Survival Signals of Hepatic Stellate Cells in Liver Regeneration Are Regulated by Glycosylation Changes in Rat Vitronectin,Especially Decreased Sialylation

The extracellular matrix (ECM) molecules play important roles in many biological and pathological processes. During tissue remodeling, the ECM molecules that are glycosylated are different from those of normal tissue owing to changes in the expression of many proteins that are responsible for glycan synthesis. Vitronectin (VN) is a major ECM molecule that recognizes integrin on hepatic stellate cells (HSCs). The present study attempted to elucidate how changes in VN glycans modulate the survival of HSCs, which play a critical role in liver regeneration. Plasma VN was purified from partially hepatectomized (PH) and sham-operated (SH) rats at 24 h after operation and non-operated (NO) rats. Adhesion of rat HSCs (rHSCs), together with phosphorylation of focal adhesion kinase, in PH-VN was decreased to one-half of that in NO- or SH-VN. Spreading of rHSCs on desialylated NO-VN was decreased to one-half of that of control VN, indicating the importance of sialylation of VN for activation of HSCs. Liquid chromatography/multiple-stage mass spectrometry analysis of Glu-C glycopeptides of each VN determined the site-specific glycosylation. In addition to the major biantennary complex-type N-glycans, hybrid-type N-glycans were site-specifically present at Asn¹⁶⁷. Highly sialylated O-glycans were found to be present in the Thr¹¹⁰–Thr¹²⁴ region. In PH-VN, the disialyl O-glycans and complex-type N-glycans were decreased while core-fucosylated N-glycans were increased. In addition, immunodetection after two-dimensional PAGE indicated the presence of hyper- and hyposialylated molecules in each VN and showed that hypersialylation was markedly attenuated in PH-VN. This study proposes that the alteration of VN glycosylation modulates the substrate adhesion to rat HSCs, which is responsible for matrix restructuring.     

Glycosylation and ligand-binding activities of rat plasma fibronectin during liver regeneration after partial hepatectomy

Fibronectin (FN) is a multifunctional glycoprotein present in the extracellular matrix (ECM) and plasma. We previously reported that the glycosylation and ligand-binding of vitronectin (VN) change markedly after partial hepatectomy (PH). Here we show the changes of FN during liver regeneration. The yields of purified sham-operated (SH-) and PH-FN were higher than that of non-operated (NO)-FN, while binding activities of FNs to ECM ligands were changed only slightly by hepatectomy. The carbohydrate concentration of PH-FN decreased to 66% of that of NO- and SH-FN. By using LC/MS(n), eight kinds of complex-type N-glycan structures were found to be present in all FNs, and bi- and trisialobiantennary glycans were the major structures. Fucosylation was markedly increased, while O-acetylation of sialic acid was found to be decreased in PH-FN. The alterations in glycosylation and biological activities of FN after PH are different from those of VN, suggesting that these glycoproteins play different biological functions in tissue remodeling.     

The changes in glycosylation after partial hepatectomy enhance collagen binding of vitronectin in plasma

Vitronectin is a multifunctional glycoprotein present in the extracellular matrix and plasma. Changes in rat vitronectin were studied during liver regeneration after partial hepatectomy. Carbohydrate concentrations of vitronectin decreased to 2/3 of sham-operated rats at 24 h after partial hepatectomy. Carbohydrate composition and lectin reactivity indicated that N-glycosylation and sialylation of vitronectin changed markedly after partial hepatectomy, while amino acid composition did not change significantly. We previously showed that deN-glycosylation of vitronectin in vitro affects collagen binding among various ligands (Yoneda et al., Biochemistry (1998) 37, 6351-6360). Vitronectins from partially hepatectomized rats at 24 h were found to exhibit markedly enhanced binding to type I collagen. The effect of sialylation on collagen binding was further examined using enzymatically deglycosylated vitronectin of nonoperated rats. Collagen binding increased by 1.2 times after deN-glycosylation of vitronectin, while it increased more than 2.9 times after desialylation. Various glycosyltransferases in liver are known to change after partial hepatectomy, including the attenuation of N-oligosaccharide transferase. The findings therefore suggest that the collagen binding of vitronectin is modulated by the alteration of peptide glycosylation caused by postoperative physiological changes of glycosyltransferases and that the change may contribute to tissue remodeling processes.     

