The low density lipoprotein receptor-related protein mediates fibronectin catabolism and inhibits fibronectin accumulation on cell surfaces

Cell-type-specific regulation of APOE levels in human neurons by the Alzheimer's disease risk gene SORL1

SORL1 is strongly implicated in the pathogenesis of Alzheimer's disease (AD) through human genetic studies that point to an association of reduced SORL1 levels with higher risk for AD. To interrogate the role(s) of SORL1 in human brain cells, SORL1 null iPSCs were generated, followed by differentiation to neuron, astrocyte, microglia, and endothelial cell fates. Loss of SORL1 led to alterations in both overlapping and distinct pathways across cell types, with the greatest effects in neurons and astrocytes. Intriguingly, SORL1 loss led to a dramatic neuron-specific reduction in APOE levels. Further, analyses of iPSCs derived from a human aging cohort revealed a neuron-specific linear correlation between SORL1 and APOE RNA and protein levels, a finding validated in human post-mortem brain. Pathway analysis implicated intracellular transport pathways and TGF- β/SMAD signaling in the function of SORL1 in neurons. In accord, enhancement of retromer-mediated trafficking and autophagy rescued elevated phospho-tau observed in SORL1 null neurons but did not rescue APOE levels, suggesting that these phenotypes are separable. Stimulation and inhibition of SMAD signaling modulated APOE RNA levels in a SORL1-dependent manner. These studies provide a mechanistic link between two of the strongest genetic risk factors for AD.

Decorin improves human pancreatic β-cell function and regulates ECM expression in vitro

Transplantation of islets of Langerhans is a promising alternative treatment strategy in severe cases of type 1 diabetes mellitus; however, the success rate is limited by the survival rate of the cells post-transplantation. Restoration of the native pancreatic niche during transplantation potentially can help to improve cell viability and function. Here, we assessed for the first time the regulatory role of the small leucine-rich proteoglycan decorin (DCN) in insulin secretion in human β-cells, and its impact on pancreatic extracellular matrix (ECM) protein expression in vitro. In depth analyses utilizing next-generation sequencing as well as Raman microspectroscopy and Raman imaging identified pathways related to glucose metabolism to be upregulated in DCN-treated cells, including oxidative phosphorylation within the mitochondria as well as proteins and lipids of the endoplasmic reticulum. We further showed the effectiveness of DCN in a transplantation setting by treating collagen type 1-encapsulated β-cell-containing pseudo-islets with DCN. Taken together, in this study, we demonstrate the potential of DCN to improve the function of insulin-secreting β-cells while reducing the expression of ECM proteins affiliated with fibrotic capsule formation, making DCN a highly promising therapeutic agent for islet transplantation.

HSP90 as a regulator of extracellular matrix dynamics

The extracellular matrix (ECM) is a dynamic and organised extracellular network assembled from proteins and carbohydrates exported from the cell. The ECM is critical for multicellular life, providing spatial and temporal cellular cues to maintain tissue homeostasis. Consequently, ECM production must be carefully balanced with turnover to ensure homeostasis; ECM dysfunction culminates in disease. Hsp90 is a molecular chaperone central to protein homeostasis, including in the ECM. Intracellular and extracellular Hsp90 isoforms collaborate to regulate the levels and status of proteins in the ECM via multiple mechanisms. In so doing, Hsp90 regulates ECM dynamics, and changes in Hsp90 levels or activity support the development of ECM-related diseases, like cancer and fibrosis. Consequently, Hsp90 levels may have prognostic value, while inhibition of Hsp90 may have therapeutic potential in conditions characterised by ECM dysfunction.

The Dual Role of Low-Density Lipoprotein Receptor-Related Protein 1 in Atherosclerosis

Low-density lipoprotein receptor–related protein-1 (LRP1) is a large endocytic and signaling receptor belonging to the LDL receptor (LDLR) gene family and that is widely expressed in several tissues. LRP1 comprises a large extracellular domain (ECD; 515 kDa, α chain) and a small intracellular domain (ICD; 85 kDa, β chain). The deletion of LRP1 leads to embryonic lethality in mice, revealing a crucial but yet undefined role in embryogenesis and development. LRP1 has been postulated to participate in numerous diverse physiological and pathological processes ranging from plasma lipoprotein homeostasis, atherosclerosis, tumor evolution, and fibrinolysis to neuronal regeneration and survival. Many studies using cultured cells and in vivo animal models have revealed the important roles of LRP1 in vascular remodeling, foam cell biology, inflammation and atherosclerosis. However, its role in atherosclerosis remains controversial. LRP1 not only participates in the removal of atherogenic lipoproteins and proatherogenic ligands in the liver but also mediates the uptake of aggregated LDL to promote the formation of macrophage- and vascular smooth muscle cell (VSMC)-derived foam cells, which causes a prothrombotic transformation of the vascular wall. The dual and opposing roles of LRP1 may also represent an interesting target for atherosclerosis therapeutics. This review highlights the influence of LRP1 during atherosclerosis development, focusing on its dual role in vascular cells and immune cells.

