Thrombospondin modulates focal adhesions in endothelial cells

Peptides derived from two separate domains of the matrix protein thrombospondin-1 have anti-angiogenic activity

Thrombospondin-1 (TSP1) is a large modular matrix protein containing three identical disulfide-linked 180-kD chains that inhibits neovascularization in vivo (Good et al., 1990). To determine which of the structural motifs present in the 180-kD TSP1 polypeptide mediate the anti-angiogenic activity, a series of protease-generated fragments were tested using several in vitro and in vivo assays that reflect angiogenic activity. The majority of the anti-angiogenic activity of TSP1 resides in the central 70-kD stalk region which alone could block neovascularization induced by bFGF in the rat cornea in vivo and inhibit both migration in a modified Boyden chamber and [3H]thymidine incorporation stimulated by bFGF in cultured capillary endothelial cells. Although TSP1 has been shown to bind active TGF beta 1, this cytokine could not account for the inhibitory effects of the stalk region of TSP1 on cultured endothelial cells. Peptides and truncated molecules were used to further localize inhibitory activity to two domains of the central stalk, the procollagen homology region and the properdin-like type 1 repeats. Trimeric recombinant TSP1 containing NH2-terminal sequences truncated after the procollagen-like module inhibited endothelial cell migration in vitro and corneal neovascularization in vivo whereas trimeric molecules truncated before this domain were inactive as was the NH2-terminal heparin-binding domain that is present in both recombinant molecules. A series of peptides from the procollagen-like region, the smallest of which consisted of residues 303-309 of TSP1, inhibited angiogenesis in vivo in the rat cornea and the migration of endothelial cells in vitro. A 19-residue peptide containing these sequences blocked vessel formation in the granulation tissue invading a polyvinyl sponge implanted into the mouse. Nineteen residue peptides derived from two of the three type 1 repeats present in the intact TSP1 molecule blocked neovascularization in vivo in the rat cornea and inhibited the migration of cultured endothelial cells with ED50's of 0.6-7 microM. One of these peptides, containing residues 481-499 of TSP1, also inhibited vessel formation in granulation tissue invading sponges in vivo. These results suggest that the large TSP1 molecule employs at least two different structural domains and perhaps two different mechanisms to accomplish a single physiological function, the inhibition of neovascularization. The definition of short peptides from each of these domains that are able to block the angiogenic process may be of use in designing targeted inhibitors of the pathological neovascularization that underlies many diseases.

Thrombospondin-platelet interactions. Role of divalent cations, wall shear rate, and platelet membrane glycoproteins

The role of thrombospondin, a multifunctional matrix glycoprotein, in platelet adhesion is controversial: both adhesive and antiadhesive properties have been attributed to this molecule. Because shear flow has a significant influence on platelet adhesion, we have assessed thrombospondin-platelet interactions both under static and flow conditions. The capacity of thrombospondin to support platelet adhesion depended upon its conformation. In a Ca(2+)-depleted conformation, such as in citrated plasma, thrombospondin was nonadhesive or antiadhesive as it inhibited platelet adhesion to fibrinogen, fibronectin, laminin, and von Willebrand factor by 30-70%. In a Ca(2+)-replete conformation, however, thrombospondin effectively supported platelet adhesion. Shear rate influenced this adhesion; percent surface coverage on thrombospondin increased from 5.4 +/- 0.3 at 0 s-1 to 41.5 +/- 6.7 at 1,600 s-1. In contrast to the extensive platelet spreading observed on fibronectin at all shear rates, platelet spreading on thrombospondin occurred only sporadically and at high shear rates. GPIa-IIa, GPIIb-IIIa, GPIV, and the vitronectin receptor, which are all proposed platelet receptors for thrombospondin, were not solely responsible for platelet adhesion to thrombospondin. These results suggest that thrombospondin may play a dual role in adhesive processes in vivo: (a) it may function in conjunction with other adhesive proteins to maintain optimal platelet adhesion at various shear rates; and (b) it may serve as a modulator of cellular adhesive functions under specific microenvironmental conditions.

SPARC, a secreted protein associated with morphogenesis and tissue remodeling, induces expression of metalloproteinases in fibroblasts through a novel extracellular matrix-dependent pathway

SPARC (osteonectin/BM40) is a secreted protein that modifies the interaction of cells with extracellular matrix (ECM). When we added SPARC to cultured rabbit synovial fibroblasts and analyzed the secreted proteins, we observed an increase in the expression of three metalloproteinases--collagenase, stromelysin, and the 92-kD gelatinase--that together can degrade both interstitial and basement membrane matrices. We further characterized the regulation of one of these metalloproteinases, collagenase, and showed that both collagenase mRNA and protein are upregulated in fibroblasts treated with SPARC. Experiments with synthetic SPARC peptides indicated that a region in the neutral alpha-helical domain III of the SPARC molecule, which previously had no described function, was involved in the regulation of collagenase expression by SPARC. A sequence in the carboxyl-terminal Ca(2+)-binding domain IV exhibited similar activity, but to a lesser extent. SPARC induced collagenase expression in cells plated on collagen types I, II, III, and V, and vitronectin, but not on collagen type IV. SPARC also increased collagenase expression in fibroblasts plated on ECM produced by smooth muscle cells, but not in fibroblasts plated on a basement membrane-like ECM from Engelbreth-Holm-Swarm sarcoma. Collagenase was induced within 4 h in cells treated with phorbol diesters or plated on fibronectin fragments, but was induced after 8 h in cells treated with SPARC. A number of proteins were transiently secreted by SPARC-treated cells within 6 h of treatment. Conditioned medium that was harvested from cultures 7 h after the addition of SPARC, and depleted of residual SPARC, induced collagenase expression in untreated fibroblasts; thus, part of the regulation of collagenase expression by SPARC appears to be indirect and proceeds through a secreted intermediate. Because the interactions of cells with ECM play an important role in regulation of cell behavior and tissue morphogenesis, these results suggest that molecules like SPARC are important in modulating tissue remodeling and cell-ECM interactions.

