The activity of N-acetylglucosaminyl transferase III (GnT III) in 7721 human heptocarcinoma cell was inhibited by two non-specific Ser/Thr protein kinase inhibitors, quercetin and trifluoperazine, and two PKC specific inhibitors, D-sphingosine and staurosporine. The change of GnT III activity paralleled that of membranous PKC (m-PKC) when the cells were treated with PMA, but not with that of cytosolic PKC (c-PKC). Quercetin, D-sphingosine and staurosporine also blocked the PMA activation of GnT-III. The inhibitory effects on m-PKC and GnT III were generally proportional to the concentration of quercetin and staurosporine. When crude GnT preparations of human and rat kidney were treated with ALP to remove the phosphate groups, the GnT III activities were significantly decreased. These results suggest that GnT III may be regulated by m-PKC directly or indirectly via the phosphorylation/dephosphorylation of the Ser/Thr residue.
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