Recombinant human factor X: high yield expression and the role of furin in proteolytic maturation in vivo and in vitro.

Factor X/Xa plays a pivotal role in the coagulation cascade and exhibits a therapeutic potential for the treatment of factor X-deficient as well as FVIII and FIX inhibitor patients. This report describes the establishment of Chinese hamster ovary cell clones expressing recombinant human factor X up to 120 microg/mL x day and 78 microg/10(6) cells x day, that is to 100-fold higher levels than reported previously. Although propeptide removal and single chain precursor to light and heavy chain processing as well as vitamin K-dependent gamma-carboxylation became impaired at these expression levels, up to 25% of the recombinant human factor X produced was active. This represents the highest functional activity ever reported for a vitamin K-dependent protein at such an expression level. Expression of recombinant human factor X in Chinese hamster ovary cells lacking the endoprotease Furin revealed that propeptide removal still occurred, whereas single chain precursor to light/heavy chain processing was abolished. This suggests that a protease different from Furin mediates propeptide removal, a unique finding compared with the other vitamin K-dependent coagulation factors. In contrast, exposure of incompletely processed rFX molecules to soluble recombinant Furin in vitro mediated both of these cleavage reactions despite the absence of a typical argP4-xP3-lys/argP2-argP1 Furin cleavage site in the propeptide, indicating relaxed specificity in vitro. Concomitantly with the degree of processing, the functional activity of recombinant human factor X increased. Interestingly, Furin was shown to even perform correct N-terminal proteolytic trimming of FX molecules truncated amino-terminal to the P3 residue in vitro. Depending on the absence or presence of warfarin in the culture media, as well as on the processing state, four distinct recombinant human factor X light chain isoforms were observed and their structure characterized. One of these light chain forms correlated with the functional activity. Finally, the distribution of the individual light chain isoforms suggests that gamma-carboxylation may be a prerequisite for propeptide removal.