A BMP-MEDIATED PATHWAY OF CHONDROCYTE BIOSYNTHETIC SUPPRESSION BY CORTICOSTEROIDS

Summary Corticosteroids (CS) are commonly administered by intra-articular injection to control the symptoms of osteoarthritis; however, CSs also suppress articular chondrocyte matrix synthesis. Both triamcinolone and methylprednisolone acetate significantly suppressed BMPs −2 and −7, and TGF-b1 expression, suggesting a mechanism by which CSs suppress articular chondrocyte matrix synthesis and cartilage homeostasis. Introduction Osteoarthritis (OA) is a common and debilitating disease that affects approximately 30% of the US population and is also a major clinical problem in companion animals. There are many drugs available to manage the symptoms of OA. Of these, intra-articular corticosteroid (CS) administration is a common and very effective anti-arthritic therapy, and is frequently administered to equine athletes. CSs exert their potent anti-inflammatory effects by blocking phospholipase A and reducing inflammatory mediator production; however, CSs also suppress matrix-biosynthetic activity of articular chondrocytes. This activity, along with ther increased joint use that symptomatic relief allows, has been linked to ‘steroid arthropathy’; a progression of arthritis driven by compromised chondrocyte homeostatic capacity. Several lines of experimental and clinical evidence emphasise the importance of TGF-b and BMP autocrine/paracrine activity in maintaining the homeostatic status of articular chondrocytes (reviewed in Oshin and Stewart 2007). This study was carried out to address the following objectives: 1) To assess the effects of CS on expression of chondro-protective TGF-β and BMP ligands in equine articular chondrocytes, and 2) To determine if exogenous BMP ligand administration can mitigate the suppressive effects of CSs on articular chondrocyte synthesis of collagen type II (Coll II) and glycosaminoglycans (sGAG). Methods Articular cartilage was collected from clinically normal joints of adult horses, euthanased for reasons other than musculoskeletal disease. Articular chondrocytes were isolated by collagenase digestion and cultured as aggregates in serum-free medium under non-adherent conditions (Stewart et al 2000). Triamcinolone acetate (TA) or methylprednisolone acetate (MPA) was added to the articular chondrocyte cultures at 10 −10 M, 10 −7 M, and 10 −5 M; comparable to in vivo exposure concentrations. Effects on Coll II, aggrecan/sGAG, BMP and TGF-b ligand expression were assessed by QPCR, Coll II ELISAs and DMMB assays. In a separate series of experiments, exogenous BMP-2 was administered to chondrocyte cultures exposed to CS supplementation, to determine whether BMP can prevent or mitigate CS-mediated suppression of matrix synthesis. Results BMP-2 and BMP-7 mRNA levels were significantly down-regulated by both CS treatments. In contrast, expression of BMPs-4 and 6 was not affected at any of the CS doses tested. TGF-b1 mRNA levels were significantly suppressed by both CSs at all doses tested. Somewhat surprisingly, TGF-b2 expression was increased by CS administration, though not significantly, while TGF-b3 expression was not affected. Exogenous BMP-2 administration (1–100 ng/ml) increased Coll II expression in the control groups but did not prevent the suppression of Coll II or sGAG synthesis in CS-treated chondrocytes. Discussion/Conclusions Both TA and MPA down-regulated BMP-2, BMP-7 and TGF-b1 mRNA expression in articular chondrocytes. These CS-mediated effects appear to be gene-specific, since BMPs-4 and 6, and TGF-bs 2 and 3 were not similarly affected. Although exogenous BMP-2 administration increased Coll II production under control conditions, this did not effectively prevent CS-mediated suppressive effects on cartilage matrix synthesis. This suggests that other elements of the articular chondrocyte BMP and/or TGF-b signaling pathways are also affected by CS administration.