Endoproteolytic activation of alpha(v) integrin by proprotein convertase PC5 is required for vascular smooth muscle cell adhesion to vitronectin and integrin-dependent signaling

Proprotein convertases regulate trafficking and maturation of key proteins within the secretory pathway

Proprotein Convertases (PCs) are serine endoproteases that regulate the homeostasis of protein substrates in the cell. The PCs family counts 9 members-PC1/3, PC2, PC4, PACE4, PC5/6, PC7, Furin, SKI-1/S1P, and PCSK9. The first seven PCs are known as Basic Proprotein Convertases due to their propensity to cleave after polybasic clusters. SKI-1/S1P requires the additional presence of hydrophobic residues for processing, whereas PCSK9 is catalytically dead after autoactivation and exerts its functions using mechanisms alternative to direct cleavage. All PCs traffic through the canonical secretory pathway, reaching different compartments where the various substrates reside. Despite PCs members do not share the same subcellular localization, most of the cellular organelles count one or more Proprotein Convertases, including ER, Golgi stack, endosomes, secretory granules, and plasma membranes. The widespread expression of these enzymes at the systemic level speaks for their importance in the homeostasis of a large number of biological functions. Among others, PCs cleave precursors of hormones and growth factors and activate receptors and transcription factors. Notably, dysregulation of the enzymatic activity of Proprotein Convertases is associated to major human pathologies, such as cardiovascular diseases, cancer, diabetes, infections, inflammation, autoimmunity diseases, and Parkinson. In the current COVID-19 pandemic, Furin has further attracted the attention as a key player for conferring high pathogenicity to SARS-CoV-2. Here, we review the Proprotein Convertases family and their most important substrates along the secretory pathway. Knowledge about the complex functions of PCs is important to identify potential drug strategies targeting this class of enzymes.

Up-regulation of PCSK6 by lipid oxidation products: A possible role in atherosclerosis

Atherosclerosis is a degenerative disease characterized by lesions that develop in the wall of large- and medium-sized arteries due to the accumulation of low-density lipoproteins (LDLs) in the intima. A growing bulk of evidence suggests that cholesterol oxidation products, known as oxysterols, and the aldehyde 4-hydroxy-2-nonenal (HNE), the major pro-atherogenic components of oxidized LDLs, significantly contribute to atherosclerotic plaque progression and destabilization, with eventual plaque rupture. The involvement of certain members of the protein convertase subtilisin/kexin proteases (PCSKs) in atherosclerosis has been recently hypothesized. Among them, PCSK6 has been associated with plaque instability, mainly thanks to its ability to stimulate the activity of matrix metalloproteinases (MMPs) involved in extracellular matrix remodeling and to enhance inflammation. In U937 promonocytic cells and in human umbilical vein endothelial cells, an oxysterol mixture and HNE were able to up-regulate the level and activity of PCSK6, resulting in MMP-9 activation as demonstrated by PCSK6 silencing. Inflammation, enhanced by these lipid oxidation products, plays a key role in the up-regulation of PCSK6 activity as demonstrated by cell pretreatment with NS-398, with epigallocatechin gallate or with acetylsalicylic acid, all with anti-inflammatory effects. For the first time, we demonstrated that both oxysterols and HNE, which substantially accumulate in the atherosclerotic plaque, up-regulate the activity of PCSK6. Of note, we also suggest a potential association between PCSK6 activity and MMP-9 activation, pointing out that PCSK6 could contribute to atherosclerotic plaque development.

Selective inhibition of N-linked glycosylation impairs receptor tyrosine kinase processing

