TLR4/MyD88 Signaling Activates Ultraviolet Irradiation-Induced Apoptosis: Outcomes and Consequences

Abstract

Ultraviolet (UV) irradiation induces DNA damage leading to the accumulation of mutations in epidermal keratinocytes (KC), and immunosuppression, which contribute to development of non-melanoma skin cancers. How the decision to undergo apoptosis is made following UV exposure is not fully understood. We hypothesize that a central mediator of TLR signaling, MyD88, determines cell fate after UV exposure. Survival after UV of immortalized bone marrow-derived macrophages (BMM) and ex vivo peritoneal macrophages (PM) from MyD88 germline-deficient mice (MyD88<super>-/-</super>) was significantly higher vs. wild-type (WT) PM. UV-induced apoptosis in PM and epidermis of MyD88<super>-/-</super> animals was decreased vs. WT. In MyD88<super>-/-</super> PM, decreased cleavage of caspase 3, PARP, and pro-necroptotic protein, RIP1, and a significant increase in pro-inflammatory TNF-α, suggest that necroptosis, rather than apoptosis, was initiated. In vivo studies confirm this hypothesis, showing low apoptosis by TUNEL and enhanced histologic inflammation in MyD88<super>-/-</super> skin sections after UV. Considering that MyD88 participates in most TLR signaling pathways, BMM from TLR2<super>-/-</super>, TLR4<super>-/-</super>, TRAM/TRIF<super>-/-</super>, and WT mice were compared for evidence of UV-induced apoptosis. Only TLR4<super>-/-</super> BMM and PM had a similar phenotype to MyD88<super>-/-</super>, suggesting that the TLR4-MyD88 axis importantly contributes to cell fate decision. We then sought to determine how alteration of this pathway affects the UV-induced damage to critical epidermal cells. In the DNFB hypersensitization model, UV-irradiated, MyD88<super>-/-</super> mice had intact ear swelling, normal IFN-γ production by T lymphocytes, and higher levels of DNFB-specific IgG2a compared to WT, which were suppressed by UV. The UV-induced emigration of antigen presenting cells from the epidermis was maintained in MyD88<super>-/-</super> mice, yet UV-induced DNA damage in the local lymph nodes was less pronounced. We then hypothesized that the aforemention preservation of PARP promotes more efficient DNA damage recognition and repair. In support of this hypothesis, human primary KC treated with MyD88 siRNA and epidermal DNA from MyD88<super>-/-</super> and TLR4<super>-/-</super> mice that were UV-irradiated had an increased resolution rate of cyclobutane pyrimidine dimers (CBPD), which can be attenuated by treatment with a PARP-inhibitor. This work provides a strong rationale for future development of topical TLR4 modulating therapies to reduce the risk of UV-induced carcinogenesis.