IL-6, IL-1β, and TNF-α only in combination influence the osteoporotic phenotype in Crohn's patients via bone formation and bone resorption

Background. Crohn ' s disease (CD) is associated with a higher prevalence of osteoporosis. The pathogenesis of bone affliction remains controversial, especially if inflammatory cytokines or glucocorticoid therapy are the main contributors. In postmenopausal osteoporosis, bone resorption is induced by IL-6, IL-1 beta and TNF-alpha. In contrast, in children with CD, IL-6 exclusively decreased bone formation without affecting bone resorption. Objectives. The objective of this study was to further clarify the pathophysiology of bone affliction in adult patients with CD with the use of an osteoblast and osteoclast cell model. Material and methods. Inflammatory cytokines IL-6, IL-1 beta, and TNF-alpha were measured in adult CD patients' serum. Mean values of these cytokines were applied with or without dexamethasone to the human cell line SCP-1 (osteoblastic cell model). Also, the effect of cytokines on primary human osteoclast differentiation and activity was determined. Results. The combined cytokine application increased the receptor activator of NF-kappa B ligand/osteoprotegerin (RANKL/OPG) ratio 2-fold after 2 and 14 days. Additional application of dexamethasone to SCP-1 cells further increased the RANKL/OPG ratio 3-fold, but decreased IL-6 and IL-1 beta expression to 10% and 50%, respectively. TNF-alpha expression was maximally suppressed to 16% by dexamethasone in the presence of cytokines. In osteoclasts, the combined cytokine treatment decreased expression of characteristic genes to approx. 30%, while increasing osteoclast resorption activity to 148%. In addition, a cytokine stimulated osteoblast cell culture-generated supernatant stimulated osteoclast resorption activity by 170%. Conclusions. Our results suggest that IL-6, IL-1 beta, and TNF-alpha only in combination induced osteoclast-stimulating activity represented by the RANKL/OPG ratio in osteoblasts. Dexamethasone further increased this effect in osteoblasts, while decreasing cytokine expression. The results in osteoclasts support a direct and osteoblast-mediated effect on bone resorption. Our in vitro results differentiate for the first time the effect of cytokines on bone turnover as measured in adult CD patients from the additional dexamethasone effect on osteoblasts as part of the pathophysiology of osteoporosis.