BIOSYNTHESIS AND EXPRESSION OF FACTOR V IN MAGAKARYOCYTES

Coagulation factor V (FV), is a single chain, multifunctional glycoprotein of Mr 350,000 which interacts with a variety of hemostatic proteins such as factor Xa, prothrombin, thrombin and protein C, on the surface of platelets and vascular endothelial cells. FV serves as both a cofactor and substrate in the generation of thrombin and plays a critical regulatory role in both physiologic hemostasis and pathologic thrombosis. The biosynthesis of FV and its subsequent expression are therefore expected to be precisely controlled and may differ in the three sites of synthesis - hepatocytes, endothelial cells, and megakaryocytes (MK). We have previously demonstrated that each guinea pig MK contains 500 times as much FV as in a platelet, as quantified by a competitive enzyme-linked-immunosorbent assay and expresses FV by cytoimmunofluorescence. De novo biosynthesis was demonstrated by incorporation of S-methionine into FV purified on a immunoaffinity column. The purified MK protein exhibited both FV coagulant activity and antigenicity. However, MK FV was more slowly activated by thrombin, more stable in the absence of Ca and exhibited a slightly higher M of 380,000 compared to plasma FV. Similar studies have documented biosynthesis in human MK. In addition, all morphologically recognizable MK enriched by elutriation from human bone marrow contained FV as documented by both monospecific polyclonal and monoclonal antibodies (MAb) to FV. All these cells bound FV since a murine MAb reacting with the light chain of FV (B38) labeled all cells. In contrast, 68% of cells synthesized FV since B10, a MAb to the activation peptide recognizing FV but not FVa, labeled this fraction. To determine whether immature nonnorphologically recognizable MK expressed FV, we identified these cells with an antiserum to human platelet glycoproteins and then probed them with B38. Seventy percent (70%) of such small cells expressed FV. In contrast, no small cells in MK colonies cloned in FV deficient medium expressed FV while only 40% of such colonies contained cells which expressed FV.To further probe the regulation of FV in MK we attempted to correlate the synthesis of FV as probed by MAb B10 with geometric mean cell diameter, stage and ploidy. No significant correlation of FV with any of these indicators of MK maturation. In contrast, preliminary studies suggest that low doses of tetradecanoyl phorbol acetate augment both the number of MK containing FV and the level of FV expressed by individual cells. Thus, FV synthesis may be regulated independent of size, stage, or ploidy and protein kinase C may play a role.To further define the molecular nature of FV in MK we found that purified FV was converted from a monomer to high Mr multimers by an enzyme derived from MK. These multimers resulting from covalent crosslinking since they were stable to SDS, 100° C and reducing agents. The responsible enzyme appeared to be MK FXIIIa since it required C, was inhibited by agents which react with the active site thiol group and was blocked by pseudoamine donor substrates such as putrescine. In addition, FXIIIa was directly demonstrated in guinea pig MK by a specific activity stain. Other investigators have established that FV became irreversibly associated with platelet cytoskeletons after exposure to thrombin. tested whether FXIIIa might mediate this association by performing ligand blotting of platelet membrane proteins using 125I-FV(FV*). Only actin of all the membrane proteins was detected by radioautography. The binding of FV* to the cytoskeleton was dependent in the presence of Ca and FXIIIa. In purified systems crosslinked complexes containing FV* or radiolabeled actin were detected in separate experiments. In whole platelets, the formation of the heteropolymer, after thrombin stimulation, was inhibited by antibodies to FXIII a chain, FV activation peptide (B10) or actin. Endogenous platelet FV was also dependent on FXIII for incorporation into the platelet cytoskeleton after thrombin stimulation. When thrombin-treated FV was crosslinked to actin only the activation peptide (150 kDa) was crosslinked. The light chain or heavy chain of FVa were not involved. Thus FXIIIa play an important role in the binding of FV in platelets to the cytoskeleton during activation and secretion.Further studies of FV in megakaryocytes are necessary to define the regulation of biosynthesis and the control of expression which dictate its critical role in hemostasis and thrombosis.