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. D NA and protein synthesis were measured as incorporation of [H-3]thymidine a nd [H-3]leucine into DNA and protein respectively. Western immunoblot was u sed to detect IGFBPs in conditioned medium and solution hybridization was u sed to measure IGFBP gene expression. IGF-I stimulated DNA synthesis with a n 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 I GFBP-4 acted in an inhibitory manner on IGF-I-induced DNA synthesis with ca lculated IC50 values of 1.6 nM and 6.2 nM respectively. IGFBP-2 (16 nM) als o inhibited the growth response to IGF-I, but this effect could only be obt ained if the two peptides were pre-incubated together for 2 h prior to addi tion to the cells. IGFBP-1, -2 and -4 inhibited IGF-I-induced protein synth esis in a similar way. Immunoblot of the incubation medium showed little de gradation of IGFBP-2 and -3 for up to 24 h. mRNA for IGFBP-2 and -4, but no t for IGFBP-1 was detected in the VSMCs. Endogenous IGFBP-2 and -4 could be detected by immunoblot in the conditioned medium but only ii it was concen trated. In conclusion, IGFBP-1, -2 and -4, of which IGFBP-2 and -4 may be l ocally derived, act as inhibitors with different potencies on IGF-I effects in VSMCs.