Retention of the Matricellular Protein SPARC in the Endoplasmic Reticulum of Chondrocytes from Patients with Pseudoachondroplasia

Pseudoachondroplasia (PSACH) is an autosomal dominant disease characterized by dwarfism, morphological irregularities of long bones and hips, and early-onset osteoarthritis. This disease has been attributed to mutations in a structural protein of the cartilage extracellular matrix (ECM), cartilage oligomeric matrix protein (COMP), which result in its selective retention in the chondrocyte rough endoplasmic reticulum (ER). Accumulation of excessive amounts of mutated COMP might reflect a defect in protein trafficking by PSACH chondrocytes. Here we identify the matricellular protein SPARC as a component of this trafficking deficit. SPARC was localized to the hypertrophic chondrocytes in the normal human tibial growth plate and in cultured control cartilage nodules. In contrast, concentrated intracellular depots of SPARC were identified in nodules cultured from three PSACH patients with mutations in COMP. The accumulated SPARC was coincident with COMP and with protein disulfide isomerase, a resident chaperone of the rough ER, whereas SPARC and COMP were not coincident in the ECM of control or PSACH nodules. SPARC-null mice develop severe osteopenia and degenerative intervertebral disc disease, and exhibit attenuation of collagenous ECM. The retention of SPARC in the ER of chondrocytes producing mutant COMP indicates a new intracellular function for SPARC in the trafficking/secretion of cartilage ECM.

Functional Analysis of the Matricellular Protein SPARC with Novel Monoclonal Antibodies

SPARC (osteonectin, BM-40) is a matricellular glycoprotein that is expressed in many embryogenic and adult tissues undergoing remodeling or repair. SPARC modulates cellular interaction with the extracellular matrix (ECM), inhibits cell adhesion and proliferation, and regulates growth factor activity. To explore further the function and activity of this protein in tissue homeostasis, we have developed several monoclonal antibodies (MAbs) that recognize distinct epitopes on SPARC. The MAbs bind to SPARC with high affinity and identify SPARC by ELISA, Western blotting, immunoprecipitation, immunocytochemistry, and/or immunohistochemistry. The MAbs were also characterized in functional assays for potential alteration of SPARC activity. SPARC binds to collagen I and laminin-1 through an epitope defined by MAb 293; this epitope is not involved in the binding of SPARC to collagen III. The other MAbs did not interfere with the binding of SPARC to collagen I or III or laminin-1. Inhibition of the anti-adhesive effect of SPARC on endothelial cells by MAb 236 was also observed. Functional analysis of SPARC in the presence of these novel MAbs now confirms that the activities ascribed to this matricellular protein can be assigned to discrete subdomains.

Revisiting the matricellular concept

The concept of a matricellular protein was first proposed by Paul Bornstein in the mid-1990s to account for the non-lethal phenotypes of mice with inactivated genes encoding thrombospondin-1, tenascin-C, or SPARC. It was also recognized that these extracellular matrix proteins were primarily counter or de-adhesive. This review reappraises the matricellular concept after nearly two decades of continuous investigation. The expanded matricellular family as well as the diverse and often unexpected functions, cellular location, and interacting partners/receptors of matricellular proteins are considered. Development of therapeutic strategies that target matricellular proteins are discussed in the context of pathology and regenerative medicine.

Overexpression of SPARC in human trabecular meshwork increases intraocular pressure and alters extracellular matrix

PURPOSE

Intraocular pressure (IOP) regulation is largely unknown. SPARC-null mice demonstrate a lower IOP resulting from increased outflow. SPARC is a matricellular protein often associated with fibrosis. We hypothesized that SPARC overexpression would alter IOP by affecting extracellular matrix (ECM) synthesis and/or turnover in the trabecular meshwork (TM).

METHODS

An adenoviral vector containing human SPARC was used to increase SPARC expression in human TM endothelial cells and perfused human anterior segments using multiplicities of infection (MOIs) 25 or 50. Total RNA from TM was used for quantitative PCR, while protein from cell lysates and conditioned media were used for immunoblot analyses and zymography. After completion of perfusion, the anterior segments were fixed, sectioned, and examined by light and confocal microscopy.

RESULTS

SPARC overexpression increased the IOP of perfused human anterior segments. Fibronectin and collagens IV and I protein levels were elevated in both TM cell cultures and within the juxtacanalicular (JCT) region of perfused anterior segments. Collagen VI and laminin protein levels were increased in TM cell cultures but not in perfused anterior segments. The protein levels of pro-MMP-9 decreased while the kinetic inhibitors of metalloproteinases, TIMP-1 and PAI-1 protein levels, increased at MOI 25. At MOI 50, the protein levels of pro-MMP-1, -3, and -9 also decreased while PAI-1 and TIMP-1 and -3 increased. Only MMP-9 activity was decreased on zymography. mRNA levels of the collagens, fibronectin, and laminin were not affected by SPARC overexpression.

CONCLUSIONS

SPARC overexpression increases IOP in perfused cadaveric human anterior segments resulting from a qualitative change the JCT ECM. Selective decrease of MMP-9 activity is likely part of the mechanism. SPARC is a regulatory node for IOP.

SPARC fusion protein induces cellular adhesive signaling

Secreted protein, acidic and rich in cysteine (SPARC) has been described as a counteradhesive matricellular protein with a diversity of biological functions associated with morphogenesis, remodeling, cellular migration, and proliferation. We have produced mouse SPARC with a FLAG-tag at the N-terminus of SPARC (Flag-SPARC, FSP) in a Bac-to-Bac baculoviral expression system. After affinity purification, this procedure yields SPARC of high purity, with an electrophoretic mobility of ∼44 kDa under reducing conditions, and ∼38–39 kDa under non-reducing conditions. Unexpectedly, FSP adsorbed to plastic supported cell attachment and spreading, in a calcium-dependent manner. The adhesive activity of native FSP was inhibited by prior incubation with anti-SPARC IgG. Cell adhesion to FSP induced the formation of filopodia and lamellipodia but not focal adhesions that were prominent on cells that were attached to fibronectin. In addition, FSP induced the tyrosine phosphorylation of FAK and paxillin in attached epithelial cells. Erk1/2 and Rac were also activated in cells attached to FSP, but at a lower level in comparison to cells on fibronectin. This study provides new insight into the biological functions of SPARC, a matricellular protein with important roles in cell-extracellualr matrix interactions.

Proteolysis of the matricellular protein hevin by matrix metalloproteinase-3 produces a SPARC-like fragment (SLF) associated with neovasculature in a murine glioma model

The matricellular SPARC-family member hevin (Sparc-like 1/SPARCL-1/SC1/Mast9) contributes to neural development and alters tumor progression in a range of mammalian models. Based on sequence similarity, we hypothesized that proteolytic digestion of hevin would result in SPARC-like fragments (SLF) that affect the activity and/or location of these proteins. Incubation of hevin with matrix metalloproteinase-3 (MMP-3), a protease known to cleave SPARC, produced a limited number of peptides. Sequencing revealed the major proteolytic products to be SPARC-like in primary structure. In gliomas implanted into murine brain, a SLF was associated with SPARC in the neovasculature but not with hevin, the latter prominent in the astrocytes encompassed by infiltrating tumor. In this model of invasive glioma that involves MMP-3 activity, host-derived SLF was not observed in the extracellular matrix adjacent to tumor cells. In contrast, it occurred with its homolog SPARC in the angiogenic response to the tumor. We conclude that MMP-3-derived SLF is a marker of neovessels in glioma, where it could influence the activity of SPARC.

Identification of a sequence in the matricellular protein SPARC that interacts with the scavenger receptor stabilin-1

SPARC (osteonectin/BM-40), a secreted matricellular protein that promotes cellular deadhesion and motility in wound healing, carcinogenesis, and inflammation, binds to the scavenger receptor stabilin-1 in alternatively activated macrophages and undergoes endocytosis and clearance from the extracellular space. Both SPARC and stabilin-1 are expressed by endothelial cells during inflammation, but their interaction in this context is unknown. We have identified a binding site on SPARC for stabilin-1 by a solid-state peptide array coupled with a modified enzyme-linked immunosorbent assay. A monoclonal antibody that recognizes the identified binding site was also characterized that could be an inhibitor for the SPARC-stabilin-1 interaction in macrophages or endothelial cells.

