The in vitro cell culture age and cell density of articular chondrocytes alter sulfated-proteoglycan biosynthesis

The effect of cell culture age and concomitant changes in cell density on the biosynthesis of sulfated‐proteoglycan by rabbit articular chondrocytes in secondary monolayer culture was studied. Low density (LD, 2 d), middle density (MD, 5‐7 d), and high density (HD, 12‐15 d) cultures demonstrated changes in cellular morphology and rates of DNA synthesis. DNA synthesis was highest at LD to MD densities, but HD cultures continued to incorporate [3H]‐thymidine. LD cultures incorporated 35SO4 into sulfated‐proteoglycans at a higher rate than MD or LD cultures. The qualitative nature of the sulfated‐proteoglycans synthesized at the different culture ages were analyzed by assessing the distribution of incorporated 35SO4 in associative and dissociative CsCI density gradients and by elution profiles on Sepharose CL‐2B. Chondrocytes deposited into the extracellular matrix (cell‐associated fraction) 35SO4‐labeled proteoglycan aggregate. More aggregated proteoglycan was found in the MD and HD cultures than at LD. A 35SO4‐labeled aggregated proteoglycan of smaller hydrodynamic size than that found in the cell‐associated fraction was secreted into the culture medium at each culture age. The proteoglycan monomer (A1D1) of young and older cultures had similar hydrodynamic sizes at all cell culture ages and cell densities. The glycosaminoglycan chains of A1D1 were hydrodynamically larger in the younger LD cultures than in the older HD cultures and consisted of only chondroitin 6 and 4 sulfate chains. A small amount of chondroitin 4,6 sulfate was detected, but no keratan sulfate was measured. The A1D2 fractions of young LD cultures contained measurable amounts of dermatan sulfate; no dermatan sulfate was found in older MD or HD cultures. These studies indicated that chondrocytes at LD synthesized a proteoglycan monomer with many of the characteristics of young immature articular cartilage of rabbits. These results also indicated that rapidly dividing chondrocytes were capable of synthesizing proteoglycans which form aggregates with hyaluronic acid. Culture age and cell density appears primarily to modulate the synthesis of glycosaminoglycan types and chain length. Whether or not these glycosaminoglycans are found on the same or different core proteins remains to be determined.