Culture shock. Selective uptake and rapid release of a novel serum protein by endothelial cells in vitro

The regulatory function of SPARC in vascular biology

SPARC is a matricellular protein, able to modulate cell/ECM interactions and influence cell responses to growth factors, and therefore is particularly attuned to contribute to physiological processes involving changes in ECM and cell mobilization. Indeed, the list of biological processes affected by SPARC includes wound healing, tumor progression, bone formation, fibrosis, and angiogenesis. The process of angiogenesis is complex and involves a number of cellular processes such as endothelial cell proliferation, migration, ECM degradation, and synthesis, as well as pericyte recruitment to stabilize nascent vessels. In this review, we will summarize current results that explore the function of SPARC in the regulation of angiogenic events with a particular emphasis on the modulation of growth factor activity by SPARC in the context of blood vessel formation. The primary function of SPARC in angiogenesis remains unclear, as SPARC activity in some circumstances promotes angiogenesis and in others is more consistent with an anti-angiogenic activity. Undoubtedly, the mercurial nature of SPARC belies a redundancy of functional proteins in angiogenesis as well as cell-type-specific activities that alter signal transduction events in response to unique cellular milieus. Nonetheless, the investigation of cellular mechanisms that define functional activities of SPARC continue to contribute novel and exciting paradigms to vascular biology.

Albumin-bound paclitaxel: a next-generation taxane

Taxanes are standard treatment for metastatic breast cancer; however, the solvents used as vehicles in these formulations cause severe toxicities. The FDA recently approved a solvent-free formulation of paclitaxel for the treatment of metastatic breast cancer that utilises 130-nanometer albumin-bound (nab) technology (Abraxane; nab-paclitaxel) to circumvent the requirement for solvents. nab-Paclitaxel utilises the natural properties of albumin to reversibly bind paclitaxel, transport it across the endothelial cell and concentrate it in areas of tumour. The proposed mechanism of drug delivery involves, in part, glycoprotein 60-mediated endothelial cell transcytosis of paclitaxel-bound albumin and accumulation in the area of tumour by albumin binding to SPARC (secreted protein, acidic and rich in cysteine). Clinical studies have shown that nab-paclitaxel is significantly more effective than paclitaxel formulated as Cremophor EL (CrEL, Taxol, CrEL-paclitaxel), with almost double the response rate, increased time to disease progression and increased survival in second-line patients. The absence of CrEL from the formulation is associated with decreased neutropenia and rapid improvement of peripheral neuropathy with nab-paclitaxel, compared with CrEL-paclitaxel. For these reasons, nab-paclitaxel can be administered using higher doses of paclitaxel than that achievable with CrEL-paclitaxel, with shorter infusion duration and without the requirement for corticosteroid and antihistamine premedication to reduce the risk of solvent-mediated hypersensitivity reactions. Taken together, these studies have demonstrated that nab technology has increased the therapeutic index of paclitaxel compared with the conventional, solvent-based formulation.

Acute L-glutamine deprivation compromises VEGF-a upregulation in A549/8 human carcinoma cells

Tumor ischemia participates in angiogenesis and cancer progression through cellular responses to hypoxia and nutrient deprivation. However, the contribution of amino acids limitation to this process remains poorly understood. Using serum-free cell culture conditions, we tested the impact of L-glutamine deprivation on metabolic and angiogenic responses in A549/8 carcinoma cells. In these cells, lowering glutamine concentration modified the cell cycle distribution and significantly induced apoptosis/necrosis. Although glutamine deprivation led to a HIF-independent increase in VEGF-A mRNA, the corresponding protein level remained low and correlated with the inhibition of protein synthesis and activation of the GCN2/eIF2alpha pathway. Limitation of glutamine availability also hampers hypoxia- and hypoglycemia-induced VEGF-A protein upregulation. Thus, glutamine deprivation may have no direct effect on VEGF-dependent angiogenesis, compared to hypoxia or to glucose deprivation, and may instead be detrimental to cancer progression by antagonizing ischemia-induced stresses.

