Received for publication January 25, 2008.
Revised February 15, 2008.
Accepted for publication February 15, 2008.

In vivo Inhibition of Serine Protease Processing Requires a High Fractional Inhibition of Cathepsin C

Abstract

Inhibition of cathepsin C, a dipeptidyl peptidase that activates many serine proteases, represents an attractive therapeutic strategy for inflammatory diseases with a high neutrophil burden. We recently showed the feasibility of blocking the activation of neutrophil elastase, cathepsin G and proteinase-3 with a single cathepsin C selective inhibitor, in cultured cells. Here we measured the fractional inhibition of cathepsin C that is required for blockade of downstream serine protease processing, in cell-based assays and in vivo. Using a radiolabeled active site probe and U937 cells, a 50% reduction of cathepsin G processing required ~50% of cathepsin C active sites to be occupied by an inhibitor. In EcoM-G cells, inhibition of 50% of neutrophil elastase activity required ~80% occupancy. Both of these serine proteases were fully inhibited at full cathepsin C active site occupancy, whereas granzyme B processing in TALL-104 cells was partially inhibited, despite complete occupancy. In vivo, leukocytes from cathepsin C +/- mice exhibited comparable levels of neutrophil elastase activity to wild-type animals, even though their cathepsin C activity was reduced by half. The chronic administration of a cathepsin C inhibitor to rats, at doses that resulted in the near complete blockade of cathepsin C active sites in bone marrow, caused significant reductions of neutrophil elastase, cathepsin G and proteinase-3 activities. Our results demonstrate that the inhibition of cathepsin C leads to a decrease of activity of multiple serine proteases involved in inflammation, but also suggest that high fractional inhibition is necessary to reach therapeutically significant effects.

Key words: Receptor binding studies, Leukocytes/Mast cells, Neuropeptides, peptidases