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.

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.

Enhanced angiogenesis characteristic of SPARC-null mice disappears with age

The impairment of angiogenesis in aging has been attributed, in part, to alterations in proteins associated with the extracellular matrix (ECM). SPARC (secreted protein acidic and rich in cysteine/osteonectin/BM-40) is a matricellular protein that regulates endothelial cell function as well as cell-ECM interactions. We have previously shown that angiogenesis, as reflected by fibrovascular invasion into subcutaneously implanted polyvinyl alcohol (PVA) sponges, is increased in SPARC-null mice (6-9 months of age) relative to their wild-type (WT) counterparts. In this study, we define the influence of aging on (a) the expression of SPARC and (b) fibrovascular invasion into sponge implants in SPARC-null and WT mice. The expression of SPARC in fibroblasts and endothelial cells derived from young donors (humans mean age less than 30 years and mice 4-6 months of age) and old donors (humans mean age over 65 years and mice 22-27 months of age) decreased 1.6 to 2.3-fold with age. Analysis of fibrovascular invasion into sponges implanted into old (22-27 months) SPARC-null and WT mice showed no differences in percent area of invasion or collagenous ECM. Moreover, sponges from old SPARC-null and WT mice contained similar levels of VEGF that were significantly lower than those from young (4-6 months) mice. In contrast to fibroblasts from young SPARC-null mice, dermal fibroblasts from old SPARC-null mice did not migrate farther, proliferate faster, or produce greater amounts of VEGF relative to their old WT counterparts. However, when stimulated with TGF-beta1, primary cells isolated from the sponge implants, and dermal fibroblasts from both old SPARC-null and WT mice, showed marked increases in VEGF secretion. These data indicate that aging results in a loss of enhanced angiogenesis in SPARC-null mice, as a result of the detrimental impact of age on cellular functions, collagen deposition, and VEGF synthesis. However, the influence of aging on these processes may be reversed, in part, by growth factor stimulation.

SPARC-null mice exhibit increased adiposity without significant differences in overall body weight

Secreted protein acidic and rich in cysteine/osteonectin/BM-40 (SPARC) is a matrix-associated protein that elicits changes in cell shape, inhibits cell-cycle progression, and influences the synthesis of extracellular matrix (ECM). The absence of SPARC in mice gives rise to aberrations in the structure and composition of the ECM that result in generation of cataracts, development of severe osteopenia, and accelerated closure of dermal wounds. In this report we show that SPARC-null mice have greater deposits of s.c. fat and larger epididymal fat pads in comparison with wild-type mice. Similar to earlier studies of SPARC-null dermis, we observed a reduction in collagen I in SPARC-null fat pads in comparison with wild-type. Although elevated levels of serum leptin were observed in SPARC-null mice, their overall body weights were not significantly different from those of wild-type counterparts. The diameters of adipocytes from SPARC-null versus wild-type epididymal fat pads were 252 +/- 61 and 161 +/- 33 microm (means +/- SD), respectively, and there was an increase in adipocyte number within SPARC-null fat pads in comparison with wild-type pads. Thus the absence of SPARC appears to result in an increase in the size of individual adipocytes as well as an increase in the number of adipocytes per fat pad. In fat pads isolated from wild-type mice, SPARC mRNA was associated with both the stromal/vascular and adipocyte fractions. We propose that SPARC limits the accumulation of adipose tissue in mice in part through its demonstrated effects on the regulation of cell shape and production of ECM.

Increased fibrovascular invasion of subcutaneous polyvinyl alcohol sponges in SPARC-null mice

The expression of SPARC (secreted protein acidic and rich in cysteine/osteonectin/BM-40) is elevated in endothelial cells participating in angiogenesis in vitro and in vivo. SPARC acts on endothelial cells to elicit changes in cell shape and to inhibit cell cycle progression. In addition, SPARC binds to and diminishes the mitotic activity of vascular endothelial growth factor. To determine the effect(s) of SPARC on angiogenic responses in vivo, we implanted polyvinyl alcohol sponges subcutaneously into wild-type and SPARC-null mice. On days 12 and 20 following implantation, SPARC-null mice showed increased cellular invasion of the sponges in comparison to wild-type mice. Areas of the sponge with the highest cell density exhibited the highest numbers of vascular profiles in both wild-type and SPARC-null animals. The endothelial component of the vessels was substantiated by immunoreactivity with three different markers specific for endothelial cells. Although sponges from SPARC-null relative to wild-type mice were populated by significantly more cells and blood vessels, an increase in the ratio of vascular to nonvascular cells was not apparent. No differences in the percentage of proliferating cells within the sponge were detected between wild-type and SPARC-null sections. However, elevated levels of vascular endothelial growth factor were associated with sponges from SPARC-null versus wild-type mice. An increase in vascular endothelial growth factor production was also observed in SPARC-null primary dermal fibroblasts relative to those of wild-type cells. In conclusion, we have shown that the fibrovascular invasion of polyvinyl alcohol sponges is enhanced in mice lacking SPARC, and we propose that increased levels of vascular endothelial growth factor account, at least in part, for this response.

A growth factor mixture that significantly enhances angiogenesis in vivo

Studies of therapeutic angiogenesis have generally focused on single growth factor strategies. However, multiple factors participate in angiogenesis. We evaluated the angiogenic potential of a growth factor mixture (GFm) derived from bovine bone. The major components of GFm (SDS-polyacrylamide gel electrophoresis, mass spectrometry, and Western blot) include transforming growth factor-beta1-3, bone morphogenic protein-2-7, and fibroblast growth factor-1. GFm was first shown to induce an angiogenic response in chorioallantoic membranes. Next, myocardial ischemia was induced in 21 dogs (ameroid) that were randomized 3 weeks later to received GFm 1 mg/ml (I), GFm 10 mg/ml (II), or placebo (P) (with investigators blinded to conditions) injected in and adjacent to ischemic myocardium. Dogs were assessed 6 weeks later using quantitative and semiquantitative measures. There were GFm concentration-dependent improvements in distal left anterior descending artery (LAD) opacification by angiography (P: 0.4 +/- 0.2, I: 1.1 +/- 0.14, II: 1.6 +/- 0.3, angiographic score p = 0.014). Histologically, there was also concentration-dependent vascular growth response of relatively large vessels (P: 0.21 +/- 0.15, I: 1.00 +/- 0.22, II: 1.71 +/- 0.18, vascular growth score p = 0.001). Resting myocardial blood flow (colored microspheres) was not significantly impaired in any group. However, maximum blood flow (adenosine) was reduced in ischemic territories and did not improve in GFm-treated hearts. GFm, a multiple growth factor mixture, is a potent angiogenic agent that stimulates large vessel growth. Although blood flow did not improve during maximal vasodilatory stress, large intramyocardial collateral vessels developed and angiographic visualization of the occluded distal LAD improved significantly. The use of multiple growth factors may be an effective strategy for therapeutic angiogenesis provided a more effective delivery strategy is devised that can achieve improved maximum blood flow potential.

