Location and characterization of the human GRP receptor expressed by gastrointestinal epithelial cells.

The exact location of normal gastrin-releasing peptide (GRP) receptor expression by epithelial cells lining the human gastrointestinal (GI) tract is not known; yet this receptor is found on upwards of 50% of GI cancers. Furthermore, the pharmacology reported for GRP receptors expressed by GI cancers varies considerably. Therefore, the purpose of this study was to determine the normal distribution of GRP receptor expression by cells lining the human GI tract, and then determine the normal pharmacology of the human receptor when ectopically expressed by the nonmalignant human colon epithelial cell line NCM460. We obtained endoscopic pinch biopsies of, and extracted the RNA from, epithelial cells lining the esophagus, stomach, jejunum, ileum, and proximal and descending colon, RT-PCR demonstrated that GRP-R expression is limited to cells lining the gastric antrum, indicating that this receptor is aberrantly expressed by GI cancers. To determine the normal pharmacology of this receptor when expressed by nonmalignant human tissues for the first time, we transfected NCM460 cells with the cDNA for the human GRP receptor. By studying three stable NCM460 cell lines expressing varying numbers of receptors, we demonstrate that agonist and antagonist binding affinity, binding kinetics, and G-protein coupling are all independent of receptor number. Finally, by comparing GRP receptors expressed by GI cancers with those on NCM460-transfected cells, we show that the pharmacology of the aberrantly expressed receptors is significantly altered. Thus, these data demonstrate that GI cancers aberrantly express GRP receptors that then behave abnormally.