Induction of prothrombin synthesis by prothrombin fragments

Anti- and procoagulant activities in factor VII-deficient subjects

The clinical feature in patients with congenital factor VII deficiency is in part dependent on the underlying genetic defect, but the mechanisms influencing the genotype-phenotype correlation remain to be fully elucidated. In addition, thromboembolic events have been reported. Compensatory mechanisms involving vitamin K-dependent factors have been suggested. We have measured anticoagulant activities in 25 factor VII-deficient subjects (factor VII activity < or =36%) and 23 age-matched controls and correlated these to the vitamin K-dependent procoagulant activities. Two of the patients had a history of thromboembolism. The factor VII-deficient patients were found to have a significantly lower protein C activity than the controls [0.84 U/ml (95% CI 0.78; 0.89) vs. 0.98 U/ml (95% CI 0.91; 1.05), P=.004]. In addition, the protein C activity was correlated to that of factor VII (r=.36; P=.014), factor IX (r=.45; P=.002) and factor X (r=.50; P=.0006), respectively. The level of prothrombin fragment 1+2 was correlated to the protein C (r=.40; P=.012) and to the factor VII activity (r=.42; P=.011). No differences between patients and controls were seen regarding total and free protein S, antithrombin, plasminogen activator inhibitor-1 (PAI-1) and tissue factor pathway inhibitor (TFPI). Seven of the patients were found to have the Factor V Leiden mutation, but none of them had experienced any thromboembolic event. The present data support the notion that compensatory hemostatic mechanisms might exist in that the protein C activity was found to be decreased in the factor VII-deficient subjects. Whether this could influence the clinical feature, including the risk of thromboembolic events in association with replacement therapy, remains to be evaluated.

Elastase regulates the synthesis of its inhibitor, alpha 1-proteinase inhibitor, and exaggerates the defect in homozygous PiZZ alpha 1 PI deficiency

The net balance of neutrophil elastase, an enzyme that degrades many components of the extracellular matrix, and its inhibitor, alpha-1-proteinase inhibitor (alpha 1 PI), is thought to be a critical determinant in the development of destructive lung disease, especially in individuals with homozygous alpha 1 PI deficiency. Synthesis and secretion of alpha 1 PI has been recently demonstrated in cells of mononuclear phagocyte lineage, including peripheral blood monocytes and tissue macrophages. In this study we show that alpha 1 PI gene expression in human monocytes and bronchoalveolar macrophages is affected by a novel mechanism, whereby elastase directly regulates the synthesis of its inhibitor. In nanomolar concentrations, neutrophil or pancreatic elastase mediates a dose- and time-dependent increase in steady state levels of alpha 1 PI mRNA and in the rate of synthesis of alpha 1 PI in human monocytes and bronchoalveolar macrophages. Antisera to neutrophil elastase or pretreatment of elastase with the serine proteinase inhibitor diisopropylfluorophosphate abrogates the effect of elastase on alpha 1 PI expression. Elastase also stimulates the synthesis of alpha 1 PI in monocytes from homozygous PiZZ alpha 1 PI-deficient individuals, but has no effect on the rate of secretion; hence, the enzyme mediates an effect on alpha 1 PI that increases the intracellular accumulation of inhibitor and exaggerates the intrinsic defect in secretion of alpha 1 PI that characterizes the homozygous PiZZ alpha 1 PI deficiency.

Turnover of factor X and of prothrombin in rabbits fed on a standard or cholesterol-supplemented diet

The turnover of prothrombin and of factor X was investigated in rabbits fed on a 1%-cholesterol-supplemented or a standard diet by studying the evolution of radioactivity in blood and in plasma from these animals after the intravenous injection of either 125I-rabbit factor X or 125I-bovine prothrombin. For factor X, half-lives and fractional pool sizes were similar for the two groups of rabbits in the extravascular, intravascular and plasma compartments. However, the equivalent plasma fractional pool size for the two groups of rabbits was only 73% of that in the intravascular compartment. The fractional catabolic rate for the hypercholesterolaemic rabbits [0.064 +/- 0.007 (of the intravascular pool)/h] was not significantly different from that in the rabbits fed on the standard diet (0.074 +/- 0.008/h). However, the absolute catabolic rate, and therefore the rate of synthesis, was significantly higher (1.261 +/- 0.141 mg/day per kg body wt. of rabbit) in the rabbits fed on the cholesterol-supplemented than that in the rabbits fed on the standard diet (0.705 +/- 0.019 mg/day per kg). The prothrombin half-lives and fractional pool sizes were similar for the two groups of rabbits in the extravascular and the intravascular compartments. The fractional catabolic rate for the hypercholesterolaemic rabbits [0.041 +/- 0.003 (of the plasma pool)/h] was not significantly different from that in the rabbits fed on the standard diet (0.035 +/- 0.003/h). However, the absolute catabolic rate and therefore the rate of prothrombin synthesis was significantly higher (3.96 +/- 0.48 mg/day per kg body wt.) in the rabbits fed on the cholesterol-supplemented than that in the rabbits fed on the standard diet (2.24 +/- 0.12 mg/day per kg).

