Vitamin K-dependent protein S in Leydig cells of human testis

Zika virus dysregulates human Sertoli cell proteins involved in spermatogenesis with little effect on tight junctions

Zika virus (ZIKV), a neglected tropical disease until its re-emergence in 2007, causes microcephaly in infants and Guillain-Barré syndrome in adults. Its re-emergence and spread to more than 80 countries led the World Health Organization in 2016 to declare a Public Health Emergency. ZIKV is mainly transmitted by mosquitos, but can persist in infected human male semen for prolonged periods and may be sexually transmitted. Testicular Sertoli cells support ZIKV replication and may be a reservoir for persistent ZIKV infection. Electrical impedance analyses indicated ZIKV infection rapidly disrupted Vero cell monolayers but had little effect upon human Sertoli cells (HSerC). We determined ZIKV-induced proteomic changes in HSerC using an aptamer-based multiplexed technique (SOMAscan) targeting >1300 human proteins. ZIKV infection caused differential expression of 299 proteins during three different time points, including 5 days after infection. Dysregulated proteins are involved in different bio-functions, including cell death and survival, cell cycle, maintenance of cellular function, cell signaling, cellular assembly, morphology, movement, molecular transport, and immune response. Many signaling pathways important for maintenance of HSerC function and spermatogenesis were highly dysregulated. These included IL-6, IGF1, EGF, NF-κB, PPAR, ERK/MAPK, and growth hormone signaling. Down-regulation of the PPAR signaling pathway might impact cellular energy supplies. Upstream molecule analysis also indicated microRNAs involved in germ cell development were downregulated by infection. Overall, this study leads to a better understanding of Sertoli cellular mechanisms used by ZIKV during persistent infection and possible ZIKV impacts on spermatogenesis.

A chimerical phagocytosis model reveals the recruitment by Sertoli cells of autophagy for the degradation of ingested illegitimate substrates

Phagocytosis and autophagy are typically dedicated to degradation of substrates of extrinsic and intrinsic origins respectively. Although overlaps between phagocytosis and autophagy were reported, the use of autophagy for ingested substrate degradation by nonprofessional phagocytes has not been described. Blood-separated tissues use their tissue-specific nonprofessional phagocytes for homeostatic phagocytosis. In the testis, Sertoli cells phagocytose spermatid residual bodies produced during germ cell differentiation. In the retina, pigmented epithelium phagocytoses shed photoreceptor tips produced during photoreceptor renewal. Spermatid residual bodies and shed photoreceptor tips are phosphatidylserine-exposing substrates. Activation of the tyrosine kinase receptor MERTK, which is implicated in phagocytosis of phosphatidylserine-exposing substrates, is a common feature of Sertoli and retinal pigmented epithelial cell phagocytosis. The major aim of our study was to investigate to what extent phagocytosis by Sertoli cells may be tissue specific. We analyzed in Sertoli cell cultures that were exposed to either spermatid residual bodies (legitimate substrates) or retina photoreceptor outer segments (illegitimate substrates) the course of the main phagocytosis stages. We show that whereas substrate binding and ingestion stages occur similarly for legitimate or illegitimate substrates, the degradation of illegitimate but not of legitimate substrates triggers autophagy as evidenced by the formation of double-membrane wrapping, MAP1LC3A-II/LC3-II clustering, SQSTM1/p62 degradation, and by marked changes in ATG5, ATG9 and BECN1/Beclin 1 protein expression profiles. The recruitment by nonprofessional phagocytes of autophagy for the degradation of ingested cell-derived substrates is a novel feature that may be of major importance for fundamentals of both apoptotic substrate clearance and tissue homeostasis.

