Expression of SPARC is correlated with altered morphologies in transfected F9 embryonal carcinoma cells

Molecular mechanisms underlying the divergent roles of SPARC in human carcinogenesis

Communication between the cell and its surrounding environment, consisting of proteinaceous (non-living material) and extracellular matrix (ECM), is important for biophysiological and chemical signaling. This signaling results in a range of cellular activities, including cell division, adhesion, differentiation, invasion, migration and angiogenesis. The ECM non-structural secretory glycoprotein called secreted protein, acidic and rich in cysteine (SPARC), plays a significant role in altering cancer cell activity and the tumor's microenvironment (TME). However, the role of SPARC in cancer research has been the subject of controversy. This review mainly focuses on recent advances in understanding the contradictory nature of SPARC in relation to ECM assembly, cancer cell proliferation, adhesion, migration, apoptosis and tumor growth.

Signaling pathways in breast cancer metastasis - novel insights from functional genomics

The advent of genomic profiling technology has brought about revolutionary changes in our understanding of breast cancer metastasis. Gene expression analyses of primary tumors have been used to predict metastatic propensity with high accuracy. Animal models of metastasis additionally offer a platform to experimentally dissect components of the metastasis genetic program. Recent integrated studies have synergized clinical bioinformatic analyses with advanced experimental methodology and begun to uncover the identities and dynamics of signaling programs driving breast cancer metastasis. Such functional genomics studies hold great promise for understanding the genetic basis of metastasis and improving therapeutics for advanced diseases.

Anti-cancer role of SPARC, an inhibitor of adipogenesis

SPARC (a secreted protein acidic and rich in cysteine) has a reputation for being potent anti-cancer and anti-obesity molecule. It is one of the first known matricellular protein that modulates interactions between cells and extracellular matrix (ECM) and is associated with the 'balance' of white adipose tissue (WAT) as well as lipogenesis and lipolysis during adipogenesis. Adipogenesis is an indication for the development of obesity and has been related to a wide variety of cancers including breast cancer, endometrial cancer, esophageal cancer, etc. Adipogenesis mainly involves ECM remodeling, changes in cell-ECM interactions, and cytoskeletal rearrangement. SPARC can also prevent hypertrophy of adipocytes and hyperplasia of adipocyte progenitors. In addition to SPARC's inhibitory role in adipogenesis, it has also been known to be involved in cell cycle, cell proliferation, cell invasion, adhesion, migration, angiogenesis and apoptosis. Molecular cancer biology and clinical biochemistry have significantly enhanced our understanding of the mechanisms that motivate the anti-cancer and anti-obesity action of SPARC. Recent studies elucidating the signaling pathways that are activated by SPARC can help develop the beneficial aspects of SPARC for cancer therapy and obesity prevention. This review focuses on the anti-cancer role of SPARC as it pertains to obesity.

Development of Secreted Protein and Acidic and Rich in Cysteine (SPARC) Targeted Nanoparticles for the Prognostic Molecular Imaging of Metastatic Prostate Cancer

Prostate cancer is the most commonly diagnosed non-skin malignancy in the United States and presents with a wide range of aggressiveness from extremely slow-growing to highly aggressive. There is a clinical need to determine the metastatic potential of the primary tumor to design the most appropriate treatment plan ranging from watchful waiting to more aggressive, invasive surgical treatments. In this study we have developed a nanoparticle based imaging agent that targets SPARC (Secreted Protein Acidic Rich in Cysteine), a molecular marker of prostate cancer metastatic potential. Previous studies by this group used phage display to identify a peptide with high binding affinity and specificity for SPARC. In this study, the SPARC-targeted peptide sequence was used to design a biomaterial with improved pharmacokinetic properties by attaching it to a biocompatible nanoparticle that is also coupled to a fluorophore for in vivo imaging. Prostate cancer cell lines with varying degrees of SPARC expression were used to show the ability of the targeted nanoparticle to bind specifically to SPARC in vitro and in vivo including the clinically relevant bone and lung metastases. We show that in vivo imaging information correlates with the metastatic potential of the prostate tumor. This prognostic information could enable doctors to stratify patients and design personalized treatment strategies.