Regulation of retinal capillary cells by basic fibroblast growth factor, vascular endothelial growth factor, and hypoxia

Vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) feature prominently in retinal neovascular diseases. Although the role of VEGF in retinal angiogenesis is well established, the importance of bFGF in this process requires further clarification. This study was undertaken to investigate the responses of retinal capillary cells (endothelial cells and pericytes) to bFGF under hypoxic conditions, as well as the potentially synergistic effects of bFGF and VEGF on the proliferation and cord formation of retinal endothelial cells. Cell proliferation was determined by cell number and by 3H-thymidine incorporation. Cord formation was assessed in three-dimensional gels of collagen type I. VEGF and bFGF increased 3H-thymidine incorporation by both cell types, an effect that was more pronounced in a hypoxic environment. Moreover, the proliferation of pericytes was stimulated to a greater extent by bFGF relative to VEGF. Endothelial migration in collagen gels, however, was induced more effectively by VEGF than by bFGF. A synergistic effect of VEGF and bFGF on cell invasion was observed in the collagen gel assay. VEGF and bFGF each augment proliferation of these cells, especially under hypoxia. We thus propose that these two cytokines have a synergistic effect at several stages of angiogenesis in the retina.

SPARC and the kidney glomerulus: matricellular proteins exhibit diverse functions under normal and pathological conditions

In the last decade, numerous studies have emphasized the important functions that matricellular proteins subserve during angiogenesis, wound healing, and the maintenance of organ and tissue integrity. Matricellular proteins are defined as a group of secreted regulatory macromolecules that are not structural components of the extracellular matrix (ECM) but rather mediate interactions between the ECM and cells. One of these matricellular proteins, termed SPARC (secreted protein acidic and rich in cysteine), is produced during the process of wound healing and is prominent in several types of injury. An excessive deposition of glomerular matrix and an elevated proliferation of certain glomerular cells characterize a variety of kidney diseases. The proliferation of these cells is associated typically with the remodeling process that occurs after kidney injury, and is, at least in part, modulated by the altered expression of ECM, various growth factors, and the elevated production of matricellular proteins (e.g., SPARC). The secretion of one or more of the matricellular proteins can lead to expansion of the glomerular basement membrane, infiltration of immunocompetent cells, and, in some cases, to a reversal of the pathological condition. However, these proteins can also contribute collectively to renal fibrosis, glomerulosclerosis, glomerulonephritis, and the eventual loss of renal function. The purpose of this review is to evaluate the multiple functions of SPARC in the kidney glomerulus under normal and pathological conditions.

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.

Lenses of SPARC-null mice exhibit an abnormal cell surface-basement membrane interface

SPARC (secreted protein acidic and rich in cysteine) is a matricellular protein involved in cell-matrix interactions. We have shown previously that mice deficient in SPARC develop posterior cortical cataract early in life that progresses to a mature opacity and capsule rupture. To evaluate the primary effects of SPARC deficiency in the lens, we examined the lenses of SPARC-null and wild-type mice by electron microscopy and immunohistochemistry to investigate whether ultrastructural abnormalities occur at the basement membrane (capsule)-lens cell interface in SPARC-null mice. The most notable feature in the lenses of SPARC-null mice, relative to wild-type animals, was the modification of the basal surface of the lens epithelial and fiber cells at the basement membrane (capsule) interface. Electron microscopy revealed numerous filopodial projections of the basal surface of the lens epithelial and fiber cells into the extracellular matrix of the anterior, posterior, and equatorial regions of the lens capsule. In 1 week old precataractous lenses, basal invasive filopodia projecting into the capsule were small and infrequent. Both the size and frequency of these filopodia increased in precataractous 3-4 week old lenses and were prominent in the cataractous 5-6 week old lenses. By rhodamine-phalloidin labeling, we confirmed the presence of basal invasive filopodia projecting into the lens capsule and demonstrated that the projections contained actin filaments. In contrast to the obvious abnormal projections at the interface between the basal surface of the lens epithelial and fiber cells and the lens capsule, the apical and lateral plasma membranes of lens epithelial cells and lens fibers in SPARC-null mice were as smooth as those of wild-type mice. We conclude that the absence of SPARC in the murine lens is associated with a filopodial protrusion of the basal surface of the lens epithelium and differentiating fiber cells into the lens capsule. The altered structures appear prior to the opacification of the lens in the SPARC-null model. These observations are consistent with one or more functions previously proposed for SPARC as a modulator of cell shape and cell-matrix interactions.

Alterations in SPARC and VEGF immunoreactivity in epithelial ovarian cancer

OBJECTIVE

Secreted protein, acidic and rich in cysteine (SPARC), is a matricellular protein that modulates cell adhesion and growth. It is thought to play a decisive role in tissue remodeling and angiogenesis. Alterations in SPARC expression have been observed in a variety of solid tumors; however, no consistent pattern of deregulation has been characterized. Vascular endothelial growth factor (VEGF) has emerged as an important regulator of tumor neovascularization. Recent work has shown that SPARC modulates the mitogenic activity of VEGF in normal endothelium. While its role in malignant transformation remains elusive, SPARC may contribute to tumor propagation and invasion. This study examines the immunoreactivity of SPARC and VEGF associated with neoplastic transformation of the ovary.

METHODS

Immunostaining for VEGF and SPARC protein was performed on 62 archival specimens.

RESULTS

Fourteen normal ovaries and 48 ovarian carcinomas were evaluated. SPARC was detected in the stroma of 63% of ovarian carcinomas. In contrast, SPARC was observed in the stroma of only 29% of normal ovaries (P = 0.02). Furthermore, SPARC was limited in normal ovaries to premenopausal patients, juxtaposed either with vesiculated follicles or within the corpus luteum. VEGF was observed in 42% of ovarian carcinomas with immunoreactivity confined to tumor cells. The level of VEGF immunoreactivity was significantly higher in ovarian carcinoma compared to normal ovary epithelium (42 vs 7%, P = 0.02).

CONCLUSIONS

Immunoreactivity of SPARC and VEGF is heightened in association with ovarian carcinoma, with a distinct distribution of SPARC in the stroma of neoplastic ovaries and VEGF within tumor cells. No obvious pattern of coincident SPARC and VEGF immunoreactivity was detected. These results indicate the possibility of an aberration in the interaction that has been described in normal endothelium between SPARC and VEGF in association with malignant transformation.

Localization of secreted protein acidic and rich in cysteine (SPARC) expression in the rat eye

Secreted protein acidic and rich in cysteine (SPARC) is a secreted glycoprotein protein which modulates cell shape and cell-matrix interactions and has been implicated in the regulation of angiogenesis, vascular permeability and cataract formation. In situ hybridization and immunohistochemical studies for SPARC were performed to determine the cell and tissue distribution of SPARC in rat eye. Studies demonstrated SPARC mRNA and protein co-localization at all sites. In the retina SPARC mRNA and protein were localized predominantly to the Müller and ganglion cells. Within the choroid, SPARC was found in vascular endothelial cells and fibroblasts; in the sclera SPARC was present in blood vessels and fibroblasts. SPARC was also present in the non-pigmented epithelial cells of the ciliary body, and in the epithelium of the lens capsule and cornea. The demonstrated anatomical distribution of SPARC in the rat eye is consistent with several of the biological functions ascribed to this matricellular protein and provides a rational basis for its examination in pathological conditions.