Sialylation of vitronectin regulates stress fiber formation and cell spreading of dermal fibroblasts via a heparin-binding site

Vitronectin (VN) plays an important role in tissue regeneration. We previously reported that VN from partial hepatectomized (PH) rats results in a decrease of sialylation of VN and de-sialylation of VN decreases the cell spreading of hepatic stellate cells. In this study, we analyzed the mechanism how sialylation of VN regulates the properties of mouse primary cultured dermal fibroblasts (MDF) and a dermal fibroblast cell line, Swiss 3T3 cells. At first, we confirmed that VN from PH rats or de-sialylated VN also decreased cell spreading in MDF and Swiss 3T3 cells. The de-sialylation suppressed stress fiber formation in Swiss 3T3 cells. Next, we analyzed the effect of the de-sialylation of VN on stress fiber formation in Swiss 3T3 cells. RGD peptide, an inhibitor for a cell binding site of VN, did not affect the cell attachment of Swiss 3T3 cells on untreated VN but significantly decreased it on de-sialylated VN, suggesting that the de-sialylation attenuates the binding activity of an RGD-independent binding site in VN. To analyze a candidate RGD-independent binding site, an inhibition experiment of stress fiber formation for a heparin binding site was performed. The addition of heparin and treatment of cells with heparinase decreased stress fiber formation in Swiss 3T3 cells. Furthermore, de-sialylation increased the binding activity of VN to heparin, as detected by surface plasmon resonance (SPR). These results demonstrate that sialylation of VN glycans regulates stress fiber formation and cell spreading of dermal fibroblast cells via a heparin binding site.     

Complex N-glycans are the major ligands for galectin-1, -3, and -8 on Chinese hamster ovary cells

Galectins are implicated in a large variety of biological functions, many of which depend on their carbohydrate-binding ability. Fifteen members of the family have been identified in vertebrates based on binding to galactose (Gal) that is mediated by one or two, evolutionarily conserved, carbohydrate-recognition domains (CRDs). Variations in glycan structures expressed on glycoconjugates at the cell surface may, therefore, affect galectin binding and functions. To identify roles for different glycans in the binding of the three types of mammalian galectins to cells, we performed fluorescence cytometry at 4 degrees C with recombinant rat galectin-1, human galectin-3, and three forms of human galectin-8, to Chinese hamster ovary (CHO) cells and 12 different CHO glycosylation mutants. All galectin species bound to parent CHO cells and binding was inhibited >90% by 0.2 M lactose. Galectin-8 isoforms with either a long or a short inter-CRD linker bound similarly to CHO cells. However, a truncated form of galectin-8 containing only the N-terminal CRD bound only weakly to CHO cells and the C-terminal galectin-8 CRD exhibited extremely low binding. Binding of the galectins to the different CHO glycosylation mutants revealed that complex N-glycans are the major ligands for each galectin except the N-terminal CRD of galectins-8, and also identified some fine differences in glycan recognition. Interestingly, increased binding of galectin-1 at 4 degrees C correlated with increased propidium iodide (PI) uptake, whereas galectin-3 or -8 binding did not induce permeability to PI. The CHO glycosylation mutants with various repertoires of cell surface glycans are a useful tool for investigating galectin-cell interactions as they present complex and simple glycans in a natural mixture of multivalent protein and lipid glycoconjugates anchored in a cell membrane.     

Structural requirements for the extracellular interaction of plasminogen activator inhibitor 1 with endothelial cell matrix-associated vitronectin.