Indomethacin and retinoic acid modify mouse intestinal inflammation and fibrosis: a role for SPARC

The mouse model of 2,4,6-Trinitrobenzene Sulfonic Acid (TNBS)-induced intestinal fibrosis allows for detailed study of the extracellular matrix changes that complicate Crohn's disease. Indomethacin induces intestinal fibrosis, while retinoic acid (RA) reduces liver fibrosis. Secreted protein acidic and rich in cysteine (SPARC), an extracellular matrix-modifying agent, may potentially link these opposing effects. Our aim was to determine the effects of indomethacin and RA and to evaluate their correlation to SPARC expression in the TNBS mouse model. CD-1 mice were randomised to TNBS enemas weekly for 2 or 8 weeks with or without indomethacin (0.2 mg/kg per day) or RA (100 microg/kg per day). At 2 weeks, indomethacin/TNBS enhanced and RA reduced inflammation, tissue destruction and fibrosis. The expression of SPARC was inversely related to fibrosis, but not to inflammation, in the TNBS-alone groups at 2 weeks; these differences were lost by 8 weeks. The results demonstrate that indomethacin increases TNBS-induced fibrosis in mice, while RA reduces it, and that SPARC may link these opposing effects.

Aberrant methylation of SPARC in human lung cancers

SPARC (secreted protein acidic and rich in cysteine) is an extracellular Ca²⁺-binding matricellular glycoprotein associated with the regulation of cell adhesion and growth. We investigated loss of expression of SPARC gene and promoter methylation in lung cancers and correlated the data with clinicopathological features. We observed loss of SPARC expression in 12 of 20 (60%) lung cancer cell lines. Treatment of expression-negative cell lines with a demethylating agent restored expression in all cases. Methylation frequencies of SPARC gene were 55% in 20 lung cancer cell lines. Primary tumours had methylation at a rate of 69% (119 of 173), while nonmalignant lung tissues (n=60) had very low rates (3%). In lung adenocarcinomas, SPARC methylation correlated with a negative prognosis (P=0.0021; relative risk 4.65, 95% confidence interval 1.75–12.35, multivariate Cox's proportional-hazard model). Immunostaining revealed protein expression in bronchial epithelium (weak intensity) and in juxtatumoral stromal tissues (strong intensity) accompanied by frequent loss in cancer cells that correlated with the presence of methylation (P<0.001). Our findings are of biological interest and potentially of clinical importance in human lung cancers.

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.

Osteonectin/SPARC is overexpressed in human hepatocellular carcinoma

Osteonectin (ON)/SPARC is a glycoprotein involved in extracellular matrix remodelling. ON expression by myofibroblasts has been reported in fibrotic human liver. As ON also plays a role in cell adhesion, differentiation, and proliferation, this study was designed to document its expression in human hepatocellular carcinoma (HCC). Tissues from 26 HCCs of various histological grades and architecture and from surrounding non-tumour liver (23 cirrhotic or fibrotic, three non-fibrotic) were tested by in situ hybridization and immunohistochemistry. Immunohistochemical detection of alpha-smooth muscle actin (alpha-SMA) was performed on serial sections or in combination with hybridization. Large amounts of ON mRNA and protein were detected in the tumour capsule, in the fibrous bands, and along capillaries within HCCs. The signal was located in cells suggestive of myofibroblasts, as confirmed by positive staining for alpha-SMA. In HCC, ON protein was always detectable, with strong staining in high-grade tumours, whereas it was mostly undetectable in non-tumour tissues. A clear difference was also shown for ON transcripts, except in a few cases with chronic active hepatitis, where ON transcripts were also expressed at a high level. Overexpression of ON transcripts in HCC vs. non-tumour liver was confirmed by RNA blot in 20/22 patients tested. In conclusion, ON is strongly expressed by the stromal myofibroblasts of human HCC, especially of high grade. This expression could play a role in tumour progression.

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.

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.

