Effect of locally infused IGF-I on femoral gene expression and bone turnover activity in old rats.

Previously, we showed that the age-dependent deficit in bone formation activity can be attributed in part to a decline in local expression of insulin-like growth factor I (IGF-I) and altered mitogenic response of old osteoprogenitor cells to IGF-I. To establish the cellular basis for using IGF-I as a possible therapeutic agent for osteoporosis, we examined the effect of locally infused (50 ng/day for 14 days) on the expression of osteoblast-related genes in femurs of old rats. Northern and dot blot analyses showed that the expression of procollagen (I), osteopontin, alkaline phosphatase, and osteocalcin was increased 0.4- to 1.5-fold in IGF-I-treated femurs as compared with control femurs. Histomorphometric analyses were carried out in parallel experiments to assess the changes in bone remodeling activity. Trabecular bone volume, trabecular number, and trabecular thickness were increased 56%, 29%, and 23%, respectively, whereas trabecular separation was reduced 26% by IGF-I treatment. IGF-I treatment increased significantly the osteoid volume, osteoid surface, osteoblast number, and osteoblast surface. Mineralizing surface and mineral apposition rate, kinetic indices of bone formation, were also stimulated by IGF-I treatment. The bone formation rate was stimulated 81% in IGF-I-treated femurs as compared with control femurs. In contrast, eroded surface and osteoclast surface, parameters associated with bone resorption, were not affected by IGF-I treatment. These findings suggest that local administration of IGF-I into femurs of old rats can stimulate the expression of matrix proteins and improve trabecular bone status by stimulating bone formation without any appreciable effect on bone resorption.