Structure, Biology, and Genetics of Factor VIII. Factor VIII Function, страница 3

Analysis of the expression of factor VIII in Chinese hamster ovary cells provided insights into its probable biosynthetic pathway (Fig. 105-2). On synthesis, factor VIII is translocated into the lumen of the endoplasmic reticulum (ER), where the signal peptide is cleaved. In the ER, addition of high-mannose oligosaccharide to multiple asparagine residues within the factor VIII molecule occurs. A significant portion of the factor VIII in the ER is bound to a resident protein of the ER, the glucose-regulated protein of 78,000 MW (GRP78), also known as immunoglobulin-binding protein, or BiP. MLID86245075 MLID88087401  35,36 BiP expression is induced by glucose deprivation, inhibition of N-linked glycosylation, or the presence of malfolded protein within the ER. MLID88175074  37,38 Increased BiP expression inhibits factor VIII secretion. MLID92224895  39 BiP exhibits a peptide-dependent ATPase activity. MLID92049699  40 Dissociation from BiP and secretion of factor VIII require unusually high levels of intracellular ATP. MLID91017519  41 It is speculated that either the BiP-mediated ATP-dependent release assists protein folding or that BiP binding prohibits improperly folded molecules exiting the ER compartment and directs them to degradation. The secretion-competent factor VIII transits to the Golgi apparatus. In the Golgi apparatus, most factor VIII is cleaved at two sites within the B domain after residues 1,313 and 1,648 to generate the heavy chains (90,000–200,000 MW) and the light chain (80,000 MW). Also within the Golgi apparatus, factor VIII is further processed by (1) modification of the asparagine-linked high-mannose-containing oligosaccharides to complex types, (2) addition of carbohydrate to multiple serine and threonine residues within the B domain, and (3) addition of sulfate to six tyrosine residues within the heavy and the light chains.

vWF promotes the association of the light and heavy chains of factor VIII and results in accumulation of stable factor VIII activity in the conditioned medium of factor VIII-producing cells. MLID88198183 MLID92042109  34,42 The heavy and light chains of factor VIII synthesized in the absence of vWF in the conditioned medium are secreted as dissociated chains that are subsequently degraded. The effect of vWF in promoting assembly and stable secretion of factor VIII in cell-culture systems may reflect the role of vWF in regulating levels of factor VIII activity in vivo. MLID83022933 MLID77207496 MLID86094303  43–47 These findings suggest that the reduction in factor VIII levels associated with vWF deficiency may result partly from the inability of factor VIII to be stabilized in the plasma on secretion from the cell.

Factor VIII and vWF are cleared with a half-life of 12 hours on infusion of the factor VII11/vWF complex into hemophilia patients. MLID83022933  43,46,47 Infusion of pure factor VIII also exhibits clearance kinetics similar to that of factor VIII/vWF complex, presumably due to rapid binding of factor VIII to plasma vWF. MLID77207496 MLID86094303  44,45 By contrast, infusion of pure factor VIII into patients with severe von Willebrand disease or into severe vWF-deficient dogs results in rapid clearance (half-life of 2.4 hours). MLID83022933 MLID86094303  43,45 These studies establish the stabilizing influence of vWF for factor VIII in the circulation. Autosomally inherited factor VIII deficiency results from genetic defects in vWF that reduce its ability to bind and stabilize factor VIII in plasma. MLID90001540 MLID92336160  48,49

Interactions of Factor VIII With Components of Hemostasis

Binding to von Willebrand Factor

vWF plays a critical role in regulating factor VIII activity by several different mechanisms. vWF stabilizes factor VIII on secretion from the cell MLID88198183 MLID92042109  34,42 and is required for its normal survival in plasma. MLID83022933 MLID77207496 MLID86094303  43–47 vWF protects factor VIII from activation by factor Xa 50 and from inactivation by activated protein C MLID89008875 MLID91115828  51,52 but does not interfere with activation by thrombin. MLID87304256 MLID93271477  53,54 Finally, vWF prevents factor VIII binding to phospholipids MLID82134732  55,56 and activated platelets. MLID92011498  57 Based on these functions, it is proposed that vWF binding to platelet receptor glycoprotein (GP)Ib brings factor VIII to the vicinity of platelets adhering to damaged endothelium. MLID92011498  57