In the present study, we report that mice given a high-fat diet rich in omega-3 fatty acid (HFFO) during UVB exposure had decreased tumor incidence, decreased the number of tumors per mouse and decreased tumor volume per mouse when compared with mice given the same amount of calories, protein, vitamins, minerals and fiber in a high-fat diet rich in omega-6 fatty acid (HFML). As we expected, there were no differences in body weight between these two groups of mice from the beginning to the end of the studies and there was also no effect on the weight of the parametrial fat pads and the thickness of the dermal fat layer between these two groups. These results suggest that in addition to a high amount of tissue fat, tissue fat composition may play an important role in UVB-induced skin carcinogenesis. Although our present study did not support this hypothesis, research data reported that dietary omega-3 and omega-6 fatty acid sources result in marked differences in epidermal cell membrane composition (17
). In future studies, we will analyze the tissue fat composition in the animals fed these two different high-fat diets. We will determine the effects of oral administration of caffeine or voluntary exercise on UVB-induced skin carcinogenesis in animals fed the HFFO diet. We expect these combination treatments would further modulate tissue fat content and synergistically decrease the tumor formation.
The first indication that dietary fat could influence ultraviolet radiation-induced skin carcinogenesis was demonstrated by Baumann and Rusch early in 1939 when they observed that animals fed high levels of fat formed UV-induced tumors more rapidly than animals fed low-fat diets (18
). Since then, a series of studies conducted mainly by Black et al.
) were begun. They observed that the degree of saturation of dietary lipids markedly influenced the UV-carcinogenic response (19
). This group and others reported an approximate linear relationship between omega-6 fatty acids intake and UV-carcinogenic expression (19
). Black et al.
) found that diets containing low-fat levels of 4% menhaden oil resulted in significantly longer tumor latent periods and lower tumor multiplicities when compared with 4% corn oil-fed animals. Our present studies with two types of high-fat diets containing 20% mixed lipids rich in omega-6 fatty acids or 10% mixed lipids plus 10% menhaden oil rich in omega-3 fatty acids provide additional data for a relationship between omega-3 or omega-6 fatty acid consumption and skin carcinogenesis in animals.
Although it has been reported that dietary omega-3 fatty acids show promise as photoprotective agents, the mechanism, especially in animal models, is not fully understood. A number of studies have focused on the effects of dietary omega-3 fatty acids to reduce the inflammatory response when compared with an equivalent dietary level of omega-6 fatty acids. It was reported that dietary omega-3 fatty acid content has a pronounced effect on prostaglandin (PGE2) levels. After 2 weeks on the respective diets, plasma PGE2 levels of 4 or 12% corn oil-fed animals were ~6-fold greater than those of 4 or 12% menhaden oil-fed groups (23
). In the animals fed 12% menhaden oil, both the UV-induced ornithine decarboxylase activity and the inflammatory response (erythema and edema) were dramatically inhibited when compared with a 12% corn oil diet (23
). A similar response was found in the dermis (23
). However, little information has been known about the effects of dietary omega-3 fatty acids to modulate the expressions of inflammatory cytokines by protein array. Our results demonstrate that after 20 weeks of UVB treatment, the HFFO diet significantly decreased the UVB-induced increases in the numbers and levels of inflammatory cytokines compared with HFML diet, especially decreased the levels of TIMP-1, LIX, sTNF RI and MIP-1γ in the epidermis. Our in vivo
study showed that high levels of TIMP-1, LIX, sTNF RI and MIP-1γ were found in tumors but not in the normal epidermis or in the normal epidermis away from the tumors. These facts suggest that these cytokines may be related to UVB-induced skin tumor formation. The present results suggest that omega-3 fatty acids present in HFFO diet may protect against UVB-induced skin carcinogenesis by decreasing proinflammatory cytokines production. It should be excluded that other ingredients in this diet may also play a role. These findings provide biomarker candidates for future mechanistic studies with these diets. We plan to confirm these findings by western blot analysis and immunohistochemical methods with the appropriate antibodies.