Endothelial cell adhesion in real time. Measurements in vitro by tandem scanning confocal image analysis

Real time measurements of cell-substratum adhesion in endothelial cells were obtained by tandem scanning confocal microscopy of sites of focal contact (focal adhesions) at the abluminal cell surface. Focal contact sites were sharply defined (low radiance levels) in the living cell such that the images could be enhanced, digitized, and isolated from other cellular detail. Sites of focal contact are the principal determinant of cell-substratum adhesion. Measurements of (a) the focal contact area and (b) the closeness of contact (inverse radiance) were used to nominally define the adhesion of a single cell or field of cells, and to record spontaneous and induced changes of cell adhesion in real time. The topography of focal contacts was estimated by calculating separation distances from radiance values using a calibration technique based on interference ring optics. While slightly closer contact was noted between the cell membrane and substratum at or near the center of each focal contact, separation distances throughout the adhesion regions were always < 50 nm. Subtraction of consecutive images revealed continuous spontaneous remodeling of individual focal adhesions in unperturbed cells during periods of < 1 min. Despite extensive remodeling of focal contact sites, however, cell adhesion calculated for an entire cell over extended periods varied by < 10%. When cytoskeletal stability was impaired by exposure to cytochalasin or when cells were exposed to proteolytic enzyme, endothelial adhesion declined rapidly. Such changes were recorded at the level of single cells, groups of cells, and at single focal adhesions. In both unperturbed and manipulated cells, the dynamics of remodeling and cell adhesion characteristics varied greatly between individual sites within the same cell; disappearance of existing sites and appearance of new ones often occurred within minutes while adjacent sites underwent minimal remodelling. Tandem scanning confocal microscopy image analysis of living cells in real time provides repetitive spatial, temporal, and quantitative information about cell adhesion. Such an approach should allow more precise quantitative analyses to be made of the interactions between extracellular matrix, adhesion proteins, integrins, and the cytoskeleton in the living cell.

The evolution of the thrombospondin gene family

Thrombospondin-1 is an adhesive glycoprotein that is involved in cellular attachment, spreading, migration, and proliferation. To date, four genes have been identified that encode for the members of the thrombospondin gene family. These four genes are homologous to each other in the EGF-like (type 2) repeats, the calcium-binding (type 3) motifs, and the COOH-terminal. The latter has been reported to be a cell-binding domain in thrombospondin-1. Phylogenetic trees have been constructed from the multisequence alignment of thrombospondin sequences from human, mouse, chicken, and frog. Two different algorithms generate comparable results in terms of the topology and the branch lengths. The analysis indicates that an early form of the thrombospondin gene duplicated about 925 million years ago. The gene duplication that produced the thrombospondin-1 and -2 branches of the family is predicted to have occurred 583 million years ago, whereas the gene duplication that produced the thrombospondin-3 and -4 branches of the family is predicted to have occurred 644 million years ago. These results indicate that the members of the thrombospondin gene family have existed throughout the evolution of the animal kingdom and thus probably participate in functions that are common to most of its members.

Differential expression of thrombospondin 1, 2, and 3 during murine development

Thrombospondin 1 is a secreted, trimeric glycoprotein that mediates interactions between cells and extracellular matrix and exhibits cell-specific effects on migration and proliferation. Recently, two additional thrombospondin genes (thrombospondin 2 and 3) have been identified. To study the functions of these proteins, we have used in situ hybridization and RNAse protection assays to compare the expression of the genes encoding thrombospondin 1, 2, and 3 during murine embryogenesis. Thrombospondin mRNAs were associated with ossification, neuronal organogenesis, and lung development, although transcripts were differentially expressed. Thrombospondin 1 was predominant from days 10 to 13. During this period, high but transient levels of expression were observed in the neural tube, head mesenchyme, and cardiac cushions. In contrast, a more constant level of thrombospondin 1 mRNA was apparent in resident megakaryocytes of the liver, as well as in circulating megakaryocytes; neither thrombospondin 2 nor 3 was detected in these cells. Thrombospondin 1 was also produced by cells of the developing kidney and gut. The expression of thrombospondin 2 was confined principally to organized connective tissue that included pericardium, pleura, perichondrium, periosteum, meninges, ligaments, and reticular dermis. Thrombospondin 2 was also produced by differentiating skeletal myoblasts and by cells of the kidney and gut. Moreover, high levels of expression were detected in blood vessels. Thrombospondin 3 mRNA was restricted to brain, cartilage, and lung. Although thrombospondin 1, 2, and 3 belong to a family of structurally related genes, the differences observed in the spatiotemporal distribution of the corresponding mRNAs indicate unique functions for these secreted proteins.

Inhibition of fibronectin binding and fibronectin-mediated cell adhesion to collagen by a peptide from the second type I repeat of thrombospondin

The platelet and extracellular matrix glycoprotein thrombospondin interacts with various types of cells as both a positive and negative modulator of cell adhesion, motility, and proliferation. These effects may be mediated by binding of thrombospondin to cell surface receptors or indirectly by binding to other extracellular matrix components. The role of peptide sequences from the type I repeats of thrombospondin in its interaction with fibronectin were investigated. Fibronectin bound specifically to the peptide Gly-Gly-Trp-Ser-His-Trp from the second type I repeat of thrombospondin but not to the corresponding peptides from the first or third repeats or flanking sequences from the second repeat. The two Trp residues and the His residue were essential for binding, and the two Gly residues enhanced the affinity of binding. Binding of the peptide and intact thrombospondin to fibronectin were inhibited by the gelatin-binding domain of fibronectin. The peptide specifically inhibited binding of fibronectin to gelatin or type I collagen and inhibited fibronectin-mediated adhesion of breast carcinoma and melanoma cells to gelatin or type I collagen substrates but not direct adhesion of the cells to fibronectin, which was inhibited by the peptide Gly-Arg-Gly-Asp-Ser. Thus, the fibronectin-binding thrombospondin peptide Gly-Gly-Trp-Ser-His-Trp is a selective inhibitor of fibronectin-mediated interactions of cells with collagen in the extracellular matrix.

A re-examination of the molecular basis of cell movement

A model for cell movement is presented. It is suggested that cells do not migrate on collagen using their VLA (very late antigen) integrins that bind this extracellular matrix protein. Rather, the cells utilize alpha v integrins to bind endogenously produced fibronectin, which binds to the underlying collagen. It is envisaged that cells proceed by a process of engagement and disengagement of alpha v integrins to the extracellular matrix, somewhat analogous to the motion of a monkey climbing a tree. Secretion of isoforms of the adhesion modulator, thrombospondin, regulates disengagement of the integrin from its ligand in migrating cells. The integrin disengagement signal is mediated by thrombospondin cross-linking the alpha v integrin to an integrin accessory molecule and thus activating protein kinases. The cross-linked receptor complex undergoes recycling back along actin stress fibres, guided by the integrin beta-subunit. After endocytosis and protein sorting the alpha v integrin is transported back to the leading edge off migrating cells in vesicles guided by the tubulin-binding capabilities of an integrin accessory molecule. Direct attachment to collagen required for processes, such as matrix contraction, is mediated by VLA integrins which displace alpha v integrins from points of attachment during integrin recycling, possibly through an alpha v beta 1 intermediary receptor.

