Mouse Skin Tumor Promotion Research Articles

Chronic Exposure to Ionizing Radiation as a Tumor Promoter in Mouse Skin

We have tested chronic exposure to 90Y beta radiation for its action as a complete tumor promoter, a stage I tumor promoter, or a stage II tumor promoter in SENCAR mouse skin. In skin initiated with a single application of 7,12,dimethylbenz[a]anthracene (DMBA, 10 nmol), chronic exposure to beta radiation as a complete promoter (0.5 Gy, twice/week, 13 weeks) produced no tumors and, when added to a complete chemical promoter (TPA), reduced tumor frequency about 30%. A similar result was observed when beta radiation was tested as a stage II promoter. DMBA-initiated mice that received chemical (12-O-tetradecanoylphorbol-13-acetate, TPA) stage I promotion followed by 13 weeks of beta-radiation exposure (0.5 Gy, twice/week) as stage II promotion produced essentially no tumors, and combining the same chronic beta-radiation exposure with chemical (mezerein) stage II promotion reduced tumor frequency about 20% when compared to a similar group that was not irradiated. Chronic beta-radiation exposure was tested two ways as a stage I tumor promoter in initiated skin that was subsequently treated with mezerein as a stage II promoter. Stage I promotion was shown to proceed with the passage of time, indicating this process occurs naturally in the absence of chemical or physical stimulation. Hyperthermia, previously shown to be a potent inhibitor of chemically stimulated stage I promotion, had no effect on the natural process, indicating at least some differences in mechanism between the two processes. The natural process was, in fact, inhibited by chemical tumor promoters, but not by radiation. In addition to the increase resulting from this natural process, tumor frequency was further increased slightly but significantly (12-15%, P less than or equal to 0.05) when chronic radiation exposure was given as a stage I promoter (0.5 Gy, twice/week, 13 weeks) subsequent to initiation, in spite of the expected 20% reduction resulting from this dose. Exposure of initiated animals to radiation (0.5 or 1.0 Gy, twice/week, 2 weeks) in addition to TPA as stage I promotion produced a similar increase in tumor frequency (P less than 0.02). At higher radiation doses, however, tumor frequency was reduced compared to unirradiated controls. In a third test as a stage I promoter, beta radiation (0.5 Gy twice/week, 4 weeks) was given prior to initiation with N-methyl-N'-nitro-N-nitrosoguanidine in animals subsequently promoted by TPA (twice/week, 13 weeks), and again the radiation slightly but significantly (P less than 0.03) increased tumor frequency compared to the unirradiated control group.(ABSTRACT TRUNCATED AT 400 WORDS)

132. The protein kinase inhibitor staurosporine induces differentiation of neoplastic mouse keratinocytes in vitro and blocks their growth as benign epidermal tumors in vivo

Salvia libanotica (sage) extract is a popular plant remedy used by Middle Eastern people to treat common complaints such as colds and abdominal pain. In this study, the chemopreventive effects of sage oil on 7,12 dimethylbenz[a]anthracene (DMBA)-initiated and 12-0-tetradecanoylphorbol-13-acetate (TPA)-promoted skin papillomas was investigated. Furthermore, its growth inhibitory and cytotoxic effects on a mouse papilloma-derived cell line (SP-1) were studied using 3H-thymidine incorporation, cell count and trypan blue dye exclusion assays. Sage oil was either applied topically to mouse skin at concentrations of 5, 50 and 100 % in acetone, injected intraperitonealy at concentrations of 4 (37 mg/ml) and 8 % (75 mg/ml) in saline or given by gavage at 100 % twice per week for 20 weeks, 20 minutes prior to each promotion treatment with TPA. The topically applied 100 % oil extract delayed tumor appearance by 4 weeks and inhibited tumor incidence and yield by 19 and 61 %, respectively, at week 20. Topical application of 50 % and 5 % sage oil inhibited tumor yield by 41 % at week 20. Tumor weight was decreased by 75 % and 80 % following treatment of mouse skin with 50 % and 100 % oil, respectively. Intraperitoneal injections and gavage treatments failed to inhibit the promotion of tumors in mouse skin, but significantly decreased tumor weight and volume. Sage oil displayed strong growth inhibitory effects on the SP-1 papilloma derived cell line following 24 hrs of treatment with estimated IC50 of 50 μg/ml. This observed growth inhibition was due to cytostatic and not cytotoxic effects. Our results suggest that the oil extract of the sage plant has potent suppressive activities against tumor promotion in mouse skin and thus could be an effective chemopreventive agent against skin cancer.

