Heterogeneity of Nucleotide Receptors in NG108‐15 Neuroblastoma and C6 Glioma Cells for Mediating Phosphoinositide Turnover

We have compared the characteristics of receptors for nucleotide analogues and the involvement of phospholipase C (PLC) in the effector mechanism in NG108‐15 neuroblastoma and C6 glioma cells. The relative potency of these analogues to stimulate inositol phosphate (IP) formation is UTP > UDP ≫ 2‐methylthio‐ATP (2‐MeSATP), GTP > ATP, CTP > ADP > UMP in NG108‐15 cells and ATP > UTP > ADP > GTP > UDP ≫ 2Me‐SATP, CTP, UMP in C6 glioma cells. α,β‐Methylene‐ATP, β,γ‐methylene‐ATP, AMP, and adenosine had little or no effect in both types of cells. The EC 50 values were 3 and 106 µ M for UTP in NG108‐15 and C6 glioma cells, respectively. The EC 50 value for ATP in C6 glioma cells was 43 µ M . 2‐MeSATP was threefold more potent than ATP in NG108‐15 cells but had little effect in C6 glioma cells at 1 m M . In NCB‐20 cells, a similar rank order of potency to that found in NG108‐15 cells, i.e., UTP ≫ GTP > ATP > CTP, was observed. In both NG108‐15 and C6 glioma cells, preincubation with ATP or UTP caused a pronounced cross‐desensitization of subsequent nucleotide‐stimulated IP production. ATP and UTP displayed no additivity in terms of IP formation at maximally effective concentrations. In contrast, endothelin‐1, bradykinin, and NaF interacted in an additive manner with either nucleotide in stimulating PI hydrolysis. Pretreatment with pertussis toxin did not affect ATP‐, UTP‐, and GTP‐stimulated IP generation in these cells, indicating that nucleotide receptors coupled to PLC by a pertussis toxin‐resistant G protein in both cell types. Short‐term treatment of the cells with protein kinase C (PKC) activators [phorbol 12‐myristate 13‐acetate (PMA) and octylindolactam V] produced a dose‐dependent inhibition of ATP‐ and UTP‐induced IP formation with a greater extent and higher susceptibility in C6 glioma cells than in NG108‐15 cells. Furthermore, a 24‐h exposure of the cells to PMA resulted in an obvious attenuation of nucleotide‐induced IP formation in C6 glioma cells but failed to change the response in NG108‐15 cells. These results suggest that distinct nucleotide receptors that respond to ATP and UTP with different selectivity exist in NG108‐15 and C6 glioma cells. These heterogeneous nucleotide receptors coupled to PLC undergo discriminative modulation by PKC. NG108‐15 and NCB‐20 neuroblastoma are two cell lines that showed the highest specificity to extracellular UTP rather than ATP among the nucleotide receptors so far studied in various cells, suggesting the presence of a pyrimidine receptor in these cells.