Proteoglycan metabolism in normal and inflammatory human macrophages

To study proteoglycan metabolism in inflammatory macrophages, primary cultures of human macrophages were cultured in the absence and presence of bacterial lipopolysaccharide (LPS). When exposed to [35S]sulfate, the cells incorporated the label almost exclusively into chondroitin sulfate proteoglycan (CSPG), which was recovered from the culture medium and the cell layer. Cells stimulated with LPS secreted approximately three times more [35]CSPG into the culture medium than control cells. Furthermore, cell adhesion was also found to promote proteoglycan secretion; when nonadherent monocytic cells were induced to adhere, the release of proteoglycan increased two times. The increased secretion seen in LPS-stimulated macrophages was partly due to increased biosynthesis, but was mostly due to increased sorting of CSPG to the secretory pathway. Only about 20% of the CSPG synthesized in unstimulated cells was secreted, whereas the corresponding figure in LPS-treated cells was 35%. In both cell types, the remaining [35S]CSPG was degraded, probably in the lysosomes. The degradation was a two-step process. First, the [35S]CSPG was rapidly cleaved to yield free glycosaminoglycan (GAG) chains (t1/2 = 15 to 30 minutes). Secondly, the GAG chains were completely depolymerized (t1/2 = 2 to 3 hours). Neither resting nor LPS-stimulated cells sorted CSPG to intracellular storage, as is evident in many hematopoietic cells. The LPS-treated cells synthesized [35S]CSPG of smaller molecular size than did control cells, with GAG chains of approximate molecular mass of 12 kD versus 16 kD in control cells. No difference was seen in the disaccharide composition of the GAG chains; both LPS-stimulated and unstimulated cells expressed a mixture of 80% to 90% chondroitin 4-sulfate and 10% to 20% chondroitin 4,6-disulfate. N-terminal sequence and Northern blot analysis indicate that the core protein of the CSPG secreted by human macrophages is serglycin.