Inhibition of endothelial cell movement by pericytes and smooth muscle cells: activation of a latent transforming growth factor-beta 1-like molecule by plasmin during co-culture

Vascular smooth muscle cell hypertrophy vs. hyperplasia. Autocrine transforming growth factor-beta 1 expression determines growth response to angiotensin II

Recent observations in our laboratory suggest that angiotensin II (Ang II) is a bifunctional vascular smooth muscle cell (VSMC) growth modulator capable of inducing hypertrophy or inhibiting mitogen-stimulated DNA synthesis. Because transforming growth factor-beta 1 (TGF beta 1) has similar bifunctional effects on VSMC growth, we hypothesized that autocrine production of TGF beta 1 may mediate the growth modulatory effects of Ang II. Indeed, this study demonstrates that Ang II induces a severalfold increase in TGF beta 1 mRNA levels within 4 h that is dependent on de novo protein synthesis and appears to be mediated by activation of protein kinase C (PKC). Ang II not only stimulates the synthesis of latent TGF beta 1, but also promotes its conversion to the biologically active form as measured by bioassay. The coincubation of VSMCs with Ang II and control IgG has no significant mitogenic effect. However, the co-administration of Ang II and the anti-TGF beta 1 antibody stimulates significantly DNA synthesis and cell proliferation. We conclude that: (a) Ang II induces increased TGF beta 1 gene expression via a PKC dependent pathway involving de novo protein synthesis; (b) Ang II promotes the conversion of latent TGF beta 1 to its biologically active form; (c) Ang II modulates VSMC growth by activating both proliferative and antiproliferative pathways; and (d) Autocrine active TGF beta 1 appears to be an important determinant of VSMC growth by hypertrophy or hyperplasia.

Cell density dependent effects of TGF-beta demonstrated by a plasminogen activator-based assay for TGF-beta

Transforming growth factor-beta 1 (TGF-beta 1) induces a decrease in plasminogen activator (PA) expression in confluent cultures of bovine aortic endothelial (BAE) cells. We describe an assay using the suppression of PA expression in confluent BAE cells by TGF-beta 1 which detects concentrations of the growth factor ranging from 5 to 200 pg/ml and has an ED50 of 15-20 pg/ml. The assay can be performed in 96-well plates and requires a minimum of 35 ul of solution per sample, thereby limiting the amount of reagents required and allowing many samples to be tested in a single assay. Here we demonstrate that the effect of TGF-beta 1 on PA expression in BAE cells depends on the length of time the cells are exposed to the growth factor and the density at which the cells are plated. In cells plated at a high density (3.5 x 10(5) cells/cm2), both 4 h and 24 h exposures to TGF-beta 1 suppress PA expression. However, with cells plated sparsely (3.5 x 10(4) cells/cm2), a 4 h exposure to TGF-beta 1 increases PA expression 2-fold, whereas a 24 h exposure results in an 85% inhibition of basal PA expression. The paradoxical stimulation of PA expression in cells at a sparse density upon 4 h exposure to TGF-beta 1 occurs in a dose-dependent manner with an ED50 of 15-20 pg/ml. This bifunctional response of PA production in cells exposed to TGF-beta 1 may have implications with regard to the role of TGF-beta 1 in angiogenesis.

Assembly of plasmin-generating proteins on the surface of human endothelial cells

Traditionally, plasmin generation has been conceptualized as a process oriented on the surface of a fibrin-containing thrombus. Recent work, however, indicated that plasminogen and its activators, tissue plasminogen activator (t-PA) and urokinase, can assemble on the surface of cultured human umbilical vein endothelial cells (HUVECs). On binding to HUVECs, plasminogen is activated by t-PA approximately 12-fold more efficiently than fluid-phase plasminogen, and is converted to a plasmin-modified form, possibly unique to cell surfaces. In addition, t-PA interacts with HUVECs at two sites. The major binding site preserves its activity and represents a true (relative molecular weight 40,000) membrane-associated exoreceptor. The low-density lipoprotein (LDL)-like lipoprotein, lipoprotein(a), is highly associated with atherosclerosis, bears striking sequence homology to plasminogen, and competes with plasminogen for cell surface binding. In summary, functional assembly of plasminogen and t-PA may represent an important thromboregulatory system.

Identification of mechanisms that may modulate the role of lipoprotein(a) in thrombosis and atherogenesis

In this report, we review recent findings concerning the identification of mechanisms that may modulate the role of lipoprotein(a), or Lp(a), in thrombosis and atherogenesis. Lp(a) binds to surface-immobilized plasmin-modified fibrin, thus providing a mechanism for incorporating Lp(a) into the vessel wall. We found that homocysteine and other sulfhydryl-containing amino acids markedly increase the binding of Lp(a) to plasmin-modified fibrin. Our results suggest that homocysteine alters the structure of Lp(a) to expose lysine-binding sites on the apolipoprotein(a) portion of the molecule, and thus provide a potential biochemical link between thrombosis and atherogenesis. We also found that transglutaminases catalyze the incorporation of primary amines into Lp(a). Studies in cell culture systems have found that Lp(a) stimulates endothelial cells to synthesize and release plasminogen activator inhibitor-1. Further, Lp(a) inhibits the activation of transforming growth factor-beta in a coculture of bovine endothelial and smooth muscle cells.

An in vitro model for cell-cell interactions

Heterotypic cell-cell interactions appear to be involved in the control of development and function in a wide variety of tissues. In the vasculature, endothelial cells and mural cells (smooth muscle cells or pericytes) make frequent contacts, suggesting a role for intercellular interactions in the regulation of vascular growth and function. We have previously grown endothelial cells and mural cells together in mixed cultures and found that heterocellular contact led to endothelial growth inhibition. However, this mixed culture system does not lend itself to the examination of the effects of contact on the phenotype of the individual cell types. We have therefore developed a co-culture system in which cells can be co-cultured across a porous membrane, permitting intercellular contact while maintaining pure cell populations. Co-culture of endothelial cells and smooth muscle cells across membranes with pore sizes of 0.02, 0.4, 0.6, and 0.8 microns maintained the two cell types as homogeneous populations, whereas smooth muscle cells migrated across the membrane through pores of 2.0 microns. Vascular cell co-culture across membranes with 0.8-microns pores resulted in the inhibition of endothelial cell proliferation and the generation of conditioned media which inhibited endothelial cell growth The arrangement of the cells in this co-culture system mimics the in vivo orientation of vascular cells in which mural cells are separated from the abluminal surface of the endothelium by a fenestrated internal elastic lamina or basement membrane. Because this co-culture system maintains separable populations of cells in contact or close proximity allowing for biochemical and molecular analyses of pure populations, it should prove useful for the study of cell-cell interactions in a variety of systems.

