Inhibition of DNA synthesis in cultures of 3T3 cells by isolated surface membranes

Cadherins and catenins in cancer: connecting cancer pathways and tumor microenvironment

Cadherin-catenin complexes are integral components of the adherens junctions crucial for cell-cell adhesion and tissue homeostasis. Dysregulation of these complexes is linked to cancer development via alteration of cell-autonomous oncogenic signaling pathways and extrinsic tumor microenvironment. Advances in multiomics have uncovered key signaling events in multiple cancer types, creating a need for a better understanding of the crosstalk between cadherin-catenin complexes and oncogenic pathways. In this review, we focus on the biological functions of classical cadherins and associated catenins, describe how their dysregulation influences major cancer pathways, and discuss feedback regulation mechanisms between cadherin complexes and cellular signaling. We discuss evidence of cross regulation in the following contexts: Hippo-Yap/Taz and receptor tyrosine kinase signaling, key pathways involved in cell proliferation and growth; Wnt, Notch, and hedgehog signaling, key developmental pathways involved in human cancer; as well as TGFβ and the epithelial-to-mesenchymal transition program, an important process for cancer cell plasticity. Moreover, we briefly explore the role of cadherins and catenins in mechanotransduction and the immune tumor microenvironment.

E-cadherin in contact inhibition and cancer

E-cadherin is a key component of the adherens junctions that are integral in cell adhesion and maintaining epithelial phenotype of cells. Homophilic E-cadherin binding between cells is important in mediating contact inhibition of proliferation when cells reach confluence. Loss of E-cadherin expression results in loss of contact inhibition and is associated with increased cell motility and advanced stages of cancer. In this review we discuss the role of E-cadherin and its downstream signaling in regulation of contact inhibition and the development and progression of cancer.

The sphingolipid receptor S1PR2 is a receptor for Nogo-a repressing synaptic plasticity

This study identifies a GPCR, S1PR2, as a receptor for the Nogo-A-Δ20 domain of the membrane protein Nogo-A, which inhibits neuronal growth and synaptic plasticity.

Nogo-A is a membrane protein of the central nervous system (CNS) restricting neurite growth and synaptic plasticity via two extracellular domains: Nogo-66 and Nogo-A-Δ20. Receptors transducing Nogo-A-Δ20 signaling remained elusive so far. Here we identify the G protein-coupled receptor (GPCR) sphingosine 1-phosphate receptor 2 (S1PR2) as a Nogo-A-Δ20-specific receptor. Nogo-A-Δ20 binds S1PR2 on sites distinct from the pocket of the sphingolipid sphingosine 1-phosphate (S1P) and signals via the G protein G₁₃, the Rho GEF LARG, and RhoA. Deleting or blocking S1PR2 counteracts Nogo-A-Δ20- and myelin-mediated inhibition of neurite outgrowth and cell spreading. Blockade of S1PR2 strongly enhances long-term potentiation (LTP) in the hippocampus of wild-type but not Nogo-A^−/− mice, indicating a repressor function of the Nogo-A/S1PR2 axis in synaptic plasticity. A similar increase in LTP was also observed in the motor cortex after S1PR2 blockade. We propose a novel signaling model in which a GPCR functions as a receptor for two structurally unrelated ligands, a membrane protein and a sphingolipid. Elucidating Nogo-A/S1PR2 signaling platforms will provide new insights into regulation of synaptic plasticity.

Recent studies have demonstrated an important role of Nogo-A signaling in the repression of structural and synaptic plasticity in mature neuronal networks of the central nervous system. These insights extended our understanding of Nogo-A's inhibitory function far beyond its well-studied role as axonal-growth inhibitor. Repression is mediated via two different Nogo-A extracellular domains: Nogo-66 and Nogo-A-Δ20. Here, we identify the G-protein coupled receptor S1PR2 as a high-affinity receptor for Nogo-A-Δ20 and demonstrate that S1PR2 binds this domain with sites different from the recently proposed S1P binding pocket. Interfering with S1PR2 activity, either pharmacologically or genetically, prevented Nogo-A-Δ20-mediated inhibitory effects. Similar results were obtained when we blocked G₁₃, LARG, and RhoA, components of the downstream signaling pathway. These findings revealed a strong increase in hippocampal and cortical synaptic plasticity when acutely interfering with Nogo-A/S1PR2 signaling, similar to previous results obtained by blocking Nogo-A. We thus provide a novel biological concept of multi-ligand GPCR signaling in which this sphingolipid-activated GPCR is also bound and activated by the high molecular weight membrane protein Nogo-A.

Ordered heterogeneity and its decline in cancer and aging

Ordered heterogeneity was introduced as a basic feature of the living state in the mid-1950s. It was later expanded to "order in the large over heterogeneity in the small" as the first principle of a theory of organisms. Several examples of ordered heterogeneity were given at the time to illustrate the principle, but many more have become apparent since then to confirm its generality. They include minimum size requirements for progressive embryological development, the errant behavior of cells liberated from tissue architecture, their sorting out to reconstitute tissues on reaggregation, and contact regulation of cell proliferation. There is increasing heterogeneity of cell growth with age, and marked heterogeneity of many characters among cells of solid epithelial tumors. Normal growth behavior is reintroduced in solitary, carcinogen-initiated epidermal cells by contact with an excess of normal epidermal cells. Contact normalization also occurs when solitary hepatocarcinoma cells are transplanted into the parenchyma of normal liver of young, but not of old, animals. The role of the plasma membrane and adhesion molecules in ordering heterogeneity is evaluated. Organizing the results in a conceptual structure helps to understand classical observations of tumor biology such as the lifetime quiescence of carcinogen-initiated epidermal cells and the marked increase of cancer incidence with age. The principle of order above heterogeneity thus provides a unifying framework for a variety of seemingly unrelated processes in normal and neoplastic development. Whereas contact between cells is required for these processes to occur, gap junctional communication is not required.

