MyD88 mediates the decision to die by apoptosis or necroptosis after UV irradiation

Abstract

UV irradiation-induced cellular damage is classically associated with apoptosis and is known to result in systemic immunosuppression. How the decision to undergo apoptosis is made following UV 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 (BMDM) and ex vivo peritoneal macrophages (PM) from MyD88 germline-deficient mice (MyD88−/−) was significantly higher than wild type (WT) PM. UV-induced apoptosis (DNA laddering) in PM and epidermis of MyD88−/− animals versus WT was decreased. In MyD88−/− PM, decreased cleavage of caspase 3, as well as pro-necroptotic protein, RIP1, and a significant increase in transcription and release of proinflammatory TNF-α, suggest that necroptosis, rather than apoptosis, has been initiated. In vivo studies confirm this hypothesis after UV, showing low apoptosis by TUNEL and inflammation in MyD88−/− skin sections. Considering that MyD88 participates in many TLR pathways, BMDM from TLR2−/−, TLR4−/− and WT mice were compared for evidence of UV-induced apoptosis. Only TLR4−/− BMDM and PM had a similar phenotype to MyD88−/−, suggesting that the TLR4– MyD88 axis importantly contributes to cell fate decision. Our study describes a new cellular consequence of MyD88 signaling after UV, and may provide rationale for therapies to mitigate UV-induced immunosuppression.

Ultraviolet Radiation Signaling through TLR4/MyD88 Constrains DNA Repair and Plays a Role in Cutaneous Immunosuppression

“…5C), CBPDs occur in MyD88 2/2 cells at levels similar to those observed in WT PMs after UVR. This supports the idea that preservation of PARP activity in UVR cells that lack an intact MyD88 signaling pathway is responsible for maintenance of DNA repair, as well as enhanced cell survival (23). Using MyD88 knockdown in cells from an XP patient, we find that increased repair of CBPDs in MyD88-deficient cells is also dependent on the presence of functional NER machinery (Fig.…”

“…Apoptotic cell fragments cause immune-suppressive reactions in APCs (35,36), but their role in UVR-induced immune suppression has not been investigated thoroughly. We demonstrated previously that cell death in the skin in vivo, as well as in cultured APCs in vitro, is nonapoptotic; instead, it proceeds by a necroptotic pathway (23). The immunosuppressive effects of UVR have far-reaching effects on the immune system; by allowing cells to die without inflammation, apoptosis caused by signaling through MyD88 may substantially contribute to this impaired immune surveillance.…”

“…Expression of TLR2 and TLR4 also were shown to be increased in epidermal cells following UVR, and an observed increase in immune signaling molecules, such as MAPK and NF-kB, is dependent on this TLR expression (22). Previously, we found that the TLR4/MyD88 signaling pathway is necessary for UV-induced apoptosis and that, without this intact pathway, UVinduced cell death is skewed to an inflammatory, necroptotic cell death (23). We hypothesized that, in addition to TLR4-deficient mice, MyD88-deficient mice are resistant to UV-induced immunosuppression and that this phenotype is a result of increased resolution of UV-induced DNA damage to skin-resident APCs.…”

Regulation of necrotic cell death: p53, PARP1 and cyclophilin D-overlapping pathways of regulated necrosis?

“…The signaling pathway of necroptosis has been reviewed in detail recently[144–146] Briefly, necroptosis can be triggered by death receptors including tumor necrosis factor 1 (TNFR1)[147,148], stimulation of Toll-like receptors (TLR3 or 4)[149,150], signaling through interferon receptors[151], or recognition of intracellular viruses by the protein DAI[152]. These pathways can induce the association of RIPK1 with RIPK3 via receptor-interacting protein–homotypic interacting motif (RHIM) RHIM-RHIM domain interactions and phosphorylation of RIPK3, which leads to aggregation of phosphorylated RIPK3 and phosphorylation of MLKL by RIPK3[153].…”

TLR4 acts as a death receptor for ultraviolet radiation (UVR) through IRAK‐independent and FADD‐dependent pathway in macrophages

“…UV irradiation is a potent immunosuppressant which is, in part, driven by the apoptosis of epidermal cells (EC) . TLR4‐MyD88 signalling is necessary for the observed UV‐induced apoptosis, and that without this intact pathway, UV‐induced cell death is skewed to necroptotic cell death …”