Inhibitors of the sulfation of proteins, glycoproteins, and proteoglycans

Tissue chaperoning—the expanded functions of fetuin-A beyond inhibition of systemic calcification

Traditionally, fetuin-A embodies the prototype anti-calcification protein in the blood, preventing cardiovascular calcification. Low serum fetuin-A is generally associated with mineralization dysbalance and enhanced mortality in end stage renal disease. Recent evidence indicates that fetuin-A is a crucial factor moderating tissue inflammation and fibrosis, as well as a systemic indicator of acute inflammatory disease. Here, the expanded function of fetuin-A is discussed in the context of mineralization and inflammation biology. Unbalanced depletion of fetuin-A in this context may be the critical event, triggering a vicious cycle of progressive calcification, inflammation, and tissue injury. Hence, we designate fetuin-A as tissue chaperone and propose the potential use of exogenous fetuin-A as prophylactic agent or emergency treatment in conditions that are associated with acute depletion of endogenous protein.

Modulating tyrosine sulfation of recombinant antibodies in CHO cell culture by host selection and sodium chlorate supplementation

BACKGROUND

Tyrosine sulfation is a post-translational modification found on many surface receptors and plays an important role in cell-cell and cell-matrix interactions. However, tyrosine sulfation of therapeutic antibodies has only been reported very recently. Because of potential potency and immunogenicity concerns, tyrosine sulfation needs to be controlled during the manufacturing process.

METHODS AND RESULTS

In this study, we explored methods to modulate antibody tyrosine sulfation during cell line development and upstream production process. We found that tyrosine sulfation levels were significantly different in various Chinese hamster ovary (CHO) cell lines due to differential expression of genes in the sulfation pathway including tyrosylprotein sulfotransferase 2 (TPST2) and the sulfation substrate transporter SLC35B2. We also screened chemical inhibitors to reduce tyrosine sulfation in CHO culture and found that sodium chlorate could significantly inhibit tyrosine sulfation while having minimal impact on cell growth and antibody production. We further confirmed this finding in a standard fed-batch production assay. Sodium chlorate at 16 mM markedly inhibited tyrosine sulfation by more than 50% and had no significant impact on antibody titer or quality.

CONCLUSION

These data suggest that we can control tyrosine sulfation by selecting CHO cell lines based on the expression level of TPST2 and SLC35B2 or adding sodium chlorate in upstream production process.

Fetuin-A – Alpha2-Heremans-Schmid Glycoprotein: From Structure to a Novel Marker of Chronic Diseases Part 2. Fetuin-A – A Marker of Insulin Resistance and Related Chronic Diseases

Fetuin-A is a secretory liver glycoprotein with multiple physiological functions such as regulation of insulin resistance, tissue calcification, bone metabolism, cellular proteolytic activity, and self-proliferative signaling.

Fetuin-A is a unique molecule which binds to the insulin receptor, modulating its sensitivity, and transducing “the physiological conditions” (serum levels of the metabolites like glucose, free fatty acids, inflammatory signals) from outside into inside the cells. Plasma fetuin-A levels correlate with reduced glucose tolerance and insulin resistance. Impaired insulin sensitivity leads to the development of metabolic syndrome, an increased risk for type 2 diabetes (T2DM), dyslipidaemias and cardiovascular diseases (CVDs). Furthermore, fetuin-A inversely correlates with inflammatory and activation biomarkers, e.g. in patients with T2DM. Thus, circulatory fetuin-A levels may have plausible predictive importance as a biomarker of risk of diabetes and negative acute phase protein. Dysregulated, it plays a crucial role in the pathogenesis of some metabolic disorders and clinical inflammatory conditions like metabolic syndrome, T2DM, CVDs, polycystic ovary syndrome (PCOS), etc.

Fetuin-A – Alpha2-Heremans-Schmid Glycoprotein: From Structure to a Novel Marker of Chronic Diseases Part 1. Fetuin-A as a Calcium Chaperone and Inflammatory Marker

Fetuin-A is a major plasma glycoprotein released mainly by the liver. Its functions include inhibition of the activity of insulin receptor, regulation of response to inflammation, inhibition of calcified matrix metabolism and ectopic mineralization, etc. Three major functional domains of fetuin-A have been identified: one similar to the Ca-binding domains, one inhibiting cysteine protease, and a domain with high affinity to insulin receptor. The fetuin-A molecule may be considered as a highly pleomorphic protein with an important impact in a variety of clinically expressed metabolic and pathological processes. It could be used as a marker in clinical practice in the future.