Diverse mechanisms for cell attachment to platelet thrombospondin

Thrombospondin-1 is a component of the extracellular matrix which is thought to play important roles in cell migration and proliferation, during embryogenesis and wound repair. To understand the basis for these activities, we are mapping the regions of the molecule with cell adhesive activity. Here, we use antagonists of specific cell binding sites, adhesion-perturbing thrombospondin monoclonal antibodies and proteolytic fragments of platelet thrombospondin, to investigate the adhesive mechanisms used by G361 melanoma cells, human intestinal smooth muscle cells (HISM), epidermal keratinocytes and MG-63 osteosarcoma cells. When attached to the same preparations of platelet thrombospondin, HISM and MG-63 cells underwent spreading, whereas G361 cells and keratinocytes did not. Attachment of all four cell types involved the carboxyterminal domain. The type 1 repeats and the amino-terminal heparin binding domain were important for stable attachment of G361, HISM and MG-63 cells, but were not involved in keratinocyte attachment. GRGDSP peptide caused near complete inhibition of HISM and MG-63 cell attachment, partially inhibited G361 attachment, but did not inhibit keratinocyte attachment. Attachment of HISM and MG-63 cells involved the alpha v beta 3 integrin. The integrity of the thrombospondin molecule was important for its adhesivity towards G361, HISM, and MG-63 cells, whereas keratinocytes attached to the 140 kDa tryptic fragment as effectively as they did to the intact molecule. These results show that cell attachment to platelet thrombospondin typically involves multiple binding interactions, but the exact profile of interactions is cell type specific. Usage of particular cell-binding sites does not predict whether cells will undergo spreading or not. These data may, in part, explain some of the current controversies surrounding the mechanisms of cell attachment to thrombospondin.

Developmental anomalies of Xenopus embryos following microinjection of SPARC antibodies

The function of SPARC (Secreted Protein, Acidic, Rich in Cysteine) in early embryonic development was assayed by microinjecting affinity-purified antibodies directed against SPARC into the blastocoel cavity of Xenopus embryos. Microinjection of SPARC antibodies did not appear to interfere with development until late neurulation. By hatching, a broad spectrum of external developmental anomalies were observable, including bent embryonic axes, accentuated ventral masses, shortened embryonic axes, and lack of visible eye pigment. Histological sections of injected embryos demonstrated that lack of visible eye pigmentation was often associated with deformities in eye development. Bending and shortening of the embryonic axis was associated with highly disorganized myotome patterns and loss of segmental boundaries. The results indicate a requirement for SPARC in the early morphological development of several tissues in Xenopus.

The in situ localization of tenascin splice variants and thrombospondin 2 mRNA in the avian embryo

Tenascin and thrombospondin belong to the growing family of extracellular matrix glycoproteins believed to have an anti-adhesive function during development. Immunohistochemistry has been used to identify these proteins in the developing central nervous system, in the matrix surrounding peripheral neurons, and in connective tissue. The antibodies used in most of these studies, however, could not distinguish between different splice variants (tenascin) nor different genetic forms (thrombospondin). For this reason, we used the reverse transcriptase polymerase chain reaction to generate DNA probes that are specific to the transcripts of high M(r) tenascin and thrombospondin 2. These probes were then used for an in situ hybridization study to determine the cellular origins of specific tenascin and thrombospondin forms throughout the development of the chick. The mRNA encoding high M(r) tenascin was found associated with motile cells and in tissues undergoing dynamic modeling: migrating glia, epithelial glia used as a substratum for migrating neurons, the growing tips of lung buds, and during osteogenesis. In contrast, the mRNAs of low M(r) tenascin were concentrated in areas of cartilage deposition and chondrocyte proliferation. Thrombospondin 2 mRNA was not detected in the developing central nervous system at any time during development by in situ hybridization. In contrast, it was found in embryonic mesenchyme, perichondrium, epimysium, and endothelial cells. Thrombospondin 2 mRNA was detected in poly(A) RNA isolated from embryonic spinal cord and cerebellum by polymerase chain reaction, though it was not detected in poly(A) RNA from the avascular retina. Thus, thrombospondin 2 mRNA may be present in the developing brain at low levels in endothelial cells or blood cells. These data support the notion that tenascin splice variants have distinct roles during development, and that thrombospondin 2 is more likely to be playing a role associated with the morphogenesis of connective tissue than neuronal development.

Thrombospondin as a mediator of cancer cell adhesion in metastasis

Thrombospondin (TSP) is a 450 kDa adhesive glycoprotein. It is present in high concentrations in the platelet alpha-granule and can readily be secreted following platelet activation where local concentrations can be increased by 3-4 orders of magnitude. TSP is also synthesized by a variety of other cells and is incorporated into their extracellular matrix. TSP is a homotrimer with a number of functional domains, at least four of which might serve as receptor recognizing regions. The amino-terminal heparin binding domain interacts with heparin, other glycosaminoglycans and glycolipids and likely recognizes specific cell surface proteoglycans. The central disulfide cross-linked region, 210 kDa non-reduced and 70 kDa reduced, contains a peptide motif CSVTCG which is apparently responsible for binding to glycoprotein IV (CD36) with high affinity. Immediately adjacent to the calcium binding region of TSP, which undergoes considerable molecular relaxation in the absence of calcium, is an RGDA sequence. TSP has been demonstrated to bind to integrins of the alpha v beta 3 and alpha IIb beta 3 class. The carboxy-terminal region of TSP also contains at least one binding epitope for a cell receptor. There are 2 well characterized genes for TSP and truncated forms of TSP have been detected which have inhibitory effects on angiogenesis. Finally, TSP can interact with fibrinogen and fibronectin, perhaps on cellular surfaces, which might serve as secondary receptor-like mechanisms for TSP binding and subsequent mediation of cell adhesion.

Immunohistochemical identification of thrombospondin in normal human brain and in Alzheimer's disease

Thrombospondin is part of a family of adhesive glycoproteins and is involved in a number of physiologic processes such as angiogenesis and neurite outgrowth. Immunohistochemical localization of thrombospondin in normal human brains was investigated in the hippocampus and inferior temporal cortex. Two antibodies (one polyclonal and one monoclonal) against thrombospondin-labeled microvessels, glial cells, and a subpopulation of pyramidal neurons. The distribution of thrombospondin staining in patients with Alzheimer's disease was found to be comparable to control subjects. However, in patients with Alzheimer's disease a subset of pyramidal neurons that may be vulnerable in Alzheimer's disease exhibited decreased staining. This decrease in the intensity of labeling might constitute a marker for a neuronal population prone to early degeneration. In addition, thrombospondin staining was demonstrated in senile plaques in Alzheimer's disease. These results suggest that thrombospondin may be involved in the process of neuronal degeneration and senile plaque formation.

