Influence of membrane-associated heparan sulfate on the internalization of the small proteoglycan decorin.

The small dermatan sulfate proteoglycan decorin is efficiently internalized by a variety of cells of mesenchymal origin. Previous studies had implicated the involvement of 51- and 26-kDa receptor proteins in this uptake process. The surface localization of these proteins has now been demonstrated by labeling with a membrane-impermeant, biotinylating reagent. The human keratinocyte cell line HaCaT exhibited only about 5% of the clearance rate of fibroblasts for exogeneously added decorin, although it was not deficient in the 51- and 26-kDa proteins. Evidence is presented that plasma membrane-associated heparan sulfate influences receptor trafficking and contributes to the low internalization rate of the receptors in keratinocytes: (i) Heparitinase digestion of intact keratinocytes led to an approximately 10-fold increase in the efficiency of decorin endocytosis. (ii) Endocytosis of decorin was increased more than 10-fold in keratinocytes in the presence of protamine, a cationic, heparan sulfate-binding protein. This effect is considered to be caused by competition between protamine and the endocytosis receptor for cell surface-associated heparan sulfate. (iii) Preincubation of keratinocytes with heparan sulfate-degrading enzymes at 37 degrees C led to a decrease of receptor proteins localized at the cell surface as judged by subsequent surface labeling at 0 degree C. (iv) An alteration of the biosynthesis of heparan sulfate proteoglycans by p-nitrophenyl-beta-xyloside was accompanied by an increased yield of intracellularly located receptor proteins. Plasma membrane-associated heparan sulfate from keratinocytes differed from the corresponding species of fibroblasts in quantity and quality. It is, therefore, suggested that the intracellular trafficking of the decorin receptor proteins is influenced by the amount and/or the composition of membrane-associated heparan sulfate.