Low Density Receptor-Related Protein 1 Interactions With the Extracellular Matrix: More Than Meets the Eye

The extracellular matrix (ECM) is a biological substrate composed of collagens, proteoglycans and glycoproteins that ensures proper cell migration and adhesion and keeps the cell architecture intact. The regulation of the ECM composition is a vital process strictly controlled by, among others, proteases, growth factors and adhesion receptors. As it appears, ECM remodeling is also essential for proper neuronal and glial development and the establishment of adequate synaptic signaling. Hence, disturbances in ECM functioning are often present in neurodegenerative diseases like Alzheimer’s disease. Moreover, mutations in ECM molecules are found in some forms of epilepsy and malfunctioning of ECM-related genes and pathways can be seen in, for example, cancer or ischemic injury. Low density lipoprotein receptor-related protein 1 (Lrp1) is a member of the low density lipoprotein receptor family. Lrp1 is involved not only in ligand uptake, receptor mediated endocytosis and lipoprotein transport—functions shared by low density lipoprotein receptor family members—but also regulates cell surface protease activity, controls cellular entry and binding of toxins and viruses, protects against atherosclerosis and acts on many cell signaling pathways. Given the plethora of functions, it is not surprising that Lrp1 also impacts the ECM and is involved in its remodeling. This review focuses on the role of Lrp1 and some of its major ligands on ECM function. Specifically, interactions with two Lrp1 ligands, integrins and tissue plasminogen activator are described in more detail.

LRP1 is required for novobiocin-mediated fibronectin turnover

Fibronectin (FN) plays a major role in the stability and organization of the extracellular matrix (ECM). We have previously demonstrated that FN interacts directly with Hsp90, as well as showing that the Hsp90 inhibitor novobiocin results in FN turnover via a receptor mediated process. However, the receptor involved has not been previously identified. LRP1 is a ubiquitous receptor responsible for the internalisation of numerous ligands that binds both Hsp90 and FN, and therefore we investigated whether LRP1 was involved in novobiocin-mediated FN turnover. FN, LRP1 and Hsp90 could be isolated in a common complex, and inhibition of Hsp90 by novobiocin increased the colocalisation of FN and LRP1. Novobiocin induced an increase (at low concentrations) followed by a loss of FN that was primarily derived from extracellular matrix-associated FN and led to a concomitant increase in intracellular FN. The effect of novobiocin was specific to LRP1-expressing cells and could be recapitulated by an LRP1 blocking antibody and the allosteric C-terminal Hsp90 inhibitor SM253, but not the N-terminal inhibitor geldanamycin. Together these data suggest that LRP1 is required for FN turnover in response to Hsp90 inhibition by novobiocin, which may have unintended physiological consequences in contexts where C-terminal Hsp90 inhibition is to be used therapeutically.

Low density lipoprotein receptor-related protein 1 couples β1 integrin activation to degradation

Low density lipoprotein receptor-related protein (LRP) 1 modulates cell adhesion and motility under normal and pathological conditions. Previous studies documented that LRP1 binds several integrin receptors and mediates their trafficking to the cell surface and endocytosis. However, the mechanism by which LRP1 may regulate integrin activation remains unknown. Here we report that LRP1 promotes the activation and subsequent degradation of β1 integrin and thus supports cell adhesion, spreading, migration and integrin signaling on fibronectin. LRP1 interacts with surface β1 integrin, binds the integrin activator kindlin2 and stimulates β1 integrin-kindlin2 complex formation. Specifically, serine 76 in the LRP1 cytoplasmic tail is crucial for the interaction with kindlin2, β1 integrin activation and cell adhesion. Interestingly, a loss of LRP1 induces the accumulation of several integrin receptors on the cell surface. Following internalization, intracellular trafficking of integrins is driven by LRP1 in a protein kinase C- and class II myosin-dependent manner. Ultimately, LRP1 dictates the fate of endocytosed β1 integrin by directing it down the pathway of lysosomal and proteasomal degradation. We propose that LRP1 mediates cell adhesion by orchestrating a multi-protein pathway to activate, traffic and degrade integrins. Thus, LRP1 may serve as a focal point in the integrin quality control system to ensure a firm connection to the extracellular matrix.

LRP1: A chameleon receptor of lung inflammation and repair

The lung displays a remarkable capability to regenerate following injury. Considerable effort has been made thus far to understand the cardinal processes underpinning inflammation and reconstruction of lung tissue. However, the factors determining the resolution or persistence of inflammation and efficient wound healing or aberrant remodeling remain largely unknown. Low density lipoprotein receptor-related protein 1 (LRP1) is an endocytic/signaling cell surface receptor which controls cellular and molecular mechanisms driving the physiological and pathological inflammatory reactions and tissue remodeling in several organs. In this review, we will discuss the impact of LRP1 on the consecutive steps of the inflammatory response and its role in the balanced tissue repair and aberrant remodeling in the lung.

Physiological, pathological, and structural implications of non-enzymatic protein-protein interactions of the multifunctional human transglutaminase 2

Transglutaminase 2 (TG2) is a ubiquitously expressed member of an enzyme family catalyzing Ca(2+)-dependent transamidation of proteins. It is a multifunctional protein having several well-defined enzymatic (GTP binding and hydrolysis, protein disulfide isomerase, and protein kinase activities) and non-enzymatic (multiple interactions in protein scaffolds) functions. Unlike its enzymatic interactions, the significance of TG2's non-enzymatic regulation of its activities has recently gained importance. In this review, we summarize all the partners that directly interact with TG2 in a non-enzymatic manner and analyze how these interactions could modulate the crosslinking activity and cellular functions of TG2 in different cell compartments. We have found that TG2 mostly acts as a scaffold to bridge various proteins, leading to different functional outcomes. We have also studied how specific structural features, such as intrinsically disordered regions and embedded short linear motifs contribute to multifunctionality of TG2. Conformational diversity of intrinsically disordered regions enables them to interact with multiple partners, which can result in different biological outcomes. Indeed, ID regions in TG2 were identified in functionally relevant locations, indicating that they could facilitate conformational transitions towards the catalytically competent form. We reason that these structural features contribute to modulating the physiological and pathological functions of TG2 and could provide a new direction for detecting unique regulatory partners. Additionally, we have assembled all known anti-TG2 antibodies and have discussed their significance as a toolbox for identifying and confirming novel TG2 regulatory functions.