Modulation of activities and RNA level of the components of the plasminogen activation system during fusion of human myogenic satellite cells in vitro

Primary cultures of human myogenic stem cells (satellite cells) mimic myogenic differentiation. During this process, the expression of the components of the plasminogen activation system underwent modulation. Activities and mRNA levels of tissue-type and urokinase-type plasminogen activator were increased in a reproducible pattern during differentiation. A modulation of the mRNA level of PAI-2 was also observed. Human satellite cells expressed a urokinase receptor and also the mRNA level of this component underwent modulation. With the exception of PAI-1 mRNA, the level of all mRNAs increased from Day 4 to Day 8, i.e., just before myoblasts fusion, and then remained high at later stages. The modulation of the plasminogen activating activity indicates that this system is directly involved in the fusion process of myogenic differentiation.

Transforming growth factor-beta stimulates the expression of desmosomal proteins in bronchial epithelial cells

Transforming growth factor-beta 1 (TGF-beta 1) has been shown to induce squamous differentiation of cultured airway epithelial cells. It has also been shown to increase expression of matrix proteins and integrin receptors in cell culture of these and other cells. However, it is unknown if TGF-beta 1 affects expression of genes encoding intercellular junctional proteins. Therefore, we have investigated the effect of TGF-beta 1 on the expression of proteins and mRNAs for desmoplakins (DPs) I and II, desmosomal plaque proteins. Fibronectin, known to be induced by TGF-beta 1 was used as a positive control and tubulin as a negative control. Twenty-four hours after TGF-beta 1 stimulation, DP I and II mRNA levels assessed by Northern blotting analysis had increased significantly (DP I mRNA, 1.8-fold, P less than 0.05; DP II mRNA, 2.4-fold, P less than 0.04), thereby indicating pretranslational regulation of DP expression. By comparison, mRNA for fibronectin increased 8.1-fold whereas mRNA for tubulin was unchanged. Immunofluorescence using the monoclonal anti-DP I and II antibodies revealed dramatic increased expression of punctate DP structures after exposure to TGF-beta 1. Immunoblot analyses with polyclonal anti-DP I antibodies showed increased levels of both DP I (250 kD) and DP II (215 kD), with the DP I increase being more pronounced (DP I, 2.5-fold; DP II, 1.4-fold at 48 h relative to controls), suggesting translational regulation by TGF-beta 1. This study therefore demonstrates the ability of TGF-beta 1 to alter cellular phenotype by altering expression of proteins involved in intercellular junctions.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification, paracrine generation, and possible function of human breast carcinoma myofibroblasts in culture

Myofibroblasts from human breast carcinomas were identified and experimentally generated in culture, and a possible function was examined. The frequency of alpha-smooth muscle actin immunoreactive cells was evaluated as a measure of myofibroblast differentiation in primary culture. Few or no alpha-smooth muscle actin-positive stromal cells (6.1 +/- 8.4%) were identified in primary cultures from normal breast tissue (n = 9). In contrast, high frequencies (68.8 +/- 15.1%) were observed in primary cultures from carcinomas (n = 19). The frequencies of myofibroblasts in primary cultures were almost identical to those obtained in the corresponding cryostat sections (69.1 vs. 68.8%). A possible precursor cell to the myofibroblast was looked for among typical fibroblasts and vascular smooth muscle cells. Purified blood vessels containing both fibroblasts and vascular smooth muscle cells were embedded in collagen gel and incubated with medium conditioned by breast epithelial cells. Fibroblasts rather than smooth muscle cells were recruited from the blood vessels. In medium conditioned by carcinoma cell lines or in co-cultures of carcinoma cell lines and purified fibroblasts, alpha-smooth muscle actin and the typical myofibroblast phenotype were induced in otherwise alpha-smooth muscle actin-negative fibroblasts. The effect of myofibroblasts on cellular movement--essential to neoplastic cells--was analyzed. Spontaneous motility of tumor cells (MCF-7) was entirely suppressed in a collagen gel assay. Under these conditions tumor cell motility was selectively mediated by direct cell-to-cell interaction between tumor cells and myofibroblasts. Under chemically defined conditions, interaction was dependent on the presence of plasminogen. Anti-plasminogen, soybean trypsin inhibitor, and anti-fibronectin partly neutralized the effect of plasminogen. It is concluded that elements of myofibroblast differentiation and function may be studied in culture.

Permeability of bovine brain microvessel endothelial cells in vitro: Barrier tightening by a factor released from astroglioma cells

It has been shown both in vivo and in culture that astrocytes communicate with brain microvessel endothelial cells (BMECs) to induce many of the blood-brain barrier characteristics attributed to these unique cells. However, the results using cultured cells are conflicting as to whether this communication is dependent upon cell-cell contact. In this study we used primary cultures of bovine BMECs grown as monolayers on polycarbonate filters to study the formation of the barrier in vitro and examine its modulation by rat C6 glioma cells. Effects were examined by treating postconfluent BMEC monolayers with medium conditioned continually by C6 cells from the basolateral side to mimic the in vivo orientation. Cell monolayer integrity was assessed using electrical resistance and by measuring diffusion of uncharged molecules. BMEC monolayers form a functionally polarized and leaky barrier, with maximal resistance of 160 omega . cm2 and significant flux of molecules of molecular weight less than 350 Da. Treatment with rat or human astroglioma cells rather than pericytoma cells or transformed fibroblasts results in a concentration-dependent 200-440% increase in electrical resistance and a coincident 50% decrease in permeability to sucrose and dextran (70 kDa). The decrease in passive diffusion is most likely due to a change in tight junctions and not to transcellular vesicular traffic. The findings support that astroglioma cells release one or more signals that are required for cultured BMECs to express a "differentiated" phenotype associated with a tighter barrier, increased gamma-glutamyl transpeptidase activity, and decreased pinocytic activity. The relative ease and quickness of this culture system makes it amenable to studies on cell-cell interaction and regulation of barrier maintenance.

