title: Characterization of ligand binding and processing by bombesin receptors in an insulin-secreting cell line.
abstract: Bombesin is a tetradecapeptide which stimulates insulin secretion in vivo by isolated islets and by HIT-T15 cells, a clonal line of hamster pancreatic-islet cells. In the present study we have used [125I-Tyr4]bombesin to characterize bombesin receptors in HIT-T15 cells. [125I-Tyr4]Bombesin binding was time- and temperature-dependent: maximum binding occurred after 45 min, 90 min and 10 h at 37, 22 and 4 degrees C respectively. Thereafter, cell-associated radioactivity declined at 37 degrees C and 22 degrees C but not at 4 degrees C. Scatchard analysis of [125I-Tyr4]bombesin binding measured at 4 degrees C showed that HIT-T15 cells contain a single class of binding sites (approximately equal to 85000/cell) with an apparent Kd of 0.9 +/- 0.11 nM. Structurally unrelated neuropeptides did not compete for [125I-Tyr4]bombesin binding. However, the relative potencies of bombesin and four bombesin analogues in inhibiting the binding of [125I-Tyr4]bombesin correlated with their ability to stimulate insulin release. Receptor-mediated processing of [125I-Tyr4]bombesin was examined by using an acid wash (0.2 M-acetic acid/0.5 M-NaCl, pH 2.5) to dissociate surface-bound peptide from the cells. Following [125I-Tyr4]bombesin binding at 4 degrees C, more than 85% of the cell-associated radioactivity could be released by acid. When the temperature was then increased to 37 degrees C, the bound radioactivity was rapidly (t1/2 less than 3 min) converted into an acid-resistant state. These results indicate that receptor-bound [125I-Tyr4]bombesin is internalized in a temperature-dependent manner. In fact, the entire ligand-receptor complex appeared to be internalized, since pretreatment of cells with 100 nM-bombesin for 90 min at 37 degrees C decreased the subsequent binding of [125I-Tyr4]bombesin by 90%. The chemical nature of the cell-associated radioactivity was determined by reverse-phase chromatography of the material extracted from cells after a 30 min binding incubation at 37 degrees C. Although 70% of the saturably bound radioactivity was co-eluted with intact [125I-Tyr4]bombesin 90% of the radioactivity subsequently dissociated from cells chromatographed as free iodide. At least some of the degradation of receptor-bound [125I-Tyr4]bombesin appeared to occur in lysosomes, since chloroquine increased the cellular accumulation of [125I-Tyr4]bombesin at 37 degrees C and slowed the release of radioactivity.(ABSTRACT TRUNCATED AT 400 WORDS)
authors.affiliation: Department of Pharmacology, Harvard Medical School, Boston, MA.
authors.forename: S L
authors.initials: SL
authors.lastname: Swope
authors.forename: A
authors.initials: A
authors.lastname: Schonbrunn
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D001839:Bombesin / Q000031:analogs & derivatives / Q000378:metabolism*; D002460:Cell Line; D002738:Chloroquine / Q000494:pharmacology; D006224:Cricetinae; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000378:metabolism*; D007700:Kinetics; D008024:Ligands; D018004:Receptors, Bombesin; D017981:Receptors, Neurotransmitter / Q000187:drug effects / Q000378:metabolism*
chemical_list: D007328:Insulin; D008024:Ligands; D018004:Receptors, Bombesin; D017981:Receptors, Neurotransmitter; D002738:Chloroquine; D001839:Bombesin