Increased activity of insulin-like growth factor (IGF) in osteoblastic cells in the presence of growth hormone (GH): positive correlation with the presence of the GH-induced IGF-binding protein BP-3.
Insulin-like growth factors (IGFs) are bound in the circulation to specific binding proteins (BP). The predominant BP is a GH-dependent glycosylated protein of 42-49 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (BP-3), whereas nonglycosylated GH-independent IGFBPs of 32 kDa and less are minor constituents. Primary cultures of rat osteoblastic cells constitutively produce IGFBP species of 32 kDa, while GH induces the accumulation of BP-3. To examine whether BP-3 could regulate the biological activity of IGF-I on osteoblasts, we compared the effects of recombinant native human IGF-I (hIGF-I) on primary cultures of osteoblasts in the presence and absence of GH. hIGF-I stimulated cell replication and alpha 1(I) collagen gene expression in a dose-dependent manner, and these effects were potentiated by the presence of GH, which increased the accumulation of BP-3. To further examine this correlation, we compared the effects of two IGF-I peptides on the osteoblastic cell line PyMS, which constitutively produces BP-3, to those in RCT-3 cells, which do not secrete this IGFBP. Using hIGF-I and [Gln3,Ala4,Tyr15,Leu16]IGF-I ([QAYL]IGF-I), a mutated IGF-I with reduced affinity to IGFBPs, we found that at equimolar concentrations hIGF-I produced higher stimulation than [QAYL]IGF-I on [3H]thymidine incorporation, cell replication, and collagen gene expression in PyMS cells. In contrast, both IGF-I peptides had similar potency in RCT-3 cells. Hence, these data show that the accumulation of BP-3 correlates with enhanced hIGF-I activity on osteoblastic cells, suggesting that BPs may act locally to augment the effects of IGF-I in bone.
Ernst
M
M
Department of Bone Biology and Osteoporosis Research Merck, Sharp, and Dohme Research Laboratories, West Point, Pennsylvania 19486.
Rodan
G A
GA
D000595:Amino Acid Sequence; D000818:Animals; D000831:Animals, Newborn; D001667:Binding, Competitive; D002455:Cell Division / Q000187:drug effects; D002460:Cell Line; D002478:Cells, Cultured; D004261:DNA Replication / Q000187:drug effects; D015870:Gene Expression / Q000187:drug effects; D013006:Growth Hormone / Q000494:pharmacology*; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000378:metabolism / Q000494:pharmacology*; D007700:Kinetics; D008969:Molecular Sequence Data; D009154:Mutation; D009842:Oligopeptides / Q000494:pharmacology; D010006:Osteoblasts / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D011347:Procollagen / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics; D051381:Rats; D011956:Receptors, Cell Surface / Q000096:biosynthesis* / Q000187:drug effects / Q000378:metabolism; D017451:Receptors, Somatomedin; D011994:Recombinant Proteins / Q000378:metabolism / Q000494:pharmacology; D013002:Somatomedins / Q000494:pharmacology*
D009842:Oligopeptides; D011347:Procollagen; D012333:RNA, Messenger; D011956:Receptors, Cell Surface; D017451:Receptors, Somatomedin; D011994:Recombinant Proteins; D013002:Somatomedins; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone