Cell density-dependent expression of chondroitin sulfate proteoglycan in cultured human monocytes

Monocytes were isolated and established in vitro at different cell densities. The incorporation of [35S]sulfate into macromolecules in monocytes (day 1 in culture) and monocyte-derived macrophages (day 5 in culture) was found to increase with decreasing cell density in approximately the same way in both day 1 and day 5 cell cultures. [35S]Sulfate was found to be incorporated almost exclusively into chondroitin sulfate proteoglycan (CSPG) in both high and low density monocyte and monocyte-derived macrophage cultures. The molecular size of the [35S]CSPGs produced by the high and low cell density cultures were not found to differ as judged by gel chromatography elution patterns. The molecular size and the structure of the glycosaminoglycan chains were found to be almost similar in high and low density day 1 and day 5 cultures. Only a small degree of proteoglycan degradation could be observed in both high and low density cultures. Furthermore, cell density-dependent differences in CSPG biosynthesis could be observed already 2 h after the establishment of the cultures, indicating that a process of "down-regulation" in high density cultures was already in operation. The glycosaminoglycan synthesis in high cell density day 1 cultures could be increased slightly following exposure to 0.5 mM benzyl-beta-D-xyloside, but not to the same level as that observed in untreated low cell density cultures. By contrast, the expression of 35S-macromolecules by cells cultured at high cell density for 5 days could be increased by xyloside treatment almost to the same level as that observed in the low density cultures.