The NF2 tumor suppressor gene product, merlin, mediates contact inhibition of growth through interactions with CD44

The neurofibromatosis-2 (NF2) gene encodes merlin, an ezrin-radixin-moesin-(ERM)-related protein that functions as a tumor suppressor. We found that merlin mediates contact inhibition of growth through signals from the extracellular matrix. At high cell density, merlin becomes hypo-phosphorylated and inhibits cell growth in response to hyaluronate (HA), a mucopolysaccharide that surrounds cells. Merlin's growth-inhibitory activity depends on specific interaction with the cytoplasmic tail of CD44, a transmembrane HA receptor. At low cell density, merlin is phosphorylated, growth permissive, and exists in a complex with ezrin, moesin, and CD44. These data indicate that merlin and CD44 form a molecular switch that specifies cell growth arrest or proliferation.

Insulin-like growth factor binding protein-4 accumulation is negatively correlated with growth rate in TM-3 cells

Cellular growth is controlled by multiple regulators, including the insulin-like growth factors (IGFs). In some cells, the IGF binding proteins (IGFBPs) are thought to be inhibitory molecules for cell growth and may be related to the process of contact inhibition. In the TM-3 (mouse Leydig) cell line, IGFBP-4 is the major IGFBP secreted into conditioned media (CM), as we have reported. In this study, we investigated cell growth, the peptide levels of IGFBP-4 in CM, and the inverse relationship between IGFBP-4 accumulation and cell growth rate. Quantification of TM-3 growth in serum-containing media demonstrated that TM-3 cell number gradually rose after plating, and plateaued when cells became confluent. The rate of cell growth fell gradually, and net cell growth stopped when cells reached confluency. IGFBP-4 peptide levels in CM, as measured by Western ligand blot, rose gradually during the culture period and plateaued when cells reached confluency. The amount of IGFBP-4 peptide level in CM correlated for cell number (IGFBP-4 accumulation rate) also rose gradually during the course of culture and plateaued. The IGFBP-4 accumulation rate was strongly negatively correlated with the rate of cell growth (r = 0.98, P < 0.001). In conclusion, our data suggest that in TM-3 cells, cell growth is related to IGFBP-4 accumulation. The negative correlation between IGFBP-4 accumulation and the rate of cell growth suggests that IGFBP-4 may be a primary regulator of TM-3 cell growth and possibly participate in the process of contact inhibition.

Evidence for a high affinity, saturable, prenylation-dependent p21Ha-ras binding site in plasma membranes

Oncogenic p21ras proteins can only exert their stimulation of cellular proliferation when plasma membrane-associated. This membrane association has an absolute requirement for post-translational modification with isoprenoids. The mechanism by which isoprenoids participate in the specific association of p21ras with plasma membranes is the subject of this report. We present in vitro evidence for a plasma membrane binding protein for p21(ras) that can recognize the isoprenoid substituent and, therefore, may facilitate the localization of p21ras.

TGF-beta 1 stimulates cultured human fibroblasts to proliferate and produce tissue-like fibroplasia: a fibronectin matrix-dependent event

During wound repair, fibroblasts accumulate in the injured area until any defect is filled with stratified layers of cells and matrix. Such fibroplasia also occurs in many fibrotic disorders. Transforming growth factor-beta (TGF-beta), a promotor of granulation tissue in vivo and extracellular matrix production in vitro, is expressed during the active fibroplasia of wound healing and fibroproliferative diseases. Under usual tissue culture conditions, normal fibroblasts grow to confluence and then cease proliferation. In this study, culture conditions with TGF-beta 1 have been delineated that promote human fibroblasts to grow in stratified layers mimicking in vivo fibroplasia. When medium supplemented with serum, ascorbate, proline, and TGF-beta was added thrice weekly to normal human dermal fibroblasts, the cells proliferated and stratified up to 16 cell layers thick within the culture dish, producing a tissue-like fibroplasia. TGF-beta stimulated both DNA synthesis as measured by 3H-thymidine uptake and cell proliferation as measured by a Hoechst dye DNA assay in these postconfluent cultures. The stratification was dependent on fibronectin assembly, as demonstrated by anti-fibronectin antibodies which inhibited both basal and TGF-beta-stimulated cell proliferation and stratification. Suppression of collagen matrix assembly in cell layers with beta-amino-proprionitrile (BAPN) did not inhibit basal or TGF-beta stimulated in vitro fibroplasia. BAPN did not interfere with fibronectin matrix assembly as judged by immunofluorescence microscopy. Thus, in concert with serum factors, TGF-beta stimulates postconfluent, fibronectin matrix-dependent, fibroblast growth creating a fibroplasia-like tissue in vitro.