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.

Expression and purification of recombinant human SPARC produced by baculovirus

SPARC (secreted protein acidic and rich in cysteine/osteonectin/BM-40), a matrix-associated protein, disrupts cell adhesion and inhibits the proliferation of many cultured cells. We report the expression of recombinant human protein (rhSPARC) in a baculovirus expression system. This procedure routinely yields approximately 1 mg of purified protein per 500 ml of culture supernate. rhSPARC produced by insect cells migrates at the appropriate molecular weight under reducing and nonreducing conditions. The rhSPARC purified from insect cell media appeared structurally similar to SPARC purified from mammalian tissue culture by the criterion of circular dichroism. In addition, a series of anti-SPARC and anti-SPARC peptide antibodies recognized insect cell rhSPARC. We also show that rhSPARC produced in this system is glycosylated and is biologically active, as assessed by inhibition of endothelial cell proliferation and induction of collagen I mRNA in mesangial cells. Significant amounts of rhSPARC can now be generated in the absence of contaminating mammalian proteins for structure/function assays of SPARC activities.

Increased expression of osteonectin/SPARC mRNA and protein in age-related human cataracts and spatial expression in the normal human lens

PURPOSE

We have previously reported increased levels of Osteonectin/SPARC transcript in age-related cataractous compared to normal human lenses. The purpose of the present study was to evaluate the corresponding levels of osteonectin/SPARC protein in age-related cataractous relative to normal lenses and to evaluate the levels of osteonectin/SPARC transcript in specific types of age-related human cataracts. The spatial expression of osteonectin/SPARC was also evaluated in normal human lenses.

METHODS

Specific types of age-related cataracts were collected and graded. Normal human lenses were microdissected into epithelia and fibers. Osteonectin/SPARC protein levels were monitored by Western immunoblotting, and transcript levels were evaluated by reverse transcriptase polymerase chain reaction (RT-PCR). Osteonectin/SPARC expression patterns were examined by RT-PCR and by immunostaining.

RESULTS

Higher levels of osteonectin/SPARC protein were detected in age-related cataractous relative to normal human lenses. Increased levels of osteonectin/SPARC transcript were also detected in posterior-subcapsular and nuclear cataractous lenses relative to normal lenses. Osteonectin/SPARC transcripts were detected in the lens epithelium but not fibers. Osteonectin/SPARC protein levels were highest in the peripheral lens epithelium.

CONCLUSIONS

Consistent with our previous studies on osteonectin/SPARC mRNA levels, osteonectin/SPARC protein levels were also elevated in cataractous compared to normal human lenses. Increased levels of osteonectin/SPARC mRNA were also found in nuclear and posterior-subcapsular cataracts relative to normal lenses. Osteonectin/SPARC expression is confined to the lens epithelium, and osteonectin/SPARC levels are highest in the peripheral lens epithelium.

Generational analysis reveals that TGF-beta1 inhibits the rate of angiogenesis in vivo by selective decrease in the number of new vessels

Quantitative analysis of vascular generational branching demonstrated that transforming growth factor-beta1 (TGF-beta1), a multifunctional cytokine and angiogenic regulator, strongly inhibited angiogenesis in the arterial tree of the developing quail chorioallantoic membrane (CAM) by inhibition of the normal increase in the number of new, small vessels. The cytokine was applied uniformly in solution at embryonic day 7 (E7) to the CAMs of quail embryos cultured in petri dishes. After 24 h the rate of arterial growth was inhibited by as much as 105% as a function of increasing TGF-beta1 concentration. Inhibition of the rate of angiogenesis in the arterial tree by TGF-beta1 relative to controls was measured in digital images by three well-correlated, computerized methods. The first computerized method, direct measurement by the computer code VESGEN of vascular morphological parameters according to branching generations G(1) through G(>/=5), revealed that TGF-beta1 selectively inhibited the increase in the number density of small vessels, N(v>/=5) (382 +/- 85 cm(-2) for specimens treated with 1 microg TGF-beta1/CAM for 24 h, compared to 583 +/- 99 cm(-2) for controls), but did not significantly affect other parameters such as average vessel length or vessel diameter. The second and third methods, the fractal dimension (D(f)) and grid intersection (rho(v)), are statistical descriptors of spatial pattern and density. According to D(f) and rho(v), arterial density increased in control specimens from 1.382 +/- 0.007 and 662 +/- 52 cm(-2) at E7 (0 h) to 1.439 +/- 0.013 and 884 +/- 55 cm(-2) at E8 (24 h), compared to 1. 379 +/- 0.039 and 650 +/- 111 cm(-2) for specimens treated with 1 microg TGF-beta1/CAM for 24 h. TGF-beta1 therefore regulates vascular pattern and the rate of angiogenesis in a unique "fingerprint" manner, as do other major angiogenic regulators that include VEGF, FGF-2 (bFGF), and angiostatin. TGF-beta1 did not stimulate angiogenesis significantly at low cytokine concentrations, which suggests that this quail CAM model of angiogenesis is not associated with an inflammatory response.

Induction of TGF-beta1 by the matricellular protein SPARC in a rat model of glomerulonephritis

Induction of TGF-beta1 by the matricellular protein SPARC in a rat model of glomerulonephritis.

BACKGROUND

SPARC has been implicated as a counteradhesive and antiproliferative protein associated with deposits of extracellular matrix in renal disease.

METHOD

We have examined the effect of recombinant SPARC containing a C-terminal His tag (rSPARC) in an acute model of mesangial cell injury that is induced in the rat by an antibody against the Thy1 antigen on the mesangial cell membrane. The recombinant protein was administered 24 hours after the induction of nephritis and was infused through day 4.

RESULTS

rSPARC was localized to the renal glomeruli of rats treated with anti-Thy1 antibody. Type I collagen and fibronectin, as well as transforming growth factor-beta1 (TGF-beta1), were increased at day 5 in rats treated with rSPARC (N = 4, P < 0.05 vs. delivery buffer), but only minimal effects were seen on mesangial cell and endothelial cell proliferation. In primary cultures of rat mesangial cells, infusion of rSPARC was associated with increases in TGF-beta1 mRNA and in total, secreted TGF-beta1 protein.

CONCLUSIONS

rSPARC stimulates expression of TGF-beta1 both in vitro and in vivo. Given the closely regulated expression of SPARC, TGF-beta1, and type I collagen in several animal models of glomerulonephritis, we propose that SPARC could be one of the major mediators of the induction of TGF-beta1 in renal disease.