Protein S

Protein S (PS) is a vitamin K-dependent plasma glycoprotein. Around 60-70% of PS in plasma is noncovalently bound to C4-binding protein (C4BP). Free PS functions as a cofactor that enhances the activity of activated protein C (APC) in the proteolytic degradation of activated factors V and VIII. PS also has a more recently described APC-independent ability to directly inhibit prothrombinase and tenase by direct binding of activated factors V, VIII, and X. Given that PS is one of the major naturally occurring inhibitors of coagulation, acquired or hereditary deficiencies of this protein result in excessive thrombin generation. As a vast array of mutations are responsible for hereditary PS deficiencies, screening for their presence by DNA testing would require sequencing each entire gene involving numerous exons. Moreover, the knowledge of the gene mutation does not offer any benefit in the treatment of thrombophilic families, so the routine molecular characterization is not indicative. These defects are detected by functional or immunological assays for free and total PS forms. Given that functional PS assays may detect some forms of PS deficiency that free PS immunoassays may miss, it is recommended to include them for initial testing along with immunoassays for free PS, although they should be used with caution. Functional PS assays are subject to multiple interference. For example in the presence of lupus anticoagulant (LA), only free PS immunoassays are recommended for initial testing. PS antigen assays are more popular with most laboratories.

Dependence on vitamin K-dependent protein S for eukaryotic cell secretion of the beta-chain of C4b-binding protein

The anticoagulant vitamin K-dependent protein S (PS) circulates in plasma in two forms, 30% free and 70% being bound to the complement regulatory protein C4b-binding protein (C4BP). The major C4BP isoform consists of 7 α-chains and 1 β-chain (C4BPβ(+)), the chains being linked by disulfide bridges. PS binds to the β-chain with high affinity. In plasma, PS is in molar excess over C4BPβ(+) and due to the high affinity, all C4BPβ(+) molecules contain a bound PS. Taken together with the observation that PS-deficient patients have decreased levels of C4BPβ(+), this raises the question of whether PS is important for secretion of the β-chain from the cell. To test this hypothesis, HEK293 cells were stably and transiently transfected with β-chain cDNA in combinations with cDNAs for PS and/or the α-chain. The concentration of β-chains in the medium increased after co-transfection with PS cDNA, but not by α-chain cDNA, suggesting secretion of the β-chains from the cells to be dependent on concomitant synthesis of PS, but not of the α-chains. Thus, β-chains that were not disulfide-linked to the α-chains were secreted in complex with PS, either as monomers or dimers. Pulse-chase demonstrated that the complexes between PS and β-chain were formed intracellularly, in the endoplasmic reticulum. In conclusion, our results demonstrate that successful secretion of β-chains depends on intracellular complex formation with PS, but not on the α-chains. This provides an explanation for the decreased β-chain levels observed in PS-deficient patients.

Down-regulation of PROS1 gene expression by 17beta-estradiol via estrogen receptor alpha (ERalpha)-Sp1 interaction recruiting receptor-interacting protein 140 and the corepressor-HDAC3 complex

Pregnant women show a low level of protein S (PS) in plasma, which is known to be a risk for deep venous thrombosis. 17Beta-estradiol (E(2)), an estrogen that increases in concentration in the late stages of pregnancy, regulates the expression of various genes via the estrogen receptor (ER). Here, we investigated the molecular mechanisms behind the reduction in PS levels caused by E(2) in HepG2-ERalpha cells, which stably express ERalpha, and also the genomic ER signaling pathway, which modulates the ligand-dependent repression of the PSalpha gene (PROS1). We observed that E(2) repressed the production of mRNA and antigen of PS. A luciferase reporter assay revealed that E(2) down-regulated PROS1 promoter activity and that this E(2)-dependent repression disappeared upon the deletion or mutation of two adjacent GC-rich motifs in the promoter. An electrophoretic mobility shift assay and DNA pulldown assay revealed that the GC-rich motifs were associated with Sp1, Sp3, and ERalpha. In a chromatin immunoprecipitation assay, we found ERalpha-Sp protein-promoter interaction involved in the E(2)-dependent repression of PROS1 transcription. Furthermore, we demonstrated that E(2) treatment recruited RIP140 and the NCoR-SMRT-HDAC3 complex to the PROS1 promoter, which hypoacetylated chromatin. Taken together, this suggested that E(2) might repress PROS1 transcription depending upon ERalpha-Sp1 recruiting transcriptional repressors in HepG2-ERalpha cells and, consequently, that high levels of E(2) leading to reduced levels of plasma PS would be a risk for deep venous thrombosis in pregnant women.