Factors affecting bovine corneal endothelial cell density in vitro

AIMS

To examine factors influencing the density and contact inhibition of bovine corneal endothelial cells cultured in vitro.

METHODS

Cell counts were performed on bovine corneal endothelial cells cultured for various times in the presence of 10% fetal calf serum, with or without varying concentrations of growth factors, 5% dextran T-500, or 2% chondroitin sulphate, at 32 degrees C or 37 degrees C, and after treatment with beta galactosidase.

RESULTS

Both basic fibroblast growth factor (FGFb) and retinal crude extract (RCE), but neither epidermal growth factor (EGF) nor acidic fibroblast growth factor (FGFa), increased endothelial cell density in vitro (p < 0.05). Continuous exposure to RCE resulted in a higher cell density than did a 24 hour pulse (p < 0.01), and higher cell densities were achieved at 37 degrees C than at 32 degrees C (p < 0.0001). In the absence of RCE, dextran T-500 increased cell density modestly (p < 0.05); in the presence of RCE, the addition of dextran T-500 had no effect on final cell density, whereas chondroitin sulphate significantly decreased final cell density (p < 0.01). In the absence of exogenous growth factors, beta galactosidase treatment resulted in a 50% increase in final cell density compared with controls (p < 0.0001).

CONCLUSIONS

Bovine corneal endothelial cell growth can be augmented under conditions different from those used in corneal preservation systems. The final cell density in a confluent monolayer can be increased by treatment with beta galactosidase, suggesting that corneal endothelial cells may be contact inhibited through a beta galactosidase sensitive receptor system.

Cell-contact mediated modulation of the sialylation of contactinhibin

Contactinhibin was found to be involved in contact-dependent inhibition of growth. The growth inhibitory activity of contactinhibin is mediated by N-linked oligosaccharides with desialylated beta-glycosidically linked, terminal galactose residues. Here we show that in sparse human fibroblasts contactinhibin was expressed in a biologically inactive, highly sialylated form both on the plasma membrane and intracellularly, while in confluent cells plasma membrane localized contactinhibin was present in a biologically active, low sialylated form. Plasma membranes were shown to contain a glycoprotein sialidase which is suggested to be engaged in the activation of contactinhibin in a cell contact-dependent manner.

Purification and stability characterization of a cell regulatory sialoglycopeptide inhibitor

Previous attempts to physically separate the cell cycle inhibitory and protease activities in preparations of a purified cell regulatory sialoglycopeptide (CeReS) inhibitor were largely unsuccessful. Gradient elution of the inhibitor preparation from a DEAE HPLC column separated the cell growth inhibitor from the protease, and the two activities have been shown to be distinct and non-overlapping. The additional purification increased the specific biological activity of the CeReS preparation by approximately two-fold. The major inhibitory fraction that eluted from the DEAE column was further analyzed by tricine-SDS-PAGE and microbore reverse phase HPLC and shown to be homogeneous in nature. Two other fractions separated by DEAE HPLC, also devoid of protease activity, were shown to be inhibitory to cell proliferation and most likely represented modified relatives of the CeReS inhibitor. The highly purified CeReS was chemically characterized for amino acid and carbohydrate composition and the role of the carbohydrate in cell proliferation inhibition, stability, and protease resistance was assessed.

The identification of a naturally occurring cell surface growth inhibitor related to a previously described bovine sialoglycopeptide

A 66-kDa sialoglycoprotein has been identified as the parental membrane molecule of an earlier described sialoglycopeptide (SGP), an 18-kDa molecule released by protease treatment of intact bovine cerebral cortex cells that was shown to be a potent inhibitor of cellular proliferation. The 66-kDa parental sialoglycoprotein (p-SGP) was purified approximately 2,400-fold, to apparent homogeneity, from bovine cerebral cortex cell membranes by its release during incubation with 3 M NaCl, preparative isoelectric focusing and lectin affinity chromatography. Although a membrane-associated molecule, the p-SGP appeared to be tightly bound to the cell membrane, since it was not released during incubations in the absence of 3 M NaCl. Incubation of the membrane preparations with 3 M urea proved to be too harsh, and the antigenicity required to follow the purification of the p-SGP was abolished. Analyses by SDS-PAGE, under reducing and nonreducing conditions, suggested that the p-SGP membrane component was a single polypeptide without subunit structure. The p-SGP was shown to be structurally related to the SGP fragment by immunoblots with IgG raised to the SGP inhibitor, and functionally related to the SGP by its ability to inhibit Swiss 3T3 proliferation at concentrations strikingly similar to that previous measured with the SGP fragment.

Neural cell adhesion molecule mediates contact-dependent inhibition of growth of near-diploid mouse fibroblast cell line m5S/1M

A near-diploid mouse fibroblast cell line m5S/1M used in this study shows a high sensitivity to contact-dependent inhibition of growth, and the addition of EGF causes both morphological change and loss of contact-dependent inhibition of growth. The m5S/1M cell and its transformants obtained by x-ray irradiation have been used to search for the cell surface glycoproteins that are responsible for the growth regulation via cell-cell interactions. Lectin blotting analyses showed that the expression of the cell surface glycoprotein of 140 kD (140KGP) is highly sensitive to the transformation induced either by x-ray irradiation or by the EGF stimulation. We purified the 140KGP and found that it was composed of two glycoproteins. The major component of 140KGP was identified as neural cell adhesion molecule (NCAM) by amino acid sequence analyses of the peptide fragments and by the cross-reactivity with anti-NCAM mAb, clone H28.1.2.3. Monoclonal antibody against 140KGP (clone LN-10) recognizes all three isoforms of NCAM expressed on m5S/1M cell and showed that the expression of NCAM was highly sensitive to the transformation. Furthermore, the immobilized LN-10 strongly inhibited the growth of actively proliferating m5S/1M cells and the LN-10 in a soluble form showed a significant growth-stimulating effect on the confluent quiescent cultures of m5S/1M cells. The results show that NCAM plays a major role in the contact-dependent inhibition of growth of m5S/1M, and that NCAM might be involved in the regulation of cell growth during embryogenesis and formation of nervous systems.

