Differential effects of SPARC and cationic SPARC peptides on DNA synthesis by endothelial cells and fibroblasts

SPARC inhibits epithelial cell proliferation in part through stimulation of the transforming growth factor-beta-signaling system

Secreted protein, acidic and rich in cysteine (SPARC) is a multifunctional secreted protein that regulates cell-cell and cell-matrix interactions, leading to alterations in cell adhesion, motility, and proliferation. Although SPARC is expressed in epithelial cells, its ability to regulate epithelial cell growth remains largely unknown. We show herein that SPARC strongly inhibited DNA synthesis in transforming growth factor (TGF)-beta-sensitive Mv1Lu cells, whereas moderately inhibiting that in TGF-beta-insensitive Mv1Lu cells (i.e., R1B cells). Overexpression of dominant-negative Smad3 in Mv1Lu cells, which abrogated growth arrest by TGF-beta, also attenuated growth arrest stimulated by SPARC. Moreover, the extracellular calcium-binding domain of SPARC (i.e., SPARC-EC) was sufficient to inhibit Mv1Lu cell proliferation but not that of R1B cells. Similar to TGF-beta and thrombospondin-1, treatment of Mv1Lu cells with SPARC or SPARC-EC stimulated Smad2 phosphorylation and Smad2/3 nuclear translocation: the latter response to all agonists was abrogated in R1B cells or by pretreatment of Mv1Lu cells with neutralizing TGF-beta antibodies. SPARC also stimulated Smad2 phosphorylation in MB114 endothelial cells but had no effect on bone morphogenetic protein-regulated Smad1 phosphorylation in either Mv1Lu or MB114 cells. Finally, SPARC and SPARC-EC stimulated TGF-beta-responsive reporter gene expression through a TGF-beta receptor- and Smad2/3-dependent pathway in Mv1Lu cells. Collectively, our findings identify a novel mechanism whereby SPARC inhibits epithelial cell proliferation by selectively commandeering the TGF-beta signaling system, doing so through coupling of SPARC-EC to a TGF-beta receptor- and Smad2/3-dependent pathway.

The collagen type XVIII endostatin domain is co-localized with perlecan in basement membranes in vivo

The C-terminal globular endostatin domain of collagen type XVIII is anti-angiogenic in a variety of experimental tumor models, and clinical trials to test it as an anti-tumor agent are already under way. In contrast, many of its cell biological properties are still unknown. We systematically localized the mRNA of collagen type XVIII with the help of in situ hybridization (ISH) and detected it in epithelial and mesenchymal cells of almost all organ systems throughout mouse development. Light and electron microscopic immunohistochemistry (IHC) revealed that the endostatin domain is a widespread component of almost all epithelial basement membranes in all major developing organs, and in all basement membranes of capillaries and blood vessels. Furthermore, quantitative immunogold double labeling demonstrated a co-localization of 50% of the detected endostatin domain together with perlecan in basement membranes in vivo. We conclude that the endostatin domain of collagen type XVIII plays a role, even in early stages of mouse development, other than regulating angiogenesis. In the adult, the endostatin domain could well be involved in connecting collagen type XVIII to the basement membrane scaffolds. At least in part, perlecan appears to be an adaptor molecule for the endostatin domain in basement membranes in vivo.

cDNA array analysis of SPARC-modulated changes in glioma gene expression

We have demonstrated that secreted protein acidic and rich in cysteine (SPARC) is highly expressed in human gliomas and it promotes glioma invasion and delays tumor growth in vitro and in vivo. cDNA array analyses were performed to determine whether SPARC, which interacts at the cell surface, has an impact on intracellular signaling and downstream gene expression changes, which might account for some of its effects on invasion and growth. Using a doxycycline (dox)-controlled gene expression system, two cDNA array analyses were performed using a parental U87T2 clone (-SPARC) transfected with the dox-controlled transactivator and a U87T2 parental-derived SPARC-transfected clone, A2b2 (+SPARC). Array analysis performed between the parental and the SPARC-transfected clone (-dox) identified 13 upregulated genes and 14 downregulated genes. With the exception of PAI-1 and MMP2, the identified genes are novel with respect to SPARC's mechanism of action. Array analysis performed using the SPARC-transfected clone ( +/- dox) identified 2 types of gene regulation; one reversible upon SPARC suppression, the other irreversible. Two of the SPARC-induced genes, BIGH3 (irreversible by dox) and PAI-1 (reversible by dox) were further studied in additional SPARC-transfected clones, human astrocytoma tissues, and human glioma cell lines by RT-PCR and Northern blot analyses. The results indicate that: (1) the array results were validated, (2) the dox regulation was validated, and (3) the differential expression identified by the array analyses was present between normal brain and in human astrocytoma tissues and cell lines. Therefore, we conclude that these cDNA array analyses provide candidate genes involved in SPARC-mediated effects on glioma cell cycle progression, signaling, and migration, and that SPARC may induce reversible and irreversible gene expression changes. Further investigation of these candidates may shed insights into SPARC's role in glioma cell proliferation and invasion, and potential use as a therapeutic target.

SPARC stimulates the synthesis of OPG/OCIF, MMP-2 and DNA in human periodontal ligament cells

BACKGROUND

Osteonectin/secreted protein, acidic and rich in cysteine (SPARC) is expressed in periodontal ligaments. Therefore, a better understanding of the action of SPARC on periodontal ligament cells could help to elucidate remodelling and repair mechanisms in periodontal tissue. In the present study, we examined the effects of human platelet-derived SPARC (hp-SPARC) on the expressions of SPARC and osteoprotegerin/osteoclastogenesis inhibitory factor (OPG/OCIF), alkaline phosphatase (ALPase) activity, matrix metalloproteinase-2 (MMP-2) production and DNA synthesis in cultures of human periodontal ligament (HPL) cells.

METHODS

HPL cells at the sixth passage were exposed to hp-SPARC. The expression of OPG/OCIF and SPARC mRNAs was examined by Northern blot analysis. The protein levels for OPG/OCIF and MMP-2 were determined by Western blot analysis. ALPase activity was measured by the method of Bessey et al. DNA synthesis was estimated by incorporation of [3H] thymidine.

RESULTS

Hp-SPARC enhanced OPG/OCIF synthesis at the protein and mRNA levels. Hp-SPARC also enhanced DNA and MMP-2 synthesis dose-dependently, but had little effect on ALPase activity and SPARC mRNA expression.

CONCLUSION

SPARC may play a role in remodelling and repair of periodontal tissue by promoting proliferation and MMP-2 production. It may also regulate osteoclast formation through OPG/OCIF in periodontal ligament cells.

Inhibition of PDGF-stimulated and matrix-mediated proliferation of human vascular smooth muscle cells by SPARC is independent of changes in cell shape or cyclin-dependent kinase inhibitors

Interactions among growth factors, cells, and extracellular matrix regulate proliferation during normal development and in pathologies such as atherosclerosis. SPARC (secreted protein, acidic, and rich in cysteine) is a matrix-associated glycoprotein that modulates the adhesion and proliferation of vascular cells. In this study, we demonstrate that SPARC inhibits human arterial smooth muscle cell proliferation stimulated by platelet-derived growth factor or by adhesion to monomeric type I collagen. Binding studies with SPARC and SPARC peptides indicate specific and saturable interaction with smooth muscle cells that involves the C-terminal Ca2+-binding region of the protein. We also report that SPARC arrests monomeric collagen-supported smooth muscle cell proliferation in the late G1-phase of the cell cycle in the absence of an effect on cell shape or on levels of cyclin-dependent kinase inhibitors. Cyclin-dependent kinase-2 activity, p107 and cyclin A levels, and retinoblastoma protein phosphorylation are markedly reduced in response to the addition of exogenous SPARC and/or peptides derived from specific domains of SPARC. Thus, SPARC, previously characterized as an inhibitor of platelet-derived growth factor binding to its receptor, also antagonizes smooth muscle cell proliferation mediated by monomeric collagen at the level of cyclin-dependent kinase-2 activity.

