EFFECT OF SELECTIVE AND NON-SELECTIVE CYSTEINE PROTEASE INHIBITORS ON THE INTRACELLULAR PROCESSING OF INTERLEUKIN 6 BY HEPG2 CELLS

The effects were measured and compared of three nonselective cysteine cathepsin inhibitors (leupeptin, trans-Epoxysuccinyl-l-Leucylamido(4-guanidino)-butane (E-64), and Z-Phe-Ala-CH2F) and a selective cathepsin B inhibitor, CA074Me, on the intracellular processing of 125I-labeled human recombinant Interleukin 6 (IL-6) by HepG2 cells. The uptake and processing of 125I-IL-6 by cells treated with inhibitors was followed over a 7-h period. All inhibitors caused an increased residence time of IL-6 inside the cell and a corresponding decrease in the output of non-trichloroacetic acid-precipitable fragments of radiolabeled protein. Maximal effect was achieved with leupeptin at 200 µM, with which the rate of IL-6 digestion was reduced to 50% that of control cells. The specific inhibitor CA074Me was the least effective in slowing the intracellular processing of IL-6. The effects of all of the inhibitors on the production of haptoglobin, either stimulated by IL-6 or basal, was negligible over a similar time period, indicating continued cell viability. The data from this model suggest that cathepsin inhibitors would not interfere with lysosomal processing to an extent which would prohibit the development of selective and potent cathepsin inhibitors for the treatment of diseases in which individual cysteine cathepsins play clearly pathophysiological roles.