Enhancement of Lysosomal Enzyme Activity by Recombinant Human Tumor Necrosis Factor and Its Role in Tumor Cell Killing in vitro

We investigated the effect of recombinant human tumor necrosis factor (TNF) on the lysosomal enzyme activity of various established cell lines in vitro. Incubation of 1 × 10(6)TNF‐sensitive mouse tumorigenic fibroblasts (L‐M cells) in the presence of TNF (100 U/ml) for 48 h increased the total (the sum of the enzyme activities in the lysosomes and the cytoplasm) acid phosphatase and /9‐ghicuronidase activities by 3.7‐ and 4.2‐fold, respectively. The same increase was observed even when 1 U/ml of TNF was added to some cultures and no further augmentation occurred at 10 or 100 U/ml. Measurement of total and free enzyme activities showed that TNF stimulation not only enhanced the total intracellular enzyme activity but also accelerated the conversion into free (cytoplasmic) enzyme activity. Addition of a lysosomotropic agent (methylamine) suppressed both the enhancement of lysosomal enzyme activity and the cytotoxicity of TNF. A similar enhancement of lysosomal enzyme activities was also detected in various TNF‐sensitive tumor cell lines, and a strong correlation (acid phosphatase: r= 0.836, β‐glucuronidase: r=0.910) was observed between the enhancement of enzyme activity and sensitivity to TNF. No such increase was detected in TNF‐resistant human diploid cells. These results show that TNF induces the activation and release of lysosomal enzymes in TNF‐sensitive cells, and suggest that such events may play an important role in TNF‐mediated cytotoxicity.