The Binding of Bombesin and Somatostatin and Their Analogs to Human Colon Cancers

Abstract

Specific receptors for bombesin/gastrin-releasing peptide, somatostatin, and EGF were investigated in 15 human colon cancer specimens. Eight of 15 clinical specimens (15%) of colon cancer showed the presence of somatostatin receptors. Octapeptide somatostatin analogs, RC-160 and RC-121, showed 10 times higher binding affinity for somatostatin receptors on colon cancer membranes than somatostatin. Analysis of ¹²⁵I-Tyr⁴-bombesin binding data revealed the presence of specific binding sites in six (40%) specimens of human colon cancer. Scatchard analysis of ¹²⁵I-labeled bombesin indicated a single class of receptors in three specimens with an apparent K_d value of 2.5 nM and two classes of receptors with high (K_d = 0.4 ± 0.2 nM) and low affinity (K_d = 1.6 ± 0.4 μM) in three other specimens. The ¹²⁵I-Tyr⁴-bombesin binding capacities in the colon cancers for high affinity binding sites were from 6 to 228 fmol/mg protein and for low affinity binding sites 76 ± 15 pmol/mg protein. None of the membrane preparations made from normal colonic mucosa specimens showed specific binding for ¹²⁵I-Tyr⁴-bombesin. Five pseudononapeptide (ψ^13–14) bombesin (6–14) antagonists, with different modifications at Positions 6 and 14, synthesized in our laboratory, inhibited the binding of ¹²⁵I-Tyr⁴-bombesin in nanomolar concentrations. No correlation was found between the degree of differentiation and the presence of binding sites for somatostatin or bombesin. Specific binding of EGF was detected in 80% of colon cancer specimens. EGF binding capacity in colon cancer membranes was on average twice as high as in normal colon mucosa (50 ± 21 vs 28 ± 14 fmol/mg protein, respectively). Specific binding sites for somatostatin and EGF, but not bombesin, were also demonstrated in human colon cancer cell line HT-29. In HCT-116 colon cancer line only EGF receptors were found. These receptor findings and our in vivo studies on inhibition of colon cancer growth support the merit of continued evaluation of somatostatin analogs and bombesin/gastrin-releasing peptide antagonists in the management of colonic carcinoma.