Modulation of mesangial cell proliferation by endothelial cells in coculture

The effects of direct cell contact between endothelial (ECs) and mesangial cells (MCs) on MCs proliferation were examined in a coculture system in vitro. Mitomycin C treated ECs (M-ECs) were plated on culture dishes and MCs were cocultured with these M-ECs. Cell number was measured at the end of days 1, 3, and 5. In the coculture system with direct contact, the growth of cocultured MCs was modulated as follows: 1) the growth of MCs was inhibited up to day 3, and 2) a high level of proliferation was observed between days 3 and 5. This biphasic pattern of growth could not be detected in coculture of fibroblasts with MCs. In coculture without direct contact, using intercup chambers, the kinetics in cell proliferation between cocultured MCs and MCs alone were essentially the same. Conditioned media derived from cocultures up to day 3 in a contact-dependent manner inhibited the 3H-thymidine uptake of MCs. From these results, it would thus appear that MCs proliferation is regulated by intercellular contact with ECs.

Isolation and characterization of a 60-70-kD plasma membrane glycoprotein involved in the contact-dependent inhibition of growth

Previous studies have shown that plasma membrane compounds are involved in the contact-dependent inhibition of growth of human diploid fibroblasts. The purification of the active plasma membrane glycoprotein is described in this report. The glycoprotein has an apparent molecular mass of 60-70 kD and, due to differential sialylation, isoelectric points between pH 5.5. and 6.2. Treatment with sialidase yielded one spot in two-dimensional gel electrophoresis with an isoelectric point of 6.3. After removal of the N-glycosidically linked oligosaccharide chains, the apparent molecular mass is reduced by approximately 22 kD. Treatment was diluted NaOH, which removes the O-glycosidically linked portion of oligosaccharides, resulted in a reduction of the apparent molecular mass by approximately 5 kD. The addition of 50 ng/ml of this glycoprotein-for which the term "contactinhibin" is proposed-in immobilized form to sparsely seeded human fibroblasts resulted in a reversible 70-80% inhibition of growth. The inhibition was not confined to human fibroblasts as other cells were also inhibited, with the exclusion of transformed cells, which are refractory to contactinhibin. The inhibitory activity was abolished by treatment with beta-galactosidase or glycopeptidase F, indicating that the glycan moiety is the biologically active part of the molecule. Confluent cultures treated with antibodies raised against contactinhibin were released from the contact-dependent inhibition of growth. In addition to enhanced saturation density, these cultures exhibited a crisscross growth pattern and the formation of foci. Immunocytochemical studies showed that contactinhibin was associated with vimentin. Furthermore, contactinhibin was found to be not expressed in a species- or organ-specific manner.

Growth control in mammalian cells by cell-cell contacts

Growth of normal diploid mammalian cells in vitro is strongly regulated by the actual cell density. Cell-cell contacts via specific plasma membrane glycoproteins whose glycan moieties interact with specific receptors has been found to be a main growth regulatory principle. Malignant growth is suggested to result from impaired function of these receptors.

Influence of the carbohydrate moiety on the growth inhibitory activity and adhesiveness of 3T3 cell plasma membranes

Treatment of 3T3 cell plasma membranes with glycosidase enzymes decreased their ability to inhibit cell growth and also decreased their binding to 3T3 cells. This suggests that carbohydrate is required for complete function of inhibitory activity and that inhibition is associated with membrane adhesion.