Global inhibition of N-linked glycosylation broadly reduces glycan occupancy on glycoproteins, but identifying how this inhibition functionally impacts specific glycoproteins is challenging. This limits our understanding of pathogenesis in the congenital disorders of glycosylation (CDG). We used selective exo-enzymatic labeling of cells deficient in the two catalytic subunits of oligosaccharyltransferase - STT3A and STT3B - to monitor the presence and glycosylation status of cell surface glycoproteins. We show reduced abundance of two canonical tyrosine receptor kinases - the insulin receptor and insulin-like growth factor 1 receptor (IGF-1R) - at the cell surface in STT3A-null cells, due to decreased N-linked glycan site occupancy and proteolytic processing in combination with increased endoplasmic reticulum localization. Providing cDNA for Golgi-resident proprotein convertase subtilisin/kexin type 5a (PCSK5a) and furin cDNA to wild-type and mutant cells produced under-glycosylated forms of PCSK5a, but not furin, in cells lacking STT3A. Reduced glycosylation of PCSK5a in STT3A-null cells or cells treated with the oligosaccharyltransferase inhibitor NGI-1 corresponded with failure to rescue receptor processing, implying that alterations in the glycosylation of this convertase have functional consequences. Collectively, our findings show that STT3A-dependent inhibition of N-linked glycosylation on receptor tyrosine kinases and their convertases combines to impair receptor processing and surface localization. These results provide new insight into CDG pathogenesis and highlight how the surface abundance of some glycoproteins can be dually impacted by abnormal glycosylation.

Proprotein convertases in atherogenesis

PURPOSE OF REVIEW

The proprotein convertases subtilisin/kexin (PCSKs) are endoproteases identified as activators of precursors from hormones and peptides. On the basis of the variety of substrates and regulation in disease, they have been recognized as mediators in atherogenesis. The discovery of PCSK9, which regulates low-density lipoprotein receptor cell membrane availability, has led to a resurgence of interest in these enzymes and their function in cardiovascular diseases.

RECENT FINDINGS

Recent data demonstrate that PCSKs are expressed in human atheroma and are regulated in animal models of atherosclerosis. In animal models, inhibition of PCSKs, such as PCSK3, affects cell proliferation and migration as well as inflammation, reducing atherosclerosis. In addition, targeting PCSK9 lowers cholesterol levels and has now been demonstrated to lessen vascular lesion formation in mice. Experimentally investigated novel anti-PCSK9 strategies include genome editing and vaccination. Furthermore, studies show that PCSKs contribute to the initiation and progression of cardiometabolic risk factors, such as insulin resistance and obesity.

SUMMARY

PCSKs affect cardiovascular diseases on multiple levels, including atherosclerotic lesion formation as well as their contribution to cardiometabolic risk factors. PCSK9 is a key regulator of plasma cholesterol levels, thereby potentially affecting atherosclerosis and has rapidly emerged as a pharmacological target.

My road to Damascus: how I converted to the prohormone theory and the proprotein convertases

My desire as a young endocrinologist to improve my clinical skills through a better knowledge of hormone chemistry led me to serendipitous discoveries and unexpected horizons. The first discovery, published in 1967, revealed that peptide hormones are derived from endoproteolytic cleavages of larger precursor polypeptides. It was the foundation of the prohormone theory. Initially thought to apply to a few hormones, the theory rapidly extended to many proteins, including neuropeptides, neurotrophins, growth and transcription factors, receptors, extracellular matrix proteins, bacterial toxins, and viral glycoproteins. Its endoproteolytic activation mechanism has become a fundamental cellular process, affecting many biological functions. It implied the existence of specific endoproteolytic enzymes. These proprotein convertases were discovered in 1990. They have been shown to play a wide range of important roles in health and disease. They have opened up novel therapeutic avenues. Inactivation of PCSK9 to reduce plasma cholesterol is currently the most promising. To make this good thing even better, I recently discovered in a French Canadian family a potent PCSK9 (Gln152His) mutation that significantly lowers plasma cholesterol and should confer cardiovascular longevity. The discovery helped me to complete the loop: "From the bedside to the bench and back to the bedside."

Embryo implantation is closely associated with dynamic expression of proprotein convertase 5/6 in the rabbit uterus

BACKGROUND

Proprotein convertase 5/6 (PC5/6) is critical for embryo implantation in women, regulating both uterine epithelial receptivity and stromal cell decidualization. PC5/6 is likewise essential for implantation in mice, but involved only in decidualization. An alternative animal model is required to address the function of PC5/6 in the uterine epithelium. This study aimed to establish whether PC5/6 is associated with embryo implantation in rabbits.

