Homer S. Black

POTENTIAL INVOLVEMENT OF FREE RADICAL REACTIONS IN ULTRAVIOLET LIGHT‐MEDIATED CUTANEOUS DAMAGE*

Free radicals are chemical species characterized by an odd number of orbital electrons or by pairs of electrons of similar directional spin isolated singly in separate orbitals. Consequently most of these agents are highly reactive and usually exhibit an extremely short half‐life, although due to steric and resonance effects some exceptions occur. Some radicals and their precursors, such as the diradical O2 which exists in the triplet state, represent a critical and essential element of normal metabolism of aerobic organisms where, under normal circumstances, controlled reduction of reactive oxygen species occurs via the cytochrome oxidase or cytochrome P‐450 mixed function monooxygenase systems. In addition to reactive oxygen species, organisms may be subjected to a wide‐range of other free radicals or their precursors, including those of both exogenous and endogenous origin. Elaborate defense mechanisms have evolved to avoid cellular damage from these highly reactive species. Enzymes, such as the superoxide dismutase, the glutathione peroxidase/reductase system, and catalase; interactions with conjugated diene systems such as those found in melanins, carotenoids, and tocopherols; and direct reduction by sulphydryl compounds, phenols, and purines represent but a few of these natural defense systems.

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.

Effect of a Low-Fat Diet on the Incidence of Actinic Keratosis

BACKGROUND Actinic keratoses are premalignant lesions and are a sensitive and important manifestation of sun-induced skin damage. Studies in animals have shown that dietary fat influences the incidence of sun-induced skin cancer, but the effect of diet on the incidence of actinic keratosis in humans is not known. METHODS We randomly assigned 76 patients with nonmelanoma skin cancer either to continue their usual diet (control group) or to eat a diet with 20 percent of total caloric intake as fat (dietary-intervention group). For 24 months, the patients were examined for the presence of new actinic keratoses by physicians unaware of their assigned diets. RESULTS At base line, the mean (+/- SD) percentage of caloric intake as fat was 40 +/- 4 percent in the control group and 39 +/- 3 percent in the dietary-intervention group. After 4 months of dietary therapy the percentage of calories as fat had decreased to 21 percent in the dietary-intervention group, and it remained below this level throughout the 24-month study period. The percentage of calories as fat in the control group did not fall below 36 percent at any time. The cumulative number of new actinic keratoses per patient from months 4 through 24 was 10 +/- 13 in the control group and 3 +/- 7 in the dietary-intervention group (P = 0.001). CONCLUSIONS In patients with a history of nonmelanoma skin cancer, a low-fat diet reduces the incidence of actinic keratosis.

INFLUENCE OF DIETARY OMEGA-6, -3 FATTY ACID SOURCES ON THE INITIATION AND PROMOTION STAGES OF PHOTOCARCINOGENESIS

To determine the segment along the carcinogenic continuum at which dietary lipid exerts its principal effect, six groups of 35 Skh‐HR‐1 hairless mice were placed on defined isocaloric diets containing either 0.75%, 12% corn oil or 12% menhaden oil as sources of omega‐6 or omega‐3 fatty acids, respectively. All animals received an 11 week course of UV‐radiation from fluorescent sunlamps. Upon termination of UV, diets of some groups were crossed‐over to either low fat, high fat, omega‐6 or omega‐3 fatty acid sources. The first tumor appeared at week 14. Life‐table analysis of the tumor incidence curves and Wilcoxon tests of tumor multiplicity provided evidence that high corn oil diets significantly (P < 0.01) enhance carcinogenic expression; that tumor enhancement by the omega‐6 fatty acid source occurs during the post‐initiation, or promotion, stage; that replacement with a low corn oil diet after UV‐initiation will negate the exacerbating effect of high corn oil; and that an omega‐3 fatty acid source inhibits UV‐carcinogenesis even at high dietary levels, although not during the post‐initiation stage.

