Properties of delta opioid receptor in neuroblastoma NS20Y: Receptor activation and neuroblastoma proliferation

Neuroblastoma NS20Y cells possess a high density of stereoselective delta opioid receptors as determined by competition binding with ³H-diprenorphine and various opioid ligands. Scatchard analysis of [³H]diprenorphine saturation binding data revealed a K(d) = 0.79 ± 0.17 nM and B(max) = 370 ± 50 fmol/mg protein. These opioid binding sites have highest affinity for delta opioid receptor selective agonists and lowest affinity for mu opioid receptor selective agonists. Agonist binding was sensitive to the presence of the monovalent cation, Na⁺. Activation of receptor with D-Ala², D-Leu⁵- enkephalin (DADLE) resulted in dose-dependent inhibition of forskolin- stimulated intracellular [³H]cAMP accumulation, which was antagonized by (- )-naloxone but not (+)-naloxone. Relative potencies of various opioid agonists to inhibit intracellular cAMP production paralleled those observed in neuroblastoma X glioma NG108-15 hybrid cells. Pretreatment of NS20Y cells with pertussis toxin (PTX) eliminated opioid agonist inhibition of adenylyl cyclase activity. Chronic DADLE treatment resulted in desensitization and down-regulation of opioid receptor. An increase in intracellular [³H]cAMP level above the control was observed in the presence of naloxone after chronic DADLE treatment. Therefore, opioid binding sites in neuroblastoma NS20Y cells possess properties of the classical delta opioid receptor type. After neuroblastoma NS20Y was growth arrested by culturing the cells in serum-free medium for 72 hr, proliferation was reinitiated by addition of fetal calf serum (FCS), 0.01% to 12%, and was monitored by either [³H]thymidine incorporation or by dye viability assay. It was demonstrated that naloxone and naltriben but not Met⁵-enkephalin could attenuate FCS- induced proliferation in a dose-dependent manner. Naltriben was 54-fold more potent than naloxone to attenuate NS20Y proliferation. The maximal level of viable cells per well was reduced (35.2 ± 1.9%) with no alteration in FCS concentration-dependent stimulation of growth. Similar inhibition by naloxone (37.3 ± 2.7%) was observed with [³H]thymidine incorporation studies. This naloxone effect was serum concentration-dependent and could be blocked by culturing NS20Y cells in the presence of both naloxone and Met⁵-enkephalin. Although pretreatment of NS20Y cells with pertussis toxin could attenuate FCS-stimulated proliferation, naloxone effect on growth was not affected by pertussis toxin pretreatment. Furthermore, the naloxone effect was not NS20Y specific. A similar naloxone effect was observed with neuroblastoma N1E115, although not with neuroblastoma X glioma NG108-15, nor human neuroblastoma SHSY5Y, cell lines that have been reported to contain delta opioid receptors. Therefore, activation of delta opioid receptor could modulate FCS-induced growth in some but not all neuroblastoma cell lines.