The 25-KiloDalton Insulin-Like Growth Factor (IGF)-Binding Protein Inhibits both Basal and IGF-I-Mediated Growth of Chick Embryo Pelvic Cartilagein Vitro

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Abstract
Insulin-like growth factor (IGF)-I stimulates the growth of many tissues, including growth plate cartilage. However, the role of IGF-binding proteins in the growth process is controversial. We purified a 25-kDa IGF-binding protein (BP-25) from amniotic fluid. We tested the effect of this BP-25 preparation on both basal and IGF-I-stimulated growth of chick embryo pelvic cartilages maintained in serum-free organ culture. Cartilage wet weight was 4.1 .+-. 0.3 mg/cartilage initially; after 3 days, BP-25 inhibited both basal and IGF-I-stimulated growth. Control cartilages weighted 7.4 .+-. 0.7 mg/cartilage, while those incubated with 100 nM BP-25 weighed 5.8 .+-. 0.5 mg/cartilage (P < 0.001 vs. control); BP-25 concentrations as low as 0.2 nM significantly inhibited basal cartilage growth. Cartilages incubated with 1.25 nM IGF-I weighted 10.4 .+-. 0.8 mg/cartilage (P < 0.001 vs. control), while those incubated with both 100 nM BP-25 and 1.25 nM IGF-I weighed 8.1 .+-. 0.5 mg/cartilage (P < 0.001 vs. cartilage incubated with IGF-I alone); BP-25 concentrations as low as 0.4 nM significantly inhibited IGF-I-stimulated cartilage growth. BP-25 also inhibited basal and IGF-I-stimulated increases in cartilage dry weight, [3H]thymidine incorporation into DNA, and 35SO4 incorporation into proteoglycan. A second BP-25 preparation, which in the presence of 1% platelet-poor plasma acts synergistically with IGF-I to stimulate DNA synthesis and cell replication of fibroblasts and smooth muscle cells in tissue culture, inhibited IGF-I-stimulated cartilage growth to the same degree as did out BP-25 preparation. In separate experiments, proteins present in serum-free medium conditioned for 3 days by chick cartilages were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, transferred to nitrocellulose, and incubated with [125I]IGF-I. This medium was found to contain two IGF-binding proteins; one appeared to be the chick equivalent of BP-25, while the other had a molecular mass similar to that of a poorly characterized human 34-kDa IGF-binding protein. We conclude that purified BP-25 inhibits the growth of chick embryo pelvic cartilage in our serum-free organ culture system. Since conditioned medium from these cartilages contains both IGF-I-like peptides and IGF-binding proteins such as BP-25, we suggest that the IGF-binding proteins present may act to downregulate the growth-promoting effects of the local IGF peptides.
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