Induction of epidermal ornithine decarboxylase activity in mouse skin exposed to biogenic silica fibers.

The present study demonstrates that biogenic silica fibers (BSF), previously shown to promote skin tumors in mice and more recently to promote the induction of mesotheliomas when injected into the pleural cavity of rats, rapidly induces epidermal ornithine decarboxylase (ODC) activity in SENCAR mice following topical application. The time course for induction of epidermal ODC by BSF was very similar to that observed following topical treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA). Maximal ODC activity was observed 4-6 h following treatment with BSF. Cycloheximide (70 mg/kg i.p.) partially inhibited (61%) the induction of ODC by BSF at 5 h. In addition, retinoic acid (RA, 5 micrograms per mouse given 30 min before BSF) effectively inhibited BSF-induced ODC by 68%, while indomethacin (100 micrograms per mouse 2 h before BSF) had little or no effect. Copper(II) bis(diisopropylsalicylate) (2 mumol 30 min before BSF), an effective inhibitor of TPA-induced ODC activity and tumor promotion, also had little or no effect on BSF-induced ODC. The work described in this paper suggests that BSF induces epidermal ODC by a very specific mechanism that exhibits both similarities and differences with that of the phorbol ester, TPA. Nevertheless, this response strongly supports the conclusion that BSF is an effective tumor promoter in mouse skin and that ODC induction is an integral part of the mechanism of action of this environmental promoter.

Studies on inhibitors of skin tumor promotion. XI. Inhibitory effects of flavonoids from Scutellaria baicalensis on Epstein-Barr virus activation and their anti-tumor-promoting activities.

To search for possible anti-tumor-promoters, fourteen flavones obtained from the root of Scutellaria baicalensis were examined for their inhibitory effects on the Epstein-Barr virus early antigen (EBV-EA) activation by a short-term in vitro assay. Among these flavones, 5,7,2'-trihydroxy- and 5,7,2',3'-tetrahydroxyflavone showed remarkable inhibitory effects on the EBV-EA activation, and the effect of the latter on Raji cell cycle was also examined by flow cytometer. These two flavones exhibited remarkable inhibitory effects on mouse skin tumor promotion in an in vivo two-stage carcinogenesis test.

Inhibitory effect of topical application of a green tea polyphenol fraction on tumor initiation and promotion in mouse skin.

A green tea polyphenol fraction was evaluated for its ability to inhibit tumor initiation by polycyclic aromatic hydrocarbons and tumor promotion by a phorbol ester in the skin of CD-1 mice. Topical application of the green tea polyphenol fraction inhibited benzo[a]pyrene- and 7,12-dimethylbenz[a]-anthracene-induced tumor initiation as well as 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced tumor promotion. Topical application of the green tea polyphenol fraction also inhibited TPA-induced inflammation, ornithine decarboxylase activity, hyperplasia and hydrogen peroxide formation. Studies with individual polyphenolic compounds in green tea indicated that topical application of (-)-epigallocatechin gallate, (-)-epigallocatechin and (-)-epicatechin gallate inhibited TPA-induced inflammation in mouse epidermis.

Further genetic analyses of skin tumor promoter susceptibility using inbred and recombinant inbred mice.

