine skin was investigated. Benzoyl peroxide (BPO) is an effective cutaneous tumor promoter acting through the generation of oxidative stress, induction of ornithine decarboxylase activity and by enhancing DNA synthesis. BPO treatment (20 mg/animal) increased cutaneous microsomal lipid peroxidation and hydrogen peroxide generation. The activity of cutaneous antioxidant enzymes, namely catalase, glutathione peroxidase, glutathione reductase and glutathione S-transferase, was decreased and the level of cutaneous glutathione was depleted. BPO treatment also induced the ornithine decarboxylase activity and enhanced the [3H]thymidine uptake in DNA synthesis in murine skin. Prophylactic treatment of mice with spearmint extract (10, 15 and 20 mg/kg) 1 hr before BPO treatment resulted in the diminution of BPO-mediated damage. The susceptibility of cutaneous microsomal membrane to lipid peroxidation and hydrogen peroxide generation was significantly reduced (P < 0.05 ). In addition, depleted levels of glutathione, inhibited activity of glutathione dependent and antioxidant enzymes were recovered to a significant level (P < 0.01, P < 0.05 and P < 0.01, respectively). Similarly, the elevated ornithine decarboxylase activity and enhanced thymidine uptake in DNA synthesis was inhibited significantly (P < 0.05 ) in a dose-dependent manner. The protective effect of spearmint was dose dependent in all parameters. The result suggests that spearmint is an effective chemopreventive agent that may suppress BPO-induced cutaneous oxidative stress, toxicity and hyperproliferative effects in the skin of mice.

Protein kinase C signaling and oxidative stress

Oxidative stress is involved in the pathogenesis of various degenerative diseases including cancer. It is now recognized that low levels of oxidants can modify cell-signaling proteins and that these modifications have functional consequences. Identifying the target proteins for redox modification is key to understanding how oxidants mediate pathological processes such as tumor promotion. These proteins are also likely to be important targets for chemopreventive antioxidants, which are known to block signaling induced by oxidants and to induce their own actions. Various antioxidant preventive agents also inhibit PKC-dependent cellular responses. Therefore, PKC is a logical candidate for redox modification by oxidants and antioxidants that may in part determine their cancer-promoting and anticancer activities, respectively. PKCs contain unique structural features that are susceptible to oxidative modification. The N-terminal regulatory domain contains zinc-binding, cysteine-rich motifs that are readily oxidized by peroxide. When oxidized, the autoinhibitory function of the regulatory domain is compromised and, consequently, cellular PKC activity is stimulated. The C-terminal catalytic domain contains several reactive cysteines that are targets for various chemopreventive antioxidants such as selenocompounds, polyphenolic agents such as curcumin, and vitamin E analogues. Modification of these cysteines decreases cellular PKC activity. Thus the two domains of PKC respond differently to two different type of agents: oxidants selectively react with the regulatory domain, stimulate cellular PKC, and signal for tumor promotion and cell growth. In contrast, antioxidant chemopreventive agents react with the catalytic domain, inhibit cellular PKC activity, and thus interfere with the action of tumor promoters.

Inhibitory effect of a flavonoid antioxidant silymarin on benzoyl peroxide-induced tumor promotion, oxidative stress and inflammatory responses in SENCAR mouse skin

In this communication, we investigate the preventive effect of a flavonoid antioxidant, silymarin, on free radical-generating skin tumor promoting agent benzoyl peroxide (BPO)-induced tumor promotion, oxidative stress and inflammatory responses in SENCAR mouse skin. Topical application of silymarin at a dose of 6 mg prior to BPO resulted in a highly significant protection against BPO-induced tumor promotion in 7,12-dimethylbenz[a]anthracene-initiated SENCAR mouse skin. The preventive effect of silymarin was evident in terms of a 70% reduction (P < 0.001) in tumor incidence, a 67% reduction (P < 0.001) in tumor multiplicity and a 44% decrease (P < 0.001) in tumor volume/tumor. In oxidative stress studies, topical application of BPO resulted in 75, 87 and 61% depletion in superoxide dismutase (SOD), catalase and glutathione peroxidase (GPX) activities in mouse epidermis, respectively. These decreases in antioxidant enzyme activities were significantly (P < 0.005-0.001) reversed by pre-application of silymarin in a dose-dependent manner. The observed effects of silymarin were 18-66, 32-72 and 20-67% protection against BPO-induced depletion of SOD, catalase and GPX activity in mouse epidermis, respectively. Silymarin pre-treatment also resulted in a dose-dependent inhibition (35-87%, P < 0.05-0. 001) of BPO-induced lipid peroxidation in mouse epidermis. In inflammatory response studies, silymarin showed a strong inhibition of BPO-induced skin edema (62-85% inhibition, P < 0.001), myeloperoxidase activity (42-100% inhibition, P < 0.001) and interleukin-1alpha protein level in epidermis (36-81% inhibition, P < 0.001). These results, together with our other recent studies, suggest that silymarin could be useful in preventing a wide range of carcinogen and tumor promoter-induced cancers.

Skin tumorigenesis by initiators and promoters of different chemical structures in lines of mice selectively bred for resistance (Car-r) or susceptibility (Car-s) to two-stage skin carcinogenesis

Carcinogenesis-resistant (Car-R) and carcinogenesis-susceptible (Car-S) mice were obtained applying a bi-directional selective breeding approach to a two-stage skin carcinogenesis protocol, using 9,10-dimethyl-1,2-benzanthracene (DMBA) as initiator and 12-O-tetradecanoylphorbol-13-acetate (TPA) as promoter. Sixteen generations of selection produced a remarkable interline difference in responsiveness to two-stage skin carcinogenesis between Car-R and Car-S: identical DMBA (25 microgram) and TPA (5 microgram) doses induced papillomas in 100% of Car-S compared with 3.3% of Car-R mice and maximal responses of 14.3 or 0.03 papillomas/mouse, respectively, despite the shorter promotion applied to Car-S (49 vs. 208 days). To define the factors determining this great difference, Car-R and Car-S mice were challenged by initiators/promoters chemically unrelated to those used for selection. Both lines were subjected to either initiation by N-methyl-N-nitrosourea (MNU) followed by TPA promotion, or promotion by benzoyl peroxide, or 1,8-dihydroxy-3-methyl-9-anthrone (chrysarobin) following DMBA initiation. Initiation with MNU induced a 10-fold tumour incidence in Car-S compared with Car-R mice, and a 32-fold difference in tumour induction rate. The 2 lines also differed markedly in susceptibility to benzoyl peroxide promotion: Car-S mice initiated with 25 microgram DMBA and promoted with 7.5 mg benzoyl peroxide showed a 12-fold tumour incidence and a 103-fold tumour induction rate compared with the corresponding Car-R group. Both lines, however, were refractory to chrysarobin promotion. The progression of papillomas to carcinomas was examined in all Car-S groups. The incidence of mice that developed carcinomas was 57% in MNU-initiated mice. Benzoyl peroxide was also able to promote carcinoma development in Car-S mice, though with a lower incidence (30.4%) than TPA.

