Effects of relaxin and estrogens on bone remodeling markers, receptor activator of NF-kB ligand (RANKL) and osteoprotegerin (OPG), in rat adjuvant-induced arthritis

Rheumatoid arthritis (RA) is characterized by joint inflammation and bone destruction. The receptor activator of nuclear factor-kappa B ligand (RANKL)-osteoprotegerin (OPG) system is important for maintaining the balance between bone resorption and formation. Both serum RANKL/OPG protein and synovial tissue RANKL/OPG mRNA ratios are elevated in patients with RA. Studies indicate that hormones of pregnancy, estrogens and relaxin, administered in combination, reduce circulating (TNF)-α and joint inflammation in a rat adjuvant-induced arthritis (AIA) model of RA. The purpose of this study was to investigate whether relaxin (RLX), alone or in combination with estrogens, regulates the bone remodeling markers RANKL and OPG in vivo and in vitro. Results show that in AIA rats, treatment with relaxin, estradiol valerate (EV) or EV in combination with relaxin had no effect on circulating RANKL. However, EV increased systemic OPG and combined treatment with EV and relaxin further increased circulating OPG in comparison to EV alone. Importantly, the RANKL/OPG protein ratio was lower in rats treated with EV or EV + RLX when compared to arthritic controls. Relaxin in combination with EV decreased local RANKL transcripts, increased OPG mRNA and decreased the RANKL/OPG mRNA ratio in joints of arthritic rats when compared to controls. RLX family peptide receptor 1 (RXFP1) gene expression in joints of AIA rats increased in response to EV and EV + RLX. In parathyroid hormone-pretreated murine UMR 106–01 osteoblast cells, 17-β-estradiol (E) and E + RLX increased RXFP1 transcripts, while RLX reduced RANKL mRNA and protein expression. However, in vitamin D-treated primary rat osteoblast cells E + RLX increased OPG protein and reduced the RANKL/OPG protein ratio. These results suggest that modulation of the RANKL-OPG system by estrogens and RLX may contribute to their antiarthritic effects on bone during pregnancy.

Research highlights
► Circulating OPG protein increased and RANKL/OPG protein ratio declined in response to EV and EV + RLX in AIA rats.
► Estrogens in combination with RLX decreased the local joint RANKL/OPG gene expression ratio in arthritic rats comparable to healthy controls.
► In primary rat osteoblast cells, E + RLX decreased the RANKL/OPG protein ratio similar to the response in AIA rats.
► Hormones of pregnancy, estrogens and RLX, by reducing the RANKL/OPG ratio are bone-protective.