Differentiation of MC3T3-E1 osteoblasts is associated with temporal changes in the expression of IGF-I and IGFBPs.
We examined the relationship between osteoblast maturation and temporal changes in the secretion of IGF-I and the IGF-binding proteins (IGFBPs) 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/micrograms DNA and 3.9 +/- 0.6 at culture (days 3 and 9) and increased progressively in postmitotic differentiating osteoblasts, reaching a maximal concentration of 13.1 +/- 1.5 ng/micrograms 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 MC3T3-E1 development. We observed a sustained increase in IGFBP-2, -4, and -5 mRNA expression between days 10-14, coincident with the onset of differentiation. IGFBP-2 and IGFBP-4 protein concentrations increased in parallel with IGFBP mRNA expression, but IGFBP-5 levels peaked between days 8-14 of culture, and declined thereafter in spite of persistent IGFBP-5 mRNA levels. These findings suggest complex transcriptional and post-transcriptional regulation of IGFBP metabolism during osteoblast development. Thus, IGF-I and IGFBP production are regulated during osteoblast development. In turn, time-dependent changes in IGF-I and modulation of IGF-I bioavailability by IGFBPs may regulate the osteoblastic developmental sequence.
Department of Pediatrics, Duke University Medical Center, Durham, NC 27710, USA.
K M
KM
Thrailkill
S R
SR
Siddhanti
J L
JL
Fowlkes
L D
LD
Quarles
D000818:Animals; D015153:Blotting, Western; D002454:Cell Differentiation; D002460:Cell Line; D017077:Culture Media, Conditioned; D015151:Immunoblotting; D018969:Insulin-Like Growth Factor Binding Proteins / Q000096:biosynthesis* / Q000235:genetics; D007334:Insulin-Like Growth Factor I / Q000096:biosynthesis* / Q000235:genetics; D007335:Insulin-Like Growth Factor II / Q000096:biosynthesis; D051379:Mice; D010006:Osteoblasts / Q000166:cytology* / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D013997:Time Factors; D015854:Up-Regulation
D017077:Culture Media, Conditioned; D018969:Insulin-Like Growth Factor Binding Proteins; D012333:RNA, Messenger; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II