ALTERED SKELETAL PATTERN OF GENE-EXPRESSION IN RESPONSE TO SPACEFLIGHT AND HINDLIMB ELEVATION

Spaceflight leads to osteopenia, in part by inhibiting bone formation. Using an animal model (hindlimb elevation) that simulates the weightl essness of spaceflight, we and others showed a reversible inhibition o f bone formation and bone mineralization. In this study, we have measu red the mRNA levels of insulin-like growth factor I (IGF-I), IGF-I rec eptor (IGF-IR), alkaline phosphatase, and osteocalcin in the tibiae of rats flown aboard National Aeronautics and Space Administration Shutt le Flight STS-54 and compared the results with those obtained from the ir ground-based controls and from the bones of hindlimb-elevated anima ls. Spaceflight and hindlimb elevation transiently increase the mRNA l evels for IGF-I, IGF-IR, and alkaline phosphatase but decrease the mRN A levels for osteocalcin. The changes in osteocalcin and alkaline phos phatase mRNA levels are consistent with a shift toward decreased matur ation, whereas the rise in IGF-I and IGF-IR mRNA levels may indicate a compensatory response to the fall in bone formation. mie conclude tha t skeletal unloading during spaceflight or hindlimb elevation resets t he pattern of gene expression in the osteoblast, giving it a less matu re profile.