The role of proteoglycans in cell adhesion, migration and proliferation

Proteoglycans comprise a part of the extracellular matrix that participates in the molecular events that regulate cell adhesion, migration and proliferation. Their structural diversity and tissue distribution suggest a functional versatility not generally encountered for other extracellular matrix components. This versatility is mainly dictated by their molecular interactions and their ability to regulate the activity of key molecules involved in several biological events. This molecular cooperativity either promotes or inhibits cell adhesion, migration and proliferation. A growing number of studies indicate that proteoglycans can play a direct role in these cellular events by functioning either as receptors or as ligands for molecules that are required for these events to occur. Such studies support a role for proteoglycans as important effectors of cellular processes that constitute the basis of development and disease.

Regulation of cell substrate adhesion: effects of small galactosaminoglycan-containing proteoglycans

Cell adhesion is a process which is initiated by the attachment of cells to specific sites in adhesive matrix proteins via cell surface receptors of the integrin family. This is followed by a reorganization of cytoskeletal elements which results in cell spreading and the formation of focal adhesion plaques. We have examined the effects of a class of small galactosaminoglycan-containing proteoglycans on the various stages of cell adhesion to fibronectin-coated substrates. Our results indicate that dermatan sulfate proteoglycans (DSPGs) derived from cartilage, as well as other related small proteoglycans, inhibit the initial attachment of CHO cells and rat embryo fibroblasts to substrates composed of the 105-kD cell-binding fibronectin fragment, but do not affect cell attachment to intact fibronectin. Although this effect involves binding of DSPGs to the substrate via the protein core, the intact proteoglycan is necessary for the observed activity. Isolated core proteins are inactive. The structural composition of the galactosaminoglycan chain does not appear to be functionally significant since both chondroitin sulfate and various dermatan sulfate proteoglycans of this family inhibit cell attachment to the fibronectin fragment. Neither the percentage of cells spread nor the mean area of spread cells adhering to substrates of intact fibronectin was significantly affected by the DSPGs. However, significantly fewer cells formed focal adhesions in the presence of DSPGs as compared with untreated control cells. These results suggest that the binding of small galactosaminoglycan-containing proteoglycans to a fibronectin substrate may affect several stages in the cell adhesion process.

Disulfides modulate RGD-inhibitable cell adhesive activity of thrombospondin

Thrombospondin (TSP) contains the Arg-Gly-Asp (RGD) sequence that is thought to be important for cell adhesion mediated by several cell-surface integrin receptors. The RGD sequence is located in the type 3 repeat region of TSP that has multiple Ca2+ binding sites and is subject to a complex intramolecular thiol-disulfide isomerization. TSP that we isolated from thrombin-activated human platelets using buffers containing 0.1 mM Ca2+, in which Cys974 is the major labeled cysteine, did not have RGD-inhibitable adhesive activity. However, one of our preparations of TSP and TSP purified following alternative procedures using greater than or equal to 0.3 mM Ca2+ did have RGD-inhibitable adhesive activity. Reduction of TSP with DTT, either before or after adsorption to surfaces, enhanced its adhesive activity. Reduced TSP supported robust cell spreading when coated at concentrations as low as 1 micrograms/ml, whereas "adhesive" TSP not treated with DTT was active at coating concentration of greater than 20 micrograms/ml and supported only modest cell spreading. Lower DTT concentrations were required for enhancement of the adhesive activity of TSP if Ca2+ was chelated with EDTA. Cellular adhesion to DTT-treated TSP was inhibited by RGD-containing peptide and by mAb to a functional site of the alpha v beta 3 integrin. Cell blots of reduced proteolytic fragments of TSP localized the adhesive activity to the RGD-containing type 3 repeat region. These results suggest a novel mechanism for regulation of integrin-ligand interactions in which the ligand can isomerize between inactive and active forms.

Colocalization of thrombospondin and syndecan during murine development

Thrombospondin is an adhesive glycoprotein that is thought to play a role in tissue genesis and repair. We have used a monoclonal anti-thrombospondin antibody, designated 5G11, to localize thrombospondin in paraformaldehyde fixed, paraffin-embedded sections of developing mouse embryos. Thrombospondin expression is observed in uterine smooth muscle, endometrial glands, the decidua, and trophoblastic giant cells during the initial phase of post-implantation development in the embryo. Cardiac myocytes and neuroepithelial cells show positive staining for thrombospondin at day 8.5 of gestation, and this expression continues throughout the development of the myocardium and central nervous system. Strong staining for thrombospondin is seen in developing bone and in the liver. Thrombospondin is also observed in developing smooth muscle and skeletal muscle, as well as in a variety of epithelia, including the epidermis, small intestinal epithelium, lens epithelium, renal tubular epithelium, and the epithelium of the developing tooth. Comparison of thrombospondin staining with that of two known cell surface receptors for thrombospondin, syndecan and the vitronectin receptor, reveals remarkable colocalization of thrombospondin and syndecan in all tissues, but almost no coexpression with the vitronectin receptor. Coexpression of thrombospondin and syndecan may play an important role in cell-cell or cell-matrix interactions during development.

Thrombospondin promotes process outgrowth in neurons from the peripheral and central nervous systems

Thrombospondin (TSP) is a prominent constituent of the extracellular matrix of the developing nervous system. We have examined the effects of TSP on the morphological differentiation of neurons. In short-term cultures (less than or equal to 24 hr) of embryonic rat sympathetic neurons, TSP stimulated neurite outgrowth, causing significant increase in the number of processes and their length. Similar effects were observed in cultures of rat dorsal root ganglion, hippocampal, and cerebral cortical neurons. Moreover, in cultures of central neurons, TSP was more effective than laminin in enhancing process extension. Analysis of long-term (5-7 days) cultures of sympathetic neurons indicated that processes formed in the presence of TSP had the cytochemical characteristics of axons. Thus, TSP can influence neuronal development by selectively enhancing axonal growth. The neurite-promoting region of the molecule was identified using a panel of monoclonal antibodies targeted to different regions of the protein. Process outgrowth could be totally inhibited with antibody A4.1, which recognizes the stalk region of TSP. These data suggest that the neurite-promoting activity is localized to a single region of the TSP molecule.