Extracellular matrix metalloproteinase 2 levels are regulated by the low density lipoprotein-related scavenger receptor and thrombospondin 2

We have recently shown that the adhesive defect observed in dermal fibroblasts derived from thrombospondin 2 (TSP2)-null mice results from an increase in matrix metalloproteinase 2 (MMP2) levels (Yang, Z., Kyriakides, T. R., and Bornstein, P. (2000) Mol. Biol. Cell 11, 3353-3364). Adhesion was restored by replacement of TSP2 and by inhibitors of MMP2 activity. In pursuing the observation that TSP2 and MMP2 interact, we now demonstrate that this interaction is required for optimal clearance of extracellular MMP2 by fibroblasts. Since TSP2 is known to be endocytosed by the scavenger receptor, low density lipoprotein receptor-related protein (LRP), we determined whether interference with LRP function affected fibroblast adhesion and/or extracellular MMP2 levels. Addition of heparin, which competes for the binding of TSP2 to LRP coreceptor proteoglycans, inhibited adhesion of control but not TSP2-null cells, and a blocking antibody to LRP as well as the LRP inhibitor, receptor-associated protein, also inhibited adhesion and increased MMP2 levels only in control fibroblasts. TSP2 did not inhibit active MMP2 directly and did not inhibit the activation of pro-MMP2. Finally, the internalization of 125I-MMP2 was reduced in TSP2-null compared with control fibroblasts. We propose that clearance of MMP2-TSP2 complexes by LRP is an important mechanism for the regulation of extracellular MMP2 levels in fibroblasts, and perhaps in other cells. Thus, some features of the phenotype of TSP2-null mice, such as abnormal collagen fibrillogenesis, accelerated wound healing, and increased angiogenesis, could result in part from increased MMP2 activity.

Matricellular proteins as modulators of cell-matrix interactions: adhesive defect in thrombospondin 2-null fibroblasts is a consequence of increased levels of matrix metalloproteinase-2

Thrombospondin 2 (TSP2)-null mice, generated by disruption of the Thbs2 gene, display a variety of connective tissue abnormalities, including fragile skin and the presence of abnormally large collagen fibrils with irregular contours in skin and tendon. In this study we demonstrate that TSP2-null skin fibroblasts show a defect in attachment to a number of matrix proteins, and a reduction in cell spreading. To investigate the molecular mechanisms responsible for these abnormal cell-matrix interactions, we compared the levels of matrix metalloproteinases (MMPs) in wild-type and mutant fibroblasts. Isolation and analysis of gelatinases from conditioned media by gelatin-agarose affinity chromatography and gelatinolytic assays demonstrated that TSP2-null fibroblasts produce a 2-fold increase in gelatinase A (MMP2) compared with wild-type cells. The adhesive defect was corrected by treatment of TSP2-null fibroblasts with soluble TSP2, with the MMP inhibitors BB94 and tissue inhibitor of metalloproteinase-2, and with a neutralizing antibody to MMP2. Moreover, stable transfection of TSP2-null fibroblasts with mouse TSP2 cDNA corrected both the adhesive defect and the altered expression of MMP2. Finally, MMP2 was shown to interact with TSP2 in a direct-binding plate assay. We conclude that TSP2 plays an important role in cell-matrix interactions, and that a deficiency in the protein results in increased levels of MMP2 that contribute to the adhesive defect in TSP2-null fibroblasts and could play a role in the complex phenotype of TSP2-null mice.

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)

Secretion of ribonucleases by normal and immortalized cells grown in serum-free culture conditions

The requirement of serum in cell culture is a major limitation for studies on secreted ribonucleases (RNases) because serum contains a high amount of ribonucleolytic activity. Defined culture condition is thus of interest to improve our knowledge of the RNase biology. We report here that cells from three different types and origins, Chinese hamster lung fibroblasts, bovine smooth muscle cells, and human endothelium-derived EA.hy926 cells, proliferate consistently in the presence of a basal medium supplemented with bovine serum albumin, high-density lipoproteins, basic fibroblast growth factor, insulin, and transferrin. Using a new quantitative radio-RNase inhibitor assay, two distinct ribonucleolytic assays, and a radioimmunoassay against angiogenin, it is shown that RNases became apparent in media conditioned by cell monolayers. Both the hamster lung fibroblast and the EA.hy926 cell lines secreted larger amounts of RNase inhibitor-interacting factors and RNase activity than normal smooth muscle cells. The serum-free medium represents an alternative way to grow these cells and allows investigation of biosynthesis and functions of RNases in culture. It should be useful to identify and quantitate unambiguously specific members of the RNase family secreted by normal versus tumor cells in culture.