The interaction of plasminogen activator inhibitor-1 (PAI-1) with its binding protein vitronectin (VN) (Declerck, P. J., De Mol, M., Alessi, M.-C., Baudner, S., Paques, E.-P., Preissner, K. T., Müller-Berghaus, G., and Collen, D. (1988) J. Biol. Chem. 263, 15454-15461) in the extracellular matrix (ECM) of cultured human endothelial cells (HUVEC) was studied. Like PAI-1, VN was found associated with the ECM as evidenced by direct antibody binding, by Western blot analysis as well as by diffuse immunofluorescence staining in permeabilized HUVEC. The specific interaction of VN with confluent monolayers of HUVEC was found to be saturable within 2-4 h at 37 degrees C only with respect to binding to the cells, while no saturable binding to the underlying ECM was observed, indicating that the majority if not all ECM-associated VN was derived from the culture medium. In contrast to PAI-1, ECM-associated VN was resistant toward glycine (pH 2.3), guanidine or urokinase treatment, suggesting that VN was tightly associated with the ECM network. Binding of recombinant PAI-1 (rPAI-1) was largely blocked by anti-VN IgG and only partly by anti-collagen IgG but not by antibodies against other ECM components, indicating that VN constitutes the primary binding protein for ECM-associated PAI-1. This contention was supported by ligand blotting experiments in which rPAI-1 was reacted with nitrocellulose replicas of electrophoretically separated ECM components. Protein band(s) (Mr = 63,000-67,000), comigrating with bovine VN (i.e. medium-derived VN) rather than with human VN were identified as major binding component(s). Moreover, binding studies with purified components revealed that PAI-1 did not show any affinity for collagen (type I/III) alone, whereas VN collagen coating was a much better template for PAI-1 binding than VN alone and that conformationally extended VN provides maximal PAI-1 binding capacity. Binding of rPAI-1 to surface-coated VN was saturable and revealed that (unlike urokinase) heparin or the synthetic peptide Gly-Arg-Gly-Asp-Ser did not inhibit PAI-1 binding. Ligand binding of rPAI-1 to nitrocellulose replicas from sodium dodecyl sulfate-polyacrylamide gels containing electrophoretically separated peptides from VN digests documented the association of PAI-1 with Mr = 10,000-20,000 fragments originating from the heparin-binding domain of VN. These results indicate that the exposure of the glycosaminoglycan-binding domain in VN may allow the concomitant binding of PAI-1 and heparin-like molecules to this region of the VN molecule.(ABSTRACT TRUNCATED AT 400 WORDS)     

Glycosylation profile of integrin alpha 3 beta 1 changes with melanoma progression

Glycosylation of integrins has been implicated in the modulation of their function. Characterisation of carbohydrate moieties of alpha(3) and beta(1) subunits from non-metastatic (WM35) and metastatic (A375) human melanoma cell lines was carried out on peptide-N-glycosidase F-released glycans using matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). beta(1) integrin subunit from both cell lines displayed tri- and tetraantennary oligosaccharides complex type glycans, but only in A375 cell line was the sialylated tetraantennary complex type glycan (Hex(7)HexNAc(6)FucSia(4)) present. In contrast, only alpha(3) subunit from metastatic cells possessed beta1-6 branched structures. Our data indicate that the beta(1) and alpha(3) subunits expressed by the metastatic A375 cell line carry beta1-6 branched structures, suggesting that these cancer-associated glycan chains may modulate tumor cell adhesion by affecting the ligand binding properties of alpha(3)beta(1) integrin. In direct ligand binding assays, alpha(3)beta(1) integrin from both cell lines binds strongly to fibronectin and to much lesser degree to placental laminin. No binding to collagen IV was observed. Enzymatic removal of sialic acid residues from purified alpha(3)beta(1) integrin stimulates its adhesion to all examined ECM proteins. Our data suggest that the glycosylation profile of alpha(3)beta(1) integrin in human melanoma cells correlates with the acquisition of invasive capacity during melanoma progression.     

Binding of plasminogen activator inhibitor type-1 to extracellular matrix of Hep G2 cells. Evidence that the binding protein is vitronectin.

Catabolism of plasminogen activators by Hep G2 cells is mediated by a specific receptor which recognizes complexes of these serine proteases with their physiological inhibitor, plasminogen activator inhibitor type-1 (PAI-1). This catabolic process is initiated by interaction of exogenous plasminogen activators with bioactive PAI-1, which is secreted and localizes in an active form to the extracellular matrix (ECM) of Hep G2 cells. We now report that vitronectin (VN) mediates the specific binding of PAI-1 to the ECM of these cells. Purified bovine or human VN competes for specific binding of PAI-1 to Hep G2 ECM, and ligand blotting reveals specific binding of PAI-1 to ECM-associated VN. Hep G2 cells secrete both VN and PAI-1, and pulse-chase studies strongly suggest that these proteins associate only following secretion. Although Hep G2 cell-derived VN does not significantly bind to ECM in vitro, 30-40% of endogenous PAI-1 binds to the ECM, even in the presence of human serum, suggesting that ECM-associated VN is entirely derived from bovine serum. PAI-1 was localized by indirect immunofluorescence to ECM beneath cells and at cell margins, whereas VN exhibited a uniform distribution throughout the growth substratum. VN associated with the ECM may confer retention and bioactivity to PAI-1, potentially facilitating both pericellular regulation of plasmin generation and the rapid hepatic clearance of plasminogen activators.     

