Transglutaminase-catalysed cross-linking: a potential mechanism for the interaction of fibrinogen, low density lipoprotein and arterial type III procollagen

Fighting the Fiber: Targeting Collagen in Lung Fibrosis

Organ fibrosis is characterized by epithelial injury and aberrant tissue repair, where activated effector cells, mostly fibroblasts and myofibroblasts, excessively deposit collagen into the extracellular matrix. Fibrosis frequently results in organ failure and has been estimated to contribute to at least one third of all global deaths. Also lung fibrosis, in particular idiopathic pulmonary fibrosis (IPF), is a fatal disease with rising incidence worldwide. As current treatment options targeting fibrogenesis are insufficient, there is an urgent need for novel therapeutic strategies. During the last decade, several studies have proposed to target intra- and extracellular components of the collagen biosynthesis, maturation, and degradation machinery. This includes intra- and extracellular targets directly acting on collagen gene products, but also such that anabolize essential building blocks of collagen, in particular glycine and proline biosynthetic enzymes. Collagen, however, is a ubiquitous molecule in the body and fulfils essential functions as a macromolecular scaffold, growth factor reservoir, and receptor binding site in virtually every tissue. This review summarizes recent advances and future directions in this field. Evidence for the proposed therapeutic targets and where they currently stand in terms of clinical drug development for treatment of fibrotic disease is provided. The drug targets are furthermore discussed in light of (1) specificity for collagen biosynthesis, maturation and degradation, and (2) specificity for disease-associated collagen. As therapeutic success and safety of these drugs may largely depend on targeted delivery, different strategies for specific delivery to the main effector cells and to the extracellular matrix are discussed.

Collagen Biosynthesis, Processing, and Maturation in Lung Ageing

In addition to providing a macromolecular scaffold, the extracellular matrix (ECM) is a critical regulator of cell function by virtue of specific physical, biochemical, and mechanical properties. Collagen is the main ECM component and hence plays an essential role in the pathogenesis and progression of chronic lung disease. It is well-established that many chronic lung diseases, e.g., chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF) primarily manifest in the elderly, suggesting increased susceptibility of the aged lung or accumulated alterations in lung structure over time that favour disease. Here, we review the main steps of collagen biosynthesis, processing, and turnover and summarise what is currently known about alterations upon lung ageing, including changes in collagen composition, modification, and crosslinking. Recent proteomic data on mouse lung ageing indicates that, while the ER-resident machinery of collagen biosynthesis, modification and triple helix formation appears largely unchanged, there are specific changes in levels of type IV and type VI as well as the two fibril-associated collagens with interrupted triple helices (FACIT), namely type XIV and type XVI collagens. In addition, levels of the extracellular collagen crosslinking enzyme lysyl oxidase are decreased, indicating less enzymatically mediated collagen crosslinking upon ageing. The latter contrasts with the ageing-associated increase in collagen crosslinking by advanced glycation endproducts (AGEs), a result of spontaneous reactions of protein amino groups with reactive carbonyls, e.g., from monosaccharides or reactive dicarbonyls like methylglyoxal. Given the slow turnover of extracellular collagen such modifications accumulate even more in ageing tissues. In summary, the collective evidence points mainly toward age-induced alterations in collagen composition and drastic changes in the molecular nature of collagen crosslinks. Future work addressing the consequences of these changes may provide important clues for prevention of lung disease and for lung bioengineering and ultimately pave the way to novel targeted approaches in lung regenerative medicine.

TGFβ-1 Induced Cross-Linking of the Extracellular Matrix of Primary Human Dermal Fibroblasts

Excessive cross-linking is a major factor in the resistance to the remodelling of the extracellular matrix (ECM) during fibrotic progression. The role of TGFβ signalling in impairing ECM remodelling has been demonstrated in various fibrotic models. We hypothesised that increased ECM cross-linking by TGFβ contributes to skin fibrosis in Systemic Sclerosis (SSc). Proteomics was used to identify cross-linking enzymes in the ECM of primary human dermal fibroblasts, and to compare their levels following treatment with TGFβ-1. A significant upregulation and enrichment of lysyl-oxidase-like 1, 2 and 4 and transglutaminase 2 were found. Western blotting confirmed the upregulation of lysyl hydroxylase 2 in the ECM. Increased transglutaminase activity in TGFβ-1 treated ECM was revealed from a cell-based assay. We employed a mass spectrometry-based method to identify alterations in the ECM cross-linking pattern caused by TGFβ-1. Cross-linking sites were identified in collagens I and V, fibrinogen and fibronectin. One cross-linking site in fibrinogen alpha was found only in TGFβ-treated samples. In conclusion, we have mapped novel cross-links between ECM proteins and demonstrated that activation of TGFβ signalling in cultured dermal fibroblasts upregulates multiple cross-linking enzymes in the ECM.