Inactivation of SPARC enhances high-fat diet-induced obesity in mice

Secreted protein, acidic and rich in cysteine (SPARC), a matricellular protein, modulates extracellular matrix assembly and turnover in many physiological processes. SPARC-null mice exhibit an increased accumulation of adipose tissue. To distinguish between the functions of SPARC in adipogenesis during development and adulthood, we studied wild-type (WT) and SPARC-null mice maintained on a normal (low-fat) or high-fat (HF) diet. On an HF diet, SPARC-null mice exhibited significantly greater weight gain, in comparison to their WT counterparts, and had an enhanced cortical bone area that was likely due to increased mechanical loading. Diet-induced obesity (DIO) was also associated with an increase in vertebral trabecular bone in WT mice, but a significant change in this parameter was not observed in SPARC-null animals. We show that SPARC inhibits mitotic clonal expansion of preadipocytes at an early stage of adipogenesis. Moreover, there were substantially diminished levels of type I collagen in SPARC-null adipose tissue, as well as a reduction in the number of cross-linked, mature collagen fibers. In the absence of SPARC, mice show enhanced DIO. In adult animals, SPARC functions in the production and remodeling of adipose tissue, as well as in the regulation of preadipocyte differentiation.

Biomaterial topography alters healing in vivo and monocyte/macrophage activation in vitro

The effect of biomaterial topography on healing in vivo and monocyte/macrophage stimulation in vitro was assessed. A series of expanded polytetrafluoroethylene (ePTFE) materials were characterized by increasing average intranodal distance of 1.2 μm (1.2-ePTFE), 3.0 μm (3.0-ePTFE), and 4.4 μm (4.4-ePTFE), but presented consistent surface chemistry with nonporous PTFE (np-PTFE). Subcutaneous implantation of 4.4-ePTFE into mice resulted in a statistically thinner capsule that appeared less organized and less dense than the np-PTFE response. In vitro, isolated monocytes/macrophages cultured on np-PTFE produced low levels of interleukin 1-beta (IL-1β), 1.2-ePTFE and 3.0-ePTFE stimulated intermediate levels, and 4.4-ePTFE stimulated a 15-fold increase over np-PTFE. Analysis of cDNA microarrays demonstrated that additional proinflammatory cytokines and chemokines, including IL-1β, interleukin 6, tumor necrosis factor alpha, monocyte chemotactic protein 1, and macrophage inflammatory protein 1-beta, were expressed at higher levels by monocytes/macrophages cultured on 4.4-ePTFE at 4 and 24 h, respectively. Expression ratios for several genes were quantified by RT-PCR and were consistent with those from the cDNA array results. These results demonstrate the effect of biomaterial topography on early proinflammatory cytokine production and gene transcription by monocytes/macrophages in vitro and decreased fibrous capsule thickness in vivo.

SPARC deficiency results in improved surgical survival in a novel mouse model of glaucoma filtration surgery

Glaucoma is a disease frequently associated with elevated intraocular pressure that can be alleviated by filtration surgery. However, the post-operative subconjunctival scarring response which blocks filtration efficiency is a major hurdle to the achievement of long-term surgical success. Current application of anti-proliferatives to modulate the scarring response is not ideal as these often give rise to sight-threatening complications. SPARC (secreted protein, acidic and rich in cysteine) is a matricellular protein involved in extracellular matrix (ECM) production and organization. In this study, we investigated post-operative surgical wound survival in an experimental glaucoma filtration model in SPARC-null mice. Loss of SPARC resulted in a marked (87.5%) surgical wound survival rate compared to 0% in wild-type (WT) counterparts. The larger SPARC-null wounds implied that aqueous filtration through the subconjunctival space was more efficient in comparison to WT wounds. The pronounced increase in both surgical survival and filtration efficiency was associated with a less collagenous ECM, smaller collagen fibril diameter, and a loosely-organized subconjunctival matrix in the SPARC-null wounds. In contrast, WT wounds exhibited a densely packed collagenous ECM with no evidence of filtration capacity. Immunolocalization assays confirmed the accumulation of ECM proteins in the WT but not in the SPARC-null wounds. The observations in vivo were corroborated by complementary data performed on WT and SPARC-null conjunctival fibroblasts in vitro. These findings indicate that depletion of SPARC bestows an inherent change in post-operative ECM remodeling to favor wound maintenance. The evidence presented in this report is strongly supportive for the targeting of SPARC to increase the success of glaucoma filtration surgery.

Processing of the matricellular protein hevin in mouse brain is dependent on ADAMTS4

The matricellular SPARC family member hevin (SPARC-like 1/SPARCL-1/SC1/Mast9) contributes to neural development and alters tumor progression in a range of mammalian models. The distribution of hevin in mouse tissues was reexamined with a novel monoclonal antibody that discriminates between hevin and its ortholog SPARC. We now report proteolysis of hevin in many tissues, with the most extensive processing in the brain. We demonstrate a cleavage site within the hevin sequence for the neural tissue proteinase ADAMTS4. Digestion of hevin by ADAMTS4 in vitro produced fragments similar to those present in brain lysates. Monoclonal antibodies revealed a SPARC-like fragment generated from hevin that was co-localized with ADAMTS4 in vivo. We show that proteolysis of hevin by ADAMTS4 in the mouse cerebellum is important for the normal development of this tissue. In conclusion, we have identified the fragmentation of hevin by ADAMTS4 in the mouse brain and propose that this specific proteolysis is integral to cell morphology and extracellular matrix deposition in the developing brain.

Lack of host SPARC enhances vascular function and tumor spread in an orthotopic murine model of pancreatic carcinoma

Utilizing subcutaneous tumor models, we previously validated SPARC (secreted protein acidic and rich in cysteine) as a key component of the stromal response, where it regulated tumor size, angiogenesis and extracellular matrix deposition. In the present study, we demonstrate that pancreatic tumors grown orthotopically in Sparc-null (Sparc(-/-)) mice are more metastatic than tumors grown in wild-type (Sparc(+/+)) littermates. Tumors grown in Sparc(-/-) mice display reduced deposition of fibrillar collagens I and III, basement membrane collagen IV and the collagen-associated proteoglycan decorin. In addition, microvessel density and pericyte recruitment are reduced in tumors grown in the absence of host SPARC. However, tumors from Sparc(-/-) mice display increased permeability and perfusion, and a subsequent decrease in hypoxia. Finally, we found that tumors grown in the absence of host SPARC exhibit an increase in alternatively activated macrophages. These results suggest that increased tumor burden in the absence of host SPARC is a consequence of reduced collagen deposition, a disrupted vascular basement membrane, enhanced vascular function and an immune-tolerant, pro-metastatic microenvironment.

Absence of thrombospondin-2 causes age-related dilated cardiomyopathy

BACKGROUND

The progressive shift from a young to an aged heart is characterized by alterations in the cardiac matrix. The present study investigated whether the matricellular protein thrombospondin-2 (TSP-2) may affect cardiac dimensions and function with physiological aging of the heart.

METHODS AND RESULTS

TSP-2 knockout (KO) and wild-type mice were followed up to an age of 60 weeks. Survival rate, cardiac function, and morphology did not differ at a young age in TSP-2 KO compared with wild-type mice. However, >55% of the TSP-2 KO mice died between 24 and 60 weeks of age, whereas <10% of the wild-type mice died. In the absence of TSP-2, older mice displayed a severe dilated cardiomyopathy with impaired systolic function, increased cardiac dilatation, and fibrosis. Ultrastructural analysis revealed progressive myocyte stress and death, accompanied by an inflammatory response and replacement fibrosis, in aging TSP-2 KO animals, whereas capillary or coronary morphology or density was not affected. Importantly, adeno-associated virus-9 gene-mediated transfer of TSP-2 in 7-week-old TSP-2 KO mice normalized their survival and prevented dilated cardiomyopathy. In TSP-2 KO animals, age-related cardiomyopathy was accompanied by increased matrix metalloproteinase-2 and decreased tissue transglutaminase-2 activity, together with impaired collagen cross-linking. At the cardiomyocyte level, TSP-2 deficiency in vivo and its knockdown in vitro decreased the activation of the Akt survival pathway in cardiomyocytes.

CONCLUSIONS

TSP-2 expression in the heart protects against age-dependent dilated cardiomyopathy.