Evolution of Src Homology 2 (SH2) Domain to Recognize Sulfotyrosine

Protein tyrosine O-sulfation is considered as the most common type of post-translational tyrosine modification in nature and plays important roles in extracellular biomolecular interactions. To facilitate the mapping, biological study, and medicinal application of this type of post-translational modification, we seek to evolve a small protein scaffold that recognizes sulfotyrosine with high affinity. We focused our efforts on the engineering of the Src Homology 2 (SH2) domain, which represents the largest class of known phosphotyrosine-recognition domain in nature and has a highly evolvable binding pocket. By using phage display, we successfully engineered the SH2 domain to recognize sulfotyrosine with high affinity. The best mutant, SH2-60.1, displayed more than 1700 fold higher sulfotyrosine-binding affinity than that of the wild-type SH2 domain. We also demonstrated that the evolved SH2 domain mutants could be used to detect sulfoprotein levels on the cell surface. These evolved SH2 domain mutants can be potentially applied to the study of protein tyrosine O-sulfation with proper experimental designs.

Recent advances in the identification of Tat-mediated transactivation inhibitors: progressing toward a functional cure of HIV

The current anti-HIV combination therapy does not eradicate the virus that persists mainly in quiescent infected CD4(+) T cells as a latent integrated provirus that resumes after therapy interruption. The Tat-mediated transactivation (TMT) is a critical step in the HIV replication cycle that could give the opportunity to reduce the size of latent reservoirs. More than two decades of research led to the identification of various TMT inhibitors. While none of them met the criteria to reach the market, the search for a suitable TMT inhibitor is still actively pursued. Really promising compounds, including one in a Phase III clinical trial, have been recently identified, thus warranting an update.

Cytosolic sulfotransferase 1A1 regulates HIV-1 minus-strand DNA elongation in primary human monocyte-derived macrophages

Background

The cellular sulfonation pathway modulates key steps of virus replication. This pathway comprises two main families of sulfonate-conjugating enzymes: Golgi sulfotransferases, which sulfonate proteins, glycoproteins, glycolipids and proteoglycans; and cytosolic sulfotransferases (SULTs), which sulfonate various small molecules including hormones, neurotransmitters, and xenobiotics. Sulfonation controls the functions of numerous cellular factors such as those involved in cell-cell interactions, cell signaling, and small molecule detoxification. We previously showed that the cellular sulfonation pathway regulates HIV-1 gene expression and reactivation from latency. Here we show that a specific cellular sulfotransferase can regulate HIV-1 replication in primary human monocyte-derived macrophages (MDMs) by yet another mechanism, namely reverse transcription.

Methods

MDMs were derived from monocytes isolated from donor peripheral blood mononuclear cells (PBMCs) obtained from the San Diego Blood Bank. After one week in vitro cell culture under macrophage-polarizing conditions, MDMs were transfected with sulfotranserase-specific or control siRNAs and infected with HIV-1 or SIV constructs expressing a luciferase reporter. Infection levels were subsequently monitored by luminescence. Western blotting was used to assay siRNA knockdown and viral protein levels, and qPCR was used to measure viral RNA and DNA products.

Results

We demonstrate that the cytosolic sulfotransferase SULT1A1 is highly expressed in primary human MDMs, and through siRNA knockdown experiments, we show that this enzyme promotes infection of MDMs by single cycle VSV-G pseudotyped human HIV-1 and simian immunodeficiency virus vectors and by replication-competent HIV-1. Quantitative PCR analysis revealed that SULT1A1 affects HIV-1 replication in MDMs by modulating the kinetics of minus-strand DNA elongation during reverse transcription.

Conclusions

These studies have identified SULT1A1 as a cellular regulator of HIV-1 reverse transcription in primary human MDMs. The normal substrates of this enzyme are small phenolic-like molecules, raising the possibility that one or more of these substrates may be involved. Targeting SULT1A1 and/or its substrate(s) may offer a novel host-directed strategy to improve HIV-1 therapeutics.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-016-0491-9) contains supplementary material, which is available to authorized users.

Sulfonation pathway inhibitors block reactivation of latent HIV-1

Long-lived pools of latently infected cells are a significant barrier to the development of a cure for HIV-1 infection. A better understanding of the mechanisms of reactivation from latency is needed to facilitate the development of novel therapies that address this problem. Here we show that chemical inhibitors of the sulfonation pathway prevent virus reactivation, both in latently infected J-Lat and U1 cell lines and in a primary human CD4+ T cell model of latency. In each of these models, sulfonation inhibitors decreased transcription initiation from the HIV-1 promoter. These inhibitors block transcription initiation at a step that lies downstream of nucleosome remodeling and affects RNA polymerase II recruitment to the viral promoter. These results suggest that the sulfonation pathway acts by a novel mechanism to regulate efficient virus transcription initiation during reactivation from latency, and further that augmentation of this pathway could be therapeutically useful.

