Role of propeptide in vitamin K-dependent gamma-carboxylation

Defining the landscape of metabolic dysregulations in cancer metastasis

Metastasis is the primary cause of cancer related deaths due to the limited number of efficient druggable targets. Signatures of dysregulated cancer metabolism could serve as a roadmap for the determination of new treatment strategies. However, the metabolic signatures of metastatic cells remain vastly elusive. Our aim was to determine metabolic dysregulations associated with high metastatic potential in breast cancer cell lines. We have selected 5 triple negative breast cancer (TNBC) cell lines including three with high metastatic potential (HMP) (MDA-MB-231, MDA-MB-436, MDA-MB-468) and two with low metastatic potential (LMP) (BT549, HCC1143). The normal epithelial breast cell line (hTERT-HME1) was also investigated. The untargeted metabolic profiling of cells and growth media was conducted and total of 479 metabolites were quantified. First we characterized metabolic features differentiating TNBC cell lines from normal cells as well as identified cell line specific metabolic fingerprints. Next, we determined 92 metabolites in cells and 22 in growth medium that display significant differences between LMP and HMP. The HMP cell lines had elevated level of molecules involved in glycolysis, TCA cycle and lipid metabolism. We identified metabolic advantages of cell lines with HMP beyond enhanced glycolysis by pinpointing the role of branched chain amino acids (BCAA) catabolism as well as molecules supporting coagulation and platelet activation as important contributors to the metastatic cascade. The landscape of metabolic dysregulations, characterized in our study, could serve as a roadmap for the identification of treatment strategies targeting cancer cells with enhanced metastatic potential.

Defective γ-Glutamyl Carboxylase Activity and Bleeding in Rambouillet Sheep

A flock of Rambouillet sheep was examined because of increased lamb mortality caused by ineffective hemostasis at parturition. Neonatal-affected lambs presented with inadequate hemostasis at the umbilicus, pale mucus membranes, and markedly prolonged activated clotting time. Affected lambs had consistently prolonged 1-stage prothrombin times and activated partial thromboplastin times that supported a defect in the common pathway or defects in both the intrinsic and extrinsic pathway of the coagulation cascade. Decreased activity of vitamin K-dependent procoagulant factors II, VII, IX, and X in male and female lambs suggested either a defect of the hepatic enzyme γ-glutamyl carboxylase, or vitamin K1 2,3 epoxide reductase. Affected lamb hepatic γ-glutamyl carboxylase activity was markedly decreased compared with that of age- and sex-matched control lambs, while vitamin K1 2,3 epoxide reductase and glucose-6-phosphatase activities were similar between an affected and normal lamb. Subcutaneous vitamin K1 supplementation did not increase vitamin K-dependent procoagulant factor activities in 3 lambs administered vitamin K1 daily. These data confirm defective γ-glutamyl carboxylase activity as the cause of impaired coagulation of sheep in this flock. This flock represents the only viable animal model of hereditarily defective γ-glutamyl carboxylase activity.

Truncated gamma-glutamyl carboxylase in rambouillet sheep

A flock of Rambouillet sheep was examined because of increased lamb mortality due to ineffective hemostasis at parturition. Decreased activities of coagulation factors II, VII, IX, and X, and severely reduced hepatic gamma-glutamyl carboxylase activity with adequate vitamin K 2,3 epoxide reductase activity was determined.(1,)(21) Parenteral vitamin K(1) supplementation did not improve vitamin K-dependent coagulation factor activities in 3 affected lambs. Affected lamb gamma-glutamyl carboxylase deoxyribonucleic acid was sequenced, and 4 single nucleotide polymorphisms (SNPs 2-5) of the gamma-glutamyl carboxylase gene were identified. Single nucleotide polymorphism-4 results in an arginine to stop codon (UGA) substitution, which prematurely terminates the peptide at residue 686 (R686Stop). This genotype (GATT/GATT) has a strong association with the coagulopathy observed in clinically affected lambs, P < 0.001. The frequency of SNP-3 in exon 11 (R486H) within the MARC 1.1 database is high in the US sheep population overall. Gamma-glutamyl carboxylase activity in hepatic microsomes from a SNP-3 homozygous lamb lacking the SNP-4 mutation (GACC/GACC) was similar to control sheep homozygous for arginine at 486 and also lacking SNP-4 (TGCC/TGCC), indicating that the R486H does not measurably impact gamma-glutamyl carboxylase activity. The remaining two SNPs (2 and 5) are located within non-coding intron sequences. These 4 SNPs allowed for determining the genotype associated with the observed fatal coagulopathy. Screening for the premature truncation (SNP-4) based on the presence of a Bbv I restriction site in clinically normal lambs but not in the homozygous affected lambs allows for detection of the heterozygous state (GATT/GACC), because carrier animals are clinically normal.