Advantages of Using Egg Yolk Antibodies in the Life Sciences: The Results of Five Studies

It has been known for over a century that specific antibodies can be extracted from the eggs of immunised chickens. However, it was only when animal welfare became a subject of public debate that the chicken was considered as an alternative source of antibodies due to the possibility of non-invasive antibody sampling. Unfortunately, the welfare of animals alone is not sufficient to attract the interest of scientists; it is therefore important to demonstrate to potential users that avian antibodies can be used successfully in a variety of scientific investigations. The particular specificity of avian antibodies would appear to be due to the phylogenetic difference between Mammalia and Aves as well as to differences between the molecular structures of avian immunoglobulin (IgY) and mammalian immunoglobulin (IgG). The use of avian antibodies has additional advantages, as a considerable quantity of antibodies can be obtained from one chicken, and because the specificity of avian antibodies often markedly differs from that of comparable mammalian antibodies. This paper aims to demonstrate the advantages of using avian antibodies by presenting the results of five separate studies. In the first study, coordinated by Rüdiger Schade, the visualisation of cholecystokinin-like immuno-reactivity in the substantia nigra of rats by using anti-cholecystokinin antibody, without the pre-treatment of colchicine, is described. The second study, headed by Albrecht Zott, describes the use of avian antibodies in the identification of modern acellular pertussis vaccines by using rocket immunoelectrophoresis. The identification of unknown vaccine batches and the comparison with reference vaccines is a prerequisite for reducing the number of animal experiments necessary for vaccine control. The third study, coordinated by Martin Gerl, investigates the specificity of antibodies directed against the N-terminal propeptide of procollagen type III (PIIINP). Among the antibodies originating from different species (rabbit, mouse and chicken), only the chicken antibody was able to respond to the PIIINP in both human and rat sera. Thus, a direct comparison between human serum samples (alcoholic liver) and serum samples derived from corresponding animal models was possible. The fourth study, coordinated by Michael Erhard, shows that egg yolk antibodies can be successfully used to manage infectious diarrhoea in young agricultural animals. The final study, led by Andreas Hlinak, describes the successful production of anti-bovine leukaemia virus antibody. This antibody could be used in several diagnostic systems (for example, enzyme immunoassays and cytology). The five studies demonstrate that avian antibodies are an attractive alternative to mammalian antibodies, not only with respect to the welfare of animals, but also with respect to scientific and economic considerations.

Insights into the heparan sulphate-dependent externalisation of transglutaminase-2 (TG2) in glucose-stimulated proximal-like tubular epithelial cells

The extracellular matrix crosslinking enzyme transglutaminase 2 (TG2) is highly implicated in tissue fibrosis that precedes end-stage kidney failure. TG2 is unconventionally secreted through extracellular vesicles in a way that depends on the heparan sulphate (HS) proteoglycan syndecan-4 (Sdc4), the deletion of which reduces experimental kidney fibrosis as a result of lower extracellular TG2 in the tubule-interstitium. Here we establish a model of TG2 externalisation in NRK-52E tubular epithelial cells subjected to glucose stress. HS-binding TG2 mutants had reduced extracellular TG2 in transfected NRK-52E, suggesting that TG2-externalisation depends on an intact TG2 heparin binding site. Inhibition of N-ethylmaleimide sensitive factor (NSF) vesicle-fusing ATPase, which was identified in the recently elucidated TG2 kidney membrane-interactome, led to significantly lower TG2-externalisation, thus validating the involvement of membrane fusion in TG2 secretion. As cyclin-G-associated kinase (GAK) had emerged as a further TG2-partner in the fibrotic kidney, we investigated whether glucose-induced TG2-externalisation was accompanied by TG2 phosphorylation in consensus sequences of cyclin-dependent kinase (CDK). Glucose stress led to intense TG2 phosphorylation in serine/threonine CDK-target. TG2 phosphorylation by tyrosine kinases was also increased by glucose. Although the precise role of glucose-induced TG2 phosphorylation is unknown, these novel data suggest that phosphorylation may be involved in TG2 membrane-trafficking.

Chronic liver injury drives non-traditional intrahepatic fibrin(ogen) crosslinking via tissue transglutaminase

