Characterization of gastrin-releasing peptide and its receptor aberrantly expressed by human colon cancer cell lines

Gastrin-releasing peptide (GRP) is a mitogen and morphogen important in the development of human colon cancers. Although epithelial cells lining the colon do not normally express GRP or its receptor (GRP-R), most human tumors express GRP-R mRNA. Yet functional protein has only been detected in 24 to 40% of colon cancers. To elucidate the reason for the difference between the expression of GRP/GRP-R mRNA and protein, we studied nine human colon cancer cell lines. Quantitative polymerase chain reaction revealed that all colon cancer cell lines expressed similar amounts of mRNA for both GRP as well as GRP-R. Yet binding studies using I-125-Tyr(4)-bombesin detected functional receptors on only five of the nine cell lines studied. Conformational fragment-length polymorphism analysis indicated that although mRNA for the ligand GRP was never mutated, mRNA for the GRP-R was always mutated. Sequencing revealed that the message for GRP-R contained between two and seven separate mutations at the nucleotide level. This resulted in 14 separate coding mutations, 2 of which were observed in more than one cell line. Each mutation was individually recreated by site-directed mutagenesis and studied in transiently transfected Chinese hamster ovary-K1 cells. Alteration of Pro(145) into a tyrosine, of Val(317) into a glutamic acid, and insertion of a 32-nucleotide segment resulting in a frameshift distal to Asp(137) all resulted in GRP receptors incapable of binding ligand. Thus, these data indicate that human colon cancers commonly express GRP and GRP-R mRNA but that receptor mutations account for the failure of functional protein to be generated.