Receptor-interacting protein kinase 3 (RIPK3) functions as a key regulator of necroptosis. Here, we report that the RIPK3 expression level is negatively regulated by CHIP (carboxyl terminus of Hsp70-interacting protein; also known as STUB1) E3 ligase-mediated ubiquitylation. Chip(-/-) mouse embryonic fibroblasts and CHIP-depleted L929 and HT-29 cells exhibited higher levels of RIPK3 expression, resulting in increased sensitivity to necroptosis induced by TNF (also known as TNFα). These phenomena are due to the CHIP-mediated ubiquitylation of RIPK3, which leads to its lysosomal degradation. Interestingly, RIPK1 expression is also negatively regulated by CHIP-mediated ubiquitylation, validating the major role of CHIP in necrosome formation and sensitivity to TNF-mediated necroptosis. Chip(-/-) mice (C57BL/6) exhibit inflammation in the thymus and massive cell death and disintegration in the small intestinal tract, and die within a few weeks after birth. These phenotypes are rescued by crossing with Ripk3(-/-) mice. These results imply that CHIP is a bona fide negative regulator of the RIPK1-RIPK3 necrosome formation leading to desensitization of TNF-mediated necroptosis.

Propionibacterium acnes (P. acnes) has been known to produce various exogenous proteases, however, their role in acne pathogenesis remains largely unknown. Proteases elicit cellular responses, at least in part, via proteinase-activated receptor-2 (PAR-2), which is known to mediate inflammation and immune response. In this study, we investigated whether proteases from P. acnes could activate PAR-2 on keratinocytes and induce pro-inflammatory cytokines, antimicrobial peptides (AMPs), and matrix metalloproteinases (MMPs) via PAR-2 signaling. We examined PAR-2 expression and protease activity in acne lesions using immunofluorescence staining and in situ zymography. The effect of the culture supernatant of P. acnes on Ca2+ signaling in immortalized keratinocytes (HaCaT) was measured using a fluorescence method. HaCaT cells were treated with P. acnes strain ATCC 6919 culture supernatant, with or without pretreatment with serine protease inhibitor or selective PAR-2 antagonist and the gene expression of pro-inflammatory cytokines, AMPs, and MMPs was detected using real-time reverse transcription-polymerase chain reaction. We found that the protease activity and PAR-2 expression were increased in acne lesions. The P. acnes culture supernatant induced calcium signaling in keratinocytes via PAR-2 and stimulated the mRNA expression of interleukin (IL)-1α, -8, tumor necrosis factor (TNF)-α, human beta defensin (hBD)-2, LL-37, MMP-1, -2, -3, -9, and -13 in keratinocytes, which was significantly inhibited by serine protease inhibitor as well as selective PAR-2 specific antagonist. These results indicate that PAR-2 plays an important role in the pathogenesis of acne by inducing inflammatory mediators in response to proteases secreted from P. acnes.

Fas-associated protein with death domain (FADD) is a pivotal component of death receptormediated extrinsic apoptosis and necroptosis. Here we show that FADD is regulated by makorin Ring Finger Protein 1 (mKRn1) E3 ligase-mediated ubiquitination and proteasomal degradation. mKRn1 knockdown results in FADD protein stabilization and formation of the rapid deathinducing signalling complex, which causes hypersensitivity to extrinsic apoptosis by facilitating caspase-8 and caspase-3 cleavage in response to death signals. We also show that mKRn1 and FADD are involved in the regulation of necrosome formation and necroptosis upon caspase inhibition. Downregulation of mKRn1 results in severe defects of tumour growth upon tumour necrosis factor-related apoptosis-inducing ligand treatment in a xenograft model using mDAmB-231 breast cancer cells. suppression of tumour growth by mKRn1 depletion is relieved by simultaneous FADD knockdown. our data reveal a novel mechanism by which fas-associated protein with death domain is regulated via an ubiquitination-induced degradation pathway. A poptosis, or programmed cell death, can be initiated by intrinsic or extrinsic pathways 1,2 . Intrinsic pathways are triggered by mitochondrial permeabilization, which results in the release of cytochrome c and the formation of an apoptosome complex, leading to caspase-9 activation 3 . Extrinsic pathways can be induced by activation of death receptors (DRs), such as TNFR1, tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) receptors and Fas/CD95 by their corresponding ligands; receptor activation triggers the formation of the death-inducing signalling complex (DISC) or TNFR complex II, which comprises caspase-8 and fas-associated protein with death domain (FADD). These complexes ultimately activate caspase-8, thereby causing extrinsic apoptosis 3,4 . In addition to inducing extrinsic apoptosis, caspase-8 and FADD are known to suppress necroptosis under various conditions [5][6][7][8][9][10][11] .DR downstream pathways are regulated in various ways. For example, cFLIP (cellular FLICE inhibitory protein) competes with caspase-8 for binding to FADD, preventing DISC formation 12 . The levels and activities of cFLIP are controlled by numerous factors including NF-κB, Akt, JNK, Srk and ITCH [13][14][15][16][17] . CARP-1 and -2, which are E3 ubiquitin ligases for caspase-8, are also known to suppress TRAIL activation 18 . Post-translational modifications such as O-glycosylation, S-nitrosylation and S-palmitoylation of DRs have also been identified to regulate death signalling 19 . However, phosphorylation of FADD, the only post-translational modification that FADD is known to undergo, is not involved in the induction of apoptosis. Rather, this modification seems to be essential for the pro-survival roles of nuclear-localized phospho-FADD [20][21][22][23] . Other post-translational modifications affecting the apoptotic and necroptotic activities of FADD have yet to be identified.Here, we show that Makorin Ring Finger Protein 1 (MKRN1), an...

Necroptosis contributes to the pathophysiology of several inflammatory, infectious and degenerative disorders. TNF-induced necroptosis involves activation of the receptor-interacting protein kinases 1 and 3 (RIPK1/3) in a necrosome complex, eventually leading to the phosphorylation and relocation of mixed lineage kinase domain like protein (MLKL). Using a high-content screening of small compounds and FDA-approved drug libraries, we identified the anti-cancer drug Sorafenib tosylate as a potent inhibitor of TNF-dependent necroptosis. Interestingly, Sorafenib has a dual activity spectrum depending on its concentration. In murine and human cell lines it induces cell death, while at lower concentrations it inhibits necroptosis, without affecting NF-κB activation. Pull down experiments with biotinylated Sorafenib show that it binds independently RIPK1, RIPK3 and MLKL. Moreover, it inhibits RIPK1 and RIPK3 kinase activity. In vivo Sorafenib protects against TNF-induced systemic inflammatory response syndrome (SIRS) and renal ischemia–reperfusion injury (IRI). Altogether, we show that Sorafenib can, next to the reported Braf/Mek/Erk and VEGFR pathways, also target the necroptotic pathway and that it can protect in an acute inflammatory RIPK1/3-mediated pathology.