Synergistic effects of epidermal growth factor and thrombin on the growth stimulation of diploid Chinese hamster fibroblasts

Enhanced osteogenic activity of bone morphogenetic protein-2 by 2-O-desulfated heparin

The objective of this study was to investigate the effect of heparin sulfate groups on the osteogenic activity of bone morphogenetic protein-2 (BMP-2) in vitro and in vivo. Three types of desulfated (DS) derivatives of heparin (2-O-DS, 6-O-DS, and N-DS) were prepared and their bioactivity in rat bone marrow derived mesenchymal stem cells (MSC) in the absence or presence of BMP-2 was evaluated. When cultured with the 2-O-DS derivative and BMP-2 MSC showed enhanced proliferation, alkaline phosphatase activity, and Runx2 mRNA expression, compared with heparin and other derivatives. A similar tendency was observed for MSC cultured on two-dimensional substrates coated with heparin or the derivatives and in three-dimensional hydrogels containing heparin or the derivatives. A binding experiment demonstrated a greater binding affinity of 2-O-DS for BMP-2 than that of heparin and the other derivatives. Following implantation into the back subcutis of mice significantly greater ectopic bone formation in terms of bone weight, amount of calcium, and histology were observed for the gelatin hydrogels incorporating 2-O-DS and containing BMP-2. In addition, the gelatin hydrogels incorporating 2-O-DS showed controlled release of BMP-2 in vitro and in vivo. These findings demonstrated that the 2-O-DS derivative of heparin has a synergistic effect on the in vitro and in vivo osteogenic activity of BMP-2.

Transcription initiation and temporal expression of thymidine kinase mRNA in Chinese hamster cells

The induction of thymidine kinase mRNA has proved to be a valuable model for understanding regulatory events at the G1/S boundary of the cell cycle (1, 2, 3). As an initial step toward characterizing the regulation of this gene in Chinese hamster cells, I have mapped the transcription start sites for TK mRNA in CHEF/18 cells. Two closely spaced sites of transcription initiation were detected downstream of a nonconsensus TATAA element in the promoter region. Using primer extension analyses, I demonstrated that the transcription initiation sites remained constant while the absolute levels of TK mRNA varied during the cell cycle in synchronized populations of Chinese hamster cells.

Thrombin is a stimulator of retinal pigment epithelial cell proliferation

Two preparations of bovine thrombin were found to stimulate DNA synthesis in cultured human retinal pigment epithelial cells. DNA synthesis was assessed by both [3H]thymidine incorporation into TCA precipitable material and nuclear labeling with [3H]thymidine. Cultures grown in the presence of thrombin for 48 hr showed a significant increase in cell number. When the concentrations of the two thrombin preparations were normalized for clotting activity, they had almost identical dose-response curves and both caused a tenfold maximal stimulation of [3H]thymidine incorporation. The EC50 for the preparation with higher specific activity was 20 ng ml(-1). Hirudin, a specific high affinity inhibitor of thrombin, completely blocked the mitogenic effect. When a maximally effective concentration of thrombin was used in combination with maximally effective concentrations of other growth factors (insulin, acidic fibroblast growth factor, platelet-derived growth factor, epidermal growth factor), they were found to be strongly synergistic in stimulating DNA synthesis. These data suggest that thrombin may act as an endocrine mediator of retinal pigment epithelial cell proliferation and participate in normal and exaggerated ocular wound healing.

Molecular aspects of mast-cell-mediated mitogenesis in fibroblasts and mesothelial cells in situ

Mast-cell-mediated mitogenesis in intact tissues is a paracrine reaction the molecular mechanisms of which still have to be elucidated. One strategy worth exploring is to study the mitogenic reaction under as defined conditions as possible. The present study demonstrates that in the virtually avascular rat mesentery, organ-cultured in a biochemically-defined medium, activation of mast cells induced a mitogenic reaction in fibroblasts and mesothelial cells, the two predominant, morphologically distinct neighboring cell types. Thus the system provides a means of studying the influence of defined molecules in the growth medium on the outcome of a mitogenic response in these two cell types in situ. It was further observed that exogenous platelet-derived growth factor (PDGF) was not essential for this mast-cell-mediated mitogenic reaction to occur in the tissue-bound fibroblasts and mesothelial cells.