Essentials Fibrin clots are often implicated in the progression of liver fibrosis. Liver fibrosis was induced in transgenic mice with defects in clot formation or stabilization. Liver fibrosis and fibrin(ogen) deposition do not require fibrin polymerization or factor XIIIa. Fibrin(ogen) is an in vivo substrate of tissue transglutaminase in experimental liver fibrosis. SUMMARY: Background Intravascular fibrin clots and extravascular fibrin deposits are often implicated in the progression of liver fibrosis. However, evidence supporting a pathological role of fibrin in hepatic fibrosis is indirect and based largely on studies using anticoagulant drugs that inhibit activation of the coagulation protease thrombin, which has other downstream targets that promote fibrosis. Therefore, the goal of this study was to determine the precise role of fibrin deposits in experimental hepatic fibrosis. Methods Liver fibrosis was induced in mice expressing mutant fibrinogen insensitive to thrombin-mediated proteolysis (i.e. locked in the monomeric form), termed FibAEK mice, and factor XIII A2 subunit-deficient (FXIII-/- ) mice. Female wild-type mice, FXIII-/- mice and homozygous FibAEK mice were challenged with carbon tetrachloride (CCl4 ) twice weekly for 4 weeks or 6 weeks (1 mL kg-1 , intraperitoneal). Results Hepatic injury and fibrosis induced by CCl4 challenge were unaffected by FXIII deficiency or inhibition of thrombin-catalyzed fibrin polymer formation (in FibAEK mice). Surprisingly, hepatic deposition of crosslinked fibrin(ogen) was not reduced in CCl4 -challenged FXIII-/- mice or FibAEK mice as compared with wild-type mice. Rather, deposition of crosslinked hepatic fibrin(ogen) following CCl4 challenge was dramatically reduced in tissue transglutaminase-2 (TGM2)-deficient (TGM2-/- ) mice. However, the reduction in crosslinked fibrin(ogen) in TGM2-/- mice did not affect CCl4 -induced liver fibrosis. Conclusions These results indicate that neither traditional fibrin clots, formed by the thrombin-activated FXIII pathway nor atypical TGM2-crosslinked fibrin(ogen) contribute to experimental CCl4 -induced liver fibrosis. Collectively, the results indicate that liver fibrosis occurs independently of intrahepatic fibrin(ogen) deposition.

Liver fibrosis: Direct antifibrotic agents and targeted therapies

Liver fibrosis and in particular cirrhosis are the major causes of morbidity and mortality of patients with chronic liver disease. Their prevention or reversal have become major endpoints in clinical trials with novel liver specific drugs. Remarkable progress has been made with therapies that efficiently address the cause of the underlying liver disease, as in chronic hepatitis B and C. Highly effective antiviral therapy can prevent progression or even induce reversal in the majority of patients, but such treatment remains elusive for the majority of liver patients with advanced alcoholic or nonalcoholic steatohepatitis, genetic or autoimmune liver diseases. Moreover, drugs that would speed up fibrosis reversal are needed for patients with cirrhosis, since even with effective causal therapy reversal is slow or the disease may further progress. Therefore, highly efficient and specific antifibrotic agents are needed that can address advanced fibrosis, i.e., the detrimental downstream result of all chronic liver diseases. This review discusses targeted antifibrotic therapies that address molecules and mechanisms that are central to fibrogenesis or fibrolysis, including strategies that allow targeting of activated hepatic stellate cells and myofibroblasts and other fibrogenic effector cells. Focus is on collagen synthesis, integrins and cells and mechanisms specific including specific downregulation of TGFbeta signaling, major extracellular matrix (ECM) components, ECM-crosslinking, and ECM-receptors such as integrins and discoidin domain receptors, ECM-crosslinking and methods for targeted delivery of small interfering RNA, antisense oligonucleotides and small molecules to increase potency and reduce side effects. With an increased understanding of the biology of the ECM and liver fibrosis and an improved preclinical validation, the translation of these approaches to the clinic is currently ongoing. Application to patients with liver fibrosis and a personalized treatment is tightly linked to the development of noninvasive biomarkers of fibrosis, fibrogenesis and fibrolysis.

Inhibitors of tissue transglutaminase

Tissue transglutaminase (TG2) catalyzes the cross-linking of proteins by the formation of isopeptide bonds between glutamine (Gln) and lysine (Lys) side chains. Although TG2 is essential for the stabilization of the extracellular matrix, its unregulated activity has been implicated in celiac disease, fibrosis, and cancer metastasis, among other disorders. Given the importance and range of TG2-related pathologies, recent work has focused on the development of potent and selective inhibitors against TG2. In this review, we present the latest and most noteworthy irreversible and reversible inhibitors of TG2, and offer perspectives for the design of future inhibitors, in the hope that lead compounds with therapeutic potential may soon be discovered.

Acyl transfer mechanisms of tissue transglutaminase

Tissue transglutaminase (TG2) is a calcium-dependent enzyme that catalyses several acyl transfer reactions. The most biologically relevant of these involve protein-bound Gln residues as an acyl-donor substrate, and either water or a primary amine as an acyl-acceptor substrate. The former leads to deamidation of Gln to Glu, whereas the latter leads to transamidation, typically resulting in protein cross-linking when the amine substrate is a protein-bound Lys residue. In this review, we present an overview of over fifty years of mechanistic studies that have led to our current understanding of TG2-mediated hydrolysis and transamidation.