INFLUENCE OF DIETARY MENHADEN OIL UPON CARCINOGENESIS AND VARIOUS CUTANEOUS REspONSES TO ULTRAVIOLET RADIATION

Abstract Recent reports suggest that eicosapentaenoic acid (EPA) inhibits growth of transplanted tumors and the formation of various chemically induced cancers in animals. Menhaden oil is a source of polyunsaturated lipid with a high content of EPA, an omega‐3 fatty acid. We sought to explore the effects of menhaden oil on photocarcinogenesis by employing the hairless mouse/UV‐carcinogenesis model. Five groups of 40 SKH‐Hr‐1 mice received (a) a semipurined equicaloric diet containing either 0.75% corn oil, 4% corn oil. 4% menhaden oil or 12% menhaden oil and (b) an escalating regimen of UV radiation to a cumulative dose of 70 J/cm2. Additional animals were employed to further examine the role of menhaden oil in acute cutaneous responses to UV, i.e., erythema, edema, and ornithine decarboxylase (ODC) induction. After a 2‐week feeding period UV‐induced ODC activity in mice receiving 12 or 20% menhaden oil was 3 to 13‐fold lower than that of corn oil fed animals. Further studies showed that edema was also markedly decreased. Animals receiving menhaden oil required twice the level of irradiance to evoke a comparable erythema as that which occurred in corn oil fed animals.

MODIFICATION OF MEMBRANE COMPOSITION, EICOSANOID METABOLISM, AND IMMUNORESPONSIVENESS BY DIETARY OMEGA‐3 AND OMEGA‐6 FATTY ACID SOURCES, MODULATORS OF ULTRAVIOLET‐CARCINOGENESIS

Abstract–Dietary sources of lipids containing predominantly n‐3 or n‐6 fatty acids (FA) have been examined for effect upon several potential pathophysiologic parameters. Epidermal, plasma, and red blood cell (RBC) membrane FA composition exhibited marked differences between animals fed the respective dietary lipid sources. Reduced levels of 18:1, 20:3 and 20:4 occurred in the n‐3 FA fed animals which exhibited significantly higher levels of 20:5 and 22:6. Approximately equal levels of 18:2 were present in animals fed either diet. Despite marked differences in RBC membrane FA composition, only marginal effect upon osmotic fragility occurred. Lower levels of 20:3 and 20:4 found in n‐3 fed animals could result from a deficit of elongase and/or A5‐desaturase activity. Whether lower 20:4 levels in n‐3 fed animals could rate‐limit eicosanoid metabolism is unknown, but epidermal capacity to metabolise arachidonic acid in these animals was found to be closely related to n‐6 FA intake. Animals fed n‐3 FA exhibited markedly lower levels of plasma PGE2, even when the diet was supplemented with n‐6 FA. In addition, UV‐radiated animals receiving the n‐3 FA source demonstrated a reduced (30%) response to inflammatory stimulus and a greater (4.5‐fold) delayed hypersensitivity (DH) to dinitrochlorobenzene than animals fed the n‐6 FA source. These data demonstrate that dietary lipid strongly influences tissue FA composition, eicosanoid metabolism, and, in the case of DH, at least one type of T‐cell mediated immune response in UV‐irradiated animals.

Lycopene and lutein inhibit proliferation in rat prostate carcinoma cells.

Abstract Consumption of lycopene, a carotenoid without provitamin A activity, has been associated with a lower risk of prostate and breast cancer. Lutein is another carotenoid that may be associated with a reduced risk of age-related macular degeneration, the leading cause of blindness in adults 65 years of age and older. Bioactive compounds such as lycopene and lutein, derived from natural plant sources, have been shown to act at low substrate levels through the action of intrinsic cytokines and growth factors and their receptors within tissues, particularly those of the fibroblast growth factor and transforming growth factor β families. The effects of grapefruit-derived and commercial lycopene and lutein preparations on androgen independent cultured malignant type II tumor cells [Dunning R3327AT3 or AT3 cells (androgen-responsive, slow-growing tumor cells with well developed epithelium and stroma)] were compared to their benign parent type I tumor epithelial cells (DTE). Results demonstrated that both lycopene, in an α -cyclodextrin water soluble carrier, and lutein inhibited malignant AT3 cells in a concentration and time-dependent manner. No such effect was observed when benign DTE cells were examined, demonstrating selective inhibition of extremely malignant AT3 prostate cancer cells relative to their benign parent. Lutein demonstrated a similar but slightly diminished response as lycopene. When cells were treated with cocktails of lycopene and lutein, no synergistic or additive effect occurred. These studies are consistent with epidemiological studies that show inverse relationships of these carotenoids with prostate cancer.

