Inhibition of neuroglandular antigen (NGA) glycosylation by phorbol ester in human melanoma cells.

NGA is a human melanoma-associated antigen recognized by a panel of murine monoclonal antibodies developed in this laboratory. NGA consists of a 23.5 kDa core protein which is glycosylated in vivo to give a family of glycoproteins (30-60 kDa). Treatment of human melanoma G361 cells with the phorbol ester PMA resulted in apparent partial inhibition of NGA glycosylation. After PMA treatment, NGA appeared as 3 different bands of 24, 29 and 34 kDa on SDS-PAGE. The 29 kDa band is similar to the one obtained by treatment with the ionophore monensin, which inhibits NGA O-glycosylation. PMA can modulate plasma membrane ion exchange, most likely by activating protein kinase C. In G361 cells PMA may produce the same net effect as monensin, by impairing transport in the Golgi complex and consequently inhibiting protein O-glycosylation through an ionophore-like effect. Treatment of G361 cells with both PMA and protein kinase C inhibitors re-established the usual NGA glycosylation pattern. Thus the observed effect of PMA on NGA glycosylation is reversible and appears to be mediated by protein kinase C activation.