To explore further the genetics of susceptibility to skin tumor promotion in inbred mice, several aspects of responsiveness to 12-O-tetradecanoylphorbol-13-acetate (TPA) were examined in C3H/He mice and segregating crosses between this mouse strain and C57BL/6 mice as well as BXD and BXH recombinant inbred (RI) strains. Dose-response relationships were established for skin tumor promotion by TPA following initiation with 7,12-dimethylbenz[a]anthracene in C3H/He and B6C3F1, as well as several other mouse stocks and strains included for comparison. The relative responsiveness to TPA skin tumor promotion was: SENCAR much greater than DBA/2 greater than C3H/He approximately B6D2F1 greater than B6C3F1 much greater than C57BL/6. Analyses of the susceptibility of B6C3F2 and B6C3F1 x C57BL/6 backcross mice suggested that a minimum of two dominant genetic loci control responsiveness to phorbol ester promotion in these mice. Further analysis of BXH and BXD RI strains suggested the presence of four distinct promotion-responsive phenotypes controlled by a minimum of two genetic loci. The existence of a 'hyper-responsive' phenotype in the sets of RI strains, however, suggests that a third, recessive locus also may play a role in controlling responsiveness to TPA promotion. At 48 h after the last of four applications of TPA, marked hyperplasia and an increase in dark basal keratinocytes were observed in C3H/He mice, whereas in B6C3F1 mice the response in these parameters was intermediate between C3H/He and C57BL/6 mice. A marked dermal inflammation, as determined by infiltration of polymorphonuclear cells, was observed in C3H/He and B6C3F1 mice, whereas little was noted in C57BL/6 mice. Furthermore, histological evaluations of selected BXD RI strains revealed a significant correlation between the magnitude of the hyperplasia response and the percentage of mice bearing tumors. The present data, in conjunction with our previous studies, confirm that the major gene(s) controlling susceptibility to tumor promoter induced by TPA in two sensitive strains (i.e. DBA/2 C3H/He) are similar or closely linked to those for induction of sustained hyperplasia. In addition, the present data provide new evidence for a model where allelic differences at a minimum of three loci contribute to gene differences in susceptibility to phorbol ester promotion DBA/2 and C3H/He versus C57BL/6 mice.

Reversal of food restriction-induced inhibition of mouse skin tumor promotion by adrenalectomy.

Restricting the food intake of laboratory mice and rats markedly reduces the incidence of spontaneous and experimentally induced cancers. Using the two-stage skin tumorigenesis model in CD-1 mice, we report now that food restriction suppresses 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulation of epidermal [3H]thymidine incorporation as well as TPA promotion of skin papillomas, whereas adrenalectomy completely reverses the inhibition in [3H]thymidine incorporation and tumor development. These results suggest that the adrenal gland may play an important role in mediating the tumor inhibitory effect of food restriction.

Measurement of neutrophil activation and epidermal cell toxicity by palytoxin and 12-O-tetradecanoylphorbol-13-acetate.

Palytoxin is a human and mouse skin irritant and complete mouse skin tumor promoter that differs from the well-studied phorbol ester class of tumor promoters in many respects. In this study, we have found palytoxin to stimulate the production of superoxide by isolated human neutrophils using electron paramagnetic resonance (EPR) spectroscopy with the spin-trap DMPO. This stimulation of oxyradical production by palytoxin is relatively weak, however, when compared to that by 12-O-tetradecanoylphorbol-13-acetate (TPA). The maximal amount of oxyradicals produced by palytoxin-stimulated neutrophils is 10(-4) mumols/10(6) neutrophils, and this stimulation requires nanomolar concentrations of palytoxin, with half maximal stimulation at concentrations of approximately 30 nM. In contrast, the tumor promoter TPA causes human neutrophils to generate in excess of 10(-3) mumols oxyradicals/10(6) neutrophils with concentrations as low as 1 nM. Toxicity to cultured human epidermal cells was observed at very low concentrations of palytoxin, with 50% loss of colony-forming efficiency observed at approximately 3 x 10(-13) M. For TPA, 50% loss of colony-forming efficiency for cultured epidermal cells requires approximately 5 nM. Thus, although palytoxin stimulates superoxide production in isolated neutrophils, epidermal cells are sensitive at much lower concentrations and are likely to be the important target cell in vivo. This is in contrast to TPA, where neutrophils are stimulated at concentrations less than those required to produce pathological effects on epidermal cells, suggesting that neutrophils may be an important target cell for TPA in vivo.