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)

SPARC regulates the expression of collagen type I and transforming growth factor-beta1 in mesangial cells

The matricellular protein SPARC is expressed at high levels in cells that participate in tissue remodeling and is thought to regulate mesangial cell proliferation and extracellular matrix production in the kidney glomerulus in a rat model of glomerulonephritis (Pichler, R. H., Bassuk, J. A., Hugo, C., Reed, M. J., Eng, E., Gordon, K. L., Pippin, J., Alpers, C. E., Couser, W. G., Sage, E. H., and Johnson, R. J. (1997) Am. J. Pathol. 148, 1153-1167). A potential mechanism by which SPARC controls both cell cycle and matrix production has been attributed to its regulation of a pleiotropic growth factor. In this study we used primary mesangial cell cultures from wild-type mice and from mice with a targeted disruption of the SPARC gene. SPARC-null cells displayed diminished expression of collagen type I mRNA and protein, relative to wild-type cells, by the criteria of immunocytochemistry, immunoblotting, and the reverse transcription-polymerase chain reaction. The SPARC-null cells also showed significantly decreased steady-state levels of transforming growth factor-beta1 (TGF-beta1) mRNA and secreted TGF-beta1 protein. Addition of recombinant SPARC to SPARC-null cells restored the expression of collagen type I mRNA to 70% and TGF-beta1 mRNA to 100% of wild-type levels. We conclude that SPARC regulates the expression of collagen type I and TGF-beta1 in kidney mesangial cells. Since increased mitosis and matrix deposition by mesangial cells are characteristics of glomerulopathies, we propose that SPARC is one of the factors that maintains the balance between cell proliferation and matrix production in the glomerulus.

Growth state-dependent regulation of plasminogen activator inhibitor type-1 gene expression during epithelial cell stimulation by serum and transforming growth factor-beta1

Transcription of the plasminogen activator inhibitor type-1 (PAI-1) gene appears to be growth state regulated in several cell types (e.g. , Ryan and Higgins, 1993, J Cell Physiol 155:376-384; Mu et al., 1998, J Cell Physiol 174:90-98). Transit of serum-stimulated normal rat kidney (NRK) epthelial cells through the first division cycle after release from quiescence (G(0)) provided a model system to assess the kinetics and mechanisms underlying PAI-1 expression in a growth "activated" phenotype. PAI-1 mRNA transcripts increased by more than 20-fold during the G(0)-->G(1) transition; induced expression had immediate-early response characteristics and abruptly declined prior to the onset of DNA synthesis. Transcriptional activity of the PAI-1 gene paralleled the steady-state mRNA abundance profile during this first synchronized growth cycle after release from quiescence. Although PAI-1 mRNA levels were up-regulated (approximately threefold) upon exposure to several different growth factors, neutralizing antibodies to transforming growth factor-beta1 (TGF-beta1) effectively attenuated the more than ninefold serum-associated PAI-1 inductive response by more than 70% (at both the mRNA transcript and protein levels). Similar to the metabolic requirements for serum-mediated PAI-1 transcription, PAI-1 induction upon addition of TGF-beta1 to quiescent NRK cell cultures was actinomycin D sensitive and resistant to cyclohexamide and puromycin, suggesting a primary mode of transcript control. The response to protein synthesis inhibitors, however, was complex. While cyclohexamide appeared to stabilize, or at least maintain, fetal bovine serum (FBS)- or TGF-beta1-stimulated PAI-1 mRNA levels, puromycin had no such affect. The amplitude and duration of induced PAI-1 expression were the same in either the presence or absence of puromycin. Cyclohexamide when used alone (i.e., in non-FBS- or TGF-beta1-treated cultures), moreover, effectively stimulated PAI-1 induction whereas puromycin was ineffective. Although TGF-beta1 was not a complete mitogen in the NRK cell system, incubation of quiescent renal cell cultures with TGF-beta1, prior to serum stimulation, resulted in a 10- to 12-fold increase in PAI-1 expression coincident with exit out of G(0). These data support a model in which PAI-1 gene expression is closely associated with creation of the growth-activated state and that cell cycle controls appear to be superimposed on the time course of the serum-induced expression of the PAI-1 gene.

Collagen accumulation is decreased in SPARC-null mice with bleomycin-induced pulmonary fibrosis

Secreted protein acidic and rich in cysteine (SPARC) has been shown to be coexpressed with type I collagen in tissues undergoing remodeling and wound repair. We speculated that SPARC is required for the accumulation of collagen in lung injury and that its absence would attenuate collagen accumulation. Accordingly, we have assessed levels of collagen in SPARC-null mice in an intratracheal bleomycin-injury model of pulmonary fibrosis. Eight- to ten-week-old SPARC-null and wild-type (WT) mice received bleomycin (0.0035 U/g) or saline intratracheally and were subsequently killed after 14 days. Relative levels of SPARC mRNA were increased 2.7-fold (P < 0.001) in bleomycin-treated WT lungs in comparison with saline-treated lungs. Protein from bleomycin-treated WT lung contained significantly more hydroxyproline (191.9 microg/lung) than protein from either bleomycin-treated SPARC-null lungs or saline-treated WT and SPARC-null lungs (147.4 microg/lung, 125.4 microg/lung, and 113. 0 microg/lung, respectively; P < 0.03). These results indicate that SPARC is increased in response to lung injury and that accumulation of collagen, as indicated by hydroxyproline content, is attenuated in the absence of SPARC. The properties of SPARC as a matricellular protein associated with cell proliferation and matrix turnover are consistent with its participation in the development of pulmonary fibrosis.

Cell cycle-dependent nuclear location of the matricellular protein SPARC: association with the nuclear matrix

Secreted protein acidic and rich in cysteine (SPARC) is a matricellular protein that inhibits cellular adhesion and proliferation. In this study, we report the detection of SPARC in the interphase nuclei of embryonic chicken cells in vivo. Differential partitioning of SPARC was also noted in the cytoplasm of these cells during discrete stages of M-phase: cells in metaphase and anaphase exhibited strong cytoplasmic immunoreactivity, whereas cells in telophase were devoid of labeling. Immunocytochemical analysis of embryonic chicken cells in vitro likewise showed the presence of SPARC in the nucleus. Furthermore, elution of soluble proteins and DNA from these cells indicated that SPARC might be a component of the nuclear matrix. We subsequently examined cultured bovine aortic endothelial cells, which initially appeared to express SPARC only in the cytoplasm. However, after elution of soluble proteins and chromatin, we also detected SPARC in the nuclear matrix of these cells. Embryonic chicken cells incubated with recombinant SPARC were seen to take up the protein and to translocate it to the nucleus progressively over a period of 17 h. These observations provide new information about SPARC, generally recognized as a secreted glycoprotein that mediates interactions between cells and components of the extracellular matrix. The evidence presented in this study indicates that SPARC might subserve analogous functions in the nuclear matrix.

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.

Expression of SPARC (secreted protein, acidic and rich in cysteine) in healing intestinal anastomoses and short bowel syndrome in rats

Due to the proposed functions in soft tissue repair, we evaluated the spatial and temporal distribution of SPARC, a counteradhesive, matricellular glycoprotein in healing intestinal anastomoses and short bowel syndrome (SBS) in rats. Intestinal anastomoses were performed in the jejunum of male Wistar rats. SBS was induced by resecting 70% of the small bowel. In situ hybridization was performed to localize SPARC mRNA and immunohistochemical studies for locating the SPARC protein. The granulation tissue in the anastomotic area exhibited immunoreactivity for SPARC at all time points. The level of expression was maximal at seven to nine days. Endothelial cells of capillaries, smooth muscle cells, fibroblastic cells, and macrophages, as well as mesothelial cells on the serosal surface, were stained. The immunoreactivity was mostly intracellular. SPARC mRNA transcripts were localized to the edges of the anastomotic area at days 1 and 4 and on the newly formed granulation tissue later. The expression of SPARC mRNA was maximal at seven days and decreased thereafter. Both in normal controls and in SBS, SPARC was expressed in endothelial cells of submucosal capillaries and in smooth muscle cells but not in epithelium. Based on the restricted temporal and spatial distribution during the healing of intestinal anastomoses and in SBS we propose that SPARC plays a significant role in intestinal repair and adaptation.