Decreased response to epidermal growth factor during cellular senescence in cultured human microvascular endothelial cells

We have previously demonstrated that epidermal growth factor (EGF) induces cell migration, tissue-type plasminogen activator synthesis, as well as tubular formation in microvascular endothelial cells from human omental tissue. In this study, we compared the responsiveness to EGF of late passaged (senescent) human omental microvascular endothelial (HOME) cells with that of early passaged (young) HOME cells. We have employed HOME cells derived from surgically resected omental samples from 14 patients. EGF-stimulated cell migration significantly more in the young cells than in the senescent cells during serial cultivation (aging) in vitro. Scatchard analysis demonstrated that the number for both high and low affinity receptors for EGF in HOME cells was decreased dramatically during serial cultivation. The expression of EGF receptor mRNA was also decreased in the senescent HOME cells. Treatment of HOME cells with EGF significantly increased cellular mRNA levels of tissue-type plasminogen activator, and two protooncogenes, c-fos and c-myc, in young HOME cells, but not in senescent HOME cells. Thus HOME cells aged in vitro show a decreased responsiveness to EGF, resulting in decreased migration of human endothelial cells. The serial cultivation of human endothelial cells in vitro may downregulate EGF receptor and decrease responsiveness to exogenous EGF, a potent angiogenic factor.

The desmoplastic response: induction of collagen synthesis by melanoma cells in vitro

When cells from the human malignant melanoma cell line, UCT-Mel 7, were injected into athymic nude mice, tumours developed that were intensely infiltrated with fibrous tissue. In an attempt to reproduce this desmoplastic response in vitro, we co-cultured fibroblasts with UCT-Mel-7 cells, and observed a 2-fold increase in the rate of fibroblast collagen synthesis. This was associated with an increase in the amount of collagen mRNA present in co-cultured fibroblasts. The stimulation was both dose- and time-dependent, with maximal stimulation at a melanoma cell:fibroblast ratio of 1:1. Analysis of the kinetics of proline incorporation into collagen showed that co-culture affected the maximal rate of proline incorporation; no effect was observed on the concentration of proline required for 50% maximal collagen synthesis. The induction of fibroblast collagen synthesis showed an absolute requirement for close proximity between the fibroblasts and the melanoma cells. No soluble fibrogenic factor released by melanoma was detected.

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.

Control of transforming growth factor-beta activity: latency vs. activation

Transforming growth factor-beta is a pluripotent regulator of cell growth and differentiation. The growth factor is expressed as a latent complex that must be converted to an active form before interacting with its ubiquitous high affinity receptors. This conversion involves the release of the mature growth factor through disruption of the non-covalent interactions with its pro-peptide or latency associated peptide. The mechanisms for this release in vivo have not been fully characterized but appear to be cell specific and might involve processes such as acidification or proteolysis. Although several factors including transcriptional regulation, receptor modulation and scavenging of the active growth factor have been implicated, the critical step controlling the biological effects of transforming growth factor-beta may be the activation of the latent molecule.

Structure and expression of the membrane proteoglycan betaglycan, a component of the TGF-beta receptor system

We describe the primary structure of rat betaglycan, a polymorphic membrane-anchored proteoglycan with high affinity for transforming growth factor-beta (TGF-beta). As deduced from its cDNA sequence, the 853 amino acid core protein of betaglycan has an extracellular domain with clustered sites for potential attachment of glycosaminoglycan chains. These chains are dispensable for TGF-beta binding to the core protein. The transmembrane region and the short cytoplasmic tail of betaglycan are very similar to these regions in human endoglin, an endothelial cell membrane glycoprotein involved in intercellular recognition. The ectodomain of betaglycan can be released as a soluble proteoglycan; a potential cleavage site near the transmembrane region is identical to the highly regulated cleavage site of the membrane-anchored transforming growth factor-alpha precursor. The unique features of betaglycan suggest important roles in cell interaction with TGF-beta.

Immunohistochemical localization of TGF beta 1, TGF beta 2, and TGF beta 3 in the mouse embryo: expression patterns suggest multiple roles during embryonic development

Isoform-specific antibodies to TGF beta 1, TGF beta 2, and TGF beta 3 proteins were generated and have been used to examine the expression of these factors in the developing mouse embryo from 12.5-18.5 d post coitum (d.p.c.). These studies demonstrate the initial characterization of both TGF beta 2 and beta 3 in mammalian embryogenesis and are compared with TGF beta 1. Expression of one or all three TGF beta proteins was observed in many tissues, e.g., cartilage, bone, teeth, muscle, heart, blood vessels, lung, kidney, gut, liver, eye, ear, skin, and nervous tissue. Furthermore, all three TGF beta proteins demonstrated discrete cell-specific patterns of expression at various stages of development and the wide variety of tissues expressing TGF beta proteins represent all three primary embryonic germ layers. For example, specific localization of TGF beta 1 was observed in the lens fibers of the eye (ectoderm), TGF beta 2 in the cortex of the adrenal gland (mesoderm), and TGF beta 3 in the cochlear epithelium of the inner ear (endoderm). Compared to the expression of TGF beta mRNA transcripts in a given embryonic tissue, TGF beta proteins were frequently colocalized within the same cell type as the mRNA, but in some cases were observed to localize to different cells than the mRNA, thereby indicating that a complex pattern of transcription, translation, and secretion for TGF beta s 1-3 exists in the mouse embryo. This also indicates that TGF beta 1, beta 2, and beta 3 act through both paracrine and autocrine mechanisms during mammalian embryogenesis.

Endothelial cell regulation by transforming growth factor-beta

Pronounced changes including growth inhibition, increased matrix deposition and suppression of cell-associated proteolytic activity, take place in endothelial cells (EC) upon the application of TGF-beta. Interrelationships between these effects have shed some light on the mechanism of action of TGF-beta and on its role in regulating EC function vis-a-vis angiogenesis. For instance, preliminary evidence has indicated that increased levels of certain matrix components may be partly responsible for the antiproliferative action of TGF-beta. In addition, TGF-beta and bFGF have opposing effects on cellular proteolytic balance which may contribute to the antagonistic effect that TGF-beta has on bFGF-induced EC growth and possibly to the anti-angiogenic effect exerted by TGF-beta under certain circumstances. Of particular interest in this regard is the fact that physical contact between EC and vascular mural cells in EC:mural cell cocultures has been found to generate active TGF-beta, thus further implicating TGF-beta in the maintenance of the quiescent, differentiated aggregation of EC as found in vascular structures in vivo. While more information is needed to define what, if any role TGF-beta plays in endothelial differentiation, it is to be noted that many of the cellular and biochemical processes affected by TGF-beta are linked to differentiation. It is therefore possible that the growth inhibition of EC by TGF-beta primes them for differentiation and/or is critical for the maintenance of a differentiated state.

