Bromodomain-containing-protein-4 and cyclin-dependent-kinase-9 inhibitors interact synergistically in vitro and combined treatment reduces post-traumatic osteoarthritis severity in mice.

OBJECTIVE: Joint injury rapidly induces expression of primary response genes (PRGs), which activate a cascade of secondary genes that destroy joint tissues and initiate post-traumatic osteoarthritis (PTOA). Bromodomain-containing-protein-4 (Brd4) and cyclin-dependent-kinase-9 (CDK9) cooperatively control the rate-limiting step of PRG transactivation, including pro-inflammatory genes. This study investigated whether Brd4 and CDK9 inhibitors suppress inflammation and prevent PTOA development in vitro and in a mouse PTOA model. METHODS: The effects of Brd4 and CDK9 inhibitors (JQ1 and Flavopiridol) on PRG and associated secondary damage were rigorously tested in different settings. Short-term effects of inflammatory stimuli (IL-1ß, IL-6, TNF) on human chondrocyte PRG expression were assessed by RT-PCR and microarray after 5-h. We quantified glycosaminoglycan release from IL-1ß-treated bovine cartilage explants after 3-6 days, and osteoarthritic changes in mice after ACL-rupture using RT-PCR (2-24hrs), in vivo imaging of MMP activity (24hrs), AFM-nanoindentation (3-7days), and histology (3days-4wks). RESULTS: Flavopiridol and JQ1 inhibitors act synergistically, and a combination of both almost completely prevented the activation of most IL-1ß-induced PRGs in vitro by microarray analysis, and prevented IL-1ß-induced glycosaminoglycan release from cartilage explants. Mice given the drug combination showed reduced IL-1ß and IL-6 expression, less in vivo MMP activity, and lower synovitis (1.5 vs 4.9) and OARSI scores (2.8 vs 6.0) than untreated mice with ACL-rupture. CONCLUSIONS: JQ1 and Flavopiridol work synergistically to reduce injury response after joint trauma, suggesting that targeting Brd4 and/or CDK9 could be a viable strategy for PTOA prevention and treatment of early OA.

Inhibition of early response genes prevents changes in global joint metabolomic profiles in mouse post-traumatic osteoarthritis.

OBJECTIVE: Although joint injury itself damages joint tissues, a substantial amount of secondary damage is mediated by the cellular responses to the injury. Cellular responses include the production and activation of proteases (MMPs, ADAMTSs, Cathepsins), and the production of inflammatory cytokines. The trajectory of cellular responses is driven by the transcriptional activation of early response genes, which requires Cdk9-dependent RNA Polymerase II phosphorylation. Our objective was to determine whether inhibition of cdk9-dependent early response gene activation affects changes in the joint metabolome. DESIGN: To model post-traumatic osteoarthritis, we subjected mice to non-invasive Anterior Cruciate Ligament (ACL)-rupture joint injury. Following injury, mice were treated with flavopiridol - a potent and selective inhibitor of Cdk9 kinase activity - to inhibit Cdk9-dependent transcriptional activation, or vehicle control. Global joint metabolomics were analyzed 1 h after injury. RESULTS: We found that injury induced metabolomic changes, including increases in Vitamin D3 metabolism, anandamide, and others. Inhibition of primary response gene activation immediately after injury largely prevented the global changes in the metabolomics profiles. Cluster analysis of joint metabolomes identified groups of injury-induced and drug-responsive metabolites. CONCLUSIONS: Metabolomic profiling provides an instantaneous snapshot of biochemical activity representing cellular responses. We identified two sets of metabolites that change acutely after joint injury: those that require transcription of primary response genes, and those that do not. These data demonstrate the potential for inhibition of early response genes to alter the trajectory of cell-mediated degenerative changes following joint injury, which may offer novel targets for cell-mediated secondary joint damage.

Inhibition of CDK9 prevents mechanical injury-induced inflammation, apoptosis and matrix degradation in cartilage explants.