Protein kinase C is differentially regulated by thrombin, insulin, and epidermal growth factor in human mammary tumor cells

The exposure of serum-deprived mammary tumor cells MCF-7 and T-47D to insulin, thrombin, and epidermal growth factor (EGF) resulted in dramatic modifications in the activity and in the translocation capacity of protein kinase C from cytosol to membrane fractions. Insulin induces a 600% activation of the enzyme after 5 h of exposure to the hormone in MCF-7 cells; thrombin either activates (200% in MCF-7) or down-regulates (in T-47D), and EGF exerts only a moderate effect. Thus, the growth factors studied modulate differentially the protein kinase C activity in human mammary tumor cells. The physiological significance of the results obtained are discussed in terms of the growth response elicited by insulin, thrombin, and EGF.

Monoclonal antibody to the thrombin receptor stimulates DNA synthesis in combination with gamma-thrombin or phorbol myristate acetate

Studies with various thrombin derivatives have shown that initiation of cell proliferation by thrombin requires two separate types of signals: one, generated by high affinity interaction of thrombin or DIP-thrombin (alpha-thrombin inactivated at ser 205 of the B chain by diisopropylphosphofluoridate) with receptors and the other, by thrombin's enzymic activity. To further study the role of high affinity thrombin receptors in initiation, we immunized mice with whole human fibroblasts and selected antibodies that blocked the binding of 125I-thrombin to high affinity receptors on hamster fibroblasts. One of these antibodies, TR-9, inhibits from 80 to 100% of 125I-thrombin binding, exhibits an immunofluorescent pattern indistinguishable from that of thrombin bound to receptors on these cells, and selectively binds solubilized thrombin receptors. By itself, TR-9 did not initiate DNA synthesis nor did it block thrombin initiation, but TR-9 addition to cells in the presence of alpha-thrombin, gamma-thrombin (0.5 microgram/ml), or PMA stimulated thymidine incorporation up to threefold over controls. In all cases, maximal stimulation was observed at concentrations of TR-9, ranging from 1 to 4 nM corresponding to concentrations required to inhibit from 30 to 100% of 125I-thrombin binding. These results demonstrate that the binding of the monoclonal antibody to the alpha-thrombin receptor can mimic the effects of thrombin's high affinity interaction with this receptor in stimulating cell proliferation.

Role of thrombin in the proliferative response of T-47D mammary tumor cells. Mitogenic action and pleiotropic modifications induced together with epidermal growth factor and insulin

The growth of the human metastatic cell line (T-47D) in a chemically defined medium (DM) is shown to be dependent on the presence of three traditional growth factors: epidermal growth factor, insulin, and transferrin. The addition of thrombin further stimulates its growth. The mitogenic action on a human mammary tumor cell line from epithelial origin is a novel action of thrombin. Cells in the DM show striking morphological changes which are dramatically enhanced by the addition of thrombin. These observations are part of a pleiotropic response to the growth factors: the protein content of the cells increases in the defined medium; the 2DG gels of the 35S- and 32P-labeled proteins show important changes in spots, several of which are probably of cytoskeletal origin. It is also shown that cells in a semisolid growth factor-supplemented medium have growth advantages over their counterparts grown with serum. All the phenotypic changes mentioned above reveal the important role of growth factors in the growth and behavior of this mammary cell line. The results obtained with thrombin indicate a new site of action of this enzyme which may be important in the metastatic spread of human mammary tumor cells.

Both protein kinase C and calcium mediate activation of the Na+/H+ antiporter in Chinese hamster embryo fibroblasts

Chinese hamster embryo fibroblast cells (CHEF/18) possess a plasma membrane-associated, amiloride-sensitive Na+/H+ antiporter that affects intracellular pH (pHi) and is activated by growth factor addition. Our results using 14C-benzoic acid distribution indicate that both epidermal growth factor (EGF) and thrombin are capable of causing rapid rises in the pHi of CHEF/18 cells. The maximal shift induced by these factors is 0.20 to 0.25 pH units above the basal unstimulated level. Distinctive differences were observed between the modes of action of these two growth factors. Sequential additions revealed that the rise in pHi due to EGF was additive with that caused by diacylglycerols (DAG), while that of thrombin was not. Furthermore, exposure of cells to the phorbol ester PMA for a prolonged period of time in order to down-regulate protein kinase C (pkC), or treatment with the pkC inhibitor H-7, abolished the pHi response to thrombin but not to EGF. In contrast, incubation of cells in nominally calcium-free medium or with the calmodulin antagonists W-7 or trifluoperazine (TFP) decreased only the ability of EGF to cause changes in pHi. These data suggest that there are two distinct mechanisms for activation of the Na+/H+ antiporter in CHEF/18 fibroblast cells and thus provide an example of the use of alternative modes for the modulation of intracellular processes.