SPARC functions as an inhibitor of adipogenesis

Adipogenesis, a key step in the pathogenesis of obesity, involves extensive ECM remodeling, changes in cell-ECM interactions, and cytoskeletal rearrangement. Matricellular proteins regulate cell-cell and cell-ECM interactions. Evidence in vivo and in vitro indicates that the prototypic matricellular protein, SPARC, inhibits adipogenesis and promotes osteoblastogenesis. Herein we discuss mechanisms underlying the inhibitory effect of SPARC on adipogenesis. SPARC enhances the Wnt/β-catenin signaling pathway and regulates the expression and posttranslational modification of collagen. SPARC might drive preadipocytes away from the status of growth arrest and therefore prevent terminal differentiation. SPARC could also decrease WAT deposition through its negative effects on angiogenesis. Therefore, several stages of white adipose tissue accumulation are sensitive to the inhibitory effects of SPARC.

Extracellular and intracellular mechanisms that mediate the metastatic activity of exogenous osteopontin

BACKGROUND

Osteopontin affects several steps of the metastatic cascade. Despite direct correlation with metastasis in experimental systems and in patient studies, the extracellular and intracellular basis for these observations remains unsolved. In this study, the authors used human melanoma and sarcoma cell lines to evaluate the effects of soluble osteopontin on metastasis.

METHODS

Exogenous osteopontin or negative controls, including a site-directed mutant osteopontin, were used in functional assays in vitro, ex vivo, and in vivo that were designed to test the extracellular and intracellular mechanisms involved in experimental metastasis.

RESULTS

In the extracellular environment, the results confirmed that soluble osteopontin is required for its prometastatic effects; this phenomenon is specific, arginine-glycine-aspartic acid (RGD)-dependent, and evident in experimental models of metastasis. In the intracellular environment, osteopontin initially induced rapid tyrosine 418 (Tyr-418) dephosphorylation of the cellular homolog of the Rous sarcoma virus (c-Src), with decreases in actin stress fibers and increased binding to the vascular endothelium. This heretofore undescribed Tyr dephosphorylation was followed by a tandem c-Src phosphorylation after tumor cell attachment to the metastatic site.

CONCLUSIONS

The results of this study revealed a complex molecular interaction as well as a dual role for osteopontin in metastasis that depends on whether tumor cells are in circulation or attached. Such context-dependent functional insights may contribute to antimetastasis strategies.

SPARC-null mice exhibit lower intraocular pressures

PURPOSE

SPARC is a matricellular protein that is highly expressed in remodeling tissues, including the trabecular meshwork and ciliary body. The hypothesis for the study was that SPARC contributes to the regulation of intraocular pressure (IOP). The IOPs of SPARC-null mice, their corresponding wild-type (WT), and heterozygous animals were compared.

METHODS

Diurnal and nocturnal IOPs of C57Bl/6x129SvJ WT, SPARC-null, and heterozygous mice were measured. Fluorophotometric measurements were made to assess aqueous turnover. Central corneal thickness (CCT) was measured using histology, ultrasound biomicroscopy, and optical coherence tomography. Iridocorneal angles were examined using light microscopy (LM).

RESULTS

During the day, the mean IOP of SPARC-null mice (n = 142, 16.9 +/- 2.4 mm Hg) was lower than that of both WT mice (n = 104, 19.9 +/- 2.9 mm Hg; P < 10(-12)), and heterozygotes (n = 38, 19.3 +/- 2.5 mm Hg; P < 10(-4)). At night, SPARC-null mice also exhibited a blunted increase in IOP in comparison to WT and heterozygous mice. CCTs were not significantly different between WT and SPARC-null mice. Heterozygous mice tended to have thicker corneas (3.4%). Fluorophotometric measurements suggest that aqueous turnover rates in SPARC-null mice are equal to if not greater than rates in WT mice. LM of the SPARC-null iridocorneal angle revealed morphology that is indistinguishable from WT.

CONCLUSIONS

SPARC-null mice have lower IOPs than do their WT counterparts with equal CCTs. The rate of aqueous turnover suggests that the mechanism is enhanced outflow resistance.

Absence of SPARC results in increased cardiac rupture and dysfunction after acute myocardial infarction

The matricellular protein SPARC (secreted protein, acidic and rich in cysteine, also known as osteonectin) mediates cell–matrix interactions during wound healing and regulates the production and/or assembly of the extracellular matrix (ECM). This study investigated whether SPARC functions in infarct healing and ECM maturation after myocardial infarction (MI). In comparison with wild-type (WT) mice, animals with a targeted inactivation of SPARC exhibited a fourfold increase in mortality that resulted from an increased incidence of cardiac rupture and failure after MI. SPARC-null infarcts had a disorganized granulation tissue and immature collagenous ECM. In contrast, adenoviral overexpression of SPARC in WT mice improved the collagen maturation and prevented cardiac dilatation and dysfunction after MI. In cardiac fibroblasts in vitro, reduction of SPARC by short hairpin RNA attenuated transforming growth factor β (TGF)–mediated increase of Smad2 phosphorylation, whereas addition of recombinant SPARC increased Smad2 phosphorylation concordant with increased Smad2 phosphorylation in SPARC-treated mice. Importantly, infusion of TGF-β rescued cardiac rupture in SPARC-null mice but did not significantly alter infarct healing in WT mice. These findings indicate that local production of SPARC is essential for maintenance of the integrity of cardiac ECM after MI. The protective effects of SPARC emphasize the potential therapeutic applications of this protein to prevent cardiac dilatation and dysfunction after MI.

HSP27 mediates SPARC-induced changes in glioma morphology, migration, and invasion

Secreted protein acidic and rich in cysteine (SPARC) regulates cell-extracellular matrix interactions that influence cell adhesion and migration. We have demonstrated that SPARC is highly expressed in human gliomas, and it promotes brain tumor invasion in vitro and in vivo. To further our understanding regarding SPARC function in glioma migration, we transfected SPARC-green fluorescent protein (GFP) and control GFP vectors into U87MG cells, and assessed the effects of SPARC on cell morphology, migration, and invasion after 24 h. The expression of SPARC was associated with elongated cell morphology, and increased migration and invasion. The effects of SPARC on downstream signaling were assessed from 0 to 6 h and 24 h. SPARC increased the levels of total and phosphorylated HSP27; the latter was preceded by activation of p38 MAPK and inhibited by the p38 MAPK inhibitor SB203580. Augmented expression of SPARC was correlated with increased levels of HSP27 mRNA. In a panel of glioma cell lines, increasing levels of SPARC correlated with increasing total and phosphorylated HSP27. SPARC and HSP27 were colocalized to invading cells in vivo. Inhibition of HSP27 mRNA reversed the SPARC-induced changes in cell morphology, migration, and invasion in vitro. These data indicate that HSP27, a protein that regulates actin polymerization, cell contraction, and migration, is a novel downstream effector of SPARC-regulated cell morphology and migration. As such, it is a potential therapeutic target to inhibit SPARC-induced glioma invasion.

SPARC inhibits adipogenesis by its enhancement of beta-catenin signaling

SPARC (secreted protein acidic and rich in cysteine) modulates interactions between cells and extracellular matrix and is enriched in white adipose tissue. We have reported that SPARC-null mice accumulate significantly more fat than wild-type mice and maintain relatively high levels of serum leptin. We now show that SPARC inhibits adipogenesis in vitro. Specifically, recombinant SPARC inhibited (a) adipocyte differentiation of stromal-vascular cells isolated from murine white adipose tissue and (b) the expression of adipogenic transcription factors and adipocyte-specific genes. SPARC induced the accumulation and nuclear translocation of beta-catenin and subsequently enhanced the interaction of beta-catenin and T cell/lymphoid enhancer factor 1. The activity of integrin-linked kinase was required for the effect of SPARC on beta-catenin accumulation as well as extracellular matrix remodeling. During adipogenesis, fusiform preadipocytes change into sphere-shaped adipocytes and convert the extracellular matrix from a fibronectin-rich stroma to a laminin-rich basal lamina. SPARC retarded the morphological changes exhibited by preadipocytes during differentiation. In the presence of SPARC, the deposition of fibronectin was enhanced, and that of laminin was inhibited; in parallel, the expression of alpha5 integrin was enhanced, and that of alpha6 integrin was inhibited. Lithium chloride, which enhances the accumulation of beta-catenin, also inhibited the expression of alpha6 integrin. These findings demonstrate a role for SPARC in adipocyte morphogenesis and in signaling processes leading to terminal differentiation.

