Integrins,insulin like growth factors,and the skeletal response to load

Bone loss during skeletal unloading, whether due to neurotrauma resulting in paralysis or prolonged immobilization due to a variety of medical illnesses, accelerates bone loss. In this review the evidence that skeletal unloading leads to bone loss, at least in part, due to disrupted insulin like growth factor (IGF) signaling, resulting in reduced osteoblast proliferation and differentiation, will be examined. The mechanism underlying this disruption in IGF signaling appears to involve integrins, the expression of which is reduced during skeletal unloading. Integrins play an important, albeit not well defined, role in facilitating signaling not only by IGF but also by other growth factors. However, the interaction between selected integrins such as αυβ3 and β1 integrins and the IGF receptor are of especial importance with respect to the ability of bone to respond to mechanical load. Disruption of this interaction blocks IGF signaling and results in bone loss. The skeletal response to mechanical load is critical for maintenance of skeletal integrity. This review will assess the interacting roles that insulin like growth factor I (IGF-I) signaling and selected integrins play in this response. Skeletal unloading results in decreased integrin expression, resistance to the anabolic actions of IGF-I, and bone loss.