SHARPIN forms a linear ubiquitin ligase complex regulating NF-κB activity and apoptosis

Abstract

SHARPIN is a ubiquitin-binding and ubiquitin-like domain-containing protein which, when mutated in mice, results in immune system disorders and multiorgan inflammation1,2. Here we report that SHARPIN functions as a novel component of the Linear Ubiquitin Chain Assembly Complex (LUBAC) and that the absence of SHARPIN causes disregulation of NF-κB and apoptotic signalling pathways, explaining the severe phenotypes displayed by chronic proliferative dermatitis in SHARPIN deficient mice. Upon binding to the LUBAC subunit HOIP, SHARPIN stimulates the formation of linear ubiquitin chains in vitro and in vivo. Co-expression of SHARPIN and HOIP promotes linear ubiquitylation of NEMO, an adaptor of the IκB kinases (IKKs) and subsequent activation of NF-κB signalling, while SHARPIN deficiency in mice causes an impaired activation of the IKK complex and NF-κB in B cells, macrophages, and mouse embryonic fibroblasts (MEFs). This effect is further enhanced upon concurrent downregulation of HOIL-1L, another HOIP-binding component of LUBAC. In addition, SHARPIN deficiency leads to rapid cell death upon TNFα stimulation via FADD- and Caspase-8-dependent pathways. SHARPIN thus activates NF-κB and inhibits apoptosis via distinct pathways in vivo.

Essential Roles of K63-Linked Polyubiquitin-Binding Proteins TAB2 and TAB3 in B Cell Activation via MAPKs

“…The cpdm mutation of the Sharpin gene (Sharpin cpdm/cpdm ), encoding another component of LUBAC, leads to the impairment in gene induction in response to TNF, CD40L, and IL-1b in MEFs and B cells. Sharpin cpdm/cpdm mice showed impaired activation of macrophages in response to TLR stimulation (33)(34)(35)(36). We also found that macrophages from Sharpin cpdm/cpdm mice showed severe defects in the production of IL-6 and IL- mune responses.…”

The ubiquitin conjugating enzyme UBE2L3 regulates TNFα-induced linear ubiquitination

“…Linear ubiquitin chains stabilize proteins and play critical roles in signaling complex assembly. The linear ubiquitin chain assembly complex (LUBAC) consists of at least 3 proteins: HOIP (also known as RNF31), HOIL-1L (also known as RBCK1), and SHARPIN [1][2][3][4][5][6]. LUBAC is the only E3 ligase known to assemble linear polyubiquitin chains.…”

“…LUBAC regulates TNFα signaling by targeting the regulatory protein, NF-κB essential modulator (NEMO), for nondegradative linear ubiquitination. Depletion of LUBAC components by RNA interference or genetic ablation attenuates cytokine-driven NF-κB signaling [1][2][3][4][5]7].…”

NIK promotes tissue destruction independently of the alternative NF-κB pathway through TNFR1/RIP1-induced apoptosis

“…3,4 The formation of this complex is initiated by the interaction of ligand-bound TNFR1 with TNF receptor-associated death domain (TRADD) and by the recruitment of the kinase receptor interacting protein kinase 1 (RIP1), the adaptor proteins TNF receptor-associated factor 2/5 and the ubiquitin ligase cellular inhibitor of apoptosis 1/2 (c-IAP1/2) and linear ubiquitin chain assembly complex (LUBAC). [5][6][7][8][9] c-IAP1/2 and LUBAC target RIP1 for K11, K63 and linear polyubiquitination priming RIP1 for the recruitment of two kinase-containing complexes, namely, TAK1/TAK1-binding protein 1 (TAB1)/TAB2 and NEMO/ inhibitor of NF-κB kinase α (IKKα)/IKKβ. 5,[10][11][12][13] Ultimately, activation of IKKβ leads to the phosphorylation and proteasomal degradation of IκBα and the nuclear translocation of NF-κB, a pathway defined as the classical, or the canonical, NF-κB pathway.…”