A prototypic matricellular protein in the tumor microenvironment--where there's SPARC, there's fire

Within the tumor microenvironment is a dynamic exchange between cancer cells and their surrounding stroma. This complex biologic system requires carefully designed models to understand the role of its stromal components in carcinogenesis, tumor progression, invasion, and metastasis. Secreted protein acidic and rich in cysteine (SPARC) is a prototypic matricellular protein at the center of this exchange. Two decades of basic science research combined with recent whole genome analyses indicate that SPARC is an important player in vertebrate evolution, normal development, and maintenance of normal tissue homeostasis. Therefore, SPARC might also play an important role in the tumor microenvironment. Clinical evidence indicates that SPARC expression correlates with tumor progression, but tightly controlled animal models have shown that the role of SPARC in tumor progression is dependent on tissue and tumor cell type. In this Prospectus, we review the current understanding of SPARC in the tumor microenvironment and discuss current and future investigations of SPARC and tumor-stromal interactions that require careful consideration of growth factors, cytokines, proteinases, and angiotropic factors that might influence SPARC activity and tumor progression.

Reduced foreign body reaction to implanted biomaterials by surface treatment with oriented osteopontin

The foreign body reaction (FBR), which leads to the encapsulation of implanted biomaterials, has been implicated in the failure of many medical devices. The protein layer that is nonspecifically adsorbed onto the implant surface immediately after implantation is thought to dictate this reaction. It is hypothesized that biomaterial surfaces having specific proteins with precisely controlled orientations will decrease the FBR. Previously, we have reported that osteopontin (OPN) adsorbed on positively charged surfaces has a preferable orientation for in vitro cell adhesion and spreading as compared to negatively charged surfaces. It is expected that coating a layer of OPN in its preferred orientation on an implant surface will decrease the FBR. In this work, in vivo studies were performed to test this hypothesis. A positively charged polymer (p(HEMA-co-AEMA)) and a negatively charged polymer (p(HEMA-co-CEA)) coated with OPN were implanted subcutaneously in wild-type mice for 7 or 28 days. Uncoated polymers were used as control. For the 7-day implants, cells on OPN-coated p(HEMA-co-AEMA) spread more than cells on the other three materials. Following 28 days of implantation the implants were explanted and the capsule thickness and vascularity around the implants were characterized. Additionally, the macrophage and foreign body giant cells (FBGCs) around the implants were quantified. It was found in this study that the modification of the positively charged polymer surface with OPN in a controlled orientation led to a reduction in the foreign body reaction as determined by capsule thickness. Our finding provides valuable information for designing better biocompatible biomaterials with improved in vivo performance.

Disruption of the thrombospondin-2 gene alters the lamellar morphology but does not permit vascularization of the adult mouse lumbar disc

Introduction

The biological basis for the avascular state of the intervertebral disc is not well understood. Previous work has suggested that the presence of thrombospondin-1 (TSP-1), a matricellular protein, in the outer annulus reflects a role for this protein in conferring an avascular status to the disc. In the present study we have examined thrombospondin-2 (TSP-2), a matricellular protein with recognized anti-angiogenic activity in vivo and in vitro.

Methods

We examined both the location and expression of TSP-2 in the human disc, and its location in the disc and bordering soft tissues of 5-month-old normal wild-type (WT) mice and of mice with a targeted disruption of the TSP-2 gene. Immunohistochemistry and quantitative histology were utilized in this study.

Results

TSP-2 was found to be present in some, but not all, annulus cells of the human annulus and the mouse annulus. Although there was no difference in the number of disc cells in the annulus of TSP-2-null mice compared with that of WT animals, polarized light microscopy revealed a more irregular lamellar collagen structure in null mouse discs compared with WT mouse discs. Additionally, vascular beds at the margins of discs of TSP-2-null mice were substantially more irregular than those of WT animals. Counts of platelet endothelial cell adhesion molecule-1-positive blood vessels in the tissue margin bordering the ventral annulus showed a significantly larger vascular bed in the tissue bordering the disc of TSP-2-null mice compared with that of WT mice (P = 0.0002). There was, however, no vascular ingrowth into discs of the TSP-2-null mice.

Conclusion

These data confirm a role for TSP-2 in the morphology of the disc and suggest the presence of other inhibitors of angiogenesis in the disc. We have shown that although an increase in vasculature was present in the TSP-2-null tissue in the margin of the disc, vascular ingrowth into the body of the disc did not occur. Our results point to the need for future research to understand the transition from the well-vascularized status of the fetal and young discs to the avascular state of the adult human disc or the small mammalian disc.

Alternatively activated macrophages regulate extracellular levels of the hormone placental lactogen via receptor-mediated uptake and transcytosis

Alternatively activated (M2) macrophages regulate immune responses and tissue remodelling. In many tissues including placenta, M2 express stabilin-1, a multidomain protein that exerts a dual role as a scavenger receptor for acetylated low density lipoprotein (acLDL) and SPARC (secreted protein acidic and rich in cysteine) and as an intracellular cargo carrier for SI-CLP. Using yeast two-hybrid screening, we identified the developmental hormone placental lactogen (PL) as a novel ligand of stabilin-1. In Chinese hamster ovary-stabilin-1 cells and M2, FACS and confocal microscopy demonstrated that stabilin-1 mediates internalization and endosomal sorting of PL. In M2 macrophages, PL was partially degraded in lysosomes; part of PL escaped degradation and was delivered to novel PL+ storage vesicles lacking endosomal/lysosomal markers. During formation, PL+ vesicles underwent transient interaction with the trans-Golgi network (TGN). Upon placement of PL-loaded M2 into PL-free medium, PL was secreted into the supernatant. Leupeptin, an inhibitor of lysosomal hydrolases, reduced PL degradation, enhanced sorting of PL into the TGN/storage vesicle pathway and increased PL secretion. Thus, processing of PL in M2 macrophages occurs either by the classical lysosomal pathway or by a novel TGN-associated trans-secretory pathway. Macrophages isolated from human placental villi efficiently endocytosed PL-FITC and transported it to the storage vesicles. Our data show that extracellular PL levels are determined by uptake, degradation, storage, and release in M2. During pregnancy PL concentration reaches 10 microg/ml in maternal circulation and stays below 0.5 microg/ml in fetal circulation. We propose that stabilin-1-positive macrophages determine the difference in PL levels between maternal and fetal circulation.

IFATS collection: Combinatorial peptides identify alpha5beta1 integrin as a receptor for the matricellular protein SPARC on adipose stromal cells

The biological features of adipose stromal (stem) cells (ASC), which serve as progenitors for differentiated cells of white adipose tissue (WAT), are still largely undefined. In an initiative to identify functional ASC surface receptors, we screened a combinatorial library for peptide ligands binding to patient-derived ASC. We demonstrate that both primary and cultured human and mouse stromal cells express a conserved receptor targeted by peptides found to mimic SPARC, a matricellular protein that is required for normal WAT development. A signaling receptor for SPARC has not as yet been determined. By using the SPARC-mimicking peptides CMLAGWIPC (termed hPep) and CWLGEWLGC (termed mPep), isolated by panning on human and mouse cells, respectively, we identified the alpha5beta1 integrin complex as a candidate receptor for SPARC. On the basis of these results, we evaluated ASC responses to SPARC or SPARC-mimicking peptide exposure. Our results suggest that extracellular SPARC binds to alpha5beta1 integrin at sites of focal adhesions, an interaction disrupting firm attachment of ASC to extracellular matrix. We propose that SPARC-mediated mobilization of ASC through its effect on alpha5beta1 integrin complex provides a functional basis for the regulation of WAT body composition by SPARC. We also show that alpha5beta1 integrin is a potential target for ASC-selective intracellular delivery of bioactive peptides and gene therapy vectors directed by the SPARC-mimicking peptides. Disclosure of potential conflicts of interest is found at the end of this article.

Altered tissue repair in hevin-null mice: inhibition of fibroblast migration by a matricellular SPARC homolog

Matricellular proteins such as hevin, secreted protein acidic and rich in cysteine, and thrombospondin-2 play an important role during tissue repair through their influence on fundamental cellular activities such as adhesion, migration, proliferation, and extracellular matrix synthesis/reorganization. We have investigated the role played by hevin during excisional and incisional cutaneous wound repair in hevin-null mice. Hevin-null animals both close and heal their skin wounds faster than wild-type animals, as evidenced by enhanced macrophage infiltration of wound beds at early time points, the earlier appearance of mature extracellular matrix, and the overall higher maturity score. In addition, fibrovascular invasion of polyvinyl alcohol sponges was more robust in hevin-null mice, a result indicating that differences in cell migration might underlie the observed alterations in wound repair. Experiments in vitro showed that hevin induced the deadhesion and inhibited the migration of primary dermal fibroblasts in a Rac-1-dependent manner. These findings indicate that the differences in wound repair between hevin-null and wild-type animals can be attributed in part to the deadhesive function of hevin and reduced cell migration within dermal wound beds in which this protein is expressed.