Induction of chondrogenic differentiation in mesenchymal stem cells by TGF-beta cross-linked to collagen-PLLA [poly(L-lactic acid)] scaffold by transglutaminase 2

Transglutaminase-mediated cross-linking has been employed to optimize the mechanical properties and stability of tissue scaffolds. We have characterized tissue transglutaminase (TG2)-mediated cross-linking as a useful tool to deliver biologically-active TGF to mesenchymal stem cells (MSCs) and direct their differentiation towards a chondrogenic lineage. TGF-β3 is irreversibly cross-linked by TG2 to collagen type II-coated poly(L-lactic acid) nanofibrous scaffolds and activates Smad phosphorylation and Smad-dependent expression of a luciferase reporter. Human bone marrow-derived MSCs cultured on these scaffolds deposit cartilaginous matrix after 14 days of culture at 50 % efficiency compared to chondrogenesis in the presence of soluble TGF-β3. These findings are significant because they suggest a novel approach for the programming of MSCs in a spatially controlled manner by immobilizing biologically active TGF-β3 via cross-linking to a collagen-coated polymeric scaffold.

Tissue transglutaminase regulates chondrogenesis in mesenchymal stem cells on collagen type XI matrices

Tissue transglutaminase (tTG) is a multifunctional enzyme with a plethora of potential applications in regenerative medicine and tissue bioengineering. In this study, we examined the role of tTG as a regulator of chondrogenesis in human mesenchymal stem cells (MSC) using nanofibrous scaffolds coated with collagen type XI. Transient treatment of collagen type XI films and 3D scaffolds with tTG results in enhanced attachment of MSC and supports rounded cell morphology compared to the untreated matrices or those incubated in the continuous presence of tTG. Accordingly, enhanced cell aggregation and augmented chondrogenic differentiation have been observed on the collagen type XI-coated poly-(L-lactide) nanofibrous scaffolds treated with tTG prior to cell seeding. These changes implicate that MSC chondrogenesis is enhanced by the tTG-mediated modifications of the collagen matrix. For example, exogenous tTG increases resistance to collagenolysis in collagen type XI matrices by catalyzing intermolecular cross-linking, detected by a shift in the denaturation temperature. In addition, tTG auto-crosslinks to collagen type XI as detected by western blot and immunofluorescent analysis. This study identifies tTG as a novel regulator of MSC chondrogenesis further contributing to the expanding use of these cells in cartilage bioengineering.

Tissue transglutaminase expression and activity in normal and glaucomatous human trabecular meshwork cells and tissues

PURPOSE

Glaucoma is a leading cause of irreversible visual impairment and blindness in the world. A major risk factor for glaucoma is elevated intraocular pressure due to increased resistance of aqueous humor outflow through the trabecular meshwork (TM). In the glaucomatous TM, there is increased accumulation of extracellular matrix (ECM) material due to a disruption of the normal balance between ECM deposition and degradation. Tissue transglutaminase (TGM2) belongs to a family of calcium-dependent enzymes that catalyze the posttranslational modification of the ECM by cross-linking proteins, thus making these proteins resistant to enzymatic and physical degradation. It is possible that the increase in ECM proteins in the glaucomatous TM is due to increased cross-linking activity of TGM2. The purpose of this study was to determine whether there are differences in expression and activity of TGM2 between normal and glaucoma TM cells and tissues.

METHODS

Normal (n = 3 NTM) and glaucomatous (n = 3 GTM) human TM cell lines were grown until confluent. Western immunoblot analysis of cell lysates was used to compare TGM2 protein levels in NTM and GTM cells. TGM2 enzyme activity between NTM and GTM cells was studied by using a biotin cadaverine assay. In addition, immunohistochemistry of three normal and three glaucomatous TM tissues was used to evaluate the in vivo expression of TGM2, fibronectin (FN) and epsilon-(gamma-glutamyl) lysine (GGEL) proteins.

RESULTS

Western blot analysis and immunohistochemistry demonstrated the presence of TGM2 protein in both NTM and GTM cells. There was an increase in TGM2 protein in GTM cells compared with NTM cells, and GTM cells also had increased in TGM2 enzyme activity compared with NTM cells. Immunohistochemical results demonstrated increased expression of TGM2 and FN in GTM tissues. FN and GGEL proteins were colocalized in GTM tissues, indicating significant cross-linking of FN by TGM2.

CONCLUSIONS

This study demonstrated that both NTM and GTM cells express TGM2. In addition, TGM2 protein levels and enzyme activities were elevated in GTM cells. There was also an increase in colocalization of FN and GGEL protein in GTM tissues. These results indicate that TGM2 may play an important role in the pathogenesis of glaucoma by cross-linking ECM proteins such as FN and thus making the ECM more resistant to degradation.