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.

A novel, quantitative model for study of endothelial cell migration and sprout formation within three-dimensional collagen matrices

Interactions between migratory endothelial cells (ECs) and surrounding extracellular matrix (ECM) are of central importance to vascular growth. Here, we present a new model of EC migration and morphogenesis within three-dimensional ECM termed "radial invasion of matrix by aggregated cells" (RIMAC). In the RIMAC model, single aggregates of defined numbers of bovine aortic ECs were embedded within small, lenticular gels of type I collagen supported by annuli of nylon mesh. Culture of the gels in nutrient media resulted in quantifiable, reproducible, radial migration of ECs into the collagen. The angiogenic proteins basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF) each stimulated migration of ECs in a concentration-dependent manner. In combination, bFGF and VEGF stimulated migration synergistically. In contrast, transforming growth factor-beta1 inhibited migration of ECs. Low concentrations (0.1-0.5 ng/ml) of VEGF induced ECs to form multicellular sprouts, some of which possessed lumen-like spaces. Mitomycin C, an inhibitor of cell proliferation, did not affect the migration of ECs into collagen induced by 0.5 ng/ml VEGF but moderately inhibited migration induced by 5 ng/ml VEGF. Increasing the density (concentration) of the collagen gel inhibited the migration of single ECs and increased the branching and anastomosis of multicellular sprouts. We conclude that the RIMAC model is a highly efficacious assay for the screening of potentially angiogenic and angiostatic compounds and, moreover, is advantageous for mechanistic studies of vascular morphogenesis.

Disruption of the Sparc locus in mice alters the differentiation of lenticular epithelial cells and leads to cataract formation

SPARC (secreted protein acidic and rich in cysteine) is a matricellular protein that regulates cellular adhesion and proliferation. In this report, we show that SPARC protein is restricted to epithelial cells of the murine lens and ends abruptly at the equatorial bow region where lens fiber differentiation begins. SPARC protein was not detected in the lens capsule or in differentiated lens fibers. SPARC-null mice developed cataracts at approximately 3-4 months after birth, at which time posterior subcapsular opacities were observed by slit lamp ophthalmoscopy. Histological analyses of ocular sections from 3-month old animals revealed several microscopic abnormalities present in the SPARC-null mice but absent from the wild-type animals. Fiber cell elongation was incomplete posteriorly and resulted in displacement of the lenticular nucleus to the posterior of the lens. Nuclear debris was present in the posterior subcapsular region of the lens, an indication of the abnormal migration and elongation of either fetal or anterior epithelial cells, and the bow region was disrupted and vacuolated. In the anterior lens, the capsule appeared to be thickened and was lined by atypical, plump cuboidal epithelium. Moreover, anterior cortical fibers were swollen. Polyacrylamide gel electrophoresis of the epithelial, cortical and nuclear fractions of wild-type and SPARC-null lenses indicated no significant differences among the alpha-, beta-, and gamma-crystallins. Expression of alphaB-crystallin appeared similar in fiber cells of wild-type and SPARC-null lenses, although the distribution of alphaB-crystallin was asymmetric in SPARC-null lenses as a result of abnormal lens fiber differentiation. No evidence of atypical extracellular matrix deposition in areas other than the capsule was detected in wild-type or SPARC-null lens at 3 months of age. We conclude that the disruption of the Sparc locus in mice results in the alteration of two fundamental processes of lens development: differentiation of epithelial cells and maturation of fiber cells.

Angiostatin diminishes activation of the mitogen-activated protein kinases ERK-1 and ERK-2 in human dermal microvascular endothelial cells

Angiostatin is an endogenous inhibitor of angiogenesis that was isolated from tumor-bearing mice. It has been established that angiostatin inhibits endothelial cell proliferation; however, the underlying mechanisms remain to be elucidated. Here we report that angiostatin reduces transiently the phosphorylation of the mitogen-activated protein kinases ERK-1 and ERK-2 in human dermal microvascular cells, but not in human vascular smooth muscle cells or human dermal fibroblasts. We demonstrate that angiostatin diminishes ERK activation by basic fibroblast growth factor and vascular endothelial growth factor. Dephosphorylation of ERK and other tyrosine-phosphorylated proteins was blocked by pretreatment of the cells with sodium meta-vanadate, an inhibitor of protein tyrosine phosphatases, indicating that angiostatin signaling may require the activity of a tyrosine phosphatase. Concentrations of angiostatin that inhibited ERK activation also inhibited basic fibroblast growth factor-stimulated collagen gel invasion by endothelial cells, but did not affect endothelial cell proliferation. We thus show that angiostatin inhibits primarily the invasion of endothelial cells and exerts minimal (if any) effects on their proliferation. Invasion is a process that involves proteolysis, adhesion and migration, all of which have been linked to ERK signaling.

SPARC/osteonectin induces matrix metalloproteinase 2 activation in human breast cancer cell lines

Activation of the matrix metalloproteinase 2 (MMP-2) has been shown to play a major role in the proteolysis of extracellular matrix (ECM) associated with tumor invasion. Although the precise mechanism of this activation remains elusive, levels of the membrane type 1-MMP (MT1-MMP) at the cell surface and of the tissue inhibitor of MMP-2 (TIMP-2) appear to be two important determinants. Induction of MMP-2 activation in cells cultivated on collagen type I gels indicated that the ECM is important in the regulation of this process. In this study, we show that SPARC/osteonectin, a small ECM-associated matricellular glycoprotein, can induce MMP-2 activation in two invasive breast cancer cell lines (MDA-MB-231 and BT549) but not in a noninvasive counterpart (MCF-7), which lacks MT1-MMP. Using a set of peptides from different regions of SPARC, we found that peptide 1.1 (corresponding to the NH2-terminal region of the protein) contained the activity that induced MMP-2 activation. Despite the requirement for MT1-MMP, seen in MCF-7 cells transfected with MT1-MMP, the activation of MMP-2 by SPARC peptide 1.1 was not associated with increased steady-state levels of MT1-MMP mRNA or protein in either MT1-MMP-transfected MCF-7 cells or constitutively expressing MDA-MB-231 and BT549 cells. We did, however, detect decreased levels of TIMP-2 protein in the media of cells incubated with peptide 1.1 or recombinant SPARC; thus, the induction of MMP-2 activation by SPARC might be due in part to a diminution of TIMP-2 protein. We conclude that SPARC, and specifically its NH2-terminal domain, regulates the activation of MMP-2 at the cell surface and is therefore likely to contribute to the proteolytic pathways associated with tumor invasion.