PDGF‑stimulated dispersal of cell clusters and disruption of fibronectin matrix on three-dimensional collagen matrices requires matrix metalloproteinase-2

Previous studies showed that morphogenic cell clustering depends on fibronectin fibrillar matrix assembly under procontractile conditions. The present study shows that disruption of fibronectin matrix necessary for dispersal of cell clusters under promigratory conditions requires matrix metalloproteinases, especially MMP-2.

Formation of cell clusters is a common morphogenic cell behavior observed during tissue and organ development and homeostasis, as well as during pathological disorders. Dynamic regulation of cell clustering depends on the balance between contraction of cells into clusters and migration of cells as dispersed individuals. Previously we reported that under procontractile culture conditions, fibronectin fibrillar matrix assembly by human fibroblasts functioned as a nucleation center for cell clustering on three-dimensional collagen matrices. Here we report that switching preformed cell clusters from procontractile to promigratory culture conditions results in cell dispersal out of clusters and disruption of FN matrix. Experiments using small interfering RNA silencing and pharmacological inhibition demonstrated that matrix metalloproteinase activity involving MMP-2 was necessary for fibronectin matrix disruption and dispersal of cell clusters.

The LDL receptor-related protein 1 (LRP1) regulates the PDGF signaling pathway by binding the protein phosphatase SHP-2 and modulating SHP-2- mediated PDGF signaling events

BACKGROUND

The PDGF signaling pathway plays a major role in several biological systems, including vascular remodeling that occurs following percutaneous transluminal coronary angioplasty. Recent studies have shown that the LDL receptor-related protein 1 (LRP1) is a physiological regulator of the PDGF signaling pathway. The underlying mechanistic details of how this regulation occurs have yet to be resolved. Activation of the PDGF receptor β (PDGFRβ) leads to tyrosine phosphorylation of the LRP1 cytoplasmic domain within endosomes and generates an LRP1 molecule with increased affinity for adaptor proteins such as SHP-2 that are involved in signaling pathways. SHP-2 is a protein tyrosine phosphatase that positively regulates the PDGFRβ pathway, and is required for PDGF-mediated chemotaxis. We investigated the possibility that LRP1 may regulate the PDGFRβ signaling pathway by binding SHP-2 and competing with the PDGFRβ for this molecule.

METHODOLOGY/PRINCIPAL FINDINGS

To quantify the interaction between SHP-2 and phosphorylated forms of the LRP1 intracellular domain, we utilized an ELISA with purified recombinant proteins. These studies revealed high affinity binding of SHP-2 to phosphorylated forms of both LRP1 intracellular domain and the PDGFRβ kinase domain. By employing the well characterized dynamin inhibitor, dynasore, we established that PDGF-induced SHP-2 phosphorylation primarily occurs within endosomal compartments, the same compartments in which LRP1 is tyrosine phosphorylated by activated PDGFRβ. Immunofluorescence studies revealed colocalization of LRP1 and phospho-SHP-2 following PDGF stimulation of fibroblasts. To define the contribution of LRP1 to SHP-2-mediated PDGF chemotaxis, we employed fibroblasts expressing LRP1 and deficient in LRP1 and a specific SHP-2 inhibitor, NSC-87877. Our results reveal that LRP1 modulates SHP-2-mediated PDGF-mediated chemotaxis.

CONCLUSIONS/SIGNIFICANCE

Our data demonstrate that phosphorylated forms of LRP1 and PDGFRβ compete for SHP-2 binding, and that expression of LRP1 attenuates SHP-2-mediated PDGF signaling events.

LRP1b shows restricted expression in human tissues and binds to several extracellular ligands, including fibrinogen and apoE-carrying lipoproteins

OBJECTIVE

To investigate low-density lipoprotein receptor-related protein 1b (LRP1b) expression in human tissues and to identify circulating ligands of LRP1b.

METHODS AND RESULTS

Using two independent RT-PCR assays, LRP1b mRNA was detected in human brain, thyroid gland, skeletal muscle, and to a lesser amount in testis but absent in other tissues, including heart, kidney, liver, lung, and placenta. Circulating ligands were purified from human plasma by affinity chromatography using FLAG-tagged recombinant LRP1b ectodomains and identified by mass spectrometry. Using this technique, several potential ligands (fibrinogen, clusterin, vitronectin, histidine rich glycoprotein, serum amyloid P-component, and immunoglobulins) were identified. Direct binding of LRP1b ectodomains to fibrinogen was verified by co-immunoprecipitation. ApoE-carrying lipoproteins were shown to bind to LRP1b ectodomains in a lipoprotein binding assay. Furthermore, binding as well as internalization of very low density lipoproteins by cells expressing an LRP1b minireceptor was demonstrated.

DISCUSSION

LRP1b expression in humans appears to be confined to few tissues, which could point out to specialized functions of LRP1b in certain organs. Most of the newly identified LRP1b ligands are well-known factors in blood coagulation and lipoprotein metabolism, suggesting a possible role of LRP1b in atherosclerosis.