Neuronal inhibition of astroglial cell proliferation is membrane mediated

Previously we have used a microwell tissue culture assay to show that early postnatal mouse cerebellar astroglia have a flattened morphology and proliferate rapidly when they are cultured in the absence of neurons, but develop specific cell-cell contacts and undergo morphological differentiation when they are co-cultured with purified granule neurons (Hatten, M. E., 1985, J. Cell Biol., 100:384-396). In these studies of cell binding between neurons and astroglia, measurement with light and fluorescence microscopy or with [35S]methionine-labeled cells indicated that the kinetics of the binding of the neurons to astroglial cells are rapid, occurring within 10 min of the addition of the neurons to the growing glia. 6 h after neuronal attachment, astroglial DNA synthesis decreases, as shown by a two- to fivefold decrease in [3H]thymidine incorporation, and glial growth ceases. No effects on astroglial cell growth were seen after adding medium conditioned by purified cerebellar neurons cultured in the absence of astroglia, by astroglia cultured in the absence of neurons, or by a mixed population of cerebellar cells. This result was unchanged when any of these media were concentrated up to 50-fold, or when neurons and astroglia were cultured in separate chambers with confluent medium. Two groups of experiments suggest that membrane-membrane interactions between granule neurons and astroglia control astroglial cell growth. First, neurons fixed with dilute amounts of paraformaldehyde (0.5%) bound to the astroglia with the same kinetics as did living cells, inhibited DNA synthesis, and arrested glial growth within hours. Second, a cell membrane preparation of highly purified granule neurons also bound rapidly to the glia, decreased [3H]thymidine incorporation two- to fivefold and inhibited astroglial cell growth. The rate of the decrease in glial growth depended on the concentration of the granule neural membrane preparation added. A similar membrane preparation from purified cerebellar astroglial cells, PC12 cells, 3T3 mouse fibroblasts, or PTK rat epithelial cells did not decrease astroglial cell growth rates. Living neurons were the only preparation that both inhibited glial DNA synthesis and induced the astroglial cells to transform from the flat, epithelial shapes they have when they are cultured without neurons to highly differentiated forms that resemble Bergmann glia or astrocytes seen in vivo. These results suggest that membrane-membrane interactions between neurons and astroglia inhibit astroglial proliferation in vitro, and raise the possibility that membrane elements involved in glial growth regulation include neuron-glial interaction molecules.

Response of osteoblastic clonal cell line (MC3T3-E1) to [Asu]eel calcitonin at a specific cell density or differentiation stage

Clone MC3T3-E1 cells isolated from newborn mouse calvaria is an osteogenic cell line which retains an ability to differentiate into osteoblastic cell in vitro. The effect of [Asu]eel calcitonin (ECT) on clonal MC3T3-E1 cells was investigated at different stages of differentiation. ECT caused an increase in alkaline phosphatase (ALP) activity. The stimulative effect was demonstrated to be dependent upon cell density or differentiation stage. At a cell density of 1.18 X 10(5)/cm2 cells were incubated with ECT for 2 days. The treatment by ECT caused an increase in ALP activity. A specific response to ECT dependent on the cell density was observed in a narrow range of cell density. Moreover this range of cell density responsible to ECT was found to be a rapid differentiation stage of MC3T3-E1 cells. These results suggest that calcitonin stimulates differentiation of osteoblast. In addition to these results, cellular adenosine-3',5'-cyclic monophosphate (cAMP) level was raised by ECT treatment at a cell density of about 1.4 X 10(5) cell/cm2 and this response was also specific for cell density. At cell density lower or higher than this density no stimulative effect by ECT was observed. On the other hand, N6,O2-dibutyryl adenosine-3',5'-cyclic monophosphate (db-cAMP) and theophylline caused an increase in ALP activity in wide cell density range. These results indicate that an increase in ALP activity by ECT is mediated by intracellular cAMP and that the specific response to ECT dependent on the cell density is regulated in the process of cAMP formation and/or in the preceding process of cAMP formation.

Modulation by the src oncogene of the effect of inhibitory diffusible factor IDF45

Density-dependent inhibition of growth has been assumed to be under the control of inhibitory molecules diffusing from dense cell cultures. Growth inhibitory factors have been fractionated or purified from medium conditioned by different cell types. In the present work, it was shown that IDF45 (inhibitory factor diffusing from 3T3 cells) decreased DNA synthesis in chick embryo fibroblasts (CEF) and was an inhibitor of CEF growth; this inhibition was reversible. Since similitudes between oncogene products and growth factors have been observed, it was of interest to compare the inhibitory effect of IDF45 upon the stimulation of DNA synthesis induced either by serum or by pp60-src. CEF infected by Ny68 virus (a mutant of Rous sarcoma virus ts for the expression of transformation) were density-inhibited at 41 degrees C, but were stimulated at this temperature by addition of 1% serum. This stimulation was 94% inhibited by IDF45. The same Ny68-infected cells could also be stimulated by transfer to 37 degrees C, the permissive temperature (in the absence of serum). The stimulation of DNA synthesis by src expression was poorly inhibited by IDF45. From our results, it appears that oncogene expression in CEF induces a loss in their sensitivity to IDF45. This would explain why transformed cells escape DDI of growth.

Membrane-associated inhibitor of DNA synthesis in senescent human diploid fibroblasts: characterization and comparison to quiescent cell inhibitor

Cell membranes prepared from senescent human diploid fibroblasts (HDF) inhibited entry into S phase by 35% when added to the medium of replicating young HDF. This membrane-associated inhibitory activity was (i) sensitive to trypsin, heat, and periodate, which suggests that the inhibitor is a glycoprotein, and (ii) not able to inhibit DNA synthesis in simian virus 40-transformed HDF, which indicates that not all types of cells are sensitive to this inhibitor. Quiescent young HDF also have a surface membrane-associated inhibitor of DNA synthesis. A comparison of the senescent HDF and quiescent HDF inhibitor activities indicates that they may have the same chemical and physical nature and the same specific activity, but their regulation is different. The inhibitory activity of quiescent young HDF is abolished within 20 hr after refeeding with fresh serum-containing medium, whereas that of senescent HDF remains unchanged. Quiescent old HDF (two or three population doublings remaining) exhibit an intermediate response to serum with approximately two-thirds of the inhibitory activity abolished. The fraction of cells in S phase at 20-24 hr post-stimulation (37% in young HDF, 24% in old HDF, and 0% in senescent HDF) is inversely proportional to inhibitor levels. This suggests that inability to neutralize the inhibitory activity in response to serum stimulation could be involved in the inability of senescent HDF to enter S phase. Disappearance of the inhibitory activity from quiescent young HDF occurs late in G1 phase. Thus, the inhibitor may play a role in determining the length of the G0 to S phase transition in these cells.

