AN EXTRACELLULAR PROTEASOME-LIKE STRUCTURE FROM C6 ASTROCYTOMA-CELLS WITH SERINE COLLAGENASE-IV ACTIVITY AND METALLO-DEPENDENT ACTIVITY ON ALPHA-CASEIN AND BETA-INSULIN

An extracellular proteasome-like (EP) structure has been isolated from serum-free media conditioned by C6 astrocytoma cells, EP has a native molecular mass of 1000 kDa and is composed of three subunits, two iso electric variants at 70 kDa and one at 65 kDa, The extracellular prote asome degraded collagen TV, alpha-casein, beta-insulin, and certain sy nthetic peptide substrates. A 68-kDa type IV collagenase, identified a s the activated form of gelatinase A, was also isolated from this medi um, The type IV collagenase activity of the proteasome was sensitive t o serine protease inhibitors, while the 68-kDa collagenase IV represen ted the matrix metalloprotease gelatinase A. The general protease acti vity of the proteasome was sensitive to metalloprotease inhibitors, We stern blot analysis indicates a sequence relationship between the 68-k Da type IV collagenase and either one or both of the 70-kDa isoelectri c variants of the proteasome; however, the two enzymes appear to be di stinct functionally, Comparison with known proteasomes indicates that EP represents a novel proteasome. The complexity of degradative enzyme s in the extracellular microenvironment implies that complete inhibiti on of tumor growth requires at least a combination of serine and metal loprotease inhibitors.