Rheumatoid arthritis (RA) is one of the most critical articular diseases with synovial hyperplasia followed by impairment of quality of life. However, the mechanism(s) that regulates synovial cell outgrowth is not fully understood. To clarify its mechanism(s), we carried out immunoscreening by using antirheumatoid synovial cell antibody and identified and cloned "Synoviolin/Hrd1", an E3 ubiquitin ligase. Synoviolin/Hrd1 was highly expressed in the rheumatoid synovium, and mice overexpressing this enzyme developed spontaneous arthropathy. Conversely, synoviolin/hrd1 +/− mice were resistant to collagen-induced arthritis by enhanced apoptosis of synovial cells. We conclude that Synoviolin/Hrd1 is a novel causative factor for arthropathy by triggering synovial cell outgrowth through its antiapoptotic effects. Our findings provide a new pathogenetic model of RA and suggest that Synoviolin/Hrd1 could be targeted as a therapeutic strategy for RA.

Synoviolin, also called HRD1, is an E3 ubiquitin ligase and is implicated in endoplasmic reticulum -associated degradation. In mammals, Synoviolin plays crucial roles in various physiological and pathological processes, including embryogenesis and the pathogenesis of arthropathy. However, little is known about the molecular mechanisms of Synoviolin in these actions. To clarify these issues, we analyzed the profile of protein expression in synoviolinnull cells. Here, we report that Synoviolin targets tumor suppressor gene p53 for ubiquitination. Synoviolin sequestrated and metabolized p53 in the cytoplasm and negatively regulated its cellular level and biological functions, including transcription, cell cycle regulation and apoptosis. Furthermore, these p53 regulatory functions of Synoviolin were irrelevant to other E3 ubiquitin ligases for p53, such as MDM2, Pirh2 and Cop1, which form autoregulatory feedback loops. Our results provide novel insights into p53 signaling mediated by Synoviolin.

RNA helicase A (RHA) is a member of an ATPase/DNA and RNA helicase family and is a homologue of Drosophila maleless protein (MLE), which regulates X-linked gene expression. RHA is also a component of holo-RNA polymerase II (Pol II) complexes and recruits Pol II to the CREB binding protein (CBP). The ATPase and/or helicase activity of RHA is required for CREB-dependent transcription. To further understand the role of RHA on gene expression, we have identified a 50-amino-acid transactivation domain that interacts with Pol II and termed it the minimal transactivation domain (MTAD). The protein sequence of this region contains six hydrophobic residues and is unique to RHA homologues and well conserved. A mutant with this region deleted from full-length RHA decreased transcriptional activity in CREB-dependent transcription. In addition, mutational analyses revealed that several tryptophan residues in MTAD are important for the interaction with Pol II and transactivation. These mutants had ATP binding and ATPase activities comparable to those of wild-type RHA. A mutant lacking ATP binding activity was still able to interact with Pol II. In CREB-dependent transcription, the transcriptional activity of each of these mutants was less than that of wild-type RHA. The activity of the double mutant lacking both functions was significantly lower than that of each mutant alone, and the double mutant had a dominant negative effect. These results suggest that RHA could independently regulate CREB-dependent transcription either through recruitment of Pol II or by ATP-dependent mechanisms.RNA helicase A (RHA) is a member of the DExH family of ATPases/helicases and catalyzes the displacement of both double-stranded RNA and DNA from 3Ј to 5Ј (32,61,63). Functional domains of RHA include two double-stranded RNA binding domains at the amino terminus known as dsRBD1 and dsRBD2. The catalytic core domain is located within the central region and contains a DExH motif. This core domain contains seven well-conserved motifs; one of them has an ATP binding site with the consensus GCGKT and FILDD, known as the A site the B site, respectively. The carboxyl terminus contains an RGG-rich region that is capable of binding singlestrand nucleic acids (62).RHA was originally isolated as a human homologue of Drosophila maleless protein (MLE), with which it has 50% sequence identity and 90% sequence similarity (33). In Drosophila, MLE colocalizes with acetylated histone H4 (8, 48). MLE is involved in sex-specific gene dosage compensation and elevates the level of transcription derived from a single X chromosome in male flies to a level equivalent to that derived from two X chromosome in the female (25,29). MLE mutants are embryonic lethal to males, indicating that MLE is an essential factor in Drosophila development.In mammals, RHA-knockout mice are embryonic lethal for homozygous RHA mutants (35). Analysis of these mice revealed that RHA is associated with differentiation of the embryonic ectoderm during gastrulation. It is possible that RHA has an i...