It was reported that TIMP-1, LIX, sTNF RI and MIP-1γ are all proinflammatory cytokines. The TIMP-1 protein is able to promote cell proliferation in a wide range of cell types and may have an antiapoptotic function. TIMP-1 plays a significant role in regulation of extracellular matrix remodeling and angiogenesis in cancers (24
). TIMP-1 concentrations were significantly increased in plasma obtained from patients with colon or rectal cancer (26
). TIMP-1 and TIMP-2 may play an important role in the pathogenesis of non-melanoma skin cancer and their expression levels are useful indicators of cutaneous cancer invasion and progression (27
). LIX has potent chemoattractant activity for neutrophils in vitro
and in vivo
and amplifies a proinflammatory cytokine response via a phosphatidylinositol 3-kinase–nuclear factor-kappaB pathway (28
). Tumor necrosis factor -α is one of the most potent proinflammatory cytokines produced by activated macrophages in response to tissue injury or chronic inflammation. Its production leads to the shedding of soluble tumor necrosis factor receptors (sTNF RI and sTNF RII) from cell membranes into the circulation. Through its proinflammatory actions, TNF-α may play a role in cancer growth and metastasis by inducing reactive oxygen species, which can cause DNA damage and inhibit DNA repair (29
). An increase in the serum level of sTNF R1 was significantly associated with a higher risk of endometrial cancer and the severity of development of multiple inflammatory-related symptoms in patients for colorectal and esophageal cancer (29
). MIP-1γ activates human granulocytes and lead to acute neutrophilic inflammation. MIP-1γ also induces the synthesis and release of other proinflammatory cytokines such as IL-1, IL-6 and TNF-α from fibroblast and macrophages. Our present studies provided evidences suggesting that TIMP-1, LIX, sTNF RI and MIP-1γ play an important role in UVB-induced skin carcinogenesis.
Although it has been demonstrated that dietary omega-3 fatty acids reduced an inflammatory response, there is little information about the effects of high-fat diets rich in omega-3 or omega-6 fatty acids on UVB-induced apoptosis in the epidermis of mice. Our previous studies showed that the mechanism of caffeine or voluntary exercise to inhibit UVB-induced skin carcinogenesis may be caused by stimulation of UVB-induced epidermal apoptosis. Our present study reported that mice fed HFFO diet rich in omega-3 fatty acid for 4 weeks also stimulated UVB-induced increases in apoptosis in the epidermis of mice compared with the HFML diet rich in omega-6 fatty acid. It is probably that these effects may contribute, at least in part, to the inhibitory effect of HFFO diet on UVB-induced carcinogenesis. The results of our studies are important since the relationship between skin carcinogenesis and increased apoptosis by HFFO diet has not been studied previously. In many situations, omega-3 fatty acids act as competitive antagonists for omega-6 fatty acids. Omega-3 fatty acids reportedly modulate oxidative stress and the highly unsaturated omega-3 fatty acids may become targets for free radical attack, resulting in the production of oxidation products and protection against UVB-induced oxidative injury (30
). In addition, dietary omega-3 and omega-6 fatty acid sources result in marked differences in epidermal cells membrane composition (17
). These altered membrane composition might be a possible mechanism through which dietary omega-3 fatty acids increases UVB-induced apoptosis in the epidermis. Furthermore, mechanistic studies are needed to confirm these hypotheses.
In summary, our results suggest that a HFFO diet rich in omega-3 fatty acids inhibits UVB-induced skin carcinogenesis in mice continually exposed to UVB. The HFFO diet decreased tumor incidence, decreased tumor number and decreased tumor size compared with animals fed an HFML premix diet rich in omega-6 fatty acids. The HFFO diet decreased the UVB-induced increases in the levels of TIMP-1, LIX and sTNF R1 as well as other several proinflammatory cytokines and stimulated UVB-induced apoptosis in the epidermis. Our results indicate that omega-3 fatty acids in an HFFO diet have beneficial effects against UVB-induced skin carcinogenesis in mice and these effects may be associated with an inhibition on UVB-induced inflammatory response.