The copper binding domain of SPARC mediates cell survival in vitro via interaction with integrin beta1 and activation of integrin-linked kinase

Secreted protein acidic and rich in cysteine (SPARC) is important for the normal growth and maintenance of the murine lens. SPARC-null animals develop cataracts associated with a derangement of the lens capsule basement membrane and alterations in lens fiber morphology. Cellular stress and disregulation of apoptotic pathways within lens epithelial cells (LEC) are linked to cataract formation. To identify molecular targets of SPARC that are linked to this disorder, we stressed wild-type (WT) and SPARC-null LEC by serum deprivation or exposure to tunicamycin. SPARC enhanced signaling by integrin-linked kinase (ILK), a serine/threonine kinase known to enhance cell survival in vitro. In response to stress, an ILK-dependent decrease in apoptosis was observed in WT relative to SPARCg-null LEC. Co-immunoprecipitation and cross-linking of cell lysates revealed enhanced levels of a SPARC-integrin beta1 complex during stress. Competition with monoclonal antibodies and peptides indicated that the copper binding domain of SPARC is required for SPARC-mediated response to stress. Inhibiting the binding and/or activity of ILK, integrin beta1, or SPARC resulted in increased apoptosis of stressed LEC. We conclude that SPARC protects cells from stress-induced apoptosis in vitro via an interaction with integrin beta1 heterodimers that enhances ILK activation and pro-survival activity.

Purification of SPARC/osteonectin

SPARC is a matricellular protein that regulates cell adhesion, extracelluolar matrix production, growth factor activity and cell cycle. This unit describes the purification of SPARC, also termed osteonectin and BM/40, from cultured mammalian cells. Additional information is presented on the purification of recombinant SPARC (rSPARC) from E. coli and from Sf9 cells, as well as its isolation from blood platelets. Assays for the activity of SPARC, de-adhesion and inhibition of cellular proliferation in vitro, are described. The expression of SPARC during remodeling and repair tissue in response to injury identifies it as a therapeutic target for the treatment of fibrotic disease, certain cancers and other disorders in which regulation of angiogenesis is a key factor.

Automated, computerized, feature-based phenotype analysis of slit lamp images of the mouse lens

Longitudinal studies of a variety of transgenic mouse models for lens development can create substantial challenges in database management and analysis. We report a novel, automated, feature-based informatics approach to screening lens phenotypes in a large database of slit lamp images. Digital slit lamp images of normal and abnormal lenses in eyes of wild type (wt), SC1 null and SPARC null transgenic mice were recorded for quantitative evaluation of their structural phenotype. The images were processed to improve the contrast of structural features that corresponded to rings of opacity and fluctuations in scattering intensity in the lenses. Measurable attributes were assigned to the features in the lens images and given as an output vector of 46 dimensions. Characteristic patterns were correlated with the structural phenotype of each mutant and wt lens and a statistical fit for each phenotype was defined. The genotype was identified correctly in nearly 85% of the slit lamp images on the basis of an automated computer analysis of the lens structural phenotype. The automated computer algorithm has the potential to evaluate a large database of slit lamp images and distinguish mouse genotypes on the basis of lens phenotypes objectively using a neural network analysis of the structural features observed in the slit lamp images. The neural network approach is a promising technology for objective evaluation of genotype/phenotype relationships based on structural features and light scattering in lenses. Further improvements in the automated method can be expected to simplify and increase the accuracy and efficiency of the feature based analysis of structural phenotypes linked to genetic variation.

Compositional differences between infant and adult human corneal basement membranes

PURPOSE

Adult human corneal epithelial basement membrane (EBM) and Descemet's membrane (DM) components exhibit heterogeneous distribution. The purpose of the study was to identify changes of these components during postnatal corneal development.

METHODS

Thirty healthy adult corneas and 10 corneas from 12-day- to 3-year-old children were studied by immunofluorescence with antibodies against BM components.

RESULTS

Type IV collagen composition of infant corneal central EBM over Bowman's layer changed from alpha1-alpha2 to alpha3-alpha4 chains after 3 years of life; in the adult, alpha1-alpha2 chains were retained only in the limbal BM. Laminin alpha2 and beta2 chains were present in the adult limbal BM where epithelial stem cells are located. By 3 years of age, beta2 chain appeared in the limbal BM. In all corneas, limbal BM contained laminin gamma3 chain. In the infant DM, type IV collagen alpha1-alpha6 chains, perlecan, nidogen-1, nidogen-2, and netrin-4 were found on both faces, but they remained only on the endothelial face of the adult DM. The stromal face of the infant but not the adult DM was positive for tenascin-C, fibrillin-1, SPARC, and laminin-332. Type VIII collagen shifted from the endothelial face of infant DM to its stromal face in the adult. Matrilin-4 largely disappeared after the age of 3 years.

CONCLUSIONS

The distribution of laminin gamma3 chain, nidogen-2, netrin-4, matrilin-2, and matrilin-4 is described in the cornea for the first time. The observed differences between adult and infant corneal BMs may relate to changes in their mechanical strength, corneal cell adhesion and differentiation in the process of postnatal corneal maturation.

Endothelial cell migration on surface-density gradients of fibronectin, VEGF, or both proteins

Cell migration is essential to many physiological processes, including angiogenesis, which is critical to the success of implanted biomaterials and tissue-engineered constructs. Gradients play an important role in cell migration. Previous work on cell migration has been mostly executed either in the concentration gradients of stimuli (e.g., VEGF) in bulk or hydrogels or on the surface-density gradients of ECM proteins (e.g., fibronectin) or small ligands (e.g., RGD). Little work has been done to investigate how cell migration responds to the surface-density gradients of growth factors. No work has been done to study how the surface gradients of both adhesive proteins and growth factors influence cell migration. In this work, we studied the effect of the surface-density gradients of fibronectin (FN), VEGF, or both proteins on endothelial cell migration. Gradients with different slopes were prepared to study how the gradient slope affects cell migration. The gradients were generated by first forming a counter-propagating C15COOH/C11OH self-assembled monolayer (SAM) gradient using a surface electrochemistry approach, followed by activating the -COOH moieties and covalently immobilizing proteins onto the surface. Fourier transform infrared spectra and X-ray photoelectron spectroscopy were used to characterize the SAM and protein gradients, respectively. A free cell migration assay using bovine aortic endothelial cells was performed on various gradient surfaces or on surfaces with uniform protein density. Results showed that cells on the surface-density gradients of FN, VEGF, or both proteins moved faster along the gradient direction than on the respective uniform control surface after 24-h cell culture. It is also shown that for each protein or protein combination, the directional cell displacement was not statistically different between two gradients with different slopes. Results show that the directional cell migration was increased by about 2-fold on the VEGF gradient as compared to the FN gradient and was further increased by another 2-fold on the combined gradients of both proteins as compared to the VEGF gradient alone. This is the first work to create surface-density gradients of VEGF and the first study to generate a combined surface gradient of growth factor and ECM protein to investigate their effect on cell migration on surfaces. This work broadens our understanding of the directional movement of endothelial cells. Our findings provide useful information for directing cell migration into tissue-engineered constructs and can be potentially used for those applications where cell migration is critical, such as angiogenesis.

An In Vitro Model of Posterior Capsular Opacity: SPARC and TGF-β2 Minimize Epithelial-to-Mesenchymal Transition in Lens Epithelium

PURPOSE

This report presents a novel model for studies of extracellular matrix (ECM) in posterior capsular opacification (PCO) in vitro. Lens epithelial cells (LEC) were cultured with an intraocular lens (IOL) on a surface of type IV collagen in an evaluation of the importance of the ECM-cell interaction in formation of PCO. Abnormal migration, proliferation, and expression of proteins associated with the epithelial-to-mesenchymal transition (EMT) that characterizes PCO were observed in the presence and absence of the matricellular protein SPARC (secreted protein, acidic and rich in cysteine), which regulates matrix-cell interactions.