Low density lipoprotein receptor-related protein 1 (LRP1) forms a signaling complex with platelet-derived growth factor receptor-beta in endosomes and regulates activation of the MAPK pathway

In addition to its endocytic function, the low density lipoprotein receptor-related protein 1 (LRP1) also contributes to cell signaling events. In the current study, the potential of LRP1 to modulate the platelet-derived growth factor (PDGF) signaling pathway was investigated. PDGF is a key regulator of cell migration and proliferation and mediates the tyrosine phosphorylation of LRP1 within its cytoplasmic domain. In WI-38 fibroblasts, PDGF-mediated LRP1 tyrosine phosphorylation occurred at 37 degrees C but not at 4 degrees C, where endocytosis is minimized. Furthermore, blockade of endocytosis with the dynamin inhibitor, dynasore, also prevented PDGF-mediated LRP1 tyrosine phosphorylation. Immunofluorescence studies revealed co-localization of LRP1 with the PDGF receptor after PDGF treatment within endosomal compartments, whereas surface biotinylation experiments confirmed that phosphorylated LRP1 primarily originates from intracellular compartments. Together, the data reveal the association of these two receptors in endosomal compartments where they form a signaling complex. To study the contribution of LRP1 to PDGF signaling, we used mouse embryonic fibroblasts genetically deficient in LRP1 and identified phenotypic changes in these cell lines in response to PDGF stimulation by performing phospho-site profiling. Of 38 phosphorylated proteins analyzed, 8 were significantly different in LRP1 deficient fibroblasts and were restored when LRP1 was expressed back in these cells. Importantly, the results revealed that LRP1 expression is necessary for PDGF-mediated activation of ERK. Overall, the studies reveal that LRP1 associates with the PDGF receptor in endosomal compartments and modulates its signaling properties affecting the MAPK and Akt/phosphatidylinositol 3-kinase pathways.

Delayed treatment with plasminogen activator inhibitor-1 decoys reduces tubulointerstitial fibrosis

We examined the capacity of delayed inhibition of plasminogen activator inhibitor-1 (PAI-1) to reduce tubulointerstitial fibrosis induced by unilateral ureteral obstruction (UUO) in mice. Small peptides mimicking parts of urokinase (uPA) and tissular plasminogen activator (tPA) and serving as decoy molecules for PAI-1 were administered daily during the late stages (3 to 8 days) of UUO. Treatment with PAI-1 decoys reduced interstitial deposition of fibronectin, collagen III and collagen IV without changes in macrophage and myofibroblast infiltration. Interestingly, while PAI-1 activity was reduced and the combined uPA and tPA activity was increased, the antifibrotic effect was obtained without modification of plasmin activity but with increased of hepatocyte growth factor (HGF) expression. We show for the first time that treatment with small PAI-1 decoy peptides reduces established tubulointerstitial fibrosis. This protective effect probably resulted from increased degradation of the extracellular matrix by an HGF dependent mechanism.

LRP1 regulates remodeling of the extracellular matrix by fibroblasts

Low density lipoprotein receptor-related protein (LRP1) is an endocytic receptor for diverse proteases, protease inhibitors, and other plasma membrane proteins, including the urokinase receptor (uPAR). LRP1 also functions in cell-signaling and regulates gene expression. The goal of this study was to determine whether LRP1 regulates remodeling of provisional extracellular matrix (ECM) by fibroblasts. To address this problem, we utilized an in vitro model in which type I collagen was reconstituted and overlaid with fibronectin. Either the collagen or fibronectin was fluorescently-labeled. ECM remodeling by fibroblasts deficient in LRP1, uPAR, or MT1-MMP was studied. MT1-MMP was required for efficient remodeling of the deep collagen layer but not involved in fibronectin remodeling. Instead, fibronectin was remodeled by a system that required urokinase-type plasminogen activator (uPA), uPAR, and exogenously-added plasminogen. LRP1 markedly inhibited fibronectin remodeling by regulating cell-surface uPAR and plasminogen activation. LRP1 also regulated remodeling of the deep collagen layer but not by controlling MT1-MMP. Instead, LRP1 deficiency or inhibition de-repressed a secondary pathway for collagen remodeling, which was active in MT1-MMP-deficient cells but not in uPAR-deficient cells. These results demonstrate that LRP1 regulates ECM remodeling principally by repressing pathways that require plasminogen activation by uPA in association with uPAR.

Polarized traffic of LRP1 involves AP1B and SNX17 operating on Y-dependent sorting motifs in different pathways

Low-density lipoprotein receptor-related protein 1 (LRP1) is an endocytic recycling receptor with two cytoplasmic tyrosine-based basolateral sorting signals. Here we show that during biosynthetic trafficking LRP1 uses AP1B adaptor complex to move from a post-TGN recycling endosome (RE) to the basolateral membrane. Then it recycles basolaterally from the basolateral sorting endosome (BSE) involving recognition by sorting nexin 17 (SNX17). In the biosynthetic pathway, Y(29) but not N(26) from a proximal NPXY directs LRP1 basolateral sorting from the TGN. A N(26)A mutant revealed that this NPXY motif recognized by SNX17 is required for the receptor's exit from BSE. An endocytic Y(63)ATL(66) motif also functions in basolateral recycling, in concert with an additional endocytic motif (LL(86,87)), by preventing LRP1 entry into the transcytotic apical pathway. All this sorting information operates similarly in hippocampal neurons to mediate LRP1 somatodendritic distribution regardless of the absence of AP1B in neurons. LRP1 basolateral distribution results then from spatially and temporally segregation steps mediated by recognition of distinct tyrosine-based motifs. We also demonstrate a novel function of SNX17 in basolateral/somatodendritic recycling from a different compartment than AP1B endosomes.

