title: Characterization of the high-affinity receptors on Swiss 3T3 cells which mediate the binding, internalization and degradation of the mitogenic peptide bombesin.
abstract: Bombesin and bombesin-related peptides such as gastrin-releasing peptide (GRP) stimulate DNA synthesis and proliferation of Swiss 3T3 cells in culture. We have used 125I-labelled [Tyr4]bombesin and 125I-labelled GRP to characterize and identify the receptors for these peptides on Swiss 3T3 cells. The binding of 125I-[Tyr4]bombesin, which retained full biological activity, was maximal between 20 and 30 min incubation at 37 degrees C, after which continued incubation led to a decline in cell-associated radioactivity. This decline was markedly slowed by the presence of lysosomal enzyme inhibitors. Specificity of the binding site was indicated by the competitive inhibition of binding by bombesin-related peptides, but not by unrelated peptides and growth factors. Scatchard analysis of binding data indicated a single class of high-affinity receptors. The calculated value for the dissociation constant (Kd) was 2.1 nM and each cell possesses approx. 240,000 receptors. Because [Tyr4]bombesin has no free amino group, 125I-GRP was used in chemical cross-linking studies. When disuccinimidyl suberate was used to covalently couple 125I-GRP to the cells, two major radiolabelled complexes were detected with molecular masses of approx. 80,000-85,000 and 140,000. The binding of 125I-[Tyr4]bombesin to the cells was pH-dependent with maximal binding at pH 6.5-7.5 and effectively no specific binding at pH values below 4.5. At 37 degrees C, cell-associated 125I-[Tyr4]bombesin quickly became resistant to removal by acidic buffers, suggesting its rapid transfer to an intracellular compartment. However, pre-incubation with unlabelled [Tyr4]bombesin did not induce down-regulation of bombesin receptors as measured by the subsequent binding of 125I-[Tyr4]bombesin. In contrast with the Swiss 3T3 cells, specific binding of 125I-[Tyr4]bombesin was not detectable in two cell lines which are biologically unresponsive to bombesin-related peptides.
authors.affiliation: AFRC Institute of Animal Physiology, Babraham, Cambridge, U.K.
authors.forename: K D
authors.initials: KD
authors.lastname: Brown
authors.forename: M S
authors.initials: MS
authors.lastname: Laurie
authors.forename: C J
authors.initials: CJ
authors.lastname: Littlewood
authors.forename: D M
authors.initials: DM
authors.lastname: Blakeley
authors.forename: A N
authors.initials: AN
authors.lastname: Corps
mesh_terms: D001839:Bombesin / Q000378:metabolism*; D002460:Cell Line; D003432:Cross-Linking Reagents; D004591:Electrophoresis, Polyacrylamide Gel; D019886:Gastrin-Releasing Peptide; D006863:Hydrogen-Ion Concentration; D007457:Iodine Radioisotopes; D008024:Ligands; D010455:Peptides / Q000378:metabolism; D018004:Receptors, Bombesin; D017981:Receptors, Neurotransmitter / Q000378:metabolism*; D013388:Succinimides
chemical_list: D003432:Cross-Linking Reagents; D007457:Iodine Radioisotopes; D008024:Ligands; D010455:Peptides; D018004:Receptors, Bombesin; D017981:Receptors, Neurotransmitter; D013388:Succinimides; D019886:Gastrin-Releasing Peptide; D001839:Bombesin; C019358:disuccinimidyl suberate