Importance of E3 ubiquitin ligase Synoviolin in fibrogenesis

Excerpt

The symptoms of rheumatoid arthritis (RA) are based on the many processes; chronic inflammation, overgrowth of synovial cells, bone and joint destruction and fibrosis. To clarify the mechanism of outgrowth of synovial cells, we carried out immunoscreening using anti-rheumatoid synovial cell antibody, and cloned 'Synoviolin'. Synoviolin, a mammalian homolog of Hrd1p/Der3p, is endoplasmic reticulum (ER)-resident E3 ubiquitin ligases with a RING motif, and is involved in ER-associated degradation (ERAD). Synoviolin is highly expressed in synoviocytes of patients with RA. Overexpression of synoviolin in transgenic mice leads to advanced arthropathy caused by reduced apoptosis of synoviocytes. We postulate that the hyperactivation of the ERAD pathway by overexpression of synoviolin results in prevention of ER-stress-induced apoptosis leading to synovial hyperplasia. Indeed, synoviolin^+/- knockout mice showed resistance to the development of collagen-induced arthritis owing to enhanced apoptosis of synovial cells. In addition, Synoviolin ubiquitinates and sequesters the tumor suppressor p53 in the cytoplasm, thereby negatively regulating its biological functions in transcription, cell-cycle regulation and apoptosis by targeting it for proteasomal degradation. Therefore Synoviolin regulates, not only apoptosis in response to ER stress, but also a p53-dependent apoptotic pathway. These studies indicate that Synoviolin is one of the causative factors of arthropathy. Further analysis using gene targeting approaches showed that in addition to its role in RA, Synoviolin is essential for embryogenesis. Synoviolin deficient (syno^-/-) mice exhibited severe anemia caused by enhancement of apoptosis in fetal liver, and the results suggested that the liver is sensitive organ for Synoviolin. …