hRpn13, a newly identified component of the 19S particle, regulates proliferation, differentiation, and function in the human osteoblast-like cell line MG63. [corrected]

ADRM1/RPN13 attenuates cartilage extracellular matrix degradation via enhancing UCH37-mediated ALK5 deubiquitination

Osteoarthritis (OA) is the most common age-related joint disorder with no effective therapy, and its specific pathological mechanism remains to be fully clarified. Adhesion-regulating molecule 1 (ADRM1) has been proven to be involved in OA progression as a favorable gene. However, the exact mechanism of ADRM1 involved in OA were unknown. Here, we showed that the ADRM1 expression decreased in human OA cartilage, destabilization of the medial meniscus (DMM)-induced mouse OA cartilage, and interleukin (IL)-1β-induced primary mouse articular chondrocytes. Global knockout (KO) ADRM1 in cartilage or ADRM1 inhibitor (RA190) could accelerate the disorders of extracellular matrix (ECM) homeostasis, thereby accelerated DMM-induced cartilage degeneration, whereas overexpression of ADRM1 protected mice from DMM-induced OA development by maintaining the homeostasis of articular cartilage. The molecular mechanism study revealed that ADRM1 could upregulate ubiquitin carboxy-terminal hydrolase 37 (UCH37) expression and bind to UCH37 to activate its deubiquitination activity. Subsequently, increased and activated UCH37 enhanced activin receptor-like kinase 5 (ALK5) deubiquitination to stabilize ALK5 expression, thereby maintaining ECM homeostasis and attenuating cartilage degeneration. These findings indicated that ADRM1 could attenuate cartilage degeneration via enhancing UCH37-mediated ALK5 deubiquitination. Overexpression of ADRM1 in OA cartilage may provide a promising OA therapeutic strategy.

Characterization of Osterix protein stability and physiological role in osteoblast differentiation

Osterix (Osx/SP7) is a C2H2 zinc finger-containing transcription factor of the SP gene family. Osx knockout mice indicate that the gene plays an essential role in osteoblast differentiation and bone formation. However, the mechanisms involved in the regulation of Osx are still poorly understood. Here, we report a novel post-translational mechanism for the regulation of Osx in mammalian cells. We found that the stability of endogenous and exogenous Osx reduced after cycloheximide treatment. In cells treated with the proteasome inhibitors MG-132 or lactacystin, both endogenous and exogenous Osx protein expression increased in a time-dependent manner. Co-immunoprecipitation (Co-IP) assays showed that both endogenous and exogenous Osx were ubiquitinated. Six lysine residues of Osx were identified as candidate ubiquitination sites by construction of point mutant plasmids and luciferase reporter assays. Furthermore, we confirmed that K58 and K230 are the ubiquitination sites of Osx by Co-IP assays and protein stability assays. Moreover, the Osx K58R and K230R mutations promoted the expression of osteoblast differentiation markers (alkaline phosphatase, collagen I and osteocalcin) and enhanced osteogenic differentiation in C2C12 cells. Taken together, our data indicate that Osx is an unstable protein, and that the ubiquitin-proteasome pathway is involved in the regulation of Osx and thereby regulates osteoblast differentiation.

Proteomics based detection of differentially expressed proteins in human osteoblasts subjected to mechanical stress

Mechanical stress is essential for bone development. Mechanical stimuli are transduced to biochemical signals that regulate proliferation, differentiation, and cytoskeletal reorganization in osteoblasts. In this study, we used proteomics to evaluate differences in the protein expression profiles of untreated Saos-2 osteoblast cells and Saos-2 cells subjected to mechanical stress loading. Using 2-D electrophoresis, MALDI-TOF mass spectroscopy, and bioinformatics, we identified a total of 26 proteins differentially expressed in stress loaded cells compared with control cells. Stress loaded Saos-2 cells exhibited significant upregulation of 17 proteins and significant downregulation of 9 proteins compared with control cells. Proteins that were most significantly upregulated in mechanically loaded cells included those regulating osteogenesis, energy metabolism, and the stress response, such as eukaryotic initiation factor 2 (12-fold), mitochondrial ATP synthase (8-fold), and peptidylprolyl isomerase A (cyclophilin A)-like 3 (6.5-fold). Among the proteins that were significantly downregulated were those involved in specific signaling pathways and cell proliferation, such as protein phosphatase regulatory (inhibitor) subunit 12B (13.8-fold), l-lactate dehydrogenase B (9.4-fold), Chain B proteasome activator Reg (Alpha) PA28 (7.7-fold), and ubiquitin carboxyl-terminal esterase L1 (6.9-fold). Our results provide a platform to understand the molecular mechanisms underlying mechanotransduction.