METHODS

The model for PCO in vitro consisted of an IOL placed on a membrane coated with collagen IV, a major constituent of the lens capsule. LECs from the lenses of wild-type (WT) and SPARC-null (SP-null) mice were cultured in the presence or absence of 10 ng/mL TGF-beta2 and 20 mug/mL recombinant human SPARC (rhSP) for up to 6 days. The migration of LECs was quantified. Labeling with BrdU and the measurement of DNA synthesis were assays for cell proliferation. Expression of the EMT markers, collagen type I, fibronectin, and alpha-smooth muscle actin were assessed using immunocytochemistry or Western immunoblots.

RESULTS

LEC migration, proliferation, and the synthesis of EMT markers were enhanced in SP-null compared with WT LECs. TGF-beta2 inhibited the migration and proliferation of both WT and SP-null LECs in the presence of rhSP. TGF-beta2 increased the production of collagen type I, fibronectin, and alpha-SMA. The responses of SP-null LECs were rescued by the addition of recombinant human (rh)SP.

CONCLUSIONS

A simple IOL culture system was useful for the evaluation of the effects of SPARC and TGF-beta2 on PCO in vitro. The action of TGF-beta2 on LEC migration and proliferation is influenced by SPARC, a regulator of matrix-cell interactions. The results indicate a functional intersection between pathways activated by TGF-beta2 and SPARC in the formation of PCO.

SPARC from olfactory ensheathing cells stimulates Schwann cells to promote neurite outgrowth and enhances spinal cord repair

Olfactory ensheathing cells (OECs) transplanted into the lesioned CNS can stimulate reportedly different degrees of regeneration, remyelination, and functional recovery, but little is known about the mechanisms OECs may use to stimulate endogenous repair. Here, we used a functional proteomic approach, isotope-coded affinity tagging and mass spectrometry, to identify active components of the OEC secreteome that differentially stimulate outgrowth. SPARC (secreted protein acidic rich in cysteine) (osteonectin) was identified as an OEC-derived matricellular protein that can indirectly enhance the ability of Schwann cells to stimulate dorsal root ganglion outgrowth in vitro. SPARC stimulates Schwann cell-mediated outgrowth by cooperative signal with laminin-1 and transforming growth factor beta. Furthermore, when SPARC-null OECs were transplanted into lesioned rat spinal cord, the absence of OEC-secreted SPARC results in an attenuation of outgrowth of specific subsets of sensory and supraspinal axons and changes the pattern of macrophage infiltration in response to the transplanted cells. These data provide the first evidence for a role for SPARC in modulating different aspects of CNS repair and indicate that SPARC can change the activation state of endogenous Schwann cells, resulting in the promotion of outgrowth in vitro, and in vivo.

A secreted isoform of ErbB3 promotes osteonectin expression in bone and enhances the invasiveness of prostate cancer cells

The propensity for prostate cancer to metastasize to bone led us and others to propose that bidirectional interactions between prostate cancer cells and bone are critical for the preferential metastasis of prostate cancer to bone. We identified previously a secreted isoform of ErbB3 (p45-sErbB3) in bone marrow supernatant samples from men with prostate cancer and bone metastasis and showed by immunohistochemical analysis of human tissue specimens that p45-sErbB3 was highly expressed in metastatic prostate cancer cells in bone. Here, we show that p45-sErbB3 stimulated mouse calvaria to secrete factors that increased the invasiveness of prostate cancer cells in a Boyden chamber invasion assay. Using gene array analysis to identify p45-sErbB3-responsive genes, we found that p45-sErbB3 up-regulated the expression of osteonectin/SPARC, biglycan, and type I collagen in calvaria. We further show that recombinant osteonectin increased the invasiveness of PC-3 cells, whereas osteonectin-neutralizing antibodies blocked this p45-sErbB3-induced invasiveness. These results indicate that p45-sErbB3 enhances the invasiveness of PC-3 cells in part by stimulating the secretion of osteonectin by bone. Thus, p45-sErbB3 may mediate the bidirectional interactions between prostate cancer cells and bone via osteonectin.

Common deleted genes in the 5q- syndrome: thrombocytopenia and reduced erythroid colony formation in SPARC null mice

The commonly deleted region (CDR) for the 5q- syndrome has been identified as a 1.5-megabase interval on human chromosome 5q32. We studied, by real-time reverse-transcription (RT)-PCR, the expression of 33 genes within the CDR that are known to be expressed in CD34+ hematopoietic stem cells. Genes in the 5q- samples that showed the most pronounced decrease in expression compared to non-5q- samples were: solute carrier family 36, member 1 (SLC36A1; 89% downregulated), Ras-GTPase-activating protein SH3 domain-binding (G3BP; 79%), antioxidant protein 1 (ATOX1; 76%), colony-stimulating factor-1 receptor precursor (CSF1R; 76%), ribosomal protein S14 (RPS14; 74%), platelet-derived growth factor receptor-beta (PDGFRB; 73%), Nef-associated factor 1 (TNIP1; 72%), secreted protein, acidic and rich in cysteine (SPARC; 71%), annexin VI (ANAX6; 69%), NSDT (66%) and TIGD (60%). We further studied the hematopoietic system in SPARC-null mice. These mice showed significantly lower platelet counts compared to wild-type animals (P=0.008). Although hemoglobin, hematocrit and mean corpuscular volume (MCV) were lower in mice lacking SPARC, differences were not statistically significant. SPARC-null mice showed a significantly impaired ability to form erythroid burst-forming units (BFU-E). However, no significant differences were found in the formation of erythroid colony-forming units (CFU-E), granulocyte/monocyte colony-forming units (CFU-GM) or megakaryocyte colony-forming units (CFU-Mk) in these animals. We conclude that many of the genes within the CDR associated with the 5q- syndrome exhibit significantly decreased expression and that SPARC, as a potential tumor suppressor gene, may play a role in the pathogenesis of this disease.

SPARC Regulates Processing of Procollagen I and Collagen Fibrillogenesis in Dermal Fibroblasts

A characterization of the factors that control collagen fibril formation is critical for an understanding of tissue organization and the mechanisms that lead to fibrosis. SPARC (secreted protein acidic and rich in cysteine) is a counter-adhesive protein that binds collagens. Herein we show that collagen fibrils in SPARC-null skin from mice 1 month of age were inefficient in fibril aggregation and accumulated in the diameter range of 60-70 nm, a proposed intermediate in collagen fibril growth. In vitro, procollagen I produced by SPARC-null dermal fibroblasts demonstrated an initial preferential association with cell layers, in comparison to that produced by wild-type fibroblasts. However, the collagen I produced by SPARC-null cells was not efficiently incorporated into detergent-insoluble fractions. Coincident with an initial increase in cell association, greater amounts of total collagen I were present as processed forms in SPARC-null versus wild-type cells. Addition of recombinant SPARC reversed collagen I association with cell layers and decreased the processing of procollagen I in SPARC-null cells. Although collagen fibers formed on the surface of SPARC-null fibroblasts earlier than those on wild-type cells, fibers on SPARC-null fibroblasts did not persist. We conclude that SPARC mediates the association of procollagen I with cells, as well as its processing and incorporation into the extracellular matrix.

Expression of integrin-linked kinase in the murine lens is consistent with its role in epithelial-mesenchymal transition of lens epithelial cells in vitro

PURPOSE

To evaluate the expression and location of integrin-linked kinase (ILK) within the mouse lens and to characterize the role of this protein during mouse lens epithelial cells (LEC) differentiation in vitro.

METHODS

Transcription levels of ILK mRNA were determined by RT-PCR in cultured cells and lens tissue. ILK protein was detected by immunoblotting, immunocytochemistry, immunohistochemistry, and immunoprecipitation. A role for ILK in the outgrowth of LEC from dissected mouse lens explants was determined by the use of ILK short interfering RNA (siRNA). Affinity-purified polyclonal anti-recombinant human ILK IgG was prepared and characterized for these experiments. A comparison of several anti-ILK antibodies was performed by immunoblotting, immunoprecipitation, and ELISA.

RESULTS

ILK was transcribed in LEC and lens fiber cells in vivo. ILK protein was expressed in the differentiating LEC at the equatorial region of the lens and, to a lesser extent, within the cortical and nuclear fiber cells. LEC in vitro produced copious ILK, which exhibited a filamentous pattern throughout the cytoplasm. The expression of ILK was increased during epithelial-mesenchymal-transition (EMT) of LEC from lens explants, whereas inhibition of ILK by siRNA delayed expression of the EMT markers smooth muscle alpha-actin and fibronectin.