Gas6 and protein S. Vitamin K-dependent ligands for the Axl receptor tyrosine kinase subfamily

Gas6 and protein S are two homologous secreted proteins that depend on vitamin K for their execution of a range of biological functions. A discrete subset of these functions is mediated through their binding to and activation of the receptor tyrosine kinases Axl, Sky and Mer. Furthermore, a hallmark of the Gas6-Axl system is the unique ability of Gas6 and protein S to tether their non receptor-binding regions to the negatively charged membranes of apoptotic cells. Numerous studies have shown the Gas6-Axl system to regulate cell survival, proliferation, migration, adhesion and phagocytosis. Consequently, altered activity/expression of its components has been detected in a variety of pathologies such as cancer and vascular, autoimmune and kidney disorders. Moreover, Axl overactivation can equally occur without ligand binding, which has implications for tumorigenesis. Further knowledge of this exquisite ligand-receptor system and the circumstances of its activation should provide the basis for development of novel therapies for the above diseases.

The Constitutive Expression of Anticoagulant Protein S Is Regulated through Multiple Binding Sites for Sp1 and Sp3 Transcription Factors in the Protein S Gene Promoter

Protein S (PS) is a vitamin K-dependent plasma protein that inhibits blood coagulation by serving as a nonenzymatic cofactor for activated protein C in the protein C anticoagulant pathway. Low PS levels are a risk factor for the development of deep venous thrombosis. The regulation of PS levels through transcriptional regulation of the PS gene was investigated in this report. A minimal PS gene promoter 370 bp upstream from the translational initiation codon was sufficient for maximal promoter activity in transient transfections regardless of the cell type. A pivotal role for Sp1 in the constitutive expression of the PS gene was demonstrated through electrophoretic mobility shift assay experiments, transient expression of mutant PS promoter-reporter gene constructs, and chromatin immunoprecipitations in HepG2 cells. At least four Sp-binding sites were identified. The two sites most proximal to the translational start codon were found to be indispensable for PS promoter activity, whereas mutation of the two most distal Sp-binding sites had a negligible influence on basal promoter activity. In addition, all other major promoter-binding proteins that were found by electrophoretic mobility shift assay could be positively identified in supershift assays. We identified binding sites for the hepatocyte-specific forkhead transcription factor FOXA2, nuclear factor Y, and the cAMP-response element-binding protein/activating transcription factor family of transcription factors. Their relevance was investigated using site-directed mutagenesis.

The physiology of vitamin K nutriture and vitamin K-dependent protein function in atherosclerosis

Recent advances in the discovery of new functions for vitamin K-dependent (VKD) proteins and in defining vitamin K nutriture have led to a substantial revision in our understanding of vitamin K physiology. The only unequivocal function for vitamin K is as a cofactor for the carboxylation of VKD proteins which renders them active. While vitamin K was originally associated only with hepatic VKD proteins that participate in hemostasis, VKD proteins are now known to be present in virtually every tissue and to be important to bone mineralization, arterial calcification, apoptosis, phagocytosis, growth control, chemotaxis, and signal transduction. The development of improved methods for analyzing vitamin K has shed considerable insight into the relative importance of different vitamin K forms in the diet and their contribution to hepatic vs. non-hepatic tissue. New assays that measure the extent of carboxylation in VKD proteins have revealed that while the current recommended daily allowance for vitamin K is sufficient for maintaining functional hemostasis, the undercarboxylation of at least one non-hemostatic protein is frequently observed in the general population. The advances in defining VKD protein function and vitamin K nutriture are described, as is the potential impact of VKD proteins on atherosclerosis. Many of the VKD proteins contribute to atherogenesis. Recent studies suggest involvement in arterial calcification, which may be influenced by dietary levels of vitamin K and by anticoagulant drugs such as warfarin that antagonize vitamin K action.

