Specific Binding and Growth Effects of Bombesin-related Peptides on Mouse Colon Cancer Cells in Vitro

In the present study, we characterized specific binding of bombesin (BBS)/gastrin-releasing peptide (GRP) to mouse colon cancer (MC-26) cells. MC-26 cells were inoculated into male BALB/c mice subdermally, and tumors were harvested from mice 21–28 days postinoculation. Tumor membranes were analyzed for binding to GRP-related peptides, using either, 25I-GRP or, 25I-tyrosine4-BBS. Under optimal binding assay conditions, BBS displaced specific binding of both 125I-GRP and l25I-tyrosine4-BBS in a dose-dependent manner, and a curvilinear displacement resulted. Specific binding data, analyzed by either a Scatchard or a Lineweaver-Burk plot, demonstrated presence of 2 classes of specific binding sites, arbitrarily named type I and type II sites. Type I sites had a high binding affinity [Ka 0.45 ± 0.05 nM (SE)] and a relatively low capacity (226 ± 27 fmol/mg membrane protein), whereas type II sites had a 10–20-fold lower binding affinity and ~6–7-fold higher capacity. BBS/GRP binding sites were specific for GRP-related peptides and demonstrated no significant binding affinity for all other unrelated peptides tested. Relative binding affinity of GRP analogues was in the order of GRP (14–27) > neuromedin C BBS GRP (1–27) > neuromedin B (for the later, P > 0.05 versus other peptides). Two BBS receptor antagonists, (D-Arg1trp7.9Leu11]substance P (spantide) and [Leu13--(CH2NH)Leu]BBS also inhibited specific binding of l25I-GRP in a dose-dependent manner. Molecular weight of GRP/BBS binding proteins on tumor membranes was determined by cross-linking methods. A major molecular form (>80–90%) (A/r 75,000) and a minor Mt -180,000 band were evident, both under reducing and nonreducing conditions. BBS (0.5–50 nM) demonstrated a significant dose-dependent growth effect on MC-26 cells in vitro, in terms of [3H]thymidine and 3-(4,5-dimethylthia-zol-2-yl)-2,5-diphenyltetrazolium bromide uptake; these studies indicate that the BBS/GRP binding sites on MC-26 cells may serve as functional receptors and mediate the growth effects of BBS on MC-26 cells.