Effects of Interleukin-4 and Interleukin-13 on Bone

Abstract: Cytokines play important roles in bone metabolism, participating in the complex interplay necessary for normal bone formation and turnover. The aim of the present thesis was to investigate the effects of two anti-inflammatory cytokines, interleukin-4 (IL-4) and interleukin-13 (IL-13) on bone. Influence of pro- and anti-inflammatory cytokines on interleukin-6 (IL-6) formation in cultured human osteoblasts (hOBs) was investigated. IL-4 and IL-13 as well as interleukin-1 (IL-1) and tumour necrosis factor alpha and beta (TNF-?/?) stimulated IL-6 secretion in hOBs. Also, IL-4 and IL-13 synergistically potentiated the effect of IL-1 and TNFs on IL-6 secretion. Effects of IL-4 and IL-13 on markers of osteoblastic activity in hOBs were investigated. IL-4 and IL-13 induced a dose-dependent increase in the formation of alkaline phosphatase (ALP) and pro-collagen type I carboxy-peptide (PICP) together with enhanced mineralization rate in hOBs. Formation of osteocalcin (OC) was unaffected. The mechanism behind inhibited proliferation by IL-4 and IL-13 in hOBs was investigated. IL-4 and IL-13 caused a dose-dependent increase in DNA-fragmentation together with escalating Caspase-3 activity in hOBs, reflecting induced apoptosis. Osteoblast apoptosis was also confirmed by TNF-?, dexamethasone and by serum starvation.The skeletal phenotype of IL-13-/-, IL-4-/-IL-13-/- and WT mice was compared. An altered cortical bone mass was detected in adult male IL-4-/-IL-13-/- mice. They displayed a reduction in cortical bone mineral content (BMC) secondary to reduced cortical thickness. Mechanical strength of the cortical bone was reduced in level with the reduction detected in BMC. Trabecular bone mineral density (tvBMD) was unaffected. Callus formation in IL-4-/-IL-13-/- and WT male mice was compared. No differences were found concerning radiological healing, biomechanical properties, callus parameters or histology. Heterotopic bone formation in IL-4-/-IL-13-/- and WT mice was compared using DXBM implants. No differences were found concerning mineralization of implants. Immuno-histology showed inhibition of autonomic nerves and lack of implant vascularization in IL-4-/-IL-13-/- mice. In summery, the two anti-inflammatory cytokines IL-4 and IL-13 influence osteoblast activity and apoptosis in vitro. They also selectively influence cortical bone formation in vivo. These findings suggest a role for IL-4 and IL-13 in osteoblast differentiation, in bone metabolism and in bone formation.