Endothelial cell spreading on fibrin requires fibrinopeptide B cleavage and amino acid residues 15-42 of the beta chain

Adhesion and spreading of cultured human umbilical vein endothelial cells on fibrin surfaces of varying structure were characterized to understand better the interactions occurring between endothelium and fibrin at sites of vascular injury. Fibrin prepared with reptilase, which cleaves only fibrinopeptide A from fibrinogen, and fibrin prepared with thrombin, which cleaves both fibrinopeptide A and fibrinopeptide B, equally supported endothelial cell adhesion. In contrast, only fibrin made with thrombin mediated endothelial cell spreading, as assessed by fluorescence microscopy of cells stained with rhodamine phalloidin to identify actin stress fibers or by scanning electron microscopy. Fibrin prepared with reptilase failed to support cell spreading. To further investigate the role of the amino terminus of the fibrin beta chain after fibrinopeptide B cleavage in promoting cell spreading, protease III from Crotalus atrox venom was used to specifically cleave the amino-terminal 42 residues of the fibrinogen B beta chain. After clotting with thrombin, this fibrin derivative lacking B beta 1-42 failed to support significant cell spreading. Spreading on fibrin was unaffected by depletion of Weibel-Palade bodies from endothelial cells, indicating that the spreading was independent of stimulated von Willebrand factor release. We conclude that endothelial cell spreading on fibrin requires fibrinopeptide B cleavage and involves residues 15-42 of the fibrin beta chain.

Rat osteoclasts adhere to a wide range of RGD (Arg-Gly-Asp) peptide-containing proteins, including the bone sialoproteins and fibronectin, via a beta 3 integrin

The ligand binding ability of rat osteoclast adhesion receptors was investigated in an attachment assay using osteoclasts disaggregated from bone. Osteoclasts adhered well to the Arg-Gly-Asp (RGD)-containing proteins osteopontin (bone sialoprotein I) and BSP (bone sialoprotein II), vitronectin, fibrinogen, von Willebrand factor, and fibronectin. Osteoclasts also adhered, but less strongly, to type I collagen. No attachment of osteoclasts was observed to thrombospondin, tenascin, laminin, or a range of non-RGD-containing bone proteins and proteins from other sources. The attachment of osteoclasts to all ligands was abolished in the presence of GRGDSP peptide, indicating the involvement of the RGD cell binding sequence in ligand binding. Attachment of osteoclasts to all substrates, with the exception of type I collagen, was also strongly inhibited by the addition of monoclonal antibody F11 to the beta 3 integrin subunit, indicating that a beta 3 integrin, probably the vitronectin receptor, was involved. Attachment to type I collagen was blocked by EDTA chelation of divalent cations and was not significantly affected by anti-beta 3 or anti-beta 1 antibodies; when taken with the inhibition by RGD peptide, this suggests the involvement of various receptors, possibly including nonintegrin collagen receptors, in the binding of osteoclasts to this protein. These results define the wide range of ligands for extracellular matrix receptors in osteoclasts in vitro. It remains to be established which of these proteins are important in osteoclast adhesion and osteoclastic bone resorption in vivo.

Inhibition of cell adhesion by high molecular weight kininogen

An anti-cell adhesion globulin was purified from human plasma by heparin-affinity chromatography. The purified globulin inhibited spreading of osteosarcoma and melanoma cells on vitronectin, and of endothelial cells, platelets, and mononuclear blood cells on vitronectin or fibrinogen. It did not inhibit cell spreading on fibronectin. The protein had the strongest antiadhesive effect when preadsorbed onto the otherwise adhesive surfaces. Amino acid sequence analysis revealed that the globulin is cleaved (kinin-free) high molecular weight kininogen (HKa). Globulin fractions from normal plasma immunodepleted of high molecular weight kininogen (HK) or from an individual deficient of HK lacked adhesive activity. Uncleaved single- chain HK preadsorbed at neutral pH, HKa preadsorbed at pH greater than 8.0, and HKa degraded further to release its histidine-rich domain had little anti-adhesive activity. These results indicate that the cationic histidine-rich domain is critical for anti-adhesive activity and is somehow mobilized upon cleavage. Vitronectin was not displaced from the surface by HKa. Thus, cleavage of HK by kallikrein results in both release of bradykinin, a potent vasoactive and growth-promoting peptide, and formation of a potent anti-adhesive protein.

Transforming growth factor-beta complexes with thrombospondin

Thrombospondin (TSP) was demonstrated to inhibit the growth of bovine aortic endothelial cells, an activity that was not neutralized by antibodies to TSP or by other agents that block TSP-cell interactions but that partially was reversed by a neutralizing antibody to transforming growth factor-beta (TGF-beta). Similar to TGF-beta, TSP supported the growth of NRK-49F colonies in soft agar in a dose-dependent manner, which required epidermal growth factor and was neutralized by anti-TGF-beta antibody. Chromatography of a TSP preparation did not separate the TGF-beta-like NRK colony-forming activity from high molecular weight protein. However, when chromatography was performed at pH 11, this activity was dissociated from TSP. These results suggest that at least some growth modulating activities of TSP are due to TGF-beta associated with TSP by strong non-covalent forces. Most of the active TGF-beta released from platelets after degranulation was associated with TSP, as demonstrated by anti-TSP immunoaffinity and gel permeation chromatography. 125I-TGF-beta binds to purified TSP in an interaction that is specific in the sense that bound TGF-beta could be displaced by TGF-depleted TSP but not significantly by native TSP, heparin, decorin, alpha 2-macroglobulin, fibronectin, or albumin. Hence, TGF-beta can bind to TSP, and the complex forms under physiological conditions. Furthermore, TSP-associated TGF-beta is biologically active, and the binding of TGF-beta to TSP may protect TGF-beta from extracellular inactivators.

Adhesion to thrombospondin by human embryonic fibroblasts is mediated by multiple receptors and includes a role for glycoprotein 88 (CD36)

Fetal embryonic fibroblasts attach and spread on thrombospondin (TSP). Adhesion is tight and focal adhesion plaques and "spots" are formed. We have investigated the receptors responsible for this adhesion. Unstimulated cells express the vitronectin receptor on their surface and this beta 3 integrin molecule contributes to adhesion. Another putative receptor for TSP, termed glycoprotein (GP) 88, which exists as a cytoplasmic pool in unstimulated cells becomes surface expressed when these cells are plated on TSP and localizes to areas of cell adhesion. Western blot analysis of cell lysate confirms GP88 as a TSP binding protein. Studies with fucoidan indicate that the heparan sulfate proteoglycan, known to function as a receptor for TSP, appears to contribute substantially to the TSP attachment of these cells and may be the receptor most important in the initial phases of TSP interaction.