Transglutaminases in disease

Transglutaminases (TGases) are enzymes that are widely used in many biological systems for generic tissue stabilization purposes. Mutations resulting in lost activity underlie several serious disorders. In addition, new evidence documents that they may also be aberrantly activated in tissues and cells and contribute to a variety of diseases, including neurodegenerative diseases such as Alzheimer's and Huntington's diseases. In these cases, the TGases appear to be a factor in the formation of inappropriate proteinaceous aggregates that may be cytotoxic. In other cases such as celiac disease, however, TGases are involved in the generation of autoantibodies. Further, in diseases such as progressive supranuclear palsy, Huntington's, Alzheimer's and Parkinson's diseases, the aberrant activation of TGases may be caused by oxidative stress and inflammation. This review will examine the role and activation of TGases in a variety of diseases.

Transglutaminase transcription and antigen translocation in experimental renal scarring

It was recently demonstrated that renal tissue transglutaminase (tTg) protein and its catalytic product the epsilon(gamma-glutamyl) lysine protein cross-link are significantly increased in the subtotal (5/6) nephrectomy model (SNx) of renal fibrosis in rats. It was proposed that the enzyme had two important physiologic functions in disease development; one of stabilizing the increased extracellular matrix (ECM) by protein cross-linking, the other in a novel form of tubular cell death. This study, using the same rat SNx model, demonstrates first by Northern blotting that expression of tTg mRNA when compared with controls is increased by day 15 (+70% increase, P < 0.05), then rises steadily, peaking at day 90 (+391%, P < 0.01), and remains elevated at 120 d (+205%, P < 0.05) when compared with controls. In situ hybridization histochemistry demonstrated that the tubular cells were the major site of the additional tTg synthesis. Immunohistochemistry on cryostat sections revealed a sixfold increase (P < 0.001) in ECM-bound tTg antigen at 90-d post-SNx, whereas in situ transglutaminase activity demonstrated by the incorporation of fluorescein cadaverine into cryostat sections indicated a 750% increase (P < 0.001) on day 90 in SNx animals. This increased activity was extracellular and predominantly found in the peritubular region. These results indicate that increased tTg gene transcription by tubular cells underlies the major changes in renal tTg protein reported previously in SNx rats, and that the presence of the epsilon(gamma-glutamyl) lysine cross-links in the extracellular environment is the result of the extracellular action of tTg. These changes may be in response to tubular cell injury during the scarring process and are likely to contribute to the progressive expansion of the ECM in renal fibrosis.

Effects of transforming growth factor-beta1 on renal extracellular matrix components and their regulating proteins

Transforming growth factor-beta1 (TGF-beta1) is widely regarded as a potent fibrogenic renal growth factor. In cell culture, TGF-beta1 has been shown to increase various extracellular matrix (ECM) proteins and tissue inhibitors of metalloproteinases (TIMP), while decreasing matrix metalloproteinases (MMP), providing the optimum environment for progressive ECM accumulation. This study, which uses the isolated perfused rat kidney (IPRK), describes for the first time in a whole kidney preparation the action of TGF-beta1 on factors associated with ECM processing. This model allows the study of the intact rat kidney with physiologic cell-cell interactions in the absence of confounding systemic influences. Left kidneys were removed from male Wistar rats by a nonischemic technique and perfused with a sterile, apyrogenic, endotoxin-free perfusate, based on the plasma volume expander Hemaccel (polygeline), at constant pressure in a recirculating IPRK system. Kidneys were perfused for 1 h either with (n = 3) or without (n = 3) recombinant human TGF-beta1 (20 ng/ml). The effects of perfusion were controlled by comparison with the nonperfused contralateral kidney (n = 6). TGF-beta1 was measured in the perfusate and urine, at the start and end of the experiment using an enzyme-linked immunosorbent assay to its biologically active form. After perfusion, sections of the kidneys were analyzed for changes in mRNA by Northern blotting. Significant increases in mRNA for fibronectin (7.5-fold, P < 0.01), heparan sulfate proteoglycan core protein (53-fold, P < 0.001), laminin beta1 (12-fold, P < 0.001), collagen alpha1(IV) (17-fold, P < 0.001), collagen alpha1(III) (fourfold, P < 0.001), and MMP9 (twofold, P < 0.05) were observed after perfusion with TGF-beta1. Measurement of TIMP1, TIMP2, TIMP3, MMP1, and MMP2 mRNA demonstrated no detectable change, whereas determination of mRNA for tissue transglutaminase, an enzyme capable of cross-linking many ECM components, showed an eightfold increase (P < 0.01). This study suggests that in the IPRK and in the absence of other exogenous growth factors, TGF-beta1 selectively increases the synthesis of ECM and tissue transglutaminase without changes that would result in the reduction of ECM degradation.