Fetuin-A regulation of calcified matrix metabolism

The final step of biomineralization is a chemical precipitation reaction that occurs spontaneously in supersaturated or metastable salt solutions. Genetic programs direct precursor cells into a mineralization-competent state in physiological bone formation (osteogenesis) and in pathological mineralization (ectopic mineralization or calcification). Therefore, all tissues not meant to mineralize must be actively protected against chance precipitation of mineral. Fetuin-A is a liver-derived blood protein that acts as a potent inhibitor of ectopic mineralization. Monomeric fetuin-A protein binds small clusters of calcium and phosphate. This interaction results in the formation of prenucleation cluster-laden fetuin-A monomers, calciprotein monomers, and considerably larger aggregates of protein and mineral calciprotein particles. Both monomeric and aggregate forms of fetuin-A mineral accrue acidic plasma protein including albumin, thus stabilizing supersaturated and metastable mineral ion solutions as colloids. Hence, fetuin-A is a mineral carrier protein and a systemic inhibitor of pathological mineralization complementing local inhibitors that act in a cell-restricted or tissue-restricted fashion. Fetuin-A deficiency is associated with soft tissue calcification in mice and humans.

Sulfation of LH does not affect intracellular trafficking

LH and FSH are produced by the same gonadotrope cells of the anterior pituitary but differ in their mode of secretion. LH secretion is primarily episodic, or regulated, while FSH secretion is primarily basal, or constitutive. The asparagine (N)-linked oligosaccharides of LH and FSH terminate with sulfate and sialic acid, respectively. TSH also contains sulfated N-linked oligosaccharides and is secreted through the regulated pathway. It has been hypothesized that sulfate plays a role in segregating LH to the regulated pathway. Using a mouse pituitary model, we tested this hypothesis by examining the secretory fate of LH from pituitaries treated with sodium chlorate, a known inhibitor of sulfation. Here we show that mouse LH is sulfated and secreted through the regulated pathway, while FSH is secreted constitutively. LH secretion from chlorate-treated pituitaries, which showed complete inhibition of sulfation, was similar to untreated pituitaries. These data suggest that the metabolic role for sulfated N-linked oligosaccharides is not for intracellular trafficking but for the extracellular bioactivity of LH.

The host cell sulfonation pathway contributes to retroviral infection at a step coincident with provirus establishment

The early steps of retrovirus replication leading up to provirus establishment are highly dependent on cellular processes and represent a time when the virus is particularly vulnerable to antivirals and host defense mechanisms. However, the roles played by cellular factors are only partially understood. To identify cellular processes that participate in these critical steps, we employed a high volume screening of insertionally mutagenized somatic cells using a murine leukemia virus (MLV) vector. This approach identified a role for 3′-phosphoadenosine 5′-phosphosulfate synthase 1 (PAPSS1), one of two enzymes that synthesize PAPS, the high energy sulfate donor used in all sulfonation reactions catalyzed by cellular sulfotransferases. The role of the cellular sulfonation pathway was confirmed using chemical inhibitors of PAPS synthases and cellular sulfotransferases. The requirement for sulfonation was mapped to a stage during or shortly after MLV provirus establishment and influenced subsequent gene expression from the viral long terminal repeat (LTR) promoter. Infection of cells by an HIV vector was also shown to be highly dependent on the cellular sulfonation pathway. These studies have uncovered a heretofore unknown regulatory step of retroviral replication, have defined a new biological function for sulfonation in nuclear gene expression, and provide a potentially valuable new target for HIV/AIDS therapy.

A genetic screen was used to identify host cell functions important for the replication of retroviruses, including human immunodeficiency viruses. These studies have uncovered a heretofore unexpected role for the cellular sulfonation pathway in an intracellular step of retroviral replication. Through the addition of sulfate groups, this pathway is responsible for modifying and regulating different types of cellular factors including proteins, lipids, carbohydrates and hormones. The role of this pathway was further confirmed by using specific chemical inhibitors. The sulfonation requirement was mapped to a step during viral DNA integration into the host genome that has a subsequent effect upon the level of expression of viral genes. These studies have uncovered a new regulatory mechanism of retroviral replication and suggest that components of the host cell sulfonation pathway might represent attractive targets for antiviral development.