Developmental expression of vitamin K-dependent gamma-carboxylase activity in zebrafish embryos: effect of warfarin

Vitamin K-dependent gamma-carboxylation is an essential posttranslational modification required for the functional activity of coagulation proteins such as factors VII, IX, X, and prothrombin. Warfarin, an inhibitor of vitamin K-dependent gamma-carboxylation, was used in earlier work on adult zebrafish to provide evidence for the presence of vitamin K-dependent carboxylase in zebrafish. Here we demonstrate the presence of vitamin K-dependent carboxylase activity in zebrafish by directly assaying the microsomal fraction prepared from adult, unfertilized eggs, and embryos from different developmental stages. Gamma-carboxylase activity was detected both before and after fertilization of embryos and the activity levels remained relatively constant from 6 h postfertilization (hpf) through other advanced stages of development. The expression of activity in the early embryos (0-6 hpf) may be due to the presence of maternal protein since the activity was detected even in the unfertilized eggs. Gamma-carboxylase activity in the eggs as well as early embryos suggested that vitamin K-dependent carboxylase is important throughout development. The detection of vitamin K-dependent carboxylase mRNA by RT-PCR and inhibitor studies using warfarin confirmed these activity results. Further, these studies provide a basis for selecting warfarin-resistant zebrafish mutants in order to find genes regulating gamma-carboxylase activity including the yet unidentified vitamin K-epoxide reductase.

Identification of residues in the Gla-domain of human factor IX involved in the binding to conformation specific antibodies

The binding of Ca2+ induces a conformational change in factor IX which can be monitored with conformation specific antibodies. Anti-FIX:Mg(II) antibodies recognize a conformational epitope (FIX') that can be induced by several metal ions such as Ca2+, Mg2+, Mn2+ and Ba2+, while anti-FIX:Ca(II) antibodies recognize a conformational epitope (FIX*) that can be only induced by Ca2+ and Sr2+ ions (Liebman et al., J. Biol. Chem., vol. 262 (1987) pp. 7605-7612). The latter conformation is essential for the function of factor IX. In this study we tried to identify residues in the Gla-domain of factor IX which are involved in binding to anti-factor IX:Mg(II) and anti-factor IX:Ca(II) antibodies. For this we substituted residues in recombinant human factor IX for those of factor X or factor VII. The substitution of residues 1-40 of factor IX by those of factor VII eliminated binding to both types of antibodies. Re-introduction of factor IX specific residues increased the binding to conformation specific anti-factor IX antibodies, but reduced the binding to conformation specific anti-factor VII antibodies, indicating that the structural integrity of the Gla-domain was not seriously affected by the mutations. We provide evidence that residues 33, 39 and 40 of human factor IX are important for binding to anti-factor IX:Mg(II) antibodies, while residues 1-11 are important for binding to anti-factor IX:Ca(II) antibodies.

Molecular cloning of avian matrix Gla protein

Matrix Gla protein plays an essential role in preventing the calcification of blood vessel walls, cartilage and other tissues. We report here the primary structure of chicken matrix Gla protein as deduced from the cDNA sequence. The avian protein exhibited the characteristic motifs previously identified in the mammalian proteins, but its amino acid sequence shared only 51-56% identity with the latter proteins. Moreover, a region proposed to function as binding site for gamma-carboxylase in the mammalian proteins was poorly conserved in the chicken protein. Our sequence data should be helpful in the design of mutational analyses which are intended to characterize functional interactions of matrix Gla proteins with other proteins.

