Insulinlike growth factor-binding protein modulates the growth response to insulinlike growth factor 1 by human gastric cancer cells

Background: This study determined whether the resistance to the mitogenic effect of insulinlike growth factor 1 (IGF-1) in AGS (we found that IGF-1 had almost no effect on the growth of AGS) cells is caused by the absence of IGF-1 receptor on the cells or by the interference of endogenous IGFs and IGF-binding protein (IGFBP). Methods: IGF-1 receptors were examined by radioligand binding assay. The protein in conditioned medium and the molecular weight of IGF-1 receptors on AGS cells were determined by affinity cross-linking with ¹²⁵I-IGF-1 followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Messenger RNAs for IGF-1, IGF-2, and IGFBP-4 were detected by Northern analysis. Results: AGS cells possessed a single class of high-affinity binding sites for IGF-1 (dissociation constant [K_d], 0.51), with a binding capacity ~ 4 × 10⁴ sites per cell. The size of the α subunit of IGF-1 receptors on cell membranes was ~ 130 kilodaltons. des(1-3) IGF-1, a truncated IGF-1 with very low affinity to IGFBPs, stimulated AGS cell growth in dose-dependent fashion. The medium conditioned by AGS cells contained IGFBPs of 27-32 and 37-42 kilodaltons. AGS cells expressed messenger (mRNA) RNAs for IGF-2 and IGFBP-4 but not for IGF-1, whereas another gastric carcinoma cell line (SIIA), whose growth is stimulated by IGF-1, expressed mRNA IGF-2 but did not express mRNA for IGF-1 or IGFBP-4: Conclusions: The relative absence of growth response of AGS cells to IGF-1 is due to the endogenously produced IGFBPs sequestering IGF-1 and preventing receptor interaction.