Evidence for participation of a calpain-like cysteine protease in cell cycle progression through late G(1) phase

Recent studies have demonstrated that cell-permeant protease inhibitors arrest human fibroblasts in late G(1). The target for the inhibitors has been claimed to be either the proteasome, or a calpain-like cysteine protease activity. In the present investigation, the progression of serum-stimulated WI-38 fibroblasts into S-phase was partially inhibited by the cell-permeant general inhibitor of cysteine proteases, E64d, but not by its non-permeant analog, E64c. Exposure of fibroblasts in late G(1) to the proteasome inhibitor, lactacystin, produced only a modest inhibition of progression into S-phase, and did not influence the extensive inhibition produced by the calpain-selective inhibitor, ZLLY-DMK. ZLLnV-CHO and ZLLL-CHO, which are reportedly selective for the proteasome, were less potent than ZLLY-DMK as inhibitors of S-phase progression. These results argue for the involvement of a calpain-like protease acting in late G(1) to allow transit into S-phase. (C) 1997 Academic Press.