Differential expression and biological effects of insulin-like growth factor-binding protein-4 and -5 in vascular smooth muscle cells.
Insulin-like growth factor-I (IGF-I) plays an important role in regulating vascular smooth muscle cell (VSMC) proliferation, migration, and apoptosis. The bioactivity of IGF-I is modulated by a group of high affinity, specific binding proteins (IGF-binding proteins; IGFBPs) that are present in the interstitial fluid. Previously, we have reported that porcine VSMCs synthesize and secrete IGF-I and several forms of IGFBPs, including IGFBP-2, IGFBP-4, and IGFBP-5. In this study, we examined the role of autocrine/paracrine secreted IGF-I in controlling the expression of IGFBP-4 and IGFBP-5 as well as the effects of these IGFBPs in modulating the cellular replication response to IGF-I. The concentrations of IGFBP-4 in the conditioned medium increased significantly from <50 ng/ml to 742 +/- 105 ng/ml. This increase was associated with a decrease in the activity of an IGF-I-regulated IGFBP-4 protease. In contrast, the synthesis of IGFBP-5 was inversely correlated with culture density, and its concentration decreased from 792 +/- 91 to 44 +/- 14 ng/ml. IGFBP-5 mRNA in sparse cultures was 3-fold higher compared with those in confluent cultures. This culture density-dependent change in IGFBP-5 mRNA correlated closely with endogenous IGF-I levels. Since treatment of VSMC with exogenous IGF-I increased IGFBP-5 mRNA levels, we neutralized the effect of endogenously secreted IGF-I with an anti-IGF-I antibody to determine if it would alter IGFBP-5 mRNA abundance. This resulted in a 4.4-fold decrease in IGFBP-5 mRNA levels. When added together with IGF-I, exogenous IGFBP-4 inhibited IGF-I-induced DNA synthesis in a concentration-dependent manner. IGFBP-5, on the other hand, potentiated the effect of IGF-I. Therefore, IGFBP-4 and IGFBP-5 appear to be differentially regulated by autocrine/paracrine IGF-I through distinct mechanisms. These two proteins, in turn, play opposing roles in modulating IGF-I action in stimulating VSMC proliferation.
Duan
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Department of Biology, University of Michigan, Natural Science Building, Ann Arbor, Michigan 48109-1048, USA. cduan@umich.edu
Clemmons
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D000818:Animals; D002478:Cells, Cultured; D005786:Gene Expression Regulation / Q000502:physiology; D018974:Insulin-Like Growth Factor Binding Protein 4 / Q000235:genetics / Q000378:metabolism*; D018975:Insulin-Like Growth Factor Binding Protein 5 / Q000235:genetics / Q000378:metabolism*; D007334:Insulin-Like Growth Factor I / Q000378:metabolism* / Q000502:physiology; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D013552:Swine
D018974:Insulin-Like Growth Factor Binding Protein 4; D018975:Insulin-Like Growth Factor Binding Protein 5; D012333:RNA, Messenger; D007334:Insulin-Like Growth Factor I