METHODS

Virgin New-Zealand white rabbits aged 3-4 moths were mated with males of the same strain, or pseudo-pregnancy induced. After mating, uterine tissues were collected over a 10 day (d) period (n = 3 per time point) for RNA, protein and histological analyses to determine the temporal and spatial uterine expression pattern of PC5/6 during the initial stages of pregnancy or induced pseudo-pregnancy.

RESULTS

PC5/6 mRNA was up-regulated just prior to embryo attachment on d6, and the elevated expression was maintained throughout implantation on d6.5-10. Western analysis revealed a preferential up-regulation of PC5/6 in the implantation sites. Immunohistochemical analysis identified that both the amount and cellular localization of PC5/6 changed with increasing pregnancy stages. Before embryo attachment, PC5/6 was low and localised in the luminal and glandular epithelium. It increased on d6.5 in the basal glands and mucosal folds, and then strongly intensified on d7-10 in the multinucleated luminal symplasma and decidual cells at the site of embryo implantation. In contrast, the pseudo-pregnant uterus displayed relatively low and static PC6 mRNA expression throughout the 10 days, with no obvious changes in either PC5/6 level or cellular localization.

CONCLUSIONS

These findings demonstrate that embryo implantation in the rabbit is closely associated with dynamic expression of uterine PC5/6, and that the rabbit may be an appropriate model to investigate the function of PC5/6 in the uterine epithelium during embryo attachment.

The molecular biology of furin-like proprotein convertases in vascular remodelling

Vascular smooth muscle cell (VSMC) proliferation and migration represent key features in atherosclerosis and restenosis. The proprotein convertases (PCs) furin and PC5 are highly expressed in human atheroma and are putatively involved in vascular lesion formation via the activation of precursor proteins, essential for cell proliferation and migration. In vitro assays have identified these PCs to govern cell functions via endoproteolytic cleavage of key substrates, including pro-integrins and pro-matrix metalloproteinases. In vivo gene expression studies of furin/PC5 and their substrates demonstrate their coordinated regulation in animal models of vascular remodelling and in human atherosclerotic lesions. Here we describe in vitro and in vivo models to investigate the function of furin/PC5 in VSMCs and in vascular lesion formation. In conjunction with the development of novel PC inhibitors, this should facilitate the development of new strategies targeting PCs in cardiovascular disease.

Experimental intrauterine growth restriction induces alterations in DNA methylation and gene expression in pancreatic islets of rats

Intrauterine growth restriction (IUGR) increases susceptibility to age-related diseases, including type 2 diabetes (T2DM), and is associated with permanent and progressive changes in gene expression. Our study was designed to test whether epigenomic dysregulation mediates the cellular memory of this intrauterine event. To test this hypothesis, we isolated pancreatic islets from control and IUGR (induced by bilateral uterine artery ligation at day 18 of fetal life) animals at 7 weeks of age. Using the HELP (HpaII tiny fragment enrichment by ligation-mediated PCR) assay, we generated the first DNA methylation map at almost 1 million unique sites throughout the rat genome in normal pancreatic islet cells, allowing us to identify the changes that occur as a consequence of IUGR. We validated candidate dysregulated loci with quantitative assays of cytosine methylation and gene expression. IUGR changes cytosine methylation at approximately 1,400 loci (false discovery rate of 4.2%) in male rats at 7 weeks of age, preceding the development of diabetes and thus representing candidate loci for mediating the pathogenesis of metabolic disease that occurs later in life. Epigenetic dysregulation occurred preferentially at conserved intergenic sequences, frequently near genes regulating processes known to be abnormal in IUGR islets, such as vascularization, beta-cell proliferation, insulin secretion, and cell death, associated with concordant changes in mRNA expression. These results demonstrate that epigenetic dysregulation is a strong candidate for propagating the cellular memory of intrauterine events, causing changes in expression of nearby genes and long term susceptibility to type 2 diabetes.

