A potent bombesin receptor antagonist inhibits bombesin-stimulated growth of mouse colon cancer cells in vitro: absence of autocrine effects

Abstrad Bombesin (BBS) exerts significant effects on the growth of a mouse colon cancer cell line (MC-26) in vitro. The presence of specific binding sites on MC-26 cells for gastrin-releasing peptide (GRP)/BBS-related peptides was recently reported by us. In the present study, we determined that the transcript size of the mRNA species that codes for GRP receptors is 9 kilobase pairs, which is similar to that reported for mouse Swiss 3T3 cells, using the complementary DNA probe for the GRP receptor gene from mouse Swiss 3T3 cells. We next examined the effeds of potent GRP receptor antagonists, D-Phe’, bombesin(6-1 3)-propylamide (DPhe’,BN(6-1 3)PA) and Leu’39-(CH2NH)Leu’4-bombesin (LL-BBS), on BBS-stimulated growth of MC-26 cells in vitro. A possible autocrine role of GRP in the growth of MC-26 cells was also investigated. MC-26 cells were inoculated s.c. into male BALB/c mice, and tumors were harvested 21-28 days postinoculation. Both DPhe’,BN(6-13)PA and LL-BBS significantly inhibited the binding of ‘251-GRP to MC-26 tumor membranes in a dose-dependent manner, with 50% inhibitory concentrations of 4.5 ± 0.52 nt.i and 87 ± 6 nM, respedively. D-Phe’,BN(6-1 3)PA similarly inhibited the specific binding of 125I-GRP, cross-linked to a ‘ -8O kilodalton binding protein on the MC-26 tumor membranes. In order to determine whether the BBS receptor antagonist, D-Phe’,BN(6-13)PA, fundioned as an antagonist or an agonist of biological fundions, we measured the bioefficacy of D-Phe’,BN(6-13)PA. Amylase release was not stimulated at all doses of DPhe’,BN(6-1 3)PA examined, but the release of amylase in response to BBS was significantly inhibited in a dosedependent manner by D-Phe’,BN(6-13)PA with a 50% inhibitory concentration of 2.90 ± 0.61 n .i. The growth of MC-26 cells in response to a maximally effedive dose of BBS (50 nM) was significantly inhibited in the presence of increasing doses of D-Phe’,BN(6-1 3)PA and LL-BBS; D-Phe’,BN(6-13)PA and LL-BBS had no significant effeds on the growth of MC-26 cells in the absence of BBS. Because the antagonists did not alter the growth of MC-26 cells in the absence of BBS, we