Warfarin, an antagonist of vitamin K, is known to disrupt the microsomal vitamin K cycle, which results in a decrease in the plasma level of protein C, an anticoagulant factor, as well as some other vitamin K-dependent coagulation factors. Here, we examined the effect of warfarin on the secretion of recombinant protein C expressed in human kidney 293 or BHK cells. In transient expression, warfarin caused a 2-4-fold decrease in the quantity of protein C secreted, compared to findings with vitamin K-treated cells. Pulse-chase experiments using stable cells showed that, although recombinant protein C was secreted in the presence of vitamin K, the decrease in the total amount of radioactivity in the warfarin-treated cells suggested intracellular degradation. This degradation depended on the concentration of warfarin and was not inhibited by an endoplasmic reticulum (ER)-Golgi transport inhibitor (brefeldin A) or by lysosomotropic inhibitors (chloroquine and NH4Cl). Thus, protein C synthesized in the presence of warfarin is probably selectively degraded, and this degradation occurs in a pre-Golgi, nonlysosomal compartment. Among the protease inhibitors tested, N-alpha-acetyl-Leu-Leu-methioninal and N-alpha-acetyl-Leu-Leu-norleucinal blocked the degradation of protein C precursor synthesized in the presence of warfarin, and the precursor accumulated intracellularly, in a dose-dependent manner. Both inhibitors, however, did not disturb the secretion of protein C precursor in the vitamin K-treated cells. Thus, a cysteine protease(s) appeared to be responsible for the degradation. Moreover, protein C synthesized in the presence of warfarin was located in the same organelle as protein disulfide isomerase, an ER-resident protein, and the intracellular protein C was sensitive to endoglycosidase H digestion. This evidence suggests that protein C synthesized in the presence of warfarin was selectively degraded by a cysteine protease(s) in the ER through a ''quality control'' mechanism.
