Objective: To explore the effect of the Heerlen polymorphism and C4b-binding protein (C4BP) on protein S catabolism in vitro and in vivo.
Methods and results: Radiolabeled protein S was efficiently bound and intracellularly degraded by THP-1 macrophages, and both processes were strongly reduced in the presence of the protein S-carrier protein C4BP. To test whether C4BP displays a similar protective effect in vivo, survival experiments were performed in mice. In the absence of C4BP, radiolabeled human protein S disappeared in a biphasic manner (mean residence time [MRT] 2 hours). However, the presence of C4BP resulted in a 4-fold prolonged survival of protein S (MRT 8 hours; P<0.0001). We also applied this experimental model to recombinant protein S-Heerlen, a naturally occurring variant that contains a Ser460Pro substitution. These clearance experiments revealed a strongly decreased survival of recombinant protein S-S460P (MRT 0.6 hours; P=0.021), which could be compensated partially by C4BP (MRT 1.4 hours; P=0.012 compared with protein S-S460P).
Conclusions: Protein S-S460P has a reduced survival in vivo, which may explain the low levels of free protein S in individuals carrying this polymorphism. Furthermore, C4BP prevents premature clearance of protein S and uses this ability to compensate the increased clearance of protein S-S460P.