The role of transglutaminase in the rat subtotal nephrectomy model of renal fibrosis

Tissue transglutaminase is a calcium-dependent enzyme that catalyzes the cross-linking of polypeptide chains, including those of extracellular matrix (ECM) proteins, through the formation of epsilon-(gamma-glutamyl) lysine bonds. This crosslinking leads to the formation of protein polymers that are highly resistant to degradation. As a consequence, the enzyme has been implicated in the deposition of ECM protein in fibrotic diseases such as pulmonary fibrosis and atherosclerosis. In this study, we have investigated the involvement of tissue transglutaminase in the development of kidney fibrosis in adult male Wistar rats submitted to subtotal nephrectomy (SNx). Groups of six rats were killed on days 7, 30, 90, and 120 after SNx. As previously described, these rats developed progressive glomerulosclerosis and tubulo-interstitial fibrosis. The tissue level of epsilon-(gamma-glutamyl) lysine cross-link (as determined by exhaustive proteolytic digestion followed by cation exchange chromatography) increased from 3.47+/- 0.94 (mean+/-SEM) in controls to 13.24+/-1.43 nmol/g protein 90 d after SNx, P </= 0.01. Levels of epsilon-(gamma-glutamyl) lysine cross-link correlated well with the renal fibrosis score throughout the 120 observation days (r = 0.78, P </= 0.01). Tissue homogenates showed no significant change in overall transglutaminase activity (14C putrescine incorporation assay) unless adjusted for the loss of viable tubule cells, when an increase from 5.77+/-0.35 to 13.93+/-4.21 U/mg DNA in cytosolic tissue transglutaminase activity was seen. This increase was supported by Western blot analysis, showing a parallel increase in renal tissue transglutaminase content. Immunohistochemistry demonstrated that this large increase in epsilon-(gamma-glutamyl) lysine cross-link and tissue transglutaminase took place predominantly in the cytoplasm of tubular cells, while immunofluorescence also showed low levels of the epsilon-(gamma-glutamyl) lysine cross-link in the extracellular renal interstitial space. The number of cells showing increases in tissue transglutaminase and its cross-link product, epsilon-(gamma-glutamyl) lysine appeared greater than those showing signs of typical apoptosis as determined by in situ end-labeling. This observed association between tissue transglutaminase, epsilon-(gamma-glutamyl) lysine cross-link, and renal tubulointerstitial scarring in rats submitted to SNx suggests that tissue transglutaminase may play an important role in the development of experimental renal fibrosis and the associated loss of tubule integrity.

Rabbit aorta and human atherosclerotic lesions hydrolyze the sphingomyelin of retained low-density lipoprotein. Proposed role for arterial-wall sphingomyelinase in subendothelial retention and aggregation of atherogenic lipoproteins

Aggregation and retention of LDL in the arterial wall are key events in atherogenesis, but the mechanisms in vivo are not known. Previous work from our laboratories has shown that exposure of LDL to bacterial sphingomyelinase (SMase) in vitro leads to the formation of LDL aggregates that can be retained by extracellular matrix and that are able to stimulate macrophage foam cell formation. We now provide evidence that retained LDL is hydrolyzed by an arterial-wall SMase activity. First, we demonstrated that SMase-induced aggregation is caused by an increase in particle ceramide content, even in the presence of excess sphingomyelin (SM). This finding is compatible with previous data showing that lesional LDL is enriched in SM, though its ceramide content has not previously been reported. To address this critical compositional issue, the ceramide content of lesional LDL was assayed and, remarkably, found to be 10-50-fold enriched compared with plasma LDL ceramide. Furthermore, the ceramide was found exclusively in lesional LDL that was aggregated; unaggregated lesional LDL, which accounted for 20-25% of the lesional material, remained ceramide poor. When [3H]SM-LDL was incubated with strips of rabbit aorta ex vivo, a portion of the LDL was retained, and the [3H]SM of this portion, but not that of unretained LDL, was hydrolyzed to [3H]ceramide by a nonlysosomal arterial hydrolase. In summary, LDL retained in atherosclerotic lesions is acted upon by an arterial-wall SMase, which may participate in LDL aggregation and possibly other SMase-mediated processes during atherogenesis.

Increased prevalence of coronary ectasia in heterozygous familial hypercholesterolemia

BACKGROUND

Although coronary atherosclerosis most commonly produces clinical effects as a result of stenosis, aneurysmal disease also occurs. We have found an increased prevalence of ectasia and aneurysmal disease in familial hypercholesterolemia (FH) suggesting a link between plasma lipoproteins and coronary aneurysms.

METHODS AND RESULTS

In 197 asymptomatic subjects with FH, we examined the prevalence of ectasia and its association with coronary risk factors. An ectatic segment was defined as one with a luminal diameter > 1.5 times that of the adjacent normal segment, excluding poststenotic dilation. Among subjects with FH, 15% had ectasia compared with 2.5% of an age- and sex-matched control group of 198 subjects without FH presenting for coronary angiography (P < .001). These control patients had significantly more severe coronary atherosclerosis than patients with FH. Ectasia was 3 times more common in men than women (P < .025). Neither age nor hypertension was predictive. Although in part reflecting the striking sex differential, ectasia was strongly associated with a lower HDL cholesterol level (P = .003), a higher LDL/HDL ratio (P = .003), and to a lesser extent, a higher LDL cholesterol level (P = .07). No association was found with plasma triglycerides or very low-density lipoprotein cholesterol levels. Among FH patients, ectasia was strongly associated with an overall index of occlusive atherosclerotic disease, based on quantitative angiography (P = .004). Intracoronary ultrasound interrogation of aneurysmal segments revealed circumferential intimal thickening.

