CCK2 RECEPTOR EXPRESSION TRANSFORMS NON-TUMORIGENIC HUMAN NCM356 COLONIC EPITHELIAL CELLS INTO TUMOR FORMING CELLS

Expression of gastrin and cholecystokinin 2 (CCK2) receptor splice variants (CCK2R and CCK2i4svR) are upregulated in human colonic adenomas where they are thought to contribute to tumor growth and progression. To determine the effects of ectopic CCK2 receptor variant expression on colonic epithelial cell growth in vitro and in vivo, we employed the non-tumorigenic colonic epithelial cell line, NCM356. Receptor expression was induced using a retroviral expression vector containing cDNAs for either CCK2i4svR or CCK2R. RT-PCR and intracellular Ca2+ ([Ca2+]i) imaging of RIE/CCK2R cells treated with conditioned media (CM) from NCM356 revealed that NCM356 cells express gastrin mRNA and secrete endogenous, biologically active peptide. NCM356 cells expressing either CCK2R or CCK2i4svR (71 and 81 fmol/mg, respectively) grew faster in vitro, and exhibited an increase in basal levels of phosphorylated ERK (pERK), compared with vector. CCK2 receptor selective antagonist, YM022, partially inhibited the growth of both receptor-expressing NCM356 cells, but not the control cells. Inhibitors of mitogen activated protein kinase pathway (MEK/ERK) or protein kinase C (PKC) isozymes partially inhibited the elevated levels of basal pERK and in vitro growth of receptor-expressing cells. Vector-NCM356 cells did not form tumors in nude mice, whereas, either CCK2 receptor-expressing cells formed large tumors. Autocrine activation CCK2 receptor variants are sufficient to increase in vitro growth and tumorigenicity of non-transformed NCM356 colon epithelial cells through a pathway involving PKC and the MEK/ERK axis. These findings support the hypothesis that expression of gastrin and its receptors in human colonic adenomas contributes to tumor growth and progression.