Articular cartilage degradation and de-differentiation of chondrocytes by the systemic administration of retinyl acetate-ectopic production of osteoblast stimulating factor-1 by chondrocytes in mice

Palovarotene reduces heterotopic ossification in juvenile FOP mice but exhibits pronounced skeletal toxicity

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder characterized by debilitating heterotopic ossification (HO). The retinoic acid receptor gamma agonist, palovarotene, and antibody-mediated activin A blockade have entered human clinical trials, but how these therapeutic modalities affect the behavior of pathogenic fibro/adipogenic progenitors (FAPs) is unclear. Using live-animal luminescence imaging, we show that transplanted pathogenic FAPs undergo rapid initial expansion, with peak number strongly correlating with HO severity. Palovarotene significantly reduced expansion of pathogenic FAPs, but was less effective than activin A inhibition, which restored wild-type population growth dynamics to FAPs. Palovarotene pretreatment did not reduce FAPs’ skeletogenic potential, indicating that efficacy requires chronic administration. Although palovarotene inhibited chondrogenic differentiation in vitro and reduced HO in juvenile FOP mice, daily dosing resulted in aggressive synovial joint overgrowth and long bone growth plate ablation. These results highlight the challenge of inhibiting pathological bone formation prior to skeletal maturation.

Identification of potent and selective retinoic acid receptor gamma (RARγ) antagonists for the treatment of osteoarthritis pain using structure based drug design

A series of triaryl pyrazoles were identified as potent pan antagonists for the retinoic acid receptors (RARs) α, β and γ. X-ray crystallography and structure-based drug design were used to improve selectivity for RARγ by targeting residue differences in the ligand binding pockets of these receptors. This resulted in the discovery of novel antagonists which maintained RARγ potency but were greater than 500-fold selective versus RARα and RARβ. The potent and selective RARγ antagonist LY2955303 demonstrated good pharmacokinetic properties and was efficacious in the MIA model of osteoarthritis-like joint pain. This compound demonstrated an improved margin to RARα-mediated adverse effects.

Chondroitin sulfate for the treatment of hip and knee osteoarthritis: current status and future trends

AIMS

Osteoarthritis (OA) is a common joint disorder and a major socio-economic burden. Chondroitin sulfate (CS), which has chondroprotective properties, is a promising candidate for the therapeutic treatment of OA. Here, we summarize current knowledge as well as future trends of CS for the treatment of hip and knee OA.

MAIN METHODS

We retrospectively reviewed pharmacokinetics, pharmacodynamics, clinical efficacy, safety and tolerability of CS for the treatment of OA.

KEY FINDINGS

The safety and tolerability of CS are confirmed. CS is effective, at least in part, for the treatment of OA, and its therapeutic benefits occur through three main mechanisms: 1) stimulation of extracellular matrix production by chondrocytes; 2) suppression of inflammatory mediators; and 3) inhibition of cartilage degeneration.

SIGNIFICANCE

CS is a safe and tolerable therapeutic agent for the management of OA. Its effects include benefits that are not achieved by current medicines and include chondroprotection and the prevention of joint space narrowing. Such positive effects of CS represent a breakthrough in the treatment of hip and knee OA.

Regulation of Sox9 activity by crosstalk with nuclear factor-kappaB and retinoic acid receptors

Introduction

Sox9 and p300 cooperate to induce expression of cartilage-specific matrix proteins, including type II collagen, aggrecan and link protein. Tumour necrosis factor (TNF)-α, found in arthritic joints, activates nuclear factor-κB (NF-κB), whereas retinoic acid receptors (RARs) are activated by retinoid agonists, including all-trans retinoic acid (atRA). Like Sox9, the activity of NF-κB and RARs depends upon their association with p300. Separately, both TNF-α and atRA suppress cartilage matrix gene expression. We investigated how TNF-α and atRA alter the expression of cartilage matrix genes.

Methods

Primary cultures of rat chondrocytes were treated with TNF-α and/or atRA for 24 hours. Levels of transcripts encoding cartilage matrix proteins were determined by Northern blot analyses and quantitative real-time PCR. Nuclear protein levels, DNA binding and functional activity of transcription factors were assessed by immunoblotting, electrophoretic mobility shift assays and reporter assays, respectively.

Results

Together, TNF-α and atRA diminished transcript levels of cartilage matrix proteins and Sox9 activity more than each factor alone. However, neither agent altered nuclear levels of Sox9, and TNF-α did not affect protein binding to the Col2a1 48-base-pair minimal enhancer sequence. The effect of TNF-α, but not that of atRA, on Sox9 activity was dependent on NF-κB activation. Furthermore, atRA reduced NF-κB activity and DNA binding. To address the role of p300, we over-expressed constitutively active mitogen-activated protein kinase kinase kinase (caMEKK)1 to increase p300 acetylase activity. caMEKK1 enhanced basal NF-κB activity and atRA-induced RAR activity. Over-expression of caMEKK1 also enhanced basal Sox9 activity and suppressed the inhibitory effects of TNF-α and atRA on Sox9 function. In addition, over-expression of p300 restored Sox9 activity suppressed by TNF-α and atRA to normal levels.

