Sharpin prevents skin inflammation by inhibiting TNFR1-induced keratinocyte apoptosis

Abstract

Linear Ubiquitin chain Assembly Complex (LUBAC) is an E3 ligase complex that generates linear ubiquitin chains and is important for tumour necrosis factor (TNF) signaling activation. Mice lacking Sharpin, a critical subunit of LUBAC, spontaneously develop inflammatory lesions in the skin and other organs. Here we show that TNF receptor 1 (TNFR1)-associated death domain (TRADD)-dependent TNFR1 signaling in epidermal keratinocytes drives skin inflammation in Sharpin-deficient mice. Epidermis-restricted ablation of Fas-associated protein with death domain (FADD) combined with receptor-interacting protein kinase 3 (RIPK3) deficiency fully prevented skin inflammation, while single RIPK3 deficiency only delayed and partly ameliorated lesion development in Sharpin-deficient mice, showing that inflammation is primarily driven by TRADD- and FADD-dependent keratinocyte apoptosis while necroptosis plays a minor role. At the cellular level, Sharpin deficiency sensitized primary murine keratinocytes, human keratinocytes, and mouse embryonic fibroblasts to TNF-induced apoptosis. Depletion of FADD or TRADD in Sharpin-deficient HaCaT cells suppressed TNF-induced apoptosis, indicating the importance of FADD and TRADD in Sharpin-dependent anti-apoptosis signaling in keratinocytes.DOI: http://dx.doi.org/10.7554/eLife.03422.001

The Inflammatory Caspases-1 and -11 Mediate the Pathogenesis of Dermatitis in Sharpin-Deficient Mice

“…This is further supported by the dichotomy in pathways contributing to skin inflammation versus systemic disease. For example, although we demonstrated that deficiency in caspase-1 and -11 imposes drastic effects on skin disease manifestation but has minimal effects on myeloid cell expansion in the spleen or splenic architecture, other investigators recently showed that genetic ablation of Mlkl from cpdm mice attenuated hepatic and splenic inflammation, whereas dermatitis persisted (44). Even diseases that are monogenic in nature can be the product of multiple interacting pathways, complicating therapeutic intervention.…”

“…It was reported recently that dermatitis of cpdm mice is initiated through TNFR1-dependent cell death, because genetic ablation of Tnfr1 from cpdm mice prevented skin disease manifestation up to 35 wk of age (45). However, although it is agreed that TNFinduced cell death is pathogenic in this context, there is a discrepancy about the primary cell death modality that drives cpdm skin pathology (39,44). Because both apoptosis and necroptosis are induced by TNF in cpdm keratinocytes ex vivo (31), it is conceivable that they are both detrimental.…”

“…Interestingly, these paradoxical pro-and anti-inflammatory states were attributed to cell type-specific roles for LUBAC, in which LUBAC deficiency led to hyperinflammation in monocytes but abrogated inflammation in fibroblasts. Cpdm bone marrow does not transfer disease to wild-type mice (44,58), and cpdm skin grafts transplanted onto nude mice retain their diseased state (59), two findings that highlight the potential pathogenic role of skin-resident cells in this context.…”

“…1G shows increased expression and processing of inflammatory caspase-1 and -11 and enhanced processing of IL-18 in the skin, indicative of inflammasome activation. Because LUBAC regulates cell death downstream of TNF signaling, which was implicated in cpdm skin disease pathogenesis (39,44,45), we examined, in parallel, the levels of cell death effectors in the skin of young cpdm mice. The kinases Ripk1 and Ripk3, which are involved in both apoptosis and necroptosis, also were induced in cpdm skin (Fig.…”

Differential Involvement of the Npl4 Zinc Finger Domains of SHARPIN and HOIL-1L in Linear Ubiquitin Chain Assembly Complex-Mediated Cell Death Protection

“…This appears to be a critical step in the activation of the IKK complex, the principal regulator of the NF-B cascade (9)(10)(11)(12). In addition, LUBAC exhibits a novel function to antagonize programmed cell death provoked by various stimuli, including tumor necrosis factor receptor (TNFR) signaling, although the precise molecular mechanisms remain unknown (13)(14)(15)(16)(17)(18).…”