ANTIMITOGENIC EFFECTS OF THE GASTRIN-CCK RECEPTORS ANTAGONISTS L-364718 AND L-365260 IN HUMAN COLON-CANCER CELL CLONE HT29-S-B6 - CELL-CYCLE ANALYSIS AND MODULATION BY SERUM

The benzodiazepine-derived gastrin-CCK antagonists L-364718 and L-3652 60 reduced the cell proliferation rate of the human colon cancer cell clone HT29-S-B6 in a dose-dependent manner, with half-effective concen trations of approximately 1 mu M (L-364718) and 5 mu M (L-365260) as e valuated after 6 days of incubation in serum-free medium. After 24 h t reatment with both antagonists, cells accumulated in G1 phase of cell cycle. The effects were reversible within 24-48 h after removal of the drugs. The growth of HT29-S-B6 cells in serum-free medium was not sti mulated by exogenous gastrin, Gly-extended gastrin or CCK. Neither gas trin, sulfated CCK-8 (0.1 nM-10 mu M), nor Gly-extended gastrin (1 mu M) was able to reverse the effects of either antagonist. Similarly, an antibody directed against the common C-terminal active moiety of matu re gastrin and CCK failed to reduce the rate of cell proliferation. In the presence of serum, 1-10 mu M L-364718 inhibited cell proliferatio n with an additional G2 arrest in this case. In contrast, high concent rations (10-50 mu M) of L-365260 induced only a weak inhibition of cel l proliferation in the presence of serum. In serum-free medium, the no n-structurally-related CCK-A antagonists PD140548 and SR27897B were in effective at concentrations lower than 10 mu M, and the CCK-B gastrin antagonist PD135158 was effective at 10 mu M. The effect of L-364718 a nd L-365260 on the proliferation of HT29-S-B6 cells are therefore medi ated by atypical receptors independent of the classical CCK-A or CCK-B subtypes.