Regulation of ganglioside metabolism by phosphorylation and dephosphorylation

Cell differentiation is frequently accompanied by alterations in the composition of gangliosides in the plasma membrane resulting from a regulation of the enzyme activities involved. The regulation of CMP-NeuAc:GM1 alpha2-3-sialyltransferase (ST-IV) and UDP-GalNAc:GM3 N-acetylgalactosaminyltransferase (Gal-NAc-T) by the degree of enzyme phosphorylation was analyzed by determination of the enzyme activity on incubation of NG108-15 cells with various protein phosphatase inhibitors (okadaic acid and orthovanadate) or protein kinase activators (phorbol ester and forskolin). Incubation with okadaic acid, but not with orthovanadate, inhibited the ST-IV activity to 45% of that of control cells with t(1/2) = 60 min for the inactivation reaction. This indicates a rapid hyperphosphorylation of ST-IV due to the inhibition of a serine/threonine-specific phosphatase. A similar rate of inactivation was found on stimulation of protein kinase C with phorbol ester. In contrast to ST-IV, the activity of GalNAc-T was increased on stimulation of intracellular phosphorylation systems. The fastest activation of GalNAc-T was achieved with forskolin, yielding up to 160% of the initial activity within 30 min of effector incubation. Up-regulation of GalNAc-T in conjunction with down-regulation of ST-IV by stimulation of phosphorylation is suggested to serve as a physiological mechanism to increase the concentration of GM1, which was found to be elevated in correlation with the cell density. This assumption was corroborated by metabolic labeling studies with radioactive ganglioside precursors indicating an enhancement of the relative amount of a-series gangliosides subsequent to GM3 on phosphorylation stimulation. In particular, the biosynthesis of GM1 was specifically elevated within 2 h of incubation with forskolin. We conclude from the overall data that the ganglioside composition during the cell differentiation of NG108-15 cells can be specifically regulated by both protein kinase A- and protein kinase C-related phosphorylation systems.