Culture-induced increase in acidic and basic fibroblast growth factor activities and their association with the nuclei of vascular endothelial and smooth muscle cells

The activity of acidic and basic fibroblast growth factor-like mitogens (aFGF, bFGF) extracted from cultured bovine aortic endothelial (BAEC) and rat aortic smooth muscle cells (SMC) was compared with that of freshly isolated cells from the same tissues. Extracts of subendothelial extracellular matrix (ECM) and cell lysates of cultured BAEC contained 4-fold more bFGF-like activity than the extracts of fresh cells. ECM and cell lysates of SMC yielded 10-fold more bFGF-like activity than the fresh cell lysates. We consistently find aFGF-like activity in both cell types. In the case of BAEC, cultured cells and ECM contained 3-fold more aFGF-like activity when compared with freshly isolated cells, whereas in cultured SMC, aFGF-like activity in cell and ECM extracts was 8-fold higher than in fresh cell extracts. The mitogens extracted from cell lysates and from the ECM are closely related to aFGF or bFGF by the criteria that they bind to heparin-sepharose and elute at 1.1 M (aFGF) or 1.5 M (bFGF) NaCl, have molecular weights of about 18,000, and react with anti-aFGF (1.1 M), or anti-bFGF (1.5 M) antibodies when analyzed by Western blots and by radioimmunoassay specific for aFGF and bFGF. This mitogenic activity is inhibited by neutralizing antibodies to aFGF and bFGF. In addition, the column fractions are potent mitogens for Balb/c 3T3 fibroblasts. Acidic and basic FGF-like mitogenic activity could also be extracted from the cell nuclei. The subcellular localization of both FGFs was visualized in both nuclei and cytoplasm with immunoperoxidase. Compared with primary SMC, secondary SMC had an increased capacity to bind 125IaFGF to high affinity receptors, while binding to freshly isolated BAEC and SMC was negligible. We conclude that FGFs are present at low levels in freshly isolated cells and that propagation in cell culture provides a stimulus for production of these mitogens.

Enzyme replacement in Fabry endothelial cells and fibroblasts: uptake experiments and electron microscopical studies

Endothelial cells are of particular interest for therapeutic strategies in Fabry's disease, because the accumulation of glycosphingolipids in the vascular endothelium as a result of alpha-galactosidase A (alpha-galA) deficiency is responsible for the major clinical manifestations of the disease. Electron microscopical observations of cultured endothelial cells obtained from the umbilical vein of a hemizygous Fabry fetus showed that the glycosphingolipids are deposited as lamellar material in the lysosomes, as has been found previously for cultured fibroblasts and many different tissues. Mannose 6-phosphate (man 6-P)-receptor mediated and Concanavalin A (ConA)-mediated uptake of purified alpha-galA was attempted in the endothelial cells as well as in cultured fibroblasts from the same fetus. Our results on high-uptake alpha-galA indicate that the endothelial cells do not internalize alpha-galA via the man 6-P receptor. Immunofluorescence studies after addition of the receptor antibody to the cells support the theory that they have no or very few man 6-P receptors on the surface. Morphological studies did not show lysosomal changes which could suggest that the enzyme is taken up into the endothelial cells; however, we found reproducible modifications of the lysosomes in Fabry fibroblasts after incubation with high-uptake alpha-galA. Cell-associated alpha-galA activity was found in both cell types, when the enzyme was added to cells preincubated with ConA; but the lectin treatment by itself induced considerable ultrastructural changes in the cytoplasm, which obscured a possible effect by the enzyme.

Molecular analysis of the cDNA for human SPARC/osteonectin/BM-40: sequence, expression, and localization of the gene to chromosome 5q31-q33

Human cDNA clones encoding the extracellular calcium-binding, acidic glycoprotein known as SPARC, osteonectin, or BM-40 were isolated from a placental cDNA library. Two polyadenylated transcripts of 2.2 and 3.0 kb were detected in human tissues and cultured cells by Northern blot analysis, and cDNAs for both transcripts were characterized. The 2133-bp sequence of the more abundant (major) transcript contains an open reading frame for 303 amino acids. The deduced polypeptide has extensive amino acid sequence identity with mouse SPARC. The larger and minor 3.0-kb cDNA has an identical coding region but utilizes a downstream polyadenylation signal. Gene localization studies have revealed a single chromosomal site at 5q31-q33 by somatic cell hybrid analysis and in situ chromosomal hybridization. Furthermore, pulsed-field gel electrophoresis of human genomic DNA cleaved with different rare-cutting restriction enzymes and hybridized with SPARC cDNA probes revealed single or double fragments of less than 50 to about 150 kb. The evidence is consistent with a single locus for SPARC in humans. The gene was found to be differentially expressed in many human tissues and in an osteogenic sarcoma, but not in other transformed cells.