Simultaneous transforming growth factor beta-tumor necrosis factor activation and cross-talk cause aberrant remodeling response and myocardial fibrosis in Timp3-deficient heart

The pleiotropic cytokines, transforming growth factor beta1 (TGFbeta1), and tumor necrosis factor (TNF) play critical roles in tissue homeostasis in response to injury and are implicated in multiple human diseases and cancer. We reported that the loss of Timp3 (tissue inhibitor of metalloproteinase 3) leads to abnormal TNF signaling and cardiovascular function. Here we show that parallel deregulation of TGFbeta1 and TNF signaling in Timp3(-/-) mice amplifies their cross-talk at the onset of cardiac response to mechanical stress (pressure overload), resulting in fibrosis and early heart failure. Microarray analysis showed a distinct gene expression profile in Timp3(-/-) hearts, highlighting activation of TGFbeta1 signaling as a potential mechanism underlying fibrosis. Neonatal cardiomyocyte-cardiofibroblast co-cultures were established to measure fibrogenic response to agonists known to be induced following mechanical stress in vivo. A stronger response occurred in neonatal Timp3(-/-) co-cultures, as determined by increased Smad signaling and collagen expression, due to increased TNF processing and precocious proteolytic maturation of TGFbeta1 to its active form. The relationship between TGFbeta1 and TNF was dissected using genetic and pharmacological manipulations. Timp3(-/-)/Tnf(-/-) mice had lower TGFbeta1 than Timp3(-/-), and anti-TGFbeta1 antibody (1D11) negated the abnormal TNF response, indicating their reciprocal stimulatory effects, with each manipulation abolishing fibrosis and improving heart function. Thus, TIMP3 is a common innate regulator of TGFbeta1 and TNF in tissue response to injury. The matrix-bound TIMP3 balances the anti-inflammatory and proinflammatory processes toward constructive tissue remodeling.

ED-B fibronectin (ED-B) can be targeted using a novel single chain antibody conjugate and is associated with macrophage accumulation in atherosclerotic lesions

It has been shown that ED-B fibronectin (ED-B) is a potential target for plaque imaging. The aim of this study was to test a novel modified single chain anti-ED-B antibody (scFv) conjugated for near infrared fluorescence imaging (NIRF) with tetrasulfonated carbocyanine-maleimide (TSC-scFv) and to examine the association of ED-B with the presence of macrophages in a murine model of atherosclerosis. Expression of ED-B was observed in plaque areas in apolipoprotein E-deficient (apoE(-/-)) mice which increased with age and plaque load. Robust imaging was possible after explantation of the aorta and demonstrated a strong NIRF signal intensity in focal aortic and brachiocephalic plaque lesions, whereas no signals were found in undiseased areas. Plaque lesion ED-B was expressed by smooth muscle cell and was closely associated to macrophage infiltrates. Although not expressed by the same cell type, there was a significant correlation (p<0.01) between ED-B and macrophage immunoreactivity. In vitro human coronary and mouse smooth muscle cells significantly increased ED-B expression after angiotensin II and TNF-alpha treatment. This study demonstrates that plaque NIRF imaging is feasible with a novel single chain antibody and that ED-B expression is closely associated with inflammation in experimental atherosclerosis.

The proprotein convertases are potential targets in the treatment of dyslipidemia

The family of the secretory proprotein convertases (PCs) comprises seven basic amino acid (aa)-specific subtilisin-like serine proteinases known as PC1/3, PC2, furin, PC4, PC5/6, PACE4 and PC7, and two other PCs, SKI-1 (subtilisin-kexin isozyme-1)/S1P (site-1 protease) and PCSK9 (proprotein convertase subtilisin kexin 9) that cleave at nonbasic residues. Except for the testicular PC4, all the other convertases are expressed in brain and peripheral organs and play a critical role in various functions including the production of diverse neuropeptides as well as growth factors and receptors, the regulation of cellular adhesion/migration, cholesterol and fatty acid homeostasis, and growth/differentiation of progenitor cells. Some of these convertases process proteins that are implicated in pathologies, including cancer malignancies, tissue regeneration, and viral infections. The implication of some of these convertases in sterol/lipid metabolism has only recently been appreciated. SKI-1/S1P activates the synthesis of cholesterol and fatty acids as well as the LDL receptor (LDLR), whereas PCSK9 inactivates the LDLR. Moreover, furin, PC5 and/or, PACE4 inactivates endothelial and lipoprotein lipases. Humans and mice exhibiting either a gain or loss of function of PCSK9 through specific point mutations or knockouts develop hypercholesterolemia and hypocholesterolemia phenotypes, respectively. A PCSK9 inhibitor in combination with statins offers a most promising therapeutic target to treat cardiovascular disorders including dyslipidemias. Specific inhibitors/modulators of the other PCs should find novel therapeutic applications in the control of PC-regulated pathologies.

