Autotaxin (NPP-2), a metastasis-enhancing motogen, is an angiogenic factor.

Autotaxin [ATX (NPP-2)], originally isolated as a tumor motility-stimulating protein, has recently been shown to augment tumor aggressiveness. Specifically, atx-transfected, ras-transformed NIH3T3 cell lines have been shown to be more invasive, tumorigenic, and metastatic than mock-transfected ras-transformed control cells. In addition, the atx-transfected ras-transformed cell lines appeared to produce tumors that were much more hyperemic than those formed by appropriate control cells. This observation led to the present study, in which we demonstrate that ATX modulates angiogenesis both directly and indirectly. We have used a murine in vivo angiogenesis model in which treated Matrigel plugs are injected s.c. into athymic nude BALB/c mice. Using the same transfected cell lines as before, we found that mixing atx-transfected ras-transformed NIH3T3 cells into the Matrigel resulted in greater new blood vessel formation than control cells. Similarly, mixing purified ATX into the Matrigel resulted in new blood vessel formation within the plug, similar to that produced by vascular endothelial growth factor. Mechanistically, ATX is not a strong chemoattractant for human endothelial cells (HUVECs); however, it strongly stimulates motility in human coronary artery smooth muscle cells. In addition, ATX stimulates HUVECs grown on Matrigel to form tubules, much like vascular endothelial growth factor. Both of these normal cell types are shown to express and secrete ATX. In HUVECs, ATX expression is up-regulated by basic fibroblast growth factor in a time-dependent manner. This up-regulation also extends to secretion of enzymatically active protein, as demonstrated by Western blot analysis and quantification of type-1 phosphodiesterase activity. These results establish the presence of ATX in HUVECs and coronary artery smooth muscle cells and specify ATX as a novel angiogenic factor, suggesting that ATX could contribute to the metastatic cascade through multiple mechanisms, perhaps by supporting an invasive microenvironment for both normal and tumor cells.

A sensitive screening assay for secreted motility-stimulating factors.

Secreted motility-stimulating factors are often expressed and secreted at low concentrations that are difficult to detect by Northern or Western blot analysis. Autotaxin (ATX) is a tumor-secreted autocrine motility-stimulating factor that has been associated with tumor invasion and metastatic potential. Although ATX has a number of enzymatic activities, it is most sensitively detected by its induced chemotactic response. After transfecting ATX cDNA into NIH3T3 fibroblasts, we developed a motility-based method to screen the resulting cloned cells for secretion of active protein. We placed the cloned and transfected cells into the bottom wells of a modified Boyden chamber and placed responding cells (A2058 human melanoma cells) into the upper wells. After overnight incubation, the membrane that separated the two chambers was removed and stained. Simple densitometry measurements were sufficiently accurate to determine which clones secreted active protein. Utilizing this method, 4 positive cell lines were chosen out of 36 tested clones. Further tests on the expanded cell lines determined that all 4 were secreting ATX. Thus, this modified Boyden chamber assay appears to provide a rapid and highly adaptable means to identify cells that secrete motility-stimulating factors.

Cyclocreatine inhibits stimulated motility in tumor cells possessing creatine kinase.

Cyclocreatine (1-carboxymethyl-2-iminoimidazolidine), an analog of creatine and a substrate for creatine kinase (EC 2.7.3.2), inhibits the stimulated motility of tumor cells which possess creatine kinase. A2058-055 human melanoma cells, transfected with a creatine kinase gene, showed an 80-90% reduction in chemotactic response to type IV collagen when incubated overnight in the presence of 10 mM cyclocreatine (p < 0.0001 for n = 8 experiments). This inhibitory effect of cyclocreatine can be partially reversed by addition of creatine to the overnight cell treatment. Non-transfected cells, with very low levels of creatine kinase, were not significantly inhibited. Further experiments utilizing type IV collagen as attractant demonstrated that cyclocreatine inhibited the chemokinetic (91%) and the haptotactic (73%) responses and the in vitro invasion of A2058-055 cells through Matrigel-coated membranes (88%). In addition, motility stimulation of A2058-055 cells by either autotaxin or fibronectin was markedly inhibited by cyclocreatine. DU-145 prostatic tumor cells, which express endogenous creatine kinase, also have a reduced motility response to either autotaxin or epidermal growth factor induced motility in the presence of cyclocreatine.