EXPERIMENTAL ULTRAVIOLET LIGHT‐CARCINOGENESIS

That some skin cancers in humans are a consequence of exposure to the UV portion of the solar emission spectrum reaching the earth is now well accepted. This premise arose from earlier clinical observations and is strongly substantiated by more recent epidemiologic studies. The evidence for such a relationship, as well as other aspects of both acute and chronic UVL damage to skin, has been reviewed (Blum, 1959; Urbach et al., 1972; Urbach, 1975; Emmett, 1973; Daniels, 1975). Because of the obvious impropriety required of human experimentation to definitively establish the etiology of actinic skin cancer, we must rely upon epidemiologic studies. Here, however, we are concerned primarily with experimental UVG carcinogenesis using animal model systems. It will be necessary, however, to draw upon experience involving humans to form experimental concepte-particularly in the case of the human malady-Xeroderma pigmentosum. We, therefore, attempt to bring the most recent developments into focus and to develop those theories of carcinogenesis that are most pertinent to UVL-induced skin cancer. To do so, the authors have chosen to select what they consider to be the major theories concerning UVLcarcinogenesis and to critically discuss each. Moreover, the organization of our discussion by separation into major theories relating to UVLcarcinogenesis should not imply that we are totally

Influence of dietary lipid upon ultraviolet-light carcinogenesis.

The effects of dietary lipid level, degree of saturation, and antioxidant supplements on ultraviolet-light (UV) carcinogenesis were studied in female albino hairless mice. Twelve groups of 42 animals each received a restricted, semipurified, isocaloric diet containing 4%, 12%, or 12% (60% hydrogenated) corn oil with or without antioxidants (2%, w/w). A regimen of escalating UV irradiation was employed until a cumulative dose of 142 J/cm2 had been delivered. Tumor development time in 50% of the population (TDT50) was derived from a cumulative distribution of time to tumor formation, which was estimated for all groups. Although there were no significant differences in TDT50s between animals receiving low and high unsaturated lipid dietary regimens, animals receiving hydrogenated corn oil demonstrated a significantly (p less than 0.01) greater TDT50 and fewer tumors per animal than those receiving either level of unsaturated corn oil. Antioxidants had no effect on TDT50s within any of the dietary groups. However, greater tumor multiplicity was observed in groups receiving unsaturated lipid and antioxidants. These data demonstrate that the degree of dietary lipid saturation modifies the carcinogenic response to UV and suggest that dietary lipid may modify the previously reported inhibitory effect of antioxidants on UV carcinogenesis. It may be concluded that adherence to dietary standards is as important as other experimental parameters when comparisons of UV effects are involved.

Influence of dietary factors on actinically-induced skin cancer

The first indication that high dietary fat intake could influence the development of ultraviolet (UV) radiation-induced skin cancer in experimental animals was reported in 1939. In the 1980s a series of animal studies showed that a high level of dietary fat intake markedly shortened the time between UV exposure and tumor appearance and increased the number of tumors that developed. Further, high levels of dietary fat affected skin cancer development at the promotional stage of UV-carcinogenesis, i.e., after the cancer causing dose of UV had been delivered. Perhaps more important, switching from a high-fat to a low-fat diet immediately after delivery of the UV-initiating dose negated the exacerbating effect of high fat intake. The latter finding suggested that dietary modification, even after a cancer-causing exposure to UV, might represent a potentially important intervention strategy in the prevention of non-melanoma skin cancer (NMSC) and provided the rationale for undertaking a dietary intervention trial. One hundred and fifteen skin cancer patients completed the 2-year clinical trial on the effect of a low-fat diet on occurrence of actinic keratosis (AK) and NMSC. Patients were randomly assigned to either continue their usual diet (control group, NI) or to adopt a diet with 20% of total caloric intake as fat (diet intervention group, DI). All patients were examined at 4-month intervals for new AK and NMSC. At baseline, the mean percent of caloric intake as fat was 40+/-4% in the NI group and 39+/-3% in the DI group. After 4 months of dietary therapy, the percent calories as fat had decreased to 21+/-7% in the DI group. The percent of calories as fat in the NI group did not drop below 37% during the study period. The cumulative number of new AK per patient from months 4 through 24 was 11.6+/-17 in the NI group and 3.2+/-6 in the DI group (P < 0.001). Numbers of new NMSC were analyzed in 8-month periods. There were no significant changes in NMSC occurrence in the NI group. However, NMSC occurrence in the DI group declined significantly (P < 0.02) in the last 8-month period. Patients in the DI group also had significantly (P < 0.01) fewer NMSC in the last 8-month period than did patients in the NI group (0.02 versus 0.26). Practical dietary advice, with respect to reduction of percent of calories as fat, could make an important contribution to the prevention and management of AK and NMSC.