Okadaic acid induces the expression of both early and secondary response genes in mouse keratinocytes.

Okadaic acid (OA), a potent mouse skin tumor promoter and inhibitor of the protein phosphatases 1 and 2A, was investigated for its effects on the expression of tumor-associated early and secondary response genes in mouse keratinocytes. Adult mice were treated topically with 12.5 nmol of OA, and the steady-state levels of various gene transcripts in the skin were determined at different times after treatment. The nuclear proto-oncogenes c-fos and c-jun are referred to as early response genes because the classical tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) induces their expression to maximal levels within 2 h after treatment. OA induced the expression of c-fos 2-72 h after treatment, with two peaks at 6 and 48 h. The steady-state level of expression of c-jun was relatively high in untreated skin, and OA induced a slight increase in its expression from 12 to 48 h after treatment. Transin and plasminogen-activator (PA) urokinase, whose induced expression peaks at least 4 h after TPA treatment, are referred to as secondary response genes. OA induced their expression more slowly than TPA. In mouse papilloma cell line 308, OA induced higher and more sustained steady-state levels of c-jun and c-fos than an equimolar dose of TPA. Transin and PA-urokinase were induced to similar levels by TPA and OA in 308 cells; however, the induction of these genes by OA was slower than induction by TPA. The existence of different patterns of induced expression of early and secondary response genes by OA and TPA suggests that these tumor promoters affect gene expression in mouse keratinocytes through different pathways.

Effect of ascorbic acid and its synthetic lipophilic derivative ascorbyl palmitate on phorbol ester-induced skin-tumor promotion in mice

The effect of topical application of ascorbic acid (AA) and ascorbyl palmitate (AP) on 12-O-tetradecanoyl-phorbol-13-acetate (TPA)-induced tumor promotion in mouse skin was investigated. A single application of TPA decreased epidermal AA by 45%. Repetitive application of 6 and 28 mumol AA with 2 nmol TPA inhibited tumor multiplicity by 39% and 76%. Repetitive application of 0.16, 0.8, and 4.0 mumol AA with 5 nmol TPA inhibited tumor multiplicity by 16%, 46%, and 91%. Because AA may be poorly absorbed cutaneously, we evaluated the effect of dietary AA. Supplementation of drinking water with AA increased epidermal ascorbic acid levels by 50%. Dietary intake of AA did not inhibit TPA-induced tumor promotion. Preliminary data suggest that the mice not receiving AA developed increased epidermal AA levels in response to the tumor promoting regimen. Recently we have found that dietary AP inhibited TPA-induced biochemical parameters associated with tumor promotion.

Inhibition of mouse skin tumor promotion by tenuazonic acid

Tenuazonic acid (TA) was topically applied to the interscapular region of Swiss albino mice at different doses before the application of 12-O-tetradecanoyl phorbol-13-acetate (TPA). Skin from the painted area was examined for ornithine decarboxylase (ODC) enzyme estimation [1]. It was observed that TA inhibited TPA induced ODC activity. The inhibitory effect of TA was also found in mouse skin tumor promotion in the two stage initiation promotion protocol. There was a remarkable delay in the latency period and decrease in the number of tumors developed and the percentage of tumor bearing animals after TA treatment.

Studies on inhibitors of skin tumor promotion, IX. Neolignans from Magnolia officinalis.

Three neolignans, known as magnolol [1], honokiol [2] and the new monoterpenylmagnolol [3], were isolated from the bark of Magnolia officinalis as inhibitors of Epstein-Barr virus early antigen (EBV-EA) activation induced by 12-O-tetradecanoylphorbol-13-acetate (TPA). The structure of 3 was determined from 2D nmr spectral data and difference nOe experiments. The MeOH extract of this plant and magnolol exhibited remarkable inhibitory effects on mouse skin tumor promotion in an in vivo two stage carcinogenesis test. This investigation indicates that these neolignans and the extract might be valuable antitumor promoters.