Extracellular matrix regulation of growth factor and protease activity

Extracellular matrices bind many growth factors, proteases, and protease inhibitors. These interactions not only localize these molecules to the pericellular environment, but also modulate their biological activities. Recent evidence suggests that some growth factors may be active in vivo primarily in complexes with extracellular matrix molecules and that this interaction may be essential to their activity.

Differential protease expression by cutaneous squamous and basal cell carcinomas

To assess the postulated role of plasminogen activation in tumor invasion, we have investigated the cellular sites of synthesis for urokinase-type (uPA) and tissue-type (tPA) plasminogen activators and their inhibitors (PAI-1 and PAI-2) in two human cutaneous neoplasia that differ in their metastatic potential. The combined use of zymography on tissue sections and in situ hybridization demonstrates that uPA is produced by malignant cells of squamous cell carcinomas (SCC) but not by basal cell carcinomas (BCC), whereas tPA is detected exclusively in nonmalignant dermal tissue. In addition, we show that SCC neoplastic cells simultaneously produce variable amounts of PAI-1, and that PAI-1 production correlates inversely with uPA enzymatic activity. These observations establish that invasive human malignant cells in vivo can activate plasminogen through uPA production during the early phases of tumor growth; they also demonstrate that the proteolytic activity of tumor cells can be modulated by the concomitant production of PAI-1. Because SCC have a higher invasive and metastatic potential than BCC, our findings lend further support to the involvement of plasminogen activation in malignant behavior.

Production of transforming growth factor beta 1 during repair of arterial injury

Repair of arterial injury produced by balloon angioplasty leads to the formation of a neointima and a narrowing of the vascular lumen. In this study, we examined the possibility that smooth muscle cells (SMC) in injured rat carotid arteries are stimulated to produce type-1 transforming growth factor-beta (TGF-beta 1) during neointima formation in vivo. Levels of TGF-beta 1 transcripts (2.4 kb) were significantly increased within 6 h after carotid injury and reached a maximum (five to sevenfold) by 24 h. Regenerating left carotids had sustained increases in TGF-beta 1 mRNA levels (about fivefold) over the next 2 wk, during which time a substantial neointimal thickening was formed. No changes in basal TGF-beta 1 mRNA levels were found in contralateral uninjured carotids at any of the times examined. Immunohistochemical studies showed that a large majority of neointimal SMC were stained for TGF-beta 1 protein in an intracellular pattern, consistent with active TGF-beta 1 synthesis in this tissue. Neointima formation and TGF-beta 1 immunoreactivity were correlated with increases in fibronectin, collagen alpha 2(I), and collagen alpha 1(III) gene expression. Infusion of purified, recombinant TGF-beta 1 into rats with a preexisting neointima produced a significant stimulation of carotid neointimal SMC DNA synthesis. These results suggest that TGF-beta 1 plays an important role as an endogenous growth regulatory factor produced by neointimal SMC themselves during progressive neointimal thickening after balloon angioplasty.

Cytokines and growth factors positively and negatively regulate interstitial collagen gene expression in human vascular smooth muscle cells

Human atheromas accumulate extracellular matrix proteins such as collagen types I and III. We tested whether cytokines or growth factors produced by cells found in human atherosclerotic plaques alter collagen gene expression in vascular smooth muscle cells (VSMCs), which produce the blood vessel matrix. Interleukin-1 (IL-1, 1-10 ng/ml) modestly increased the synthesis of collagens I and III (measured by tritiated proline incorporation into specific electrophoretic bands), whereas transforming growth factor-beta (TGF-beta) or platelet-derived growth factor (PDGF) markedly stimulated production of these interstitial collagens. Interferon gamma (IFN-gamma), a product of activated T cells found in atheromas, selectively alters several VSMC functions. For example, this cytokine reduces growth of VSMCs, decreases alpha-actin gene expression, and induces VSMC expression of class II histocompatibility antigens. We report here that IFN-gamma also inhibits basal as well as IL-1-, PDGF-, or TGF-beta-stimulated collagen I and III synthesis by human VSMCs. TGF-beta, the most potent stimulator of collagen synthesis studied here, raised the level of collagen III mRNA in VSMCs 4.8-fold (determined by densitometry of Northern blots), whereas exposure to both TGF-beta and IFN-gamma reduced this mRNA to 0.5 of basal level. Locally produced cytokines and growth factors may thus modify matrix accumulation during atherogenesis by stimulating or suppressing expression of interstitial collagen mRNA and protein by VSMCs.

Transformation-dependent activation of urokinase-type plasminogen activator by a plasmin-independent mechanism: involvement of cell surface membranes

Transformed cells produce elevated levels of the urokinase-type plasminogen activator (u-PA), which has been linked with the invasive or migratory phenotype of these cells. The u-PA is secreted and normally maintained in the inactive, single-chain form (scu-PA) and it has been assumed that natural activation occurs via a plasmin-mediated cleavage converting scu-PA to the active, two-chain form (tcu-PA). We now demonstrate that secreted scu-PA in Rous sarcoma virus-transformed chicken embryo fibroblast (RSVCEF) cultures is activated by an endogenous, plasmin-independent mechanism. Normal CEFs and CEFs infected with a temperature-sensitive RSV mutant and incubated at the nonpermissive temperature do not activate scu-PA. Conditioned medium harvested from plasmin-free cultures of RSVCEFs contains active tcu-PA as determined by two independent methods. The scu-PA is progressively converted with time in culture and requires the presence of intact cells or a plasma membrane-enriched fraction. When added to RSVCEF cultures, a synthetic peptide corresponding to residues 20-41 of the growth factor domain of chicken u-PA blocks the conversion to tcu-PA, and scu-PA accumulates in the cultures. These results suggest that scu-PA is secreted by cells, becomes bound to a u-PA receptor, and is proteolytically converted to active tcu-PA by a catalytic mechanism on the surface of RSV-transformed fibroblasts.