Intracellular pH is increased after transformation of Chinese hamster embryo fibroblasts

These studies reveal that a series of tumorigenic Chinese hamster embryo fibroblast (CHEF) cell lines maintain an internal pH (pHi) that is 0.12 +/- 0.04 pH unit above that of the nontumorigenic CHEF/18 parental line. This increase of pHi in the tumorigenic CHEF cells is not due to autocrine growth factor production or to the persistent activation of pathways previously shown to modulate Na+/H+-antiporter activity present in the CHEF/18 line. These findings suggest that the defect in pHi regulation in the tumorigenic CHEF/18 derivatives lies in the Na+/H+ antiporter itself. Further studies to determine the biological significance of an increased pHi show that the external pH (pHo)-dependence curve for initiation of DNA synthesis in the tumorigenic CHEF lines is shifted by approximately 0.2 pH unit toward acidic values relative to that of the nontumorigenic CHEF/18 parent. These data show a critical role for pHi in the regulation of DNA synthesis in Chinese hamster embryo fibroblasts and demonstrate that aberrations in pHi can contribute to the acquisition of altered growth properties.

Thrombin-receptor occupancy initiates a transient increase in cAMP levels in mitogenically responsive hamster (NIL) fibroblasts

Previous studies from our laboratory have shown that thrombin mitogenesis requires both high-affinity receptor occupancy and enzymic activity. Combined addition of DIP-inactivated-thrombin, which retains the ability to bind to thrombin receptors, and enzymically active gamma-thrombin generates a complete set of signals sufficient to initiate cell proliferation. Several possible signals, including stimulation of ion fluxes and phosphoinositide turnover, appear to be stimulated by thrombin's enzymic activity, but not by receptor occupancy. We now report that alpha-thrombin and DIP-thrombin stimulate an early, transient increase of 60 to 200% in intracellular levels of cAMP. This stimulation occurs at low mitogenic concentrations of alpha-thrombin where less than half the receptors are occupied. Enzymically active gamma-thrombin, which stimulates other types of signals, has no stimulatory effects on cAMP. Thus, this effect appears to be generated by high-affinity interaction of thrombin with its cell-surface receptors. Artificially increasing cAMP levels within these cells, however, cannot replace the requirement for thrombin-receptor occupancy in completing the mitogenic stimulation. Therefore, thrombin-receptor occupancy may generate additional, as yet unidentified, required signals.

Phosphoinositides in mitogenesis: neomycin inhibits thrombin-stimulated phosphoinositide turnover and initiation of cell proliferation

Thrombin stimulates 32Pi incorporation into phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bis-phosphate (PIP2), and phosphatidylinositol (PI), and initiates DNA synthesis in hamster (NIL) fibroblasts at a half-maximal concentration of 125 ng/ml. Neomycin, which binds PIP2 and PIP, inhibits both thrombin-stimulated initiation of cell proliferation and 32P pI incorporation into at concentrations above 2 mM without affecting thrombin binding, thymidine uptake, or cellular protein synthesis. At lower concentrations, neomycin inhibits thrombin-stimulated release of inositol 1,4,5-trisphosphate (IP3), by selectively binding PIP2, but does not inhibit 32P incorporation into PI or initiation of DNA synthesis. Phosphoinositide recycling and diacylglycerol release therefore appear necessary for initiation of cell proliferation by thrombin. IP3-stimulated Ca++ mobilization may not be required for thrombin mitogenesis, however, since neomycin can block IP3 release without inhibiting initiation.