Aging and survival of cutaneous microvasculature

The growth and turnover of blood vessels in the skin is fundamental in normal development, wound repair, hair follicle cycling, tumor cell metastasis, and in many different states of cutaneous pathology. Whereas many investigations are focused on mechanisms of angiogenesis in the skin, the influence of cellular aging and replicative senescence (i.e., the inability, after a critical number of population doublings, to replicate) on microvascular remodeling events has received relatively less attention. In this article, we review the clinical and pathologic relationships associated with cutaneous vascular aging and update current knowledge of endothelial cell survival characteristics. A hypothesis is presented in which endothelial cell aging and survival are linked to molecular mechanisms controlling cell proliferation, quiescence, apoptosis, and cellular senescence. We review recent results demonstrating how activation of telomerase in human dermal microvascular endothelial cells affects their durability both in vitro and in vivo and conclude by linking these studies with current concepts involving endothelial cell precursors, control of postnatal somatic cell telomerase activity, and murine model systems.

SPARC (secreted protein acidic and rich in cysteine) induces apoptosis in ovarian cancer cells

Secreted protein acidic and rich in cysteine (SPARC) is an extracellular Ca(2+)-binding matricellular glycoprotein that associates with cell populations undergoing migration, morphogenesis, and differentiation. Studies on endothelial cells have established that its principal functions in vitro are counteradhesion and antiproliferation. The mechanism(s) underlying these antitumor effects is unknown. In this study, we showed that SPARC expression in ovarian cancer cells is inversely correlated with the degree of malignancy. The immunohistochemical data presented here confirmed the importance of diminished SPARC expression in ovarian cancer development. Treating human ovarian surface epithelial cells and ovarian cancer cells with SPARC revealed that as SPARC inhibits the proliferation of both normal and cancer cells, it induces apoptosis only in cancer cells. This observation indicates that down-regulation of SPARC is essential for ovarian carcinogenesis as cancer cells become sensitized to the apoptotic activity of SPARC during malignant transformation. We also showed here the first direct evidence that putative SPARC receptors are present on ovarian epithelial cells. Their levels are higher in human ovarian surface epithelial cells than cancer cells. Binding of SPARC to its receptor is likely to trigger tissue-specific signaling pathways that mediate its tumor suppressing functions. Decrease in ligand-receptor interaction by the down-regulation of SPARC and/or its receptor is essential for ovarian carcinogenesis.

SPARC modulates cell growth, attachment and migration of U87 glioma cells on brain extracellular matrix proteins

We have identified secreted protein acidic and rich in cysteine (SPARC) as a potential glioma invasion-promoting gene. To determine whether SPARC alters the growth, attachment, or migration of gliomas, we have used U87T2 and doxycycline-regulatable SPARC-transfected clones to examine the effects of SPARC on (1) cell growth, (2) cell cycle progression, (3) cell attachment, and (4) cell migration, using growth curves, flow cytometry, attachment, and migration analyses on different brain ECMs, including collagen IV, laminin, fibronectin, vitronectin, hyaluronic acid, and tenascin. Our data indicate that SPARC delays tumor cell growth in the log phase of the growth curve. The clones secreted different levels of SPARC. The clone secreting the lowest level of SPARC was associated with a higher percentage of cells in G2M, whereas the clones secreting the higher levels of SPARC were associated with a greater percentage of cells in G0/G1. In comparison to the parental U87T2 clone, the SPARC-transfected clones demonstrated increased attachment to collagen, laminin, hyaluronic acid, and tenascin, but not to vitronectin or fibronectin. SPARC-transfected clones also demonstrated altered migration on the different extracellular matrix proteins. The modulation of migration, either positive or negative, was associated with changes in the level of secreted SPARC. These data suggest that SPARC may modulate glioma proliferation and invasion by modulating both the growth and migration of glioma cells.

Reduced expression of laminin alpha 3 and alpha 5 chains in non-small cell lung cancers

The basement membrane is considered to act as a barrier which hinders cancer cells from invading the surrounding stroma. In order to assess changes in essential components during neoplasia in the lung, we immunohistochemically studied distribution patterns of laminins alpha 3 and alpha 5 in 40 adenocarcinomas and 8 squamous cell carcinomas. The a 5 chain was generally preserved at the periphery, frequently disrupted in foci with alveolar collapse and absent in foci of fibroblastic proliferation within adenocarcinomas. Fragmentation and absence of laminin alpha 3 chain were more prominent than for alpha 5 chain. Laminin alpha 3 chain was partially fragmented or absent in peripheral areas of adenocarcinomas, being significantly different from alpha 5 chain. Non-small cell lung cancers with reduced alpha 5 chain showed a tendency for greater lymph node metastasis. In cultured normal air way epithelial cells, both laminin alpha 3 and alpha 5 chains were found to be expressed by northern analysis. Eleven of the twelve cultured lung cancer cell lines did not express alpha 3 chain and expression of alpha 5 chain was reduced in three. Quantitative RT-PCR analysis also demonstrated expression of laminin alpha 3 chain in adenocarcinoma tissues to be significantly lower than in normal lung tissues. These results suggest that expression of laminin alpha chains is often reduced in lung cancer cells and this might contribute to basement membrane fragmentation and subsequent proliferation of stromal elements, as well as play some role in the process of cancer cell invasion.

SPARC, a matricellular protein: at the crossroads of cell-matrix communication

SPARC is a multifunctional glycoprotein that belongs to the matricellular group of proteins. It modulates cellular interaction with the extracellular matrix (ECM) by its binding to structural matrix proteins, such as collagen and vitronectin, and by its abrogation of focal adhesions, features contributing to a counteradhesive effect on cells. SPARC inhibits cellular proliferation by an arrest of cells in the G1 phase of the cell cycle. It also regulates the activity of growth factors, such as platelet-derived growth factor (PDGF), fibroblast growth factor (FGF)-2, and vascular endothelial growth factor (VEGF). The expression of SPARC in adult animals is limited largely to remodeling tissue, such as bone, gut mucosa, and healing wounds, and it is prominent in tumors and in disorders associated with fibrosis. The crystal structure of two of the three domains of the protein has revealed a novel follistatin-like module and an extracellular calcium-binding (EC) module containing two EF-hand motifs. The follistatin-like module and the EC module are shared by at least four other proteins that comprise a family of SPARC-related genes. Targeted disruption of the SPARC locus in mice has shown that SPARC is important for lens transparency, as SPARC-null mice develop cataracts shortly after birth. SPARC is a prototypical matricellular protein that functions to regulate cell-matrix interactions and thereby influences many important physiological and pathological processes.

SPARC, a matricellular protein: at the crossroads of cell-matrix

SPARC is a multifunctional glycoprotein that belongs to the matricellular group of proteins. It modulates cellular interaction with the extracellular matrix (ECM) by its binding to structural matrix proteins, such as collagen and vitronectin, and by its abrogation of focal adhesions, features contributing to a counteradhesive effect on cells. SPARC inhibits cellular proliferation by an arrest of cells in the G1 phase of the cell cycle. It also regulates the activity of growth factors, such as platelet-derived growth factor (PDGF), fibroblast growth factor (FGF)-2, and vascular endothelial growth factor (VEGF). The expression of SPARC in adult animals is limited largely to remodeling tissue, such as bone, gut mucosa, and healing wounds, and it is prominent in tumors and in disorders associated with fibrosis. The crystal structure of two of the three domains of the protein has revealed a novel follistatin-like module and an extracellular calcium-binding (EC) module containing two EF-hand motifs. The follistatin-like module and the EC module are shared by at least four other proteins that comprise a family of SPARC-related genes. Targeted disruption of the SPARC locus in mice has shown that SPARC is important for lens transparency, as SPARC-null mice develop cataracts shortly after birth. SPARC is a prototypical matricellular protein that functions to regulate cell-matrix interactions and thereby influences many important physiological and pathological processes.

