Expression of cannabinoid receptors by human articular chondrocytes

Osteoarthritis (OA) is a progressive degenerative disease of joints, causing pain and disability. Although all joint tissues are affected, changes in articular cartilage are predominant. Recent studies have shown that cannabinoid receptors are expressed in bone and the endocannabinoid system may be involved in the regulation of bone turnover. Two, cannabinoid receptors have been identified; CB1 and CB2. A third receptor, GPR55, also binds to certain cannabinoid agonists. The aim of this study was undertaken to assess whether functional cannabinoid receptors were expressed by chondrocytes, the cells of cartilage, and to establish whether they may have a role in osteoarthritis.

Expression of CB1, CB2 and GPR55 receptors by normal and OA chondrocytes was assessed by RT-PCR and Western blotting. To investigate potential functions of the receptors, chondrocytes were stimulated with specific CB1 and CB2 agonists and phosphorylation of Akt, and the MAPkinases ERK and p38 was investigated.

Normal chondrocytes expressed CB2 and GPR55 receptors, and OA chondrocytes showed expression of all three receptors. Normal chondrocytes stimulated by the CB1 and CB2 agonists did not show up-regulation of the signaling pathways investigated. Stimulation of OA chondrocytes with a CB2 agonist whilst showing no effect on Akt or ERK phosphorylation increased p38 phosphorylation. These studies show that chondrocytes derived from normal and OA human articular cartilage express cannabinoid receptors but potential roles in regulating pathological processes in OA remain to be defined.