Pancreatic cancer is the fourth most common cancer-related mortality in the United States, and the ninth most common cause of death from cancer worldwide. The opioid growth factor (OGF), [Met5]-enkephalin, inhibits the growth of human pancreatic adenocarcinoma in vitro and in vivo, and acts in a receptor-mediated fashion. Ligand binding assays using PANC-1 human pancreatic tumor cells and [3H]-[Met5]-enkephalin were performed to identify and characterize the receptor responsible for the growth-regulatory effects of OGF in pancreatic cancer. Specific and saturable binding was detected, and a Scatchard analysis revealed that the data were consistent for a single binding site with a binding affinity of 1.2+/-0.3 nM and a binding capacity of 36.4+/-4.1 fmol/mg protein. Subcellular fractionation studies showed that binding was restricted to the nuclear fraction. Competition experiments revealed that cold [Met5]-enkephalin was the most effective ligand at displacing [3H]-[Met5]-enkephalin; ligands for mu, delta, and kappa opioid receptors exhibited little or no competition. Binding was detected in 3 other human pancreatic tumor cell lines. Receptor number in xenografts of Capan-1 was decreased 8.6-fold compared to the same cells grown in tissue culture. Binding to radiolabeled [Met5]-enkephalin was detected in pancreatic cancers obtained from surgical resections. Binding capacity, but not binding affinity, was 7.1-fold greater in normal pancreatic tissues than in pancreatic neoplasia. The function, pharmacological and biochemical characteristics, distribution, and subcellular location of OGF binding in human pancreatic cancer were consistent with the OGF receptor (OGFr). In addition, human pancreatic cancer appears to have a low number of receptors for OGF, having the net effect of diminishing control of cellular replicative events.
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