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1 Research School of Biosciences, University of Kent at Canterbury, Canterbury, Kent CT2 7NJ; and 2 School of Biosciences, University of Westminster, London W1W 6UW, United Kingdom
Protein kinase C (PKC) regulation of
L-ascorbic acid transport mediated by the
Na+/ascorbic acid transporters, hSVCT1 and hSVCT2,
expressed in COS-1 cells was studied using recombinant
carboxyl-terminal V5 epitope-tagged forms of the transporters. The PKC
activator phorbol 12-myristate 13-acetate (PMA) caused a time-dependent
and concentration-dependent decrease (40-60%) in ascorbic acid
transport activity. Effects of PMA were not observed with the inactive
phorbol ester 4
-phorbol and were reversed by treatment of the cells
with the PKC-specific inhibitor Ro-31-8220. Kinetically, the
reduction in hSVCT1 and hSVCT2 activity arose from a decrease in
maximal velocity with no change in the apparent affinity. Western blot
and confocal microscopy analyses indicated that the total pool of
hSVCT1 or hSVCT2 proteins expressed in the transfected COS-1 cells
remained unaffected by PMA treatment. For hSVCT1 the decrease in
L-ascorbic acid correlated with a redistribution of the
transporter from the cell surface to intracellular membranes. However,
for hSVCT2 there was no apparent change in transporter distribution,
suggesting that the PKC-dependent modulation of L-ascorbic
acid transport mediated by hSVCT2 was the result of reduced catalytic
transport efficiency.
vitamin C transport; human SVCT1 and SVCT2; protein kinase C; transport regulation; protein trafficking
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