CONCLUSIONS

Analysis of ILK expression, localization, and activity in the mouse lens and cultured LEC is substantially facilitated by the generation of a multi-functional, polyclonal, affinity-purified anti-ILK antibody. Expressed in most tissues and cells lines, ILK is unexpectedly restricted to the equatorial LEC and differentiated fiber cells of the mouse lens. The occurrence of ILK expression with LEC differentiation is consistent with the positive regulatory function of ILK, which is revealed in a model of EMT in vitro. This is the first study to show the expression of ILK in the lens and its unique distribution pattern within cultured lens epithelia.

Tumor-suppressor function of SPARC-like protein 1/Hevin in pancreatic cancer

SPARC-like protein 1 (SPARCL1), a member of the SPARC family, is downregulated in various tumors. In the present study, the expression and localization of SPARCL1 were analyzed in a wide range of nontumorous and neoplastic pancreatic tissues by quantitative reverse transcription-polymerase chain reaction, laser capture microdissection, microarray analysis, and immunohistochemistry. For functional analysis, proliferation and invasion assays were used in cultured pancreatic cancer cells. Pancreatic ductal adenocarcinoma (PDAC) and other pancreatic neoplasms exhibited increased SPARCL1 mRNA levels compared to those of the normal pancreas. SPARCL1 mRNA levels were low to absent in microdissected and cultured pancreatic cancer cells, and promoter demethylation increased SPARCL1 levels only slightly in three of eight cell lines. SPARCL1 was observed in small capillaries in areas of inflammation/tumor growth and in some islet cells. In PDAC, 15.4% of vessels were SPARCL1-positive. In contrast, the percentage of SPARCL1-positive vessels was higher in chronic pancreatitis and benign and borderline pancreatic tumors. Recombinant SPARCL1 inhibited pancreatic cancer cell invasion and exerted moderate growth-inhibitory effects. In conclusion, SPARCL1 expression in pancreatic tissues is highly correlated with level of vascularity. Its anti-invasive effects and reduced expression in metastasis indicate tumor-suppressor function.

Inhibition of melanoma cell motility by the snake venom disintegrin eristostatin

Eristostatin, an RGD-containing disintegrin isolated from the venom of Eristicophis macmahoni, inhibits lung or liver colonization of melanoma cells in a mouse model. In this study, transwell migration and in vitro wound closure assays were used to determine the effect of eristostatin on the migration of melanoma cells. Eristostatin significantly impaired the migration of five human melanoma cell lines. Furthermore, it specifically inhibited cell migration on fibronectin in a concentration-dependent manner, but not that on collagen IV or laminin. In contrast, eristostatin was found to have no effect on cell proliferation or angiogenesis. These results indicate that the interaction between eristostatin and melanoma cells may involve fibronectin-binding integrins that mediate cell migration. Mutations to alanine of seven residues within the RGD loop of eristostatin and four residues outside the RGD loop of eristostatin resulted in significantly less potency in both platelet aggregation and wound closure assays. For six of the mutations, however, decreased activity was found only in the latter assay. We conclude that a different mechanism and/or integrin is involved in these two cell activities.

SPARC is a VCAM-1 counter-ligand that mediates leukocyte transmigration

VCAM-1 is a cell surface molecule, which has been shown to mediate leukocyte adhesion to the endothelium and subsequent transmigration. Although VCAM-1 regulates adhesion through its interaction with VLA-4, VLA-4 does not play a role in VCAM-1-dependent diapedesis, an observation suggesting the presence of a second ligand for VCAM-1. We now report a novel interaction between VCAM-1 and secreted protein acidic and rich in cysteine (SPARC), which induces actin cytoskeletal rearrangement and intercellular gaps, physiological processes known to be important for leukocyte transmigration. The binding of leukocyte-derived SPARC to VCAM-1 was demonstrated to be necessary for leukocyte transmigration through endothelial monolayers (diapedesis) in vitro, and furthermore, SPARC null mice have abnormalities in leukocyte recruitment to the inflamed peritoneum in vivo. These findings provide new insight into the mechanisms of transendothelial leukocyte migration and suggest a potential, targetable interaction for therapeutic intervention.

Regulation of secreted protein acidic and rich in cysteine during adipose conversion and adipose tissue hyperplasia

OBJECTIVE

To explore the regulation of secreted protein acidic and rich in cysteine (SPARC) expression and its role in adipose tissue.

RESEARCH METHODS AND PROCEDURES

We studied the regulation of SPARC expression in transgenic mice expressing the human beta3 and alpha2 adrenergic receptors on a murine beta3 adrenergic receptor null background that became obese under a high-fat diet mainly as a result of adipose tissue hyperplasia. Furthermore, we analyzed its expression in human adipose tissue and its regulation during adipocyte differentiation.

RESULTS

SPARC protein in adipose tissue was increased in obese transgenic mice compared with control mice, indicating that SPARC expression was associated with adipose tissue hyperplasia. Both SPARC mRNA and protein were detected in human adipose tissue. Comparing adipocytes and vascular stroma, we found that SPARC expression was mainly associated with the adipocyte fraction. Consistent with this, SPARC transcript increased during differentiation of human primary preadipocytes. 3T3-L1 preadipocytes showed an increase in SPARC expression in differentiated cells but with biphasic expression during the process. After induction in committed cells, SPARC mRNA and protein levels declined as differentiation began and returned to elevated levels in fully differentiated adipocytes.

DISCUSSION

SPARC expression correlated with adipose tissue hyperplasia and adipogenesis. Therefore, SPARC seems to play a role in adipose tissue physiology as it is involved in growth and differentiation.

Gene expression profiling of human ovarian tumours

There is currently a lack of reliable diagnostic and prognostic markers for ovarian cancer. We established gene expression profiles for 120 human ovarian tumours to identify determinants of histologic subtype, grade and degree of malignancy. Unsupervised cluster analysis of the most variable set of expression data resulted in three major tumour groups. One consisted predominantly of benign tumours, one contained mostly malignant tumours, and one was comprised of a mixture of borderline and malignant tumours. Using two supervised approaches, we identified a set of genes that distinguished the benign, borderline and malignant phenotypes. These algorithms were unable to establish profiles for histologic subtype or grade. To validate these findings, the expression of 21 candidate genes selected from these analyses was measured by quantitative RT–PCR using an independent set of tumour samples. Hierarchical clustering of these data resulted in two major groups, one benign and one malignant, with the borderline tumours interspersed between the two groups. These results indicate that borderline ovarian tumours may be classified as either benign or malignant, and that this classifier could be useful for predicting the clinical course of borderline tumours. Immunohistochemical analysis also demonstrated increased expression of CD24 antigen in malignant versus benign tumour tissue. The data that we have generated will contribute to a growing body of expression data that more accurately define the biologic and clinical characteristics of ovarian cancers.

Matricellular hevin regulates decorin production and collagen assembly

Matricellular proteins such as SPARC, thrombospondin 1 and 2, and tenascin C and X subserve important functions in extracellular matrix synthesis and cellular adhesion to extracellular matrix. By virtue of its reported interaction with collagen I and deadhesive activity on cells, we hypothesized that hevin, a member of the SPARC gene family, regulates dermal extracellular matrix and collagen fibril formation. We present evidence for an altered collagen matrix and levels of the proteoglycan decorin in the normal dermis and dermal wound bed of hevin-null mice. The dermal elastic modulus was also enhanced in hevin-null animals. The levels of decorin protein secreted by hevin-null dermal fibroblasts were increased by exogenous hevin in vitro, data indicating that hevin might regulate both decorin and collagen fibrillogenesis. We also report a decorin-independent function for hevin in collagen fibrillogenesis. In vitro fibrillogenesis assays indicated that hevin enhanced fibril formation kinetics. Furthermore, cell adhesion assays indicated that cells adhered differently to collagen fibrils formed in the presence of hevin. Our observations support the capacity of hevin to modulate the structure of dermal extracellular matrix, specifically by its regulation of decorin levels and collagen fibril assembly.

