The UV radiation in sunlight is the primary cause of skin cancer (1
). In addition, UV radiation is immunosuppressive, and the immune suppression induced by UV radiation is a major risk factor for skin cancer induction (7
). Because we previously demonstrated that PAF and 5-HT2A
receptor binding plays an essential role in UV-induced immune suppression (11
), we tested the hypothesis that PAF and 5-HT2A
receptor antagonists could block UV-induced photocarcinogenesis. Here we show that PAF and 5-HT2A
receptor antagonists block skin cancer induction and progression. In addition, we demonstrate that injecting PAF and 5-HT2A
receptor antagonists into mice treated with a single dose of UV radiation, blocked UV-induced skin damage, including, sunburn cell formation, apoptosis, and cytokine secretion. These data, and our previous findings, indicating that PAF and 5-HT2A
receptor antagonists block UV-induced immune suppression (11
), indicate that these reagents are working at multiple levels to block photocarcinogenesis.
In the carcinogenesis studies, we tested the effect of PAF and 5-HT2A
receptor antagonists on both tumor induction and tumor progression. In both cases, the use of multiple, structurally unrelated PAF and serotonin receptor antagonists blocked carcinogenesis. The data presented in is particularly important because it more closely mimics the human model of skin cancer treatment. Generally, a patient comes into the skin cancer clinic with a obvious lesion, it is removed, and the patient is advised to stay out of the sun, apply sunscreen when he/she does go outside, and if available, use some type of chemopreventive agent to prevent the development of a second skin cancer. Our experiment was designed to mimic this situation. Once a papilloma was evident, the UV exposure was halted, and the mice were injected with the PAF or the 5-HT2A
receptor antagonists. We noted significant suppression in the development of second tumors. The number of tumors generated in this experiment is somewhat lower than that reported by others (19
). Two reasons may explain this fact. First, we used a sub-erythemal UV dose, one-half the minimal erythemal dose, in these experiments. Here again, this was done to more closely model human exposure to sunlight, which generally does not include multiple successive sun burning episodes (i.e., Monday, Wednesday, Friday) over a span of 25 to 35 weeks. Second, in our analysis we only counted skin cancers (squamous cell carcinoma) and not papillomas, as is often done in other experiments of this type. Because not every papilloma progresses into a skin cancer, and it is common to find regression of papillomas, even in the presence of continual UV exposure, we only counted tumors in our experiments.
The data presented in indicates that the PAF and 5-HT2A
receptor antagonists work synergistically to block skin cancer induction. In this experiment, the mice were injected with 500 pmol of the receptor antagonists, rather than the normal 500 to 1000 nmol dose. Dose response experiments carried out in the past indicated that injecting 500 pmol of either PAF or 5-HT2A
receptor antagonists had no effect in UV-induced immune suppression (11
). We noted the same here, injecting the mice with 500 pmol of ketanserin or CV-3988 did not prevent skin cancer induction. When however, the mice were injected with a cocktail of both drugs (500 pmol of each), we noted substantial suppression of skin cancer induction, indicating that the drugs are working synergistically. The mechanism is not known, but we suspect that UV-induced cis
-UCA is binding to a cell in the skin, perhaps a dermal mast cell, causing it to secrete PAF, which induces COX-2 up-regulation and PGE2
secretion, leading to immune suppression (25
) and photocarcinogenesis (18
). Studies are in progress to test this hypothesis.
Our data suggests that one mechanism by which 5-HT2A
and PAF receptor antagonists block UV-induced skin cancer induction is by preventing the initial damage to the skin. We used an acute exposure to UV radiation in hairless mice, with a relatively low dose of UVB radiation, to examine the effects of the receptor antagonists on sunburn cell formation, apoptosis, and cytokine production. In all cases treating the mice with the 5-HT2A
and/or the PAF receptor antagonists prevented UV-induced skin damage. Our finding that PAF receptor binding plays a role in UV-induced apoptosis confirms and extends previous reports (26
). Similarly, studies with cardiac and neuronal tissues have documented a role for 5-HT2A
receptor binding in apoptosis (28
). Our data is the first to implicate 5-HT2A
receptor binding in apoptosis induction in the skin after exposure to the common environmental carcinogen, UV radiation.
The reversal of apoptosis in our system may at first appear to be counter-intuitive, in that drugs that prevent apoptosis also block skin cancer induction. Note however, that we measured the effect these drugs have on UV-induced skin damage after a single acute exposure to a subcarcinogenic dose of UV radiation. We suggest that the effect here may be similar to what has been reported regarding the up-regulation of the tumor suppressor gene, p53 after UV exposure (22
). Initially, UV-damage up-regulates normal p53 protein, which serves to suppress tumor induction by controlling apoptosis. After continual UV exposure, the gene is mutated and the tumor suppressing function is aborted. We have never looked at the effect of PAF and 5-HT receptor antagonists on apoptosis after multiple UV exposures, so it is impossible to project what would happen. We have only used these findings to conclude that these drugs can block initial UV-induced skin damage. Any other conclusion would be an over interpretation of the data.
In summary, we show that PAF and 5-HT2A receptor antagonists inhibit UV-induced skin cancer induction and progression. In addition, we show that PAF and 5-HT2A receptor antagonists inhibit UV-induced skin damage, in that they prevent cytokine release, and prevent UV-induced apoptosis after a single exposure to UV radiation. These findings suggest that PAF and 5-HT2A receptor antagonists affect UV-induced carcinogenesis at two distinct levels: First by preventing UV-induced damage in the skin. Second, by preventing UV-induced immune suppression. Our findings indicate that PAF and 5-HT2A receptor antagonists maybe considered as novel chemopreventive agents for sunlight-induced skin cancer.