Basic fibroblast growth factor regulates IGF-I binding proteins in the clonal osteoblastic cell line MC3T3-E1

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Abstract
In previous studies, we reported that basic fibroblast growth factor (bFGF) regulates insulin-like growth factor messenger RNAs and protein levels in the osteoblastic MC3T3-E1 cells. In the present study, we examined the expression of insulin-like growth factor binding proteins (IGFBPs) in MC3T3-E1 cells and determined whether bFGF altered IGFBP mRNAs and protein levels. Since previous studies suggested that IGFBPs can inhibit DNA synthesis stimulated by IGF-I, we wondered whether the mitogenic effect of bFGF was altered by exogenous IGFBP-3. Confluent MC3T3-E1 cells were serum-deprived for 24 h and then treated with bFGF for 6–24 h. In control cultures, MC3T3-E1 cells expressed the mRNAs for IGF-I, IGF-II, and IGFBP-2, 4, 5, and 6 but not IGFBP-1 or 3. A 24 h treatment with bFGF at 10−8 M decreased IGF-I mRNA by 97%, IGF-II mRNA by 73%, IGFBP-2 by 64%, IGFBP-4 by 73%, IGFBP-5 by 95%, and IGFBP-6 by 65%. The inhibitory effect of bFGF on IGF-I and IGFBP mRNA levels was not altered by aphidicolin, an inhibitor of cell replication. bFGF 10 nM decreased IGF-I levels determined by radioimmunoassay after acidification by 45% and 72% at 24 and 48 h, respectively. Western ligand blot for IGF binding proteins revealed that MC3T3-E1 cells expressed IGFBPs of 24, 30, and 34 kD. Treatment with bFGF 10−8 M decreased the levels of the 24 and 30 kD band at 24 h but increased the 34 kD band. Western immunoblot revealed that the 24 kD protein was IGFBP-4 and the 34 kD band was IGFBP-2. bFGF at 10−9-10−8 M increased thymidine incorporation into DNA (TdR) in a dose-dependent manner. When exogenous IGFBP-3 was added to the cultures there was a significant reduction in DNA synthesis while the mitogenic effect of bFGF was not blocked. In summary, bFGF not only regulates IGF-I mRNA and protein levels but it also regulates the IGF-II mRNA and mRNA and protein levels of the IGFBPs expressed in MC3T3-E1 cells. However, the mitogenic effect of bFGF may be independent of endogenous IGF-I. These results may be important in understanding the role of bFGF in bone cell function.
Funding Information
  • Public Health Service (AR-38933, AR-42368)

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