Endogenous opioid systems regulate growth of neural tumor cells in culture

Endogenous opioid systems (i.e., opioids and opioid receptors) play a role in neural cancer. Using a tissue culture system of S20Y murine neuroblastoma to assess the effects of opioids on growth, [Met⁵]-enkephalin was the most potent compound to influence cell replication. With a median effective concentration of 10^-10 M, this peptide inhibited cell proliferation in a stereospecific and naloxone-reversible manner. [Met⁵]-Enkephalin depressed both DNA synthesis and mitosis. [Met⁵]-Enkephalin was detected in neuroblastoma cells by radioimmunoassay, and was found to increase in concentration in culture media over time, suggesting that these cells produced the peptide. Immunocytochemistry showed [Met⁵]-enkephalin-like activity in the cortical cytoplasm, but not the cell nucleus, of neuroblastoma cells. Binding of [³H]-[Met⁵]-enkephalin was specific and saturable, and Scatchard analysis yielded a K_d of 1.2 ± 0.1 nM and a binding capacity of 50.2 ± 4.3 fmol/mg protein. [Met⁵]-Enkephalin also depressed the growth of N115 murine neuroblastoma, SK-N-MC human neuroblastoma, and HT-1080 human fibrosarcoma. These results indicate that [Met⁵]-enkephalin, a naturally occurring pentapeptide that is derived from proenkephalin A, is a potent inhibitor of cell growth. Since cancer cells produce [Met⁵]-enkephalin, and contain a binding site to this ligand, endogenous opioid systems appear to control cell proliferation by an autocrine mechanism.