MK2 regulates the early stages of skin tumor promotion

The association between inflammation and tumorigenesis is well recognized. Mitogen-activated protein kinase-activated protein kinase-2 (MK2) is known to play a pivotal role in inflammatory processes. Here, we studied the effect of MK2-deficiency and tumor necrosis factor (TNF)-alpha-deficiency on skin tumor development in mice using the two-stage chemical carcinogenesis model. We found that MK2(-/-) mice developed significantly fewer skin tumors compared with both TNF-alpha(-/-) and wild-type mice when induced by initiation with 7,12-dimethylbenz[a]anthracene (DMBA) and by promotion with 12-O-tetradecanoylphorbol-13-acetate (TPA). The TPA-induced inflammatory response was reduced in both, TNF-alpha(-/-) mice and MK2(-/-) mice, but most pronounced in TNF-alpha(-/-) mice, indicating that a reduced inflammatory response was not the only explanation for the inhibited tumorigenesis seen in MK2(-/-) mice. Interestingly, increased numbers of apoptotic cells were detected in the epidermis of MK2(-/-) mice compared with TNF-alpha(-/-) and wild-type mice, suggesting an additional role of MK2 in the regulation of apoptosis. This was further supported by: (i) increased levels of the tumor suppressor protein p53 in MK2(-/-) mice after DMBA/TPA treatment compared with controls, (ii) reduced phosphorylation (activation) of the negative p53 regulator, murine double minute 2 in MK2(-/-) mouse keratinocytes in vitro and (iii) a significant decrease in the DMBA/TPA induced apoptosis in cultured MK2(-/-) keratinocytes transfected with p53 small interfering RNA. Taken together, these findings demonstrate a dual role of MK2 in the early stages of tumor promotion through regulation of both the inflammatory response and apoptosis of DNA-damaged cells. These results also identify MK2 as a putative target for future skin carcinoma therapy.