Elevated plasma factor VIII in a mouse model of low-density lipoprotein receptor–related protein deficiency

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Abstract
It has been established that low-density lipoprotein receptor-related protein (LRP) is involved in the cellular uptake and degradation of coagulation factor VIII (FVIII) in vitro. To address the physiologic role of LRP in regulating plasma FVIII in vivo, we used cre/loxP–mediated conditional LRP- deficient mice (MX1cre+LRPflox/flox). Upon inactivation of the LRP gene, MX1cre+LRPflox/flox mice had significantly higher plasma FVIII as compared with control LRPflox/floxmice (3.4 and 2.0 U/mL, respectively; P < .001). Elevated plasma FVIII levels in MX1cre+LRPflox/flox mice coincided with increased plasma von Willebrand factor (VWF) (2.0 and 1.6 U/mL for MX1cre+LRPflox/flox and control LRPflox/flox mice, respectively; P < .05). Elevation of plasma FVIII and VWF persisted for at least 6 weeks after inactivation of the LRP gene. Upon comparing plasma FVIII and VWF in individual mice, we observed an increase of the FVIII/VWF ratio in MX1cre+LRPflox/flox mice as compared with control LRPflox/flox mice. Administration of either a vasopressin analog or an endotoxin resulted in increased plasma VWF, but not FVIII. In clearance experiments, MX1cre+LRPflox/flox mice displayed a 1.5-fold prolongation of FVIII mean residence time. Adenovirus-mediated overexpression of the 39-kDa receptor–associated protein (RAP) in normal mice resulted in a 3.5-fold increase of plasma FVIII. These data confirm that the regulation of plasma FVIII in vivo involves a RAP-sensitive mechanism. Surprisingly, plasma FVIII in MX1cre+LRPflox/flox mice increased 2-fold after RAP gene transfer. We propose that RAP-sensitive determinants other than hepatic LRP contribute to the regulation of plasma FVIII in vivo.