Insulin-like growth factors inhibit interstitial collagenase synthesis in bone cell cultures

Abstract

Insulin-like growth factor-I (IGF-I) and IGF-II are among the most prevalent growth factors secreted by bone cells and are presumed to act as autocrine regulators of bone formation. We recently demonstrated that IGFs inhibit bone collagen degradation, and we postulated that they may either inhibit the expression of interstitial collagenase or stimulate the synthesis of tissue inhibitors of metalloproteinase-1 (TIMP-1), -2, or -3. We tested the effects of IGF-I and -II on collagenase and TIMP-1, -2, and -3 expression in cultures of osteoblast-enriched cells from 22-day-old fetal rat calvariae (Ob cells). Steady state messenger RNA (mRNA) levels were determined by Northern blot analysis, and collagenase concentrations were determined in the culture medium by a specific immunoassay. After 2-6 h of treatment, IGF-I and -II decreased collagenase transcripts by up to 80%. IGF-I was a more potent inhibitor than IGF-II, because it was active at doses as low as 10 nM, whereas a dose of 100 nM was required to observe the IGF-II effect. In addition, IGF-I and -II opposed the stimulatory effect of retinoic acid on collagenase transcripts. Immunoreactive collagenase levels were not detectable in control or IGF-treated cultures, but IGF-I and -II decreased the levels induced by retinoic acid by 70-90%. The protein synthesis inhibitor cycloheximide superinduced collagenase transcripts, and IGF-I or -II decreased this mRNA induction to levels similar to, but not lower than, those observed in control cultures. The effects of IGF-I and -II on collagenase transcripts were not modified by the DNA synthesis inhibitor hydroxyurea at 1 mM. Neither IGF-I nor IGF-II modified the expression of TIMP-1, -2, or -3 mRNA in Ob cells. TIMP protein levels were not determined, and our study does not exclude a translational or posttranslational effect of IGF. In conclusion, IGF-I and -II decrease interstitial collagenase transcripts as well as induced protease levels in Ob cells, and this effect may contribute to their inhibitory actions on bone collagen degradation.