The gamma-carboxylation recognition site is sufficient to direct vitamin K-dependent carboxylation on an adjacent glutamate-rich region of thrombin in a propeptide-thrombin chimera

The propeptides of the vitamin K-dependent proteins contain a gamma-carboxylation recognition site that is required for gamma-glutamyl carboxylation. To determine whether the propeptide is sufficient to direct carboxylation, two mutant prothrombin species were expressed and characterized with regard to posttranslational gamma-carboxylation. A double point mutant, in which serine substituted for cysteines 17 and 22 disrupted a conserved loop formed by a disulfide bond, was fully carboxylated when expressed in Chinese hamster ovary cells. A propeptide/thrombin chimeric protein, constructed by deleting the Gla, aromatic amino acid stack, and kringle domains of prothrombin, has the signal peptide and propeptide juxtaposed to a glutamate-rich COOH-terminal region of prothrombin, residues 249-530. Of the 8 glutamic acid residues contained within the first 40 residues of the NH2 terminus adjacent to the propeptide, at least seven were fully carboxylated as demonstrated by direct gamma-carboxyglutamic acid analysis of the alkaline hydrolysate and by NH2-terminal sequence analysis. These results indicate that the gamma-carboxylation recognition site within the prothrombin propeptide in a prothrombin propeptide-thrombin chimeric protein is sufficient to direct gamma-carboxylase-catalyzed carboxylation of adjacent glutamic acid residues in a glutamate-rich region of thrombin that is not normally gamma-carboxylated. Furthermore, the disulfide loop in the Gla domain of prothrombin is not required for complete carboxylation.

Genomic Sequence and Transcription Start Site for the Human γ-Glutamyl Carboxylase

The human gene for γ-glutamyl carboxylase is 13 kb in length and contains 15 exons. Transcription starts at a cytosine 217 base pair upstream of the first codon. There are two major transcripts in all tissues examined. They are distinguished by the presence of an Alu sequence in the 3′ nontranslated end of the longer species. Relative mRNA levels for 12 bovine tissues are presented.

Modification of the N-terminus of human factor IX by defective propeptide cleavage or acetylation results in a destabilized calcium-induced conformation: effects on phospholipid binding and activation by factor XIa

The propeptide of human coagulation factor IX (FIX) directs the gamma-carboxylation of the first 12 glutamic acid residues of the mature protein into gamma-carboxyglutamic acid (Gla) residues. The propeptide is normally removed before secretion of FIX into the blood. However, mutation of Arg-4 in the propeptide abolishes propeptide cleavage and results in circulating profactor IX in the blood. We studied three such genetic variants, factor IX Boxtel (Arg-4-->Trp), factor IX Bendorf (Arg-4-->Leu) and factor IX Seattle C (Arg-4-->Gln). These variant profactor IX molecules bind normally to anti-FIX:Mg(II) antibodies, which indicates that the mutations do not seriously affect gamma-carboxylation. Metal ion titration of the binding of variant profactor IX to conformation-specific antibodies demonstrates that the calcium-induced conformation is destabilized in the variant molecules. Also the binding of FIX Boxtel to phospholipids and its activation by factor XIa requires a high (>5 mM) calcium concentration. The three-dimensional structure of the Gla domain of FIX in the presence of calcium indicates that the acylation of the amino-terminus, rather than the presence of the propeptide, was responsible for the destabilization of the calcium-induced conformation. In order to confirm this, the alpha-amino group of Tyr1 of FIX was acetylated. This chemically modified FIX showed a similar destabilization of the calcium-induced conformation to variant profactor IX. Our data imply that the amino-terminus of FIX plays an important role in stabilizing the calcium-induced conformation of the Gla domain of FIX. This conformation is important for the binding to phospholipids as well as for the activation by factor XIa. Our results indicate that mutations in FIX that interfere with propeptide cleavage affect the function of the protein mainly by destabilizing the calcium-induced conformation.

Multi-site-specificity of the vitamin K-dependent carboxylase: in vitro carboxylation of des-gamma-carboxylated bone Gla protein and Des-gamma-carboxylated pro bone Gla protein