CONCLUSIONS

Coronary ectasia is more prevalent in patients with FH than in other patients with coronary atherosclerosis and shows a strong inverse association with HDL cholesterol levels. This suggests that disordered lipoprotein metabolism in FH may predispose patients to aneurysmal coronary artery disease.

Increase in epsilon(gamma-glutamyl)lysine crosslinks in atherosclerotic aortas

Portions of aortas from normal and atherosclerotic rabbits and from human autopsy subjects were washed and separated into layers which were subjected to exhaustive proteolytic digestion. The digests were assayed for epsilon(gamma-glutamyl)lysine crosslinks by a two-stage high performance liquid chromatography (HPLC) procedure. Crosslink concentrations in intima-media from rabbits where more than 15% of the aorta lumen surface was lesioned are greater than in normal aortas or aortas with less than 15% of the surface lesioned. Higher crosslink concentrations occur in fibrolipid plaques from human aortas than in intima-media layers of equal thickness from non-lesioned areas of the same aortas. Much of the crosslink in fibrolipid plaques occurs in the proteins which float at d < 1.18 g/ml. Non-lesioned areas of intima-media from aortas with fatty streaks or plaques have higher crosslink concentrations than intima-media from aortas with no lesions. In normal and lesioned intimas thinner than 0.2 mm, the concentration of the crosslink is lower than in the subjacent media. These findings indicate that increased epsilon(gamma-glutamyl)lysine crosslinking occurs in the atherosclerotic aorta and is associated principally with smooth muscle cells. It is suggested that the crosslinked products may be involved in retention of lipoproteins and the increase in collagen production.

Increased epsilon(gamma-glutamyl)lysine crosslinking associated with increased protein synthesis in the inner layers of healing rat skin wounds

Three days after biopsy wounds were made in the dorsal skin of rats the animals were killed and explants of wounded and unwounded skin were incubated for 7 h with either [3H]glutamine or [3H]lysine. Both incubated and fresh control explants were then dissected into three layers which were homogenized, extracted, digested and then assayed for epsilon (gamma-glutamyl)lysine. The concentration of epsilon(gamma-glutamyl)lysine was greater in all three wounded layers than in the corresponding unwounded layers. The concentration in the wounded middle (dermal) layer and in the unwounded middle layer of younger rats was greater than in the unwounded outer (keratinized) layer, which has previously been shown to contain epsilon(gamma-glutamyl)lysine crosslinks. The incorporation of label from both [3H]glutamine and [3H]lysine into buffer-insoluble protein of the middle and inner (muscle) layers was much greater in the wounded explants than in the unwounded. Except for [3H]lysine in the inner layer there was also an increase in the fraction of incorporated label which was converted to epsilon(gamma-glutamyl)lysine. These results show that increased protein biosynthesis during repair in the wounded explants is associated with increased formation of epsilon(gamma-glutamyl)lysine. In addition, they indicate that the crosslink is involved in some process in the middle and inner layers which is distinct from its known function in keratinization of the epidermis.

Purification of porcine aminoterminal propeptide of type III procollagen from lymph and use for lymphatic clearance studies in pigs

To investigate the lymphatic transport of the aminoterminal propeptide of type III procollagen (PIIINP) we established a thoracic duct-venous shunt in 6 pigs. Porcine PIIINP was purified, characterised, and compared with human PIIINP to ensure the suitability of the radioimmunoassay of human PIIINP for measurements in pigs. SDS-PAGE and radioimmunoinhibition assays show human and porcine PIIINP to be similar, thus indicating that the assay of human PIIINP is also reliable for determinations on pig serum and lymph. Intact PIIINP, as identified by gel filtration, accounted for 60% and 40% of the total PIIINP immunoreactivity in lymph and serum, respectively. The higher amount of total immunoreactivity and proportion of intact PIIINP in lymph compared with serum support the hypothesis that intact PIIINP is transported from peripheral tissue into the circulation by lymph. Two days after the shunt was established, the lymph was collected quantitatively hour-by-hour for 24 h. The flow was higher during the light periods than in the dark (p less than 0.01). The PIIINP concentration varied inversely with the flow, being higher in the dark hours (p less than 0.03). However, the total collected amount of PIIINP in lymph did not differ during the light and dark periods. Serum PIIINP remained unchanged over the 24 h. The lymphatic clearance of total PIIINP immunoreactive components was 6.2 ml serum/min and the lymphatic clearance of intact PIIINP was 9.1 ml serum/min, equal to 7 and 10 times the plasma volume/24 h, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Irreversible inhibition of transglutaminases by sulfonium methylketones: optimization of specificity and potency with omega-aminoacyl spacers

