Induction of mouse epidermal ornithine decarboxylase activity and skin tumors by 7,12-dimethylben

Inhibition by retinoids of neoplastic transformation in vitro: cellular and biochemical mechanisms

We have demonstrated that non-toxic concentrations of retinoids can cause a dose-dependent inhibition of 3-methylcholanthrene-induced transformation of C3H/10T1/2 cells. On removal of the retinoid, transformed foci appear after a latent period of about four weeks at the same frequency as observed in controls treated with carcinogen only. Reasoning that this activity is compatible with the stabilization of the carcinogen-initiated state, we have succeeded in isolating from carcinogen-treated cultures a cell line which in the presence of retinyl acetate is similar to the parental 10T1/2 cells, but without retinyl acetate transforms at a high frequency after a latent period of about four weeks. Retinyl acetate treatment of this cell line (INIT/10T1/2) and the parental 10T1/2 cells induces an ultra-normal phenotype. In retinoid-deprived INIT/10T1/2 cells, the first sign of transformation (an increased thymidine-labelling index) occurs 16 days after retinyl acetate removal. We have detected by two-dimensional electrophoresis that concomitant with this there is an increase in phosphorylation of a protein of Mr 34 000 (34K) which may be associated with the cytoskeleton. This phosphoprotein has been found in all transformed lines examined. A second phosphoprotein, of about 38K, has also been detected in transformed cells. Retinyl acetate treatment of transformed cells alters the isoelectric point of this protein, a change compatible with decreased phosphorylation. Alkali-resistant phosphorylation, presumably on tyrosine, has been found on a second 34K protein of transformed cells. Retinyl acetate treatment specifically decreases this phosphorylation. Mechanisms for the altered tyrosine phosphorylation induced by retinyl acetate are as yet unresolved, but the decrease could be due to altered levels of substrate, kinase or phosphorylase. In view of the apparent role of tyrosine kinases as mediators of growth factors and as oncogene products, we consider the activity of retinoids as modulators of tyrosine phosphorylation to be of great potential significance.

Epidermal expression of the translation inhibitor programmed cell death 4 suppresses tumorigenesis

Programmed cell death 4 (Pdcd4) is a novel repressor of in vitro transformation. Pdcd4 directly inhibits the helicase activity of eukaryotic translation initiation factor 4A, a component of the translation initiation complex. To ascertain whether Pdcd4 suppresses tumor development in vivo, we have generated transgenic mice that overexpress Pdcd4 in the epidermis (K14-Pdcd4). K14-regulated Pdcd4 expression caused a neonatal short-hair phenotype due to early catagen entry compared with matched wild-type siblings. In response to the 7,12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA) mouse skin carcinogenesis protocol, K14-Pdcd4 mice showed significant reductions in papilloma formation, carcinoma incidence, and papilloma-to-carcinoma conversion frequency compared with wild-type mice. The translational efficiency of an mRNA engineered to form a structured 5' untranslated region (UTR) was attenuated in primary keratinocytes when Pdcd4 was overexpressed. Pdcd4 inhibited by 46% TPA-induced activator protein-1 (AP-1)-dependent transcription, an event required for tumorigenesis. CDK4 and ornithine decarboxylase (ODC) are candidates for Pdcd4-regulated translation as their mRNAs contain 5'structured UTRs. In K14-Pdcd4 primary keratinocytes expressing activated Ha-Ras to mimic DMBA-initiated epidermis, ODC and CDK4 protein levels were decreased by 40% and 46%, respectively. Expression of a protein encoded by 5' unstructured mRNA showed no change. These results extend to an in vivo model the observations that Pdcd4 inhibits both translation initiation and AP-1 activation while decreasing benign tumor development and malignant progression. The K14-Pdcd4 mice seem to validate translation initiation as a novel target for cancer prevention.

Topical application of the immunosuppressant tacrolimus accelerates carcinogenesis in mouse skin

BACKGROUND

Tacrolimus, produced by the fungus Streptomyces tsukabaensis, is a potent macrolide immunosuppressant widely used in liver and kidney transplantation. Topical tacrolimus has recently been found to be an effective treatment for atopic dermatitis (AD).