Plasma-protein production by rat hepatoma cells in culture, their variants and revertants

A series of subclones of the H4II line of the Reuber H35 rat hepatoma produce substantial amounts of three plasma proteins, transferrin, alpha 1-antitrypsin and fibrinogen. Immunocytochemical staining demonstrated that each of these proteins is synthesized by essentially every cell of these cell populations. Cells of dedifferentiated variant clones either cease to produce the proteins, or exhibit a substantial reduction that is accompanied by variability in the synthetic activity of individual cells of the population. As previously observed with regard to angiotensinogen production, the variant clones clearly divide into two categories: those that show only a reduction in synthesis are able to give rise to revertants, whereas the negative clones fail to do so. Revertant cells exhibit a dramatic restoration of the synthesis of plasma proteins, which in some cases, exceeds by severalfold the rates seen in the differentiated clones of origin. In addition, the revertant cells synthesize alpha-fetoprotein, a function that is not expressed by H4II cells or its daughter subclones. Immunocytochemical staining revealed that, with regard to several plasma proteins including albumin, fibrinogen and alpha-fetoprotein, the cell populations of revertant clones are very heterogeneous, for only a fraction of the cells synthesizes each protein. Hybrid cells resulting from several types of crosses, exhibited extinction of the plasma proteins, the exception being transferrin, whose production was maintained, but at a reduced level and in only a fraction of the cells. Taken together, our results show that the expression of albumin and transferrin can be dissociated from one to another, and from that of fibrinogen, alpha 1-antitrypsin and angiotensinogen.

Studies on the secretion of serum proteins from rat hepatoma cells

We have used crossed immunoelectrophoresis to identify and establish the relative amounts of serum proteins secreted by a differentiated cell line (Fao) derived from a Reuber H35 rat hepatoma. Our results show that these cells secrete at least 15 plasma proteins. Ten of these: albumin, alpha 1-antitrypsin, alpha 1-lipoprotein, alpha 1-macroglobulin, alpha 1-antichymotrypsin, GC-globulin (transcalciferin), fibronectin, hemopexin, transferrin and the C3 component of complement have been identified. To examine the feasibility of using the Fao cell line as a model for studies on the regulation of hepatic protein secretion, we measured the relative amounts of 10 serum proteins secreted into the growth medium after exposure of these cells to dibutyryl cyclic AMP, hydrocortisone and a combination of both compounds. We also examined the effects of growth temperature (33.5 degrees, 37 degrees and 39 degrees C) and the removal of fetal calf serum from the growth medium on the relative amounts of these proteins secreted. We found that the rates of secretion of most of the serum proteins were altered by one or more of the treatments used in these experiments. In addition, detectable levels of secretion of three serum proteins, fibronectin and two unidentified, occurred only under certain of the experimental conditions. These results demonstrate that the pattern of proteins secreted from Fao cells can be experimentally altered and indicate that this cell line may be a useful model for studies on the control of hepatic protein secretion.

Biosynthesis of abnormally glycosylated alpha 1-antitrypsin by a human hepatoma cell line

The human hepatoma cell line PLC/PRF/5 synthesized and secreted a functional alpha 1-antitrypsin (alpha 1-AT) glycoprotein with normal molecular size but retarded electrophoretic mobility. The total process of translation, glycosylation and export required about 40 min and followed the same synthetic pattern as seen in rat hepatocytes, i.e., a signal peptide is cleaved cotranslationally; a core-glycosylated protein in the high-mannose form is formed in the rough endoplasmic reticulum, trimmed, and a stable complex-glycosylated alpha 1-AT is found intracellularly prior to export. alpha 1-AT export to medium was delayed by tunicamycin, inhibited by cycloheximide but unaffected by colchicine. After addition of exogenous alpha 1-AT to culture medium, neither negative nor positive feedback induction of synthesis could be demonstrated. Electrophoretic techniques indicated the presence of atypical, highly branched but incompletely sialylated carbohydrate chains in the hepatoma cell-derived alpha 1-AT. The accumulation of intracellular alpha 1-AT inclusions seen in the endoplasmic reticulum may reflect an imbalance between a high rate of polypeptide synthesis and terminal glycosylation.

The vitamin K-dependent carboxylation reaction

Gammacarboxyglutamic acid (Gla) is an abnormal amino acid, which occurs in a number of proteins. It was discovered about 10 years ago in the four vitamin K-dependent blood clotting factors and it could be demonstrated that Gla is formed in a post-translational modification step, which requires a carboxylating enzyme system (carboxylase) and vitamin K. Since at the time of this discovery the earlier mentioned clotting factors were the only proteins known to be synthesized in a vitamin K-dependent way, it has been assumed for many years that the blood clotting system was unique in this respect. Recently it has been demonstrated, however, that vitamin K-dependent carboxylase is not restricted to the liver (the place of synthesis of the clotting factors) but that it is also present in other tissues such as lung, kidney, spleen and testis. Moreover, numerous Gla-containing proteins have been detected, although in most cases their function is not wholly understood. It seems that (like for instance the glycosylation) the vitamin K-dependent carboxylation is a normal post-translational modification, which is required for the correct function of a certain class of Ca2+-binding proteins.