Complement inhibitor C4b-binding protein-friend or foe in the innate immune system?

The complement system constitutes an important component of the defence against foreign organisms, functioning both in innate and adaptive immune systems. It is potentially harmful also to the own organism and is therefore tightly regulated by a number of membrane-bound and soluble factors. C4b-binding protein (C4BP) is a potent circulating soluble inhibitor of the classical and lectin pathways of complement. In recent years, the relationships between the structure of C4BP and its functions have been elucidated using a combination of computer-based molecular analysis and recombinant DNA technologies. Moreover, two novel functions have recently been ascribed to C4BP. One is the ability of C4BP to localize complement regulatory activity to the surface of apoptotic cells via its interaction with the membrane-binding vitamin K-dependent protein S. The other is the ability of C4BP to act as a survival factor for B cells due to an interaction with CD40. The complement regulatory activity of C4BP is not only beneficial because it is also explored by pathogens such as Neisseria gonorrhoeae, Bordetella pertussis, Streptococcus pyogenes, Escherichia coli K1, and Candida albicans, that bind C4BP to their surfaces. This contributes to the serum resistance and the pathogenicity of these bacteria. In this review, the structural requirements and functional importance of the interactions between C4BP and its various ligands are discussed.

A review of the technical, diagnostic, and epidemiologic considerations for protein S assays

OBJECTIVES

To review the state of the art relating to protein S deficiency as a risk factor for thrombosis and to make recommendations regarding the use of protein S measurements in the assessment of thrombotic risk in individual patients and families.

DATA SOURCES, EXTRACTION, AND SYNTHESIS

Selection criteria were developed for the inclusion of publications from 1985 to 2001 based on the relevant literature concerned with the systematic review of diagnostic tests. Minimal selection criteria were agreed on and the articles stratified into level 1 if they met these criteria and level 2 if they did not meet these criteria. The included articles were reviewed by the authors and abstracted onto predetermined data collection forms. These forms were then scored and recommendations based on level 1 studies. As described elsewhere, results of discussions at the College of American Pathologists Conference XXXVI on Diagnostic Issues in Thrombophilia were used to revise the manuscript into its final form.

CONCLUSIONS

Consensus was reached on 16 recommendations for the use of protein S assays in the assessment of thrombotic risk in individuals and families. Two themes run through the conclusions. First, protein S assays are the most technically problematic of the assays reviewed at this conference. Second, only 2 papers evaluating the diagnostic use of protein S assays met our level 1 inclusion criteria. These 2 problems point out the need for better standardized assays and rigorous studies of the diagnostic utility of these assays.

Synthesis of gamma-carboxylated polypeptides by alpha-cells of the pancreatic islets

gamma-Carboxylated proteins were detected in normal human pancreas by immunohistochemistry with a monoclonal antibody (M3B) specific for gamma-carboxyglutamyl residues. Staining appeared to be localized to the glucagon-secreting alpha-cells in the islets of Langerhans. Consistent with this, sections from a glucagonoma were stained much more intensely with the M3B antibody than those from an insulinoma. A murine alpha-cell line (alphaTC1 Clone 9) was cultured and gamma-carboxylated polypeptides, identified immunologically as prothrombin, protein S and (tentatively) Gas6, were isolated from the intracellular compartment by chromatography on an M3B-coupled resin. As in liver, prothrombin is synthesized by alpha-cells as a gamma-carboxylated zymogen that can be cleaved by ecarin to form an active serine protease that is inhibited by hirudin. The pancreas thus appears to be a novel site of synthesis for certain vitamin K-dependent proteins.