Endostatins derived from collagens XV and XVIII differ in structural and binding properties, tissue distribution and anti-angiogenic activity

Endostatin is a fragment of the C-terminal domain NC1 of collagen XVIII that inhibits angiogenesis and tumor growth. We report the characterization of a collagen XV endostatin analogue and its parent NC1 domain, obtained by recombinant expression in mammalian cells. Both NC1 domains contain a trimerization domain, a hinge region that is more sensitive to proteolysis in collagen XVIII and the endostatin domain. Unlike endostatin-XVIII, endostatin-XV does not bind zinc or heparin, which is explained by the crystal structure of endostatin-XV. The collagen XV and XVIII fragments inhibited chorioallantoic membrane angiogenesis induced by basic fibroblast growth factor (FGF-2) or vascular endothelial growth factor (VEGF), but there are striking differences depending on which cytokine is used and whether free endostatins or NC1 domains are applied. The collagen XV and XVIII fragments showed a similar binding repertoire for extracellular matrix proteins. Differences were found in the immunohistological localization in vessel walls and basement membrane zones. Together, these data indentify endostatin-XV as an angiogenesis inhibitor, which differs from endostatin-XVIII in several important functional details.

Expression and characterization of SPARC in human lens and in the aqueous and vitreous humors

SPARC (secreted protein, acidic and rich in cysteine) is a matricellular glycoprotein that regulates morphogenesis, cellular proliferation, and differentiation. SPARC is a critical factor in the development and maintenance of lens transparency in mice. SPARC-null mice develop lenticular opacity at an early age that progresses gradually to mature cataract. Despite the high level of homology between the mouse and human genes, little is known about SPARC in the human lens. We have studied the expression of SPARC protein in human lens and surrounding ocular tissues from normal human donors (60-70 years old). Immunohistochemical and immunoblot analyses were conducted on lens, aqueous humor, vitreous, ciliary epithelium, pigment epithelium, cornea and retina. The epithelia and capsule of the lens contained SPARC, whereas the cortical and nuclear fibers did not. In contrast, the aqueous humor and vitreous, which provide nutrients to the lens and regulate its development and function, contained significant amounts of SPARC. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of extracts of various ocular tissues revealed bands of 43 and 29 kD after disulfide bond reduction that were reactive with anti-SPARC IgG. Despite the presence of protease inhibitors during sample preparation, we observed cleavage of intact SPARC to a 29 kD fragment, a peptide reported in other tissues and attributed to endogenous proteolysis. In addition, bands of molecular mass 150 and 200 kD were present that appeared to be disulfide-bonded complexes of SPARC monomers. Human cornea, ciliary epithelium, pigment epithelium and retina also contained SPARC. The presence of SPARC in the aqueous humor and vitreous, as well as in the lens, indicates a functional importance of SPARC in adult human eye as well as in lens development.

Down-regulation of the extracellular matrix protein SPARC in vSrc- and vJun-transformed chick embryo fibroblasts contributes to tumor formation in vivo

In vitro transformation of primary cultures of chick embryo fibroblasts by the membrane-bound vSrc or the nuclear vJun oncoproteins is correlated with a down-regulation of the secreted glycoprotein SPARC (also called BM-40 or osteonectin). This protein is a nonstructural component of the extracellular matrix that is thought to regulate cell-matrix interaction during development, wound repair, and carcinogenesis. Its precise function remains unclear. To estimate the contribution of SPARC down-regulation to the major aspects of the transformed phenotype, we have reexpressed this protein from a self-replicating retrovirus Rcas, designated R-SPARC, in the transformed cultures. These R-SPARC-infected cultures display the following main properties: (i) they accumulate the SPARC protein to a level identical to or only slightly higher than the level in normal chick embryo fibroblasts; (ii) they retain the main phenotypic properties characteristic of in vitro transformed cells, that is, altered morphology, capacity to grow in a reduced amount of serum, and capacity to develop colonies from single cells in agar; (iii) they display a clearly reduced capacity to develop local fibrosarcomas in vivo. Taken together, these data strongly suggest that down-regulation of SPARC contributes to the transformed phenotype triggered by vSrc and vJun in primary avian fibroblasts, by facilitating in vivo tumorigenesis.

Extracellular matrix-associated protein Sc1 is not essential for mouse development

Sc1 is an extracellular matrix-associated protein whose function is unknown. During early embryonic development, Sc1 is widely expressed, and from embryonic day 12 (E12), Sc1 is expressed primarily in the developing nervous system. This switch in Sc1 expression at E12 suggests an importance for nervous-system development. To gain insight into Sc1 function, we used gene targeting to inactivate mouse Sc1. The Sc1-null mice showed no obvious deficits in any organs. These mice were born at the expected ratios, were fertile, and had no obvious histological abnormalities, and their long-term survival did not differ from littermate controls. Therefore, the function of Sc1 during development is not critical or, in its absence, is subserved by another protein.

SPARC, a matricellular glycoprotein with important biological functions

SPARC (secreted protein, acidic and rich in cysteine) is a unique matricellular glycoprotein that is expressed by many different types of cells and is associated with development, remodeling, cell turnover, and tissue repair. Its principal functions in vitro are counteradhesion and antiproliferation, which proceed via different signaling pathways. SPARC consists of three domains, each of which has independent activity and unique properties. The extracellular calcium binding module and the follistatin-like module have been recently crystallized. Specific interactions between SPARC and growth factors, extracellular matrix proteins, and cell surface proteins contribute to the diverse activities described for SPARC in vivo and in vitro. The location of SPARC in the nuclear matrix of certain proliferating cells, but only in the cytosol of postmitotic neurons, indicates potential functions of SPARC as a nuclear protein, which might be involved in the regulation of cell cycle progression and mitosis. High levels of SPARC have been found in adult eye, and SPARC-null mice exhibit cataracts at 1-2 months of age. This animal model provides an excellent opportunity to confirm and explore some of the properties of SPARC, to investigate cataractogenesis, and to study SPARC-related family proteins, e.g., SC1/hevin, a counteradhesive matricellular protein that might functionally compensate for SPARC in certain tissues.(J Histochem Cytochem 47:1495-1505, 1999)

Angiogenesis inhibitor endostatin is a distinct component of elastic fibers in vessel walls

Theendothelial cell inhibitor endostatin (22 kDa) is part of the carboxyl-terminal globular domain of collagen XVIII and shows a widespread tissue distribution. Immunohistology of adult mouse tissues demonstrated a preferred localization in many vessel walls and some other basement membrane zones. A strong immunogold staining was observed across elastic fibers in the multiple elastic membranes of aorta and other large arteries. Staining was less strong along sparse elastic fibers of veins and almost none was observed in the walls of arterioles and capillaries. Strong evidence was also obtained for some intracellular and basement membrane associations. Immunogold double staining of elastic fibers showed a close colocalization of endostatin with fibulin-2, fibulin-1, and nidogen-2, but not with perlecan. Reasonable amounts of endostatin could be extracted from aorta and skin by EDTA, followed by detergents, with aorta being the richest source of the inhibitor identified so far. Solubilizations with collagenase and elastase were approximately fivefold less efficient. Immunoblots of aortic extracts detected major endostatin components of 22-25 kDa whereas skin extracts also contained some larger components. Solid-phase assays demonstrated distinct binding of recombinant mouse endostatin to the fibulins and nidogen-2, consistent with their tissue colocalization. Together, the data indicate several different ways for endostatin to be associated with the extracellular matrix, and its release may determine biological activation. This also defines a novel function for some elastic tissues.