TGF beta 1 and TGF beta 2 are potential growth regulators for low-grade and malignant gliomas in vitro: evidence in support of an autocrine hypothesis

Low-grade astrocytomas, anaplastic astrocytomas and glioblastomas in vitro were found to ubiquitously produce the mRNA of transforming growth factor-beta (TGF beta). TGF beta 1 and TGF beta 2 mRNA were expressed to a lesser degree among the hyperdiploid malignant gliomas. By radioreceptor assay of conditioned medium, TGF beta was secreted predominantly in latent form, in both latent and active form, or only in active form within a panel of low-grade and malignant gliomas. The TGF beta receptor (types I, II, and III) was evident among the glioma lines. Many near-diploid gliomas were growth-inhibited by TGF beta 1 and TGF beta 2 in vitro. Most hyperdiploid glioblastomas showed a positive mitogenic response to exogenous TGF beta 1 and TGF beta 2. A synergistic or additive mitogenic interaction with epidermal growth factor and insulin was observed among some. Under serum-free conditions, anti-TGF beta antibody neutralized the expected growth-regulatory effect of endogenous TGF beta, thus establishing the specificity of the response in vitro. TGF beta 1 also enhanced the clonogenicity of certain gliomas which had been growth-stimulated in monolayer. Thus, basic elements in support of an autocrine hypothesis have been demonstrated: TGF beta mRNA was expressed among low-grade and malignant gliomas, TGF beta was secreted in latent and/or active form into conditioned media and appeared to serve as an endogenous regulator of glioma proliferation in vitro. The mitogenic response, either positive or negative, correlated with the degree of anaplasia and karyotypic divergence.

Activation of human platelet-derived latent transforming growth factor-beta 1 by human glioblastoma cells. Comparison with proteolytic and glycosidic enzymes

Transforming growth factor-beta (TGF-beta), a regulator of cell growth and differentiation, is secreted by most cultured cells in latent form (L-TGF-beta). Activation of L-TGF-beta can be achieved by various physico-chemical treatments, including acidification, alkalinization, heating and chaotropic agents. Proposed physiological activators include proteinases and glycosidases, which, however, only lead to limited activation (15-20% of the total TGF-beta activity after acidic activation). In the present study L-TGF-beta 1 partially purified from human platelets was not activated by treatment with neuraminidase or the proteinases plasmin, endoproteinase Arg-C, elastase and chymotrypsin. The mechanism of activation of L-TGF-beta was further assessed by using the human glioblastoma cell line 308, which releases biologically active TGF-beta 2. Factor(s) secreted by 308 glioblastoma cells were found to be able to activate partially purified L-TGF-beta 1 from human platelets. Our finding may prove to constitute a physiologically relevant mechanism for the activation of latent forms of TGF-beta in vivo.

Lipoprotein (a) inhibits the generation of transforming growth factor beta: an endogenous inhibitor of smooth muscle cell migration

Conditioned medium (CM) derived from co-cultures of bovine aortic endothelial cells (BAECs) and bovine smooth muscle cells (BSMCs) contains transforming growth factor-beta (TGF-beta) formed via a plasmin-dependent activation of latent TGF-beta (LTGF beta), which occurs in heterotypic but not in homotypic cultures (Sato, Y., and D. B. Rifkin. 1989. J. Cell Biol. 107: 1199-1205). The TGF-beta formed is able to block the migration of BSMCs or BAECs. We have found that the simultaneous addition to heterotypic culture medium of plasminogen and the atherogenic lipoprotein, lipoprotein (a) (Lp(a)), which contains plasminogen-like kringles, inhibits the activation of LTGF-beta in a dose-dependent manner. The inclusion of LDL in the culture medium did not show such an effect. Control experiments indicated that Lp(a) does not interfere with the basal level of cell migration, the activity of exogenous added TGF-beta, the release of LTGF-beta from cells, the activation of LTGF-beta either by plasmin or by transient acidification, or the activity of plasminogen activator. The addition of Lp(a) to the culture medium decreased the amount of plasmin found in BAECs/BSMCs cultures. Similar results were obtained using CM derived from cocultures of human umbilical vein endothelial cells and human foreskin fibroblasts. These results suggest that Lp(a) can inhibit the activation of LTGF-beta by competing with the binding of plasminogen to cell or matrix surfaces. Therefore, high plasma levels of Lp(a) might enhance smooth muscle cell migration by decreasing the levels of the migration inhibitor TGF-beta thus contributing to generation of the atheromatous lesions.

Endothelium and glomerular growth

The role of glomerular endothelium has been barely considered, let alone investigated, but this is in large part due to the extreme difficulty in isolating and growing it in culture. The glomerular capillary is part of the arterial system and is better perceived as a "hemiarteriole." The mesangium and its response to hemodynamic events in the glomerular capillary is equivalent to the smooth muscle cell wall of an arteriole. The capability of cytokines, derived from the endothelium, to act in an autocrine and paracrine manner is clearly demonstrated by angiogenesis. The glomerular endothelial cell is likely to play a crucial role in the glomerulus in health and disease.

A cell-type specific and enhancer-dependent silencer in the regulation of the expression of the human urokinase plasminogen activator gene

A transcriptional silencer has been identified in the 5' regulatory region of the human urokinase plasminogen activator (uPA) gene. This region is able to block transcription from the human u-PA as well as the rabbit beta-globin promoters in a cell type specific and orientation independent way. The silencer is enhancer dependent and is active in two cell lines (HeLa and CV-1) which produce little if any uPA, but not in the high uPA producer PC3. Silencing activity and enhancer dependence can be separated: the silencing activity has been localized to the DNA fragment -660 to -536, while the enhancer dependence is located in the -536 to -308 fragment. The DNA sequence of the silencer region contains an element that closely resembles the TGF-beta responsive negative element TIE.