Role of the benzoyloxyl radical in DNA damage mediated by benzoyl peroxide

Benzoyl peroxide (BzPO) is both a tumor promoter and progressor in mouse skin; however, BzPO is neither an initiator nor a complete carcinogen in this tissue. Although not mutagenic, BzPO has been observed to produce strand breaks in DNA of exposed cells. These actions are presumed to be mediated by free-radical derivatives of BzPO. Previous studies suggested that the metabolism of BzPO in keratinocytes proceeds via the initial cleavage of the peroxide bond, yielding benzoyloxy radicals which, in turn, can either fragment to form phenyl radicals and carbon dioxide or abstract H atoms from biomolecules to yield benzoic acid. Benzoic acid is the major stable metabolite of BzPO produced by keratinocytes. In the present study we have investigated the role of BzPO and its metabolites in the generation of strand scissions in a cell-free system using phi X-174 plasmid DNA. In this system BzPO produced DNA damage that was dose-dependent over a concentration range of 0.1-1 mM and required the presence of copper but not other transition metals. By contrast, benoic acid did not produce DNA damage in this system, either in the presence or in the absence of copper. The inclusion of spin trapping agents, such as N-tert-butyl-alpha-phenylnitrone (PBN), 3,5-dibromo-4-nitrosobenzenesulfonate, and nitrosobenzene, in incubations was found to significantly reduce the extent of DNA damage generated via the copper-mediated activation of BzPO. Electron paramagnetic resonance spectroscopy studies suggested that the primary radical trapped by PBN following copper-mediated decomposition of BzPO was the benzoyloxy radical.(ABSTRACT TRUNCATED AT 250 WORDS)

Free radical-derived quinone methide mediates skin tumor promotion by butylated hydroxytoluene hydroperoxide: expanded role for electrophiles in multistage carcinogenesis.

Free radical derivatives of peroxides, hydroperoxides, and anthrones are thought to mediate tumor promotion by these compounds. Further, the promoting activity of phorbol esters is attributed, in part, to their ability to stimulate the cellular generation of oxygen radicals. A hydroperoxide metabolite of butylated hydroxytoluene, 2,6-di-tert-butyl-4-hydroperoxyl-4-methyl-2,5-cyclohexadienone (BHTOOH), has previously been shown to be a tumor promoter in mouse skin. BHTOOH is extensively metabolized by murine keratinocytes to several radical species. The primary radical generated from BHTOOH is a phenoxyl radical that can disproportionate to form butylated hydroxytoluene quinone methide, a reactive electrophile. Since electrophilic species have not been previously postulated to mediate tumor promotion, the present study was undertaken to examine the role of this electrophile in the promoting activity of BHTOOH. The biological activities of two chemical analogs of BHTOOH, 4-trideuteromethyl-BHTOOH and 4-tert-butyl-BHTOOH, were compared with that of the parent compound. 4-Trideuteromethyl-BHTOOH and 4-tert-butyl-BHTOOH have a reduced ability or inability, respectively, to form a quinone methide; however, like the parent compound, they both generate a phenoxyl radical when incubated with keratinocyte cytosol. The potency of BHTOOH, 4-trideuteromethyl-BHTOOH, and 4-tert-butyl-BHTOOH as inducers of ornithine decarboxylase, a marker of tumor promotion, was commensurate with their capacity for generating butylated hydroxytoluene quinone methide. These initial results were confirmed in a two-stage tumor promotion protocol in female SENCAR mice. Together, these data indicate that a quinone methide is mediating tumor promotion by BHTOOH, providing direct evidence that an electrophilic intermediate can elicit this stage of carcinogenesis.

