Inhibition of ADP-ribosyltransferase increases synthesis of Gsα in neuroblastoma × glioma hybrid cells and reverses iloprost-dependent heterologous loss of fluoride-sensitive adenylate cyclase

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Exposure of NG108-15 cells to 50 mM nicotinamide [an inhibitor of mono(ADP-ribosyl)transferase] for 18 hr led to an increase in membrane associated Gsα measured either as cholera toxin substrate or by immunoblotting with a specific antiserum. Prolonged exposure of NG108-15 cells to iloprost is followed by homologous loss of iloprost sensitivity, and heterologous loss of fluoride-dependent activation of adenylate cyclase. Nicotinamide reversed the loss of fluoride sensitivity, but failed to restore iloprost-dependent activation of adenylate cyclase. These results with nicotinamide in NG108-15 cells contrasted with those from platelets, which also exhibit heterologous desensitization of fluoride sensitivity following prolonged exposure to iloprost. Treatment of platelets with 50 mM nicotinamide for 18 hr led to an increase of 75.0 ± 19.4% in the amount of membrane associated cholera toxin substrate. However, there was no associated increase in the abundance of Gsα as determined by immunoblotting. Furthermore, in platelets there was no restoration by nicotinamide of the iloprost-dependent loss of fluoride-sensitive adenylate cyclase activity. It follows that heterologous desensitization in platelets is accompanied by inactivation of Gsα, which is retained within the plasma membrane in its inactive state.. The nicotinamide-dependent increase in the abundance of membrane associated cholera toxin substrate and immunoreactive Gsα in NG108-15 cells is associated with an increase of 72.0 ± 20.3% in the levels of mRNA encoding Gsα. The capacity of nicotinamide to increase the abundance of membrane associated Gsα was reversed when the cells were cultured in the presence of 20 μg/mL cycloheximide. These results suggest that the ability of nicotinamide to increase the abundance of Gsα in NG108-15 cells is mediated by de novo protein synthesis.