Insulin-like growth factor binding proteins-2 to -6 are expressed by human vascular smooth muscle cells

We have investigated the expression and secretion of insulin-like growth factor binding proteins (IGFBPs-1 to -6) in human vascular smooth muscle cells (hVSMCs) cultured from human renal arteries. Solution hybridization was used to determine IGFBP mRNA levels and Western immunoblot to detect the corresponding peptides. The hVSMCs expressed mRNAs for IGFBPs-2 to -6; IGFBP-1 mRNA was not detected. IGFBPs-3, -4 and -6 mRNAs were the most abundant, IGFBP-5 was also highly expressed, whereas the IGFBP-2 mRNA was just above the limit of detection. Serum starvation for 48 h significantly decreased the mRNA levels of IGFBPs-2 to -5 and tended to decrease IGFBP-6 mRNA also. IGFBPs-2, -4, -5 and -6 peptides could be detected in conditioned medium, but IGFBP-3 peptide was not detected. IGFBP-4 was the only peptide detected without any concentration step. Low-molecular-mass immunoreactive degradation products were found for IGFBPs-2 and -4. Exogenous IGFBPs-1, -3 and -4 in concentrations of 50 ng/ml inhibited DNA synthesis induced by 1 nM IGF-I, whereas IGFBPs-2, -5 and -6 had no significant inhibitory effects at this concentration. We conclude from these results that all IGFBPs except IGFBP-1 are expressed in hVSMC. Our results indicate that locally produced, in addition to circulating, IGFBPs may have an important role in the regulation of hVSMC.