SYNTHESIS. PROCESSING AND SECRETION OF HUMAN FACTOR VIII IN MAMMALIAN CELLS: REQUIREMENT FOR VON WILLEBRAND FACTOR

In the plasma factor VIII exists as a complex with von Willebrand factor (vWF). The cloning of the cDNA for factor VIII has provided the ability to develop mammalian cell lines which express high levels of factor VIII by using appropriatate expression plasmids and DNA cotransformation with selectable markers. We have studied the synthesis, processing, and secretion of factor VIII expressed in baby hamster kidney cells and in Chinese hamster ovary cells by 35S-methionine pulse and chase labeling and analysis by immunoprecipitation with specific antibodies which recognize the light and heavy chains of factor VIII. In both mammalian cell lines, factor VIII is synthesized as a primary translation product of 230 kDa. A significant amount remains within the endoplasmic reticulum in a stable complex with a glucose regulated protein of 78 kDa. The remainder traverses into the Golgi compartment where it is cleaved to the heavy and light chain forms. Very shortly thereafter the mature factor VIII appears in the conditioned media as the mature heavy and light chain species. Very little single chain factor VIII is secreted into the conditioned media. The accumulation of factor VIII in the conditioned media requires the presence of vWF factor. In the absence of vWF, the factor VIII appears as unassociated heavy and light chains which are rapidly degraded. Bovine, porcine, or human 3WF all effectively stabilize human factor VIII expressed in these rodent cell lines. These results suggest the presence of vWF promotes factor VIII chain association which stabilizes the factor VIII to proteolysis.