Overview

Recovery of IGF-1 Signaling in Bone by Skeletal Reloading (Postdoctoral Fellowship)

Principal Investigator:
Roger Long, M.D.

Organization:
University of California, San Francisco

Prolonged spaceflight causes low bone-mineral density (osteopenia) because in microgravity, bones do not bear weight, leading to a decrease in bone formation. Research indicates that microgravity-induced osteopenia may be caused by a disruption in the interaction between an integral protein receptor on cell membranes and a receptor for a bone formation growth factor. NSBRI Postdoctoral Fellow Dr. Roger K. Long has designed a project to test the elements required to activate these receptors and reverse bone formation resistance. He hopes to determine whether a prescription of intermittent weight-bearing with an infusion of growth factor works as a countermeasure treatment to prevent bone loss associated with spaceflight.

NASA Taskbook Entry

Technical Summary

Prolonged spaceflight causes osteopenia due to decreased bone formation secondary to impaired osteoblast proliferation and increased osteoblast apoptosis. Hindlimb unloading by tail suspension, a model for skeletal unloading of spaceflight, causes osteoblast precursor and skeletal tissue resistance to the effects of insulin-like growth factor-1 (IGF-1). The nature of this resistance is characterized by decreased activation of the IGF-1 receptor and downstream signaling pathways. Osteoblast precursors isolated from unloaded bones demonstrate decreased expression of integrins, putative mechanosensors that bind extracellular matrix proteins, and in vitro treatment of normal osteoblasts with echistatin, an integrin antagonist, recreates the phenomena of unloading-induced IGF-1 resistance. Additionally, mechanical stimulation of human osteoblasts activates the IGF-1 receptor and augments the receptor response to IGF-1. These effects are abrogated by echistatin treatment. These findings suggest that integrin receptors have a role in mechanical regulation of IGF-1 receptor in osteoblasts.

Our hypothesis is that interaction of integrin and IGF-1 receptor signaling cascades is required for IGF-1 activation of its receptor and intact IGF-1 signaling in osteoblasts. Mechanical loading stimulates the formation of an integrin/IGF-1 receptor complex, thus, enhancing IGF-1 signaling and enabling mechanically induced osteoblast proliferation and bone formation. To test the hypothesis, we proposed the following specific aims:

Determine the means by which skeletal reloading regulates skeletal response to IGF-1. Tail suspended rats treated with IGF-1 will be skeletally reloaded to preserve bone mass and osteoblast IGF-1 signaling. Integrity of IGF-1 signaling will be correlated with the interaction between integrins and the IGF-1 receptor.
Determine the mechanism for regulation of the osteoblast IGF-1 receptor by integrins in response to mechanical loading by pulsatile fluid flow. Formation of the integrin and IGF-1 receptor complex in human osteoblasts will be stimulated by fluid flow loading in order to determine the components within the complex and their function.

At the completion of this project, we established the critical regulatory role of integrins in IGF-1 signaling of osteoblasts. Mechanical stimulation of human osteosarcoma (HOS) cells activates the IGF-1R, including the proliferative mitogen-activated protein kinases (MAPK) signal pathway and the anti-apoptotic serine-threonine protein kinase (Akt) signal pathway, in a ligand independent manner. The sensitivity of HOS cells and its IGF-1R to mechanical stimulation requires and is influenced by integrin receptor signaling. This is illustrated by impaired activation of IGF-1R induced by echistatin and augmented activation of IGF-1R by stimulation of integrin receptors with exposure to their ligands. The results of siRNA knockdown experiments indicate that both integrin beta 1 and beta 3 are required for normal activation of the IGF-1R by treatment with either ligand or mechanical stimulation. The integrin regulation of IGF-1 signaling in osteoblasts allows osteoblasts to sense and respond to mechanical load, providing a link between mechanical loading, bone formation and skeletal integrity.

The feasibility of intermittent cyclic skeletal loading and IGF-1 infusion to prevent unloading-induced bone loss was evaluated. Tail suspended rats treated with intermittent short-duration cyclic loading totaling only 42 minutes over two weeks had significantly blunted unloading-induced trabecular bone loss. The mechanism of the enhanced response of the unloaded bones to cyclic loading is due to augmentation of bone formation rather than reduction in osteoclast activity, and these responses were not affected by IGF-1 treatment. However, within the cortical bone compartment, the intermittent loading-induced stimulation of periosteal bone formation was further augmented by treatment with IGF-1. These findings demonstrate the complex nature of the skeletal response to mechanical loading in that in cancellous bone, the threshold for mechanical stimulation-induced bone formation in the unloaded state is lower than required to prevent unloading-induced osteoclast-mediated bone loss.And, IGF-1 has a limited role in modulating the response of cancellous bone to mechanical loading but works synergistically with mechanical loading in cortical bone at the periosteal bone surface. The complexity of these differing skeletal responses to mechanical and pharmacologic treatments and the interactions between the integrin and IGF-1 signaling cascades in osteoblasts will need to be considered in the design of potential targets for countermeasure treatments to preserve the effects of mechanical loading and prevent the bone loss of spaceflight.

Earth Applications

The research project outlined in my postdoctoral fellowship focuses on understanding the role of the hormone insulin-like growth factor 1 (IGF-1) in the skeletal response to skeletal unloading and loading. Previous work from our lab has demonstrated that skeletal unloading by tail suspension, a disuse osteoporosis model that not only mimics the weightlessness of spaceflight but the skeletal unloading associated with prolonged bedrest or neurological injury, causes a resistance to the anabolic effects of IGF-1 both in vivo and in vitro. A disruption of integrin signaling has been associated with the resistance of the IGF-1 receptor, and the role of the interaction between these two signaling pathways will be explored. A membrane-associated complex of the integrin and IGF-1 receptors would enable mechanically-induced bone formation and osteoblast proliferation via enhanced IGF-1 signaling. Understanding the interaction between integrin receptors, with their ability to sense mechanical load, and the powerfully anabolic effects of IGF-1 is vital for efforts to maximize bone accrual and minimize bone loss associated with skeletal unloading. We propose to test the hypothesis that the skeletal response to mechanical load requires the formation of a complex including selected integrins and IGF-1R. If this hypothesis turns out to be valid, efforts to maintain the integrin signaling pathway will prove efficacious in preventing the skeletal loss of responsiveness to IGF-1 associated with skeletal unloading. We envision that the use of intermittent skeletal loading, to maintain the integrity of the integrin and IGF-1 receptor interaction and thus the skeletal sensitivity to IGF-1, combined with IGF-1 therapy could be incorporated into hospital stays to prevent acute bone loss. This combination could also be used in rehabilitation programs to stimulate recovery of bone loss from acute injury or illness.

This project's funding ended in 2009