Holland N, Escriva H. Phylogenetic analysis of Amphioxus genes of the proprotein convertase family, including aPC6C, a marker of epithelial fusions during embryology

The proprotein convertases (PCs) comprise a family of subtilisin-like endoproteases that activate precursor proteins (including, prohormones, growth factors, and adhesion molecules) during their transit through secretory pathways or at the cell surface. To explore the evolution of the PC gene family in chordates, we made a phylogenetic analysis of PC genes found in databases, with special attention to three PC genes of the cephalochordate amphioxus, the closest living invertebrate relative to the vertebrates. Since some vertebrate PC genes are essential for early development, we investigated the expression pattern of the C isoform of the amphioxus PC6 gene (aPC6C). In amphioxus embryos and larvae, aPC6C is expressed at places where epithelia fuse. Several kinds of fusions occur: ectoderm-to-ectoderm during neurulation; mesoderm-to-ectoderm during formation of the preoral ciliated pit; and endoderm-to-ectoderm during formation of the mouth, pharyngeal slits, anus, and external opening of the club-shaped gland. Presumably, at all these sites, aPC6C is activating proteins favoring association between previously disjunct cell populations.

PACE4 expression in mouse basal keratinocytes results in basement membrane disruption and acceleration of tumor progression

Collagen type IV degradation results in disruption and breakdown of the normal basement membrane architecture, a key process in the initiation of tumor microinvasion into the connective tissue. PACE4, a proprotein convertase, activates membrane type matrix metalloproteinases (MT-MMPs) that in turn process collagenase type IV. Because PACE4 is overexpressed in skin carcinomas and in vitro overexpression of PACE4 resulted in enhanced invasiveness, we investigated whether or not in vivo PACE4 expression leads to the acquisition of invasiveness and increased tumorigenesis. Two transgenic mouse lines were designed by targeting PACE4 to the epidermal basal keratinocytes. Transgenic keratinocytes showed increased processing of MT1-MMP and MT2-MMP resulting in collagenase IV activation and collagen type IV degradation. Higher collagenolytic activity partially disrupted normal basement membrane architecture favoring epithelial endophytic growth into the dermis and accelerating invasion and metastasis after chemical carcinogenesis. PACE4 overexpression resulted in enhanced susceptibility to carcinogenesis and tumor progression pointing to a new target for blocking tumor cell invasiveness.

Proprotein convertases furin and PC5: targeting atherosclerosis and restenosis at multiple levels

Several growth factors, chemokines, adhesion molecules, and proteolytic enzymes important for cell-cell/cell-matrix interactions in atherosclerosis and restenosis are initially synthesized as inactive precursor proteins. Activation of proproteins to biologically active molecules is regulated by limited endoproteolytic cleavage at dibasic amino acid residues. This type of activation typically requires the presence of suitable proprotein convertases (PCs). The PC-isozymes furin and PC5 are expressed in human atherosclerotic lesions and have been found to be up-regulated, following vascular injury in animal models in vivo. In vitro, these PCs can regulate vascular smooth muscle cell and macrophage functions and signaling events, through activation of pro-alpha-integrins and/or pro-membrane-type matrix metalloproteinases. Integrins link the cytoskeleton with the extracellular matrix and mediate bidirectional signaling and mechanotransduction, whereas matrix metalloproteinases are the major matrix-degrading enzymes. Both activities are required for cell recruitment to the intima. Furthermore, cleavage of extracellular matrix molecules by matrix metalloproteinases potentially contributes to weakening of the fibrous cap, promoting plaque rupture. Based on these recent in vitro and in vivo data, furin and PC5 are potential contributors to the initiation, progression, and complications of atherosclerosis and restenosis. Targeting these PCs may provide future anti-atherosclerotic therapies.

Screening of hepatocyte proteins binding to F protein of hepatitis C virus by yeast two-hybrid system

AIM: To investigate the biological function of F protein by yeast two-hybrid system.

