Search

Tumor suppressor Smad4 is the common signaling effector in the transforming growth factor β (TGF-β) superfamily. Phosphorylated regulatory Smads (R-Smads) interact with Smad4, and the complex translocates into the nucleus to regulate gene transcription. Proper TGF-β signaling requires precise control of Smad functions. Smurfs have been shown to mediate the degradation of R-Smads but not the common- partner Smad4. We report a novel mechanism of Smad4 degradation. Jab1 interacts directly with Smad4 and induces its ubiquitylation for degradation. Jab1 was initially identified as a co-activator of c-jun, and it also induces degradation of cell cycle inhibitor p27 and tumor suppressor p53. Ectopic expression of Jab1 decreased endogenous Smad4 steady-state levels. The 26S proteasome inhibitors lactacystin and MG132 reduced the degradation rate of Smad4 protein. Examination of the effects of JAB1-induced Smad4 degradation indicates that Jab1 inhibited TGF-β-induced gene transcription. Our data suggest that Jab1 antagonizes TGF-β function by inducing degradation of Smad4 through a distinct degradation pathway. [ABSTRACT FROM AUTHOR]

Erosion of cartilage and bone is a hallmark of rheumatoid arthritis (RA). This study was undertaken to explore the roles of hyperproliferating synovial fibroblasts and macrophages in abnormal osteoclast formation, using the recently described BXD2 mouse model of RA.Cell distribution in the joints was analyzed by immunohistochemistry, using tartrate‐resistant acid phosphatase (TRAP) staining to identify osteoclasts. To identify the defective cells in BXD2 mice, mouse synovial fibroblasts (MSFs) were cultured with bone marrow–derived macrophages. Osteoclast formation was assayed by TRAP staining and bone resorption pit assay, and the cytokine profiles of the MSFs and macrophages were determined by quantitative real‐time polymerase chain reaction and enzyme‐linked immunosorbent assay.In BXD2 mice, TRAP‐positive osteoclasts were found at sites of active bone erosion, in close proximity to hyperproliferating synovial fibroblasts. On coculture, MSFs from BXD2 mice, but not C57BL/6 mice, produced high levels of RANKL messenger RNA, induced macrophages to form osteoclasts, and actively eroded bone slices, through a mechanism(s) that could be blocked by pretreatment with osteoprotegerin. Although macrophages from BXD2 mice expressed higher basal levels of tumor necrosis factor α (TNFα), interleukin‐1β (IL‐1β), and IL‐6 than those from C57BL/6 mice, abnormal osteoclast formation was not due to enhanced sensitivity of the BXD2 mouse macrophages to RANKL. TNFα, produced by both BXD2 MSFs and BXD2 mouse macrophages, had a strong stimulatory effect on RANKL expression.BXD2 MSFs produce RANKL and induce the development of osteoclasts from macrophages. The enhanced production of RANKL is possibly due to autocrine stimulation, together with paracrine stimulation by factors produced by macrophages. [ABSTRACT FROM AUTHOR]