SPARC deficiency leads to early-onset cataractogenesis

PURPOSE

To determine the role of SPARC (secreted protein, acidic, and rich in cysteine) in cataractogenesis by examining mice deficient in a matricellular protein SPARC.

METHODS

Mice were rendered SPARC-deficient by a targeted disruption of the gene. Slit-lamp microscopy and histology were used to examine the eyes of SPARC-null and wild-type mice from birth to 14 months of age.

RESULTS

SPARC-null mice developed opacities in the posterior cortex of the eye as early as 1.5 months after birth. The diffuse cataracts appeared to progress toward the anterior cortex and reached maturity in many animals by 3.5 months of age. Early stages of cataractogenesis in SPARC-null mice included inhibition of normal lens fiber cell differentiation, degeneration of fiber cells, vacuole formation at the equator, and liquefaction of the cortex. No cataracts were detected in wild-type mice up to the age of 8 months.

CONCLUSIONS

The early onset of cataracts in SPARC-null mice establishes that the gene is essential to the maintenance of lens transparency.

SPARC (BM-40, osteonectin) inhibits the mitogenic effect of vascular endothelial growth factor on microvascular endothelial cells

SPARC (secreted protein, acidic and rich in cysteine) is a matricellular protein that modulates cell adhesion and proliferation and is thought to function in tissue remodeling and angiogenesis. In this study, we demonstrate that SPARC inhibits DNA synthesis by >90% in human microvascular endothelial cells (HMEC) stimulated by the endothelial cell mitogen vascular endothelial growth factor (VEGF). Peptides derived from SPARC domain IV, which contains a disulfide-bonded EF-hand sequence and binds to endothelial cells, mimicked the effect of native SPARC. The inhibition was also observed with a peptide from the follistatin-like domain II, whereas peptides from SPARC domains I and III had no effect on VEGF-stimulated DNA synthesis. The inhibition of HMEC proliferation was mediated in part by the binding of VEGF to SPARC. The binding of 125I-VEGF to HMEC was reduced by SPARC and SPARC peptides from domain IV in a concentration-dependent manner. In a radioimmune precipitation assay, peptides from SPARC domains II and IV each competed with native SPARC for its binding to VEGF. It has been reported that VEGF stimulates the tyrosine phosphorylation and activation of mitogen-activated protein kinases Erk1 and Erk2. We now show that SPARC reduces this phosphorylation in VEGF-stimulated HMEC to levels of unstimulated controls. SPARC thus modulates the mitogenic activity of VEGF through a direct binding interaction and reduces the association of VEGF with its cell-surface receptors. Moreover, an additional diminution of VEGF activity by SPARC is accomplished through a reduction in the tyrosine phosphorylation of mitogen-activated protein kinases.

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.

Secreted protein, acidic and rich in cysteine (SPARC) and thrombospondin in the developing follicle and corpus luteum of the rat

In adult mammals, growth of new vasculature from extant blood vessels (angiogenesis) is rare in the absence of pathology. However, nonpathogenic angiogenesis occurs in the cycling ovary when the avascular postovulatory follicle transforms into a highly vascularized corpus luteum (CL). To improve our understanding of molecular mechanisms that regulate nonpathogenic vascular growth, we characterized the expression of two secreted matricellular proteins associated with angiogenesis, SPARC and thrombospondin (TSP), in postovulatory preluteal follicles and CL of hormone-primed immature rats. By indirect immunofluorescence with specific antibodies, we found SPARC in the cytoplasOFFf granulosa cells and thecal cells of preluteal follicles, in connective tissue cells of the ovarian interstitium, and in the oocyte nucleus. Administration of a luteinizing stimulus (chorionic gonadotropin) increased the expression of SPARC in granulosa cells. TSP was prominent in the basement membranes of growing follicles. Many cells in the early vascularizing CL expressed both SPARC and TSP. Neovascularization of CL was accompanied by expression of SPARC in nascent vessels and concentration of TSP in central avascular areas. In mature CL, steroidogenic luteal cells expressed both SPARC and TSP. Luteal cells of regressing CL retained SPARC to a variable degree but did not express TSP. The observed changes in expression of SPARC and TSP during development of the CL support distinct roles for these matricellular proteins in nonpathological morphogenesis and angiogenesis.

Transforming growth factor-beta1 induces apoptotic cell death in cultured retinal endothelial cells but not pericytes: association with decreased expression of p21waf1/cip1

Transforming growth factor-beta1 (TGF-beta1) regulates a variety of cellular functions. In several types of cells, for example, it acts as a growth inhibitor and an inducer of apoptotic cell death. Although one of the important modulators in retinal vascular development and retinal neovascularization, the effects of TGF-beta1 on retinal microvascular cells are not fully defined. We have found that proliferation of both bovine retinal endothelial cells (EC) and pericytes was inhibited by TGF-beta1 in a concentration-dependent manner. However, only retinal EC lost viability after exposure to increasing concentrations of TGF-beta1 (up to 10 microg/ml) in the presence of 2% fetal bovine serum. Dying EC exhibited the morphological and biochemical characteristics of apoptosis. Fragmented nuclei and chromatin condensation were apparent after staining with the fluorochrome Hoechst 33258 and the reagent ApopTag; moreover, gel electrophoresis of DNA from TGF-beta1-treated EC demonstrated degradation of chromatin into the discrete fragments typically associated with apoptosis. The addition of anti-TGF-beta1 neutralizing antibody abolished the apoptotic cell death induced by TGF-beta1. Because not all the EC in a given culture died after exposure to TGF-beta1, we separated the apoptosis-sensitive cells from those resistant to TGF-beta1 -mediated apoptosis and determined the expression of several proteins associated with this apoptotic pathway. Apoptosis of EC mediated by TGF-beta1 was associated with a decreased level of the cyclin-dependent kinase inhibitor p21waf1/cip1, compared with that observed in the apoptosis-resistant cells. In contrast, the translation product of the tumor-suppressor gene p53 was increased in the TGF-beta1-treated apoptotic cells. Thus, we propose that p21waf1/cip1 and p53 function in distinct pathways that are protective or permissive, respectively, for the apoptotic signals mediated by TGF-beta1.

