Role of Linear Ubiquitination in Health and Disease

Abstract

The covalent attachment of ubiquitin to target proteins is one of the most prevalent post-translational modifications, regulating a myriad of cellular processes including cell growth, survival, and metabolism. Recently, a novel RING E3 ligase complex was described, called linear ubiquitin assembly complex (LUBAC), which is capable of connecting ubiquitin molecules in a novel head-to-tail fashion via the N-terminal methionine residue. LUBAC is a heteromeric complex composed of heme-oxidized iron-responsive element-binding protein 2 ubiquitin ligase-1L (HOIL-1L), HOIL-1L-interacting protein, and shankassociated RH domain-interacting protein (SHARPIN). The essential role of LUBAC-generated linear chains for activation of nuclear factorkB (NF-kB) signaling was first described in the activation of tumor necrosis factor-a receptor signaling complex. A decade of research has identified additional pathways that use LUBAC for downstream signaling, including CD40 ligand and the IL-1b receptor, as well as cytosolic pattern recognition receptors including nucleotide-binding oligomerization domain containing 2 (NOD2), retinoic acid-inducible gene 1 (RIG-1), and the NOD-like receptor family, pyrin domain containing 3 inflammasome (NLRP3). Even though the three components of the complex are required for full activation of NF-kB, the individual components of LUBAC regulate specific cell type-and stimuli-dependent effects. In humans, autosomal defects in LUBAC are associated with both autoinflammation and immunodeficiency, with additional disorders described in mice. Moreover, in the lung epithelium, HOIL-1L ubiquitinates target proteins independently of the other LUBAC components, adding another layer of complexity to the function and regulation of LUBAC. Although many advances have been made, the diverse functions of linear ubiquitin chains and the regulation of LUBAC are not yet completely understood. In this review, we discuss the various roles of linear ubiquitin chains and point to areas of study that would benefit from further investigation into LUBACmediated signaling pathways in lung pathophysiology.

Linear ubiquitin assembly complex regulates lung epithelial–driven responses during influenza infection

“…It has been reported that IFN-α and IFN-γ can regulate the expression of LUBAC components in SHARPIN-deficient mice (43). In our mod-HOIL-1L has several LUBAC-independent functions, such as acting as an E3 ubiquitin ligase to target proteins such as PKCζ for degradation as seen in adaptation to hypoxia and cancer (23,31,39). Nondenaturing native gel electrophoresis of lysates from WT AT2 cells or A549 cells exposed to WSN in vitro revealed that HOIL-1L runs consistent with the 600 kDa molecular weight of LUBAC (25) ( Figure 4C and Supplemental Figure 5K).…”

“…In support of these observations, an SNP predicted to result in an amino acid change in the "catalytic core" of HOIP was associated with enhanced NEMO linear ubiquitination and robust downstream NF-κB activation (Figure 8). Loss of LUBAC activity has been shown to have cell typespecific effects (23,(58)(59)(60). For example, patients with LUBAC deficiency present with both autoinflammation and immunodeficiency.…”

“…Thus, we propose that the IFN-mediated regulation of HOIL-1L is a host maladaptation to IAV infection, as we observe significant protection in SPC Cre /HOIL-1L fl/fl mice. While HOIL-1L does not directly contribute to linear ubiquitin chain formation (23,27), its binding within LUBAC masks a ubiquitination site on HOIP to prevent its proteasomal degradation and releases HOIP's autoinhibitory fold to maximize LUBAC activity (23,26,27,32). Thus, increased abundance of HOIL-1L may contribute to the enhanced stability and activity of LUBAC.…”

“…The linear ubiquitin assembly complex (LUBAC) is essential for NF-κB activation, targeting the NF-κB essential modulator (NEMO) for linear ubiquitination (20)(21)(22)(23)(24). LUBAC is an E3 ligase complex composed of the accessory protein Shank-associated RH domain-interacting protein (SHARPIN) as well as two RINGin-between-RING ligases: heme-oxidized iron-responsive element-binding protein 2 ubiquitin ligase-1L (HOIL-1L) and the catalytically active component HOIL-1-interacting protein (HOIP) (20,25,26).…”

HIF and HOIL-1L–mediated PKCζ degradation stabilizes plasma membrane Na,K-ATPase to protect against hypoxia-induced lung injury

“…the transcription of heme-oxidized IRP2 ubiquitin ligase 1L (HOIL-1L), which acts as the E3 ubiquitin ligase for PKCζ, targeting it for proteasomal degradation (20,25). HOIL-1L together with HOIL-1-interacting protein (HOIP) and Shank-associated RH-domain-interacting protein (SHARPIN) form the linear ubiquitination assembly complex (LUBAC) (26)(27)(28)(29). We and others have found that, when acting independently of LUBAC, HOIL-1L adds Lys-48-linked chains and serves as an ubiquitin E3 ligase (20,30).…”