Protection against 12-O-tetradecanoylphorbol-13-acetate induced skin-hyperplasia and tumor promotion, in a two-stage carcinogenesis mouse model, by the 2,3-dimethyl-6(2-dimethylaminoethyl)-6H-indolo-[2,3-b]quinoxaline analogue of ellipticine

The effects of topical applications of 2,3-dimethyl-6(2-dimethylaminoethyl)-6H-indolo-[2,3-b]quinoxaline (B-220), on 12-O-tetradecanoylphorbol-13-acetate (TPA) or benzoylperoxide (BPO) induced promotion of skin tumors and hyperplasia were studied in female SENCAR mice. Papillomas were induced by initiation with 7,12-dimethylbenz[a]anthracene (DMBA), followed by promotion biweekly with TPA or BPO. Administration of B-220 1 h before TPA promotion resulted in a prolonged latency period of tumor appearance and a significantly reduced (up to 15% of positive controls) papilloma yield at 20 weeks. Moreover, if B-220 treatment was terminated after 20 weeks and TPA treatment continued, papilloma development resumed indicating that initiated tumor cells were still present but were unable to grow with B-220 present. If B-220 pretreatment was not given during the first 10 weeks of TPA promotion, incidence at 20 weeks was not reduced but tumor multiplicity was still decreased. In addition a marked reduction of the TPA induced sustained epidermal hyperplasia was observed in the long term experiment. Neither the inflammatory response nor the increase in the number of apoptotic cells seen in short term experiment after a single TPA treatment were inhibited by B-220. B-220 administration before BPO promotion had no effect on the appearance of BPO induced papillomas or epidermal hyperplasia, suggesting that TPA and BPO promote tumor formation via at least partially different mechanisms. In experiments where B-220 was applied topically 1 h before DMBA initiation, little or no effect was seen. No morphological changes in mouse skin due to long term exposure (two times/week, 39 weeks) to B-220 were found. In conclusion, we present evidence that B-220 is a potent inhibitor of mouse skin tumor promotion by TPA, but has little effect on the initiation step or the survival of initiated cells.

Enhancement of susceptibility to diverse skin tumor promoters by activation of the insulin-like growth factor-1 receptor in the epidermis of transgenic mice

Insulin-like growth factor-1 (IGF-1) and its receptor are believed to play an important role in mitogenesis and neoplastic transformation. The purpose of this study was to further examine the role of IGF-1 during tumor promotion in mouse skin. HK1.IGF1 transgenic mice, which overexpress IGF-1 in epidermis via the human keratin 1 promoter, were previously shown to be hypersensitive to skin tumor promotion by 12-O-tetradecanoylphorbol-13-acetate (TPA). We examined these mice for their sensitivity to diverse classes of tumor-promoting agents. HK1.IGF-1 transgenic mice initiated with 7,12-dimethylbenz[a]anthracene were more sensitive to treatment with a wide variety of tumor promoters, including chrysarobin, okadaic acid, and benzoyl peroxide, which resulted in more rapid development of tumors and a dramatic increase in the number of tumors per mouse compared with corresponding non-transgenic mice treated with the same compounds. Histological analyses of skin from HK1.IGF-1 mice treated with various tumor promoters revealed that these mice were also more sensitive to the induction of epidermal hyperplasia and cell proliferation. Analysis of the IGF-1 receptor (IGF-1r) and epidermal growth factor (EGFr) in the epidermis of TPA-treated HK1.IGF-1 transgenic and non-transgenic mice revealed that both receptors were activated (hyperphosphorylated on tyrosine residues), and the level of activation was higher in transgenic mice. The mechanism for the increased sensitivity of HK1.IGF-1 mice to tumor promoters may involve cooperation between the IGF-1r and EGFr signaling pathways. Our data suggest that IGF-1r signaling may play an important role in the process of tumor promotion by diverse classes of tumor promoters.

A hairless mouse model for assessing the chronic toxicity of topically applied chemicals

An enormous number of synthetic chemicals are incorporated in topical drugs, cosmetics and toiletries. These have the potential to cause irritant reactions when chronically applied to human skin. In predictive tests for assessing the irritancy potential of these chemicals, haired species, especially rabbits, guinea pigs and mice, have figured prominently. Customarily these tests, including the renowned Draize rabbit test, have entailed a single acute exposure or at most daily exposures over a few weeks. Estimation of inflammation and tissue injury in these models have relied on visual assessment. We submit that this approach is no longer acceptable. Visual assessments are unreliable. Reactions which are scored equivalently by the naked eye may differ strikingly when examined histologically. Moreover, tissue injury may be present in clinically normal skin. Short-term results. even when abetted by routine histological evaluations, cannot predict the degree of injury from long-term exposures. Cosmetics and toiletries, for example, are used daily for decades, often over most of the lifespan of persons who are well groomed. We present the hairless mouse as a convenient, reliable model for assessing the chronic toxicity of diverse chemicals. Histological examination enables a detailed description of the different tissue components which participate in the complex cascade of changes that comprise the inflammatory response.

Effect of iron overload on the benzoyl peroxide-mediated tumor promotion in mouse skin

In this communication, we report that iron overload augments benzoyl peroxide (BPO)-mediated tumor promotion in 7,12-dimethylbenz[a]anthracene (DMBA)-initiated mouse skin. Female albino Swiss mice were overloaded with iron and tumors were initiated by applying a single topical application of DMBA. A week after the initiation, promoting agent, BPO, was applied three times/week for 46 weeks. The appearance of the first tumor (papilloma) and the number of tumors/mouse were recorded. When compared to the control group, the iron-overloaded mice showed an increased incidence of tumors at various time intervals. In iron-overloaded animals, tumors appeared earlier and also the number of tumors/mouse was significantly higher. These data could be correlated with the iron levels of mouse skin in the two groups. Further, BPO-mediated induction in ornithine decarboxylase (ODC) activity and [3H]thymidine incorporation in cutaneous DNA were higher in the iron overload group. In addition, in iron-overloaded mice, cutaneous lipid peroxidation (LPO) and xanthine oxidase (XOD) activities were higher, whereas catalase activity was reduced. Similar to papilloma induction, a significant increase in carcinoma yield and incidence was observed in iron-overloaded animals. Based on this study, we propose that iron overload significantly increases the tumor promotion and progression potential of BPO. We suggest that oxidative stress generated by iron overload is responsible for the augmentation of BPO-mediated cutaneous tumorigenesis.

