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Insulin-like growth factor-I (IGF-I) is a ubiquitous peptide that regulates cellular growth and differentiation and is involved in vascular proliferative responses. The effects of IGF-I are modulated by several IGF-I binding proteins (IGFBPs), including IGFBP-4, the main IGFBP produced by vascular smooth muscle cells (VSMCs). We have previously shown that angiotensin II (Ang II)-induced and thrombin-induced mitogenesis in VSMCs is dependent on autocrine IGF-I. In addition, we have demonstrated that IGF-I and IGFBP-4 mRNA levels are upregulated in the hypertensive aorta of abdominally coarcted rats, a high-renin hypertension model. To obtain further insight into the IGF-I system and to specifically study changes in IGFBP-4, a known inhibitor of IGF-I action, VSMCs were incubated with Ang II or thrombin. Compared with control, Ang II induced an 87+/-2% downregulation of IGFBP-4 mRNA levels at 24 hours, with a 61+/-6% decrease of IGFBP-4 levels, as determined by Western ligand blot analysis. Thrombin had the same depressor effects (87+/-2% for the mRNA levels and 61+/-3% for the protein levels). Ang II and thrombin coincubation with (125)I-IGFBP-4 in the conditioned media failed to reveal any increase in fragmentation, indicating that proteolytic cleavage of IGFBP-4 was not involved in the observed effects. Exogenous recombinant human IGFBP-4 decreased thrombin-induced DNA synthesis of human aortic VSMCs by 64%, whereas anti-IGFBP-4 antibody potentiated thrombin-induced DNA synthesis. These data suggest that downregulation of IGFBP-4 expression in VSMCs may play a critical role in vascular growth response to Ang II and thrombin in normal and diseased states, by increasing the bioavailability of IGF-I for its cell-surface receptor.
Anwar et al. (Tue,) studied this question.