Developmental expression of LC3alpha and beta: absence of fibronectin or autophagy phenotype in LC3beta knockout mice

Murine light chain 3 (LC3) exists as two isoforms, LC3alpha and beta: LC3beta is an RNA-binding protein that enhances fibronectin (FN) mRNA translation, and is also a marker of autophagy. We report embryonic expression patterns for LC3alpha and LC3beta, with some overlap but notable differences in the brain, and in tissues of non-neuronal origin. LC3beta knockout (-/-) mice develop normally without a compensatory increase in LC3alpha. LC3beta-/- embryonic fibroblasts (MEFs) exhibit reduced FN synthesis but maintain wild type (WT) levels of FN protein. No significant changes in proteins associated with FN turnover, i.e., caveolin-1, LRP-1, or matrix metalloproteinases were identified. Autophagosomes form in amino acid-starved LC3beta-/-MEFs, and Caesarean-delivered pups survive as long as WT pups without an increase in LC3-related proteins linked to autophagy. These results suggest novel compensatory mechanisms for loss of LC3beta, ensuring proper FN accumulation and autophagy during fetal and neonatal life.

Regulation of tumor necrosis factor receptor-1 and the IKK-NF-kappaB pathway by LDL receptor-related protein explains the antiinflammatory activity of this receptor

Low-density lipoprotein receptor-related protein (LRP-1) functions in endocytosis and in cell signaling directly (by binding signaling adaptor proteins) or indirectly (by regulating levels of other cell-surface receptors). Because recent studies in rodents suggest that LRP-1 inhibits inflammation, we conducted activity-based protein profiling experiments to discover novel proteases, involved in inflammation, that are regulated by LRP-1. We found that activated complement proteases accumulate at increased levels when LRP-1 is absent. Although LRP-1 functions as an endocytic receptor for C1r and C1s, complement protease mRNA expression was increased in LRP-1-deficient cells, as was expression of inducible nitric oxide synthase (iNOS) and interleukin-6. Regulation of expression of inflammatory mediators was explained by the ability of LRP-1 to suppress basal cell signaling through the I kappaB kinase-nuclear factor-kappaB (NF-kappaB) pathway. LRP-1-deficient macrophages, isolated from mice, demonstrated increased expression of iNOS, C1r, and monocyte chemoattractant protein-1 (MCP-1); MCP-1 expression was inhibited by NF-kappaB antagonism. The mechanism by which LRP-1 inhibits NF-kappaB activity involves down-regulating cell-surface tumor necrosis factor receptor-1 (TNFR1) and thus, inhibition of autocrine TNFR1-initiated cell signaling. TNF-alpha-neutralizing antibody inhibited NF-kappaB activity selectively in LRP-1-deficient cells. We propose that LRP-1 suppresses expression of inflammatory mediators indirectly, by regulating TNFR1-dependent cell signaling through the I kappaB kinase-NF-kappaB pathway.

Constitutively activated dystrophic muscle fibroblasts show a paradoxical response to TGF-beta and CTGF/CCN2

Transforming growth factor beta (TGF-beta) and connective tissue growth factor (CTGF) have been described to induce the production of extracellular matrix (ECM) proteins and have been reported to be increased in different fibrotic disorders. Skeletal muscle fibrosis is a common feature of Duchenne muscular dystrophy (DMD). The mdx mouse diaphragm is a good model for DMD since it reproduces the muscle degenerative and fibrotic changes. Fibronectin (FN) and proteoglycans (PG) are some of the ECM proteins upregulated in dystrophic conditions. In view of understanding the fibrotic process involved in DMD we have isolated fibroblasts from dystrophic mdx diaphragms. Here we report that regardless of the absence of degenerative myofibers, adult mdx diaphragm fibroblasts show increased levels of FN and condroitin/dermatan sulfate PGs synthesis. Fibroblasts isolated from non fibrotic tissue, such as 1 week old mice diaphragms or skin, do not present elevated FN levels. Furthermore, mdx fibroblast conditioned media is able to stimulate FN synthesis in control fibroblasts. Autocrine TGF-beta signaling was unaltered in mdx cells. When control fibroblasts are exposed to TGF-beta and CTGF, FN increases as expected. Paradoxically, in mdx cells it decreases in a concentration dependent manner and this decrease is not due to a downregulation of FN synthesis. According to this data we hypothesize that a pathological environment is able to reprogram fibroblasts into an activated phenotype which can be maintained through generations.

Perindoprilat modulates the activity of lipoprotein receptor-related protein in human mesangial cells

Low density lipoprotein receptor-related protein (LRP) is a multifunctional endocytic receptor implicated in the modulation of a number of cellular processes, including the turnover of proteases and the degradation of extracellular matrix proteins. As such, it can play a key role in the control of fibrosis. The aim of this investigation was to ascertain whether the anti-fibrotic effects exerted by the angiotensin-converting enzyme inhibitor (ACE-I) perindoprilat on macrophage-conditioned medium (MPCM)-injured human mesangial cells can be modulated by this receptor. Addition of receptor-associated protein to MPCM-injured mesangial cells with and without ACE-I increased the amount of tissue plasminogen activator protein detected in mesangial cell culture supernatants without affecting the protein levels of plasminogen activator inhibitor-1. The ability of ACE-I to reduce fibronectin was diminished in the presence of receptor-associated protein. ACE-I induced an increase in mesangial cell MMP9 mRNA, but reduced the MMP9 enzyme activity detected in mesangial cell supernatants. Mesangial cell lysates from ACE-I-treated cells were able to bind immobilized fibronectin at higher dilutions than cell lysates from untreated cells. Flow cytometry showed that MPCM induced an increase in LRP surface expression in mesangial cells over that in control cells and that this expression was further increased by ACE-I treatment. The increase in LRP expression in response to ACE-I was also observed by Western blotting. Northern blot analysis of RNA extracted from cells following a 24-h exposure to MPCM with and without ACE-I demonstrated that there was no change in LRP mRNA expression upon ACE-I treatment. In conclusion, we show that ACE-I treatment is able to modulate mesangial cell-surface expression of LRP, providing an additional mechanism whereby ACE-Is can mediate anti-fibrotic actions independent of their hemodynamic actions.