Comparison of the skin tumor-promoting potential of different organic peroxides in SENCAR mice

The skin tumor-promoting activities of three organic peroxides were evaluated and compared to the activity of benzoyl peroxide, a well-characterized tumor promoter. Two of the compounds (di-t-butyl peroxide and dicumyl peroxide) were dialkyl peroxides and the other (di-m-chlorobenzoyl peroxide) was a diacyl peroxide. These compounds were selected based on a previous study in which we evaluated their capacity to induce epidermal hyperplasia, ornithine decarboxylase activity, and dark basal keratinocytes, which have been reliable short-term markers of tumor promotion. Dicumyl peroxide was a weak tumor promoter despite its high activity in inducing hyperplasia. Like benzoyl peroxide, di-m-chlorobenzoyl peroxide generally had intermediate activity as an inducer of short-term markers of tumor promotion and was a moderately effective tumor promoter. However, compared to benzoyl peroxide, di-m-chlorobenzoyl peroxide was more toxic to the skin, which may have limited its tumor-promoting activity. The final compound, di-t-butyl peroxide, which was essentially inactive in short-term assays, was also totally inactive in promoting papillomas or carcinomas in initiated skin. Tumor-promoting efficacy generally showed an inverse association with thermal stability for the compounds tested, suggesting that the rate of formation of free radicals is a key factor contributing to tumor promotion by organic peroxides. However, a number of other factors can potentially affect the activity of different organic peroxides as tumor promoters. Each compound evaluated had a different spectrum of activities, and these compounds should be useful for studying mechanisms of organic peroxide-induced tumor promotion.

Insights into the mechanism of action of benzoyl peroxide as a tumor promoter

A comparison of the mechanism of action of benzoyl peroxide, a tumor promoter was studied in three different cell lines i.e. NIH 3T3, HDCS and A431. Benzoyl peroxide was found to mediate its effect by inducing poly ADP-ribosylation in all the three cell types studied but to different extents, with histone H1 serving as a common acceptor for poly ADP-ribose. It also stimulated the activities of the antioxidant enzymes CuZn superoxide dismutase and catalase in NIH 3T3 and HDCS cells, but not in A431. Alterations in the expression of c-jun and c-fos were observed in NIH 3T3 and A431 cells. Benzoyl Peroxide appeared to mediate its effect via genetic and epigenetic mechanisms.

Benzoyl peroxide-induced damage to DNA and its components: direct evidence for the generation of base adducts, sugar radicals, and strand breaks

Benzoyl peroxide is a known tumor promoter and progression agent in mouse skin, though it is not an initiator or complete carcinogen. Previous studies have suggested that this activity may be due to the generation of strand breaks in cells exposed to this compound. This may be as a result of free radical generation, though there is controversy as to which radicals are responsible for this damage; previous workers have variously implicated benzoyloxyl (PhCO2,) phenyl (Ph.), and hydroxyl radicals (HO.) as the initiating agent. In the present study a detailed examination of the radicals generated on reaction of benzoyl peroxide with Cu(I) has been carried out by electron paramagnetic resonance (EPR) spectroscopy and spin trapping; the results obtained are consistent with the formation of PhCO2. and Ph. but not HO. The subsequent reactions of these benzoyl peroxide-derived radicals with nucleobases, sugars, nucleosides, nucleotides, RNA, and DNA have been examined and the intermediate species have been identified in many cases. Comparison of these data with those obtained with Ph. alone has allowed the reactions of PhCO2. and Ph. to be distinguished. Evidence has been obtained which is consistent with both the addition of these radicals to the C5-C6 double bond of the pyrimidines to give adduct species, and hydrogen abstraction from the sugar rings. The former process is the major reaction for nucleosides and nucleotides. Studies with RNA and DNA also provide strong evidence for the formation of base adducts, though the exact identity of the species detected in these cases could not be determined due to the complexity of the spectra. Hydrogen abstraction at the sugar-phosphate backbone is also believed to occur with these substrates as strand breakage is observed; the extent of the latter is dependent on the radical flux and the attacking species, with PhCO2. appearing to be a much more effective inducer of fragmentation than Ph. The nature of the species detected with all the substrates examined, with the exception of the isolated sugars where essentially random attack by both radicals is observed, suggests that of the two possible radicals generated by benzoyl peroxide, PhCO2. and Ph., it is the former which is responsible for the majority of the observed degradation. The results obtained in this study are consistent with the genetic damage produced by this compound being due to the formation of both strand breaks and high yields of altered bases via the formation of base adducts.

Regression and progression characteristics of papillomas induced by chrysarobin in SENCAR mice

The present study was designed to test the effects of a free radical generating tumor promoter, chrysarobin (1,8-dihydroxy-3-methyl-9-anthrone), on the growth and progression of papillomas generated in the skin of SENCAR mice. In the first set of experiments, papillomas were generated by initiation with 6.4 microg of 7,12-dimethylbenz[a]anthracene (DMBA) followed by promotion with once-weekly applications of 52.8 microg chrysarobin for 10 weeks. The fate of individual papillomas was then monitored for a 20 week interval following cessation of promoter treatment. Five weeks after the cessation of chrysarobin treatment, the papilloma response reached a maximum of 13.2 papillomas/mouse. By the end of the 20 week interval 19% and 18% of the papillomas had regressed or coalesced respectively. A three-stage treatment protocol was also utilized to test the ability of chrysarobin to enhance the progression of pre-existing papillomas to squamous cell carcinomas (SCCs). In stage I, mice were initiated with 0.5 microg of DMBA. In stage II, mice were promoted with twice-weekly applications of 1 or 2 microg of 12-0-tetradecanoylphorbol-13-acetate (TPA) for 15 weeks. Then, in stage III, mice were treated with acetone, TPA (1 or 2 microg), chrysarobin (52.8 microg) or benzoyl peroxide (BzPo; 20 mg) for the next 45 weeks. The mean number of papillomas per mouse at plateau was very similar for all groups. The carcinoma incidence was also similar for all groups regardless of the treatment protocol used, as was the mean number of carcinomas per mouse. The ratio of papillomas that converted to SCCs in mice treated with chrysarobin during stage III was not significantly different from the acetone controls or any of the other treatment groups (P > 0.05, Kruskal-Wallis analysis). In addition, BzPo did not enhance the progression of papillomas to SCCs under the current experimental conditions. Collectively, the results indicate that papillomas promoted by chrysarobin have growth properties similar to those promoted by TPA under similar experimental conditions. Furthermore, despite its ability to generate free radical intermediates, chrysarobin does not enhance the malignant progression of pre-existing papillomas induced by TPA treatment.

