Bombesin receptors in a human duodenal tumor cell line: binding properties and function

The bombesin family of peptides elicit numerous biological responses in the gut, including stimulation of cell proliferation, and have been implicated as growth factors in a variety of gastrointestinal tumors. Even though these peptides and their receptors are distributed throughout the gastrointestinal tract, there are few cell lines available as model systems to study bombesin action in gastrointestinal cells. In this study, we have characterized functional bombesin receptors in a human duodenal cancer cell line, HuTu-80. The binding of [125I-Tyr4]bombesin to intact cells at 4 degrees C reached equilibrium by 6 h. Scatchard analysis of [125I-Tyr4]bombesin binding showed that HuTu-80 cells contained a single class of high affinity binding sites (5900 +/- 1960/cell; Kd = 80 +/- 20 pM). [125I-Tyr4]bombesin binding was inhibited by bombesin receptor agonists and antagonists with the following order of potencies: gastrin-releasing peptide (GRP) = GRP-(14-27) = bombesin > [DPhe6]bombesin(6-13)ethylamide > [Leu13 psi-(CH2NH)Leu14]bombesin > neuromedin B. Photoaffinity cross-linking studies, in which N-5-azido-2-nitrobenzoyloxysuccinimide was used to covalently couple [125I]GRP(14-27) to cells at 4 degrees C, resulted in the specific labeling of a broad band with an apparent molecular mass of 66,000 daltons. Consistent with the presence of high affinity receptors, bombesin increased the formation of inositol phosphates in HuTu-80 cells in a dose-dependent manner (concentration eliciting half-maximal effect, 290 +/- 70 pM). However, under conditions where both insulin and serum increased [3H]thymidine incorporation into DNA, 10 nM bombesin had no effect either alone or in the presence of insulin. Bombesin also had no effect on colony formation by HuTu-80 cells in soft agar. Furthermore, the bombesin receptor antagonist, [Leu13 psi(CH2NH)Leu14]bombesin, did not inhibit [3H]thymidine incorporation or clonal growth either in the absence or in the presence of serum. Together, these results show that HuTu-80 cells contain high affinity bombesin receptors of the GRP subtype. These receptors are functionally coupled to second messenger production but do not stimulate cell proliferation.