Inhibitory effect of curcumin and some related dietary compounds on tumor promotion and arachidonic acid metabolism in mouse skin

Topical application of curcumin, the major yellow pigment in turmeric and curry, has a potent inhibitory effect on 12- O-tetradecanoylphorbol-13-acetate (TPA)-induced tumor promotion in mouse skin. The structurally related compounds chlorogenic acid, caffeic acid and ferulic acid are less potent inhibitors. Curcumin is a potent inhibitor of TPA-induced ornithine decarboxylase activity and inflammation in mouse skin whereas chlorogenic acid, caffeic acid and ferulic acid are only weakly active or inactive. Curcumin is a potent inhibitor of arachidonic acid-induced inflammation in vivo in mouse skin, and this compound is also a potent inhibitor of epidermal lipoxygenase and cyclooxygenase activity in vitro. Although chlorogenic acid is only weakly active as an inhibitor of epidermal lipoxygenase activity and TPA-induced ear inflammation, it is more active than caffeic acid and ferulic acid. The inhibitory effects of curcumin, chlorogenic acid, caffeic acid and ferulic acid on TPA-induced tumor promotion in mouse skin parallel their inhibitory effects on TPA-induced epidermal inflammation and epidermal lipoxygenase and cyclooxygenase activities. Examination of the structural features of curcumin required for its biological activity indicate that free hydroxyl groups on the benzene rings are not required for inhibition of TPA-induced ornithine decarboxylase activity and inflammation in mouse skin.

Modulation of tumor promotion in mouse skin by the food additive citral (3,7-dimethyl-2,6-octadienal)

Citral inhibits the formation of retinoic acid from retinol in mouse epidermis. Since skin-carcinogenesis is sensitive to retinoid status, and retinoic acid may be the active form of vitamin A in the epidermis, citral was tested for its ability to modulate tumor promotion in a two-stage skin-carcinogenesis study in hairless mice. The dorsal skins of female skh/hr1 mice were initiated with 0.1 mumol dimethylbenzanthracene, and tumors were promoted by twice-weekly application of 10 nmol of tetradecanoylphorbol-13-acetate (TPA) for 20 weeks. Prior to each TPA application groups were dosed with 0, 1 mumol, or 10 mumol citral. Citral had a dose-dependent inhibitory effect on tumor-production in the TPA promoted groups. At 10 weeks of promotion the percentage of mice with tumors were 88%, 72% and 60%, for the 0, 1 and 10 mumol citral treated groups, and the numbers (mean +/- S.D.) of tumors per affected animal were 7.3 +/- 6.6, 3.9 +/- 4.2, and 3.7 +/- 3.5, respectively. At 15 weeks of promotion the tumor incidence was 96%, 96% and 84%, respectively, and the number of tumors per affected animal were 9.5 +/- 6.8, 7.2 +/- 4.6 and 4.5 +/- 3.3, respectively. Again the affected mice in the high dose citral group had significantly fewer tumors. When the study was terminated at 20 weeks of promotion all mice had at least one tumor, but the number of tumors per affected mouse were lower in the citral treated groups. These findings concur with the proposal that there is a retinoid requirement for skin tumor promotion, and establishes that anti-retinoids have potential uses as modulators of carcinogenesis.

Effects of dietary calcium and vitamin D3 on tumor promotion in mouse skin

The effects of low, adequate, and supplemental intake of calcium and vitamin D3 on 12-O-tetradecanoylphorbol-13-acetate (TPA) skin tumor promotion were examined. Administration of the experimental diets was started one week before the first TPA application to the 7,12-dimethylbenz[a]anthracene-initiated dorsal skin of female Sencar mice. Neither dietary calcium in a range from 0.15% to 2.0% of the diet as calcium carbonate nor vitamin D3 ranging from 200 to 4,000 IU/kg diet resulted in modulation of the skin papilloma response in terms of incidence, number per mouse, or size distribution of tumors. There were also no effects of the varied levels of calcium or vitamin D3 on mouse body weights, serum calcium, or TPA induction of epidermal ornithine decarboxylase activity. These results indicate that dietary administration of a wide range of doses of calcium or vitamin D does not alter the serum calcium levels and, therefore, does not appear capable of altering skin tumor promotion. These results are in contrast to reports that demonstrate antineoplastic activity for both calcium ion and active hormonal vitamin D, either in control of epidermal cell proliferation and/or differentiation or inhibition of carcinogenesis.

