Dual regulation of tyrosine protein kinase and protein kinase C on N-acetylglucosaminyltransferase V

The effects of the epidermal growth factor(EGF), a stimulator of tyrosine protein kinase (TPK), and phorbol-12-myristate-13-acetate (PMA), a stimulator of protein kinase C(PKC), on the activity of N-acetylglucosaminyltransferase V (GnT-V) were studied in human hepatocarcinoma cell line 7721 in order to elucidate the regulation of TPK and PKC on GnT-V, It was found that the GnT-V activity obviously increased after treatment of the cells with EGF or PMA for 48 h, A non-specific protein kinase inhibitor, quercetin, inhibited the activities of TPK and PKC (inhibited mainly the membranous TPK and PKC) as well as GnT-V simultanously, Moreover, quercetin completely eliminated the stimulating effect of EGF or PMA on GnT-V, When Tyrohostin-25, a specific inhibitor of TPK, or sphingosine, the specific inhibitor of PKC, was used separately to substitute for quercetin, the induction effect of EGF or PMA on GnT-V was only partially eliminated. However, when both Tyrphostin-25 and sphingosine were added to the culture medium, the elevation of GnT-V caused by EGF or PMA was entirely blocked. Cycloheximide, a well-known inhibitor of protein synthesis, showed an effect similar to the inhibition of protein kinases; it not only inhibited the basal activity of GnT-V, but also abolished the inducing stimulation of GnT-V by EGF or PMA, These results indicate that EGF or PMA regulates the activity of GnT-V via protein kinases, and GnT-V is regulated by dual mechanism of membranous TPK and PKC, Membranous TPK is more important than membranous PKC in the regulation of GnT-V.