A novel assay of angiogenesis in the quail chorioallantoic membrane: stimulation by bFGF and inhibition by angiostatin according to fractal dimension and grid intersection

In a novel assay of angiogenesis in the quail chorioallantoic membrane (CAM), we measured vascular pattern and angiogenic rate after homogeneous exposure of the entire vascular tree to recognized modulators of vessel growth. In comparison to phosphate-buffered saline (PBS)-treated controls, the vascular stimulator, basic fibroblast growth factor (bFGF or FGF-2), increased the rate of angiogenesis by a maximum of 72%, whereas a recently discovered angiogenic inhibitor, angiostatin, decreased the rate of vascular growth by a maximum of 68%. The perturbants were applied in PBS to the CAM of 7-day-old embryos (E7) cultured in petri dishes, and the embryos were cultured further until fixation at E8 or E9. For morphometry of the quasi-two-dimensional CAM vasculature, digital images of arterial endpoints from the middle region of the CAM were acquired in grayscale at a magnification of 10x, binarized to black/white, and skeletonized. The pattern of vessel branching was assessed by measurement of the fractal dimension (Df), and vessel density (rhov), with the method of grid intersection. Correlations between these two statistical techniques were linear (r2 ranged from 0.967 to 0.985). For skeletonized images at E9, Df and rhov of bFGF-treated samples were 1.55 +/- 0.01 and 782 +/- 26/cm2, respectively (relative to 1.49 +/- 0.02 and 583 +/- 60/cm2 for controls), and of angiostatin-treated samples, 1.43 +/- 0.02 and 424 +/- 74/cm2 (relative to 1.50 +/- 0.02 and 616 +/- 59/cm2 for controls). To establish normalization values for rates of angiogenesis, we analyzed untreated CAMs of E6 to E12. From E7 to E10 in skeletonized images, Df increased linearly from 1.37 +/- 0.01 to 1.54 +/- 0.01 and rhov from 311 +/- 67 to 746 +/- 124/cm2 (in both cases, r2 = 1.000). Thus, the rates of normal angiogenic growth as measured by Df and rhov were 0.06/day and 138/cm2-day, respectively. From E10 to E12, Df and rhov declined slightly. Differences between the vasculature of untreated and PBS-treated CAMs were statistically insignificant. In conclusion, vascular branching pattern and density in the quail CAM were stimulated by bFGF and inhibited by angiostatin. We quantified these changes with statistical significance by Df and rhov, which are expressed relative to the rates of normal developmental angiogenesis measured for the two parameters in untreated quail embryos.

Growth factors reverse the impaired sprouting of microvessels from aged mice

Aging is accompanied by impaired angiogenesis and deficient expression of several angiogenic growth factors. To test the hypothesis that replacement of these factors would improve angiogenesis in aged animals, we cultured microvessels derived from the epididymal fat pad of aged and young mice ("aged" and "young" microvessels) in three-dimensional collagen gels for 2 weeks and measured their sprouting (formation of branch points) in response to fetal bovine serum (FBS), endothelial cell growth supplement (ECGS), and the specific growth factors transforming growth factor-beta1 (TGF-beta1), vascular endothelial growth factor (VEGF), insulin-like growth factor-1 (IGF-1), and basic fibroblast growth factor (bFGF). In the presence of culture medium with 1% FBS (Minimal medium), sprouting of aged microvessels was significantly less than sprouting of young microvessels. The addition of high levels of FBS and ECGS to Minimal medium enhanced the sprouting of microvessels from aged mice to a greater degree than that of young mice, such that the difference between the two age groups was no longer significant. Formation of branch points by aged microvessels was also significantly increased by Minimal medium supplemented with TGF-beta1, bFGF, IGF-1, or VEGF (listed in order of highest to lowest stimulation). Sprouts generated in the presence of VEGF possessed a particularly high percentage of endothelial cells. Mitomycin C did not diminish the degree of sprouting induced by TGF-beta1, VEGF, or IGF-1, a result indicating that early stages of angiogenesis, including formation of branch points, do not require cell division. From our findings in vitro, we propose that age-related deficiencies in angiogenesis in vivo are likely to be due, in part, to a decrease in angiogenic growth factors in the extracellular milieu.

Regulation of SPARC expression during early Xenopus development: evolutionary divergence and conservation of DNA regulatory elements between amphibians and mammals

SPARC (Secreted Protein, Acidic, Rich in Cysteine/osteonectin/BM-40) is a highly conserved metal-binding extracellular matrix (ECM) glycoprotein which is first expressed by Xenopus embryos during late gastrulation/early neurulation (stage 12/13), by presumptive notochord and somitic cells. When animal cap explants of stage 9 embryos were cultured in vitro, SPARC expression was not detected until sibling embryos reached late neurula stage (stage 19). Addition of activin, a potent dorsal mesoderm inducer, to animal caps resulted in SPARC being expressed by the time sibling embryos reached stage 16. While basic fibroblast growth factor (bFGF), a ventral mesoderm inducer, had modest effects on SPARC mRNA expression, the combination of both activin and bFGF was synergistic. The appearance, however, of SPARC transcripts 11 h after the addition of activin and bFGF, indicates that unknown intermediates were likely to be involved in activating SPARC expression. In order to identify the potential intermediate regulatory factors which may activate and control SPARC expression, we examined the genomic organization of the 5' end of the Xenopus SPARC gene. No significant homology to the equivalent region that is highly conserved in the mouse, bovine and human SPARC genes was observed. Thus, while mammalian SPARC promoters lack TATA or CAAT boxes, the Xenopus gene contains a consensus TATA box. Moreover, promoter-proximal GGA-box repeats necessary for high level expression of mammalian SPARC are absent in Xenopus. When reporter constructs containing the 5' flanking region of the Xenopus gene were microinjected into two-cell embryos, 868 bp of 5' flanking DNA was sufficient to mimic the temporal and tissue-specific pattern of SPARC expression observed in whole embryos. While a bovine SPARC promoter reporter construct containing 740 bp of the 5' flanking DNA was expressed at a significant level in Xenopus embryos, significant differences in the cell-type expression of the reporter genes were obtained between the bovine and Xenopus constructs. The data indicate that zygotic activation of SPARC mRNA is mediated by regulatory factors acting downstream of major mesoderm induction events. The high DNA sequence conservation at the 5' end of mammalian SPARC genes is not conserved in Xenopus. These differences led to differences in their ability to direct tissue-specific gene expression in early Xenopus embryos.

Neovascularization in aged mice: delayed angiogenesis is coincident with decreased levels of transforming growth factor beta1 and type I collagen

Angiogenesis, the growth of new vessels from existing microvasculature, is delayed in aged animals. In this study we asked whether this impairment might be due, in part, to changes in the expression of a growth factor, transforming growth factor-beta1 (TGF-beta1), and a matrix protein, type I collagen, which have been shown to regulate angiogenesis in vivo. We implanted polyvinyl alcohol sponges subcutaneously in the dorsa of young and aged mice and examined the sponges 7 to 21 days later for the presence of invasive fibrovascular bundles. Blood vessel ingrowth and proliferative activity were assessed by immunostain for von Willebrand factor and Ki-67, respectively. The fibrovascular bundles were also analyzed for TGF-beta1 and type I collagen. Relative to young mice, angiogenic invasion of sponges in aged mice was similar at 7 days, was diminished significantly (70%) at 14 days, but was again similar by 21 days after implantation. The expression of TGF-beta1 and type I collagen mRNA and protein in fibrovascular bundles was coincident but was also delayed (42 to 47%) at 14 days in the aged mice. Moreover, levels of active TGF-beta1 were decreased (48%) in the sera of aged relative to young mice. The delay in angiogenesis in aged mice was thus associated with decreased expression of TGF-beta1 and type I collagen by neovascular bundles. We conclude that changes in the levels of growth factors and proteins in the extracellular matrix contribute to impaired angiogenesis in aging.