Effect of diverse tumor promoters on the expression of gap-junctional proteins connexin (Cx)26, Cx31.1, and Cx43 in SENCAR mouse epidermis

The inhibition of gap-junctional intercellular communication (GJIC) between initiated and surrounding normal cells by tumor promoters is believed to be important in the promotion stage of carcinogenesis. Therefore, we examined the effect of skin-tumor promoters on the expression of the gap-junctional proteins connexin (Cx) 26, Cx43, and Cx31.1 in SENCAR mouse skin. Animals were treated with 12-0-tetradecanoylphorbol-13-acetate (TPA) (8.3 nmol), okadaic acid (OA) (2.5 nmol), chrysarobin (220 nmol), or benzoyl peroxide (BzPo) (83 micromol). Northern blot and immunofluorescence analyses revealed that keratinocytes in adult mouse skin expressed Cx31.1 and Cx43 but not Cx26. All four of the skin-tumor promoters switched on the Cx26 gene, transiently increased expression of Cx43, and significantly inhibited the expression of Cx31.1. The time courses for changes in Cx26, Cx3l. 1, and Cx43 mRNA levels coincided in most cases and in general corresponded well to the time-response curves for hyperplastic changes in mouse skin. The peaks of Cx26 and Cx43 expression and Cx31.1 inhibition appeared 12 h after TPA application and 24 h after OA and chrysarobin application. BzPo elevated the levels of Cx26 and Cx43 transcripts later (peak at 2-4 d). In tumor promoter-treated skin, Cx26 and Cx43 plaques were on the plasma membrane of most keratinocytes. Cx31.1 staining was much weaker than in untreated epidermis. Thus, tumor promoters induce a large change in the expression of several Cxs, which in turn may affect both the level of GJIC and the sensitivity of GJlC to regulatory factors.

The use of initiated cells as a test system for the detection of inhibitors of gap junctional intercellular communication

The effects of five non-mutagenic carcinogens--Aroclor 1260, benzoyl peroxide (BP), phenobarbital (PB), 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and 1,1'-(2,2,2-trichloroethylidene)bis[4-chlorobenzene] (DDT)--on gap junctional intercellular communication (GJIC) were tested in a cell line consisting of initiated cells (3PC). Four agents suspected of tumor promotion activity--o-anisidine, clofibrate, L-ethionone and d-limonene--were also tested for their effects on GJIC. Finally sodium fluoride (NaF), whose carcinogenic property is still unclear, was tested for its effects on GJIC in the 3PC cell line. Four of the five selected tumor promoters (Aroclor 1260, BP, DDT and TPA) decreased GJIC between these initiated epidermal cells. The four non-mutagenic carcinogens with tumor-promoting activity in vivo (o-anisidine, clofibrate, L-ethionine and d-limonene) all inhibited GJIC, whereas NaF had no effect. Seven compounds (o-anisidine, Aroclor 1260, BP, DDT, L-ethionine, d-limonene and TPA) had a dose-dependent as well as time-dependent inhibitory effect on GJIC. Under the experimental conditions used, clofibrate showed only a dose-related inhibition of GJIC. PB showed no inhibitory effect on GJIC in the 3PC cell line. In order to determine the role of biotransformation in the tumor-promoting activity of PB, its effect on GJIC was also examined in the presence of an Aroclor 1254-induced rat liver homogenate (S9 mix) and in the hepatoma cell line HepG2. In the presence of rat liver homogenate PB decreased GJIC in the 3PC cell line, whereas in the HepG2 cells PB showed a time- and dose-dependent inhibitory effect. To study the potential differences in susceptibility of cells representing different stages in the process of tumor formation, the effect of the selected tumor promoters on GJIC was also investigated in primary mouse keratinocytes and in a mouse skin carcinoma-derived cell line (CA3/7). Primary keratinocytes were sometimes more (BP and clofibrate) and sometimes less sensitive (ethionine and limonene) for inhibitory effects on GJIC compared to the effects in the cell line 3PC. Except for TPA and anisidin, GJIC between the CA3/7 cells was less affected by the selected agents compared to the 3PC cell line. These results show that, during the process of tumor formation the susceptibility of cells to inhibition of GJIC by tumor promoters is variable. Overall the CA3/7 cells are less sensitive compared to 3PC cells. The susceptibility of primary keratinocytes is variable compared to 3PC cells, depending on the agent used. These results also show that GJIC is a valid parameter for testing the tumor-promoting activity of compounds. Finally, this study demonstrates that mouse keratinocyte cell lines could serve as an in vitro model for the detection of non-mutagenic carcinogens with diverse target organs in vivo. For this use the cell line consisting of initiated cells (3PC) is more sensitive than the carcinoma-derived cell line CA3/7.

Generation of DNA base modification following treatment of cultured murine keratinocytes with benzoyl peroxide

Benzoyl peroxide (BzPO) is a free radical generating compound that acts as a tumor promoter and progressor in mouse skin. BzPO is cleaved in the presence of copper to produce benzoyloxyl and phenyl radicals. Treatment of mutation reporter plasmids with BzPO and copper yields predominantly single-strand breaks and G-->T transversion mutations. To explore the role of base modifications in the possible mammalian mutagenicity of BzPO the formation of 8-hydroxy-2'-deoxyguanosine (8-OHdG) within the DNA of cultured murine keratinocytes was investigated. Treatment with 10 microM BzPO produced a maximum 3-fold increase in levels of 8-OHdG versus vehicle controls within 1-2 h, with significant levels of 8-OHdG persisting 6 h after initial exposure to BzPO. Pretreatment with the copper chelator bathocuproine disulfonic acid reduced the levels of 8-OHdG generated by BzPO to near background. However, treatment with the iron chelator desferal did not. The stable metabolic product of BzPO benzoic acid was ineffective in producing 8-OHdG. Depletion of cellular glutathione with L-buthionine-(S,R)-sulfoximine increased the amount of BzPO-generated 8-OHdG, while supplementation with glutathione monoethyl ester reduced the number of 8-OHdG molecules formed. Collectively, these results suggest that BzPO at non-cytotoxic concentrations undergoes copper-dependent activation to a reactive product to generate 8-OHdG within cultured murine keratinocytes.

Benzoyl peroxide cytotoxicity evaluated in vitro with the human keratinocyte cell line, RHEK-1

The human keratinocyte cell line, RHEK-1, was used to evaluate the cytotoxicity of benzoyl peroxide (BZP). As determined with the neutral red (NR) cytotoxicity assay, the 24-h midpoint (NR50) toxicity values, in mM, were 0.11 for BZP and 29.5 for benzoic acid, the stable metabolite of BZP. Irreversible cytotoxicity occurred after a 1-h exposure to 0.15 mM BZP and greater. When exposed to BZP for 7 days, a lag in growth kinetics was first observed at 0.06 mM BZP. Damage to the integrity of the plasma membrane was evident, as leakage of lactic acid dehydrogenase occurred during a 4-h exposure to BZP at 0.05 mM and greater. Intracellular membranes were also affected, as extensive vacuolization, initially perinuclear but then spreading throughout the cytoplasm, was noted in BZP-stressed cells. The generation of reactive free radicals from BZP was suggested by the following: the intracellular content of glutathione was lowered in cells exposed to BZP; cells pretreated with the glutathione-depleting agent, chlorodinitrobenzene, were hypersensitive to a subsequent challenge with BZP; lipid peroxidation by BZP was inducible in the presence of Fe2+; and cells previously maintained in a medium amended with vitamin E, an antioxidant, were more resistant to BZP, showed less lipid peroxidation in the presence of BZP+Fe2+ and did not develop the extensive intracellular vacuolization as compared to non-vitamin E maintained cells.

