Identification of a Crucial Site for Synoviolin Expression

Synoviolin is a molecule cloned from synoviocytes of patients with rheumatoid arthritis (RA) and characterizes RA synovial cells (RASCs) based on its high expression level in these cells (4). Indeed, immunohistochemical analysis showed marked expression of Synoviolin in synovial tissue of RA patients relative to that of patients with osteoarthritis. Other studies indicated that Synoviolin is an endoplasmic reticulum (ER)-resident membrane protein and is the human homologue of the yeast E3-ubiquitin ligase (Hrd1p), which functions as an ER-associated degradation (ERAD) system in yeast (7, 21, 59). Furthermore, Synoviolin was found to have an E3 ligase activity and to function in the ERAD system, similar to Hrd1p (4, 28, 33, 37). The biological role of Synoviolin was first investigated with transgenic mice. Interestingly, Synoviolin caused arthropathy with synovial hypertrophy in over 30% of transgenic mice, which was associated with significant suppression of apoptosis (4). In contrast, destruction of the synoviolin gene heterozygote, i.e., 50% half gene dosage mice, was almost completely protective against collagen-induced arthritis (CIA) due to enhanced apoptosis of synovial cells (4). These results confirm the involvement of Synoviolin in the onset of arthropathy and that synoviolin gene dosage correlates significantly with the onset of arthropathy; i.e., increased expression of Synoviolin appears to be important for synovium overgrowth and triggering of arthropathy (4). In other studies, we also demonstrated that the synoviolin gene is involved in the maintenance of embryonic life, since homozygote mice deficient in synoviolin died in utero at 13.5 days postconception (dpc) because of aberrant apoptosis (60). Furthermore, in a culture system using small interfering RNA (siRNA), down-regulation of the synoviolin gene was vulnerable to various ER stress reagents such as tunicamycin, thapsigargin, and dithiothreitol, leading to apoptosis, whereas overexpression conversely rescued the apoptosis (4). These results indicate that alternation of the Synoviolin expression level can modulate the resistance to apoptosis caused by disruption of ERAD function. Furthermore, reduction of constitutive expression of the synoviolin gene could result in deterioration of ER homeostasis, consequently leading to a breakdown of cellular homeostasis and eventual apoptosis of the cell. Since most cells are exposed to a flux of newly synthesized proteins even under physiological conditions and consequently some of these proteins accumulate as misfolded and unfolded proteins in the ER, Synoviolin has to eliminate such proteins in order to maintain ER homeostasis, namely, to protect against any disruption of cellular homeostasis. Therefore, constitutive expression of Synoviolin might be responsible for maintaining ER homeostasis for cell survival in vivo. The aforementioned findings emphasize the importance of transcriptional regulation of the synoviolin expression level in cellular homeostasis. The present study was designed to determine the mechanism(s) involved in the transcriptional regulation of synoviolin expression in the cells.