The Sjögren’s Syndrome-Associated Autoantigen Ro52 Is an E3 Ligase That Regulates Proliferation and Cell Death

Abstract

Patients affected by Sjögren’s syndrome and systemic lupus erythematosus (SLE) carry autoantibodies to an intracellular protein denoted Ro52. Although the serologic presence of Ro52 autoantibodies is used clinically for diagnostic purposes, the function of the protein or why it is targeted as an autoantigen in several rheumatic conditions has not been elucidated. In this study, we show that the expression of Ro52 is significantly increased in PBMC of patients with Sjögren’s syndrome and SLE, and demonstrate that Ro52 is a RING-dependent E3 ligase involved in ubiquitination. Overexpression of Ro52, but not of Ro52 lacking the RING domain, in a mouse B cell line lead to decreased growth in steady state and increased cell death after activation via the CD40 pathway. The role of Ro52 in activation-mediated cell death was further confirmed as a reduction in Ro52 expression restored cell viability. These findings suggest that the increased expression of the Ro52 autoantigen in patients may be directly involved in the reduced cellular proliferation and increased apoptotic cell death observed in Sjögren’s syndrome and SLE, and may thus contribute to the autoantigenic load and induction of autoimmune B and T cell responses observed in rheumatic patients.

Anti-Ro52 Autoantibodies from Patients with Sjögren's Syndrome Inhibit the Ro52 E3 Ligase Activity by Blocking the E3/E2 Interface

“…In this study, we addressed the question whether patient-derived autoantibodies can inhibit the function of their corresponding antigen, as well as the mechanism for such inhibition. For this investigation, we studied the effect of autoantibodies on the autoantigen and immunomodulatory protein, Ro52, which we recently identified as an E3 ubiquitin ligase (7).…”

“…Ubiquitination Assays-The ubiquitination assays were performed as described previously (7). In short, 0.50 M MaBPRo52 or MaBP-Ro52 mutant proteins were used in each reaction.…”

“…A B30.2 domain is located in the C-terminal region (6). Several TRIM family members, including Ro52, have been identified as E3 ubiquitin ligases involved in the ubiquitination process (7)(8)(9)(10). Ubiquitination is a covalent post-translational modification of proteins by the 76-amino acid residue polypeptide ubiquitin, leading either to proteolysis in the proteasome, internalization from membranes, or functional alteration (11,12).…”

Expression of the Immune Regulator Tripartite-Motif 21 Is Controlled by IFN Regulatory Factors

“…A large number of TRIM genes are induced by type I or type II IFNs (e.g., during viral or bacterial infections) (20,21). Interestingly, some TRIM genes are also overexpressed in systemic autoimmune diseases such as systemic lupus erythematosus and Sjögren's syndrome, suggesting that TRIM proteins participate in the pathogenesis of these diseases (22)(23)(24). Trim21 is induced by both type I and type II IFNs and is overexpressed in patients with systemic lupus erythematosus and Sjögren's syndrome compared with healthy controls (23,24).…”

Autoantibodies from mice exposed to Libby amphibole asbestos bind SSA/Ro52-enriched apoptotic blebs of murine macrophages

“…The asbestos induced SLE-like disease is characterized by the production of AAs that recognize the SSA/Ro52 autoantigen. SSA/Ro52 is a newly characterized RING-finger-type E3 ubiquitin ligase (Espinosa et al 2006;Wada and Kamitani 2006), which in unstimulated cells localizes to the cytoplasm (Ohlsson et al 2002). Autoantibodies against SSA/Ro52 are commonly found in patients with SLE (Hassan et al 2002;Hoffman et al 2004;Popovic et al 2007;Routsias et al 2006).…”