Identification and characterization of zeta-opioid receptor in human colon cancer.

Opioid growth factor (OGF, [Met5]enkephalin) inhibits the growth of human colon cancer in nude mice in a receptor-mediated fashion. Ligand binding assays using HT-29 human colon cancer tissue and [3H][Met5]enkephalin were performed to characterize the receptor responsible for the growth-regulatory effects of OGF in colon cancer. Specific and saturable binding was detected, and Scatchard analysis revealed that the data were consistent for a single binding site with a binding affinity of 15.4 +/- 2.0 nM and a binding capacity of 364.8 +/- 25.7 fmol/mg protein. Subcellular fractionation studies revealed that binding was restricted to the nuclear fraction. Competition experiments showed that cold [Met5]enkephalin was the most effective ligand at displacing [3H][Met5]enkephalin. Binding to radiolabeled [Met5]enkephalin also was detected in colon cancers obtained from surgical resections. The function, pharmacological and biochemical characteristics, distribution, and subcellular location of this OGF receptor in human colon cancer are consistent with the zeta-opioid receptor.