Cysteine proteinases and inhibitors in inflammation: their role in periodontal disease.

Cellular and molecular events during the development of inflammatory disease are accompanied by the release of host lysosomal cysteine proteinases (CPs) affecting not only degradation of matrix proteins but possibly also antigen processing and chemotaxis of neutrophils. Activity measurements of Cat B and Cat L could not be used as an accurate indicator of disease activity in individual patients, although average values were higher in patients with more advanced periodontal inflammation. In contrast, simultaneous decrease of cystatin C and alpha 2-macroglobulin (alpha 2-M) in inflamed gingiva and gingival fluid, respectively, might be useful diagnostic/prognostic factors. While the total and the free form of alpha 2-M in gingival fluid decreased with the progression of the disease, the complexed alpha 2-M form was hardly detectable. This indicates an increased consumption of this inhibitor by various proteinases and clearance of protease: alpha 2-M complexes by macrophages. Elevated serum levels of alpha 2-M were found in patients with more pronounced disease, suggesting a systemic host response. In addition, high levels of stefin A and moderate levels of kininogen were observed in gingival tissue homogenates. Stefin A was also found to play a role in the inhibition of neutrophil chemotaxis. In addition, other proteinases which are released at inflammatory sites from neutrophils, macrophages, lymphocytes, and/or bacteria may degrade the cystatins, thereby further increasing CP activities. Increased CP activity may inactivate serine protease inhibitors, leading to the so-called "proteolytic burst."

Granulocyte-macrophage colony-stimulating factor induces human melanoma-cell migration.

Tumor metastasis is the primary cause of death for cancer patients. The metastatic cascade requires successful tumor cell invasion into and through vascular and parenchymal barriers. We have shown that autocrine motility factor (AMF, autotaxin) and the insulin-like growth factors (IGFs) induce tumor-cell migration. Since granulocyte-macrophage colony-stimulating factor (GM-CSF) has been shown to prime neutrophils for chemotaxis, we have therefore studied the influence of GM-CSF upon tumor cells and report that GM-CSF stimulates migration of these cells in a dose-dependent fashion. The ED50 for A2058 human melanoma cell line chemotaxis to GM-CSF is approx. 60 pM. The motile response to GM-CSF was additive to that of IGF-I and AMF, both of which are potent attractants for tumor cells. Pre-treatment of cells for 2 hr with non-toxic concentrations of pertussis toxin (PT) or amiloride resulted in a 50% inhibition of chemotaxis to GM-CSF. Therefore, GM-CSF, through PT- and amiloride-sensitive signal pathways, is a potent attractant for melanoma cells, the response to which is additive to that of other attractants. The presence of the GM-CSF receptor in A2058 melanoma cells was indicated by Northern-blot analysis which identified message transcripts of 2.1 and 3.0 kb. These data emphasize the versatility of the melanoma cell migration response to an array of cytokines, including GM-CSF.

The role of autotaxin and other motility stimulating factors in the regulation of tumor cell motility.

Active cellular motility is required for tumor cell penetration of the basement membrane and the interstitial stroma during the transition from in situ to invasive carcinoma. Multiple factors, both autocrine and paracrine in origin, appear to influence this motile response. Recently, a potent new cytokine with molecular mass 120 kDa has been purified to homogeneity from a human melanoma cell line (A2058). This new protein, termed autotaxin (ATX), is a basic glycoprotein with pI approximately 7.7. ATX is active in the picomolar range, stimulating pertussis toxin sensitive chemotactic and chemokinetic responses by the same cell line that produces it. Sequence information, obtained on 11 purified tryptic peptides (114 residues), confirmed that the protein is unique with no significant homology to growth factors or previously described motility factors. It is hypothesized that an autocrine motility factor, such as ATX, could play a role in the initiation of the metastatic cascade by stimulating tumor cells to move away from the primary tumor. Other motility stimulating factors, such as components of the extracellular matrix or growth factors, could then influence both the time course and the localization of tumor cell spread.

Cytoskeletal agents inhibit motility and adherence of human tumor cells.

