DIFFERENTIATION OF MC3T3-E1 OSTEOBLASTS IS ASSOCIATED WITH TEMPORAL CHANGES IN THE EXPRESSION OF ICF-I AND IGFBPS

We examined the relationship between osteoblast maturation and tempora l changes in the secretion of IGF-I and the IGF-binding proteins (IGFB Ps) in the MC3T3-E1 model of osteoblast development. IGF-I was present at low levels in conditioned media in proliferating preosteoblasts (3 .7 +/- 1.7 ng/mu g DNA and 3.9 +/- 0.6 at culture (days 3 and 9) and i ncreased progressively in postmitotic differentiating osteoblasts, rea ching a maximal concentration of 13.1 +/- 1.5 ng/mu g DNA by day 25 of culture. We also observed an increase in IGF-I mRNA expression. Using Western ligand blot and immunoblot techniques, we found that IGFBP-2, -4, and -5 also displayed temporal differences in expression during M C3T3-E1 development. We observed a sustained increase in IGFBP-2, -4, and -5 mRNA expression between days 10-14, coincident with the onset o f differentiation. IGFBP-2 and IGFBP-4 protein concentrations increase d in parallel with IGFBP mRNA expression, but IGFBP-5 levels peaked be tween days 8-14 of culture, and declined thereafter in spite of persis tent IGFBP-5 mRNA levels. These findings suggest complex transcription al and post-transcriptional regulation of IGFBP metabolism during oste oblast development. Thus, IGF-I and IGFBP production are regulated dur ing osteoblast development. In turn, time-dependent changes in IGF-I a nd modulation of IGF-T bioavailability by IGFBPs may regulate the oste oblastic developmental sequence.