Sulfonium methylketones, of structure Cbz-Phe-NH(CH2)nCOCH2S+ (CH3)2, n > 2, are specific and potent inactivators of transglutaminases. The length of the -(CH2)n-spacer moiety, n = 1-5, is a critical determinant for both the specificity and potency of the inactivator. The dipeptidyl analog Cbz-Phe-Gly-(CH2)nS+ (CH3)2, n = 1, is a more powerful inactivator of the thiol proteinase cathepsin B, k/K > 3 x 10(5) M-1 min-1, than of transglutaminases, ki(app)/Ki(app) < 1.5 x 10(4) M-1 min-1. In contrast, the gamma-aminobutyryl analog, n = 3, is a very potent transglutaminase inactivator with ki(app)/Ki(app) = 3.1 x 10(6) M-1 min-1, but does not inactivate cathepsin B. In cell studies, the gamma-aminobutyryl and epsilon-aminohexyl analogs inhibited the transglutaminase-mediated process of ionophore-induced cross-linked envelope formation by human malignant keratinocytes and the order of potency was related to that found for enzyme inhibition. The sulfonium methylketones, in equilibrium with the resonance stabilized ylides, are chemically inert towards glutathione under ambient conditions demonstrating the potential utility of this novel class of transglutaminase inhibitors for the study of enzyme inhibition in cellular environments.

Hepatic extraction of the aminoterminal propeptide of type III procollagen before and after bile duct ligation in pigs

The aminoterminal propeptide of type III procollagen is extracted from the circulation by the liver, and PIIINP is found in bile. This study was performed in order to investigate whether biliary excretion contributes substantially to the hepatic extraction of circulating PIIINP. Hepatic extraction before and during a 4-h period after ligation of the common bile duct was assessed from serum PIIINP concentrations in a systemic artery, the portal vein and a hepatic vein of seven healthy anaesthetized pigs. Seven sham-operated anaesthetized pigs served as controls. Ligation of the bile duct did not cause a decrease in the hepatic extraction ratio of circulating PIIINP. The PIIINP serum levels of the cholestatic pigs and of the controls were similar throughout the investigation period. The PIIINP concentrations in bile were only 10% of the corresponding serum values. Gel filtration of sera showed that the lower PIIINP concentration in the hepatic vein, as compared to the artery and the portal vein was due to a selective decrease in the concentration of the intact propeptide. The study shows that biliary excretion does not contribute significantly to the hepatic extraction of circulating PIIINP in the normal liver. Furthermore, the hepatic extraction of circulating PIIINP preferentially affects the intact propeptide, rather than the somewhat larger PIIINP related molecule in serum.

Cross-linked A alpha.gamma chain hybrids serve as unique markers for fibrinogen polymerized by tissue transglutaminase

Notwithstanding the high degree of amino acid sequence homologies between human factor XIIIa on the one hand and intracellular transglutaminases (protein-glutamine:amine gamma-glutamyltransferase, EC 2.3.2.13) from guinea pig liver or human erythrocytes on the other, we find that the two sets of enzymes differ remarkably in the mode of cross-linking the same protein substrate--i.e., human fibrinogen. In the program of polymerization with factor XIIIa, production of the known gamma-gamma' homologous chain pairs is the dominant feature, whereas with either intracellular transglutaminase, a series of hitherto unidentified A alpha.gamma hybrid chain combinations, designated A alpha p gamma q (p and q = 1, 2, 3...), is generated and practically no gamma-gamma' dimers are formed. Two-dimensional electrophoresis is particularly useful for demonstrating the production of A alpha p gamma q structures by protein staining as well as by immunoblotting against specific antibodies to the A alpha and gamma chains of fibrinogen. These findings should aid in deciding whether the direct cross-linking of fibrinogen by transglutaminase might contribute to thrombotic processes in addition to the thrombin- and factor XIIIa-dependent pathway of clot formation.

Lipoprotein binding of crosslinked type III collagen aminopropeptide and fractions of its antigen in blood

When [125I] labelled bovine type III collagen aminopropeptide (PIIIP) is incubated with tissue transglutaminase (TGase) mixed with hyperlipemic rabbit plasma and subjected to ultracentrifugation the labelled fraction with density less than 1.2 g/ml is larger than when either lipoprotein or TGase is omitted. Chromatography of the fraction with density less than 1.2 g/ml shows the presence of peaks which are not present in the denser material. Since their elution positions indicate that they have higher molecular weights than PIIIP it is concluded that they consist of [125I]PIIIP which had been crosslinked by TGase and bound to lipoprotein. Low concentrations of similar low density, high molecular weight PIIIP antigens were found in normal human plasma and pooled sera from angiography subjects. In two out of seven infarct patients an unusually large fraction of the PIIIP antigen in the serum was found in a very high molecular weight peak containing low density material. It is speculated that this may arise from atherosclerotic lesions.