Conclusion

NF-κB and RARs converge to reduce Sox9 activity and cartilage matrix gene expression, probably by limiting the availability of p300. This process may be critical for the loss of cartilage matrix synthesis in inflammatory joint diseases. Therefore, agents that increase p300 levels or activity in chondrocytes may be useful therapeutically.

Targeting pleiotropin to treat osteoarthritis

Pleiotropin (PTN) is a secreted heparin-binding peptide expressed in mesodermal and neuroectodermal cells during development, but rarely in adult tissues. Although PTN is abundant in fetal or juvenile cartilage, it is undectable in mature cartilage. However, PTN is re-expressed in chondrocytes in early stages of osteoarthritis where it is detectable in situ and in synovial fluids from patients. PTN enhances chondrogenesis by stimulation of extra-cellular matrix synthesis, reduction of degrading matrix metalloproteases and induction of their inhibitors; PTN also slightly reduces pro-inflammatory factors, such as nitric oxide and vascular endothelial growth factor. Furthermore, PTN stimulates chondrocyte clustering and proliferation. Thus, PTN appears to mediate repair and protective processes in osteoarthritic cartilage and appears to be a promising factor to treat osteoarthritis.

Effects of pleiotrophin, a heparin-binding growth factor, on human primary and immortalized chondrocytes

OBJECTIVE

Pleiotrophin (PTN) is a secreted heparin-binding peptide expressed in mesodermal and neuroectodermal cells during development, but rarely in adult tissues. In fetal and juvenile, but not in mature cartilage, PTN is abundant. Furthermore, PTN is re-expressed in chondrocytes in early stages of osteoarthritis (OA). Since little is known about the functions of PTN in cartilage, we investigated the occurrence of PTN receptors in human articular cartilage in situ and PTN effects on human primary and immortalized chondrocytes in vitro.

METHODS

Receptor expression and regulation was monitored by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and immunohistochemistry. PTN effects and signal transduction were studied by electrophoretic mobility shift, Boyden chamber cell migration and proliferation assays, effects on gene expression by real time RT-PCR and that on nitric oxide (NO) by the Griess reaction.

RESULTS

Of the putative PTN signaling receptors, immortalized and primary chondrocytes (pc) expressed the anaplastic lymphoma kinase (ALK), less the receptor-type protein tyrosine phosphatase zeta/beta (PTPzeta). ALK expression was upregulated upon ligand exposure. PTN stimulation activated the AP-1 (activator protein-1) transcription factor and altered gene expression. Prolonged stimulation induced PTN mRNA expression slightly, reduced vascular endothelial growth factor (VEGF) mRNA as well as NO production. Whereas mRNA expression of matrix metalloproteinases (MMPs) MMP-1 and MMP-13 was reduced, their inhibitors TIMP-1 and TIMP-2 were induced. Furthermore, PTN stimulated chondrocyte migration and proliferation.

CONCLUSIONS

These results show that PTN is an autocrine growth factor in cartilage. We suggest that PTN may be involved in the clustering and proliferation of chondrocytes observed in the early stages of OA.

Subchondral bone in osteoarthritis: a biologic link with articular cartilage leading to abnormal remodeling

PURPOSE OF REVIEW

This review deals with new findings highlighting the concept of cross-talk between subchondral bone tissue and articular cartilage that may be crucial for the initiation and/or progression of osteoarthritis. In this review, new factors either produced by subchondral bone tissue or modifying osteoblast metabolism, yet implicated in osteoarthritis, are discussed.

RECENT FINDINGS

The development of cartilage degeneration is concomitant with subchondral bone thickness in osteoarthritis, whereas it is related to higher subchondral bone activity and dysregulation in the synthesis of bone proteins. As an immediate consequence, homotrimers of type 1 collagen are formed that could lead to undermineralization of this tissue. This dysregulation also leads to abnormal production of different factors by osteoblasts such as prostaglandins, leukotrienes, and growth factors. Because microcracks or neovascularization provide a link between the subchondral bone tissue and articular cartilage, these factors could contribute to the abnormal remodeling of osteoarthritic cartilage.

SUMMARY

These findings have an immediate implication for research because new tools need to be developed to study the subchondral bone-cartilage functional unit. Moreover, it could lead to a possible cure for osteoarthritis because this pathology should be considered both a bone and cartilage disease.