Joint injury often leads to post-traumatic osteoarthritis (PTOA). Acute injury responses to trauma induce production of pro-inflammatory cytokines and catabolic enzymes, which promote chondrocyte apoptosis and degrade cartilage to potentiate PTOA development. Recent studies show that the rate-limiting step for transcriptional activation of injury response genes is controlled by cyclin-dependent kinase 9 (CDK9), and thus it is an attractive target for limiting the injury response. Here, we determined the effects of CDK9 inhibition in suppressing the injury response in mechanically-injured cartilage explants. Bovine cartilage explants were injured by a single compressive load of 30 % strain at 100 %/s, and then treated with the CDK9 inhibitor Flavopiridol. To assess acute injury responses, we measured the mRNA expression of pro-inflammatory cytokines, catabolic enzymes, and apoptotic genes by RT-PCR, and chondrocyte viability and apoptosis by TUNEL staining. For long-term outcome, cartilage matrix degradation was assessed by soluble glycosaminoglycan release, and by determining the mechanical properties with instantaneous and relaxation moduli. Our data showed CDK9 inhibitor markedly reduced injury-induced inflammatory cytokine and catabolic gene expression. CDK9 inhibitor also attenuated chondrocyte apoptosis and reduced cartilage matrix degradation. Lastly, the mechanical properties of the injured explants were preserved by CDK9 inhibitor. Our results provide a temporal profile connecting the chain of events from mechanical impact, acute injury responses, to the subsequent induction of chondrocyte apoptosis and cartilage matrix deterioration. Thus, CDK9 is a potential disease-modifying agent for injury response after knee trauma to prevent or delay PTOA development.

Musculoskeletal changes following non-invasive knee injury using a novel mouse model of post-traumatic osteoarthritis.

OBJECTIVE: Post-traumatic osteoarthritis (PTOA) is a common consequence of traumatic joint injury, with 50% of anterior cruciate ligament (ACL) rupture patients developing PTOA within 10-20 years. Currently accepted mouse models of PTOA initiate symptoms using various methods, none of which faithfully mimic clinically-relevant injury conditions. In this study we characterize a novel non-invasive mouse model of PTOA that injures the ACL with a single load of tibial compression overload. We utilize this model to determine the time course of articular cartilage and subchondral bone changes following knee injury. DESIGN: Mice were euthanized 1, 3, 7, 14, 28, or 56 days after non-invasive knee injury. Knees were scanned using micro-computed tomography (µCT) in order to quantify subchondral trabecular bone, subchondral bone plate, and non-native bone formation (heterotopic ossification). Development of osteoarthritis (OA) was graded using the osteoarthritis research society international (OARSI) scale on histological sections of injured and uninjured knees. RESULTS: Following injury we observed a rapid loss of trabecular bone in injured knees compared to uninjured knees by 7 days post-injury, followed by a partial recovery of trabecular bone to a new steady state by 28 days post-injury. We also observed considerable non-native bone formation by 56 days post-injury. Grading of histological sections revealed deterioration of articular cartilage by 56 days post-injury, consistent with development of mild OA. CONCLUSIONS: This study establishes a novel mouse model of PTOA, and describes the time course of musculoskeletal changes following knee injury, helping to establish the window of opportunity for preventative treatment.

Comparative analysis with collagen type II distinguishes cartilage oligomeric matrix protein as a primary TGFß-responsive gene.

OBJECTIVE: This study aims to investigate the regulation of expression of Cartilage oligomeric matrix protein (COMP), which is predominately expressed by chondrocytes and functions to organize the extracellular matrix. Mutations in COMP cause two skeletal dysplasias: pseudoachondroplasia and multiple epiphyseal dysplasia. The mechanism controlling COMP expression during chondrocyte differentiation is still poorly understood. DESIGN: Primary human bone marrow-derived stem cells were induced to differentiate into chondrocyte by pellet cultures. We then compared the temporal expression of COMP with the well-characterized cartilage-specific Type II collagen (Col2a1), and their response to transforming growth factor (TGF)ß and Sox trio (Sox5, 6, and 9) stimulation. RESULTS: COMP and Col2a1 expression are differentially regulated by three distinct mechanisms. First, upregulation of COMP mRNA precedes Col2a1 by several days during chondrogenesis. Second, COMP expression is independent of high cell density but requires TGF-ß1. Induction of COMP mRNA by TGF-ß1 is detected within 2h in the absence of protein synthesis and is blocked by specific inhibitors of the TGFß signaling pathway; and therefore, COMP is a primary TFGß-response gene. Lastly, while Col2a1 expression is intimately controlled by the Sox trio, overexpression of Sox trio fails to activate the COMP promoter. CONCLUSION: COMP and Col2a1 expression are regulated differently during chondrogenesis. COMP is a primary response gene of TGFß and its fast induction during chondrogenesis suggests that COMP is suitable for rapidly accessing the chondrogenic potential of stem cells.