A calcium-binding motif in SPARC/osteonectin inhibits chordomesoderm cell migration during Xenopus laevis gastrulation: evidence of counter-adhesive activity in vivo

Secreted protein, acidic, rich in cysteine (SPARC) is a Ca2+-binding, counter-adhesive, extracellular glycoprotein associated with major morphogenic events and tissue remodeling in vertebrates. In Xenopus laevis embryos, SPARC is expressed first by dorsal mesoderm cells at the end of gastrulation and undergoes complex, rapid changes in its pattern of expression during early organogenesis. Another study has reported that precocious expression of SPARC by injection of native protein into the blastocoele cavity of pregastrula embryos leads to a concentration-dependent reduction in anterior development. Thus, normal development requires that the timing, spatial distribution, and/or levels of SPARC be regulated precisely. In a previous study, we demonstrated that injection of a synthetic peptide corresponding to the C-terminal, Ca2+-binding, EF-hand domain of SPARC (peptide 4.2) mimicked the effects of native SPARC. In the present investigation, peptide 4.2 was used to examine the cellular and molecular bases of the phenotypes generated by the aberrant presence of SPARC. Exposure of late blastula embryos to LiCl also generated a concentration-dependent reduction in anterior development; therefore, injections of LiCl were carried out in parallel to highlight the unique effects of peptide 4.2 on early development. At concentrations that caused a similar loss in anterior development (60-100 ng peptide 4.2 or 0.25-0.4 microg LiCl), LiCl had a greater inhibitory effect on the initial rate of chordomesoderm cell involution, in comparison with peptide 4.2. However, as gastrulation progressed, peptide 4.2 had a greater inhibitory effect on prospective head mesoderm migration than that seen in the presence of LiCl. Moreover, peptide 4.2 and LiCl had distinct influences on the expression pattern of dorso-anterior markers at the neural and tail-bud stages of development. Scanning electron microscopy showed that peptide 4.2 inhibited spreading of migrating cells at the leading edge of the involuting chordomesoderm. While still in close proximity to the blastocoele roof, many of the cells appeared rounded and lacked lamellipodia and filopodia extended in the direction of migration. In contrast, LiCl had no effect on the spreading or shape of involuting cells. These data are the first evidence of a counter-adhesive activity for peptide 4.2 in vivo, an activity demonstrated for both native SPARC and peptide 4.2 in vitro.

Primary mesenchymal cells isolated from SPARC-null mice exhibit altered morphology and rates of proliferation

SPARC (secreted protein acidic and rich in cysteine)/BM 40/osteonectin is a matricellular protein shown to function as a counteradhesive factor that induces cell rounding and as an inhibitor of cell proliferation. These activities have been defined in cell culture, in which interpretation has been complicated by the presence of endogenous SPARC. We therefore sought to determine whether cell shape and proliferation would be affected by the absence of SPARC. Mesangial cells, fibroblasts, and aortic smooth muscle cells were isolated from SPARC-null and age-matched, wild-type mice. In contrast to wild-type cells, SPARC-null mesangial cells exhibited a flat morphology and an altered actin cytoskeleton. In addition, vinculin-containing focal adhesions were distributed over the center of SPARC-null cells, whereas in wild-type cells, the number of focal adhesions was reduced, and these structures were restricted largely to the cell periphery. Although the SPARC-null fibroblasts did not display overt differences in cell morphology, the cells responded to exogenous recombinant SPARC by rounding up in a manner similar to that of wild-type fibroblasts. Thus, the expression of endogenous SPARC is not required for the response of cells to SPARC. Additionally, SPARC-null mesangial cells, fibroblasts, and smooth muscle cells proliferated faster than their respective wild-type counterparts. Null cells also showed a greater sensitivity to the inhibition of cell cycle progression by the addition of recombinant SPARC. The increased proliferation rate of SPARC-null cells appeared to be mediated, at least in part, by an increase in the cell cycle regulatory protein cyclin A. We conclude that the expression of SPARC influences the cellular architecture of mesangial cells and that SPARC plays a role in the regulation of cell cycle in mesangial cells, fibroblasts, and smooth muscle cells.

Follistatin: a multifunctional regulatory protein

Follistatin was first described in 1987 as a follicle-stimulating hormone inhibiting substance present in ovarian follicular fluid. We now know that this effect of follistatin is only one of its many properties in a number of reproductive and nonreproductive systems. A majority of these functions are facilitated through the affinity of follistatin for activin, where activin's effects are neutralized through its binding to follistatin. As such, the interplay between follistatin and activin represents a powerful regulatory mechanism that impinges on a variety of cellular processes within the body. In this review we focus on the biochemical characteristics of follistatin and its interaction with activin and discuss the emerging role of these proteins as potent tissue regulators in the gonad, pituitary gland, pregnancy membranes, vasculature, and liver. Consideration is also given to the larger family of proteins that contain follistatin-like modules, in particular with regard to their functional and structural implications.

SPARC inhibits endothelial cell adhesion but not proliferation through a tyrosine phosphorylation-dependent pathway

SPARC, a counteradhesive matricellular protein, inhibits endothelial cell adhesion and proliferation, but the pathways through which these activities are blocked are not known. In this study, we used inhibitors of major signaling proteins to identify mediators through which SPARC exerts its counteradhesive and antiproliferative functions. Pretreatments with the general protein tyrosine kinase (PTK) inhibitors, herbimycin A and genistein, protected against the inhibitory effect of SPARC on bovine aortic endothelial (BAE) cell spreading by more than 60%. Similar pretreatments with PTK inhibitors significantly blocked the diminishment of focal adhesions by SPARC in confluent BAE cell monolayers, as determined by the formation of actin stress-fibers and the distribution of vinculin in focal adhesion plaques. Inhibition of endothelial cell cycle progression by SPARC and a calcium-binding SPARC peptide, however, was not affected by PTK inhibitors. Inhibition of DNA synthesis by SPARC was not reversed by inhibitors of the activity of protein kinase C (PKC), or of cAMP-dependent protein kinase (PKA), but was sensitive to pertussis (and to a lesser extent, cholera) toxin. The counteradhesive effect of SPARC on endothelial cells is, therefore, mediated through a tyrosine phosphorylation-dependent pathway, whereas its antiproliferative function is dependent, in part, on signal transduction via a G protein-coupled receptor.

Structure, function and tissue forms of the C-terminal globular domain of collagen XVIII containing the angiogenesis inhibitor endostatin

The C-terminal domain NC1 of mouse collagen XVIII (38 kDa) and the shorter mouse and human endostatins (22 kDa) were prepared in recombinant form from transfected mammalian cells. The NC1 domain aggregated non-covalently into a globular trimer which was partially cleaved by endogenous proteolysis into several monomers (25-32 kDa) related to endostatin. Endostatins were obtained in a highly soluble, monomeric form and showed a single N-terminal sequence which, together with other data, indicated a compact folding. Endostatins and NC1 showed a comparable binding activity for the microfibrillar fibulin-1 and fibulin-2, and for heparin. Domain NC1, however, was a distinctly stronger ligand than endostatin for sulfatides and the basement membrane proteins laminin-1 and perlecan. Immunological assays demonstrated endostatin epitopes on several tissue components (22-38 kDa) and in serum (120-300 ng/ml), the latter representing the smaller variants. The data indicated that the NC1 domain consists of an N-terminal association region (approximately 50 residues), a central protease-sensitive hinge region (approximately 70 residues) and a C-terminal stable endostatin domain (approximately 180 residues). They also demonstrated that proteolytic release of endostatin can occur through several pathways, which may lead to a switch from a matrix-associated to a more soluble endocrine form.

SPARC/osteonectin mRNA is induced in blood vessels following injury to the adult rat cerebral cortex

Recently we described the pattern of expression of the anti-adhesive glycoprotein SPARC/osteonectin in the developing and adult brain. SPARC mRNA was present in developing blood vessels during neurogenesis, but was not detected in the mature vasculature. We have now examined the effect of a lesion to the adult rat cerebral cortex on the expression of SPARC by in situ hybridization. SPARC mRNA was increased in the zone proximal to the wound at 3 to 10 days after cortical brain injury. During this period, SPARC was induced in mature blood vessels close to the lesion site and in blood vessels which develop following injury. These results suggest a role for SPARC in the process of angiogenesis following injury to the adult cerebral cortex.