Regeneration and the mechanism of epidermal tumor promotion

Chemically induced epidermal carcinogenesis is often divided into two stages: initiation, which involves the conversion of some epidermal cells into latent neoplastic cells, and promotion, which allows the evolution of this neoplastic change into the formation of a neoplasm. The hallmark of epidermal tumor promotion is the transformation of the normal epidermis into a hyperplastic epidermis. A major unanswered question about epidermal tumor promotion is whether the epidermal hyperplasia that characterizes promoted skin is a regenerative epidermal hyperplasia resulting from damage produced by the promoter. The opinion currently held is that the epidermal hyperplasia produced by tumor promoters is not simply a regenerative epidermal hyperplasia and possesses characteristics which a regenerative hyperplasia does not have, enabling it to evolve into an epidermal neoplasm. The purpose of this review is to present recent evidence which strongly suggests that promoter-induced epidermal hyperplasia is a regenerative hyperplasia. Three principal lines of evidence are reviewed. The first demonstrates that an epidermal regenerative hyperplasia repeatedly produced by wounding or abrasion can promote epidermal carcinogenesis in the initiated skin of mice. The second line of evidence demonstrates that the epidermal hyperplasia produced by the application of 12-O-tetradecanoyl-phorbol-13-acetate (TPA), the most powerful and widely used promoter of skin carcinogenesis, is preceded by damage to the epidermis. This strongly suggests that the epidermal hyperplasia which ensues is a regenerative hyperplasia. Thirdly, evidence is presented which demonstrates that hyperplasia-producing agents which do not promote, produce an epidermal hyperplasia which is different from that produced by tumor promoters. Finally, the review discusses the evidence which suggests that the production of a hyperplasia may be the mechanism for tumor promotion in other organs, such as the liver, bladder, and intestine.

Anchorage-independent growth of normal human mesothelial cells: a sensitive bioassay for EGF which discloses the absence of this factor in fetal calf serum

This laboratory recently reported that normal human mesothelial cells require epidermal growth factor (EGF) and hydrocortisone (HC), in addition to fetal calf serum and a complex defined medium component, in order to grow optimally in surface culture. We report here that this normal cell type also forms large colonies at high efficiency in semi-solid medium, but exhibits more stringent serum and EGF requirements for anchorage-independent than for surface growth. Mesothelial cells are unable to divide at all in semi-solid medium without added EGF or with less than 2% serum, whereas they grow slowly but progressively in surface culture under such conditions. In semi-solid medium containing 20% serum and HC, mesothelial cells are stimulated to divide by the addition of as little as 30 pg/ml purified EGF. Human urine or male mouse plasma could substitute for purified EGF, yielding growth commensurate with the levels of EGF in these biological fluids previously measured by others using radioreceptor and radioimmune assays. Thus growth of mesothelial cells in semi-solid medium can serve as a highly sensitive assay of EGF biological activity which is unaffected by the presence of serum proteins. In addition, our results demonstrate that fetal calf serum does not provide mitogenic levels of EGF to cultured cells, raising the question of the identity of plasma and serum mitogens.

Glycosaminoglycans and the control of cell surface proteinase activity

It is postulated that the metabolically variable fine structure of pericellular heparan glycosaminoglycans affects the ability of these molecules to influence cell proliferation-associated proteinase-catalysed reactions occurring at cell surfaces. Evidence suggesting the possibility of a wide repertoire of glycosaminoglycan-mediated positive and negative effects on such reactions is reviewed. It is suggested that clinical administration of compounds related chemically to heparins might usefully modulate cell proliferation-associated proteinase activity.

Effects of epidermal growth factor on osteoblastic cells in vitro

The effect of epidermal growth factor (EGF) on clone MC3T3-El cells that have osteoblastic activity was examined by phase-contrast microscopy and electron microscopy; hydroxyproline content, collagen synthesis, collagen pattern, and alkaline phosphatase (ALP) activity were also determined. We found that EGF (0.4 ng/ml) transformed the cells from their normal polygonal shape to a spindle-like morphology by 8 h. This hormone also caused dose-related suppression of hydroxyproline content and ALP activity which was detectable 2 days and 1 day, respectively, after EGF addition. Indomethacin did not affect hydroxyproline content and ALP activity, suggesting that the effect of EGF on the cells may not be mediated by prostaglandins. Epidermal growth factor at concentrations of 2 to 50 ng/ml significantly decreased collagen synthesis in the cells, whereas protein synthesis was stimulated. Electron microscopy demonstrated that collagen fiber formation was also reduced by EGF; an immature type of fibril was observed compared with the typical cross-striated one in the controls. Moreover, the hormone treatment also resulted in the appearance of type III collagen in addition to the type I already present in the cells. These suppressive effects of EGF on MC3T3-El cells in vitro suggest that this hormone may be involved in bone remodelling in vivo as well.