Vitronectin's basic domain is a syndecan ligand which functions in trans to regulate vitronectin turnover

During the process of tissue remodeling, vitronectin (Vn) is deposited in the extracellular matrix where it plays a key role in the regulation of pericellular proteolysis and cell motility. In previous studies we have shown that extracellular levels of vitronectin are controlled by receptor-mediated endocytosis and that this process is dependent upon vitronectin binding to sulfated proteoglycans. We have now identified vitronectin's 12 amino acid "basic domain" which is contained within the larger 40 amino acid heparin binding domain, as a syndecan binding site. Recombinant vitronectins representing wild type vitronectin (rVn) and vitronectin with the basic domain deleted (rVnDelta347-358) were prepared in a baculoviral expression system. The rVn as well as a glutathione S-transferase (GST) fusion protein, consisting of vitronectin's 40 amino acid heparin binding domain (GST-VnHBD), exhibited dose dependent binding to HT-1080 cell surfaces, which was attenuated following deletion of the basic domain. In addition, GST-VnHBD supported both HT-1080 and dermal fibroblast cell adhesion, which was also dependent upon the basic domain. Similarly, ARH-77 cells transfected with syndecans -1, -2, or -4, but not Glypican-1, adhered to GST-VnHBD coated wells, while adhesion of these same cells was lost following deletion of the basic domain. HT-1080 cells were unable to degrade rVnDelta347-358. Degradation of rVnDelta347-358 was completely recovered in the presence of GST-VnHBD but not in the presence of GST-VnHBDDelta347-358. These results indicate that turnover of soluble vitronectin requires ligation of vitronectin's basic domain and that this binding event can work in trans to regulate vitronectin degradation.

The binding of the circumsporozoite protein to cell surface heparan sulfate proteoglycans is required for plasmodium sporozoite attachment to target cells

The major surface protein of malaria sporozoites, the circumsporozoite protein, binds to heparan sulfate proteoglycans on the surface of hepatocytes. It has been proposed that this binding event is responsible for the rapid and specific localization of sporozoites to the liver after their injection into the skin by an infected anopheline mosquito. Previous in vitro studies performed under static conditions have failed to demonstrate a significant role for heparan sulfate proteoglycans during sporozoite invasion of cells. We performed sporozoite attachment and invasion assays under more dynamic conditions and found a dramatic decrease in sporozoite attachment to cells in the presence of heparin. In contrast to its effect on attachment, heparin does not appear to have an effect on sporozoite invasion of cells. When substituted heparins were used as competitive inhibitors of sporozoite attachment, we found that sulfation of the glycosaminoglycan chains at both the N- and O-positions was important for sporozoite adhesion to cells. We conclude that the binding of the circumsporozoite protein to hepatic heparan sulfate proteoglycans is likely to function during sporozoite attachment in the liver and that this adhesion event depends on the sulfated glycosaminoglycan chains of the proteoglycans.

Secretion pattern, ultrastructural localization and function of extracellular matrix molecules involved in eggshell formation

The chicken eggshell is a composite bioceramic containing organic and inorganic phases. The organic phase contains, among other constituents, type X collagen and proteoglycans (mammillan, a keratan sulfate proteoglycan, and ovoglycan, a dermatan sulfate proteoglycan), whose localization depends on a topographically defined and temporally regulated deposition. Although the distribution of these macromolecules in the eggshell has been well established, little is known about their precise localization within eggshell substructures and oviduct cells or their pattern of production and function during eggshell formation. By using immunofluorescent and immuno-ultrastructural analyses, we examined the distribution of these macromolecules in oviduct cells at different post-oviposition times. To understand the role of proteoglycan sulfation on eggshell formation, we studied the effects of inhibition of proteoglycan sulfation by treatment with sodium chlorate. We showed that these macromolecules are produced by particular oviduct cell populations and at precise post-oviposition times. Based on the precise ultrastructural localization of these macromolecules in eggshell substructures, the timing of the secretion of these macromolecules by oviduct cells and the effects on eggshell formation caused by the inhibition of proteoglycan sulfation, the putative role of mammillan is in the nucleation of the first calcite crystals, while that of ovoglycan is to regulate the growth and orientation of the later forming crystals of the chicken eggshell.

Cytotoxicity and genotoxicity of methyleugenol and related congeners-- a mechanism of activation for methyleugenol

Methyleugenol is a substituted alkenylbenzene found in a variety of foods, products, and essential oils. In a 2-year bioassay conducted by the National Toxicology Program, methyleugenol caused neoplastic lesions in the livers of Fischer 344 rats and B6C3F(1) mice. We were interested in the cytotoxicity and genotoxicity caused by methyleugenol and other alkenylbenzene compounds: safrole (a known hepatocarcinogen), eugenol, and isoeugenol. The endpoints were evaluated in cultured primary hepatocytes isolated from male Fischer 344 rats and female B6C3F(1) mice. Cytotoxicity was determined by measuring lactate dehydrogenase (LDH) release, while genotoxicity was determined by using the unscheduled DNA synthesis (UDS) assay. Rat and mouse hepatocytes showed similar patterns of toxicity for each chemical tested. Methyleugenol and safrole were relatively non-cytotoxic, but caused UDS at concentrations between 10 and 500 microM. In contrast, isoeugenol and eugenol produced cytotoxicity in hepatocytes with LC50s of approximately 200-300 microM, but did not cause UDS. Concurrent incubation of 2000 microM cyclohexane oxide (CHO), an epoxide hydrolase competitor, with a non-cytotoxic concentration of methyleugenol (10 microM) resulted in increased cytotoxicity but had no effect on genotoxicity. However, incubation of 15 microM pentacholorophenol, a sulfotransferase inhibitor, with 10 uM methyleugenol resulted in increased cytotoxicity but had a significant reduction of genotoxicity. These results suggest that methyleugenol is similar to safrole in its ability to cause cytotoxicity and genotoxicity in rodents. It appears that the bioactivation of methyleugenol to a DNA reactive electrophile is mediated by a sulfotransferase in rodents, but epoxide formation is not responsible for the observed genotoxicity.

