Role for membrane and secreted insulin-like growth factor-binding protein-2 in the regulation of insulin-like growth factor action in lung tumors.
The insulin-like growth factors (IGFs) have been implicated in the autocrine and/or paracrine growth of a number of tumor types, including lung tumors. Importantly, insulin-like growth factor-binding proteins (IGFBPs), which both enhance and inhibit the physiological and biological actions of the IGFs, have been shown to be secreted in vitro by a wide range of tumors. In particular, IGFBP-2 is frequently produced by human tumor cells, suggesting that this protein may be an important determinant of IGF action in tumors. In the present study, we investigated IGFBP-2 effects in lung tumor cells by examining the influence of IGFBP-2 on IGF-receptor interaction and the biological actions of IGF-I and IGF-II. Affinity cross-linking studies demonstrated expression of type-I and type-II IGF receptors on small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC) cells and the presence of abundant membrane-associated IGFBP in SCLC cells but not in NSCLC cells. An antiserum specific for IGFBP-2 was used in immunoprecipitation and immunoblotting studies which demonstrated that the membrane-associated IGFBP identified by affinity cross-linking in SCLC cells is IGFBP-2. In NSCLC cells, both IGF-I and IGF-II bound predominantly to IGF-I receptors, whereas in SCLC cells binding was principally to surface-associated IGFBP-2. SCLC cells failed to respond to IGF-I and -II stimulation in a DNA synthesis assay. For NSCLC cells, IGF-II was a more potent stimulator of DNA synthesis than IGF-I. Soluble IGFBP-2 inhibited the binding of radiolabeled IGF-I and -II to both SCLC and NSCLC cells in a concentration-dependent manner and inhibited IGF-stimulated DNA synthesis in NSCLC cells. These observations indicate that both soluble and membrane-associated IGFBP-2 compete with IGF receptors for ligand binding and, thus, are likely to be important determinants of IGF responsiveness. The findings of the present study also indicate that the type-I receptor on NSCLC cells contains a high-affinity binding site for IGF-II which presumably mediates the biological effects of IGF-II in these cells, thereby implicating IGF-II in the autocrine/paracrine growth of NSCLC.
Reeve
J G
JG
Medical Research Council Clinical Oncology and Radiotherapeutics Unit, Medical Research Council Centre, Cambridge, United Kingdom.
Morgan
J
J
Schwander
J
J
Bleehen
N M
NM
D018288:Carcinoma, Small Cell / Q000378:metabolism*; D002352:Carrier Proteins / Q000302:isolation & purification / Q000378:metabolism* / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D018971:Insulin-Like Growth Factor Binding Protein 2; D007334:Insulin-Like Growth Factor I / Q000302:isolation & purification / Q000378:metabolism* / Q000494:pharmacology; D007335:Insulin-Like Growth Factor II / Q000302:isolation & purification / Q000378:metabolism* / Q000494:pharmacology; D007457:Iodine Radioisotopes; D007700:Kinetics; D008175:Lung Neoplasms / Q000378:metabolism*; D008970:Molecular Weight; D017526:Receptor, IGF Type 1 / Q000378:metabolism*; D017527:Receptor, IGF Type 2 / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism / Q000494:pharmacology; D014407:Tumor Cells, Cultured
D002352:Carrier Proteins; D018971:Insulin-Like Growth Factor Binding Protein 2; D007457:Iodine Radioisotopes; D017527:Receptor, IGF Type 2; D011994:Recombinant Proteins; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II; D017526:Receptor, IGF Type 1