DIFFERENTIAL EXPRESSION AND BIOLOGICAL EFFECTS OF INSULIN-LIKE GROWTHFACTOR-BINDING PROTEIN-4 AND PROTEIN-5 IN VASCULAR SMOOTH-MUSCLE CELLS

Insulin-like growth factor-I (IGF-I) plays an important role in regula ting vascular smooth muscle cell (VSMC) proliferation, migration, and apoptosis, The bioactivity of IGF-I is modulated by a group of high af finity, specific binding proteins (IGF-binding proteins; IGFBPs) that are present in the interstitial fluid, Previously, we have reported th at porcine VSMCs synthesize and secrete IGF-I and several forms of IGF BPs, including IGFBP-2, IGFBP-4, and IGFBP-5, In this study, we examin ed the role of autocrine/paracrine secreted IGF-I in controlling the e xpression of IGFBP-4 and IGFBP-5 as well as the effects of these IGFBP s in modulating the cellular replication response to IGF-I. The concen trations 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 cult ure 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 chang e in IGFBP-5 mRNA correlated closely with endogenous IGF-I levels. Sin ce treatment of VSMC with exogenous IGF-I increased IGFBP-5 mRNA level s, we neutralized the effect of endogenously secreted IGF-I with an an ti-IGF-I antibody to determine if it would alter IGFBP-5 mRNA abundanc e. This resulted in a 4.4-fold decrease in IGFBP-5 mRNA levels. When a dded together with IGF-I, exogenous IGFBP-4 inhibited IGF-I-induced DN A 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 th rough distinct mechanisms. These two proteins, in turn, play opposing roles in modulating IGF-I action in stimulating VSMC proliferation.