AB0064 PHARMACOLOGICAL INHIBITION OF IL-6/JAK/STAT AXIS INCREASES MUSCLE MASS IN AN EXPERIMENTAL MODEL OF SARCOPENIA ASSOCIATED TO RHEUMATOID ARTHRITIS

Background Sarcopenia is a frequent comorbidity of rheumatoid arthritis (RA) due to an imbalance in muscle remodelling with dominance of catabolism over anabolism. IL-6/JAK/STAT axis is of particular importance in this regulation because of its dual activity. According to local and acute factors, it promotes muscle healing and hypertrophy, whereas in chronic inflammation, this signalling substantially increases muscle catabolism activating pro-atrophic pathways involving atrogene expression and ubiquitin proteasome system (UPS). JAK/STAT blockade suppresses muscle wasting induced by IL-6, through STAT3 activation. Interestingly, different clinical trials have shown that JAK inhibitors produce an asymptomatic increase in serum creatine kinase (CK) and creatinine levels in RA patients, suggesting an impact on muscle. Objectives To evaluate the effect of JAK inhibition and its effect in muscle remodelling in an experimental model that accurately mimics human rheumatoid sarcopenia. Methods An experimental model of antigen-induced arthritis (AIA) was carried out in 14 rabbits by immunization against ovalbumin followed by 4 intra-articular (i.a) injections of this protein. One week after the first i.a injection, 7 of these rabbits received tofacitinib (TOFA, orally 10mg/kg/day) for 2 wk. Animals were euthanized one day after the last i.a injection, when tibialis anterior (TA), extensor digitorum longus (EDL) and gastrocnemius (GN) were isolated from both hind limbs. Histological changes and ATPase staining were analysed in TA, while the number of myonuclei was evaluated in EDL fibres by immunofluorescence studies. C-reactive protein (CRP) and myostatin (MSTN) serum concentration were determined by ELISA. The gene expression of proinflammatory cytokines (IL-1β, IL-6, TNF-α, MCP-1), atrogenes (Atrogin-1, MuRF-1) and MSTN was measured by quantitative PCR, while proliferative muscle marker (PAX-7), differentiation muscle markers (MyoD, Myogenin), pSTAT3, pSTAT1, MSTN and CK protein expression was analysed by western blot in GN. Creatine presence in GN was analysed by a colorimetric assay. Results A significant increase in body weight was observed in the AIA+TOFA group vs. AIA at the end of the study. A significant increase in TA cross-sectional area and diameter in the AIA+TOFA, in comparison to AIA, was found, while the decrease in the area of type II fibres observed in the AIA animals was not noticed in AIA+TOFA. Moreover, the number of myonuclei in the EDL that was increased in the AIA group when compared to healthy animals, was significantly decreased in AIA+TOFA. However, systemic inflammation measured by CRP was not modified by TOFA treatment at the end of the study, in comparison to AIA, and so did serum MSTN concentration. TOFA evoked a significant reduction in the gene expression of IL-6, MCP-1, atrogin-1 and MuRF-1 in GN in comparison to untreated AIA rabbits. Notably, AIA+TOFA showed higher protein levels of CK and lower creatine compared to AIA in muscle. Simultaneously, no differences in the proliferative marker PAX-7 were found between groups, while AIA rabbits showed an increase in the differentiation markers MyoD and Myogenin in the muscle that was prevented in AIA+TOFA. Conclusion These data provide novel insights into the effects of JAK inhibitors in muscle remodelling during rheumatoid cachexia. Despite an elevated systemic inflammatory status, JAK inhibition was able to rapidly increase muscle mass through attenuating IL-6/JAK/STAT activation, decreasing atrogene expression, and restoring to baseline the muscle cell differentiation markers in the tissue. The increase in muscle mass was accompanied by an increase in CK presence, supporting the role of CK as a valuable marker of muscle gain following treatment with JAK inhibitors. However, the low levels of MCP-1 and the disorganization of muscle fibre myonuclei raise the question of whether these treatments could trigger a complete regeneration of muscle fibre cell or just hypertrophy of cells already present. Disclosure of Interests Ismael Bermejo: None declared, Sandra Pérez-Baos: None declared, Juan Pablo Medina: None declared, Paula Gratal: None declared, Ivan Prieto-Potín: None declared, Raquel Largo: None declared, Gabriel Herrero-Beaumont Grant/research support from: The Bone and Joint Research Unit has received funding from several pharmaceutical laboratories (Pfizer, LILLY...).