Focal adhesion integrity is downregulated by the alternatively spliced domain of human tenascin

Tenascin, together with thrombospondin and SPARC, form a family of matrix proteins that, when added to bovine aortic endothelial cells, caused a dose-dependent reduction in the number of focal adhesion-positive cells to approximately 50% of albumin-treated controls. For tenascin, a maximum response was obtained with 20-60 micrograms/ml of protein. The reduction in focal adhesions in tenascin-treated spread cells was observed 10 min after addition of the adhesion modulator, reached the maximum by 45 min, and persisted for at least 4 h in the continued presence of tenascin. This effect was fully reversible, was independent of de novo protein synthesis, and was neutralized by a polyclonal antibody to tenascin. Monoclonal antibodies to specific domains of tenascin (mAbs 81C6 and 127) were used to localize the active site to the alternatively spliced segment of tenascin. Furthermore, a recombinant protein corresponding to the alternatively spliced segment (fibronectin type III domains 6-12) was expressed in Escherichia coli and was active in causing loss of focal adhesions, whereas a recombinant form of a domain (domain 3) containing the RGD sequence had no activity. Chondroitin-6-sulfate effectively neutralized tenascin activity, whereas dermatan sulfate and chondroitin-4-sulfate were less active and heparan sulfate and heparin were essentially inactive. Studies suggest that galactosaminoglycans neutralize tenascin activity through interactions with cell surface molecules. Overall, our results demonstrate that tenascin, thrombospondin, and SPARC, acting as soluble ligands, are able to provoke the loss of focal adhesions in well-spread endothelial cells.

Relationship between neuronal migration and cell-substratum adhesion: laminin and merosin promote olfactory neuronal migration but are anti-adhesive

Regulation by the extracellular matrix (ECM) of migration, motility, and adhesion of olfactory neurons and their precursors was studied in vitro. Neuronal cells of the embryonic olfactory epithelium (OE), which undergo extensive migration in the central nervous system during normal development, were shown to be highly migratory in culture as well. Migration of OE neuronal cells was strongly dependent on substratum-bound ECM molecules, being specifically stimulated and guided by laminin (or the laminin-related molecule merosin) in preference to fibronectin, type I collagen, or type IV collagen. Motility of OE neuronal cells, examined by time-lapse video microscopy, was high on laminin-containing substrata, but negligible on fibronectin substrata. Quantitative assays of adhesion of OE neuronal cells to substrata treated with different ECM molecules demonstrated no correlation, either positive or negative, between the migratory preferences of cells and the strength of cell-substratum adhesion. Moreover, measurements of cell adhesion to substrata containing combinations of ECM proteins revealed that laminin and merosin are anti-adhesive for OE neuronal cells, i.e., cause these cells to adhere poorly to substrata that would otherwise be strongly adhesive. The evidence suggests that the anti-adhesive effect of laminin is not the result of interactions between laminin and other ECM molecules, but rather an effect of laminin on cells, which alters the way in which cells adhere. Consistent with this view, laminin was found to interfere strongly with the formation of focal contacts by OE neuronal cells.

Anti-adhesive molecules of the extracellular matrix

The prototype extracellular matrix glycoproteins had been identified on the basis of their activity in promoting cell adhesion and spreading. Recently, more and more evidence is accumulating that the reverse effect of extracellular matrix proteins, namely the inhibition of cell adhesion and spreading, may be equally important for proper cell function during morphogenesis and development. Several anti-adhesive proteins have been described and their mechanisms of action are being investigated.

Platelet-derived interleukin 1 induces human endothelial adhesion molecule expression and cytokine production

Interleukin 1 (IL-1) plays a central role in the regulation of the body's response to infectious and inflammatory stimuli. Recent evidence has shown that human platelets express a cell associated form of this proinflammatory cytokine very rapidly following activation. Since one of the earliest events in inflammation is frequently the rapid adhesion of platelets to injured endothelium, it was of interest to determine whether platelets express IL-1 in a functionally relevant form that can alter the phenotype of human endothelial cells in vitro. Thrombin activated platelets induced significant expression of the adhesion molecule intercellular adhesion molecule 1, as well as secretion of the IL-1 inducible cytokines IL-6 and granulocyte macrophage colony stimulating factor by cultured human umbilical cord and saphenous vein endothelial cells. This was inhibited by prior treatment of the platelets with antibody specific for IL-1. These results suggest that platelet delivered IL-1 might initiate and regulate some of the earliest phases of the inflammatory response. An additional observation of interest was differential induction of endothelial leucocyte adhesion molecule 1 by activated platelets on saphenous vein but not umbilical vein but not umbilical vein endothelial cells, which suggests functional heterogeneity of the endothelial cells.

A second thrombospondin gene in the mouse is similar in organization to thrombospondin 1 but does not respond to serum

A second, expressed thrombospondin (TSP) gene, Thbs2, has been identified in the mouse. The exon/intron organization of Thbs2 is highly conserved in comparison with Thbs1 in that exon size and the pattern of interruption of the reading frame by introns are preserved, but there is a marked divergence in coding sequence, primarily in the first 7 exons. On the other hand, the DNA and translated amino acid sequences of exons coding for the type I, II, and III repeats in the two TSPs are far better conserved. Thbs2 is located on chromosome 17, band A3, whereas Thbs1 was found on chromosome 2, band F. In marked contrast to Thbs1, the Thbs2 gene is not induced by serum in NIH 3T3 cells; promoter sequences in the two genes are also very different. It is therefore likely that the two TSPs perform related but distinct functions.