Inhibition of mouse skin tumor promotion by adriamycin and daunomycin in combination with verapamil or palmitoylcarnitine

The anti-cancer drugs Adriamycin (ADR) and Daunomycin (DAU) alone were unable to inhibit the promotion of skin papillomas by repeated applications of 8.5 nmol of 12-O-tetradecanoylphorbol-13-acetate (TPA) in 7,12-dimethylbenz(a)anthracene (DMBA)-initiated mice. Pretreatments with 50 μg of ADR also failed to alter the tumor-promoting activities of smaller doses of TPA. Therefore, the effects of the anthracycline antibiotics on skin tumor promotion were evaluated in combination with the Ca 2+ antagonist verapamil (VRP) and the protein kinase C (PKC) inhibitor palmitoylcarnitine (PC), compounds known to circumvent drug resistance. When applied simultaneously with each promotion treatment with 8.5 nmol of TPA, 2.5 mg of VRP inhibited the number of papillomas/mouse by 26%. But the combination of VRP + 50 μg of ADR or DAU inhibited the yields of papillomas by 50 or 47%, respectively, suggesting that VRP was required to reveal the antitumorpromoting activities of otherwise ineffective drugs. Similarly, the promotion of skin tumors by TPA was inhibited synergistically by the combinations of 2 μmol of PC + 50 μg of ADR or DAU. For instance, ADR and DAU had no effects alone but inhibited the incidence of skin papillomas by 78 and 86%, respectively, in the presence of PC, a compound which alone inhibited the tumor incidence by only 44%. The results indicate that ADR and DAU are effective against the promoting component of skin carcinogenesis only if they are applied in combination with Ca 2+ antagonists or PKC inhibitors at a time when they can inhibit the early biochemical effects induced by TPA.

Mouse skin ornithine decarboxylase induction and tumor promotion by cyclohexane

Cyclohexane, a frequently used solvent in industry, was assessed for its tumorigenic potential on mouse skin following multistage initiation-promotion protocols. The activity of ornithine decarboxylase (ODC), a marker of tumor promotion was found to be induced by the topical application of cyclohexane. This ODC induction was dependent on the dose of cyclohexane used and the duration of application. Effect of protein synthesis inhibitors and the modifiers of tumor promotion on the cyclohexane induced ODC activity was also studied. ODC induction was inhibited by cycloheximide and also, up to some extent, by actinomycin D. Inhibitors of stage II tumor promotion showed more effect on the ODC induction by cyclohexane as compared to the inhibitors of stage I tumor promotion. In chronic animal bioassay experiments topical application of cyclohexane to DMBA initiated mouse skin resulted in just 10% of tumor bearing animals while prior application of TPA for two weeks resulted in 45% of tumor bearing animals. Collectively, the present study demonstrates that cyclohexane is more effective as a stage II tumor promoter over mouse skin and possibly affects the biochemical events at the molecular level.

The non-phorbol ester tumor promoter okadaic acid does not promote morphological transformation or inhibit junctional communication in hamster embryo cells

Okadaic acid is a potent non-phorbol ester mouse skin tumor promoter. Unlike the phorbol ester tumor promoters, okadaic acid is unable to promote the induction of morphological transformation in Syrian hamster embryo cell colonies. On the contrary, okadaic acid seems to counteract the effect of phorbol esters on transformation. Also unlike phorbol ester tumor promoters, okadaic acid does not inhibit intercellular communication, neither in primary hamster embryo cells, nor in the phorbol ester sensitive cell line BPNi. Furthermore, okadaic acid has no effect on the reoccurrence of communication following removal of 12-O-tetradecanoylphorbol-13-acetate.