Phosphodiesterase 10A upregulation contributes to pulmonary vascular remodeling

Phosphodiesterases (PDEs) modulate the cellular proliferation involved in the pathophysiology of pulmonary hypertension (PH) by hydrolyzing cAMP and cGMP. The present study was designed to determine whether any of the recently identified PDEs (PDE7-PDE11) contribute to progressive pulmonary vascular remodeling in PH. All in vitro experiments were performed with lung tissue or pulmonary arterial smooth muscle cells (PASMCs) obtained from control rats or monocrotaline (MCT)-induced pulmonary hypertensive (MCT-PH) rats, and we examined the effects of the PDE10 inhibitor papaverine (Pap) and specific small interfering RNA (siRNA). In addition, papaverine was administrated to MCT-induced PH rats from day 21 to day 35 by continuous intravenous infusion to examine the in vivo effects of PDE10A inhibition. We found that PDE10A was predominantly present in the lung vasculature, and the mRNA, protein, and activity levels of PDE10A were all significantly increased in MCT PASMCs compared with control PASMCs. Papaverine and PDE10A siRNA induced an accumulation of intracellular cAMP, activated cAMP response element binding protein and attenuated PASMC proliferation. Intravenous infusion of papaverine in MCT-PH rats resulted in a 40%-50% attenuation of the effects on pulmonary hypertensive hemodynamic parameters and pulmonary vascular remodeling. The present study is the first to demonstrate a central role of PDE10A in progressive pulmonary vascular remodeling, and the results suggest a novel therapeutic approach for the treatment of PH.     

Dual roles of the Cardin-Weintraub motif in multimeric Sonic hedgehog

The fly morphogen Hedgehog (Hh) and its mammalian orthologs, Sonic, Indian, and Desert hedgehog, are secreted signaling molecules that mediate tissue patterning during embryogenesis and function in tissue homeostasis and regeneration in the adult. The function of all Hh family members is regulated at the levels of morphogen multimerization on the surface of producing cells, multimer release, multimer diffusion to target cells, and signal reception. These mechanisms are all known to depend on interactions of positively charged Hh amino acids (the Cardin-Weintraub (CW) motif) with negatively charged heparan sulfate (HS) glycosaminoglycan chains. However, a precise mechanistic understanding of these interactions is still lacking. In this work, we characterized ionic HS interactions of multimeric Sonic hedgehog (called ShhNp) as well as mutant forms lacking one or more CW residues. We found that deletion of all five CW residues as well as site-directed mutagenesis of CW residues Lys(33), Arg(35), and Lys(39) (mouse nomenclature) abolished HS binding. In contrast, CW residues Arg(34) and Lys(38) did not contribute to HS binding. Analysis and validation of Shh crystal lattice contacts provided an explanation for this finding. We demonstrate that CW residues Arg(34) and Lys(38) make contact with an acidic groove on the adjacent molecule in the multimer, suggesting a new function of these residues in ShhNp multimerization rather than HS binding. Therefore, the recombinant monomeric morphogen (called ShhN) differs in CW-dependent HS binding and biological activity from physiologically relevant ShhNp multimers, providing new explanations for functional differences observed between ShhN and ShhNp.     

Chromatofocusing fails to separate hFSH isoforms on the basis of glycan structure