Angiogenesis and tumor metastasis

Angiogenesis, the recruitment of new blood vessels, is an essential component of the metastatic pathway. These vessels provide the principal route by which tumor cells exit the primary tumor site and enter the circulation. For many tumors, the vascular density can provide a prognostic indicator of metastatic potential, with the highly vascular primary tumors having a higher incidence of metastasis than poorly vascular tumors. Tumor angiogenesis is regulated by the production of angiogenic stimulators including members of the fibroblast growth factor and vascular endothelial growth factor families. In addition, tumors may activate angiogenic inhibitors such as angiostatin and endostatin that can modulate angiogenesis both at the primary site and at downstream sites of metastasis. The potential use of these and other natural and synthetic angiogenic inhibitors as anticancer drugs is currently under intense investigation. Such agents may have reduced toxicity and be less likely to generate drug resistance than conventional cytotoxic drugs. Clinical trials are now underway to develop optimum treatment strategies for antiangiogenic agents.

Basic fibroblast growth factor destabilizes osteonectin mRNA in osteoblasts

Osteonectin (secreted protein acidic and rich in cysteine, 40-kDa basement membrane) is a glycoprotein abundantly expressed in bone and in other tissues undergoing active remodeling. Fibroblast growth factors (FGFs) are important in skeletal development and fracture repair, events associated with extracellular matrix remodeling. We used the murine osteoblastic cell line MC3T3 to determine whether basic FGF (bFGF) regulates osteonectin expression in bone. Northern blot analysis showed that bFGF decreased osteonectin transcripts in a dose- and time-dependent manner. This regulation was independent of the mitogenic effect of bFGF but was dependent on new protein synthesis. Immunoprecipitation of [35S]methionine-cysteine osteoblast-conditioned medium and cell layer proteins showed that bFGF decreased osteonectin synthesis. Nuclear runoff assays failed to reveal regulation of osteonectin gene transcription by bFGF. However, bFGF dramatically decreased the stability of osteonectin mRNA in transcriptionally arrested osteoblasts. This destabilization of osteonectin mRNA may be one means by which bFGF regulates extracellular matrix remodeling.

Crystal structure of a pair of follistatin-like and EF-hand calcium-binding domains in BM-40

BM-40 (also known as SPARC or osteonectin) is an anti-adhesive secreted glycoprotein involved in tissue remodelling. Apart from an acidic N-terminal segment, BM-40 consists of a follistatin-like (FS) domain and an EF-hand calcium-binding (EC) domain. Here we report the crystal structure at 3.1 A resolution of the FS-EC domain pair of human BM-40. The two distinct domains interact through a small interface that involves the EF-hand pair of the EC domain. Residues implicated in cell binding, inhibition of cell spreading and disassembly of focal adhesions cluster on one face of BM-40, opposite the binding epitope for collagens and the N-linked carbohydrate. The elongated FS domain is structurally related to serine protease inhibitors of the Kazal family. Notable differences are an insertion into the inhibitory loop in BM-40 and a protruding N-terminal beta-hairpin with striking similarities to epidermal growth factor. This hairpin is likely to act as a rigid spacer in proteins containing tandemly repeated FS domains, such as follistatin and agrin, and forms the heparin-binding site in follistatin.

Manipulating the aggregation and oxidation of human SPARC in the cytoplasm of Escherichia coli

Human SPARC (secreted protein acidic and rich in cysteine), an extracellular matrix protein containing 14 cysteine residues, was found to partition equally between soluble and insoluble cellular fractions when overexpressed in the Escherichia coli cytoplasm. While the growth temperature and medium pH had little effect on inclusion body formation, co-overproduction of the dnaKJ operon, but not of the groE operon, suppressed aggregation at the expense of intracellular accumulation. Although both forms of the protein were fully reduced in wild-type cells, 70% to 85% of soluble and insoluble SPARC could be converted into oxidized species in a thioredoxin reductase (trxB) null mutant following incubation on ice. Approximately 15% to 20% of SPARC exhibited the electrophoretic mobility of the biologically active protein. Overproduction of the dnaKJ operon in trxB cells decreased the formation of disulfide-bonded SPARC multimers in the aggregated material but not in its soluble counterpart. Our results suggest that the activity responsible for disulfide bond formation in trxB mutants acts at the post-translational level and is able to freely diffuse within inclusion bodies.

Ectopic expression of SPARC in Xenopus embryos interferes with tissue morphogenesis: identification of a bioactive sequence in the C-terminal EF hand

SPARC is a matricellular Ca(2+)-binding glycoprotein that exhibits both counteradhesive and antiproliferative effects on cultured cells. It is secreted by cells of various tissues as a consequence of morphogenesis, response to injury, and cyclic renewal and/or repair. In an earlier study with Xenopus embryos we had shown a highly specific and regulated pattern of SPARC expression. We now show that ectopic expression of SPARC before its normal embryonic activation produces severe anomalies, some of which are consistent with the functions of SPARC proposed from studies in vitro. Microinjection of SPARC RNA, protein, and peptides into Xenopus embryos before endogenous embryonic expression generated different but overlapping phenotypes. (a) Injection of SPARC RNA into one cell of a two-cell embryo resulted in a range of unilateral defects. (b) Precocious exposure of embryos to SPARC by microinjection of protein into the blastocoel cavity was associated with certain axial defects comparable to those obtained with SPARC RNA. (c) SPARC peptides containing follistatin-like and copper-binding sequences were without obvious effect, whereas SPARC peptide 4.2, corresponding to a disulfide-bonded, Ca(2+)-binding domain, was associated with a reduction in axial structures that led eventually to complete ventralization of the embryos. Histological analysis of ventralized embryos indicated that the morphogenetic events associated with gastrulation might have been inhibited. Microinjection of other Ca(2+)-binding glycoproteins, such as osteopontin and bone sialoprotein, resulted in phenotypes that were unique. We probed further the structural correlates of this region of SPARC in the context of tissue development. Co-injection of peptide 4.2 with Ca2+ or EGTA, and injection of peptide 4.2K (containing a mutated consensus Ca(2+)-binding sequence), demonstrated that the developmental defects associated with peptide 4.2 were independent of Ca2+. However, the disulfide bridge in this region of SPARC was found to be critical, as injection of peptide 4.2AA, a mutant lacking the cystine, generated no axial defects. We have therefore shown for the first time in vivo that the temporally inappropriate presence of SPARC is associated with perturbations in tissue morphogenesis. Moreover, we have identified at least one bioactive region of SPARC as the C-terminal disulfide-bonded, Ca(2+)-binding loop that was previously shown to be both counteradhesive and growth-inhibitory.

Pieces of eight: bioactive fragments of extracellular proteins as regulators of angiogenesis

Specific stages of angiogenesis are regulated by extracellular proteins. This review discusses the endogenous proteolysis of eight of these proteins and the release of polypeptide fragments that have biological activities different from those of the native, parent protein. The generation of natural cleavage products could provide a precise mechanism for the regulation of angiogenesis. Although further experimental confirmation of this mechanism is needed, this leitmotif offers an attractive explanation, in part, for the complex role of proteolysis in vascular morphogenesis.

The expression of the secreted protein acidic and rich in cysteine (SPARC) is associated with the neoplastic progression of human melanoma

SPARC (secreted protein acidic and rich in cysteine) is an extracellular protein associated with tissues exhibiting high rates of cell proliferation and matrix remodeling. The current work shows that the human melanoma cell lines IIB-MEL-LES, IIB-MEL-IAN, and IIB-MEL-J and different human metastatic melanomas expressed high levels of SPARC mRNA and protein. By western blot analysis we detected a single secreted 42-kDa band in human diploid fibroblasts-conditioned medium and a 45- to 40-kDa doublet in the three melanoma cell lines and all the metastatic melanomas tested. Part of the melanoma samples and cell lines showed an additional doublet of 36-34 kDa. SPARC mRNA was expressed by the three established cell lines, 14 metastatic melanoma samples, and tumors raised in nude mice, and no spliced variants were found. The heterogeneous pattern of SPARC secreted by human melanoma cells is the result of post-translational glycosylation and a specific extracellular leupeptin-inhibitable cleavage. Unlike human fibroblasts, melanoma cells did not overexpress SPARC on addition of TGF-beta. Immunohistochemical analysis showed that SPARC was strongly expressed in 100% of primary melanomas (7 of 7) and metastatic melanomas (29 of 29), moderately expressed in most of the positive dysplastic nevi (13 of 14), and only weakly expressed in nevocellular nevi (4 of 25). Normal melanocytes did not express SPARC. The data suggest that the expression of SPARC is associated with the neoplastic progression of human melanoma.

