In search of the source of factor VIII

The cellular site of synthesis of factor VIII in the circulation has long been disputed, but 2 papers in this issue of Blood by Fahs et al¹  and Everett et al²  finally identify the cell type that makes factor VIII in the liver and, by implication, in the rest of the body.

To understand why this quest was so prolonged, one needs to go back to the technologies of the 1970s and 1980s. Factor VIII is a cofactor for conversion of factor X to factor Xa by activated factor IX and is the rarest of all the clotting factors in blood. Not surprisingly, it was the last but one of the classical blood coagulation factors to yield to the advance of molecular biology (in 1984, 2 years before tissue factor and von Willebrand factor). To purify enough protein for biochemical characterization was beyond the limits of technology available to researchers up to 1980. Then, progressively new techniques enabled even the rarest of proteins to be isolated. Thus, factor VIII was purified and sequenced and its gene cloned.³  Earlier efforts to localize the organs making factor VIII showed that the liver of a normal dog transplanted into a dog with hemophilia corrected the bleeding tendency by elevating the dog’s blood factor VIII level up to 50% of normal.⁴  In the reverse experiment, a normal dog with a hemophilic liver still maintained a factor VIII level of 50%. Later, we found that a hepatocyte-rich cell fraction from human donor liver contained factor VIII messenger RNA and factor VIII antigen,⁵  but there remained lingering doubt about the actual cell type involved. Studies in rodents showed that liver endothelium contained factor VIII antigen,⁶  and the original cross transplantation work demonstrated that cells other than hepatocytes must be able to make half the factor VIII circulating in blood.

Recently, it has been shown convincingly that factor VIII activity and antigen are confined to cells sorted individually from liver that carry markers restricted to endothelium.⁷  The papers published in this issue^1,2  complete the story using sophisticated cellular biology methods not even dreamt of in the 1980s. In one study using a Cre/lox-dependent conditional knockout model, hepatocyte-specific factor VIII knockout mice are indistinguishable from controls, whereas efficient endothelial knockout models display a severe hemophilic phenotype with no detectable plasma FVIII activity. In the other study, Lman1 conditional knockout mice were generated to characterize the FVIII secretion profiles of endothelial cells and hepatocytes. Because Lman1 is essential for export of factor VIII from cellular sites of synthesis, it was again shown that only if endothelial cells have no Lmann1 does the factor VIII level fall and hence that they are the primary biosynthetic source of murine FVIII. Conversely, when hepatocytes made no Lmann1, the plasma FVIII pool was unaffected. Taken together, these studies conclusively demonstrate that hepatocytes have no role in factor VIII biosynthesis, a role that is fulfilled by endothelial cells in the liver and elsewhere.

This result must raise the question of why hepatocytes synthesize every other circulating factor necessary for clot formation yet specifically omit one crucial cofactor in the enzymatic cascade leading to thrombin formation and conversion of fibrinogen to fibrin. Several explanations come to mind. Perhaps it is dangerous to have a complete cascade in one cell, with risk of intracellular fibrin formation and adverse consequences for hepatocytes. Against this is the observation that expression of factor VIII at high levels in the hepatocytes of mice receiving gene transfer had no such effect.⁸  Alternatively, it may be that having factor VIII made throughout the circulation in cells that also make the carrier molecule von Willebrand factor is efficient and also allows for localized targeted release that occurs when those cells are damaged or in response to certain physiological stimuli such as norepinephrine and exercise. Whatever the evolutionary explanation may be, it is satisfying to know at last the true source of factor VIII.