Background The Single immunoglobin interleukin-1 (IL-1)-related receptor (Sigirr), also known as IL-1R8, has been shown to exhibit broad anti-inflammatory effects against inflammatory diseases including acute lung injury, while molecular regulation of IL-1R8/Sigirr protein stability has not been reported. This study is designed to determine whether stabilization of IL-1R8/Sigirr by a deubiquitinating enzyme (DUB) is sufficient to suppress inflammatory responses and lessen lung inflammation. Methods A molecular signature of ubiquitination and degradation of IL-1R8/Sigirr was determined using a receptor ligation chase model. The anti-inflammatory effects on USP13 were investigated. USP13 knockout mice were evaluated for stabilization of IL-1R8/Sigirr and disease phenotype in an acute lung injury model. Findings IL-1R8/Sigirr degradation is mediated by the ubiquitin-proteasome system, through site-specific ubiquitination. This effect was antagonized by the DUB USP13. USP13 levels correlate directly with IL-1R8/Sigirr, and both proteins were reduced in cells and tissue from mice subjected to inflammatory injury by the TLR4 agonist lipopolysaccharide (LPS). Knockdown of USP13 in cells increased IL-1R8/Sigirr poly-ubiquitination and reduced its stability, which enhanced LPS-induced TLR4 signaling and cytokine release. Likewise, USP13-deficient mice were highly susceptible to LPS or Pseudomonas aeruginosa models of inflammatory lung injury. IL-1R8/Sigirr overexpression in cells or by pulmonary viral transduction attenuated the inflammatory phenotype conferred by the USP13 −/− genotype. Interpretation Stabilization of IL-1R8/Sigirr by USP13 describes a novel anti-inflammatory pathway in diseases that could provide a new strategy to modulate immune activation. Fund This study was supported by the US National Institutes of Health (R01HL131665, HL136294 to Y.Z., R01 GM115389 to J.Z.).

The transforming growth factor β-1 (TGFβ-1) signaling pathway plays a central role in the pathogenesis of pulmonary fibrosis. Two TGFβ-1 receptors, TβRI and TβRII, mediate this pathway. TβRI protein stability, as mediated by the ubiquitin/de-ubiquitination system, has been well studied; however, the molecular regulation of TβRII still remains unclear. Here we reveal that a de-ubiquitinating enzyme, USP11, promotes TGFβ-1 signaling through de-ubiquitination and stabilization of TβRII. We elucidate the role that mitoxantrone (MTX), an USP11 inhibitor, has in the attenuation of TGFβ-1 signaling. Inhibition or downregulation of USP11 results in increases in TβRII ubiquitination and reduction of TβRII stability. Subsequently, TGFβ-1 signaling is greatly attenuated, as shown by the decreases in phosphorylation of SMAD2/3 levels as well as that of fibronectin (FN) and smooth muscle actin (SMA). Overexpression of USP11 reduces TβRII ubiquitination and increases TβRII stabilization, thereby elevating phosphorylation of SMAD2/3 and the ultimate expression of FN and SMA. Further, elevated expression of USP11 and TβRII were detected in lung tissues from bleomycin-challenged mice and IPF patients. Therefore, USP11 may contribute to the pathogenesis of pulmonary fibrosis by stabilization of TβRII and promotion of TGFβ-1 signaling. This study provides mechanistic evidence for development of USP11 inhibitors as potential antifibrotic drugs for pulmonary fibrosis.

Cyclic adenosine monophosphate (cAMP) response element–binding protein (CREB)–binding protein (CBP) is a histone acetyltransferase that plays a pivotal role in the control of histone modification and the expression of cytokine-encoding genes in inflammatory diseases, including sepsis and lung injury. We found that the E3 ubiquitin ligase subunit FBXL19 targeted CBP for site-specific ubiquitylation and proteasomal degradation. The ubiquitylation-dependent degradation of CBP reduced the extent of lipopolysaccharide (LPS)–dependent histone acetylation and cytokine release in mouse lung epithelial cells and in a mouse model of sepsis. Furthermore, we demonstrated that the deubiquitylating enzyme USP14 (ubiquitin-specific peptidase 14) stabilized CBP by reducing its ubiquitylation. LPS increased the stability of CBP by reducing the association between CBP and FBXL19 and by activating USP14. Inhibition of USP14 reduced CBP protein abundance and attenuated LPS-stimulated histone acetylation and cytokine release. Together, our findings delineate the molecular mechanisms through which CBP stability is regulated by FBXL19 and USP14, which results in the modulation of chromatin remodeling and the expression of cytokine-encoding genes.

Lysophosphatidic acid receptor 1 (LPA1) is a druggable target for treating pulmonary inflammatory diseases. However, the molecular regulation of LPA1 stability, a factor that critically impacts its biological activity, remains largely unknown. Here we identify two enzymes that regulate the balance of LPA1 ubiquitination and deubiquitination. Ubiquitin E3 ligase Nedd4L targets LPA1 for its site specific ubiquitination and degradation in the lysosome. Nedd4L negatively regulates LPA-LPA1-mediated cytokine release. The stability of LPA1 is up-regulated by ubiquitin-specific protease 11 (USP11), which deubiquitinates LPA1 and enhances LPA1-mediated pro-inflammatory effects. LPA1 is associated with USP11 in quiescent cells, while LPA treatment triggers LPA1 dis-association with USP11 and in turn binding to Nedd4L. Knockdown or inhibition of USP11 reduces LPA1 stability, levels of LPA1, and LPA1-CD14 interaction complex; thereby diminishing both LPA- and LPS-induced inflammatory responses and lung injury in preclinical murine models. Thus, our findings identify an ubiquitin E3 ligase and a deubiquitinating enzyme responsible for regulation of LPA1 stability and biological activities. This study provides potential targets for the development of anti-inflammatory molecules to lessen lung injury.

Transforming growth factor β-1 (TGFβ-1)-induced phosphorylation of transcription factors Smad2 and Smad3 plays a crucial role in the pathogenesis of idiopathic pulmonary fibrosis (IPF). However, the molecular regulation of Smad2/Smad3 proteins stability remains a mystery. Here, we show that ubiquitin carboxyl-terminal hydrolase-L5 (UCHL5 or UCH37) de-ubiquitinates both Smad2 and Smad3, up-regulates their stability, and promotes TGFβ-1-induced expression of profibrotic proteins, such as fibronectin (FN) and α-smooth muscle actin (α-SMA). Inhibition or down-regulation of UCHL5 reduced Smad2/Smad3 levels and TGFβ-1-induced the expression of FN and α-SMA in human lung fibroblast. We demonstrate that Smad2 and Smad3 ubiquitination was diminished by over-expression of UCHL5, while it was enhanced by inhibition or down-regulation of UCHL5. UCHL5 is highly expressed in IPF lungs. UCHL5, Smad2, and Smad3 levels were increased in bleomycin-injured lungs. Administration of UCHL5 inhibitor, b-AP15, reduced the expression of FN, type I collagen, Smad2/Smad3, and the deposition of collagen in lung tissues in a bleomycin-induced model of pulmonary fibrosis. Our studies provide a molecular mechanism by which UCHL5 mitigates TGFβ-1 signaling by stabilizing Smad2/Smad3. These data indicate that UCHL5 may contribute to the pathogenesis of IPF and may be a potential therapeutic target.