Suppression of SPARC expression by antisense RNA abrogates the tumorigenicity of human melanoma cells

Acquisition of invasive/metastatic potential is a key event in tumor progression. Cell surface glycoproteins and their respective matrix ligands have been implicated in this process. Recent evidence reveals that the secreted glycoprotein SPARC (secreted protein, acidic and rich in cysteine) is highly expressed in different malignant tissues. The present study reports that the suppression of SPARC expression by human melanoma cells using a SPARC antisense expression vector results in a significant decrease in the in vitro adhesive and invasive capacities of tumor cells, completely abolishing their in vivo tumorigenicity. This is the first evidence that SPARC plays a key role in human melanoma invasive-metastatic phenotype development.

SPARC is expressed in renal interstitial fibrosis and in renal vascular injury

Tubulointerstitial inflammation and fibrosis are critical determinants for renal function and prognosis in a variety of human nephropathies. Yet, the pathophysiology of the injury remains obscure. We investigated the expression of SPARC (secreted protein acidic and rich in cysteine) by immunohistochemistry and in situ hybridization in experimental models characterized by tubulointerstitial fibrosis and matrix expansion in rats. SPARC is a secreted glycoprotein that has been demonstrated to affect cellular interaction with matrix proteins, modulate cell proliferation, bind to and/or inhibit growth factors such as PDGF and bFGF, and regulate angiogenesis. Interstitial expression of SPARC was most prominent in passive Heyman nephritis (PHN), chronic cyclosporine A (CsA) nephropathy, and the remnant kidney model and, to a lesser extent, in angiotensin II (Ang II)-infused animals. SPARC protein and mRNA were substantially increased at sites of tubulointerstitial fibrosis/matrix expansion. In the PHN model, SPARC protein was expressed by interstitial fibroblasts that also produced alpha-smooth muscle actin ("myofibroblasts") and correlated both temporally (r = 0.97) and spatially with sites of type I collagen deposition. Interstitial cell proliferation preceded the development of interstitial fibrosis, and maximal SPARC expression (d15) coincided with the initial decline in interstitial proliferation. In the Ang II-infusion model, which is characterized by arteriolopathy and tubulointerstitial injury, an increase in SPARC protein and mRNA was also seen in injured blood vessels. SPARC was shown to be expressed by vascular smooth muscle cells and also by cells in the adventitia of hypertrophied arteries. In summary, SPARC was transiently expressed by interstitial fibroblasts at sites of tubulointerstitial injury and fibrosis, and by smooth muscle cells and cells in the adventitia of injured arteries in the Ang II-model. In addition to its proposed role in extracellular matrix deposition. the antiproliferative properties of SPARC might contribute to the resolution of interstitial fibroblast proliferation in the PHN model.

The exon structure of the mouse Sc1 gene is very similar to the mouse Sparc gene

Sc1 and Sparc are two extracellular proteins sharing similarity in their carboxyl terminus, with 63% identity over a 232-amino-acid region. We have cloned and mapped the genomic locus of mouse Sc1. The mouse Sc1 gene contains 11 exons spanning approximately 35 kb of DNA. The genomic structure (exon/intron boundaries) of Sc1 exons 6 to exon 11 is identical to those of the similar portion of the Sparc gene. This suggests that Sc1 and Sparc originated from a common ancestral gene. Using fluorescence in situ hybridization analysis, Sc1 was localized to band 5E4 of mouse chromosome 5.

Type I collagen-deficient Mov-13 mice do not retain SPARC in the extracellular matrix: implications for fibroblast function

The Mov-13 strain of mice was created by the insertion of the murine Moloney leukemia virus into the first intron of the alpha 1 (I) collagen gene. Consequently, Mov-13 embryos do not transcribe alpha 1 (I) collagen mRNA and lack type I collagen protein in the extracellular matrix (ECM). Homozygotes die within 12-14 days of embryonic development, in part from the rupture of large blood vessels, and also exhibit deficiencies in hematopoesis and assembly of the ECM (Lohler et al. [1984] Cell 38:597-607). Several matricellular proteins, proteoglycans, and growth factors bind to type I collagen, e.g., fibronectin, secreted protein acidic and rich in cysteine (SPARC), decorin, and transforming growth factor-beta. Here we investigate the expression and function of SPARC in the absence of type I collagen. We show that fibroblasts isolated from Mov-13 homozygous, heterozygous, and wild-type embryos transcribed and translated SPARC mRNA in vitro. However, accumulation of extracellular SPARC was severely affected in the tissues of Mov-13 homozygotes, whereas extracellular deposition of the secreted glycoproteins fibronectin and type III collagen was not altered. Since SPARC has been shown to be a regulator of cell shape, the functional consequences of the absence of extracellular SPARC were evaluated in collagen gel contraction assays. Fibroblasts isolated from homozygous Mov-13 mice did not contract native type I collagen gels as efficiently as fibroblasts from heterozygous littermates; however, addition of exogenous SPARC enhanced the contraction of collagen by homozygous Mov-13 fibroblasts. The stimulatory effect of SPARC was blocked by antibodies specific for the amino terminus of the protein. These results provide evidence that type I collagen is one of the major extracellular proteins that binds SPARC in vivo. Furthermore, the capacity of fibroblasts to contract ECM in vitro is enhanced by extracellular SPARC. We therefore propose that the remodeling of ECM by cells in vivo is regulated in part by a specific interaction between SPARC and type I collagen.

Periodontal ligament cell population: the central role of fibroblasts in creating a unique tissue

BACKGROUND

Fibroblasts are the predominant cells of the periodontal ligament (PL) and have important roles in the development, function, and regeneration of the tooth support apparatus. Biological processes initiated during the formation of the PL contribute to the long-lasting homeostasic properties exhibited by PL fibroblast populations.

DEVELOPMENT

The formation of the PL is likely controlled by epithelial-mesenchymal and epithelial hard tissue interactions, but the actual mechanisms that contribute to the development of cellular lineages in the PL are unknown. Fibroblasts in the normally functioning PL migrate through the tissue along collagen fibres to cementum and bone and in an apico-coronal direction during tooth eruption. ADULT TISSUE: Cell kinetic experiments have shown that PL fibroblasts comprise a renewal cell system in steady-state and the progenitors can generate multiple types of more differentiated, specialized cells. Progenitor cell populations of the PL are enriched in locations adjacent to blood vessels and in contiguous endosteal spaces. In normally functioning periodontal tissues, there is a relatively modest turnover of cells in which apoptotic cell death balances proliferation. Large increases of cell formation and cell differentiation occur after application of orthodontic forces or wounding. As PL cells comprise multiple cellular phenotypes, it has been postulated that after wounding, the separate phenotypes repopulating the site will ultimately dictate the tissue form and type.

CONCLUSIONS

PL fibroblasts play an essential role in responses to mechanical force loading of the tooth by remodelling and repairing effete or damaged matrix components. In consideration of the important roles played by fibroblasts in PL homeostasis, they could be described as "the architect, builder, and caretaker" of the periodontal ligament.

Enhancement of SPARC (osteonectin) synthesis in arthritic cartilage. Increased levels in synovial fluids from patients with rheumatoid arthritis and regulation by growth factors and cytokines in chondrocyte cultures

OBJECTIVE

To investigate the roles of SPARC (secreted protein, acidic and rich in cysteine) (osteonectin) in arthritis, using cartilage and synovium specimens and synovial fluids (SF) from patients with rheumatoid arthritis (RA) or osteoarthritis (OA), and to examine the effects of cytokines, growth factors, and hormones on SPARC synthesis by chondrocytes in culture.