SPARC is expressed by ganglion cells and astrocytes in bovine retina

SPARC (secreted protein, acidic and rich in cysteine)/osteonectin is a matricellular, counteradhesive glycoprotein that disrupts cell-matrix interactions, interacts with growth factors and components of extracellular matrix, and modulates the cell cycle, but appears to subserve only minor structural roles. SPARC is expressed in a variety of tissues during embryogenesis and remodeling and is believed to regulate vascular morphogenesis and cellular differentiation. Although usually limited in normal adult tissues, SPARC is expressed at significant levels in the adult central nervous system. Using a monoclonal antibody against bovine bone osteonectin, we have determined the localization of SPARC in newborn (3-day-old) and adult (4-8-year-old) normal bovine retinas. SPARC was present in the soma of ganglion cells and strong reactivity was found in ganglion cell axons. Muller cells displayed no immunoreactivity, but SPARC was present in retinal astrocytes that were identified by the astrocyte marker glial fibrillary acidic protein (GFAP). Newborn calf retina showed a staining pattern similar to that of adult retina but exhibited significantly reduced levels of SPARC. Minimal levels of SPARC protein were also detected in some capillaries of the inner retina of both newborn and adult animals, whereas large vessels were negative. The presence of SPARC in the retina was confirmed by Western blotting of retinal extracts. These data indicate that SPARC originating from bot h neurons and glia of the inner retina may be an important modulator of retinal angiogenesis. The increased expression of SPARC in adult relative to newborn retinal tissue also indicates that SPARC has an ongoing role in the maintenance of retinal functions.

Regulation of human monocyte matrix metalloproteinases by SPARC

SPARC (secreted protein, acidic and rich in cysteine), also called osteonectin or BM-40, is a collagen-binding glycoprotein secreted by a variety of cells and is associated with functional responses involving tissue remodeling, cell movement and proliferation. Because SPARC and monocytes/macrophages are prevalent at sites of inflammation and remodeling in which there is connective tissue turnover, we examined the effect of SPARC on monocyte matrix metalloproteinase (MMP) production. Treatment of human peripheral blood monocytes with SPARC stimulated the production of gelatinase B (MMP-9) and interstitial collagenase (MMP-1). Experiments with synthetic peptides indicated that peptide 3.2, belonging to the alpha helical domain III of SPARC, is the major peptide mediating the MMP production by monocytes. SPARC and peptide 3.2 were also shown to induce prostaglandin synthase (PGHS)-2 as determined by Western and Northern blot analyses. The increase in PGHS-2 stimulated by SPARC or peptide 3.2 correlated with substantially elevated levels of prostaglandin E2 (PGE2) and other arachidonic acid metabolites as measured by radioimmunoassay and high performance liquid chromatography (HPLC), respectively. Moreover, the synthesis of MMP was dependent on the generation of PGE2 by PGHS-2, since indomethacin inhibited the production of these enzymes and their synthesis was restored by addition of exogenous PGE2 or dibutyryl cAMP (Bt2cAMP). These results demonstrate that SPARC might play a significant role in the modulation of connective tissue turnover due to its stimulation of PGHS-2 and the subsequent release of PGE2, a pathway that leads to the production of MMP by monocytes.

Cloning and expression of murine SC1, a gene product homologous to SPARC

A number of cDNAs (SC1, QR1, and hevin) have been shown to be similar to SPARC (secreted protein acidic and rich in cysteine), a matricellular protein that regulates cell adhesion, cell cycle, and matrix assembly and remodeling. These proteins are 61-65% identical in the final 200 residues of their C-termini; their N-terminal sequences are related but more divergent. All have an overall acidic pl, with a follistatin-like region that is rich in cysteine, and a Ca+2 binding consensus sequence at the C-terminus. Using degenerate primers representing the most highly conserved region in SPARC, SC1, and QR1, we identified a 300-BP SC1 clone in a primary polymerase chain reaction (PCR) screen of a mouse brain cDNA library. This cDNA was used to obtain a full-length clone, which hybridized to a 2.8-KB RNA abundant in brain. Mouse SC1 displays a similarity of 70% to mouse SPARC at the amino acid level. Northern blot and RNAse protection assays revealed a 2.8-KB mRNA expressed at moderate levels (relative to brain) in mouse heart, adrenal gland, epididymis, and lung, and at low levels in kidney, eye, liver, spleen, submandibular gland, and testis. In contrast to SPARC, in situ hybridization showed expression of SC1 mRNA in the tunica media and/or adventitia of medium and large vessels; transcripts were not detected in capillaries, venules, or large lymphatics. The distribution of transcripts for SC1 was also different from that of SPARC in several organs, including adrenal gland, lung, heart, liver, and spleen. Moreover, SC1 mRNA was not evident in endothelium cultured from rat heart, bovine fetal and adult aorta, mouse aorta, human omentum, and bovine retina. Cultured smooth muscle cells and fibroblasts also failed to express SC1 mRNA. The absence of SC1 transcript in cultured cells indicates that the SC1 gene is potentially sensitive to regulatory factors in serum or to a three-dimensional architecture conferred by the extracellular matrix that is lacking in vitro. In conclusion, the expression of SPARC and SC1 appears to be coincident in specific tissues (e.g., adrenal gland and brain), but these proteins exhibit distinct expression patterns in most organs of the mouse. Because SC1 and SPARC are structurally similar and exhibit counteradhesive effects on cultured cells, their overlapping and/or adjacent expression in most tissues predicts that one protein might compensate functionally, at least in part, for the other.

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.

Differential modulation of cell adhesion by interaction between adhesive and counter-adhesive proteins: characterization of the binding of vitronectin to osteonectin (BM40, SPARC)

Heparin-binding forms of vitronectin, a multifunctional adhesive glycoprotein, are associated with the extracellular matrix (ECM) at different locations in the body and serve to promote cell adhesion and the regulation of pericellular proteolysis at sites of angiogenesis. In the present study we characterized the interactions of vitronectin with the counter-adhesive protein osteonectin (also termed SPARC or BM40). Osteonectin and vitronectin were both found associated with the ECM of cultured endothelial cells and were localized in vessel wall sections of kidney tissue. In vitro, the heparin-binding multimeric isoform of vitronectin bound to immobilized osteonectin in a saturable manner with half-maximal binding at 30-40 nM. Preincubation of plasma vitronectin with plasminogen activator inhibitor 1 (PAI-1), which provoked multimer formation, induced the binding of vitronectin to osteonectin. Binding was optimal at physiological ionic strength, and binary complexes were stabilized by tissue transglutaminase-mediated cross-linking. In a concentration-dependent fashion, PAI-1, CaCl2, heparin and heparan sulphate, but not other glycosaminoglycans, interfered with the binding of vitronectin to osteonectin. Using vitronectin-derived synthetic peptides as well as mutant forms of recombinant osteonectin, we found that the heparin-binding region of vitronectin interacted with the C-terminal region of osteonectin that contains a high-affinity Ca2+-binding site with counter-adhesive properties. Adhesion of cultured endothelial cells was partly abrogated by osteonectin and was correspondingly reversed by vitronectin in a concentration-dependent manner. These results indicate that specific interactions between vitronectin and osteonectin modulate cell adhesion and might thereby regulate endothelial cell function during angiogenesis.

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.