Rôle du récepteur REG/EXTL3 dans l’inflammation et son implication possible dans l’ostéoarthrose (OA)

Osteoarthritis (OA) is an articular disease with a particularly high incidence in the elderly. This disease is characterized by the progressive degeneration of the cartilage followed by subchondral bone remodelling and a change in the soft tissues of the joint. Local chronic pain and joint malfunction are generally attributed to the inflammation of the synovial membrane, which in itself has been shown to significantly contribute to the pathogenesis of OA. In fact, the synthesis and expression of many proteolytic enzymes which degrade cartilage matrix are regulated by numerous cytokines originating from these inflammation sites. Two pro-inflammatory cytokines, the tumor necrosis factor alpha (TNF-α) and the interleukine-1β (Il-1β), play a major role in triggering inflammation associated with OA. These cytokines act on synoviocytes and chondrocytes by activating the transcription factor NF-κB, which in turn activates the cytokines’ genes. This positive regulating loop amplifies and maintains inflammatory responses. Recently, studies have shown that the over-expression of the REG receptor/EXTL3, a transmembranous receptor, enhances the activity of cytokine TNF-α in the activation of NF-κB. Unfortunately the mechanism involved in this process is still unknown. In addition, levels of EXTL3 and its ligand REG1B observed in OA patients suggest their possible involvement in the development of OA. Our goal was to study and elucidated the mechanisms used by EXTL3 to amplify NF-κB activation by TNF-α, as well as to examine whether the same phenomenon is occurring with IL-β. A human chondrocytes cell line called C28/I2 as experimental model. The techniques used for the current study were transfection assays, immunoflorescence (IF), immunoprecipitation (IP), and Western blotting (WB). Our transfection data have shown that EXTL3 was able to enhance NF-κB activity induced by TNF-α as well as by IL-1β. This result suggests that the enhanced NF-κB activity by EXTL3 is not specific to TNF-α. The IP experiments with TNFR1 and TRAF2 revealed the presence of EXTL3 in TNF-α/TNFR1 complex which is formed in the plasma membrane. Also, IF assay in combination with confocal microscopy allowed us to detect TNFR1/TRAF2/EXTL3 co-localisation on the nuclear membrane, suggesting the formation of TNF-α/TNFR1 complex on both the nuclear and plasma membranes. Somehow, REG1B, an EXTL3 ligand, and glucosamine were able to inhibit the formation of this complex at the plasma membrane. They were also able to abolish NF-κB activity enhanced by EXTL3. These results suggest that not only EXTL3 recruitment in the TNF-α/TNFR1 complex is required to amplify NF-κB activation by TNF-α, but also that REG1B ligand and glucosamine have the ability to modulate this activation by reducing or inhibiting EXTL3 and TNFR1 interactions. This study’s data represents a major advance in the understanding of molecular events controlling NF-κB activation by pro-inflammatory cytokines. These results could lead to the development of new therapeutics targets, in the treatment of disorders associated to OA and involving recurrent activation of NF-κB.

Keyphrases: inflammation, NF-κB, EXTL3, REG, glucosamine, Osteoarthritis