Specific inhibition of endothelial cell proliferation by isolated endothelial plasma membranes

Cultures of human vascular endothelial cells were used to study the phenomenon of density-dependent inhibition of cell growth. Endothelial cells were disrupted by nitrogen cavitation, and a plasma membrane-enriched fraction was prepared by differential centrifugation followed in some cases by sucrose density gradient fractionation. Membrane suspension was added to low-density early-passage endothelial cultures grown in microwells. Hemocytometer cell counts and 6 hr 3H-thymidine pulses were performed in triplicate wells at varying intervals. Plasma membranes suppressed cell proliferation in a reversible, dose-dependent fashion. Increasing the ambient concentration of endothelial cell growth factor did not alter the inhibitory effect. The antiproliferative effect was sensitive to heat and trypsin and to incubation with 0.1 M sodium carbonate, pH 11.5. Membrane vesicles selectively derived from the apical cell surface also suppressed proliferation. This phenomenon showed at least some specificity for cell type and species in both human and bovine models. Therefore, cell-cell contact is capable of regulating endothelial cell proliferation in vitro despite the presence of available growth surfaces and of optimally supportive culture medium.

Contact inhibition of growth of human diploid fibroblasts by immobilized plasma membrane glycoproteins

The human embryonic fibroblasts used in this study show pronounced inhibition of growth when reaching a critical cell density. High cell density and growth inhibition has previously been mimicked by the addition of glutaraldehyde-fixed cells or of isolated plasma membranes to sparsely seeded proliferating fibroblasts (Wieser, R. J., R. Heck, and F. Oesch, 1985, Exp. Cell Res., 158:493-499). In this report, we describe the successful solubilization of the growth-inhibiting glycoproteins and their covalent coupling to silicabeads (10 microns), which had been derivatized with 3-isothiocyanatopropyltriethoxysilane. The beads, bearing the plasma membrane proteins, were added to sparsely seeded, actively proliferating fibroblasts, and growth was measured by the determination of cell number or of incorporation of [3H]thymidine into DNA. The growth was inhibited in a concentration-dependent manner, whereby 50% inhibition was achieved with 0.3 micrograms of immobilized protein added to 5 X 10(3) cells. Terminal galactose residues of plasma membrane glycoproteins with N-glycosydically bound carbohydrates were responsible for the inhibition of growth. Dense cultures of human fibroblasts are characterized by an accelerated synthesis of procollagen type III. We have found that this cellular response can also be induced by the addition of immobilized plasma membrane glycoproteins to sparsely seeded cells. These observations support the conclusion that the addition of immobilized plasma membrane glycoproteins to sparsely seeded fibroblasts mimics the situation occurring at high cell density. These results show that cell-cell contacts via plasma membrane glycoproteins carrying terminal galactose residues are important for the regulation of the proliferation of cultured human fibroblasts and presumably of the accelerated synthesis of collagen type III.

Effects of cholera toxin and isobutylmethylxanthine on growth of human fibroblasts

Cholera toxin produced a dose-dependent decrease in the restimulation of G0/G1 traverse in density-arrested human fibroblasts but did not inhibit the stimulation of cells arrested in G0 after serum starvation at low density. In addition, cholera toxin did not inhibit the proliferation of sparse logarithmically growing human fibroblasts, even when low concentrations of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) were also present. However, the final density to which sparse cells grew was limited by cholera toxin, when added either alone or together with low concentrations of IBMX. In contrast, high concentrations of the phosphodiesterase inhibitor alone produced a profound inhibition in the growth of sparse human fibroblasts. IBMX produced an inhibition both in the G1 and in the G2 phases of the cell cycle by a mechanism(s) that was not related to the magnitude of the increases in adenosine 3',5'-cyclic monophosphate concentrations.

Effects of EGF and thrombin on inositol-containing phospholipids of cultured fibroblasts: stimulation of phosphatidylinositol synthesis by thrombin but not EGF

The effects of growth factors on inositol-containing phospholipids were investigated to test the hypothesis that alterations in their metabolism are involved in mitogenic stimulation. Thrombin and EGF stimulated comparable increases in the synthesis (30-50%) and degradation (20-40%) of phosphatidylinositol 4-monophosphate (DPI) and phosphatidylinositol 4,5-bisphosphate (TPI) in a cell line which is mitogenically responsive to both growth factors. The increases in synthesis were time and dose dependent in a manner which was consistent with their involvement in mitogenesis; the increases were observed only under conditions where a mitogenic response occurred. While it has been suggested that an increased synthesis of phosphatidylinositol (PI) is coupled to the stimulation of DPI and TPI synthesis, we found that thrombin stimulated an early synthesis PI but EGF did not. To further evaluate the involvement of PI in thrombin-stimulated cell division we determined the time and dose dependence of the stimulated PI synthesis and found that it also occurred in a manner which was consistent with its involvement in thrombin-stimulated cell division. Furthermore, the stimulated PI synthesis was not observed with nonmitogenic proteases or in cell lines which were not responsive to thrombin. These results demonstrate that the metabolism of DPI and TPI appears closely related to the mitogenic response generated by EGF and thrombin. However, an early stimulation of PI synthesis is not coupled to this metabolism and is not necessary for mitogenic stimulation by EGF. Thus, a stimulation of PI synthesis is not a valid measure of alterations in inositol-containing phospholipids and what has been termed the "PI response."