The vitamin K-dependent carboxylase processes multiple glutamic acid residues to gamma-carboxyglutamic acid (Gla) residues in a limited number of proteins. The targeted proteins are synthesized with an amino-terminal propeptide which has been shown to play an important role in gamma-carboxylation. The specificity of the enzyme for each potential Gla site, the direction of carboxylation, and the influence of a bound propeptide on these events are not understood. Des-gamma-carboxy forms of bone Gla protein (BGP), which contain potential Gla residues at positions 17, 21, and 24, were employed as model substrates to determine the multi-site-specificity of the enzyme. Recombinant bovine des-gamma-carboxylated proBGP (rdproBGP) and heat-decarboxylated BGP (dBGP), lacking a propeptide, were used as substrates for a bovine liver carboxylase, and the in vitro reaction products were analyzed for the formation of 14CO2 Gla. The di-Gla species was found to be the predominant product of in vitro carboxylation of both rdproBGP and dBGP at less than saturating concentrations of each substrate. Carboxylation of both substrates occurred preferentially at the more C-terminal potential Gla sites, residues 21 and 24. A similar pattern of carboxylation was observed with a rat bone cell carboxylase, suggesting no species or tissue variation in the enzyme specificity. Some tricarboxylated product accumulated during carboxylation of rdproBGP but not dBGP, suggesting that the covalently bound propeptide directs more complete carboxylation of the Gla domain. In addition, monocarboxylated rdproBGP was found to accumulate in the absence but not in the presence of a free noncovalently attached propeptide, indicating that free propeptide affects more efficient carboxylation of rdproBGP.

Mutagenesis of vitamin K-dependent carboxylase demonstrates a carboxyl terminus-mediated interaction with vitamin K hydroquinone

The gamma-glutamyl carboxylase and vitamin K epoxidase activities of a series of mutants of bovine vitamin K-dependent carboxylase with progressively larger COOH-terminal deletions have been analyzed. The recombinant wild-type (residues 1-758) and mutant protein carboxylases, Cbx 711, Cbx 676, and Cbx 572, representing residues 1-711, 1-676, and 1-572, respectively, were expressed in baculovirus-infected Sf9 cells. Wild-type carboxylase had a Km for the substrate Phe-Leu-Glu-Glu-Leu (FLEEL) of 0.87 mM; the carboxylation of FLEEL was stimulated 2.5-fold by proPT18, the propeptide of prothrombin. Its Km for vitamin K hydroquinone was 23 microM and the specific epoxidase activity of the carboxylase was 938 pmol vitamin KO/30 min/pmol of carboxylase. Cbx 711, which was also stimulated by proPT18, had a Km for FLEEL, a Km for vitamin K hydroquinone, and a specific epoxidase activity that was comparable to the wild-type carboxylase. In contrast Cbx 572 lacked both carboxylase and epoxidase activities. Although Cbx 676 had a normal carboxylase active site in terms of the Km for FLEEL and its stimulation by proPT18, the Km for vitamin K hydroquinone was 540 microM, and the specific epoxidase activity was 97 pmol KO/30 min/pmol of Cbx 676. The catalytic efficiencies of Cbx 676 for glutamate carboxylation and vitamin K epoxidation were decreased 15- and 400-fold, respectively, from wild-type enzyme reflecting the requirement for formation of an activated vitamin K species for carboxylation to occur. These data indicate that the truncation of COOH-terminal segments of the carboxylase had no effect on FLEEL or propeptide recognition, but in the case of Cbx 676, selectively affected the interaction with vitamin K hydroquinone and the generation of epoxidase activity. These data suggest that a vitamin K epoxidase activity domain may reside near the COOH terminus while the carboxylase active site domain resides toward the NH2 terminus.

Profactor IX: the propeptide inhibits binding to membrane surfaces and activation by factor XIa

The gamma-carboxylase recognition site in the propeptide of profactor IX signals the gamma-carboxylation of specific glutamic acid residues in the adjacent Gla domain during factor IX biosynthesis. To study posttranslational processing of the vitamin K-dependent blood coagulation factors and the properties of processing intermediates, we have isolated an incompletely processed factor IX species, profactor IX, from the medium of heterologous mammalian cells expressing the human factor IX cDNA. Profactor IX was purified by sequential immunoaffinity chromatography using antibodies specific for the propeptide and antibodies specific for the well-carboxylated factor IX species. This purified profactor IX preparation was fully gamma-carboxylated and contained the N-terminal propeptide, but it exhibited no factor IX procoagulant activity. Profactor IX was not cleaved following incubation with factor XIa. In contrast to mature factor IX, profactor IX did not demonstrate Ca(II)-dependent binding to acidic phospholipid vesicles, nor can the membrane binding surface be expressed, as detected by antibodies specific for this epitope. The propeptide of profactor IX can be removed in vitro by a specific endopeptidase, furin/PACE, yielding factor IX, which can be converted to fully active factor IXa by factor XIa and which binds normally to acidic phospholipid vesicles. These results indicate that fully gamma-carboxylated profactor IX is biologically inactive due to the presence of the propeptide.