Amino acid residues in thrombin-sensitive region and first epidermal growth factor domain of vitamin K-dependent protein S determining specificity of the activated protein C cofactor function

Human protein S (PS) potentiates the anticoagulant activity of human but not bovine activated protein C (APC), whereas bovine PS is a cofactor to APC from both species. The structural requirements for the specificity of the APC cofactor function of human PS are located in its thrombin-sensitive region (TSR) and the first epidermal growth factor (EGF1)-like module. To elucidate which residues in these two modules determine the specificity of the APC cofactor activity, 41 human PS mutants were expressed. All mutants were cofactors to human APC and some also to bovine APC. Residues in TSR (positions 49 and 52) and EGF1 (residues 97 and 106) together determined the specificity of the APC cofactor function, whereas substitution of individual residues did not change specificity. Bovine PS, and mutants expressing cofactor activity to bovine APC, stimulated phospholipid binding of bovine APC. In contrast, human PS and mutants lacking cofactor activity to bovine APC failed to support binding of bovine APC to phospholipids. These data indicate that residues in TSR and EGF1 cause the specificity of the APC cofactor activity and support the concept that key residues in these two modules interact with APC on the phospholipid surface.

Extrahepatic expression and regulation of protein C in the mouse

Activated protein C (APC) acts as an anticoagulant by inhibiting coagulation factors Va and VIIIa. Although the liver appears to be the primary site of protein C (PC) synthesis, the demonstration that other components of this system are produced extrahepatically raises the possibility that PC itself is synthesized in other tissues. We therefore used quantitative reverse transcription-polymerase chain reaction, in situ hybridization, and immunohistochemistry to screen various murine tissues for PC expression. Relatively high levels of PC mRNA were detected in the kidney (35% of liver) and testis (22% of liver). PC mRNA and antigen were demonstrated in tubular epithelial cells in the renal cortex, in spermatogenic cells in the testis, and in epithelial cells in the epididymis. Low but significant levels of PC mRNA were detected in the epididymis (1.7% of the level in liver), brain (1.1% of liver), and lung (0.8% of liver). PC antigen was demonstrated in bronchial epithelial cells in the lung, in pyramidal neurons in the cerebrum, and in Purkinje cells in the cerebellum. The extrahepatic expression of PC mRNA (ie, in the kidney) was significantly decreased in mice with renal disease (eg, in MRL lpr/lpr mice with autoimmune lupus nephritis, in db/db mice with diabetic nephropathy, and in endotoxin-treated mice with acute renal injury). The decreased renal expression of PC may contribute to the increased procoagulant potential of the kidney during septic and inflammatory processes and to the progression of kidney disease associated with these conditions.

Binding site for C4b-binding protein in vitamin K-dependent protein S fully contained in carboxy-terminal laminin-G-type repeats. A study using recombinant factor IX-protein S chimeras and surface plasmon resonance

The interaction between vitamin K-dependent protein S and the C4b-binding protein (C4BP) was studied using surface plasmon resonance and genetic engineering. The affinity, as well as association and dissociation rates of the complex, was measured for human and bovine protein S at five different calcium concentrations. The binding to C4BP of six protein hybrids containing different parts of coagulation factor IX and protein S was studied in the absence and presence of calcium. The results show that dissociation of the human protein S-C4BP complex is extremely slow in the presence of > or = 10 microM calcium (k(off) = 7 x 10(-6) s(-1)) and the association rate constant is k(on) = 7 x 10(4) M(-1) s(-1). Human and bovine protein S were found to bind to human C4BP with the same affinity, K(D) = 0.1 nM, but the rates of association and dissociation were higher for the bovine protein S (k(on) = 2 x 10(5) M(-1) s(-1), k(off) = 2 x 10(-5) s(-1)). In the absence of calcium, the affinity for C4BP was reduced by a factor of 65 for human protein S and by a factor of 40 for bovine protein S. The decreased affinity could be mainly attributed to an increased off-rate (12-17-fold), while the on-rate decreased 3-4-fold. The studies using chimeric proteins show that the portion of protein S that is responsible for binding to C4BP is fully contained in the two laminin-G-type repeats, which are homologous to the sex hormone binding globulin (SHBG). All hybrids that contain the laminin-G-type repeats bind to C4BP with the same affinity as recombinant protein S, whereas hybrids lacking these repeats show no detectable binding to C4BP. The present data also suggest that the effect of calcium on the C4BP-binding properties is mediated by calcium binding site(s) in the laminin-G-type repeats.