Control of myogenic differentiation by fibroblast growth factor is mediated by position in the G1 phase of the cell cycle

We have used the expression of the muscle form of creatine phosphokinase (M-CPK) to assay myogenic differentiation in the cloned muscle cell line BC3Hl. BC3Hl cells express M-CPK when arrested in the G0 portion of the cell cycle. Addition of the anionic form of brain fibroblast growth factor (B-FGF) rapidly represses synthesis of M-CPK with a half-time of 7 h. Even though B-FGF is not mitogenic for the cells, it causes quiescent BC3Hl cells to exit from the G0 portion of the cell cycle, and to accumulate at a new restriction point approximately 4 to 6 h in the G1 portion of the cell cycle. The repression of M-CPK synthesis by B-FGF is reversible upon removal of B-FGF, and cells which have re-initiated expression of M-CPK have also returned to the G0 portion of the cell cycle. The primary control of M-CPK expression by B-FGF appears to be at the level of gene transcription. We conclude that arrest of cells at G0 but not at other positions in the G1 phase of the cell cycle provides permissive conditions for the expression of muscle-specific proteins, and that defined polypeptide growth factors, in this case B-FGF, are important in the control of the expression of muscle-specific proteins.

Inhibition of lymphocyte proliferation by detergent-solubilized mouse liver membranes

The role of phenomena analogous to fibroblast contact inhibition in lymphocyte growth regulation is controversial, although it is clear that direct cell-cell contact is vital to immunoregulation and accessory cell function. An extract of mouse liver plasma membrane proteins, referred to as suppressive liver extract (SLE), that suppresses the growth of 3T3 fibroblasts also inhibited the mitogen-induced proliferation of murine lymphocytes. A dose of 20 micrograms/ml SLE was less than 95% suppressive of proliferation in both mouse T and mouse B cells treated with a variety of mitogens. B cell growth factor, while increasing DNA synthesis overall in mitogen-stimulated B cells, did not change the extent of SLE suppression, which suggests that the SLE does not interfere with lymphocyte-growth factor interactions. In exploring a sequence of B cell activation events, we discovered that SLE had no effect on the early activation event of increased phosphatidylinositol turnover. Blastogenesis, however, was inhibited in mitogen-stimulated, SLE-treated B cells. The maximum suppressive effect was observed if the SLE was added within 8-12 h of the mitogenic stimulus. SLE did not affect the viability of cells in culture. These results point to a possible unity of regulatory mechanisms between contact inhibition in fibroblasts and the processes of immunoregulation.

The role of membrane-membrane interactions in the regulation of endothelial cell growth

A cell surface preparation from confluent endothelial cells can inhibit DNA synthesis of actively growing endothelial cells. The decrease in the rate of [3H]thymidine incorporation is concentration dependent and levels off at 47% of the control. The preparation has no affect on the growth of vascular smooth muscle cells. A similar preparation from smooth muscle cells does not show inhibitory activity with either endothelial or smooth muscle cells. The inhibition of growth can also be demonstrated by a decrease in thymidine index and growth rate. The inhibition is transient and after 48 h, the growth rate is similar to that of the control. In a wound edge assay, both migration and proliferation are inhibited. The inhibitory activity is partially labile to trypsin and abolished by pepsin, heating at 100 degrees C, or reduction. Cell surface iodination and analysis of the proteins removed by urea treatment by SDS polyacrylamide gel electrophoresis show at least 11 bands with apparent molecular weights from 250,000 to 18,000. These radiolabeled proteins, as well as the active component of the cell surface preparation, are sedimentable at 100,000 g for 1 h. They are both solubilized in 30 mM octyl glucoside but not by treatment with 0.1 M sodium carbonate, pH 11.5. These results suggest that the activity is due to a cell-surface membrane fraction and may provide a basis for studying the mechanism of density-dependent inhibition of growth in a normal cell of defined origin.

Characterization of a coronary vasoconstrictor produced by cultured endothelial cells

The vasoactive effects of media obtained from bovine aortic endothelial cells (EC) in culture were directly tested on isolated rings of the porcine left anterior descending coronary artery. Increasing concentrations of EC-conditioned culture media resulted in progressive dose-dependent increments in isometric tension in porcine, bovine, and canine coronary arteries; the response did not require an intact endothelium. Control (nonconditioned) media and that conditioned by fibroblasts or vascular smooth muscle cells in culture had negligible effects on vessel tone. The vasoconstriction required extracellular Ca2+ and was unaffected by inhibitors of cyclooxygenase and lipoxygenase or by antagonists to the alpha- or beta-adrenergic, serotonergic, histaminergic, or cholinergic receptor systems. Calibrated gel filtration of the media indicated a molecular weight of 8,500 for the vasoactive factor; treatment of the EC-conditioned media with either sodium dodecyl sulfate, trypsin, alkali, or with acid hydrolysis completely abolished the vasoconstrictive effect. These findings and others now provide evidence for the existence of an EC-derived polypeptide vasoconstrictor that may be important in the regulation of vascular smooth muscle contractility.