Benzoyl peroxide acts as a promoter of radiation induced malignant transformation in vitro

The experiments reported here demonstrate that benzoyl peroxide (BP) can promote radiation induced transformation in vitro. BP is shown to be capable of generating free radicals, determined by the kinetics of hydroxylation as measured by fluorescence of coumarin-3-carboxylic acid. Although the mechanisms involved in the BP enhancement of radiation transformation are unknown, we hypothesize that lipid peroxidation produced by benzoyl radicals in the vicinity of membrane associated unsaturated lipids could contribute to the promotion of transformation in vitro.

Glutathione depletion potentiates 12-O-tetradecanoyl phorbol-13-acetate(TPA)-induced inhibition of gap junctional intercellular communication in WB-F344 rat liver epithelial cells: relationship to intracellular oxidative stress

Treatment of WB-F344 liver epithelial cells with buthionine sulfoximine (BSO, 100 microM) for 24 h caused a greater than 95% depletion in cellular glutathione (GSH) and potentiated the ability of 12-O-tetradecanoyl phorbol-13-acetate (TPA) to inhibit gap junctional intercellular communication (GJIC) between the cells (IC50 shifted from 5 microM to 2 microM). Similarly, acute depletion of GSH by up to 30%, either with the thiol oxidant diamide or with BSO, also potentiated the inhibitory effect of the phorbol ester on GJIC. The treatment of the control cells with TPA caused a concomitant increase in the accumulation of oxidation products of 2',7'-dichlorofluorescein (DCF), indicating elevated production of oxidants in the cells during the blockade of GJIC. The depletion of GSH over a 24 h period with BSO itself increased the flux of oxidants in the cells but did not inhibit GJIC. Treatment of these GSH-depleted cells with TPA caused an additive elevation in the accumulation of oxidised DCF metabolites. Direct application of H2O2 (25-200 microM) or benzoyl peroxide (25-150 microM) to the control cells for 60 min caused weak, dose-dependent inhibitions of gap junctional communication in these cells but these responses were accompanied by the induction of acute, sub-lethal cytotoxicity. The depletion of GSH from the cells did not potentiate these responses to the peroxides but did facilitate synergistic inhibition of gap junctional communication in response to both TPA and sub-toxic doses of either peroxide. The results of the above studies indicate that oxidants are produced in WB-F344 cells in response to TPA and that these function in a co-operative manner with other cellular responses to the phorbol ester in the inhibition of gap junctional communication. This may explain why priming the cells for the induction of oxidative stress by the depletion of GSH potentiates the inhibitory activity of TPA on gap junctional communication.

Benzoyl peroxide: an integrated human safety assessment for carcinogenicity

Topical benzoyl peroxide has been used in the treatment of acne for over 30 years, with no reports of adverse effects that could be related to skin carcinogenesis. Two case-control epidemiological studies have found a lack of association between the specific use of benzoyl peroxide and skin cancer. In addition to these findings in humans, 23 carcinogenicity studies in rodents with benzoyl peroxide, including 16 employing topical application, have yielded negative results. An increase in skin carcinomas was reported in 1 study in which benzoyl peroxide in acetone was applied to the skin of SENCAR mice for a 1-year period; however, this study did not employ adequate control groups to fully understand the unusual findings, and the results were inconsistent with those of 6 other similar studies. While benzoyl peroxide is not a complete carcinogen in animals and has weak or no mutagenic potential, it has been found to be a tumor promoter in mouse skin using experimental two-stage models of carcinogenesis. Consistently positive results have been obtained in tumor promotion studies in which SENCAR mice were exposed to initiating doses of potent experimental carcinogens followed by promotion with benzoyl peroxide in acetone. Negative results have been obtained in similar studies with commercial formulations. However, the results of promotion studies with benzoyl peroxide do not carry significant weight for human safety assessment as evidenced by (i) the absence of demonstrated carcinogenicity in humans of a number of rodent tumor promoters despite long-term human exposure; (ii) the observation that tumor promotion in mouse skin occurs only under specific experimental conditions and predominantly in highly sensitive strains; (iii) clinical use scenarios markedly different from the conditions resulting in tumor promotion in mouse skin; and (iv) the significant physiological differences between mouse and human skin. Thus, to date, available scientific evidence does not allow the results of these rodent promotion studies to be meaningfully applied to human safety assessment. As such, significant scientific progress must be made before human safety estimations can be derived from rodent promotion data.(ABSTRACT TRUNCATED AT 400 WORDS)

Mast cell phenotypic changes in skin of mice during benzoyl peroxide-induced tumor promotion

The behavior of the mast cell population was analyzed during the sequential changes that normal mice skin undergoes during experimental two-stage carcinogenesis. Our study reveals that the number of mast cells increased during the promotion period but that this alteration is confined to the 30-microns-wide strip below the epidermis. A different mast cell phenotype appeared in this area, compatible with an MMC-like phenotype. During the carcinogenesis process, the mast cell population is comprised of two distinct subpopulations that appeared simultaneously in the same tissue, i.e. connective tissue mast cells, normally found in the skin of mice, and the newly formed mucosal mast cell-like cells, currently found in gastrointestinal mucosa.