Molecular characterization, enzymatic analysis, and purification of murine proprotein convertase-1/3 (PC1/PC3) secreted from recombinant baculovirus-infected insect cells

A cDNA coding for the murine proprotein convertase-1 (mPC1 also known as mPC3 or mSPC3) was inserted into the Autographa californica nuclear polyhedrosis virus. Following infection of Spodoptera frugiperda cells, the recombinant N-glycosylated protein is secreted into the cell culture medium from which it can be purified to homogeneity as a fully enzymatically active enzyme. Two major secreted molecular forms of mPC1 with apparent molecular weights of 85 and 71 kDa, respectively, and a minor one of 75 kDa are immunodetected in the medium. Automated NH2-terminal sequencing reveals that all three forms result from processing at the predicted zymogen activation site whereas both the 75- and the 71-kDa forms are truncated at their COOH-terminus. Labeling by an active-site titrant demonstrates that the 85-kDa form is optimally labeled at near neutral pH whereas the COOH-truncated forms are optimally labeled at acidic pH. Additionally it is shown that the 85-kDa mPC1 is transformed into the COOH-truncated forms following in vitro incubation at acidic pH levels and in presence of calcium. Concomitantly, the transformation from 85 to 71 kDa is accompanied by a 10- to 40-fold increase in enzymatic activity upon assaying at pH 6.0. The 71-kDa form can be recovered after purification at a level of 1 to 1.5 mg per liter of cell culture medium and is enzymatically stable only in the pH range from 5.0 to 6.5. Cells treated with tunicamycin show a drastically reduced secretion of the convertase in the medium but are not affected by swainsonine and deoxymannojirimycin. Finally, the 85-kDa secreted mPC1 is shown to be sulfated.

Degradation of distinct forms of multimeric vitronectin by human fibroblasts

The plasma protein vitronectin is thought to be an important regulator of extravascular plasminogen activation. In previous studies we have shown that a disulfide stabilized multimeric form of vitronectin is endocytosed and degraded by fibroblast cells (T.S. Panetti, P.J. McKeown-Longo, J. Biol. Chem. 268 (1993) 11988-11993; P.J. McKeown-Longo, T.S. Panetti, in: K.T. Preissner, S. Rosenblatt, C. Kost, J. Wegerhoff, D.F. Mosher (Eds.), Biology of Vitronectins and their Receptors, Elsevier Science Publishers, Amsterdam, 1993, pp. 111-118). The preparation of multimeric vitronectin used in these earlier studies was in the form of high molecular weight disulfide-bonded aggregates which were stable in sodium dodecyl sulfate (SDS). To address the question of whether vitronectin needed to be in the form of disulfide stabilized multimers in order to be endocytosed, a multimeric vitronectin, which was not disulfide stabilized, was prepared from vitronectin that had been treated with reducing agent and alkylated with iodoacetamide. The resulting protein migrated as a 65/75 kDa protein on SDS gels in the absence of reducing agent, confirming that this form of vitronectin was no longer stabilized into disulfide-bonded aggregates. However, the protein was still multimeric when analyzed by native gels and could be converted to SDS stable multimers by cross-linking agents. This result demonstrated that reduced and alkylated vitronectin aggregates into multimeric forms which are not stable in SDS. Similar to disulfide stabilized multimers, alkylated multimers of vitronectin bound to sulfated proteoglycans in the extracellular matrix and were endocytosed and degraded. Degradation of both forms of vitronectin was inhibited with arginine-glycine-aspartic acid peptides, an anti-alphavbeta5 antibody and heparin. Chloroquine and wortmannin were also able to inhibit degradation of both forms of vitronectin, indicating that both multimeric forms were following the same endocytic and degradative pathway. These results suggest that the organization of vitronectin into a multimeric form which will be recognized for endocytosis does not require disulfide bond stabilization. This study further suggests that recognition of vitronectin for endocytosis is dependent upon its conversion from a monomeric to a multivalent form (C.E. Wilkins-Port, P.J. McKeown-Longo, Mol. Biol. Cell 8:S:64A (1997).