Cytoskeleton in TFG-beta- and bFGF-modulated endothelial monolayer repair

Endothelial cell proliferation and migration in vitro is depressed by transforming growth factor beta (TFG-beta) and enhanced by basic fibroblast growth factor (bFGF) treatment. This study examines interactions between cytoskeletal changes and cell proliferation in regenerating endothelial monolayers treated with bFGF, TFG-beta, and both factors. As previously described by others, monolayer regeneration is enhanced by bFGF and reduced by TFG-beta. Endothelial cell morphology is altered by TFG-beta treatment. Cells lose their cobblestone appearance and assume a pleomorphic shape. Actin microfilament staining is modified in both intact and regenerating TFG-beta-treated monolayers as well. There is a loss of dense peripheral band staining and an enhancement in staining intensity of cytoplasmic stress fibers. No such alterations are seen in bFGF-treated cultures. Cell proliferation at the wound edge, as indicated by bromodeoxyuridine incorporation, is inhibited by TGF-beta. Although monolayer repair is modulated by growth factor treatment, centrosome reorientation and microtubule staining patterns are not altered by either factor. Thus these factors appear to have effects on a mechanism(s) other than centrosome reorientation which may be involved in repair of denuded endothelial monolayers.

Interaction of monocytes with cocultures of human aortic wall cells involves interleukins 1 and 6 with marked increases in connexin43 message

Medium from cocultures of human aortic endothelial cells (HAEC) and smooth muscle cells (HASMC) taken from the same donor contained approximately two- to fourfold more macrophage colony-stimulating factor, granulocyte/macrophage colony-stimulating factor, and up to 5.1-fold more transforming growth factor beta than could be accounted for by the sum of the activities of media from equivalent numbers of HAEC and HASMC cultured separately. After pulse labeling, immunoprecipitated [35S]fibronectin and [14C]collagen were also found to be substantially increased in the coculture compared to the sum of HAEC and HASMC cultured separately. The cocultivation of HAEC and HASMC resulted in a 2.7-fold increase in connexin43 messenger RNA. When direct physical contact between HAEC and HASMC was prevented by a membrane that was permeable to medium, the levels of [35S]fibronectin and [14C]collagen in the coculture were significantly reduced. Monocytes cultured alone contained low levels of [35S]fibronectin and [14C]collagen but when added to the coculture there was up to a 22-fold increase in [35S]fibronectin and a 1.9-fold increase in [14C]collagen compared to the coculture alone. The increase in fibronectin was prevented in the presence of neutralizing antibody to interleukin 1 and antibody to interleukin 6 by 45% and 67%, respectively. Addition of monocytes to cocultures also induced the levels of mRNA for connexin43 by 2.8-fold. We conclude that the interaction of HAEC, HASMC, and monocytes in coculture can result in marked increases in the levels of several biologically important molecules and that increased gap junction formation between the cells and interleukins 1 and 6 may be partially responsible for these changes.

In vitro and in vivo activation of endothelial cells by colony-stimulating factors

This study was designed to identify the set of functions activated in cultured endothelial cells by the hematopoietic growth factors, granulocyte colony-stimulating factor (G-CSF) and granulocyte macrophage-colony-stimulating factor (GM-CSF), and to compare them with those elicited by prototypic cytokines active on these cells. Moreover, indications as to the in vivo relevance of in vitro effects were obtained. G-CSF and GM-CSF induced endothelial cells to proliferate and migrate. In contrast, unlike appropriate reference cytokines (IL-1 and tumor necrosis factor, IFN-gamma), G-CSF and GM-CSF did not modulate endothelial cell functions related to hemostasis-thrombosis (production of procoagulant activity and of platelet activating factor), inflammation (expression of leukocyte adhesion molecule-1 and production of platelet activating factor), and accessory function (expression of class II antigens of MHC). Other colony-stimulating factors (IL-3 and macrophage-colony-stimulating factor) were inactive on all functions tested. In comparison to basic fibroblast growth factor (bFGF), G-CSF and GM-CSF induced lower maximal proliferation of endothelial cells, whereas migration was of the same order of magnitude. G-CSF and GM-CSF stimulated repair of mechanically wounded endothelial monolayers. Exposure to both cytokines induced shape changes and cytoskeletal reorganization consistent with a migratory phenotype. To explore the in vivo relevance of the in vitro effects of these cytokines on endothelium, we studied the angiogenic activity of human G-CSF in the rabbit cornea. G-CSF, but not the heat-inactivated molecule, had definite angiogenic activity, without any sign of inflammatory reactions. G-CSF was less active than bFGF. However, the combination of a nonangiogenic dose of bFGF with G-CSF resulted in an angiogenic response higher than that elicited by either individual cytokines. Thus, G-CSF and GM-CSF induce endothelial cells to express an activation/differentiation program (including proliferation and migration) related to angiogenesis.

The stimulatory effect of PDGF on vascular smooth muscle cell migration is mediated by the induction of endogenous basic FGF

The migration of arterial smooth muscle cells from the media to the intima is a crucial event for the development of the atherosclerotic lesion, and platelet derived growth factor (PDGF) is thought to play an important role in this process. Here we report that the spontaneous migration of bovine smooth muscle (BSM) cells is dependent on endogenously produced basic fibroblast growth factor (bFGF). PDGF stimulates the migration of BSM cells and its effect is abolished by affinity purified anti-bFGF antibody. PDGF induces bFGF mRNA in BSM cells. These results indicate that the effect of PDGF on the migration of BSM cells may be mediated by the induction of endogenous bFGF.

Tumor necrosis factor and epidermal growth factor modulate migration of human microvascular endothelial cells and production of tissue-type plasminogen activator and its inhibitor

Epidermal growth factor (EGF) induces tubular formation of cultured human microvascular endothelial (HME) cells in the gel matrix containing collagen, and tumor necrosis factor (TNF) disrupts the tubular formation (Mawatari et al. (1989) J. Immunol. 143, 1619-1627). Here we studied the effects of EGF and TNF on endothelial cell migration and on the production of proteases. Confluent HME cells, when wounded with a razor blade, moved into the denuded space. This migration was stimulated by EGF and inhibited by TNF in this assay and in the Boyden chamber assay. Antibody against tissue-type plasminogen activator (t-PA) inhibited the EGF-stimulated cell migration in both assays by approximately 70%, but antibody against urokinase-type plasminogen activator (u-PA) could not inhibit its migration. Quantitative immunoreactive assays showed an approximately three- to fourfold increase of t-PA at 6 to 12 h after EGF addition, and TNF inhibited the production of t-PA by 50%. Northern blot analysis showed increased expression of t-PA mRNA by EGF alone in a time- and dose-dependent manner, whereas TNF alone inhibited its expression in a time- and dose-dependent manner. Northern blot analysis showed a significant increase of plasminogen activator inhibitor-1 (PAI-1) mRNA when EGF or TNF was present. Stimulation by EGF of cell migration of HME cells and its inhibition by TNF appear to be closely correlated with the cellular modulation of t-PA and PAI-1 activities.