Osteonectin is an alpha-granule component involved with thrombospondin in platelet aggregation

We previously showed that thrombospondin, a major alpha-granule glycoprotein of human platelets, forms a specific complex with osteonectin, a phosphoglycoprotein originally described in bone that is also present in human platelets. The storage organelles and the function of osteonectin in platelets are still unknown. In this study, using electron microscopy in combination with immunogold staining, the major storage organelle for platelet-secreted proteins, the alpha-granules. Furthermore, osteonectin was qualitatively and quantitatively assessed by studying normal platelets and the platelets from a patient with gray platelet syndrome. Gray platelet syndrome is a rare congenital bleeding disorder characterized by a selective deficiency in morphologically recognizable platelet alpha-granules and in the alpha-granule secretory proteins. Binding of an iodinated antiosteonectin monoclonal antibody to gray platelet proteins transferred to nitrocellulose from SDS-polyacrylamide gels showed no band corresponding to osteonectin compared to control platelets. Using a polyclonal antiosteonectin antibody-based radioimmunoassay, gray platelets contained 0.2 +/- 0.03 ng osteonectin per 10(6) platelets, which is only 20% of the normal platelet content of osteonectin (0.93 +/- 0.16 ng per 10(6) platelets). Study of the localization of osteonectin to the surface of human platelets demonstrated that a radioiodinated antiosteonectin polyclonal antibody bound specifically to thrombin-stimulated platelets but not to resting platelets. Binding was concentration-dependent, saturable (1710 +/- 453 binding sites per platelet, Kd = 1 microM), and inhibited by an excess of cold antiosteonectin polyclonal antibody. No binding was observed on the surface of thrombin-stimulated gray platelets. To gain further insights into the role of osteonectin released from activated platelets, the effect of an antiosteonectin polyclonal antibody was tested on the aggregation of washed platelets. F(ab')2 fragments from the antiosteonectin polyclonal antibody inhibited in a dose-dependent manner the aggregation of collagen-stimulated, washed human platelets without affecting collagen-induced platelet serotonin release. To characterize the mechanism through which antiosteonectin F(ab')2 fragments inhibit platelet aggregation, the expression of endogenous thrombospondin (TSP) on the surface of thrombin-activated platelets was studied using 125I-labeled anti-TSP monoclonal antibody P10. The endogenous surface expression of TSP to thrombin-stimulated platelets was significantly inhibited in the presence of antiosteonectin F(ab')2 fragments (6286 +/- 2065 molecules of P10 per platelet) compared to 11,230 +/- 766 molecules of P10 per platelet in the presence of nonimmune F(ab')2 fragments.(ABSTRACT TRUNCATED AT 400 WORDS)

Thrombospondins

The thrombospondins are a family of proteins generated by alternative splicing and gene duplication, which contain binding sites for many soluble proteins and up to five cellular receptors. This family of modular proteins functions in regulation of cellular migration and proliferation as manifested in development, wound healing, angiogenesis and tumorigenesis.

Identification and partial characterization of angiogenesis bioactivity in the lower respiratory tract after acute lung injury

Survival after acute lung injury (ALI) depends on prompt alveolar repair, a process frequently subverted by the development of granulation tissue within the alveolar airspace. Immunohistochemical examination of the intraalveolar granulation tissue confirmed that capillaries as well as myofibroblasts were the principal cellular constituents. We therefore hypothesized that angiogenesis factors would be present on the air-lung interface after ALI. To evaluate this hypothesis, bronchoalveolar lavage fluid from patients with ALI (n = 25) and patient controls (n = 8) was examined for angiogenesis bioactivity by its ability of induce endothelial cell migration. While lavage fluid from controls had no bioactivity, lavage fluid from 72% of patients with ALI promoted endothelial cell migration. Heparin affinity, ion exchange, and gel filtration chromatography resolved the bioactivity into at least two moieties. One appeared identical to the well characterized endothelial cell growth factor, basic fibroblast growth factor. The other was a 150-kD non-heparin binding protein that mediated endothelial cell migration and attachment in vitro, and the growth of new vessels in vivo. These data are consistent with the hypothesis that the growth of capillaries into the alveolar airspace results from angiogenesis factors present on the alveolar surface of the lung after ALI.

Diverse forms of stress result in changes in cellular levels of osteonectin/SPARC without altering mRNA levels in osteoligament cells

The osteonectin/SPARC gene has been shown to possess motifs for a heat shock element and metal responsiveness. Also, the expression of the protein has been associated with culture stress in endothelial cells. In the present study, osteoligament (OL) cells derived from the patellar ligament were subjected to diverse forms of stress that included (a) exposure to sodium arsenite, (b) heat shock, (c) cadmium ion, and (d) the amino acid analog, AZC. Osteonectin/SPARC levels in OL cells were determined by Western blot analyses, and immunoprecipitation using antiosteonectin antibodies. Expression of osteonectin/SPARC mRNA was determined by Northern analysis using a 1.5 kb EcoRI restriction fragment of bovine osteonectin cDNA. These studies reveal that osteonectin/SPARC is produced following diverse forms of stress, however, the levels are lower than observed in unchallenged OL cells. In all instances, the mRNA levels were comparable to control cells. These studies indicate that expression of osteonectin/SPARC mRNA is tightly controlled in OL cells and that the protein may be regulated at the level of protein translation.

Thrombospondin exerts an antiangiogenic effect on cord formation by endothelial cells in vitro

The response of endothelial cells to angiogenic stimuli has been shown to be influenced by the extracellular microenvironment. We tested whether thrombospondin, an extracellular matrix protein, modulated the spontaneous formation of cords by endothelial cells in vitro. Despite continued proliferation, a decrease in secreted thrombospondin was detected in cord-containing, as compared with subconfluent, cultures of both aortic and microvascular endothelial cells. Consistent with this trend, mRNA levels of thrombospondin decreased by factors of 16 in aortic and 60 in microvascular cultures that contained endothelial cords. Since thrombospondin was immunolocalized to fibrillar arrays that appeared to be associated with endothelial cords, we added anti-thrombospondin IgG to cord-forming cultures to limit the availability of the protein during this process. In the presence of anti-thrombospondin antibodies, there was a 33-50% increase in cord formation. These results suggest that thrombospondin is an inhibitor of angiogenesis in vitro and are consistent with its proposed roles as a destabilizer of endothelial cell focal contacts and as an inhibitor of endothelial cell proliferation.

Thrombospondin-induced adhesion of human platelets

Washed human unactivated platelets attached and spread on thrombospondin (TSP)-coated microtiter plates. Platelet adhesion was promoted by divalent cations Mn2+, Mg2+, and Ca2+ as compared to buffer having all divalent cations complexed with EDTA. TSP-dependent adhesion was inhibited by anti-TSP fab fragments, an anti-TSP monoclonal antibody, an RGD-containing peptide, complex-specific anti-glycoprotein (GP)IIb-IIIa monoclonal antibodies (A2A9 or AP-2) and anti-VLA-2 monoclonal antibodies (6F1 and Gi9), but not by rabbit preimmune fab fragments, mouse IgG, an anti-GPIIIa monoclonal antibody, or monoclonal antibodies against either the human vitronectin receptor, glycocalicin, or GPIV. At saturating concentrations, anti-GPIIb-IIIa inhibited adhesion by 40-60%. Glanzman's thrombasthenic platelets, which lack GPIIb-IIIa, adhered to TSP to the same extent as anti-GPIIb-IIIa-treated normal platelets or 40-60% as well as untreated normal platelets. Antibody 6F1 (5-10 micrograms/ml) inhibited platelet adhesion of both normal and thrombasthenic platelets by 84-100%. Both VLA-2 antibodies also inhibited collagen-induced platelet adhesion, but had no effect on fibronectin-induced adhesion of normal platelets. These data indicate that platelets specifically adhere to TSP and that this adhesion is mediated through GPIIb-IIIa and/or VLA-2.