OBJECTIVES

Because of the well-known association between T-cell immunosuppression and an increased risk of carcinogenesis, we investigated the effect of topical tacrolimus on skin carcinogenesis in 117 mice.

METHODS

Approximately 8 cm2 of the shaved dorsal skin of 7-week-old female CD-1 mice was treated with 7,12-dimethylbenz[alpha]anthracene (DMBA) dissolved in acetone, which is in general use as a tumour initiator, or acetone alone, on day 1 of the experiment, followed by promoting treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA) with or without tacrolimus, or acetone with or without tacrolimus, for 20 weeks. The mice were divided into six treatment groups: (1) DMBA followed by acetone; (2) DMBA followed by TPA; (3) DMBA followed by acetone + tacrolimus; (4) DMBA followed by TPA + tacrolimus; (5) acetone followed by acetone + tacrolimus; and (6) acetone followed by acetone (control).

RESULTS

The induction of skin tumours was significantly greater in the TPA-treated groups than in the absence of TPA. However, after 14 weeks there was marked synergy between tacrolimus and the DMBA/TPA regimen, with 0.47 +/- 0.13 (mean +/- SD) new tumours per mouse per week in group 4 vs. 0.10 +/- 0.025 in group 2 (P < 0.01), and 0.01 +/- 0.002 in group 3. A significant reduction in the CD4/CD8 ratio was found in axillary and inguinal lymph nodes in tacrolimus-treated mice, supporting the presumption that the immunosuppressive effect of the drug was responsible for its effect in promoting tumorigenesis. The major increase in tumours caused by topical tacrolimus was of papillomas, not squamous cell carcinomas. Papillomas are uncommon in humans, and are benign. However, 8.5% of the tumours found in the experiment were squamous cell carcinomas, and a considerable synergy between topical tacrolimus and conventional carcinogens was observed, raising the spectre of some risk of skin carcinogenesis in AD patients undergoing prolonged treatment with tacrolimus.

CONCLUSIONS

Caution and careful surveillance are required with regard to skin lesions in patients treated with tacrolimus for prolonged periods.

Effects of dietary N-(4-hydroxyphenyl)retinamide on N-nitrosomethylbenzylamine metabolism and esophageal tumorigenesis in the Fischer 344 rat

BACKGROUND

9-cis-Retinoic acid (9-cis-RA) and N-(4-hydroxyphenyl)retinamide (4-HPR) are effective chemopreventive agents against epithelial tumors in the oral cavity, breast, and prostate. We tested the inhibitory activity of these retinoids against N-nitrosomethylbenzylamine (NMBA)-induced tumorigenesis in the rat esophagus.

METHODS

Male Fischer 344 rats were randomly assigned to receive diets either lacking or containing 9-cis-RA or 4-HPR for 1 week before tumor initiation with NMBA and then for the duration of the study. NMBA metabolism, O(6)-methylguanine adduct formation, and cytochrome P450 messenger RNA (mRNA) expression in the esophagi of the rats were studied to investigate the mechanisms by which dietary 4-HPR affects tumorigenesis. All statistical tests were two-sided.

RESULTS

Dietary 4-HPR resulted in a dose-dependent and statistically significant enhancement (P<.05) of tumorigenesis in response to NMBA. In two different tumor bioassays, the mean tumor multiplicity for rats fed the highest concentration of dietary 4-HPR (0.8 g/kg diet) was increased by 5.9 tumors (95% confidence interval [CI] = 1.7 to 10.1 tumors) and 6.7 tumors (95% CI = 5.6 to 7.8 tumors) compared with the mean tumor multiplicity for rats that received the control diet lacking 4-HPR. Animals fed diets containing 9-cis-RA displayed no statistically significant increase in tumorigenesis. Compared with animals fed a diet lacking 4-HPR, animals fed 4-HPR had increased NMBA metabolism in esophageal explant cultures and had higher levels of O(6)-methylguanine DNA adducts and CYP2A3 mRNA in their esophagi.