Effect of coumadin-induced coagulopoietin plasma on vitamin K-dependent carboxylation of liver microsomes

Coumadin-treated rabbits have a humoral substance(s) (coagulopoietin) which is capable of elevating vitamin K-dependent coagulation factors when injected into recipient rabbits (Karpatkin & Karpatkin, 1973). Biologic levels of coagulation factors II, V, VII and X; immunologic levels of factors II and X; and vitamin K-dependent liver microsomal carboxylase activity were measured in recipient rabbits receiving coumadin-induced coagulopoietin plasma. Factor II biologic activity increased 3.5-fold compared to the increase in immunologic activity. Factor X biologic activity increased 1.7-fold compared to the increase in immunologic activity. This indicates an increase in specific activity of factors II and X. Coumadin-induced coagulopoietin plasma had no effect on vitamin K-dependent liver microsomal carboxylase activity in vitro. However, livers obtained from recipient animals treated with coumadin-induced coagulopoietin plasma enhanced their carboxylase activity (compared to control animals) 2.4-fold employing endogenous microsomal precursor for carboxylation, and 6.2-fold employing synthetic substrate, phe-leu-glu-glu-val. Thus, coumadin-induced coagulopoietin plasma enhances the biologic activity of vitamin K-dependent coagulation factors II, VII, and X as well as the ex vivo vitamin K-dependent carboxylase activity of liver microsomes.

Plasma protein induction by isolated hepatocytes

Hepatocytes can be maintained in culture for periods of a few hours to many days. This review summarizes the metabolic characteristics of these cultures and describes their use in studying the regulation of plasma protein synthesis. Hormones selectively stimulate the synthesis of certain proteins. Cortisol stimulates the synthesis of fibrinogen and other acute-phase proteins; whereas, insulin stimulates albumin synthesis. In the latter case insulin increases the rate of a nuclear process. Mediators elaborated by leukocytes stimulate acute-phase protein synthesis in hepatocytes. Plasmin-generated fibrin peptides stimulate fibrinogen synthesis via a leukocytic mediator. Lipoprotein synthesis is stimulated by fatty acids and is inhibited by albumin and other macromolecules. These and other processes are susceptible to detailed analysis using sub-cellular fractions (mRNA, nuclei, transcription factors, etc.) isolated from hepatocytes. Studies on fetal or embryonic hepatocytes and hepatomas are yielding information on the regulation of secretory protein synthesis during development and following neoplastic transformation.

Characterization of the Mr difference between secreted murine fourth component of complement and the major plasma form: evidence for carboxyl-terminal cleavage of the alpha chain

The alpha-chain of murine fourth component of complement (C4) secreted by cells in vitro and in vivo has a Mr that is larger by approximately equal to 4,000 than that of the alpha-chain of the principal form of C4 in plasma. By using in vivo labeling of C4 with [35S]methionine, C4 was shown to be first synthesized with the higher Mr ("secreted") alpha-chain, which was then quickly processed (t1/2 approximately equal to 1 hr) extracellularly to the mature ("plasma") C4 possessing the lower Mr alpha-chain. Both forms of C4 were functional as assayed by the ability of their alpha-chains to be cleaved by the protease C1, to bind methylamine, and to undergo denaturation-dependent autolysis. When secreted C4 and plasma C4 were activated to C4b, the Mr difference of 4,000 was maintained in the alpha'-chains. The Mr difference was localized to the carboxyl-terminal autolytic fragment of the alpha-chain and was unaffected by the removal of carbohydrate. C4 from resident peritoneal macrophage cultures could be converted to the plasma form by incubation with heparin/plasma. This conversion could be blocked by EDTA or 1,10-phenanthroline. These data suggest that an enzyme, presumably a neutral proteinase present in mouse plasma, cleaves the carboxyl terminus of newly synthesized C4 alpha-chains, thereby creating the major form of C4 in plasma.

Characteristics of vitamin K-dependent carboxylating systems from human liver and placenta

Bovine liver vitamin K-dependent carboxylase was compared with that obtained from human liver and placenta. Human liver microsomal preparations contained more endogenous substrate than did bovine preparations, but no differences were found between the two types of hepatic enzyme. This observation demonstrates that the bovine liver carboxylating enzyme system is a good model system which will help us to understand vitamin K action in man. Placental carboxylase differed from the liver systems because only vitamin K hydroquinone and not vitamin K quinone could be used as a coenzyme for the carboxylation reaction. Obviously, vitamin K reductase was absent in these preparations.