Different inhibitory actions of IGFBP-1, -2 and -4 on IGF-I effects in vascular smooth muscle cells

IGF-I is involved in the regulation of metabolism, growth and migration of vascular smooth muscle cells (VSMCs). We have studied how IGFBP-1, -2 and -4 modulate IGF-I-induced DNA and protein synthesis in cultured rat VSMCs. DNA and protein synthesis were measured as incorporation of [3H]thymidine and [3H]leucine into DNA and protein respectively. Western immunoblot was used to detect IGFBPs in conditioned medium and solution hybridization was used to measure IGFBP gene expression. IGF-I stimulated DNA synthesis with an EC50 of 44 pM, reaching a maximal effect at 1 nM. An IGF-I concentration of 1 nM was subsequently used in the experiments with IGFBPs. IGFBP-1 and IGFBP-4 acted in an inhibitory manner on IGF-I-induced DNA synthesis with calculated IC50 values of 1.6 nM and 6.2 nM respectively. IGFBP-2 (16 nM) also inhibited the growth response to IGF-I, but this effect could only be obtained if the two peptides were pre-incubated together for 2 h prior to addition to the cells. IGFBP-1, -2 and -4 inhibited IGF-I-induced protein synthesis in a similar way. Immunoblot of the incubation medium showed little degradation of IGFBP-2 and -4 for up to 24 h. mRNA for IGFBP-2 and -4, but not for IGFBP-1 was detected in the VSMCs. Endogenous IGFBP-2 and -4 could be detected by immunoblot in the conditioned medium but only if it was concentrated. In conclusion, IGFBP-1, -2 and -4, of which IGFBP-2 and -4 may be locally derived, act as inhibitors with different potencies on IGF-I effects in VSMCs.