METHODS: We constructed F protein bait plasmid by cloning the gene of F protein into pGBKT7, then recombinant plasmid DNA was transformed into yeast AH109 (a type). The transformed yeast AH109 was mated with yeast Y187 (α type) containing liver cDNA library plasmid in 2×YPDA medium. Diploid yeast was plated on synthetic dropout nutrient medium (SD/-Trp-Leu-His-Ade) containing X-α-gal for selection and screening. After extracting and sequencing plasmids from positive (blue) colonies, we underwent sequence analysis by bioinformatics.

RESULTS: Thirty-six colonies were selected and sequenced. Among them, 11 colonies were zymogen granule protein, 5 colonies were zinc finger protein, 4 colonies were zinc-α-2-glycoprotein, 1 colony was sialyltransferase, 1 colony was complement control protein factor I, 1 colony was vitronectin, and 2 colonies were new genes with unknown function.

CONCLUSION: The yeast two-hybrid system is an effective method for identifying hepatocyte proteins interacting with F protein of hepatitis C virus. F protein may bind to different proteins.

The P2Y2 nucleotide receptor interacts with alphav integrins to activate Go and induce cell migration

Extracellular ATP and UTP induce chemotaxis, or directed cell migration, by stimulating the G protein-coupled P2Y(2) nucleotide receptor (P2Y(2)R). Previously, we found that an arginine-glycine-aspartic acid (RGD) integrin binding domain in the P2Y(2)R enables this receptor to interact selectively with alpha(v)beta(3) and alpha(V)beta(5) integrins, an interaction that is prevented by mutation of the RGD sequence to arginine-glycine-glutamic acid (RGE) (Erb, L., Liu, J., Ockerhausen, J., Kong, Q., Garrad, R. C., Griffin, K., Neal, C., Krugh, B., Santiago-Perez, L. I., Gonzalez, F. A., Gresham, H. D., Turner, J. T., and Weisman, G. A. (2001) J. Cell Biol. 153, 491-501). This RGD domain also was found to be necessary for coupling the P2Y(2)R to G(o)- but not G(q)-mediated intracellular calcium mobilization, leading us to investigate the role of P2Y(2)R interaction with integrins in nucleotide-induced chemotaxis. Here we show that mutation of the RGD sequence to RGE in the human P2Y(2)R expressed in 1321N1 astrocytoma cells completely prevented UTP-induced chemotaxis as well as activation of G(o), Rac, and Vav2, a guanine nucleotide exchange factor for Rac. UTP also increased expression of vitronectin, an extracellular matrix protein that is a ligand for alpha(v)beta(3)/beta(5) integrins, in cells expressing the wild-type but not the RGE mutant P2Y(2)R. P2Y(2)R-mediated chemotaxis, Rac and Vav2 activation, and vitronectin up-regulation were inhibited by pretreatment of the cells with anti-alpha(v)beta(5) integrin antibodies, alpha(v) integrin antisense oligonucleotides, or the G(i/o) inhibitor, pertussis toxin. Thus, the RGD-dependent interaction between the P2Y(2)R and alpha(v) integrins is necessary for the P2Y(2)R to activate G(o) and to initiate G(o)-mediated signaling events leading to chemotaxis.

The cysteine-rich domain of the secreted proprotein convertases PC5A and PACE4 functions as a cell surface anchor and interacts with tissue inhibitors of metalloproteinases

The proprotein convertases PC5, PACE4 and furin contain a C-terminal cysteine-rich domain (CRD) of unknown function. We demonstrate that the CRD confers to PC5A and PACE4 properties to bind tissue inhibitors of metalloproteinases (TIMPs) and the cell surface. Confocal microscopy and biochemical analyses revealed that the CRD is essential for cell surface tethering of PC5A and PACE4 and that it colocalizes and coimmunoprecipitates with the full-length and C-terminal domain of TIMP-2. Surface-bound PC5A in TIMP-2 null fibroblasts was only observed upon coexpression with TIMP-2. In COS-1 cells, plasma membrane-associated PC5A can be displaced by heparin, suramin, or heparinases I and III and by competition with excess exogenous TIMP-2. Furthermore, PC5A and TIMP-2 are shown to be colocalized over the surface of enterocytes in the mouse duodenum and jejunum, as well as in liver sinusoids. In conclusion, the CRD of PC5A and PACE4 functions as a cell surface anchor favoring the processing of their cognate surface-anchored substrates, including endothelial lipase.

