Expression of
gastrin and
cholecystokinin 2 (CCK(2)) receptor splice variants (CCK(2)R and CCK(2i4sv)R) are upregulated in human colonic
adenomas where they are thought to contribute to
tumor growth and progression. To determine the effects of ectopic CCK(2) 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 CCK(2i4sv)R or CCK(2)R. RT-PCR and intracellular Ca(2+) ([Ca(2+)](i)) imaging of RIE/CCK(2)R 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 CCK(2)R or CCK(2i4sv)R (71 and 81 fmol/mg, respectively) grew faster in vitro, and exhibited an increase in basal levels of phosphorylated ERK (pERK), compared with vector. CCK(2) 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 CCK(2) receptor-expressing cells formed large
tumors. Autocrine activation CCK(2) 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.