Elements within the first 17 amino acids of human osteonectin are responsible for binding to type V collagen

The region in human osteonectin (ON) responsible for binding to type V collagen has been identified as the first 17 NH2-terminal residues. This conclusion is based upon binding studies with deletion mutants of ON produced in Escherichia coli, in which parts of the first 17 amino acids have been removed. Wild-type ON from E. coli and mammalian cell-derived nonglycosylated ON bind identically to type V collagen and at least twice as effectively as mammalian cell-derived N-glycosylated ON. In previous studies, it was shown that N-glycosylation at residue 99 significantly reduces the capacity of ON to bind to type V collagen. Results reported in this communication demonstrate that the actual binding site on ON for type V collagen is distal from the site of N-glycosylation in terms of amino acid sequence but may be proximal in the folded, fully glycosylated, three-dimensional structure. Consistent with this conclusion is the ability of a synthetic peptide consisting of amino acids 1-17 to specifically inhibit the binding of ON to type V collagen.

TGF-beta 1 induces the expression of type I collagen and SPARC, and enhances contraction of collagen gels, by fibroblasts from young and aged donors

Fibroblasts have a major role in the synthesis and reorganization of extracellular matrix that occur during wound repair. An impaired biosynthetic or functional response of these cells to stimulation by growth factors might contribute to the delayed wound healing noted in aging. We, therefore, compared the responses of dermal fibroblasts from young and elderly individuals (26, 29, 65, 89, 90, and 92 years of age) to transforming growth factor-beta 1 (TGF-beta 1) with respect to: (1) the synthesis of type I collagen and SPARC (two extracellular matrix proteins that are highly expressed by dermal fibroblasts during the remodeling phase of wound repair) and (2) the contraction of collagen gels, and in vitro assay of wound contraction. With the exception of one young donor, all cultures exposed for 44 hours to 10 ng/ml TGF-beta 1 exhibited a 1.6- to 5.5-fold increase in the levels of secreted type I collagen and SPARC, relative to untreated cultures, and exhibited a 2.0- to 6.2-fold increase in the amounts of the corresponding mRNAs. Moreover, the dose-response to TGF-beta 1 (0.1-10 ng/ml), as determined by synthesis of type I collagen and SPARC mRNA, was as vigorous in cells from aged donors as in cells from a young donor. In assays of collagen gel contraction, fibroblasts from all donors were stimulated to a similar degree by 10 ng/ml TGF-beta 1. In conclusion, cells from both young and aged donors exhibited similar biosynthetic and contractile properties with exposure to TGF-beta 1. It therefore appears that the impaired wound healing noted in the aged does not result from a failure of their dermal fibroblasts to respond to this cytokine.

Structure and molecular regulation of bone matrix proteins

The organic matrix of bone contains several protein families, including collagens, proteoglycans, and glycoproteins, all of which may be extensively modified by posttranslational events, such as phosphorylation and sulfation. Many of the glycoproteins contain Arg-Gly-Asp (RGD), the integrin-binding sequence, within their structure, whereas other constituent proteins contain gamma-carboxyglutamic acid. The deposition of bone matrix by cells in the osteoblastic lineage is regulated by extrinsic factors, such as systemic and local growth factors and physical forces, and factors that are intrinsic to the cell, such as position in the cell cycle, maturational stage, and developmental age of the donor. Recent studies of several bone matrix gene promoters have identified cis- and trans-acting elements that are responsible for gene activity, although the precise sequence of regulatory events is not known. Development of in vitro assays, coupled with studies of the appearance of these proteins during development in vivo, provides insight into the functions of these proteins during the various stages of bone metabolism. Potential roles for these proteins include proliferation and maturation of stem cells, formation of matrix scaffolding elaborated by bone-forming cells, modeling, and remodeling. Changes in the functional properties of the extracellular matrix may be involved in a variety of disease processes, including osteoporosis and oral bone loss.

The Caenorhabditis elegans homologue of the extracellular calcium binding protein SPARC/osteonectin affects nematode body morphology and mobility