Furin-Like Proprotein Convertases Are Central Regulators of the Membrane Type Matrix Metalloproteinase–Pro-Matrix Metalloproteinase-2 Proteolytic Cascade in Atherosclerosis

BACKGROUND

Accumulation of macrophages and their in situ expression of matrix metalloproteinases (MMPs) are important determinants of plaque stability. Activation of membrane-bound MT1-MMP, the major activator of pro-MMP-2, requires intracellular endoproteolytic cleavage of its precursor protein. This type of activation typically requires suitable furin-like proprotein convertases (PCs), specifically furin and PC5. The present study was done to investigate the function of MT1-MMP as well as furin-like PCs in mononuclear inflammatory cells.

METHODS AND RESULTS

Macrophage differentiation of human monocytic THP-1 cells was accompanied by increased expression of furin, PC5, and MT1-MMP. Some pro-MMP-2 activation was found in macrophages, but pro-MMP-2 level or activation was not enhanced after stimulation with the proinflammatory mediators tumor necrosis factor-alpha or lipopolysaccharide. However, culturing of macrophages in conditioned medium from serum-starved vascular smooth muscle cells, which constitutively secrete pro-MMP-2, resulted in a strong pro-MMP-2 activation. Inhibition of furin-like PCs with the specific pharmacological inhibitor decanoyl-RVKR-chloromethylketone (dec-CMK) inhibited MT1-MMP activation in macrophages. Dec-CMK or furin-specific small interfering RNA significantly inhibited macrophage MT1-MMP-dependent activation of vascular smooth muscle cell-derived pro-MMP-2. Flow cytometry demonstrated that human circulating monocytes express furin and PC5, and MT1-MMP and immunohistochemistry revealed their colocalization in macrophages in advanced human atherosclerotic lesions.

CONCLUSIONS

Furin-like PCs (furin and PC5) play a central role in a MT-MMP-MMP-2 proteolytic cascade, involving provision of macrophage MT1-MMP for the activation of pro-MMP-2 synthesized by other cells. Furin and PC5 are expressed in human peripheral blood mononuclear cells and colocalize with MT1-MMP in macrophages in the atherosclerotic plaque, supporting the hypothesis that they are potential targets in atherosclerosis.

Immunohistochemical localization of subtilisin/kexin-like proprotein convertases in human atherosclerosis

Integrins are heterodimeric alpha/beta receptors that link the cytoskeleton with the extracellular matrix, thereby regulating several cell functions important in atherosclerosis. In vitro, the subtilisin/kexin-like proprotein convertases (PCs), namely PC5 and furin, have been shown to be responsible for the endoproteolytic activation of the alpha(v) integrin subunit. Based on their cleavage activity, these PCs are potential targets in atherosclerosis. In the present study, we investigated the localization of furin and PC5 in different stages of human atherosclerosis. Immunohistochemical analysis of furin and PC5 revealed their presence in vascular smooth-muscle cells and endothelial cells in atherosclerotic and non-atherosclerotic lesions. However, in the more advanced lesions, furin and PC5 staining was significantly expressed in macrophages/foam cells. In vitro, THP-1 derived macrophages contained furin and PC5, and maturation of monocytes to macrophages was accompanied by enhanced alpha(v)beta3 cell-surface expression. Inhibition of furin/PC5 with the specific pharmacological furin-like PC-inhibitor dec-CMK inhibited alpha(v) endoproteolytic activation but did not abolish alpha(v)beta3 cell-surface expression. This indicates that furin/PC5 is required for alpha(v) endoproteolytic activation but not for alpha(v) routing and sorting to the cell surface. In conclusion, our study demonstrates that furin and PC5 are significantly expressed in mononuclear cells in advanced human atherosclerotic lesions, where they regulate alpha(v) endoproteolytic activation.

