Processing and release of tumor necrosis factor alpha

Tumor necrosis factor alpha (TNFalpha) is synthesized as a transmembrane precursor form that is proteolytically processed and released as the soluble mature form. In human monocytes and monocytic cell lines, the production, processing, and release of TNFalpha are co-induced by certain activators, such as lipopolysaccharide. To investigate the mechanism of TNFalpha processing and release, we established a cell line which constitutively produced TNFalpha, by transfecting the TNFalpha precursor form cDNA into NIH/3T3 cells. In these cells, the processing and release of TNFalpha were augmented by phorbol 12-myristate 13-acetate (PMA), mediated through a protein kinase C (PKC) signalling pathway. Various protease inhibitors were tested and it was found that matrix metalloproteinase (MMP) inhibitors blocked the processing and release of TNFalpha both in the absence and presence of PMA. This result is compatible with the recent reports that MMP are involved in the processing and release of TNFalpha. In contrast, 3,4-dichloroisocoumarin, N alpha-p-tosyl-L-phenylalanine chloromethane, iodoacetamide, and o-phenanthroline inhibited the processing and release of TNFalpha only in the presence of PMA, suggesting that serine proteases requiring SH for their activity, a combination of serine proteases and cysteine proteases, or MMP, may be involved in the PKC-mediated induction of TNFalpha processing and release.