Restoration of synthesis of sulfoglucuronylglycolipids in cerebellar granule neurons promotes dedifferentiation and neurite outgrowth

Sulfoglucuronyl carbohydrate (SGC) linked to the terminal moiety of neolacto-oligosaccharides is expressed in several glycoproteins of the immunoglobulin superfamily involved in neural cell-cell recognition as well as in two sulfoglucuronylglycolipids (SGGLs) of the nervous system. SGGLs and SGC-containing glycoproteins are temporally and spatially regulated during development of the nervous system. In the cerebellum, the expression of SGC, particularly that of SGGLs, is biphasic. Several studies have suggested that the initial rise and decline in the levels of SGGLs and SGC-containing proteins correlated with the migration of granule neurons from the external granule cell layer to the internal granule cell layer and their subsequent maturation, whereas the later rise and continued expression of SGGLs in the adult was associated with their localization in the Purkinje neurons and their dendrites in the molecular layer. Here it is shown by immunocytochemical methods that the expression of SGC declined progressively in granule neurons isolated from cerebella of increasing age. The decline in the expression of SGC in granule neurons was also shown with time in culture. These results correlated with the previously shown declining activity of the regulatory enzyme lactosylceramide N-acetylglucosaminyltransferase (GlcNAc-Tr) with age in vivo and in isolated granule neurons in culture. GlcNAc-Tr synthesizes a key precursor, lactotriosylceramide, involved in the biosynthesis of SGGL-1. The down-regulated synthesis of SGGLs in the mature granule neurons was shown by immunocytochemical and biochemical methods to be restored when a precursor, glucuronylneolactotetraosylceramide (GGL-1), which is beyond the GlcNAc-Tr step, was exogenously provided to these cells. The biological effect of such restoration of the synthesis of SGGLs in the mature granule neurons leads to cell aggregation and enhanced proliferation of neurites, amounting to dedifferentiation.

Evidence for the role of proteoglycans in cation-mediated gene transfer

We report evidence that gene complexes, consisting of polycations and plasmid DNA enter cells via binding to membrane-associated proteoglycans. Treatment of HeLa cells with sodium chlorate, a potent inhibitor of proteoglycan sulfation, reduced luciferase expression by 69%. Cellular treatment with heparinase and chondroitinase ABC inhibited expression by 78% and 20% with respect to control cells. Transfection was dramatically inhibited by heparin and heparan sulfate and to a smaller extent by chondroitan sulfate B. Transfection of mutant, proteoglycan deficient Chinese hamster ovary cells was 53 x lower than of wild-type cells. For each of these assays, the intracellular uptake of DNA at 37 degrees C and the binding of DNA to the cell membrane at 4 degrees C was impaired. Preliminary transfection experiments conducted in mutant and wild-type Chinese hamster ovary cells suggest that transfection by some cationic lipids is also proteoglycan dependent. The variable distribution of proteoglycans among tissues may explain why some cell types are more susceptible to transfection than others.

Heparan sulfate proteoglycans function in the binding and degradation of vitronectin by fibroblast monolayers

Vitronectin, a 75-kDa plasma protein is also found in the extracellular matrix, where it is believed to promote cell adhesion and migration. In addition to its role in adhesion, matrix vitronectin is also believed to function as an opsonin promoting the clearance of thrombin-serpin complexes from the matrix. Vitronectin is cleared from the matrix by receptor-mediated endocytosis followed by lysosomal degradation, suggesting that cells can regulate the levels of vitronectin present in the matrix. However, the mechanism by which plasma vitronectin associates with the extracellular matrix remains unclear. Studies were conducted to define the binding site(s) for vitronectin in fibroblast cell layers. Sodium chlorate, a competitive inhibitor of proteoglycan sulfation, produced a dose-dependent decrease in both binding and degradation of vitronectin. This inhibition was reversible in that removal of chlorate returned both binding and degradation of vitronectin to near control levels within 24 h. The binding of vitronectin to cell layers was not dependent on cells because vitronectin bound directly to isolated matrix. Isolated matrices prepared from cell layers treated with sodium chlorate also exhibited a dose-dependent decrease in vitronectin binding, consistent with the binding site for vitronectin in the matrix being sulfated proteoglycans. Binding and degradation of vitronectin were also sensitive to the addition of exogenous heparin, suggesting that the heparin binding domain of vitronectin was mediating binding to the matrix. Incubating fibroblast monolayers with heparinase III resulted in a 40% decrease in binding and degradation of vitronectin. Taken together, the above findings suggest that vitronectin's binding to the matrix and its subsequent degradation are dependent on heparan sulfate proteoglycans.