The extracellular matrix-associated protein, SPARC (osteonectin [Secreted Protein Acidic and Rich in Cysteine]), modulates cell adhesion and induces a change in cell morphology. SPARC expression in mammals is developmentally regulated and is highest at sites of extracellular matrix assembly and remodeling such as parietal endoderm and bone. We have isolated cDNA and genomic DNA clones encoding the Caenorhabditis elegans homologue of SPARC. The gene organization is highly conserved, and the proteins encoded by mouse, human, and nematode genes are about 38% identical. SPARC consists of four domains (I-IV) based on predicted secondary structure. Using bacterial fusion proteins containing nematode domain I or the domain IV EF-hand motif, we show that, like the mammalian proteins, both domains bind calcium. In transgenic nematodes expressing a SPARC-lacZ fusion gene, beta-galactosidase staining accumulated in a striated pattern in the more heavily stained muscle cells along the body. Comparison of the pattern of transgene expression to unc-54-lacZ animals demonstrated that SPARC is expressed by body wall and sex muscle cells. Appropriate levels of SPARC are essential for normal C. elegans development and muscle function. Transgenic nematodes overexpressing the wild-type SPARC gene were abnormal. Embryos were deformed, and adult hermaphrodites had vulval protrusions and an uncoordinated (Unc) phenotype with reduced mobility and paralysis.

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

SPARC (secreted protein, acidic and rich in cysteine), also known as osteonectin, is an extracellular Ca+2-binding glycoprotein that inhibits the incorporation of [3H]-thymidine and delays the onset of S-phase in synchronized cultures of bovine aortic endothelial (BAE) cells. This effect appears not to be dependent on the functional properties of SPARC associated with changes in cell shape or inhibition of cell spreading. In this study we investigate the conditions under which cell cycle modulation occurs in different types of cells. Human umbilical vein endothelial cells, a transformed fetal BAE cell line, and bovine capillary endothelial cells exhibited a sensitivity to SPARC and a cationic peptide from a non-Ca+2-binding region of SPARC (peptide 2.1, 0.2-0.8 mM) similar to that observed in BAE cells. In contrast, human foreskin fibroblasts and fetal bovine ligament fibroblasts exhibited an increase in the incorporation of [3H]-thymidine in the presence of 25 microM-0.2 mM peptide 2.1; inhibition was observed at concentrations in excess of 0.4 mM. This biphasic modulation could be further localized to a sequence of 10 amino acids comprising the N-terminal half of peptide 2.1. A synthetic peptide from another cationic region of SPARC (peptide 2.3) increased [3H]-thymidine incorporation by BAE cells and fibroblasts in a dose-dependent manner. In endothelial cells, a stimulation of 50% was observed at a concentration of 0.01 mM; fibroblasts required approximately 100-fold more peptide 2.3 for levels of stimulation comparable to those obtained in endothelial cells. The observation that SPARC and unique SPARC peptides can differentially influence the growth of fibroblasts and endothelial cells in a concentration-dependent manner suggests that SPARC might regulate proliferation of specific cells during wound repair and remodeling.

SPARC antagonizes the effect of basic fibroblast growth factor on the migration of bovine aortic endothelial cells

Migration of endothelial cells is requisite to wound repair and angiogenesis. Since the glycoprotein SPARC (secreted protein, acidic and rich in cysteine) is associated with remodeling, cellular migration, and angiogenesis in vitro, we questioned whether SPARC might influence the motility of endothelial cells. In this study we show that, in the absence of serum, exogenous SPARC inhibits the migration of bovine aortic endothelial cells induced by bFGF. Similar results were obtained from two different assays, in which cell migration was measured in a Boyden chamber and in monolayer culture after an experimental wound. Without bFGF, the migration of endothelial cells was unaffected by SPARC. The inhibitory effect of SPARC on cell motility was dose-dependent, required the presence of Ca2+, was mimicked by synthetic peptides from the N- and C-terminal Ca(2+)-binding domains of the protein, and was not seen in the presence of serum. Modulation of the activities of secreted and cell-associated proteases, including plasminogen activators and metalloproteinases, appeared not to be responsible for the effects that we observed on the motility of endothelial cells. Moreover, a molecular interaction between SPARC and bFGF was not detected, and SPARC did not interfere with the binding of bFGF to high-affinity receptors on endothelial cells. Finally, in culture medium that contained serum, SPARC inhibited the incorporation of [3H]-thymidine into newly synthesized DNA, both in the absence and presence of bFGF. However, DNA synthesis was not affected by SPARC when the cells were plated on gelatin or fibronectin in serum-free medium. We propose that the combined action of a serum factor and SPARC regulates both endothelial cell proliferation and migration and coordinates these events during morphogenetic processes such as wound repair and angiogenesis.