Cell-surface transglutaminase undergoes internalization and lysosomal degradation: an essential role for LRP1

Tissue transglutaminase functions as a protein crosslinking enzyme and an integrin-binding adhesion co-receptor for fibronectin on the cell surface. These activities of transglutaminase and the involvement of this protein in cell-matrix adhesion, integrin-mediated signaling, cell migration and matrix organization suggest a precise and efficient control of its cell-surface expression. We report a novel mechanism of regulation of surface transglutaminase through internalization and subsequent lysosomal degradation. Constitutive endocytosis of cell-surface transglutaminase depends on plasma membrane cholesterol and the activity of dynamin-2, and involves both clathrin-coated pits and lipid rafts or caveolae. Furthermore, the key matrix ligands of transglutaminase, fibronectin and platelet-derived growth factor, promote its endocytosis from the cell surface. Our results also indicate that transglutaminase interacts in vitro and on the cell surface with the major endocytic receptor, low-density lipoprotein receptor-related protein 1, and demonstrate the requirement for this receptor in the endocytosis of transglutaminase. Finally, a deficiency of this endocytic receptor or blockade of endo-lysosomal function upregulate transglutaminase expression on the cell surface, leading to increased cell adhesion and matrix crosslinking. These findings characterize a previously unknown pathway of transglutaminase internalization and degradation that might be crucial for regulation of its adhesive and signaling functions on the cell surface and reveal a novel functional link between cell-matrix adhesion and endocytosis.

The low density lipoprotein receptor-related protein functions as an endocytic receptor for decorin

Decorin is a small leucine-rich proteoglycan that modulates the activity of transforming growth factor type beta and other growth factors and thereby influences the processes of proliferation and differentiation in a wide array of physiological and pathological reactions. Hence, understanding the regulatory mechanisms of decorin activity has broad implications. Here we report that the extracellular levels of decorin are controlled by receptor-mediated catabolism, involving the low density lipoprotein receptor family member, low density lipoprotein receptor-related protein (LRP). We show that decorin is endocytosed and degraded by C2C12 myoblast cells and that both processes are blocked by suppressing LRP expression using short interfering RNA. The same occurs with CHO cells, but not with CHO cells genetically deficient in LRP. Finally, we show that LRP-null CHO cells, transfected to express mini-LRP polypeptides containing either the second or fourth LRP ligand-binding domains, carry out decorin endocytosis and lysosomal degradation. These findings point to LRP-mediated catabolism as a new control pathway for the biological activities of decorin, specifically for its ability to influence extracellular matrix signaling.

Urokinase-type plasminogen activator receptor regulates a novel pathway of fibronectin matrix assembly requiring Src-dependent transactivation of epidermal growth factor receptor

Previous studies have indicated that the urokinase-type plasminogen activator receptor (uPAR) can functionally interact with integrins thereby modulating integrin activity. We have previously demonstrated that treatment of fibroblasts with the uPAR ligand, P25, results in an increase in the activation of the beta1 integrin and a 35-fold increase in fibronectin matrix assembly (Monaghan, E., Gueorguiev, V., Wilkins-Port, C., and McKeown-Longo, P. J. (2004) J. Biol. Chem. 279, 1400-1407). Experiments were conducted to address the mechanism of uPAR regulation of matrix assembly. Treatment of fibroblasts with P25 led to an increase in the activation of the epidermal growth factor receptor (EGFR) and a colocalization of activated EGFR with beta1 integrins in cell matrix contacts. The effects of P25 on matrix assembly and beta1 integrin activation were inhibited by pretreatment with EGFR or Src kinase inhibitors, suggesting a role for both Src and EGFR in integrin activation by uPAR. Phosphorylation of EGFR in response to P25 occurred on Tyr-845, an Src-dependent phosphorylation site and was inhibited by PP2, the Src kinase inhibitor, consistent with Src kinase lying upstream of EGFR and integrin activation. Cells null for Src kinases also showed a loss of P25-induced matrix assembly, integrin activation, and EGFR phosphorylation. These P25-induced effects were restored following Src re-expression. The effects of P25 were specific for uPAR as enhanced matrix assembly by P25 was not seen in uPAR-/- cells, but was restored upon uPAR re-expression. These data provide evidence for a novel pathway of fibronectin matrix assembly through the uPAR-dependent sequential activation of Src kinase, EGFR, and beta1 integrin.

Regulation of the composition of the extracellular matrix by low density lipoprotein receptor-related protein-1: activities based on regulation of mRNA expression

Low density lipoprotein receptor-related protein-1 (LRP-1) is a catabolic receptor for extracellular matrix (ECM) structural proteins and for proteins that bind to ECM. LRP-1 also is implicated in integrin maturation. In this study, we applied a proteomics strategy to identify novel proteins involved in ECM modeling that are regulated by LRP-1. We show that LRP-1 deficiency in murine embryonic fibroblasts (MEFs) is associated with increased levels of type III collagen and pigment epithelium-derived factor, which accumulate in the substratum surrounding cells. The collagen receptor, uPAR-AP/Endo-180, is also increased in LRP-1-deficient MEFs. Human LRP-1 reversed the changes in protein expression associated with LRP-1 deficiency; however, the endocytic activity of LRP-1 was not involved. Instead, regulation occurred at the mRNA level. Inhibition of c-Jun amino-terminal kinase (JNK) blocked type III collagen expression in LRP-1-deficient MEFs, suggesting regulation of JNK activity as a mechanism by which LRP-1 controls mRNA expression. The ability of LRP-1 to regulate expression of the factors identified here suggests a role for LRP-1 in determining blood vessel structure and in angiogenesis.