Protein tyrosine sulfation, 1993--an update

Sulfation is the most abundant post-translational modification of tyrosine residues and occurs in many soluble and membrane proteins passing through the secretory pathway of metazoan cells. The sulfation reaction is catalysed by tyrosylprotein sulfotransferase, a membrane-bound enzyme of the trans-Golgi-network. Tyrosylprotein sulfotransferase has been purified and its substrate specificity characterized. Tyrosine sulfation has been shown to be important for protein-protein interactions occurring during the intracellular transport of proteins and upon their secretion.

3'-Phosphoadenosine 5'-phosphosulfate biosynthesis and the sulfation of cholecystokinin by the tyrosylprotein-sulfotransferase in rat brain tissue

This article resumes the work we have accomplished in the past few years. Cholecystokinin sulfation is an important post-translational modification necessary for the biological activity of this peptide hormone. The tyrosyl protein sulfotransferase (TPST) activity from rat cerebral cortex was characterized. TPST activity is most probably responsible for the endogenous sulfation of CCK. TPST reaction kinetic properties were studied using radiolabeled 3'-phosphoadenosine 5'-phosphosulfate (PAPS) and the non-sulfated peptide acceptor terbutyloxycarbonyl-cholecystokinin octapeptide (BocCCK-8(ns)) as substrates, and brain microsomes as the enzyme source. The BocCCK-8 sulfating reaction data is consistent with the idea that TPST forward reaction follows an ordered Bi Bi mechanism. PAPS biosynthesis and availability was studied in slices from rat cerebral cortex incubated in the presence of [35S]sulfate. There is a rapid and dynamic turnover of the steady-state level of PAPS in brain cells which is decreased by depolarizing agents such as potassium, veratridine and glutamate. Furthermore, the presence of a membrane-bound PAPS biosynthesis inhibitor was observed. These results are discussed in view of the biological importance that the cell sulfating pathways might play in nerve cell activity.

The relationship between proteoglycan synthesis in Swarm chondrocytes and pathways of cellular energy and UDP-sugar metabolism

The effect of anaerobic culture conditions and various metabolic inhibitors on 35S-proteoglycan synthesis, UDP-sugar pools, and the ATP pool were investigated in confluent, primary, day 1 cultures of Swarm chondrosarcoma chondrocytes. (i) Incubation under a nitrogen atmosphere for 6 h did not affect 35S-proteoglycan synthesis or the pool size for UDP-glucuronate, other UDP-sugars, or ATP. Incubation with 5 mM sodium azide brought about a 40% reduction of proteoglycan synthesis in the first 30 min but no further change over the subsequent 90 min. UDP-Glucuronate, other UDP-sugar pools, and the ATP level were not affected by azide treatment. The results indicate that proteoglycan synthesis and its energy requirements can be supported entirely by anaerobic metabolism in these cells. (ii) 35S-Proteoglycan synthesis, UDP-sugar production, and nucleotide triphosphate pools were inhibited in a concentration-dependent fashion with sodium iodoacetate. A > 70% reduction of the ATP pool after 30 min treatment suggests that glycolysis is a major target for iodoacetate. Lactate production was inhibited by 40% after 3 h treatment with 10(-4) M iodoacetate. (iii) Glutamine deprivation resulted in a 60% contraction in the UDP-N-acetylhexosamine pool and markedly inhibited 35S-proteoglycan and 3H-protein synthesis. At the same time the UDP-glucose pool expanded to 200%, but the UDP-glucuronate pool was unchanged. The sum of the UDP-N-acetylhexosamine and UDP-hexose pools remained constant. Restoration of glutamine to previously depleted cultures resulted in excessive expansion of the UDP-N-acetylhexosamine pool and excessive contraction of the UDP-hexose pool before both adjusted to normal levels. The UDP-xylose pool was very small. No increases were observed during inhibition of proteoglycan synthesis induced by glutamine deprivation. (iv) 6-Diazo-5-oxo-L-norleucine (DON), a glutamine analogue and amino transferase inhibitor, induced a further contraction of the UDP-N-acetylhexosamine pool and a further decrease in proteoglycan synthesis in glutamine-deprived cultures. Thus cultures use endogenous glutamine during exogenous glutamine deprivation. DON unaccountably stimulated expansion of the UDP-glucuronate pool by 180%, irrespective of whether glutamine was present or not.

