Glycine-extended gastrin exerts growth- promoting effects on human colon cancer cells

Background: Since human colon cancers often contain significant quantities of progastrin-processing intermediates, we sought to explore the possibility that the biosynthetic precursor of fully processed amidated gastrin, glycine-extended gastrin, may exert trophic effects on human colonic cancer cells. Materials and Methods: Binding of radiolabeled glycine-extended and amidated gastrins was assessed on five human cancer cell lines: LoVo, HT 29, HCT 116, Colo 320DM, and T 84. Trophic actions of the peptides were assessed by increases in [H]thymidine incorporation and cell number. Gastrin expression was determined by northern blot and radioimmunoassay. Results: Amidated gastrin did not bind to or stimulate the growth of any of the five cell lines. In contrast, saturable binding of radiolabeled glycine- extended gastrin was seen on LoVo and HT 29 cells that was not inhibited by amidated gastrin (10 M) nor by a gastrin/CCK(B) receptor antagonist (PD 134308). Glycine-extended gastrin induced a dose-dependent increase in [H]thymidine uptake in LoVo (143 ± 8% versus control at 10 M) and HT 29 (151 ± 11% versus control at 10 M) cells that was not inhibited by PD 134308 or by a mitogen-activated protein (MAP) or ERK kinase (MEK) inhibitor (PD 98509). Glycine-extended gastrin did stimulate jun-kinase activity in LoVo and HT 29 cells. The two cell lines expressed the gastrin gene at low levels and secreted small amounts of amidated gastrin and glycine-extended gastrin into the media. Conclusions: Glycine-extended gastrin receptors are present on human colon cancer cells that mediate glycine-extended gastrin's trophic effects via a MEK-independent mechanism. This suggests that glycine-extended gastrin and its novel receptors may play a role in colon cancer cell growth.