A role for decorin in cutaneous wound healing and angiogenesis

Decorin is known to influence tissue tensile strength and cellular phenotype. Therefore, decorin is likely to have an impact on tissue repair, including cutaneous wound healing. In this study, cutaneous healing of both excisional and incisional full-thickness dermal wounds was studied in decorin-deficient (Dcn(-/-)) animals. A statistically significant delay in excisional wound healing in the Dcn(-/-) mice occurred at 4 and 10 days postwounding and, in incisional wounds at 4, 10, and 18 days when compared with wild-type (Dcn(-/-)) controls. Fibrovascular invasion into polyvinylalcohol sponges was significantly increased by day 18 in Dcn(-/-) mice relative to Dcn(+/+) mice. The 18-day sponge implants in the Dcn(-/-) mice showed a marked accumulation of biglycan when compared with the corresponding implants in Dcn(+/+) mice. Thus, regulated production of decorin may serve as an excellent therapeutic approach for modifying impaired wound healing and harmful foreign body reactions.

Absence of SPARC in lens epithelial cells results in altered adhesion and extracellular matrix production in vitro

The matricellular protein SPARC (also known as osteonectin and BM-40) is expressed abundantly in lens epithelium. That SPARC-null mice exhibit early cataractogenesis, indicates a role for SPARC in the maintenance of lens transparency. Comparison of cultured wild-type and SPARC-null lens epithelial cells revealed significant changes in adhesion to different substrates. SPARC-null lens cells displayed enhanced attachment and spreading, focal adhesion formation, and resistance to trypsin detachment in comparison to wild-type cells. In the absence of SPARC, there was increased deposition of the ECM protein laminin-1 (LN-1). Proteins associated with focal adhesions were increased in SPARC-null versus wild-type lens cells: levels of alpha6-integrin heterodimers, talin, and paxillin phosphorylated on tyrosine were enhanced significantly, as was the association of beta1-integrin with talin and paxillin. Restoration of the wild-type phenotype in SPARC-null cultures was accomplished through genetic rescue by stable transfection of SPARC cDNA. Our findings indicate that SPARC is counter-adhesive for murine lens epithelial cells and demonstrate that multiple factors contribute to this activity. We also identify SPARC as a modulator of LN-1 secretion and deposition by these cells, an activity important in epithelial cell-ECM interactions in the ocular lens.

Novel Function of Alternatively Activated Macrophages: Stabilin-1-Mediated Clearance of SPARC

The matricellular protein SPARC (secreted protein acidic and rich in cysteine) has been implicated in development, differentiation, response to injury, and tumor biology by virtue of its regulation of extracellular matrix production/assembly and its antiadhesive and antiproliferative effects on different cell types. Despite numerous biological activities described for SPARC, cell surface receptors for this protein have not been identified. By phage display and in vitro-binding assays, we now show that SPARC interacts with stabilin-1, a scavenger receptor expressed by tissue macrophages and sinusoidal endothelial cells. The interaction is mediated by the extracellular epidermal growth factor-like region of stabilin-1 containing the sequence FHGTAC. Using FACS analysis and confocal microscopy, we demonstrate that stabilin-1 internalizes and targets SPARC to an endosomal pathway in Chinese hamster ovary cells stably transfected with this receptor. In human macrophages, stabilin-1 expression is required for receptor-mediated endocytosis of SPARC. SPARC was efficiently endocytosed by alternatively activated macrophages stimulated by IL-4 and dexamethasone, but not solely by Th1 or Th2 cytokines. A time course of ligand exposure to alternatively activated macrophages revealed that stabilin-1-mediated endocytosis of SPARC was followed by its targeting for degradation, similar to the targeting of acetylated low density lipoprotein, another stabilin-1 ligand. We propose that alternatively activated macrophages coordinate extracellular matrix remodeling, angiogenesis, and tumor progression via stabilin-1-mediated endocytosis of SPARC and thereby regulate its extracellular concentration.

Loss of SPARC-mediated VEGFR-1 suppression after injury reveals a novel antiangiogenic activity of VEGF-A

VEGF-A promotes angiogenesis in many tissues. Here we report that choroidal neovascularization (CNV) incited by injury was increased by excess VEGF-A before injury but was suppressed by VEGF-A after injury. This unorthodox antiangiogenic effect was mediated via VEGFR-1 activation and VEGFR-2 deactivation, the latter via Src homology domain 2-containing (SH2-containing) tyrosine phosphatase-1 (SHP-1). The VEGFR-1-specific ligand placental growth factor-1 (PlGF-1), but not VEGF-E, which selectively binds VEGFR-2, mimicked these responses. Excess VEGF-A increased CNV before injury because VEGFR-1 activation was silenced by secreted protein, acidic and rich in cysteine (SPARC). The transient decline of SPARC after injury revealed a temporal window in which VEGF-A signaling was routed principally through VEGFR-1. These observations indicate that therapeutic design of VEGF-A inhibition should include consideration of the level and activity of SPARC.

Absence of SPARC in murine lens epithelium leads to increased deposition of laminin-1 in lens capsule

PURPOSE

To investigate the role of SPARC in the regulation of the expression and deposition of extracellular matrix (ECM) proteins in the lens capsule.

METHODS

Wild-type (SP+/+) and SPARC-null (SP-/-) mice of embryonic day (E)14 to 3 months of age were examined. Transcript levels of lens basement membrane (BM) components were analyzed by semiquantitative RT-PCR with mRNA from lens epithelia. Expression of ECM proteins in lens capsule and lens epithelium was analyzed by immunohistochemistry and Western blot analysis. Cell attachment was assessed by lens epithelial explant culture. Coimmunoprecipitation was performed to identify intracellular protein interactions.

RESULTS

From postnatal day 5 to 3 months of age, SPARC-null lens capsules exhibited higher levels of laminin-1 deposition relative to their wild-type counterparts, as revealed by immunohistochemistry and immunoblot analysis. An uneven and aggregated distribution of laminin-1 protein was apparent in the anterior region of SPARC-null lens capsules. SPARC and laminin-1 were expressed abundantly in the endoplasmic reticulum (ER) of lens epithelial cells. Coimmunoprecipitation identified that SPARC associates with laminin-1 before laminin secretion. Furthermore, increased laminin-1 in lens capsule promoted the attachment of lens epithelial explants in culture.

CONCLUSIONS

SPARC affects the secretion and deposition of laminin-1 protein in lens epithelial cells. Because abnormal deposition of laminin-1 in the lens BM could influence lens epithelial cell adhesion and fiber cell differentiation, the authors propose that SPARC is important to lens homeostasis through its regulation of lens BM matrix organization.

Chaperone-like activity revealed in the matricellular protein SPARC

SPARC (Secreted Protein, Acidic and Rich in Cysteine) is a matricellular glycoprotein that modulates cell proliferation, adhesion, migration, and extracellular matrix (ECM) production. In this report chaperone-like activity of SPARC was identified in a thermal aggregation assay in vitro. Ultraviolet circular dichroism (UVCD) spectroscopy determined that SPARC was stable at temperatures up to 50 degrees C. Unfolding and aggregation of the chaperone target protein, alcohol dehydrogenase (ADH), were initiated at 50 degrees C. SPARC inhibited the thermal aggregation of ADH in a concentration-dependent manner, with maximal inhibition at a 1:4 molar ratio of SPARC:ADH. Synergy between the chaperone-like activities of SPARC and alphaB-crystallin, a small heat shock protein and molecular chaperone in the lens, was observed in SPARC-alphaB-crystallin double -/- mice.

Secreted protein acidic and rich in cysteine (SPARC/osteonectin/BM-40) binds to fibrinogen fragments D and E, but not to native fibrinogen

Secreted protein acidic and rich in cysteine (SPARC/osteonectin/BM-40) is a matricellular protein that functions in wound healing. Fibrinogen is a plasma protein involved in many aspects of wound healing, such as inflammation, fibrosis and thrombosis. In this study, the binding of SPARC to both native and plasmin-cleaved fibrinogen under physiological conditions was examined by the use of a surface plasmon resonance (SPR) biosensor. We show that SPARC binds to plasmin-cleaved fibrinogen, but not to native fibrinogen. SPARC binds to both fibrinogen fragments D and E fg D and fg E with similar dissociation constants (8.67 x 10(-8) M for Fg D and 1.61 x 10(-7) M for Fg E). Results from endothelial cell proliferation assays show that the binding of SPARC to Fg E suppressed the inhibition of proliferation by SPARC, whereas the binding of SPARC to Fg D did not influence the activity of SPARC on the cell cycle. The interaction of SPARC with fibrinogen fragments D and E, which are produced as a result of proteolytic activation of fibrinolysis, reveals potential storage sites in provisional extracellular matrix for SPARC during the wound healing process and indicates a regulatory role of SPARC in fibrinolysis and angiogenesis.