Activated protein C resistance: from phenotype to genotype and clinical practice

The anticoagulant protein C system is an important regulator of the blood coagulation process. Its targets are the procoagulant cofactors factor Va and factor VIIIa, which are cleaved and inactivated by activated protein C, protein S and intact factor V working as cofactors. Genetic defects of protein C or protein S were, together with antithrombin III deficiency, the previously established major causes of familial venous thromboembolism. However, these abnormalities are found in less than 5-10% of patients with thrombosis. Inherited resistance to activated protein C was recently identified as a major risk factor for venous thromboembolism. The activated protein C-resistance phenotype is found in 20-60% of the patients with venous thrombosis, depending on selection criteria and on the prevalence of activated protein C-resistance in the population. The frequency of activated protein C-resistance is 2-10% in the normal populations studied so far. In more than 90% of cases, the molecular background for the activated protein C-resistance is a single point mutation in the factor V gene, which predicts substitution of an arginine at position 506 by a glutamine. Mutated factor V is activated by thrombin or factor Xa in the normal way, but impaired inactivation of mutated factor Va by activated protein C results in a life-long hypercoagulability. Owing to the high prevalence of activated protein C-resistance in the population, it occasionally occurs in patients with deficiency of protein S, protein C or antithrombin III. Individuals with combined defects suffer more severely from thrombosis, and often at a younger age, than those with single defects, suggesting thrombophilia to be a multigenetic disease.

Stimulation of sky receptor tyrosine kinase by the product of growth arrest-specific gene 6

Sky (also called Rse, Brt, and Tyro3) is a member of a subfamily of related receptor tyrosine kinases, including Axl/Ufo/Ark and c-Eyk/Mer. We obtained evidence that Gas6 (the product of growth arrest-specific gene 6) is a ligand of the Sky receptor tyrosine kinase. Gas6, but not protein S (an anticoagulant protein structurally similar to Gas6), specifically bound to the soluble form of Sky (Sky-Fc), composed of the extracellular domain of Sky fused to the Fc domain of human immunoglobulin G1. The native and recombinant Gas6, but not protein S, stimulated tyrosine phosphorylation of Sky ectopically expressed in Chinese hamster ovary cells. Stimulation of Sky in response to Gas6 was inhibited by Sky-Fc. The half-maximal concentration of Gas6 that stimulated Sky was about 1 nM. Thus, Gas6 as a ligand for Sky specifically binds to and stimulates Sky receptor tyrosine kinase.

Expression and functional characterization of chimeras between human and bovine vitamin-K-dependent protein-S-defining modules important for the species specificity of the activated protein C cofactor activity

Vitamin-K-dependent protein S is an anticoagulant plasma protein functioning as a cofactor to activated protein C (APC) in the degradation of factors Va and VIIIa. The APC-cofactor function of protein S is species specific, as human protein S potentiates the anticoagulant activity of human but not that of bovine APC, whereas bovine protein S is a cofactor to APC from both species. To elucidate which modules in protein S determine the species specificity, in vitro mutagenesis was used to construct six recombinant chimeric molecules between human and bovine protein S. Wild-type human and bovine protein S and the chimeras were expressed in 293 cells and the recombinant proteins purified by monoclonal antibody affinity chromatography. The recombinant proteins were found to be post-translationally modified, they bound C4b-binding protein and were functionally active as cofactors to APC. Chimeras having both the thrombin-sensitive region (TSR) and the first epidermal-growth-factor-(EGF)-like module of bovine origin expressed APC-cofactor activity similar to that of bovine protein S. Those chimeras, in which TSR or EGF1 derived from different species, manifested APC-cofactor activity similar to that of human protein S, i.e. they did not express cofactor activity to bovine APC. These data indicate that sequence differences in the TSR and EGF1 of human and bovine protein S cause the species specificity of the APC-cofactor activity. The data support the concept that these two modules of protein S interact with APC on the surface of negatively charged phospholipids.