INSULIN-LIKE GROWTH-FACTORS INHIBIT INTERSTITIAL COLLAGENASE SYNTHESIS IN BONE CELL-CULTURES

Insulin-Like growth factor-I (IGF-I) and IGF-II are among the most pre valent growth factors secreted by bone cells and are presumed to act a s autocrine regulators of bone formation. We recently demonstrated tha t IGFs inhibit bone collagen degradation, and we postulated that they may either inhibit the expression of interstitial collagenase or stimu late the synthesis of tissue inhibitors of metalloproteinase-1 (TIMP-1 ), -2, or -3. We tested the effects of IGF-I and -II on collagenase an d TIMP-1, -2, and -3 expression in cultures of osteoblast-enriched cel ls from 22-day-old fetal rat calvariae (Ob cells). Steady state messen ger RNA (mRNA) levels were determined by Northern blot analysis, and c ollagenase concentrations were determined in the culture medium by a s pecific immunoassay. After 2-6 h of treatment, IGF-I and -II decreased collagenase transcripts by up to 80%. IGF-I was a more potent inhibit or than IGF-II, because it was active at doses as low as 10 nM, wherea s a dose of 100 nM was required to observe the IGF-II effect. In addit ion, IGF-I and -II opposed the stimulatory effect of retinoic acid on collagenase transcripts. Immunoreactive collagenase levels were not de tectable in control or IGF-treated cultures, but IGF-I and -II decreas ed the levels induced by retinoic acid by 70-90%. The protein synthesi s inhibitor cycloheximide superinduced collagenase transcripts, and IG F-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 synthe sis inhibitor hydroxyurea at 1 mM. Neither IGF-I nor IGF-II modified t he expression of TIMP-1, -2, or -3 mRNA in Ob cells. TIMP protein leve ls were not determined, and our study does not exclude a translational or posttranslational effect of IGF. In conclusion, IGF-I and -II decr ease 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.