Cell Biology and Metabolism
An Extracellular Proteasome-like Structure from C6 Astrocytoma Cells with Serine Collagenase IV Activity and Metallo-dependent Activity on α-Casein and β-Insulin (∗)

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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 isoelectric variants at 70 kDa and one at 65 kDa. The extracellular proteasome degraded collagen IV, α-casein, β-insulin, and certain synthetic peptide substrates. A 68-kDa type IV collagenase, identified as the activated form of gelatinase A, was also isolated from this medium. The type IV collagenase activity of the proteasome was sensitive to serine protease inhibitors, while the 68-kDa collagenase IV represented the matrix metalloprotease gelatinase A. The general protease activity of the proteasome was sensitive to metalloprotease inhibitors. Western blot analysis indicates a sequence relationship between the 68-kDa type IV collagenase and either one or both of the 70-kDa isoelectric variants of the proteasome; however, the two enzymes appear to be distinct functionally. Comparison with known proteasomes indicates that EP represents a novel proteasome. The complexity of degradative enzymes in the extracellular microenvironment implies that complete inhibition of tumor growth requires at least a combination of serine and metalloprotease inhibitors.

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This work was supported in part by the Brain Tumor Foundation of Canada, the Victoria Hospital Research Development Fund, and the Royal Arch Masons of Canada, “Release a Miracle Donation to Medical Research.” The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

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Recipient of a Brain Tumor Foundation of Canada Fellowship.