Human small cell lung cancer cells express high affinity naloxone-insensitive [125I]-endorphin binding sites

Previous reports have demonstrated that beta-endorphin stimulates the clonal growth of human small cell lung carcinoma (SCLC) cell lines. In this study, the human SCLC lines, NCI-H69, NCI-H345, and NCI-N417, were observed to be highly-enriched in saturable, high-affinity binding sites which are labeled by [125I]beta-endorphin. In contrast to conventional opioid receptors, [125I]beta-endorphin SCLC binding was insensitive to naloxone and other mu, delta, or kappa opioid ligands. Further analysis of the NCI-H69 cells demonstrated that specific (naloxone-insensitive) binding was dependent on receptor concentration, reversible, sensitive to sodium ion, but insensitive to the GTP analogue, Gpp(NH)p. These results suggest a role for naloxone-insensitive beta-endorphin in modulating SCLC metabolism.