METHODS

SPARC in cartilage and synovium was immunostained with monoclonal antibodies. SPARC synthesis by cultured chondrocytes was measured by Northern blot analysis, immunoblotting, and sandwich enzyme-linked immunosorbent assay.

RESULTS

SPARC was identified in numerous chondrocytes in the superficial and middle zones and in regenerating chondrocytes of RA and OA joints, whereas such staining was absent in these zones of normal cartilage, except for weak signals from a few chondrocytes in the deep zone. In addition, SPARC synthesis was enhanced in synovial cells of RA and OA joints. The average SPARC level in SF was 10-fold higher in the RA than in the OA population. In rabbit articular chondrocyte cultures, administration of transforming growth factor beta 1 (TGF beta 1) and bone morphogenetic protein 2 increased SPARC levels at 24-48 hours, whereas interleukin-lbeta (IL-1 beta), IL-1 alpha, tumor necrosis factor alpha, lipopolysaccharide, phorbol myristate acetate, basic fibroblast growth factor, and dexamethasone decreased SPARC levels at 24-72 hours. TGF beta increased SPARC messenger RNA (mRNA) levels at 24 hours, whereas IL-1 beta caused a marked decrease in SPARC mRNA levels at 24 hours. Furthermore, IL-1 decreased the glycosylation of SPARC.

CONCLUSION

These findings suggest that various growth factors and cytokines, including TGF beta 1 and IL-1 beta, regulate the production of SPARC by chondrocytes at pre- and posttranslational levels, and that SPARC synthesis is markedly enhanced in arthritic joints.

Osteonectin gene expression in fibrotic liver

Osteonectin is an extracellular matrix (ECM) protein which is secreted by various cell types, and regulates tissue remodeling and cell proliferation. In the present study, we have examined the expression of osteonectin in fibrotic liver. Osteonectin transcripts were undetectable in normal liver, however, the abundant expression of the osteonectin gene was detected in fibrotic liver. The transcripts of osteonectin were only detected in hepatic lipocytes and the number of lipocytes was increased in fibrotic liver. These results suggested that in fibrotic liver, enhanced osteonectin expression may play an important role in liver fibrosis.

QRI, a retina-specific gene, encodes an extracellular matrix protein exclusively expressed during neural retina differentiation

Neural retina development results from growth arrest of neuroectodermal precursors and differentiation of postmitotic cells. The QRI gene is specifically expressed in Müller retinal glial cells. Its expression coincides with the stage of withdrawal from the cell cycle and establishment of differentiation and is repressed upon induction of retinal cell proliferation by the v-src gene product. In this report, we show that the QR1 gene encodes several glycosylated proteins that are secreted and can either associate with the extracellular matrix or remain diffusible in the medium. By using pulse-chase experiments, the 100-103 kDa forms seem to appear first and are specifically incorporated into the extracellular matrix, whereas the 108 and 60 kDa polypeptides appear later and are detected as soluble forms in the culture medium. We also report that expression of the QR1 gene is developmentally regulated in the chicken. Its mRNA is first detectable at embryonic day 10, reaches a maximal level at embryonic day 15 and is no longer detected at embryonic day 18. Immunolocalization of the QR1 protein in chicken retina sections during development shows that expression of the protein parallels the differentiation pattern of post-miotic cells (in particular Müller cells and rods), corresponding to the two differentiation gradients in the retina: from the ganglion cell layer to the inner nuclear layer and outer nuclear layer, and from the optic nerve to the iris. At embryonic day 10, expression of the QR1 protein(s) is restricted to the optic nerve region and the inner nuclear layer, colocalizing with Müller cell bodies. As development proceeds, QR1 protein localization spreads towards the iris and towards the outer nuclear layer, following Müller cell elongations towards the photoreceptors. Between embryonic days 16 and 18, the QR1 protein is no longer detectable in the optic nerve region and is concentrated around the basal segment of the photoreceptors in the peripheral retina. Our results suggest a role for the QR1 gene product in the process of growth arrest and establishment of photoreceptor differentiation.

Conformational changes of bovine bone osteonectin induced by interaction with calcium

To clarify calcium-induced conformational changes in bovine bone osteonectin, the protein was labeled with fluorescein isothiocyanate (FITC) in the presence and absence of calcium. By calcium titration using fluorescence spectrometry, it was demonstrated that FITC-osteonectin labeled in the presence of 2 mM CaCl2 showed a much higher affinity for calcium ions than did that labeled in the absence of calcium ions. The midpoint for completion of the increase in the intrinsic fluorescence (K0.5) of the two were 1 x 10(-7) M and 5 x 10(-7) M, respectively. By tryptic digestion and isolation of the fluorescent peptide of both FITC-osteonectins, the site of FITC-labeling was determined to be Lys174. Furthermore, it was found that the efficacy of labeling in this specific binding site was three times higher in the FITC-osteonectin labeled in the presence of 2 mM CaCl2 than in that labeled in the absence of calcium. The results indicate that in the presence of 2 mM CaCl2 the microenvironment around Lys174 of osteonectin was more open to modification than in the absence of calcium.

Expression of SPARC during development of the chicken chorioallantoic membrane: evidence for regulated proteolysis in vivo

SPARC is a secreted glycoprotein that has been shown to disrupt focal adhesions and to regulate the proliferation of endothelial cells in vitro. Moreover, peptides resulting from the proteolysis of SPARC exhibit angiogenic activity. Here we describe the temporal synthesis, turnover, and angiogenic potential of SPARC in the chicken chorioallantoic membrane. Confocal immunofluorescence microscopy revealed specific expression of SPARC protein in endothelial cells, and significantly higher levels of SPARC were observed in smaller newly formed blood vessels in comparison to larger, developmentally older vessels. SPARC mRNA was detected at the earliest stages of chorioallantoic membrane morphogenesis and reached maximal levels at day 13 of embryonic development. Interestingly, steady-state levels of SPARC mRNA did not correlate directly with protein accumulation; moreover, the protein appeared to undergo limited degradation during days 10-15. Incubation of [125I]-SPARC with chorioallantoic membranes of different developmental ages confirmed that extracellular proteolysis occurred during days 9-15, but not at later stages (e.g., days 17-21). Comparison of peptides produced by incubation with chorioallantoic membranes with those generated by plasmin showed an identical pattern of proteolysis. Plasmin activity was present throughout development, and in situ zymography identified sites of plasminogen activator activity that corresponded to areas exhibiting high levels of SPARC expression. Synthetic peptides from a plasmin-sensitive region of SPARC, between amino acids 113-130, stimulated angiogenesis in the chorioallantoic membrane in a dose-dependent manner; in contrast, intact SPARC was inactive in similar assays. We have shown that SPARC is expressed in endothelial cells of newly formed blood vessels in a manner that is both temporally and spatially restricted. Between days 9 and 15 of chorioallantoic membrane development, the protein undergoes proteolytic cleavage that is mediated, in part, by plasmin. SPARC peptides released specifically by plasmin induce angiogenesis in vivo. We therefore propose that SPARC acts as an intrinsic regulator of angiogenesis in vivo.

Neoplastic progression of human colorectal cancer is associated with overexpression of the stromelysin-3 and BM-40/SPARC genes

The interaction of neoplastic cells with the extracellular matrix is a critical event for the initiation of cancer invasion and metastasis. This study was designed to evaluate the potential implication of stromelysin-3 (ST3), a newly identified member of the matrix-degrading metalloproteinase family, and of BM-40/SPARC, a glycoprotein associated with the extracellular matrix, during the progression of human colorectal cancers. We analyzed the relative abundance of ST3 and BM-40/SPARC transcripts by Northern blot, and their distribution by in situ hybridization, in normal mucosa, benign adenomas, and primary colorectal adenocarcinomas and their liver metastases. The ST3 and BM-40/SPARC transcripts were overexpressed in primary colorectal cancers and their liver metastases compared to non-neoplastic mucosa. These transcripts were localized in stromal fibroblasts adjacent to the neoplastic foci. Overexpression of ST3 correlated with the progression of human colorectal tumors toward local invasion and liver metastasis. Induction of these genes also occurred in diverticulitis and digestive neoplasms such as gastric and esophageal carcinomas.