Analysis of growth factor "relaxation" in Chinese hamster lung fibroblasts required for tumoral expression

The Chinese hamster lung fibroblast line, CCl39, displays the properties characteristic of normal secondary cultures of Chinese hamster fibroblasts including: reversible G0 growth arrest (less than 2% labeled nuclei), anchorage dependence, and high serum-growth factor dependence. Injection of CCl39 cells, or anchorage-independent variants, in nude mice leads to tumor formation; however, as we have previously shown (Pérez-Rodriguez et al., 1981b), the resulting tumor clones no longer possess the high serum dependence of injected CCl39 cells. Hormonal growth restraints imposed by the host create an in vivo selection for diminished, or "relaxed," growth factor requirement. To characterize this growth factor "relaxation" further, we have analyzed the mitogenic response of parental CCl39 cells, anchorage-independent clones, and selected tumoral derivatives, to purified growth factors. Two highly purified growth factors, thrombin and insulin, together fulfill the growth factor requirements of CCl39 cells; thrombin (1 U/ml) stimulates the reinitiation of DNA synthesis in G0-arrested CCl39 cells, and insulin (10 micrograms/ml) maximally potentiates this stimulation to the level obtained with 10% fetal calf serum. First, we found no correlation between loss of anchorage dependence and growth factor relaxation. Second, we found that A71 (anchorage independent), a tumoral variant of CCl39 capable of growth arrest, and tumor-derived cells all display an increased sensitivity to thrombin and a diminished requirement for the potentiating action of insulin. Examination of thrombin binding to CCl39, A51 (nontumoral, anchorage independent), and A71 cells revealed that the increased sensitivity to thrombin of A71 cells is not attributable to an alteration in thrombin cell surface receptor number or affinity for thrombin. Rather, under standard conditions of serum or growth factor removal (30 hr), A71 cells maintain a metabolically elevated growth-arrested state, different from that of their nontumoral counterparts. Consequently, much lower concentrations of growth factors are needed to induce a proliferative response in these tumoral cells.

Cells regulate their mitogenic response to thrombin through release of protease nexin

We previously reported that human and mouse fibroblast-like cells release into their growth medium a protein that we termed protease nexin. Protease nexin forms a covalent acyl linkage with thrombin and certain other serine proteases via the protease active site and mediates their binding, internalization and degradation by cells. Binding of thrombin-protease nexin to cells is mediated by the protease nexin portion of the complex to a high-affinity cellular binding site. As thrombin is a potent mitogen for a variety of fibroblast-like cells in culture, we examined whether protease nexin itself regulates thrombin-stimulated cell division. Recently, we showed that heparin virtually blocked the binding of thrombin-protease nexin complexes to both mouse and human cells without affecting the ability of these cells to respond to thrombin. Thus, protease nexin does not appear to be a positive modulator in thrombin-induced cell division. Here, we show that protease nexin negatively regulates the mitogenic response of cells in culture to thrombin.

Cleavage of cell surface proteins by thrombin

This study was based on our previous findings that the mitogenic action of thrombin on cultured fibroblasts can result from interaction of thrombin with the cell surface in the absence of internalization, and that the proteolytic activity of thrombin is required for stimulation of cell division. This prompted us to look for thrombin-mediated cleavages using 2-dimensional gel electrophoresis of labeled cell surface proteins. Surface membrane components were labeled by 3 procedures: 1) proteins were labeled by lactoperoxidase-catalyzed iodination using 125I-; 2) galactose and galactosamine residues of glycoproteins were oxidized with galactose oxidase and reduced with 3H-NaBH4; and 3) glycoproteins were metabolically labeled by incubating cells with 3H-fucose. labeling with the first 2 procedures was carried out after thrombin treatment; in contrast, cells metabolically labeled with 3H-fucose were subsequently treated with thrombin to look for proteolytic cleavages. Collectively, these studies indicated that only about 5 cell surface proteins were thrombin-sensitive, consistent with the high specificity of this protease. Each of the labeling procedures revealed a thrombin-sensitive cell surface glycoprotein which was identified as fibronectin by immunoprecipitation experiments. In addition, cell surface proteins of about 140K and 55K daltons were thrombin-sensitive. However, cell surface proteins of about 45K daltons and 130K to 150K daltons were increased after thrombin treatment. These experiments were conducted on an established line of Chinese hamster lung cells with the eventual goal of studying thrombin-mediated cleavages of cell surface proteins in a large number or in cloned populations derived from this line that are either responsive or unresponsive to the mitogenic action of thrombin. This approach should permit identification of proteolytic cleavages tha are necessary for thrombin-stimulated cell division.