Regulation of matrix metalloproteinase (MMP) activity by the low-density lipoprotein receptor-related protein (LRP). A new function for an “old friend”

Matrix metalloproteinases (MMPs) are essential contributors to a microenvironment that promotes tumour progression. During the two last decades, inhibition of MMPs has become the focus of considerable interest for cancer therapy, and numerous synthetic metalloproteinase inhibitors have been developed by the pharmaceutical industry. However, clinical trials have shown disappointing efficacy or unexpected toxicity and new targets are thus eagerly awaited. The identification of endocytic clearance of several MMPs by the low-density lipoprotein receptor-related protein (LRP) might provide insight into novel strategies for controlling MMP level during malignant processes. This review attempts to summarize recent aspects on the cellular and molecular basis of LRP-mediated endocytic disposal of MMPs.

Fibronectin matrix turnover occurs through a caveolin-1-dependent process

Extracellular matrix remodeling occurs during development, tissue repair, and in a number of pathologies, including fibrotic disorders, hypertension, and atherosclerosis. Extracellular matrix remodeling involves the complex interplay between extracellular matrix synthesis, deposition, and degradation. Factors that control these processes are likely to play key roles in regulating physiological and pathological extracellular matrix remodeling. Our data show that fibronectin polymerization into the extracellular matrix regulates the deposition and stability of other extracellular matrix proteins, including collagen I and thrombospondin-1 (Sottile and Hocking, 2002. Mol. Biol. Cell 13, 3546). In the absence of continual fibronectin polymerization, there is a loss of fibronectin matrix fibrils, and increased levels of fibronectin degradation. Fibronectin degradation occurs intracellularly after endocytosis and can be inhibited by chloroquine, an inhibitor of lysosomal degradation, and by caveolae-disrupting agents. Down-regulation of caveolin-1 by RNAi inhibits loss of fibronectin matrix fibrils, fibronectin internalization, and fibronectin degradation; these processes can be restored by reexpression of caveolin-1. These data show that fibronectin matrix turnover occurs through a caveolin-1-dependent process. Caveolin-1 regulation of fibronectin matrix turnover is a novel mechanism regulating extracellular matrix remodeling.

Low density lipoprotein receptor-related protein mediates endocytic clearance of pro-MMP-2.TIMP-2 complex through a thrombospondin-independent mechanism

The low density lipoprotein receptor-related protein (LRP) mediates the endocytic clearance of various proteinases and proteinase.inhibitor complexes, including thrombospondin (TSP)-dependent endocytosis of matrix metalloproteinase (MMP)-2 (or gelatinase A), a key effector of extracellular matrix remodeling and cancer progression. However, the zymogen of MMP-2 (pro-MMP-2) mostly occurs in tissues as a complex with the tissue inhibitor of MMPs (TIMP-2). Here we show that clearance of the pro-MMP-2.TIMP-2 complex is also mediated by LRP, because addition of receptor-associated protein (RAP), a natural LRP ligand antagonist, inhibited endocytosis and lysosomal degradation of (125)I-pro-MMP-2.TIMP-2. Both TIMP-2 and the pro-MMP-2 collagen-binding domain independently competed for endocytosis of (125)I-pro-MMP-2.TIMP-2 complex. Surface plasmon resonance studies indicated that pro-MMP-2, TIMP-2, and pro-MMP-2.TIMP-2 directly interact with LRP in the absence of TSP. LRP-mediated endocytic clearance of (125)I-pro-MMP-2 was inhibited by anti-TSP antibodies and accelerated upon complexing with TSP-1, but these treatments had no effect on (125)I-pro-MMP-2.TIMP-2 uptake. This implies that mechanisms of clearance by LRP of pro-MMP-2 and pro-MMP-2.TIMP-2 complex are different. Interestingly, RAP did not inhibit binding of (125)I-pro-MMP-2.TIMP-2 to the cell surface. We conclude that clearance of pro-MMP-2.TIMP-2 complex is a TSP-independent two-step process, involving (i) initial binding to the cell membrane in a RAP-insensitive manner and (ii) subsequent LRP-dependent (RAP-sensitive) internalization and degradation.

Low density lipoprotein receptor-related protein-1 promotes beta1 integrin maturation and transport to the cell surface

Low density lipoprotein receptor-related protein-1 (LRP-1) mediates the endocytosis of multiple plasma membrane proteins and thereby models the composition of the cell surface. LRP-1 also functions as a catabolic receptor for fibronectin, limiting fibronectin accumulation in association with cells. The goal of the present study was to determine whether LRP-1 regulates cell surface levels of the beta(1) integrin subunit. We hypothesized that LRP-1 may down-regulate cell surface beta(1) by promoting its internalization; however, unexpectedly, LRP-1 expression was associated with a substantial increase in cell surface beta(1) integrin in two separate cell lines, murine embryonic fibroblasts (MEFs) and CHO cells. The total amount of beta(1) integrin was unchanged because LRP-1-deficient cells retained increased amounts of beta(1) in the endoplasmic reticulum (ER). Expression of human LRP-1 in LRP-1-deficient MEFs reversed the shift in subcellular beta(1) integrin distribution. Metabolic labeling experiments demonstrated that the precursor form of newly synthesized beta(1) integrin (p105) is converted into mature beta(1) (p125) more slowly in LRP-1-deficient cells. Although low levels of cell surface beta(1) integrin, in LRP-1-deficient MEFs, were associated with decreased adhesion to fibronectin, the subcellular distribution of beta(1) integrin was most profoundly dependent on LRP-1 only after the cell cultures became confluent. A mutagen-treated CHO cell line, in which LRP-1 is expressed but retained in the secretory pathway, also demonstrated nearly complete ER retention of beta(1) integrin. These studies support a model in which LRP-1 either directly or indirectly promotes maturation of beta(1) integrin precursor and thereby increases the level of beta(1) integrin at the cell surface.

