Insulin-like growth factor-I (IGF-I) and transforming growth factor-beta 1 release IGF-binding protein-3 from human fibroblasts by different mechanisms.
Human neonatal fibroblasts in monolayer culture secrete insulin-like growth factor-binding proteins (IGFBPs), which may modulate IGF action. To examine whether an increase in extracellular concentrations of IGFBPs in response to IGF-I is due to the release of cell-associated IGFBPs, we measured secreted and cell-associated IGFBP-3 immunologically in fibroblast monolayers treated with IGF-I and IGF analogs with altered affinities for the IGF receptors and IGFBPs. IGFBP-3 in medium conditioned by fibroblasts treated with IGF-I was significantly increased (P < 0.05) compared with that in medium from untreated cultures; concomitantly, cell-associated IGFBP-3 was significantly decreased (P < 0.05). [Ser24]IGF-I (reduced affinity for IGF receptors) also increased secreted IGFBP-3 and decreased cell-associated IGFBP-3. In contrast, IGFBP-3 concentrations in medium conditioned by fibroblasts treated with B-chain IGF-I (reduced affinity for IGFBPs) were not significantly increased, and cell-associated IGFBP-3 was unchanged. Heparin, which releases proteins attached to cell surface proteoglycans, increased medium concentrations of IGFBP-3 and decreased IGFBP-3 binding to fibroblasts. An IGFBP of 29-31 kilodaltons (kDa) showed a pattern of regulation similar to that of IGFBP-3, while a third IGFBP, of 24 kDa, was decreased in IGF-I- and [Ser24]IGF-I-conditioned medium and unchanged by B-chain IGF-I and heparin. Preincubation with transforming growth factor-beta 1 (TGF beta 1), which stimulates fibroblast IGFBP-3 production, or human serum-derived IGFBP-3 did not increase cell-associated IGFBP-3. Analysis of total RNA isolated from fibroblasts revealed that IGFBP-3 mRNA was increased by TGF beta 1, but not by IGF-I. These data suggest that IGFs and TGF beta 1 release fibroblast IGFBPs by distinct mechanisms: IGFs by binding and subsequent release of cell-associated IGFBP-3 and 29- to 31-kDa IGFBP, and TGF beta 1 by increased de novo synthesis of IGFBP-3.
Martin
J L
JL
Department of Endocrinology, Royal Prince Alfred Hospital, Camperdown, New South Wales, Australia.
Ballesteros
M
M
Baxter
R C
RC
D002352:Carrier Proteins / Q000276:immunology / Q000378:metabolism*; D003470:Culture Media; D005347:Fibroblasts / Q000378:metabolism; D006493:Heparin / Q000494:pharmacology; D006801:Humans; D007106:Immune Sera / Q000276:immunology; D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D011863:Radioimmunoassay; D012867:Skin / Q000166:cytology / Q000378:metabolism*; D013002:Somatomedins / Q000378:metabolism; D016212:Transforming Growth Factor beta / Q000494:pharmacology*
D002352:Carrier Proteins; D003470:Culture Media; D007106:Immune Sera; D018969:Insulin-Like Growth Factor Binding Proteins; D013002:Somatomedins; D016212:Transforming Growth Factor beta; D007334:Insulin-Like Growth Factor I; D006493:Heparin