Cytoskeletal agents have been demonstrated to inhibit stimulated motility and substrate adherence by the human tumor cell line, A2058. cis-tubulozole, taxol, and cytochalasin D were tested for their effects on chemotaxis in response to a tumor cytokine, autocrine motility factor, and on adherence to several substrata: laminin- and gelatin-coated dishes as well as tissue culture plastic. Cytochalasin D, which inhibits microfilament polymerization, abolished stimulated motility. Taxol, which stabilizes microtubules, decreased stimulated motility to a greater degree than cis-tubulozole, which inhibits microtubular polymerization. In contrast, cis-tubulozole had the greatest inhibitory effect on adherence with a gelatin substratum more affected (100% inhibition) than tissue culture plastic (90%) or laminin substratum (52%). Taxol affected adherence in the same order but less than cis-tubulozole. Cytochalasin D had no significant effect on adherence to laminin with moderate inhibition of adherence to tissue culture plastic or gelatin. These data suggest that, in these tumor cells, microfilaments are more crucial for motility than adherence, but the dynamic polymerization and depolymerization of microtubules are required for both types of cellular activities.

Identification, purification, and partial sequence analysis of autotaxin, a novel motility-stimulating protein.

Autotaxin (ATX) is a potent human motility-stimulating protein that has been identified in the conditioned medium from A2058 melanoma cells. This protein has been purified to homogeneity utilizing a strategy involving five column steps. Homogeneity of ATX was verified by two-dimensional gel electrophoresis. The molecular size of ATX is 125 kDa, and it has an isoelectric point of 7.7 +/- 0.2. Purified ATX was digested with cyanogen bromide and trypsin, and the resulting ATX peptides were purified by reverse-phase high performance liquid chromatography. Eleven peptides were subjected to amino acid sequence analysis, and 114 residues were identified. The partial amino acid sequences and the amino acid composition obtained for ATX show that it does not exhibit any significant homology to known growth factors or previously described motility factors. At picomolar concentrations, ATX stimulates both random and directed migration of human A2058 melanoma cells. Pretreatment of the melanoma cells with pertussis toxin abolishes the response to purified ATX, indicating that ATX stimulates motility through a receptor acting via a pertussis toxin-sensitive G protein.

A possible role for cysteine proteinase and its inhibitors in motility of malignant melanoma and other tumour cells.

The metastasis of malignant tumour cells depends on their rapid replication, and their ability to adhere to the matrix of a biological barrier such as basement membrane, to degrade the matrix, and to migrate through this more permeable barrier. Secreted enzymes, including the cysteine proteinases cathepsins B and L, are known to degrade basement membrane components. Using a barrier-free substratum we studied the possible role of cysteine proteinases in influencing the motility per se of metastatic cells. We found that stefins, the natural inhibitors of cysteine proteinases, markedly decreased the stimulated motility of both human melanoma cells and W256 carcinosarcoma cells at low concentrations (0.5 microM). A stefin also inhibited melanoma cell adherence, but to a lesser extent than motility. Additionally, synthetic inhibitors (E-64, diazomethyl ketones) of cysteine proteinases were found to depress stimulated motility of W256 cells. These results suggest that cysteine proteinases and their inhibitors may have a direct role in the development of a migratory response per se in tumour cells.

Comparison of autocrine mechanisms promoting motility in two metastatic cell lines: human melanoma and ras-transfected NIH3T3 cells.

Tumor-cell migration plays an essential role in invasion into surrounding tissues and the formation of metastatic colonies in distant organs. Metastatic human A2058 melanoma and ras-transfected NIH3T3 cells produce autocrine motility factors (AMFs) which stimulate their own motility, and the A2058 cell AMF (AMF/A2058) has been purified. In this study, we partially purified the AMF produced by N-ras-transfected NIH3T3 cells (AMF/NIH3T3) and compared it with AMF/A2058. The two AMFs differed in their gel filtration patterns and heat stability, although both elicited migration of N-ras-transfected NIH3T3 cells. The receptor for AMF/A2058 in A2058 cells is linked to a pertussis-toxin-sensitive GTP-binding protein. Pre-treatment of N-ras-transfected NIH3T3 cells with pertussis toxin also specifically blocked the promotion of motility by AMF/A2058, but did not affect the activity of AMF/NIH3T3. Stimulation of N-ras-transfected NIH3T3 cells by both AMFs elicited an additive response. Thus, the autocrine mechanisms of these two metastatic tumor cell lines are different with regard to the AMF molecules, receptors, and signal transduction pathways.

Tumor cell motility.

Tumor cell motility is required for invasion and metastasis. The locomotory machinery of the cell includes cell projections called pseudopodia which are regulated by a complicated linkage between cell surface receptors or sensors and the internal cytoskeleton. Recently a new class of motility stimulating cytokines have been identified. These cytokines can function as autocrine motility factors and require a pertussis toxin sensitive G protein pathway to transduce a random motile response.

Cell motility, a principal requirement for metastasis.

In studying the role of motility in the metastasis of tumor cells, we have described an autocrine motility factor. This agent, which stimulates random motility, probably contributes to the initial dissociation of the cells from the primary tumor mass. Extracellular matrix components, via several different mechanisms, may facilitate the crossing of biological barriers by the cells prior to the entry into the circulation. In locating at new sites, the tumor cells may be induced to exit from the circulation in response to attractants such as IGFs that could emanate from the target organ. These same growth factors could then stimulate cellular proliferation for another metastatic cycle. It is quite probable that detection of AMF may provide a new tool in cancer diagnosis. The complete characterization of AMF may also yield valuable therapeutic approaches: design of low molecular size antagonists of the attractants and antibodies that might be effective therapeutically as well as diagnostically. It seems clear, in any event, that immobilizing the tumor cell may be a crucial step in inhibiting metastasis.

Glycolysis as primary energy source in tumor cell chemotaxis.

The energy requirements via glycolytic pathways were directly measured in migrating tumor cells. Motility in the metastatic human melanoma cell line A2058, stimulated by insulinlike growth factor I (IGF-I), depends on glycolysis in the presence of glucose as its principal source of energy. Motility in glucose-free medium was 75% reduced and utilized mitochondrial respiration (inhibited by oligomycin). With increasing (physiologic) glucose concentrations, there was a dramatic shift to anaerobic glycolysis as the energy source and 93% elimination of the oligomycin inhibition of motility. Oxamate, an inhibitor of glycolysis, inhibited motility at all glucose concentrations. CO2 production from glycolysis and from the hexose monophosphate shunt was measured in migrating tumor cells. The time course and glucose-dose dependence of glycolytic CO2 production correlated directly with motility. In contrast, mitochondrial CO2 production was inversely related to glucose concentration. A monoclonal antibody for the IGF-I receptor inhibited both motility and glycolytic CO2 production, indicating that both processes are receptor mediated.

Heterogeneity of the motility responses in malignant tumor cells: a biological basis for the diversity and homing of metastatic cells.

Tumor metastasis requires highly motile cells that can respond to appropriate stimuli. A2058 human melanoma cells were shown previously to secrete a highly potent autocrine motility factor (AMF) that stimulates chemokinetic movement. We have shown that the insulin polypeptides (IPs; insulin-like growth factors I and II [IGF-I, -II] and insulin) stimulated A2058 cell chemotaxis and chemokinesis. We now report that the IPs and AMF stimulate locomotion in other human malignant cell lines. Insulin (100 nM) induced motility of up to 50% of the magnitude of the AMF response in human carcinoma lines MDA-231 (breast), T24 (bladder), and OVCAR3 (ovarian). The tumorigenic and metastatic 5R Haras-transfected rat embryo fibroblast cell line responded to insulin with both chemotaxis and chemokinesis and was 100% of that seen for AMF. The ED50 for IGF-I in the carcinoma cell lines was in the order of I nM, but the magnitude of the responses at this concentration was 40% of the AMF-stimulated response, with the exception of the A2058 cells, which were maximally stimulated at I nM. IGF-II induced maximal motility of 75 to 130% of the AMF-stimulated response in the carcinoma lines with ED50 of less than or equal to 10 nM. IGF-II-stimulated motility in the carcinoma lines was predominantly chemotactic by modified checkerboard analysis. Cell pretreatment with pertussis toxin inhibited 90-100% of AMF-induced motility, whereas migration to the IPs was not pertussis toxinsusceptible. In growth studies, IGF-I induced mitogenesis up to 140% of basal media control growth. In general, maximal growth stimulation was seen at 100 nM IGF-I, and optimal migration was seen at 10 nM IGF-I. The IGFs are secreted by normal stroma in a number of organs that are common sites for primary and metastatic disease. Therefore, we suggest that IPs may be important homing and mitogenic signals for tumor cells in the process of invasion and metastasis and that the differential motility stimulation and respective mechanisms of action by these physiologically important agents may underlie the diversity of the metastatic process.

Signal transduction for chemotaxis and haptotaxis by matrix molecules in tumor cells.

Transduction of signals initiating motility by extracellular matrix (ECM) molecules differed depending on the type of matrix molecule and whether the ligand was in solution or bound to a substratum. Laminin, fibronectin, and type IV collagen stimulated both chemotaxis and haptotaxis of the A2058 human melanoma cell line. Peak chemotactic responses were reached at 50-200 nM for laminin, 50-100 nM for fibronectin, and 200-370 nM for type IV collagen. Checkerboard analysis of each attractant in solution demonstrated a predominantly directional (chemotactic) response, with a minor chemokinetic component. The cells also migrated in a concentration-dependent manner to insoluble step gradients of substratum-bound attractant (haptotaxis). The haptotactic responses reached maximal levels at coating concentrations of 20 nM for laminin and type IV collagen, and from 30 to 45 nM for fibronectin. Pretreatment of cells with the protein synthesis inhibitor, cycloheximide (5 micrograms/ml), resulted in a 5-30% inhibition of both chemotactic and haptotactic responses to each matrix protein, indicating that de novo protein synthesis was not required for a significant motility response. Pretreatment of cells with 50-500 micrograms/ml of synthetic peptides containing the fibronectin cell-recognition sequence GRGDS resulted in a concentration-dependent inhibition of fibronectin-mediated chemotaxis and haptotaxis (70-80% inhibition compared to control motility); negative control peptide GRGES had only a minimal effect. Neither GRGDS nor GRGES significantly inhibited motility to laminin or type IV collagen. Therefore, these results support a role for the RGD-directed integrin receptor in both types of motility response to fibronectin. After pretreatment with pertussis toxin (PT), chemotactic responses to laminin, fibronectin, and type IV collagen were distinctly different. Chemotaxis to laminin was intermediate in sensitivity; chemotaxis to fibronectin was completely insensitive; and chemotaxis to type IV collagen was profoundly inhibited by PT. In marked contrast to the inhibition of chemotaxis, the hepatotactic responses to all three ligands were unaffected by any of the tested concentrations of PT. High concentrations of cholera toxin (CT; 10 micrograms/ml) or the cAMP analogue, 8-Br-cAMP (0.5 mM), did not significantly affect chemotactic or haptotactic motility to any of the attractant proteins, ruling out the involvement of cAMP in the biochemical pathway initiating motility in these cells. The sensitivity of chemotaxis induced by laminin and type IV collagen, but not fibronectin, to PT indicates the involvement of a PT-sensitive G protein in transduction of the signals initiating motility to soluble laminin and type IV collagen.(ABSTRACT TRUNCATED AT 400 WORDS)

Autocrine motility factor stimulates a three-fold increase in inositol trisphosphate in human melanoma cells.

The biochemical pathways through which tumor cell locomotion is mediated are poorly understood. Autocrine motility factor (AMF), which is produced by and stimulates motility in A2058 human melanoma cells, was used to characterize phosphoinositide (PtdIns) metabolism activated in association with tumor cell motility. AMF stimulated up to a 400% increase in de novo incorporation of 3H-myo-inositol into cellular lipids beginning 40 minutes after exposure. In cells prelabeled with 3H-myo-inositol, AMF stimulated a 200% increase in total inositol phosphates (inositol monophosphate, InsP1; inositol bisphosphate, InsP2; inositol trisphosphate, InsP3) after 90 minutes of exposure, with a 300% maximal increase in InsP3 at 120 minutes. InsP1 and InsP2 were maximally increased 130% of control values. Treatment with AMF stimulated a parallel dose-dependent increase in both motility and PtdIns levels. We have shown previously that the A2058 motile response to AMF is inhibited markedly by cell pretreatment with pertussis toxin (PT). Inositol phosphate production was inhibited by a 2-hour pretreatment of cells with PT (0.5 microgram/ml). PT treatment of A2058 membranes was associated with ADP-ribosylation of a 40-kDa protein consistent with the presence of an alpha subunit of a guanine nucleotide-binding protein (G protein). These data indicate that AMF elicits increases in cell motility and phosphoinositide metabolism via a PT-sensitive G protein signal transduction pathway.

The type I insulin-like growth factor receptor is a motility receptor in human melanoma cells.