CONCLUSIONS

Dietary 4-HPR enhances tumorigenesis in response to NMBA in the rat esophagus by increasing tumor initiation events. Dietary 4-HPR may exert paradoxical effects at some sites, such as the aerodigestive tract, by modulating the bioactivation of carcinogens in target tissues.

A 60-Hz magnetic field increases the incidence of squamous cell carcinomas in mice previously exposed to chemical carcinogens

The exposure of tumor-bearing SENCAR mice to a magnetic field of 2 mT at a frequency of 60 Hz for 52 weeks, was found to increase the rate of malignant conversion in chemically-induced dorsal skin papillomas. Detailed histopathology revealed the presence of squamous cell papillomas and squamous cell carcinomas in both sham and magnetic field exposed mice at week 52. However, of the nine mice assessed as having squamous cell carcinomas, eight came from the group exposed to magnetic fields, a difference which is statistically significant at P = 0.03.

Multistage carcinogenesis in mouse skin

The mouse skin model of multistage carcinogenesis has for many years provided a conceptual framework for studying carcinogenesis mechanisms and potential means for inhibiting specific stages of carcinogenesis. The process of skin carcinogenesis involves the stepwise accumulation of genetic change ultimately leading to malignancy. Initiation, the first step in multistage skin carcinogenesis involves carcinogen-induced genetic changes. A target gene identified for some skin tumor initiators is c-Ha-ras. The second step, the promotion stage, involves processes whereby initiated cells undergo selective clonal expansion to form visible premalignant lesions termed papillomas. The process of tumor promotion involves the production and maintenance of a specific and chronic hyperplasia characterized by a sustained cellular proliferation of epidermal cells. These changes are believed to result from epigenetic mechanisms such as activation of the cellular receptor, protein kinase C, by some classes of tumor promoters. The progression stage involves the conversion of papillomas to malignant tumors, squamous cell carcinomas. The accumulation of additional genetic changes in cells comprising papillomas has been correlated with tumor progression, including trisomies of chromosomes 6 and 7 and loss of heterozygosity. The current review focuses on the mechanisms involved in multistage skin carcinogenesis, a summary of known inhibitors of specific stages and their proposed mechanisms of action, and the relevance of this model system to human cancer.

Modulation of constitutive cytochrome P-450 expression in vivo and in vitro in murine keratinocytes as a function of differentiation and extracellular Ca2+ concentration

A procedure was developed for the per cell estimation of cytochrome P-450-dependent monooxygenase activities in cultures and whole cell suspensions of murine epidermal keratinocytes (MEKs). Murine keratinocytes cultured in medium containing less than or equal to 0.04 mM Ca2+ can be induced to differentiate by raising medium Ca2+ concentrations to 1.2 mM. The per cell activities of the monooxygenases 7-ethoxyresorufin O-deethylase (7-ER) and 7-ethoxycoumarin O-deethylase (7-EC) were elevated greater than or equal to 2090% and approximately 460%, respectively, within 13-24 hr of Ca2+ shift. These increases could be completely suppressed by supplementation of culture medium with actinomycin D or cycloheximide immediately prior to Ca2+ shift. After prolonged culture in low Ca2+ medium, some MEKs detached from the monolayer. These detached cells had the characteristics of differentiating MEKs but did not have elevated 7-EC or 7-ER activities. Percoll gradient centrifugation of freshly isolated dorsal skin MEKs was used to prepare four subpopulations that differed in their stages of terminal differentiation. 7-EC and 7-ER activities varied among these subpopulations and correlated with the degree of MEK differentiation. Specifically, the lowest and highest per cell activities (greater than 7-fold difference) were in the basal and most differentiated spinous cell populations, respectively. Collectively, the current studies demonstrate that in vivo P-450 activities are markedly different in proliferating and differentiating MEKs and suggest that constitutive P-450 expression may be modulated as a function of changes in Ca2+ concentration that occur during keratinocyte terminal differentiation.

Effects of epidermal growth factor, fibroblast growth factor, retinoic acid and serum on anchorage-dependent and anchorage-independent growth of HRRT cells

The effects of EGF, FGF, RA and serum on anchorage-dependent and anchorage-independent growth of HRRT cells were studied. The five different types of serum tested in the present work induced a dose dependent rise in anchorage-independent growth in aggregates. FCS, SBCS and RS also supported colony formation in soft agar, whereas BS and HS had no significant effect. EGF and FGF stimulated anchorage-dependent growth of HRRT cells in monolayers. The peptide growth factors were also found to induce phenotypic transformation of the nonneoplastic HRRT cells, as measured by anchorage-independent growth in soft agar as well as in aggregates. At equimolar concentrations EGF was much more effective than FGF. The stimulating effect of EGF and FGF on cell proliferation in the aggregate form was markedly inhibited by RA. Treatment of HRRT cells with the highest noncytotoxic concentration of RA, 2 x 10(-7) M, reduced the stimulating effect of both growth factors by about 60%.

Topical retinoids: in vivo predictive assays

Vitamin A is important for epithelial cell proliferation and differentiation and therefore has been used as a therapy for a variety of skin diseases. Synthetic vitamin A derivatives, called retinoids, have been used to treat a number of clinical skin conditions, including cystic acne and various forms of ichthyosis, Darier's disease, and psoriasis. Because of the toxicity of existing retinoids, newer vitamin A derivatives have been synthesized and must be evaluated in preclinical assays. Retinoids have profoundly differing biologic properties that can be evaluated selectively by studying their efficacy in different in vivo animal assays. This review will discuss some of the assays that may be useful predictors of retinoid efficacy.

Enhancement of beta-propiolactone tumorigenesis in mouse skin by pretreatment with the anti-inflammatory steroid fluocinolone acetonide

Using a two-step carcinogenesis protocol, SENCAR mice were initiated with 25 micrograms 7,12-dimethylbenz[a] anthracene (DMBA) and were then treated twice weekly with either (a) 0.5 mg beta-propiolactone (BPL) or (b) 1 microgram fluocinolone acetonide (FA) followed in 30 min by 0.5 mg BPL. The tumor incidence for the group receiving FA prior to BPL was significantly greater than for BPL alone (P less than 0.0005). Under these experimental conditions, BPL alone showed neither promoting activity nor complete carcinogenic activity. These results were not anticipated, but the reasons for their occurrence are being explored.

Growth inhibition of transformed cells correlates with their junctional communication with normal cells

The growth of various chemically and virally transformed cell types in culture is inhibited when they are in contact with normal cell types. We show that this growth inhibition is contingent on the presence of junctional communication between the normal and transformed cells (heterologous communication), as probed with a 443 dalton microinjected fluorescent tracer. In cell combinations where heterologous communication is weak or absent there is no detectable growth inhibition; the inhibition appears when communication is induced by cyclic AMP-dependent phosphorylation, and only then. In cell combinations where heterologous communication is spontaneously strong, the growth inhibition is present, but it is abolished when the communication is blocked by retinol or retinoic acid. The cell-to-cell membrane channels of gap junctions are the likely conduits of the signals for this growth control.

Hepatic ornithine decarboxylase activity in the rat as influenced by retinyl acetate and ethanol or tryptophan

Ethanol and tryptophan have been demonstrated earlier to induce a rapid stimulation of hepatic ornithine decarboxylase (ODC) activity in overnight-fasted rats. In this study the effect of the administration of retinyl acetate prior to administering ethanol or tryptophan was investigated. The levels of ODC activity in the livers of control and experimental rats were assayed in vitro by measuring the release of 14CO2 from DL-[1-14C]ornithine. Intraperitoneal administration of retinyl acetate (1 microgram/100 g body wt) 1 hr before tube feeding ethanol (0.75 g as a 50% solution/100 g body wt) or L-tryptophan (30 mg in 3 ml water/100 g body wt) and 3 hr before killing caused an enhanced stimulation of hepatic ODC activity compared to that when each agent was administered alone. In vitro [14C]leucine incorporation into protein using hepatic microsomes of tryptophan-treated rats with or without retinyl acetate was increased in comparison with that of controls while decreases were observed when using microsomes of ethanol-treated rats with or without retinyl acetate. Although retinyl acetate has been reported earlier to inhibit the stimulation of hepatic ODC activity due to a variety of agents, including some agents known as carcinogens or promoters, it did not act in this manner against the acute administration of ethanol or tryptophan.

