Identification and characterization of opioid growth factor receptor in human pancreatic adenocarcinoma

Pancreatic cancer is the fourth most common cancer-related mortality in the United States, and the ninth most common cause of death from cancer worldw ide. The opioid growth factor (OGF), [Met(5)]-enkephalin, inhibits the grow th of human pancreatic adenocarcinoma in vitro and in vivo, and acts in a r eceptor-mediated fashion. Ligand binding assays using PANC-1 human pancreat ic tumor cells and [H-3]-[Met(5)]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 bi nding site with a binding affinity of 1.2+/-0.3 nM and a binding capacity o f 36.4+/-4.1 fmol/mg protein. Subcellular fractionation studies showed that binding was restricted to the nuclear fraction. Competition experiments re vealed that cold [Met(5)]-enkephalin was the most effective ligand at displ acing [H-3]-[Met(5)]-enkephalin; ligands for mu, delta, and kappa opioid re ceptors 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 [Met(5)]-enkephalin was detected in pancreatic ca ncers obtained from surgical resections. Binding capacity, but not binding affinity, was 7.1-fold greater in normal pancreatic tissues than in pancrea tic neoplasia. The function, pharmacological and biochemical characteristic s, distribution, and subcellular location of OGF binding in human pancreati c cancer were consistent with the OGF receptor (OGFr). In addition, human p ancreatic cancer appears to have a low number of receptors for OGF, having the net effect of diminishing control of cellular replicative events.