D Schlaeger | PubMed Health

PubMed1990

IGF-I production by mouse osteoblasts.

Mouse osteoblasts contain and secrete insulinlike growth factor I (IGF-I), which can be measured by radioimmunoassay after separation from endogenous IGF-I binding activity. Our studies indicate that IGF-I is produced by all bone cell populations prepared by sequential digestion of mouse calvaria with collagenase and protease. Furthermore, relatively small amounts of IGF-I are cell associated, and IGF-I is recovered primarily in the cell medium after 24 h of culture. Basal IGF-I secretion is also density dependent, and secretion per cell is approximately 20-fold higher when cultures are inoculated at 0.125 versus 1.0 x 10(5) cells per cm2. Growth hormone increased the secretion of IGF-I only in cells released in the earlier stages of digestion. These growth hormone-responsive populations were previously shown to differ from late released cells in that they show a lower expression of the osteoblastic phenotype, harbor more EGF receptors per cell, and have a higher proliferative response to low doses of exogenous IGF-I and EGF. These data reaffirm the presence of different subclasses of bone cells in populations obtained by sequential digestion of bone and suggest that growth hormone stimulates IGF-I secretion by immature osteoblasts.

Animals↗

PubMed1989

Mitogenic responses to and binding of insulin-like growth factor 1 and/or epidermal growth factor by bone cells.

Populations of cells with different biochemical characteristics can be obtained following enzymatic digestion of newborn mouse calvaria. Previous studies had suggested an enrichment for periosteal fibroblasts in early released cells, based on their proliferative response to PGE2 and for osteoblasts in late released cells due to their high cAMP response to PTH. Intermediate cells were presumed to be a mixture of those two phenotypes. We have continued our characterization of these populations by studying their mitogenic responses to IGF-1 and EGF since these polypeptide growth factors have been reported to stimulate proliferation of immature bone cells in organ cultures of bone. [3H]Thymidine incorporation into acid insoluble radioactivity was stimulated by IGF-1 in all populations but the greatest increase was in intermediate populations. These latter cells were also the most highly responsive to EGF, which had a small growth stimulating effect on early released cells and no significant effect on late released cells. In bone cells both low and high affinity EGF binding sites were identified (Kd approximately 4 nM and 0.5 nM) and the total EGF binding sites/cell were approximately two-fold higher in intermediate than in early and late released populations. These data suggest that among sequentially released populations of bone cells the intermediate populations may be enriched for immature bone cells from the proliferative zone of bone.

Animals↗