Stimulatory effect of 1 alpha, 25-dihydroxyvitamin D3 on the formation of skin tumors in mice treated chronically with 7,12-dimethylbenz[a]anthracene

The effect of 1 alpha, 25-dihydroxyvitamin D3 (1 alpha, 25-(OH)2D3) and its 24,24-difluoro analog on the formation of skin tumors in mice was evaluated in a complete carcinogenesis model using 7,12-dimethylbenz[a]anthracene (DMBA) as the carcinogen. Twice weekly topical application of 0.25-0.50 nmol of 1 alpha, 25-(OH)2D3 or 0.05-0.10 nmol of the difluoro analog of 1 alpha, 25-(OH)2D3 1 hour prior to treatment with 50 nmol DMBA stimulated tumor formation several fold compared to animals receiving DMBA alone. Topical application of 0.50 nmol of 1 alpha, 25-(OH)2D3 24 hours after treatment with DMBA, or half of this dose of the vitamin D3 metabolite, applied 1 hour before and 24 hours after treatment with DMBA, also stimulated tumor formation several fold. These results are in marked contrast to the potent inhibitory effect of 1 alpha, 25-(OH)2D3 and its difluoro analog on the formation of skin tumors in mice promoted by 12-O-tetradecanoylphorbol-13-acetate.

Clinical implications of current studies in carcinogenesis

Carcinogenesis probably proceeds through a succession of cellular events. Understanding of these events may provide a rational basis for the development of anticarcinogenic treatments. These will be designed to reverse or delay the evolution of a tumor before the stage at which invasion develops. The design and conduct of trials of such agents will be easiest if they are aimed at relatively late stages in the carcinogenic process. Recent research on viral and cellular oncogenes, growth factors, and the cellular mechanism of action of phorbol ester tumor promoters raises the hope that each will be understood and related to the others by their effects on different components of a set of central controls for cellular growth and differentiation. This may ultimately provide a means for rational design of anticarcinogenic treatment. Understanding is still very far from complete, however, and we are still a long way from potential clinical application. An immediate alternative to the long-term rational approach is an empirical one based, for example, on the use of retinoids. The design and interpretation of empirically based trials of anticarcinogenic agents requires careful thought.

Induction of ornithine decarboxylase and augmentation of tyrosine aminotransferase activity by N-hydroxy-2-acetylaminofluorene and 2-acetylaminofluorene in rat liver. Influence of sex, retinylacetate, indomethacin, and pentachlorophenol

IP injection in rats of 2-acetylaminofluorene (AAF) or N-hydroxy-2-acetylaminofluorene (N-OH-AAF) resulted in a transient increase of hepatic ornithine decarboxylase (ODC) and tyrosine aminotransferase (TAT) activity. Maximal activity of ODC was observed 4 hr and of TAT 3 hr after administration of either AAF or N-OH-AAF. A lag-time of 2 hr preceded the increase of ODC and TAT activity. N-OH-AAF dependent ODC induction displayed an almost linear dose-response in the dose range up to 94.1 mumol/kg bw (body weight) when the ODC activity was measured at its maximum 4 hr after administration. Elevation of the dose N-OH-AAF to 126 mol/kg bw resulted in a lower ODC induction. Administration of doses AAF to 31.4 mumol did not change ODC activity. At doses up to 126 mumol/kg bw ODC induction increased linear. TAT induction increased linear in the dose range 15.7-94.1 mumol N-OH-AAF and 31.4-94.1 mumol AAF/kg. Lowering the dose of AAF did not result in a lower ODC or TAT activity. Judged by the effects of actinomycin D or cycloheximide administered 1 hr prior to AAF or N-OH-AAF, the in vivo induction of rat liver ODC activity by AAF and N-OH-AAF appeared to be under transcriptional control, whereas augmentation of TAT activity under influence of AAF or N-OH-AAF appeared the result of (post) translational events. Induction of ODC by AAF or N-OH-AAF was not significantly changed by indomethacin, was slightly increased by pentachlorophenol (PCP) and was synergistically enhanced by retinylacetate (RA). The increase of TAT activity was stimulated by PCP and RA. The effect of PCP indicates that N-sulfonoxy-2-acetylaminofluorene is most probably not involved in the induction of ODC. AAF appeared more effective hepatic ODC inducer in females than males and moreover more effective than N-OH-AAF in females. N-OH-AAF had stronger ODC inducing capacity in males than females. Similar observations were made with respect to TAT activity. When induction of ODC is indicative for a tumor promoting property then the data presented here suggest that tumor promotion of the complete carcinogens AAF and N-OH-AAF is not mediated by N-O-sulfation; this might be due to other metabolic conversions.

