Effects of interleukin-17A on osteogenic differentiation of isolated human mesenchymal stem cells

Objectives: Rheumatoid arthritis (RA) is characterized by defective bone repair and excessive destruction and ankylosing spondylitis (AS) by increased ectopic bone formation with syndesmophytes. Since TNF-alpha and IL-17A are involved in both diseases, this study investigated their effects on the osteogenic differentiation of isolated human bone marrow-derived mesenchymal stem cells (hMSCs). Methods: Differentiation of hMSCs into osteoblasts was induced in the presence or absence of IL-17A and/or TNF-alpha Matrix mineralization (MM) was evaluated by alizarin red staining and alkaline phosphatase (ALP) activity. mRNA expression was measured by qRT-PCR for bone morphogenetic protein (BMP)-2 and Runx2, genes associated with osteogenesis, DKK-1, a negative regulator of osteogenesis, Schnurri-3 and receptor activator of nuclear factor kappa B ligand (RANKL), associated with the cross talk with osteoclasts, and TNF-alpha receptor type land TNF-alpha receptor type II (TNFRII). Results: TNF-alpha alone increased both MM and ALP activity. IL-17A alone increased ALP but not MM. Their combination was more potent. TNF-alpha alone increased BMP2 mRNA expression at 6 and 12 h. These levels decreased in combination with IL-17A at 6 h only. DKK-1 mRNA expression was inhibited by TNF-alpha and IL-17A either alone or combined. Supporting an imbalance toward osteoblastogenesis, RANKL expression was inhibited by TNF-alpha and IL-17A. However, TNF-alpha but not IL-17 alone decreased Runx2 mRNA expression at 6 h. In parallel, TNF-alpha but not IL-17 alone increased Schnurri-3 expression with a synergistic effect with their combination. This may be related to an increase of TNFRII overexpression. Conclusion: IL-17 increased the effects of TNF-alpha on bone matrix formation by hMSCs. However, IL-17 decreased the TNF-alpha-induced BMP2 inhibition. Synergistic interactions between TNF-alpha and IL-17 were seen for RANKL inhibition and Schnurri-3 induction. Such increase of Schnurri-3 may in turn activate osteoclasts leading to bone destruction as in RA. Conversely, in the absence of osteoclasts, this could promote ectopic bone formation as in AS.