Copper-dependent autocleavage of glypican-1 heparan sulfate by nitric oxide derived from intrinsic nitrosothiols

Abstract

Cell surface heparan sulfate proteoglycans facilitate uptake of growth-promoting polyamines (Belting, M., Borsig, L., Fuster, M. M., Brown, J. R., Persson, L., Fransson, L.-Å., and Esko, J. D. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 371-376). Increased polyamine uptake correlates with an increased number of positively charged N-unsubstituted glucosamine units in the otherwise polyanionic heparan sulfate chains of glypican-1. During intracellular recycling of glypican-1, there is an NO-dependent deaminative cleavage of heparan sulfate at these glucosamine units, which would eliminate the positive charges (Ding, K., Sandgren, S., Mani, K., Belting, M., and Fransson, L.-Å. (2001) J. Biol. Chem. 276, 46779-46791). Here, using both biochemical and microscopic techniques, we have identified and isolated S-nitrosylated forms of glypican-1 as well as slightly charged glypican-1 glycoforms containing heparan sulfate chains rich in N-unsubstituted glucosamines. These glycoforms were converted to highly charged species upon treatment of cells with 1 mM L-ascorbate, which releases NO from nitrosothiols, resulting in deaminative cleavage of heparan sulfate at the N-unsubstituted glucosamines. S-Nitrosylation and subsequent deaminative cleavage were abrogated by inhibition of a Cu2+/Cu+ redox cycle. Under cell-free conditions, purified S-nitrosylated glypican-1 was able to autocleave its heparan sulfate chains when NO release was triggered by L-ascorbate. The heparan sulfate fragments generated in cells during this autocatalytic process contained terminal anhydromannose residues. We conclude that the core protein of glypican-1 can slowly accumulate NO as nitrosothiols, whereas Cu2+ is reduced to Cu+. Subsequent release of NO results in efficient deaminative cleavage of the heparan sulfate chains attached to the same core protein, whereas Cu+ is oxidized to Cu2+.