Modulation by retinyl acetate of microfilament bundle formation in C3H/10T1/2 cells

Retinyl acetate has been previously shown to inhibit carcinogen-induced neoplastic transformation in 10T1/2 cells and to accentuate many aspects of the nontransformed phenotype. Scanning electron microscopy of logarithmic phase 10T1/2 cells treated for 3 days with 0.3 micrograms/ml retinyl acetate revealed that this treatment caused extensive flattening of cells to the plastic substrate. In contrast the tumor promoter tetradecanoyl phorbol acetate, which antagonizes the antineoplastic activity of retinyl acetate, caused cell rounding and completely inhibited the action of retinyl acetate on cell morphology. During this same time course, the formation of microfilament bundles was also found to be modulated by retinyl acetate. Transmission electron micrographs of unsectioned peripheral regions of flattened cells showed that while the unit density of microfilament bundles was not influenced, the thickness of bundles, particularly those with a diameter of 100 nm or more, was increased by retinyl acetate. Tetradecanoyl phorbol acetate had little effect on microfilament bundle diameters but did partially antagonize the action of retinyl acetate. To determine if this increase was associated with an increase in total actin/cell, total cell proteins, and proteins not extractable by glycerol-triton extraction, were subjected to sodium dodecylsulfate/ polyacrylamide gel electro-phoresis. It was found that while total cellular actin was not increased by retinyl acetate, the proportion of nonextractable actin (which includes microfilament bundles) increased from 65% to 88% of total actin. This increase was not inhibited by inhibitors of protein or RNA synthesis. These studies again demonstrate that retinyl acetate accentuates the nontransformed phenotype of 10T1/2 cells; it is hypothesized that these actions are related to the antineoplastic activity of retinoids.

Monooxygenase activity of human epithelial cells (HeLa): comparison of the response to treatment by arylhydrocarbons and diterpene esters

Arylhydrocarbons are inducers of monooxygenase activity in epithelial cells in culture (Hela) whereas diterpene esters are not. This is one more example of a specific biochemical difference - here in gene activation - exhibited by tumor initiators and tumor promoters of the classes of compounds assayed.

Papillomas induced by initiation-promotion differ from those induced by carcinogen alone

The discovery of a two-stage mechanism of carcinogenesis in mouse skin, with initiation and promotion as independent components, provided new approaches to the study of the development of neoplasms in experimental animals and humans. However, it is not clear how the carcinogen and promoting agent are involved at different steps in the development of papillomas and carcinomas. Here we have used cell markers in mice to study the mechanism of development of papillomas induced by the classical method of two-stage skin tumorigenesis (initiation with subtumorigenic doses of a carcinogen followed by promotion with phorbol esters) and by multiple treatments with initiating doses of a carcinogen. Our results show that papillomas induced by repeated carcinogen applications arise from significantly more cells than those induced by the carcinogen-promoter regimen.

