Liver-derived IGF-I is permissive for ovariectomy-induced trabecular bone loss

Introduction Estrogen deficiency results in trabecular bone loss, associated with T-cell proliferation in the bone marrow. Insulin-like growth factor I (IGF-I) is involved in the regulation of both bone metabolism and lymphopoiesis. A major part of serum IGF-I is derived from the liver. The aim of the present study was to investigate the role of liver-derived IGF-I for ovariectomy (ovx)-induced trabecular bone loss. Materials and methods Mice with adult liver-specific IGF-I inactivation (LI-IGF-I−/−) and wild type mice (WT) were either ovx or sham operated. After 5 weeks, the skeletal phenotype was analyzed by pQCT and microCT. The bone marrow cellularity was analyzed using FACS technique, and mRNA levels were quantified using real-time PCR. Results Ovx resulted in a pronounced reduction in trabecular bone mineral density (−52%, P < 0.001), number (−45%, P < 0.01) and thickness (−13%, P < 0.01) in WT mice while these bone parameters were unaffected by ovx in LI-IGF-I−/− mice. Furthermore, ovx increased the number of T-cells in the bone marrow of the femur in WT but not in LI-IGF-I−/− mice. Interleukin 7 (IL-7) has been reported to stimulate the formation and function of osteoclasts by inducing the expression of receptor activator of NF-κB ligand (RANKL) on T-cells. IL-7 mRNA levels and the RANKL/osteoprotegerin ratio in bone were increased by ovx in WT but not in LI-IGF-I−/− mice. Conclusions Liver-derived IGF-I is permissive for ovx-induced trabecular bone loss. Our studies indicate that IGF-I might exert this permissive action by modulation of the number of T-cells and the expression of IL-7, which in turn is of importance for the RANKL/OPG ratio and consequently osteoclastogenesis in the bone marrow.