Inhibition of pro-cholecystokinin (CCK) sulfation by treatment with sodium chlorate alters its processing and decreases cellular content and secretion of CCK 8

Pro-cholecystokinin (CCK) has three sulfated tyrosine residues. Sulfation of the tyrosine residue in CCK 8 is known to be important for its activity at CCK A receptors. The role of these sulfated tyrosines in the sorting and processing of pro-CCK was examined by treatment of CCK-secreting rat thyroid medullary carcinoma cells with 10 nM sodium chlorate (a non-toxic inhibitor of tyrosine sulfation). This treatment caused a 50% decrease in the cellular content of immunoreactive CCK and an 80% decrease in its secretion. Sephadex G-50 chromatography of cellular extracts and culture media showed a selective depletion of CCK 8. There was a comparative sparing of CCK 33 and larger molecular forms in cellular extracts which was not observed in the media. These results suggest that the sulfation of the tyrosines of pro-CCK is clearly important for the correct sorting and/or processing of pro-CCK. The pattern of immunoreactive CCK peptides seen with chlorate treatment is consistent with the substrate specificity of a recently identified putative CCK cleaving enzyme and suggests that unsulfated pro-CCK is not efficiently processed to CCK 8 in vivo. The large decrease in CCK content and secretion observed with sodium chlorate may also be due to inefficient sorting of unsulfated pro-CCK into secretory vesicles.

Involvement of prostanoids in the regulation of angiogenesis by polypeptide growth factors

Polypeptide growth factors (PGFs), mainly those of the fibroblast growth factor (FGF) family, have been shown to be capable of regulating angiogenesis. Although many data have been accumulated during this last year on the mechanism of action of PGF, little is known about a possible identification of second messengers signalling to the cell the occupancy of the receptor by its ligand. We have previously proposed that arachidonic acid or its derivatives may play a role as PGF second messengers. In the present paper we described a modification of the chorioallanthoic membrane (CAM) technique, involving the use of labelled sulphate to follow the angiogenic process. Thus we have been able to correlate morphological observation of CAMs development with incorporation of labelled sulphate in a stable form. Here we show that, as expected, PGF as endothelial cell growth factor (ECGS) or basic fibroblast growth factor (bFGF) potentiate the incorporation of radioactivity into CAMs at concentrations which for bFGF are of the order of 1.5 micrograms/egg. This effect can be correlated to the generation of prostanoids by two kinds of approach: A) PGE1 injected into eggs was capable of strongly increasing labelling of CAMs; B) Indomethacin had a dramatic effect on embryo survival as well as on CAM development, decreasing both at very low concentration (50 survival rate observable at 2 micrograms/egg). Finally vanadate, which is known to inhibit tyrosine phosphatase, was capable of potentiating the effect of PGF on angiogenesis. Thus it appears that products of the prostaglandin H synthase pathway behave as mediators of PGF control of angiogenesis.

Sulfation of tyrosine residues increases activity of the fourth component of complement

Sulfation of tyrosine residues recently has been recognized as a biosynthetic modification of many plasma proteins and other secretory proteins. Effects of this site-specific modification on protein function are not known, but the activity of several peptides such as cholecystokinin is greatly augmented by sulfation. Here, we examine the role of sulfation in the processing and activity of C4 (the fourth component of complement), one of the few proteins in which sites and stoichiometry of tyrosine sulfation have been characterized. Our results, with C4 as a paradigm, suggest that sulfation of tyrosine residues can have major effects on the activity of proteins participating in protein-protein interactions. Sulfation of C4 synthesized by Hep G2 cells was blocked by incubating the cells with NaClO3 and guaiacol. These sulfation inhibitors did not alter secretion or other steps in the processing of C4. However, hemolytic activity of C4 was decreased more than 50%. The inhibitors' effect on C4 activity was prevented by adding Na2SO4 to restore sulfation of C4. Activity of C3, a complement component homologous to C4 but lacking tyrosine sulfate residues, was minimally reduced (19%) by the inhibitors. Decreased hemolytic activity of nonsulfated C4 apparently resulted from impaired interaction with complement subcomponent C1s (EC 3.4.21.42), the protease that physiologically activates C4. Purified C1s was able to cleave nonsulfated C4, but approximately 10-fold higher concentrations of C1s were required for that cleavage than to yield equivalent cleavage of sulfated C4. Our results suggest that activation of C4, a central component in the classical pathway of complement activation, is influenced by the level of sulfation of the protein. Thus, sulfation of C4 provides a potential locus for physiological or pharmacological modulation of complement-mediated opsonization and inflammation.

Chlorate: a reversible inhibitor of proteoglycan sulfation

Bovine aorta endothelial cells were cultured in medium containing [3H]glucosamine, [35S]sulfate, and various concentrations of chlorate. Cell growth was not affected by 10 mM chlorate, while 30 mM chlorate had a slight inhibitory effect. Chlorate concentrations greater than 10 mM resulted in significant undersulfation of chondroitin. With 30 mM chlorate, sulfation of chondroitin was reduced to 10% and heparan to 35% of controls, but [3H]glucosamine incorporation on a per cell basis did not appear to be inhibited. Removal of chlorate from the culture medium of cells resulted in the rapid resumption of sulfation.