Enhanced Photocarcinogenesis in Interleukin-12–Deficient Mice

Abstract

UV-induced DNA damage is the basis for the development of UV-mediated skin cancer because reduction of DNA damage lowers the risk for photocarcinogenesis. The cytokine interleukin (IL)-12 was shown to exhibit the capacity to reduce UV-induced DNA damage presumably via induction of nucleotide excision repair. Because IL-12 is also produced in the skin, we wondered whether endogenous IL-12 protects from photocarcinogenesis. Therefore, we used knockout mice that lack the IL-12p40 chain and thus do not secrete biologically active IL-12. IL-12p40 knockout (IL-12p40-/-) and wild-type (wt) mice were exposed thrice weekly to UV. Skin biopsies obtained after 6 weeks revealed significantly increased numbers of sunburn cells in IL-12p40-/- mice. Additionally, a higher load of UV-induced pyrimidine dimers could be detected in the skin of UV-exposed IL-12p40-/- mice. Staining of epidermal sheets with an antibody against the tumor suppressor gene p53 revealed a higher number of p53 patches in the skin of IL-12p40-/- mice. After approximately 200 days, first skin tumors developed. Kaplan-Meier analysis indicated a significantly increased probability of tumor development in the IL-12p40-/- mice. In addition, the number of tumors developing in the individual mice was significantly higher in IL-12p40-/- mice than in wt mice. Tumors obtained in IL-12p40-/- mice grew faster than those obtained from wt mice on inoculation into nu/nu mice. This was confirmed in an electrophysiologic assay evaluating the intrinsic invasive potency of tumor cells. Together, these data indicate that IL-12 deficiency is associated with an increased risk to develop UV-induced skin cancer, implying that endogenous IL-12 may protect from photocarcinogenesis.

Interleukin-12 and photocarcinogenesis

“…In this case, the intensity of the dot-blot indicating the presence of genomic DNA containing CPDs, of samples obtained from wild-type mice 24 hours after UV exposure was significantly lower than of samples obtained from wild-type mice immediately after UV exposure. In contrast, the intensity of the dot-blot of samples obtained from IL-12 KO mice 24 hours after UV exposure was not significantly lower than that of samples obtained immediately after UV exposure (Meeran et al, 2006c;Maeda et al, 2006). These observations further support the evidence that wild-type mice are able to rapidly remove or repair UV-induced DNA damage in the form of CPDs whereas IL-12 KO mice are not.…”

“…However, in samples obtained 24 hours after UVB exposure, the numbers of CPD-positive cells were significantly lower in the wild-type mice compared to the number of CPD-positive cells obtained immediately after UV exposure. In contrast, the number of CPDpositive cells in IL-12 KO mice 24 hours after UVB exposure was not significantly lower than the number detected immediately after UV exposure (Meeran et al, 2006a;Meeran et al, 2006b;Meeran et al, 2006c;Maeda et al, 2006). Moreover, the spontaneous DNA repair was greater in wild-type mice than IL-12 KO mice.…”

“…In another experiment, IL-12p40 -/-and their wild-type mice were exposed to UVB three times a week for four weeks, and thereafter mice were sacrificed, skin samples obtained and subjected to immunohistochemical analysis of CPDs. The staining of the nuclei for CPDs was more pronounced in skin of UV-exposed IL-12p40 -/-mice than the skin of wild-type mice (Maeda et al, 2006). This further indicates that the amounts of UV-induced CPDs are higher in the absence of IL-12 presumably due to decreased DNA repair.…”

“…To investigate this feature, tumor cells were isolated from skin tumors developed in IL-12 KO mice and their wild-type counterparts and subjected to analysis of their proliferative capacity. The proliferative capacity of the cultured tumor cells obtained from IL-12 KO mice was significantly higher than that of the cultured tumor cells obtained from the tumors of wild-type mice when analyzed using an MTT assay and colony formation assay (Maeda et al, 2006). In addition, the levels of expression of the proliferationspecific protein, cyclin D1, were higher in the UVB-induced tumors than in age-matched skin samples from unirradiated control mice of the same strain, and the levels of cyclin D1 were markedly higher in the UVB-induced tumors of IL-12 KO mice than the UVB-induced tumors of their wild-type counterparts (Katiyar et al, unpublished data).…”

“…To define the role of IL-12 in photocarcinogenesis, IL-12 KO mice (IL-12p35 -/-or IL-12p40 -/-) and their wild-type counterparts were subjected to a photocarcinogenesis protocol (Meeran et al, 2006c;Maeda et al, 2006). The tumor incidence (percentage of mice with tumors), tumor multiplicity (total number of tumors/group) and tumor size (mean tumor size in mm 3 /tumor in a group of 20 mice) were recorded on weekly basis.…”