Regulation of motility in bovine brain endothelial cells

Scatter factor (SF) is a fibroblast-derived cytokine which stimulates motility of epithelial and vascular endothelial cells. We used a quantitative assay based on migration of cells from microcarrier beads to flat surfaces to study the regulation of motility in bovine brain endothelial cells (BBEC). Peptide growth factors (EGF, ECGF, basic FGF) did not stimulate migration. Tumor promoting phorbol esters (PMA, PDD) markedly stimulated migration, while inactive phorbol esters (4a-PDD, phorbol-13,20-diacetate) did not affect migration. Both SF- and PMA-stimulated migration were inhibited by 1) TGF-beta; 2) protein kinase inhibitors (e.g., staurosporine, K-252a); 3) activators of the adenylate cyclase signaling pathway (e.g., dibutyryl cyclic AMP, theophylline); 4) cycloheximide; and 5) anti-cytoskeleton agents (e.g., cytochalasin B, colcemid). However, PMA and SF pathways were distinguishable: 1) PMA induced additional migration at saturating SF concentrations; 2) the onset of migration-stimulation was immediate for PMA and delayed for SF; and 3) down-modulation of protein kinase C (PKC) ablated PMA but not SF responsiveness. Assessment of PKC by (3H)-phorbol ester (PDBu) binding and by immunoblot showed 1) scatter factor does not cause significant redistribution or down-modulation of PDBu binding or alpha-PKC; and 2) PDBu mediates redistribution and down-modulation of both binding and alpha-PKC. These findings suggest two pathways for BBEC motility: a PKC-dependent pathway and an SF-stimulated/PKC-independent pathway.

Cellular activation of latent transforming growth factor beta requires binding to the cation-independent mannose 6-phosphate/insulin-like growth factor type II receptor

The activation of latent transforming growth factor beta (LTGF-beta) normally seen in cocultures of bovine aortic endothelial and bovine smooth muscle cells can be inhibited by coculturing the cells with either mannose 6-phosphate (Man-6-P) or antibodies directed against the cation-independent Man-6-P/insulin-like growth factor type II receptor (anti-Man-6-PR). This result was established by measuring the ability of coculture conditioned medium (formed with or without Man-6-P or anti-Man-6-PR) to suppress bovine aortic endothelial cell migration and protease production, activities previously shown to be related to transforming growth factor beta activity. The inhibition by Man-6-P is dose dependent, with maximal inhibition seen at 100 microM and is specific because mannose 1-phosphate and glucose 6-phosphate do not interfere with activation of LTGF-beta. The inhibitory effect of anti-Man-6-PR is also specific and dose dependent; maximal inhibition of activation occurs at 400 micrograms/ml. Control experiments indicate that Man-6-P and anti-Man-6-PR do not interfere with the basal level of migration of bovine aortic endothelial cells, the migration observed when exogenous transforming growth factor beta is added, the activation of transforming growth factor beta by plasmin or transient acidification, and the release of LTGF-beta. Thus, binding to the cation-independent Man-6-P/insulin-like growth factor type II receptor appears to be a requirement for activation of LTGF-beta.

Chondrocytes inhibit endothelial sprout formation in vitro: evidence for involvement of a transforming growth factor-beta

Using a quantitative in vitro model of spontaneous endothelial sprout formation, we have attempted to define physiological inhibitors of angiogenesis from hyaline cartilage, a tissue whose antiangiogenic properties have been well described. The model consists of embedding bovine microvascular endothelial cell aggregates into fibrin or collagen gels, which results in the formation of radially growing sprouts. When chondrocytes derived from the permanent cartilagenous region of the chick embryo sternum are cocultured with the endothelial cell aggregates, sprout formation is markedly inhibited. Addition of anti-TGF-beta antibodies to the cocultures significantly reduced the inhibitory effect of chondrocytes on sprout formation. Chondrocyte-conditioned medium or exogenously added TGF-beta 1 have a similar albeit transient inhibitory effect. Depletion of TGF-beta from chondrocyte conditioned medium with anti-TGF-beta antibodies and solid-phase protein-A significantly decreases the inhibition of sprout formation. These results demonstrate that a chondrocyte-derived TGF-beta-like molecule inhibits capillary sprout formation in vitro and suggest that the antiangiogenic properties of cartilage may at least in part, be mediated by TGF-beta.

Vascular cell responses to TGF-beta 3 mimic those of TGF-beta 1 in vitro

The vascular cell responses to the type 3 isoform of transforming growth factor-beta (TGF-beta 3) were studied using bovine aortic endothelial (BAECs) and smooth muscle cells (BASMCs) as well as rat epididymal fat pad microvascular endothelia (RFCs). Four distinct bioassays indicated that TGF-beta 3 elicits results that do not differ significantly from those of the TGF-beta 1 isoform in all three cell populations. Inhibition of proliferation by TGF-beta 3 at a 5-day time point ranged from 85% on BAECs, to 55% and 53% on RFCs and BASMCs, respectively. The effects of TGF-beta 3 and TGF-beta 1 on cell migration were also found to be similar; migration of large vessel endothelial cells was inhibited 35%, while migration of smooth muscle cells was enhanced 30%. TGF-beta 1 and TGF-beta 3 also had equivalent effects on neovascularization while a 10-fold higher concentration of TGF-beta 2 was required to elicit a similar response. Experimentation to decipher cell surface binding by the different isoforms revealed that iodinated TGF-beta 1 bound to the surface of all three vascular cell types can be competed off in similar fashion by either TGF-beta 1 or TGF-beta 3; however, competition with TGF-beta 2 produced unique binding profiles dependent upon the cell type examined. In summary, both the TGF-beta 1 and TGF-beta 3 isoforms of the transforming growth factor-beta family evoke comparable responses in proliferation, migration, angiogenic and cell surface binding assays using three distinct vascular cell types, while the biofunctions of TGF-beta 2 on these cells are distinct.