Copper-dependent site-specific mutagenesis by benzoyl peroxide in the supF gene of the mutation reporter plasmid pS189

Benzoyl peroxide (BzPO) enhances tumor promotion and malignant conversion in mouse epidermis. DNA damage may contribute to these processes. BzPO reacts with Cu(I) to produce the benzoyloxyl radical, which in turn causes strand breaks in plasmid DNA. In this study we investigated whether BzPO with or without Cu(I) caused promutagenic DNA damage in the supF gene of the mutation reporter plasmid pS189 replicating in human Ad293 cells. Exposure of pS189 in vitro to BzPO (0.1-1 mM) inhibited plasmid replication; however, addition of Cu(I) (0.1 mM) did not augment BzPO-induced plasmid toxicity. Exposure to BzPO with or without 0.1 mM Cu(I) was also associated with a concentration-dependent increase in mutation frequency, up to > 100-fold above the spontaneous mutation frequency. Supplemental Cu(I) was not required for mutagenesis; however, it both raised the maximal mutation frequency observed and lowered the threshold concentration of BzPO necessary to discern mutagenesis above the spontaneous background. Neither the hydroxyl radical scavengers mannitol or DMSO, the spin trap N-tert-butyl-alpha-phenylnitrone, nor reduced glutathione altered BzPO/Cu(I)-induced mutagenesis; however, mutagenesis was suppressed by the chelator EDTA. Twenty-four of 32 individual BzPO/Cu(I)-induced mutants characterized by sequencing contained point mutations; 22/25 point mutations occurred at G-C base pairs. There were five large deletions and four small deletions. Three additional BzPO-induced mutants contained four point mutations, all occurring at G-C base pairs. Two BzPO/Cu(I)-induced mutational clusters at d(pGGG)-d(pCCC) sites were observed. These data suggest that BzPO may interact with Cu(I) bound to G-C base pairs in DNA to produce site-specific promutagenic DNA damage.

Studies of skin toxicity in vitro: dose-response studies on JB6 cells

There are many reasons for developing in vitro tests of toxicity including cost, speed, studies of mechanisms, and studies utilizing human cells and tissues. The present study focuses on the development of in vitro tests to predict in vivo toxicity by comparing them to data from the literature. A broad spectrum of model toxic compounds was evaluated for toxicity on mouse skin JB6 cells in culture. These included mercuric chloride, sodium lauryl sulfate, formaldehyde, dimethyl sulfoxide, benzoyl peroxide, and ionomycin, all of which have been proven to be positive in the Draize test or in cutaneous toxicity studies. Cell viability was evaluated every 15 min for up to 1 hr, and then after 24 hr of treatment using the Trypan Blue exclusion method; morphological changes were evaluated using phase-contrast and transmission electron microscopy. Dose- and time-dependent cell death and morphological changes were observed at concentrations ranging from 10(-14) to 10(-2) M. Arbitrary rankings were assigned based on 1) IC50 value estimated from the present data, and 2) in vivo toxicity reported in the Registry of Toxic Effects of Chemical Substances. Good correlation between in vitro and in vivo toxicity based on arbitrary rankings was observed. Thus, these findings suggest that the JB6 cell culture model can be used for predicting in vivo toxicity. In the future, it may be possible to utilize this system for the study of intracellular ionized calcium ([Ca2+]i), and the expression of oncogenes as early indicators of toxicity.

The effect of dietary lipid on skin tumor promotion by benzoyl peroxide: comparison of fish, coconut and corn oil

Fish or vegetable oils were fed during the promotion stage of a mouse skin carcinogenesis model in order to investigate the effects of dietary fat on tumor development. Two weeks after initiation with 10 nmol dimethylbenz[a]anthracene, SENCAR mice were divided into five groups and maintained on one of the following semipurified diets containing 10% total fat and varying the type of fat: 8.5% coconut oil (CT)/1.5% corn oil (CO); 1% menhaden oil (MO)/7.5% CT/1.5% CO; 4% MO/4.5% CT/1.5% CO; 8.5% MO/1.5% CO; or 10% CO. Promotion with twice-weekly applications of 40 mg benzoyl peroxide was begun 2 weeks later and continued for 52 weeks. No statistically significant differences in kcal food consumed or body weights were observed between diet groups. Papilloma latency, incidence and yield differed among the diet groups with the group fed the 8.5% CT/1.5% CO diet having the shortest latency and highest papilloma incidence and number. In addition, carcinoma latency and incidence was assessed and the first carcinoma appeared in the group fed 8.5% CT/1.5% CO after 20 weeks of benzoyl peroxide treatment; this group yielded the highest carcinoma incidence throughout the study. In comparison, the group fed the 10% CO diet had the longest latency period, and among the lowest papilloma and carcinoma incidence and fewest tumors. In parallel studies, ornithine decarboxylase activity, vascular permeability and hyperplasia were elevated in the epidermis of benzoyl peroxide-treated mice but the extent of the response did not correlate with the different rates of tumor formation observed among the diet groups. These data indicate that dietary fat modulates tumor promotion by benzoyl peroxide in this skin carcinogenesis model with the predominantly saturated fat diet producing the highest rates of papilloma and carcinogen formation and the polyunsaturated fat diet the lowest.

Induction of short-term markers of tumor promotion by organic peroxides

Experiments from different laboratories have shown that benzoyl peroxide (BzPo) and other organic peroxides are effective tumor promoters in the mouse skin two-stage carcinogenesis system. In the present paper we have studied the short-term effect of six other organic peroxides, which have not been previously assayed as skin tumor promoters. These compounds were chosen for their molecular diversity, the type of radical predicted to be formed, solubility and availability. The parameters evaluated in this study include a series of short-term markers of tumor promotion, hyperplasia, induction of dark basal keratinocytes and induction of ornithine decarboxylase activity. After single applications the biological activity of the compounds was: m-chloroperoxybenzoic acid greater than di-m-methylbenzoyl peroxide greater than dicumyl peroxide greater than O,O-t-butyl-O-(2-ethylhexyl)mono-peroxycarbonate greater than benzoyl peroxide greater than di-m-chlorobenzoyl peroxide greater than di-t-butyl peroxide greater than t-butyl hydroperoxide. After multiple applications, the order of activity of the compounds was: dicumyl peroxide greater than di-m-methyl-benzoyl peroxide greater than O,O-t-butyl-O-(2-ethylhexyl)monoperoxy carbonate greater than m-chloroperoxybenzoic acid greater than di-m-chlorobenzoyl peroxide greater than t-butyl hydroperoxide greater than benzoyl peroxide greater than di-t-butyl peroxide. The difference of activity among the different compounds did not seem to correlate directly with the chemical stability of the compound; it is more likely that the activity depends on different factors such as percutaneous absorption, metabolism, and the rate of free radical formation in vivo. The data presented here further support the association between free radicals and tumor promotion since all of the compounds, with the exception of one, were active in inducing the short-term markers of tumor promotion. It will also establish conditions for future tumor experiments.