Independent induction and inhibition of ornithine decarboxylase and aryl hydrocarbon hydroxylase activities in rat epidermis

Changes in the activities of ornithine decarboxylase (ODC) and aryl hydrocarbon hydroxylase (AHH) were investigated in rat epidermis after wounding the skin and application of 7,12-dimethylbenz(a)anthracene (DMBA), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and several enzyme inhibitors. Wounding of the skin by vigorous shaving led to a marked induction of ODC activity with a peak at 6 hr. Topical application of a single dose of tetradecanoylphorbol-13-acetate to wounded skin did not affect the activities of ODC and AHH. Application of single large dose (2.5 mg) of DMBA increased AHH activity 7-fold without affecting ODC activity. DL-alpha-difluoromethyl ornithine, a specific irreversible inhibitor of ODC, almost completely abolished ODC activity but did not inhibit DMBA- or TCDD-induced AHH activity. Several potential modifiers, including retinoic acid, indomethacin, 1,3-diamino-2-propranol, alpha-naphthoflavone, and SKF 525 A had unequal effects on ODC and AHH activities. These data indicate that ODC and AHH induction processes in the epidermis are independent biochemical events that are not causally related.

Retinoic acid promotion of papilloma formation in mouse skin

Retinoic acid is a weak promotor of skin tumorigenesis in Charles River CD-1 mice. Multiple papillomas were seen in 17% of the mice treated 3 times weekly with 5.1 micrograms retinoic acid for 20 weeks after initiation by a single treatment with 50 micrograms 7,12-dimethylbenz[alpha]anthracene (DMBA). These results suggest the necessity of a more thorough evaluation of retinoids as tumor promoters before their serious consideration as anti-cancer agents in man.

Retinoic acid modulation of ultraviolet light-induced epidermal ornithine decarboxylase activity

Irradiation of skin with ultraviolet light of sunburn range (UVB) leads to a large and rapid induction of the polyamine biosynthetic enzyme ornithine decarboxylase in the epidermis. Induction of epidermal ornithine decarboxylase also occurs following application of the tumor promoting agent 12-0-tetradecanoylphorbol-13 acetate and topical retinoic acid is able to block both this ornithine decarboxylase induction and skin tumor promotion. In the studies described below, topical application of retinoic acid to hairless mouse skin leads to a significant inhibition of UVB-induced epidermal ornithine decarboxylase activity. The degree of this inhibition was dependent on the dose, timing, and frequency of the application of retinoic acid. To show significant inhibition of UVB-induced ornithine decarboxylase the retinoic acid had to be applied within 5 hr of UVB irradiation. If retinoic acid treatment was delayed beyond 7 hr following UVB, then no inhibition of UVB-induced ornithine decarboxylase was observed. The quantities of retinoic acid used (1.7 nmol and 3.4 nmol) have been shown effective at inhibiting 12-0-tetradecanoyl phorbol-13 acetate induced ornithine decarboxylase. The results show that these concentrations of topical retinoic acid applied either before or immediately following UVB irradiation reduces the UVB induction of epidermal ornithine decarboxylase. The effect of retinoic acid in these regimens on UVB-induced skin carcinogenesis is currently under study.

Photocarcinogenesis: an overview

This paper reviews factors that have been reported to influence photocarcinogenesis in laboratory animals. Such factors include the sensitivity of the test animals, the amount of the ultraviolet radiation (UVR) delivered, the mode of its delivery, and interactions of other radiations or of chemicals in the process of carcinogenesis. New data are presented in these areas: reduction in the size of each unit dose (and thus an increase in dosing frequency) increases the carcinogenic effectiveness of a given lifetime dose; certain inbred strains of albino hairless mice exhibit heritable differences in their susceptibility; several chemicals are known to enhance photocarcinogenesis, but they appear to have so little in common, either structurally or functionally, that they offer limited guidance about which other compounds may be effective in this way. Prevention of long-term UVR effects on skin is a desirable goal; development of personal UVR dosimeters will aid in defining the quantitative nature of the problem; improved sunscreens should provide the means to achieve significant reduction in the incidence of UVR-induced human skin cancer.