SPARC mediates focal adhesion disassembly in endothelial cells through a follistatin-like region and the Ca(2+)-binding EF-hand

SPARC is a one of a group of extracellular matrix proteins that regulate cell adhesion through a loss of focal adhesion plaques from spread cells. We previously reported that SPARC reduced the number of bovine aortic endothelial (BAE) cells positive for focal adhesions [Murphy-Ullrich et al. (1991): J Cell Biol 115:1127-1136]. We have now characterized the effect of SPARC on the cytoskeleton of BAE cells. Addition of SPARC to spread BAE cells caused a dose-dependent loss of focal adhesion-positive cells, that was maximal at approximately 1 microgram/ml (0.03 microM). Consistent with the loss of adhesion plaques as detected by interference reflection microscopy, vinculin appeared diffuse and F-actin was redistributed to the periphery of cells incubated with SPARC. However, the distribution of the integrin alpha v beta 3 remained clustered in a plaque-like distribution. These data, and the observation that SPARC binds to BAE cells but not to the extracellular matrix, indicate that SPARC acts via interactions with cell surface molecules and not by steric/physical disruption of integrin-extracellular matrix ligands. To determine the region(s) of SPARC that mediate a loss of focal adhesions, we tested peptides from the four distinct regions of SPARC. The cationic, cysteine-rich peptide 2.1 (amino acids 54-73) and the Ca(2+)-binding EF-hand-containing peptide 4.2 (amino acids 254-273) were active in focal adhesion disassembly. Furthermore, antibodies specific for these regions neutralized the focal adhesion-labilizing activity of SPARC. These results are consistent with previous data showing that peptide 2.1 and 4.2 interact with BAE cell surface proteins and indicate that the loss of focal adhesions from endothelial cells exposed to SPARC is a receptor-mediated event.

Inhibition of endothelial cell proliferation by SPARC is mediated through a Ca(2+)-binding EF-hand sequence

SPARC (secreted protein, acidic and rich in cysteine, also known as osteonectin and BM-40) is a metal-binding glycoprotein secreted by a variety of cultured cells and characteristic of tissues undergoing morphogenesis, remodeling, and repair. Recently it has been shown that SPARC inhibits the progression of the endothelial cell cycle in mid-G1, and that a synthetic peptide (amino acids 54-73 of secreted murine SPARC, peptide 2.1) from a cationic, disulfide-bonded region was in part responsible for the growth-suppressing activity [Funk and Sage (1991): Proc Natl Acad Sci USA 88:2648-2652]. Moreover, SPARC was shown to interact directly with bovine aortic endothelial (BAE) cells through a C-terminal EF-hand sequence comprising a high-affinity Ca(2+)-binding site of SPARC and represented by a synthetic peptide (amino acids 254-273) termed 4.2 [Yost and Sage (1993): J Biol Chem 268:25790-25796]. In this study we show that peptide 4.2 is a more potent inhibitor of DNA synthesis that acts cooperatively with peptide 2.1 to diminish the incorporation of [3H]-thymidine by both BAE and bovine capillary endothelial (BCE) cells. At concentrations of 0.019-0.26 mM peptide 4.2, thymidine incorporation by BAE cells was decreased incrementally, relative to control values, from approximately 100 to 10%. Although somewhat less responsive, BCE cells exhibited a dose-responsive decrement in thymidine incorporation, with a maximal inhibition of 55% at 0.39 mM. The inhibitory effect of peptide 4.2 was essentially independent of heparin and basic fibroblast growth factor and was blocked by anti-SPARC peptide 4.2 IgG, but not by antibodies specific for other domains of SPARC. To identify residues that were necessary for inhibition of DNA synthesis, we introduced single amino acid substitutions into synthetic peptide 4.2 and tested their activities and cell-surface binding characteristics on endothelial cells. Two peptides displayed null to diminished effects in the bioassays that were concentration-dependent: peptide 4.2 K, containing an Asp258 --> Lys substitution, and peptide 4.2 AA, in which the two disulfide-bonded Cys (positions 255 and 271) were changed to Ala residues. Peptide 4.2 K, which failed to fulfill the EF-hand consensus formula, exhibited an anomalous fluorescence emission spectrum, in comparison with the wild-type 4.2 sequence, that was indicative of a compromised affinity for Ca2(+).(ABSTRACT TRUNCATED AT 400 WORDS)

Wound repair in the context of extracellular matrix

Wound repair requires a continually evolving network of interactions among cells, cytokines and the extracellular matrix. Cell-surface integrins provide a mechanical connection between matrix components and the cytoskeleton, and integrins can transduce an astonishing variety of signals along pathways that may intercept the pathways triggered by cytokine receptors.

Hepatic sinusoidal endothelial cell G1/S arrest correlates with severity of alcoholic liver injury in the rat

BACKGROUND/AIMS

Capillarization of the hepatic sinusoid occurs in alcoholic liver disease. Because endothelial cell proliferation is relevant to capillarization, we used the intragastric feeding rat model to evaluate the relationship between pathological liver injury and endothelial cell proliferation.

METHODS

Male Wistar rats (225-250 g) were fed liquid diets containing corn oil and ethanol for periods ranging between 1 week and 2 months. At the time the rats were killed, the severity of pathological injury and endothelial cell proliferation using anti-rat proliferating cell nuclear antigen/antibody was evaluated.

RESULTS

Two distinctly different populations of proliferating sinusoidal lining cells were identified; one population showed relatively weak granular staining consistent with cells arrested at the G1/S boundary. The other population of cells showed strong granular staining of the nucleoplasm and nucleoli (cells in mid to late S phase). A strong correlation (r = 0.85; P < 0.01) was obtained between pathological severity and G1/S-arrested endothelial cells. There was no correlation between cells in S phase.

CONCLUSIONS

The presence of an increased number of G1/S-arrested endothelial cells in animals with severe pathological change suggests that stimuli are present for both endothelial cell proliferation and G1/arrest. The identification of these stimuli could lead to a better understanding of the pathogenesis of alcoholic liver disease.

SPARC is a source of copper-binding peptides that stimulate angiogenesis

SPARC is a transiently expressed extracellular matrix-binding protein that alters cell shape and regulates endothelial cell proliferation in vitro. In this study, we show that SPARC mRNA and protein are synthesized by endothelial cells during angiogenesis in vivo. SPARC and peptides derived from a cationic region of the protein (amino acids 113-130) stimulated the formation of endothelial cords in vitro; moreover, these peptides stimulated angiogenesis in vivo. Mapping of the active domain demonstrated that the sequence KGHK was responsible for most of the angiogenic activity; substitution of the His residue decreased the effect. We found that proteolysis of SPARC provided a source of KGHK, GHK, and longer peptides that contained these sequences. Although the Cu(2+)-GHK complex had been identified as a mitogen/morphogen in normal human plasma, we found KGHK and longer peptides to be potent stimulators of angiogenesis. SPARC113-130 and KGHK were shown to bind Cu2+ with high affinity; however, previous incubation with Cu2+ was not required for the stimulatory activity. Since a peptide from a second cationic region of SPARC (SPARC54-73) also bound Cu2+ but had no effect on angiogenesis, the angiogenic activity appeared to be sequence specific and independent of bound Cu2+. Thus, specific degradation of SPARC, a matrix-associated protein expressed by endothelial cells during vascular remodeling, releases a bioactive peptide or peptides, containing the sequence (K)GHK, that could regulate angiogenesis in vivo.