Urokinase receptor deficiency accelerates renal fibrosis in obstructive nephropathy

The urokinase cellular receptor (uPAR) recognizes the N-terminal growth factor domain of urokinase-type plasminogen activator (uPA) and is expressed by several cell types. The present study was designed to test the hypothesis that uPAR regulates the renal fibrogenic response to chronic injury. Groups of uPAR wild-type (+/+) and deficient (-/-) mice were investigated between 3 and 14 d after unilateral ureteral obstruction (UUO) or sham surgery. Not detected in normal kidneys, uPAR mRNA was expressed in response to UUO in the +/+ mice. By in situ hybridization, uPAR mRNA transcripts were detected in renal tubules and interstitial cells of the obstructed uPAR+/+ kidneys. The severity of renal fibrosis, based on the measurement of total collagen (13.5 +/- 1.5 versus 9.8 +/- 1.0 microg/mg kidney on day 14; -/- versus +/+) and interstitial area stained by Masson trichrome (22 +/- 4% versus 14 +/- 3% on day 14; -/- versus +/+) was significantly greater in the uPAR-/- mice. In the absence of uPAR, renal uPA activity was significantly decreased compared with the wild-type animals after UUO (62 +/- 20 versus 135 +/- 13 units at day 3 UUO; 74 +/- 17 versus 141 +/- 16 at day 7 UUO; 98 +/- 20 versus 165 +/- 10 at day 14 UUO; -/- versus +/+). In contrast, renal expression of several genes that regulate plasmin activity were similar in both genotypes, including uPA, tPA, PAI-1, protease nexin-1, and alpha2-antiplasmin. Worse renal fibrosis in the uPAR-/- mice appears to be TGF-beta-independent, as TGF-beta activity was actually reduced by 65% in the -/- mice despite similar renal TGF-beta1 mRNA levels. Significantly lower levels of the major 2.3-kb transcript and the 69-kd active protein of hepatocyte growth factor (HGF), a known anti-fibrotic growth factor, in the uPAR-/- mice suggests a potential link between HGF and the renoprotective effects of uPAR. These data suggest that renal uPAR attenuates the fibrogenic response to renal injury, an outcome that is mediated in part by urokinase-dependent but plasminogen-independent functions.

Regulation of Rac1 activation by the low density lipoprotein receptor-related protein

The low density lipoprotein receptor-related protein (LRP-1) binds and mediates the endocytosis of multiple ligands, transports the urokinase-type plasminogen activator receptor (uPAR) and other membrane proteins into endosomes, and binds intracellular adaptor proteins involved in cell signaling. In this paper, we show that in murine embryonic fibroblasts (MEFs) and L929 cells, LRP-1 functions as a major regulator of Rac1 activation, and that this activity depends on uPAR. LRP-1-deficient MEFs demonstrated increased Rac1 activation compared with LRP-1-expressing MEFs, and this property was reversed by expressing the VLDL receptor, a member of the same gene family as LRP-1, with overlapping ligand-binding specificity. Neutralizing the activity of LRP-1 with receptor-associated protein (RAP) increased Rac1 activation and cell migration in MEFs and L929 cells. The same parameters were unaffected by RAP in uPAR-/- MEFs, prepared from uPAR gene knockout embryos, and in uPAR-deficient LM-TK- cells. Untreated uPAR+/+ MEFs demonstrated substantially increased Rac1 activation compared with uPAR-/- MEFs. In addition to Rac1, LRP-1 suppressed activation of extracellular signal-regulated kinase (ERK) in MEFs; however, it was Rac1 (and not ERK) that was responsible for the effects of LRP-1 on MEF migration. Thus, LRP-1 regulates two signaling proteins in the same cell (Rac1 and ERK), both of which may impact on cell migration. In uPAR-negative cells, LRP-1 neutralization does not affect Rac1 activation, and other mechanisms by which LRP-1 may regulate cell migration are not unmasked.

Fibronectin polymerization regulates the composition and stability of extracellular matrix fibrils and cell-matrix adhesions

Remodeling of extracellular matrices occurs during development, wound healing, and in a variety of pathological processes including atherosclerosis, ischemic injury, and angiogenesis. Thus, identifying factors that control the balance between matrix deposition and degradation during tissue remodeling is essential for understanding mechanisms that regulate a variety of normal and pathological processes. Using fibronectin-null cells, we found that fibronectin polymerization into the extracellular matrix is required for the deposition of collagen-I and thrombospondin-1 and that the maintenance of extracellular matrix fibronectin fibrils requires the continual polymerization of a fibronectin matrix. Further, integrin ligation alone is not sufficient to maintain extracellular matrix fibronectin in the absence of fibronectin deposition. Our data also demonstrate that the retention of thrombospondin-1 and collagen I into fibrillar structures within the extracellular matrix depends on an intact fibronectin matrix. An intact fibronectin matrix is also critical for maintaining the composition of cell-matrix adhesion sites; in the absence of fibronectin and fibronectin polymerization, neither alpha5beta1 integrin nor tensin localize to fibrillar cell-matrix adhesion sites. These data indicate that fibronectin polymerization is a critical regulator of extracellular matrix organization and stability. The ability of fibronectin polymerization to act as a switch that controls the organization and composition of the extracellular matrix and cell-matrix adhesion sites provides cells with a means of precisely controlling cell-extracellular matrix signaling events that regulate many aspects of cell behavior including cell proliferation, migration, and differentiation.