Insulin-like growth factor-I from a perspective of bone regeneration

Abstract

The insulin-like growth factor I (IGF-I) is produced by osteoblasts and many other cell types. IGF-I is abundant in bone matrix and mediates some of the effects of growth hormone (GH). The present investigations have focused on IGF-I actions during bone regeneration. We have studied the IGF-I expression during fracture healing and during endochodral bone formation after demineralized bone matrix (DBM) implantation. We have also demonstrated that hyaluronan (HA) is a suitable slow release vehicle for IGF-I. Subsequently, we have investigated the potential effects of local administration of slow release IGF-I in attempts to influence endochondral bone formation, and in attempts to facilitate the healing of critical sized calvarial defects.

The expression of IGF-I mRNA was studied in rats during the healing process of closed tibial fractures, and expression of IGF-I mRNA and IGF-II mRNA were also analyzed during endochondral bone formation in intramuscular DBM implants. Solution hybridization/RNase protection assays were used to detect the specific mRNA transcripts in callus, fracture adjacent muscle (the anterior tibial muscle), contralateral non-affected tibial bone and muscle, and in harvested endochondral bone formation samples, respectively. The present studies show that fracture healing is associated with an increased local production of IGF-I. IGF-I is presented in various local cell types such as regenerating muscle cells, chondroblasts and osteoblasts. The lack of GH, using hypophysectomized rats, did not seem to decrease the expression of IGF-I. Indomethacin treatment during the event of fracture healing decreased and retarded the IGF-I expression. During endochondral bone formation, IGF-I mRNA showed a transient increase peaking on day 3 following DBM implantation. IGF-II mRNA reached the highest levels day 11-12. Old rats expressed lower IGF-I mRNA levels compared to young animals. In vitro and in vivo experiments were performed to investigate if HA can be used as a vehicle for the slow release delivery of IGF-I. Being a normal constituent in the body and also being enriched during normal wound and fracture healing, we find HA to be a suitable slow release vehicle for IGF-I. The HA concentration and presence of cross-links are parameters that influence the release properties of HA/IGF-I mixtures.

Endochondral bone formation was studied after intramuscular implantation of DBM implants, and attempts were made to influence bone formation by local addition of various doses of IGF-I dissolved in hyaluronan. A localized delivery of such IGF-I formulations did not increase bone formation in young rats. In old rats, bone formation was increased by IGF-I at a dose of 3000 ng/implant. The negative effect of corticosteroids on endochondral bone formation in young rats was partially reversed by IGF-I.

The healing of circular critical sized, Ø 4.5 mm, calvarial defects in young rats, was studied after the local addition of IGF-I dissolved in hyaluronan. Treatment with 50 [my]g IGF-I showed a significant stimulatory effect on the healing of the calvarial defects compared to controls and 5 [my]g IGF-I. In summary, our data suggests that increments of IGF-I production could be of functional significance for the formation of bone. This thesis also identifies hyaluronan as a useful vehicle for IGF-I and demonstrates that exogenous local IGF-I delivery has stimulatory effects on direct bone formation and during certain conditions on endochondral bone formation.