Follicle-stimulating hormone (FSH) glycosylation is regulated by feedback from the gonads, resulting in an array of glycans associated with FSH preparations derived from pools of pituitary or urine extracts. FSH glycosylation varies due to inhibition of FSHbeta N-glycosylation, elaboration of 1-4 branches possessed by mature N-glycans, and the number and linkage of terminal sialic acid residues. To characterize FSH glycosylation, FSH isoforms in pituitary gland extracts and a variety of physiological fluids are commonly separated by chromatofocusing. Variations in the ratios of immunological and biological activities in the resulting FSH isoform preparations are generally attributed to changes in glycosylation, which are most often defined in terms of sialic acid content. Using Western blotting to assess human FSHbeta glycosylation inhibition revealed 30-47% nonglycosylated hFSHbeta associated with four of six hFSH isoform preparations derived by chromatofocusing. Glycopeptide mass spectrometry assessment of glycan branching in these isoforms extensively characterized two N-glycosylation sites, one at alphaAsn52, the critical glycan for FSH function, and the other at betaAsn24. With two to four N-glycans per FSH molecule, many combinations of charges distributed over these sites can provide the same isoelectric point. Indeed, several glycans were common to all isoform fractions that were analyzed. There was no trend showing predominantly monoantennary glycans associated with the high-pI fractions, nor were predominantly tri- and tetra-antennary glycans associated with low-pI fractions. Thus, differences in receptor binding activity could not be associated with any specific glycan type or location in the hormone. FSH aggregation was associated with reduced receptor binding activity but did not affect immunological activity. However, as gel filtration indicated sufficient heterodimer was present in each isoform preparation to generate complete inhibition curves, the near total loss of receptor binding activity in several preparations could not be explained by aggregation alone, and the mechanism remains unknown.     

Prolidase activity in fibroblasts is regulated by interaction of extracellular matrix with cell surface integrin receptors

Prolidase (EC 3.4.13.9) is a ubiquitously distributed imidodipeptidase that catalyzes the hydrolysis of C-terminal proline or hydroxyproline containing dipeptides. The enzyme plays an important role in the recycling of proline for collagen synthesis and cell growth. An increase in enzyme activity is correlated with increased rates of collagen turnover indicative of extracellular matrix (ECM) remodeling, but the mechanism linking prolidase activity and ECM is poorly understood. Thus, the effect of ECM-cell interaction on intracellular prolidase activity is of special interest. In cultured human skin fibroblasts, the interaction with ECM and, more specifically, type I collagen mediated by the β₁ integrin receptor regulates cellular prolidase activity. Supporting evidence comes from the following observations: 1) in sparse cells with a low amount of ECM collagen or in confluent cells in which ECM collagen was removed by collagenase (but not by trypsin or elastase) treatment, prolidase activity was decreased; 2) this effect was reversed by the addition of type I collagen or β₁ integrin antibody (agonist for β₁ integrin receptor); 3) sparse cells (with typically low prolidase activity) showed increased prolidase activity when grown on plates coated with type I collagen or on type IV collagen and laminin, constituents of basement membrane; 4) the relative differences in prolidase activity due to collagenase treatment and subsequent recovery of the activity by β₁ integrin antibody or type I collagen treatment were accompanied by parallel differences in the amount of the enzyme protein recovered from these cells, as shown by Western immunoblot analysis. Thus, we conclude that prolidase activity responded to ECM metabolism (tissue remodeling) through signals mediated by the integrin receptor. J. Cell. Biochem. 67:166–175, 1997. Published 1997 Wiley-Liss, Inc.     

Collagen modulates gene activation of plasminogen activator system molecules

Skin extracellular matrix (ECM) molecules regulate a variety of cellular activities, including cell movement, which are central to wound healing and metastasis. Regulated cell movement is modulated by proteases and their associated molecules, including the serine proteases urinary-type plasminogen activator (uPA) and tissue-type plasminogen activator (tPA) and their inhibitors (PAIs). As a result of wounding and loss of basement membrane structure, epidermal keratinocytes can become exposed to collagen. To test the hypothesis that during wounding, exposed collagen, the most abundant ECM molecule in the skin, regulates keratinocyte PA and PAI gene expression, we utilized an in vitro model in which activated keratinocytes were cultured in dishes coated with collagen or other ECM substrates. tPA, uPA, and PAI-1 mRNA and enzymatic activity were detected when activated keratinocytes attached to fibronectin, vitronectin, collagen IV, and RGD peptide. In contrast, adhesion to collagen I and collagen III completely suppressed expression of PAI-1 mRNA and protein and further increased tPA expression and activity. Similarly, keratinocyte adhesion to laminin-1 suppressed PAI-1 mRNA and protein expression and increased tPA activity. The suppressive effect of collagen I on PAI-1 gene induction was dependent on the maintenance of its native fibrillar structure. Thus, it would appear that collagen- and laminin-regulated gene expression of molecules associated with plasminogen activation provides an additional dimension in the regulation of cell movement and matrix remodeling in skin wound healing.     

