High-uptake and low-uptake forms of proteoglycans secreted by arterial smooth muscle cells

Cultured arterial smooth muscle cells synthesize and secrete two types of sulfated proteoglycans, designated as proteoglycan A and B, into the culture medium. They are isolated as immunologically distinct monomers with relative molecular masses of 280 X 10(3) and 180 X 10(3) and are characterized as chondroitin-sulfate-rich (A) and dermatan-sulfate-rich (B) proteoglycans. Both proteoglycan A and B were labelled with [35S]sulfate and used for studies of endocytosis. Uptake of proteoglycan B by arterial smooth muscle cells shows saturable kinetics. At saturation (500 microM) one cell may endocytose up to 1.5 X 10(6) proteoglycan B molecules/h. Proteoglycan A is internalized at a 10-fold lower rate. No saturation kinetics were observed at high proteoglycan A concentrations (500 microM). Endocytosis of proteoglycan B in the presence of an excess of proteoglycan A and vice versa suggest that proteoglycan A and B do not compete for the same receptor site. Free hyaluronate or chondroitin sulfate do not inhibit the uptake of proteoglycan B or A. The results suggest that proteoglycan B is internalized by arterial smooth muscle cells via a high-affinity receptor-mediated process, whereas proteoglycan A is taken up by fluid endocytosis and/or by low-affinity endocytotic processes.