Cloning and overexpression of TGF-beta 1 cDNA in a mammary adenocarcinoma: in vitro and in vivo effects

It has been suggested that transforming growth factor beta (TGF-beta) may be a potential negative autocrine growth regulator of carcinomas including mammary carcinomas. To directly test this hypothesis we have cloned and expressed human TGF-beta 1 cDNA in a murine mammary adenocarcinoma which is normally growth-inhibited by addition of exogenous TGF-beta in vitro. A number of transfectants over-expressing the foreign TGF-beta 1 mRNA were selected and compared to transfectants which did not overexpress the exogenous TGF-beta 1 cDNAS. Cell lines overexpressing the transfected TGF-beta 1 mRNA were found to produce total levels of TGF-beta 7 to 10 fold greater than the parental cells or control transfected clones. However, when levels of active fractions of TGF-beta were compared in cell lines overexpressing TGF-beta 1 to those which did not, no differences were found. This suggests that the activation mechanism is not necessarily induced or altered by increasing levels of latent TGF-beta 1 production in a given tumor cell line. The basal in vitro doubling time of TGF-beta 1 overexpressing clones was identical to the control populations. Similarly, in vivo tumor growth rates after s.c. injection were similar to that of the parental line. Thus the precise role of TGF-beta in mediating either the in vitro or in vivo growth control of a sensitive mammary adenocarcinoma cell line remains unclear. It may be that cellular over-secretion of latent TGF-beta must be coupled with enhanced cellular TGF-beta activation prior to any observed effect on growth rate in vitro or in vivo; this latter event may constitute the "rate-limiting" step of TGF-beta activity on tumor behavior.

Vascular cells respond differentially to transforming growth factors beta 1 and beta 2 in vitro

Transforming growth factor beta 1 (TGF-beta 1) and beta 2 (TGF-beta 2) are equipotent in many cell systems studies thus far. Recent data, however, show different effects elicited by these two growth factors in specific biologic systems. This investigation compares the effects of TGF-beta 1 and TGF-beta 2 bovine aortic endothelial cells (BAECs), rat epididymal fat pad microvascular endothelium (RFCs), and bovine aortic smooth muscle cells (BASCs). In two-dimensional cultures, proliferation of BAECs, BASMCs, and RFCs were all inhibited by TGF-beta 1, while in response to TGF-beta 2, BASMCs were fully inhibited, RFCs were modestly inhibited, and BAECs were unaffected. Bovine aortic endothelial cell migration was significantly inhibited by TGF-beta 1, but only slightly inhibited by TGF-beta 2. In contrast, BASMC migration was enhanced by TGF-beta 1 and was not affected by TGF-beta 2. In three-dimensional cultures, RFCs were stimulated to undergo in vitro angiogenesis in response to TGF-beta 1 and TGF-beta 2 at 10-fold higher concentrations. Three distinct receptor assays demonstrated the presence of type I and type II TGF-beta 1 cell-surface-binding proteins on BAECs, BASMCs, and RFCs. Labeled TGF-beta 1 was competed off completely with 100-fold molar excess unlabeled TGF-beta 1, but only partially with equivalent excess unlabeled TGF-beta 2. Furthermore the ratios of type I to type II TGF-beta receptors in these three vascular cell types vary from 1:1 in BAECs to 1.5:1 in RFCs to 3:1 in BASMCs and can be correlated with the differences noted in cellular responses to TGF-beta 1 and TGF-beta 2 in proliferation, migration, and in vitro angiogenic assays. These findings support the hypothesis that there are different responses to the TGF-beta s, depending on the cell type and experimental conditions as well as the TGF-beta concentration and isoform used.

Angiogenesis in arteries: review

The inner parts of the walls of large blood vessels do not normally contain intrinsic vasculature. In pathologic conditions such as arteriosclerosis or thrombosis, angiogenesis occurs, and may have significant clinical consequences. This review attempts to relate the little that is known about the factors specific to vascular walls which regulate angiogenesis to more general knowledge of the phenomenon.

Smooth muscle cell growth factors

While the roles of the platelet-derived growth factors (PDGFs) in vascular smooth muscle cells (SMCs) continue to be elucidated, these cells, especially in their activated 'synthetic' state, have also been found to express, and proliferate in response to, many of the other families of polypeptide growth factors, such as the fibroblast growth factors. Other stimulators of DNA synthesis, and particularly of SMC hypertrophy, include the vasoconstrictor hormones such as angiotensin II, as well as physical forces, especially stretch or tension. For many of these ligands, multiple receptors have been identified and their means of signal transduction are being characterized rapidly. Regulatory regions of these genes are being identified as are transcription factors. Complex post-transcriptional regulation has also been shown by the findings that some growth factors are phosphorylated, or translocated to the nucleus or the extracellular matrix. Inhibitors have also been identified. These include some prostaglandins, calcium antagonists, agonists that activate guanylate and adenylate cyclases, inhibitors of angiotensin-converting enzyme, interferon gamma, and heparin. Future studies are likely to show that tyrosine phosphatases and recessive oncogenes also regulate growth. The existence of so many autocrine/paracrine mitogens--together with some experimental data--suggests some redundancy in the system as well as some additive effects. Redundancy may limit the efficacy of antibodies to a single growth factor to block cell proliferation. Their evolutionary conservation implies some unique roles for each growth factor but these have not been apparent from in vitro studies to date. Further insights are apt to come from the increasing recognition that growth factors have other effects--on cell attachment, migration, survival, production of extracellular matrix, thrombosis, vaso-constriction, regulation of cytokine synthesis, and inhibition of growth. Many of these effects may prove to be context-dependent, as with the case of growth inhibition by transforming growth factor-beta. Studies in monolayer cultures may not obtain the same results as studies using cocultures of endothelial and smooth muscle cells, or 3-dimensional matrix cultures, organ cultures, or in the intact animal. In vivo descriptive studies of growth factors expressed in vascular embryogenesis, hypertension, atherosclerosis, acute balloon injury and thrombosis are being supplemented by interventions such as infusions with growth factors, antibodies, and toxin conjugates. These studies, and studies using transgenic mice and homologous recombination, should yield information as to mechanisms and may also suggest new therapies.