Models of human platelet thrombospondin in solution. A dynamic light-scattering study

The translational diffusion coefficient (D20,w) of human platelet thrombospondin was measured by dynamic light-scattering. D20,w, measured in 20 mM-Hepes buffer, pH 7.4, containing 350 mM-NaCl and 2 mM-CaCl2, was 1.73(+/- 0.02) x 10(-7) cm2.s-1. After removal of bound Ca2+ by addition of EDTA, D20,w decreased to 1.56(+/- 0.04) x 10(-7) cm2.s-1; this was not a consequence of aggregation. D20,w showed little sensitivity to NaCl concentration between 130 and 550 mM. Through hydrodynamic analysis combining D20,w and other parameters taken from the literature, two major types of models for thrombospondin can be proposed: either classic compact models (i.e. low degree of hydration) such as prolate or oblate ellipsoids with a high axial ratio (greater than 20) or models of low axial ratio made of multiple subunits with significant cavities (i.e. high degree of hydration).

Actin-membrane interaction in focal adhesions

Focal adhesions are regions of the plasma membrane where cells in tissue culture adhere strongly to the underlying extracellular matrix, and which at their cytoplasmic face serve to anchor bundles of actin microfilaments. They provide an experimental model for studying the links between the cytoskeleton and the extracellular matrix. Members of the integrin family of extracellular matrix receptors are prominent components, spanning the membrane in focal adhesions, but there is evidence that other membrane components are also needed for these structures to form. A number of proteins are concentrated at the cytoplasmic face of focal adhesions. Recent efforts have sought to determine the links between actin and the integrin cytoplasmic domains. Using in vitro binding assays, two potential bridges between actin and integrin have been identified. One involves talin, which has recently been shown to bind actin directly. The other involves the actin-binding protein, alpha-actinin, which has been found to interact with several integrins. The physiological significance of these two potential bridges between actin and integrin remains to be determined in vivo.

Functional mapping of SPARC: peptides from two distinct Ca+(+)-binding sites modulate cell shape

Using synthetic peptides, we have identified two distinct regions of the glycoprotein SPARC (Secreted Protein Acidic and Rich in Cysteine) (osteonectin/BM-40) that inhibit cell spreading. One of these sites also contributes to the affinity of SPARC for extracellular matrix components. Peptides representing subregions of SPARC were synthesized and antipeptide antibodies were produced. Immunoglobulin fractions of sera recognizing an NH2-terminal peptide (designated 1.1) blocked SPARC-mediated anti-spreading activity. Furthermore, when peptides were added to newly plated endothelial cells or fibroblasts, peptide 1.1 and a peptide corresponding to the COOH terminal EF-hand domain (designated 4.2) inhibited cell spreading in a dose-dependent manner. These peptides exhibited anti-spreading activity at concentrations from 0.1 to 1 mM. The ability of peptides 1.1 and 4.2 to modulate cell shape was augmented by an inhibitor of protein synthesis and was blocked by specific antipeptide immunoglobulins. In addition to blocking cell spreading, peptide 4.2 competed for binding of [125I]SPARC and exhibited differential affinity for extracellular matrix molecules in solid-phase binding assays. The binding of peptide 4.2 to matrix components was Ca+(+)-dependent and displayed specificities similar to those of native SPARC. These studies demonstrate that both anti-spreading activity and affinity for collagens are functions of unique regions within the SPARC amino acid sequence. The finding that two separate regions of the SPARC protein contribute to its anti-spreading activity lead us to propose that multiple regions of the protein act in concert to regulate the interactions of cells with their extracellular matrix.

Role of the extracellular matrix protein thrombospondin in the early development of the mouse embryo

The distribution of the extracellular matrix protein thrombospondin (TSP) in cleavage to egg cylinder staged mouse embryos and its role in trophoblast outgrowth from cultured blastocysts were examined. TSP was present within the cytoplasm of unfertilized eggs; in fertilized one- to four-cell embryos; by the eight-cell stage, TSP was also densely deposited at cell-cell borders. In the blastocyst, although TSP was present in all three cell types; trophectoderm, endoderm, and inner cell mass (ICM), it was enriched in the ICM and at the surface of trophectoderm cells. Hatched blastocysts grown on matrix-coated coverslips formed extensive trophoblast outgrowths on TSP, grew slightly less avidly on laminin, or on a 140-kD fragment of TSP containing its COOH terminus and putative cell binding domains. There was little outgrowth on the NH2 terminus heparin-binding domain. Addition of anti-TSP antibodies (but not GRGDS) to blastocysts growing on TSP strikingly inhibited outgrowth. Consistent with its early appearance and presence in trophoblast cells during implantation, TSP may play an important role in the early events involved in mammalian embryogenesis.

Thrombospondin gene expression is associated with mitogenesis in 3T3 cells: induction by basic fibroblast growth factor

Growth factor-depleted Swiss 3T3 cells responded to basic fibroblast growth factor (bFGF) with a burst of mitogenesis and with a rapid and marked increase in thrombospondin (TS) mRNA levels. mRNA levels for the alpha 1 chain of type I collagen and for fibronectin were unaffected. At early times following stimulation (0-2 h), "superinduction" of TS mRNA by inhibition of protein synthesis with cycloheximide was not observed, and the increase in TS mRNA could be attributed primarily to an increase in transcription rate of the TS gene. However, at later times (4-8 h) the combination of cycloheximide and bFGF superinduced TS mRNA levels, suggesting the existence of a labile inhibitor of transcription or a short-lived RNase that might be produced in response to prolonged treatment with bFGF. In contrast to its stimulatory effect on 3T3 cells, bFGF did not stimulate the proliferation of mouse muscle BC3H1 cells nor did it cause an increase in TS mRNA levels, but BC3H1 cells do respond to bFGF by inhibition of myogenic differentiation. We propose, on the basis of these and other findings, that TS facilitates the progression of some anchorage-dependent cells through the cell cycle.