Human U937 cell surface peptidase activities: characterization and degradative effect on tumor necrosis factor‐α

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Abstract
Surface peptidase activities on the human monocytic lineage cell line U937 were characterized. Two diisopropyl phosphofluoridate (DFP)‐inhibitable serine peptidases were identified by differences in their hydrolytic activities on chromogenic peptides: one removed tripeptides from the free NH2‐terminal end of the synthetic peptide Ala‐Ala‐Phe‐p‐nitroanilide (pNA) and was not inhibited by inhibitors of metallo‐, cysteic‐, and aspartic‐proteinases, or by those of elastase‐, trypsin‐ and chymotrypsin‐like enzymes, suggesting the presence of a hitherto unidentified serine tripeptidyl endopeptidase; the other peptidase catalyzed the release of Gly‐Pro from Gly‐Pro‐pNA and was inhibited by DFP, phenylmethyl sulfonyl fluoride and diprotin A, thus resembling dipeptidyl peptidase IV (DPP IV) with respect to its substrate specificity and inhibitor profile. A group of N‐exoaminopeptidase activities specifically inhibited by bestatin, was also detected when Ala‐, Leu‐, Arg‐ and Lys‐pNA were used as substrates. The activities were surface associated and not secreted as determined by extracellular location of product and enzymatic recovery in highly purified U937 cell membranes. Peripheral monocytes and macrophages were found to virtually exhibit identical levels of these two classes of peptidase activities when compared to those detected on U937 cells. The relative contributions of these hydrolytic enzymes to the cleavage of bioactive and radioiodinated cytokines including tumor necrosis factor‐α (TNF‐α), interleukin‐1α and interferon‐γ was next examined. The results indicated that N‐aminopeptidases do not appear to participate in the catabolism of any tested cytokine. In contrast, the most interesting finding was that both serine peptidases participate in TNF‐α degradation. Analysis of the final proteolytic digestion products demonstrated the disappearance of the native 17‐kDa molecule TNF‐α, and the concomitant release of biologically inactive fragments of ≤2 kDa. Together, these observations indicate new roles for both the DPP IV‐like enzyme and the tripeptidyl endopeptidase located at the surface of human monocytic cells, including the regulation of the extracellular TNF‐α concentration. Thus, the identification of functional ectopeptidases provides insight into their potential role in both normal and malignant monocytic function.