Evidence for a common genetic pathway controlling susceptibility to mouse skin tumor promotion by diverse classes of promoting agents

The present study has compared different mouse stocks and strains with known sensitivity to phorbol ester skin tumor promotion for their sensitivities to skin tumor promotion by a prototypic organic peroxide (benzoyl peroxide, BzPo) and anthrone (chrysarobin, Chr) tumor promoter. Following initiation with either 7,12-dimethylbenz(a)anthracene and/or N-methyl-N'-nitro-N-nitrosoguanidine, groups of mice were promoted with several different doses of each promoting agent. Among mice selectively bred for sensitivity to phorbol ester promotion, the order of sensitivity to BzPo was inbred SENCAR (SSIn) greater than SENCAR greater than CD-1. With Chr as the promoter, the order of sensitivity was SENCAR greater than SSIn greater than CD-1. Concurrent tumor promotion experiments examined the responsiveness of two common inbred mouse strains, DBA/2 and C57BL/6. The phorbol ester-responsive mouse strain, DBA/2, was more sensitive to skin tumor promotion by Chr than was C57BL/6 at all doses tested but was clearly less sensitive than both SENCAR and SSIn mice. Finally, DBA/2 and C57BL/6 mice were similar in their responsiveness to BzPo promotion, but again both of these inbred strains were significantly less sensitive than were SSIn and SENCAR mice to this organic peroxide type of skin tumor promoter. Histological evaluations comparing SENCAR and C57BL/6 mice revealed that a major difference between these strains in response to multiple Chr and BzPo treatments was in the inflammatory response (measured by edema formation). Unlike 12-O-tetradecanoylphorbol-13-acetate, Chr and BzPo did not induce dramatic differences in the epidermal hyperplasia (as measured by epidermal thickness) in these two mouse lines. The results presented in this paper suggest that there is a common pathway controlling susceptibility to skin tumor promotion by 12-O-tetradecanoylphorbol-13-acetate, BzPo, and chrysarobin. These results are discussed in terms of a possible genetic model(s) for skin tumor promotion in mice.

Malignant conversion of UV radiation and chemically induced mouse skin benign tumors by free-radical-generating compounds

It is known that the free-radical-generating compound, benzoyl peroxide (BPO) enhances malignant conversion of murine skin benign papillomas into carcinomas. To further define the role of free radicals in malignant conversion, we studied the effect of various free-radical-generating compounds on the conversion of benign papillomas into carcinomas in murine skin. Papillomas were induced in Sencar mice by 7,12-dimethylbenz[a]anthracene (DMBA) initiation and 12-O-tetradecanoylphorbol-13-acetate (TPA) promotion and in SKH-1 hairless mice by biweekly exposure to UVB radiation. After 20 and 27 weeks respectively, papilloma yield stabilized and no new tumors developed. Skin application of acetone, TPA, BPO, 2,2-azobis(2-amidino-propane) (ABP) and tert-butyl hydroperoxybenzoic acid (BPB) increased the rate of malignant conversion in both Sencar and SKH-1 mice. In general, the rate of malignant conversion was faster in UVB-induced tumors compared with DMBA-induced tumors. The relative efficacy of each agent was similar in both groups and was in the order: acetone less than TPA less than BPB less than BPO less than ABP. Our data suggest that free-radical-generating compounds may accelerate the malignant conversion of benign papillomas into carcinoma, indicating that epigenetic mechanism(s) may also be involved in this process.

Photocarcinogenesis promotion studies with benzoyl peroxide (BPO) and croton oil

Previous studies demonstrated that BPO can promote chemically initiated tumor formation in SENCAR mice. In addition, a number of chemicals have been shown to promote and/or enhance UVR induced carcinogenesis. This study examined the effect of BPO on UVR initiated tumor formation. One hundred and forty-eight Uscd mice received 270 mJ/cm2 of UVB radiation to the posterior halves of their backs 3 times a week for 8 weeks. Four weeks later the mice were divided into 4 groups. Group I received croton oil in acetone applications to the back 5 times a week for the duration of the study. Group II received acetone, Group III received the BPO diluent, and Group IV received the BPO in an aqueous diluent applications as in Group I. One mouse in Group II (acetone) and one in Group IV (BPO) developed tumors in unirradiated skin. In the UVR initiated skin 38% of the survivors developed tumors in Group I (croton oil), whereas 5% did in Group II (acetone), 8% in Group III (BPO base), and 8% group IV (BPO). Thus under the circumstances of this study croton oil did promote UV initiated tumor formation but BPO did not. These results are consistent with those recently reported by Iversen.

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 it was demonstrated on hairless mouse skin that 5% benzoyl peroxide (BP) in a gel (Panoxyl), or gel alone, applied just before UV radiation had a protective effect against 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) irradiation with or without additional treatment with Panoxyl or gel in order 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 and the mice were irradiated the next day; in others, Panoxyl or gel was applied 5-30 min after UV irradiation. Enhancement of DMBA-induced carcinogenesis in hr/hr mice by the gel alone (assumed to be inert) was unexpected, and hence one group of hr/hr mice was first given 51.2 micrograms DMBA in acetone and thereafter treated twice a week with gel alone. All mice were tested and observed for skin tumors and other lesions for 52 weeks. Neither Panoxyl nor gel influenced UV tumorigenesis or carcinogenesis under these experimental conditions. In hr/hr mice there was this time no enhancement of DMBA-induced tumorigenesis by the gel, and a slight reduction of carcinogenesis. In addition, several groups of SENCAR mice (which have been bred for high sensitivity to skin carcinogenesis) were also treated, with acetone alone, with a single application of DMBA alone, with Panoxyl alone, or with DMBA followed by treatment with the ointment gel or with Panoxyl twice a week throughout the experiment. In SENCAR mice there was no difference between the results of treatment with DMBA followed by Panoxyl, or DMBA followed by gel, and both substances tended to reduce the tumorigenicity of DMBA alone, and Panoxyl or gel showed no tumorigenicity of their own. The total dose of UV used in this study was lower than that used in the first study. This reduction in dose significantly increased the tumorigenic effect of UV.

Benzoyl peroxide promotes the formation of melanotic tumors in the skin of 7,12-dimethylbenz[a]anthracene-initiated Syrian golden hamsters

A two-stage carcinogenesis experiment was performed in Syrian golden hamsters using a single intragastric initiation with 10 mg/kg body weight of 7,12-dimethylbenz[a]anthracene (DMBA) and repetitive topical promotion on the back skin with two different doses (80 and 160 mg, respectively) of benzoyl peroxide in 1 ml acetone. Benzoyl peroxide was administered three times per week over a period of 16 months. The treatment with benzoyl peroxide alone leads to both a generalized hyperpigmentation and skin scaling without formation of any tumors. DMBA initiation alone induces a moderate number of melanotic foci and a small number of palpable melanotic tumors. Both lesions are located in the dermis. Papillomas develop in the epithelia of the tongue, esophagus and especially of the forestomach but not, however, on the back skin. The combined treatment with DMBA and both doses of benzoyl peroxide drastically increases the incidence of dermal melanotic foci and at late stages also that of melanotic tumors. Therefore, in addition to its known ability to promote papilloma and carcinoma formation in the back skin of mice, benzoyl peroxide is also able to promote the formation of melanotic tumors in the dermis of hamsters.

