Involvement of both heparanase and plasminogen activator in lymphoma cell‐mediated degradation of heparan sulfate in the subendothelial extracellular matrix

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Abstract
The effect of plasminogen on the ability of highly metastatic ESb mouse lymphoma cells to degrade heparan sulfate (HS) in the subendothelial extracellular matrix (ECM) was studied. A metabolically sulfate‐labeled ECM was incubated with the lymphoma cells, and labeled degradation products were analyzed by gel filtration on Sepharose 6B. Heparanase‐mediated release of low‐Mr (0.5 < Kav < 0.85) HS cleavage products was stimulated fourfold in the presence of plasminogen. Incubation of plasminogen alone with the ECM resulted in its conversion into plasmin, which released high‐Mr (Kav < 0.33) labeled proteoglycans from the ECM. Heating the ECM (80°C, 1 hr) abolished its ability to convert plasminogen into plasmin, yet plasminogen stimulated, through its activation by the ESb plasminogen activator, heparanase‐mediated release of low‐Mr HS fragments. Heparin inhibited both the basal and plasminogen‐stimulated degradation of HS side chains but not the total amount of labeled material released from the ECM. In contrast, aprotinin inhibited the plasminogen‐stimulated release of high‐ as well as low‐Mr material. In the absence of plasminogen, degradation of heated ECM by ESb cells was completely inhibited by aprotinin, but there was only a partial inhibition of the degradation of native ECM and no effect on the degradation of soluble HS proteoglycan. These results demonstrate that proteolytic activity and heparanase participate synergistically in the sequential degradation of ECM HS and that the ESb proteolytic activity is crucial for this degradation when the ECM‐associated protease is inactivated. Plasminogen may serve as a source for the proteolytic activity that produces a more accessible substrate to the heparanse.