Osteoarthritis (OA) is the most common human joint disease. Recent studies suggest that an abnormal subchondral bone metabolism is intimately involved in the genesis of this disease. Bone remodelling is tightly regulated by a molecular triad composed of OPG/RANK/RANKL. RANKL exists as 3 isoforms: RANKL1, 2, and 3. RANKL1 and 2 enhance osteoclastogenesis whereas RANKL3 inhibits this phenomenon. We previously reported that human OA subchondral bone osteoblasts can be discriminated into two subgroups according to their level of PGE2 [low (L) or high (H)]. Moreover, we also showed that L-OA osteoblasts express higher levels of total RANKL compared to H-OA osteoblasts. In this study, we investigated the level of membranous RANKL, comparing L- and H-OA subchondral bone osteoblasts, as well as its modulation by osteotropic factors. The impact of the modulation of RANKL1 and 3 on the membranous RANKL level was also studied.
Gene expression was determined using real-time PCR for RANKL1 and semi-quantitative PCR for RANKL3. Membranous RANKL was measured by flow cytometry. The modulation of membranous RANKL and RANKL isoforms was monitored on the L- and H-OA osteoblasts and also following treatment with osteotropic factors, including vitamin D3 (50 nM), IL-1beta (100 pg/ml), TNF-alpha (5 ng/ml), PGE2 (500 nM), PTH (100 nM), IL-6 (10 ng/ml) and IL-17 (10 ng/ml).
Membranous RANKL levels were significantly increased in L-OA osteoblasts compared to normal (p<0.01) and H-OA (p<0.05). The gene expression level of the RANKL1 profile was reminiscent of the membranous RANKL level. Although RANKL3 gene expression was lower on the H-OA osteoblasts than on normal and L-OA osteoblasts (p<0.03), the overall outcome favoured RANKL1. Treatment with the tested factors showed a significant increase in membranous RANKL on the L-OA osteoblasts, with the exception of PTH and IL-17. Interestingly in this subpopulation, the RANKL3 gene expression level was significantly increased upon PTH and IL-17 treatment. No effect of the tested osteotropic factors was found on the H-OA.
Our findings showed that the normal, L- and H-OA subchondral bone osteoblasts differentially express membranous RANKL and RANKL isoforms, and that treatment with osteotropic factors generally favours increased membranous localization of RANKL on L-OA compared to H-OA osteoblasts. This phenomenon appears to take place through differential modulation of each RANKL isoform.