Insulin-like growth factors I and II (IGF-I and II) and insulin are chemotactic agents for the human melanoma cell line A2058. As shown in this report, the motility receptor mediating this response is the heterodimeric type I IGF receptor. These three factors are able to compete with 125I-labeled IGF-I for binding to the cell surface with IC50 values equal to approximately 2 (IGF-I), approximately 150 (IGF-II), and approximately 300 nM (insulin). Cross-linking of 125I-IGF-I to the cell surface with disuccinimidyl suberate followed by analysis with sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography reveals a 130-kDa protein (reduced) consistent with the alpha component of a type I receptor and a 38-kDa protein which does not bind insulin, and thus could be another IGF-I cell surface binding protein. The anti-IGF-I receptor monoclonal antibody (alpha IR-3) also competes with labeled IGF-I in binding experiments. In contrast, a control monoclonal antibody, matched to alpha IR-3 with respect to IgG subclass, has no significant effect on IGF-I binding. While alpha IR-3 inhibits the motility induced by IGF-I, IGF-II, and insulin, pertussis toxin (0.01-1.0 micrograms/ml) has no significant effect on the motility induced by the insulin-like growth factors or insulin on this cell line. Therefore, the type I IGF receptor appears to mediate a highly potent pertussis toxin-insensitive motility response to IGF-I, IGF-II, and insulin. In contrast, motility induced by the autocrine motility factor, a cytokine produced by the A2058 cells, is not affected by alpha IR-3 but is extremely sensitive to pertussis toxin. When mixtures of autocrine motility factor and IGF-I are employed to induce chemotaxis, the resulting motility is greater than that induced by either agent alone. These data indicate that motility in this melanoma cell line can be initiated through multiple receptors that stimulate the cells by separate transduction pathways. This capability to respond to multiple stimuli could enhance the metastatic potential.

Detection of autocrine motility factor in urine as a marker of bladder cancer.

Cell locomotion is an essential requirement for invasion and metastasis of malignant cells. We have previously described the characterization of a 50-kilodalton autocrine motility factor (AMF), a cytokine that stimulates motility in human tumor cells. In this study, we investigated the elaboration of this factor in vivo by human bladder carcinoma and in vitro by a cultured transitional cell carcinoma (TCC) of the bladder cell line T24P. Urine samples from patients with bladder cancer were assayed for their capacity to stimulate migration of tumor cells. Comparing all TCC cases (22 patients) with all nonmalignant diagnoses (27 patients), we found a statistically significant (P less than .001) difference in the motility values. Invasive TCC cases (15 patients) were significantly (P less than .002) higher in regard to motility values compared with noninvasive TCC cases (8 patients), including one case of carcinoma in situ. In follow-up screening studies evaluating TCC recurrence, the recurrent tumors (9 patients) were higher (P less than .001) in regard to motility values than the tumor-free cases (11 patients). Furthermore, T24P cells showed a dose-dependent motile response to their own serum-free conditioned medium as well as to the AMF present in the urine of TCC patients. This finding is consistent with the source of AMF in the urine of these patients being the cancer itself. An enzyme-linked immunosorbent assay (ELISA) for AMF was also developed. Values determined by ELISA correlated well with the motility values measured separately. These data support the potential usefulness of AMF as a urine marker for bladder TCC.

Insulin-like growth factors stimulate chemotaxis in human melanoma cells.

Insulin and insulin-like growth factors stimulate motility in the highly metastatic human melanoma cell line, A2058. Insulin-like growth factor-I (IGF-I) is the most potent with a maximal response at a concentration of 10 nM compared to the activities of insulin and insulin-like growth factor-II (IGF-II) which peak at 300-400 nM. Using checkerboard analysis, the responses to IGF-I and insulin are predominantly chemotactic, although insulin had a significant chemokinetic component. Pertussis toxin does not inhibit the response to any of these polypeptides. However, in previous studies, it was shown that the motile response to autocrine motility factor from these same A2058 cells was markedly inhibited by pertussis toxin. 125I-labelled IGF-I binds saturably and specifically to the A2058 cells. Scatchard analysis indicates a high binding affinity (Kd approximately 3 x 10(-10) M) and an estimated 5000 receptors/cell. These studies indicate that in addition to their mitogenic properties, certain growth factors may profoundly enhance metastasis of tumor cells by their ability to induce motility.