Enhanced malignant progression of mouse skin tumors by the free-radical generator benzoyl peroxide

Chemical carcinogenesis in mouse skin can be divided into the processes of initiation, promotion, and progression. The free-radical generator benzoyl peroxide is moderately active during the promotion stage. Repetitive treatment of mouse benign skin tumors (papillomas) with benzoyl peroxide (20 mg, twice weekly) increased the number of cumulative carcinomas per group by 325% and the number of keratoacanthomas by 44% compared to tumor-bearing Sencar mice treated with the promoter 12-O-tetradecanoylphorbol-13-acetate. The lack of increase in the number of cumulative papillomas per group due to benzoyl peroxide treatment suggests that benzoyl peroxide enhanced the progression of preexisting papillomas. The ability of benzoyl peroxide to enhance the progression of benign tumors to cancer should be considered when determining the human risk from exposure to this widely used chemical agent; in addition, biological assays specifically testing malignant progression may be essential and beneficial for determining an agent's carcinogenic risk.

Carcinogenesis studies with benzoyl peroxide (Panoxyl gel 5%)

Several groups of hairless mice were given UV radiation with and without pretreatment with 7,12-dimethylbenz(a)anthracene (DMBA), 5% benzoyl peroxide in a gel (Panoxyl), and gel alone, in various combinations, with appropriate control groups included, in order to see whether benzoyl peroxide, which is known to enhance chemical skin carcinogenesis after a single, small dose of DMBA, also enhances UV carcinogenesis. The mice were observed for skin tumors, and all skin lesions were histologically investigated. The percentage of tumor-bearing animals with time is called the tumor rate, the total number of tumors occurring is called the tumor yield. Continual treatment with 5% benzoyl peroxide in gel twice a week, with or without a short pretreatment period of UV radiation resulted in only 2 skin carcinomas, which is remarkable, but not significant. Both Panoxyl and gel alone enhanced tumorigenicity significantly in animals pretreated with a single dose of 51.2 micrograms DMBA. There was no difference between the enhancement caused by Panoxyl and the gel as regards the tumor rate, but when measured as final tumor yield, Panoxyl was slightly more tumor-enhancing than gel alone. However, both Panoxyl and gel protected significantly against UV tumorigenesis (all tumors). There was no difference between the protective effect of the 2 types of treatment. Neither Panoxyl nor gel alone influenced significantly UV skin carcinogenesis (malignant tumors). It is concluded that under these experimental conditions both Panoxyl and gel alone tend to protect against the tumorigenicity and do not enhance the carcinogenicity of UV radiation in hairless mice, whereas both gel and Panoxyl enhance chemical carcinogenesis. The carcinogenic mechanisms may be different for UV and chemical carcinogenesis, respectively.

Association of DNA strand breaks with accelerated terminal differentiation in mouse epidermal cells exposed to tumor promoters

We have studied the appearance of single strand breaks (SSB) in DNA of mouse keratinocytes exposed in vitro to various tumor promoters. Mouse basal keratinocytes were selectively cultured in low calcium medium, prelabeled with [14C]thymidine, exposed to test agents, and SSB quantified by alkaline elution. 12-O-Tetradecanoylphorbol-13-acetate (TPA) caused a dose-dependent (10(-9)-10(-7) M) increase in SSB after 24 h but not after shorter exposures. DNA containing TPA-induced SSB was found only in cells which had detached from the culture plate as a consequence of TPA-induced terminal differentiation. Attached cells, resistant to the differentiation-inducing effects of TPA, had the low level of SSB found in DNA from vehicle-treated control cells. Attached cells were resistant to the formation of SSB and to induced differentiation when reexposed to TPA. Other tumor-promoting phorbol esters, mezerein and retinyl phorbol acetate, also produced SSB in detached cells, whereas phorbol or resiniferatoxin caused neither SSB or cell detachment. Retinoic acid, which blocks the induction of differentiation by TPA, inhibited the production of SSB by TPA; however, fluocinolone acetonide, chymostatin, catalase, or superoxide dismutase blocked neither TPA-induced SSB nor terminal differentiation. Epidermal cell lines resistant to TPA-induced differentiation were also resistant to SSB production by TPA. Benzoyl peroxide (BP) (10(-4) M) induced SSB in basal keratinocytes within 1 h, and attached cells showed extensive SSB by 12 h. Retinoic acid had only a slight effect on BP-induced SSB, and 1 of 3 TPA-resistant cell lines developed SSB when exposed to BP. These results suggest that TPA-induced SSB in epidermal cells are an indirect consequence of the induction of terminal differentiation, whereas BP produces SSB by a more direct mechanism of DNA damage.

Allergic and irritant potential of benzoyl peroxide

Benzoyl peroxide (BP) is shown to be a weak allergen. Of 25 guinea pigs, only 5 were sensitized in the TINA test. Before treatment, only 2 of 172 acne patients gave a positive patch test with 1% BP, but not with 0.1% BP. The following results suggest that BP is a strong irritant. The irritant potential ID50 in rabbits was 2.52%, and the IT50 was 2.31 days with 5% BP, 4.34 days with 1% and 8.35 days with 0.1% BP. 29% of acne patients and non-exposed controls had a positive patch test with 5% BP. However, only 11 of 155 acne patients had clinical signs of intolerance, which settled despite continued use in 10 cases. The possible reasons for this dichotomy are explored. Neither the irritant nor allergenic properties of BP impaired its clinical use.

Effects of formaldehyde, acetaldehyde, benzoyl peroxide, and hydrogen peroxide on cultured normal human bronchial epithelial cells

The effects of several aldehydes and peroxides on growth and differentiation of normal human bronchial epithelial cells were studied. Cells were exposed to formaldehyde, acetaldehyde, benzoyl peroxide (BPO), or hydrogen peroxide (HPO). The effect of each agent on the following parameters was measured: (a) clonal growth rate; (b) squamous differentiation; (c) DNA damage; (d) ornithine decarboxylase activity; (e) nucleic acid synthesis; (f) aryl hydrocarbon hydroxylase activity; and (g) arachidonic acid and choline release. None of the agents were mitogenic, and their effects were assessed at concentrations which reduced growth rate (population doublings per day) to 50% of control. The 50% of control concentrations for the 6-h exposure were found to be 0.065 mM BPO, 0.21 mM formaldehyde, 1.2 mM HPO, and 30 mM acetaldehyde. BPO-exposed cells were smaller than controls (median cell planar area, 620 sq microns versus 1150 sq microns), and acetaldehyde-exposed cells were larger than controls (median cell planar area, 3200 sq microns). All agents increased the formation of cross-linked envelopes and depressed RNA synthesis more than DNA synthesis. HPO caused DNA single-strand breaks, while formaldehyde and BPO caused detectable amounts of both single-strand breaks and DNA-protein cross-links. Other effects included increased arachidonic acid and choline release due to HPO. The similarities and differences of the effects of these aldehydes and peroxides to those caused by tumor promoters are discussed.