Dual role of matrix metalloproteinases (matrixins) in intimal thickening and atherosclerotic plaque rupture

Intimal thickening, the accumulation of cells and extracellular matrix within the inner vessel wall, is a physiological response to mechanical injury, increased wall stress, or chemical insult (e.g., atherosclerosis). If excessive, it can lead to the obstruction of blood flow and tissue ischemia. Together with expansive or constrictive remodeling, the extent of intimal expansion determines final lumen size and vessel wall thickness. Plaque rupture represents a failure of intimal remodeling, where the fibrous cap overlying an atheromatous core of lipid undergoes catastrophic mechanical breakdown. Plaque rupture promotes coronary thrombosis and myocardial infarction, the most prevalent cause of premature death in advanced societies. The matrix metalloproteinases (MMPs) can act together to degrade the major components of the vascular extracellular matrix. All cells present in the normal and diseased blood vessel wall upregulate and activate MMPs in a multistep fashion driven in part by soluble cytokines and cell-cell interactions. Activation of MMP proforms requires other MMPs or other classes of protease. MMP activation contributes to intimal growth and vessel wall remodeling in response to injury, most notably by promoting migration of vascular smooth muscle cells. A broader spectrum and/or higher level of MMP activation, especially associated with inflammation, could contribute to pathological matrix destruction and plaque rupture. Inhibiting the activity of specific MMPs or preventing their upregulation could ameliorate intimal thickening and prevent myocardial infarction.

XPACE4 is a localized pro-protein convertase required for mesoderm induction and the cleavage of specific TGFbeta proteins in Xenopus development

XPACE4 is a member of the subtilisin/kexin family of pro-protein convertases. It cleaves many pro-proteins to release their active proteins, including members of the TGFbeta family of signaling molecules. Studies in mouse suggest it may have important roles in regulating embryonic tissue specification. Here, we examine the role of XPACE4 in Xenopus development and make three novel observations: first, XPACE4 is stored as maternal mRNA localized to the mitochondrial cloud and vegetal hemisphere of the oocyte; second, it is required for the endogenous mesoderm inducing activity of vegetal cells before gastrulation; and third, it has substrate-specific activity, cleaving Xnr1, Xnr2, Xnr3 and Vg1, but not Xnr5, Derriere or ActivinB pro-proteins. We conclude that maternal XPACE4 plays an important role in embryonic patterning by regulating the production of a subset of active mature TGFbeta proteins in specific sites.

Proprotein convertases regulate insulin-like growth factor 1-induced membrane-type 1 matrix metalloproteinase in VSMCs via endoproteolytic activation of the insulin-like growth factor-1 receptor

The IGF-1 receptor (IGF-1R) and MT1-MMP are synthesized as larger precursor proproteins, which require endoproteolytic activation by the proprotein convertases (PCs) furin/PC5 to gain full biological activity. The aim of this study was to investigate the contribution of PCs to IGF-1R and/or MT1-MMP activation in vascular smooth muscle cells (VSMCs) as well as VSMC proliferation/migration, which are key elements in vascular remodeling. Furin and PC5 mRNAs and proteins were found in VSMCs. Inhibition of furin-like PCs with the specific pharmacological inhibitor dec-CMK inhibited IGF-1R endoproteolytic activation. Inhibition of IGF-1R maturation abrogated IGF-induced IGF-1R autophosphorylation, PI3-kinase and MAPK induction, as well as VSMC proliferation (p<0.05 vs. controls), whereas it had no effect of PDGF-stimulated signaling pathways or cell growth. Both, IGF-1 and PDGF-BB, induced MT1-MMP expression, but only IGF-1-mediated MT1-MMP induction was inhibited by dec-CMK. Induction of MMP-2 by IGF-1 was inhibited by the PI3-kinase inhibitor wortmannin, but not by the MEK-inhibitor PD98059. Dec-CMK inhibited VSMC chemotaxis comparable to the effects of the MMP-inhibitor GM6001 (both p<0.05 vs. controls), supporting that MMPs are involved. In conclusion, this study demonstrates that targeting furin-like PCs and thus inhibiting IGF-1R activation is a novel target to inhibit IGF-1-mediated signaling and cell functions, such as IGF-1-induced MT1-MMP/MMP-2 in VSMCs.

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