Effects of matrix proteins on the expression of matrix metalloproteinase-2, -9, and -14 and tissue inhibitors of metalloproteinases in human cytotrophoblast cells during the first trimester.

The activity of matrix metalloproteinases (MMPs) specifies the ability of the trophoblast cell to degrade extracellular matrix (ECM) substrates. Usually the process of normal human placentation involves a coordinated interaction between the fetal-derived trophoblast cells and their microenvironment in the uterus. In this study, the effects of ECM proteins on the expression of MMP-2, -9, and -14 (membrane-type MMP-1); and the production of tissue inhibitors of metalloproteinase (TIMP) types -1, -2, and -3 have been investigated. Cytotrophoblast cells at 9 or 10 wk of gestation were cultured on various ECM coated dishes under serum-free conditions. Gelatin zymography analysis showed that cells grown on fibronectin (FN), laminin (LN), and vitronectin (VN) secreted more MMP-9 (about 1.5- to 3-fold more) than cells cultured on collagen I (Col I), whereas the secretion of MMP-9 by cells cultured on collagen IV (Col IV) was only half that by the cells on Col I. Northern Blot analysis gave the same results as zymography, indicating that expression of the MMP-9 gene in cytotrophoblast cells can be affected by matrix proteins. There was no significant difference in the expression of MMP-2 either at protein or mRNA levels among the cells cultured on the different matrix substrates. The expression of MMP-14 was regulated in a manner similar to that of MMP-2. Using ELISA, we detected higher levels of TIMP-1 in the culture medium of cells grown on VN, LN, and FN compared with that grown on Col I. But the expression of TIMP-3 mRNA was remarkably inhibited by VN, and ECM proteins had no effect on TIMP-1 and TIMP-2 mRNA expression. It was also observed that cultured cytotrophoblast cells expressed the corresponding receptors for the tested matrix proteins, such as integrins alpha(1), alpha(5), alpha(6), beta(1), and beta(4). Furthermore, the adhesiveness of cytotrophoblast cells on Col I, Col IV, FN, and LN was increased by 62%, 45%, 21%, and 22%, respectively, when compared with adhesiveness on VN. Isolated cytotrophoblast cells remained stationary when cultured on dishes coated with Col I and Col IV, but they assumed a more motile morphology and aggregated into a network when cultured on LN and VN. These data indicate that human trophoblast cells interact with their microenvironment to control their behavior and function.     

Asparagine-linked glycosylation of human chymotrypsin C is required for folding and secretion but not for enzyme activity

Human chymotrypsin C (CTRC) plays a protective role in the pancreas by mitigating premature trypsinogen activation through degradation. Mutations that abolish activity or secretion of CTRC increase the risk for chronic pancreatitis. The aim of the present study was to determine whether human CTRC undergoes asparagine-linked (N-linked) glycosylation and to examine the role of this modification in CTRC folding and function. We abolished potential sites of N-linked glycosylation (Asn-Xaa-Ser/Thr) in human CTRC by mutating the Asn residues to Ser individually or in combination, expressed the CTRC mutants in HEK 293T cells and determined their glycosylation state using PNGase F and endo H digestion. We found that human CTRC contains a single N-linked glycan on Asn52. Elimination of N-glycosylation by mutation of Asn52 (N52S) reduced CTRC secretion about 10-fold from HEK 293T cells but had no effect on CTRC activity or inhibitor binding. Overexpression of the N52S CTRC mutant elicited endoplasmic reticulum stress in AR42J acinar cells, indicating that N-glycosylation is required for folding of human CTRC. Despite its important role, Asn52 is poorly conserved in other mammalian CTRC orthologs, including the rat which is monoglycosylated on Asn90. Introduction of the Asn90 site in a non-glycosylated human CTRC mutant restored full glycosylation but only partially rescued the secretion defect. We conclude that N-linked glycosylation of human CTRC is required for efficient folding and secretion; however, the N-linked glycan is unimportant for enzyme activity or inhibitor binding. The position of the N-linked glycan is critical for optimal folding, and it may vary among the otherwise highly homologous mammalian CTRC sequences.