Loss of reciprocal modulations of growth and liver function of hepatoma cells in culture by contact with cells or cell membranes

In mature rat hepatocytes in primary culture, many metabolic functions and cell growth are controlled reciprocally by cell density, and this reciprocal regulation is mediated by a cell-surface modulator through cell-cell contact. Cultured RY-121B cells from Reuber hepatoma and MH1C1 cells from Morris hepatoma, which retain some liver-specific functions, did not show any cell-density dependency of either cell growth or hepatocyte-specific functions, such as induction of tyrosine aminotransferase by dexamethasone. However, when RY-121B cells were cocultured with a low density of rat hepatocytes as monolayers in direct contact, they exerted contact-dependent control of DNA synthesis and of differentiated function of the hepatocytes. Furthermore, plasma membranes from various tumor cells including these hepatoma cells had strong modulator activity on primary cultures of normal rat hepatocytes, and their activity mimicked the reciprocal effects of cell density on DNA synthesis and induction of tyrosine aminotransferase. On the contrary, addition of plasma membranes from normal adult rat liver to sparse cultures of RY-121B or MH1C1 cells did not cause any inhibition of active DNA synthesis or enhancement of induction of tyrosine aminotransferase in these cells. These results suggest that hepatoma cells have lost cell density-dependent regulations of many cellular activities and cell growth because they have lost the ability to respond to the cell surface modulator, although they retain modulator activity on their plasma membranes.

Growth-inhibitory activity of lymphoid cell plasma membranes. II. Partial characterization of the inhibitor

We have shown that plasma membranes from lymphoid cells have inhibitory activity for the growth of normal lymphocytes and lymphoid tumor cells (Stallcup, K. C., A. Dawson, and M. F. Mescher, J. Cell Biol. 99:1221-1226). This growth-inhibitory activity has been found to co-purify with major histocompatibility complex class I antigens (H-2K and D) when these cell surface glycoproteins are isolated from detergent lysates of cells by affinity chromatography on monoclonal antibody columns. When incorporated into liposomes, the affinity-purified H-2 antigens inhibited the growth of both normal lymphocytes and tumor cells at concentrations of 1-3 micrograms/ml. Inhibition was readily reversed upon removal of the liposomes from the cell cultures, even after several days of exposure of cells to the inhibitor. Inhibitory activity was insensitive to protease digestion or heat treatment, indicating that it was not due to the H-2 glycoproteins. This was confirmed by the demonstration that inhibitory activity could be separated from the H-2 protein by gel filtration in the presence of deoxycholate and could be extracted from membranes or H-2 antigen preparations with organic solvents. The results demonstrate that the growth-inhibitory component(s) of the plasma membrane is a minor lipid or lipid-like molecule which retains activity in the absence of other membrane components. The findings reported here and in the preceding article suggest that this novel membrane component may have a role in control of lymphoid cell growth, possibly mediated by cell contacts.

Growth-inhibitory activity of lymphoid cell plasma membranes. I. Inhibition of lymphocyte and lymphoid tumor cell growth

Membranes isolated from normal spleen cells or lymphoid tumor cells were found to inhibit in vitro growth of several murine tumor cell lines including a B cell hybridoma, a thymoma, and a mastocytoma. 50% inhibition occurred at membrane protein concentrations of 60-100 micrograms/ml. A similar concentration dependence was found for inhibition of [3H]-thymidine incorporation by tumor cells and for the lipopolysaccharide-induced mitogenic response of normal spleen cells. The inhibitory activity co-purified with the plasma membrane upon fractionation of crude membranes. Membrane solubilization with deoxycholate followed by dialysis to remove the detergent gave good recovery of inhibitory activity in the resulting reconstituted membranes. Membrane-mediated growth inhibition resulted from a decreased rate of proliferation and not from increased cell death. A toxic effect of the membranes was further ruled out by the finding that increasing the fetal calf serum content of the medium could substantially reverse the growth inhibition. Thus, the plasma membrane of lymphoid cells contains a component that can slow or stop the growth of cells in culture. This membrane component may have a role in cell contact-mediated regulation of growth.

Identification of a 34,000-dalton mitogenic protein associated with plasma membranes from human A431 epidermoid carcinoma cells

We present evidence for a discrete 34,000-Da polypeptide with mitogenic activity, associated with plasma membranes from human A431 carcinoma cells. Plasma membranes from A431 cells are highly mitogenic for quiescent fibroblasts. A significant fraction of the membrane-associated activity can be released by treatment with a high concentration of salt and is relatively acid stable. Incubation of 125I-labeled salt-extracted proteins with target fibroblasts results in preferential binding of a 34,000-Da protein--i.e., greater than 90% of the cell-bound radioactivity is associated with the 34,000-Da polypeptide. Studies correlating the mitogenic activity with the cell-binding 34,000-Da protein indicate that this protein is the acid-stable, peripherally attached, mitogenically active component of A431 membranes. Available data suggest that this protein may be mitogenically active in the nanomolar concentration range. Some properties of this protein are described.

Growth control in cultured 3T3 fibroblasts: neutralization and identification of a growth-inhibitory factor by a monoclonal antibody

A fibroblast growth regulator, which inhibits the growth and division of proliferating fibroblasts, has been isolated from medium conditioned by exposure to density-inhibited mouse 3T3 cells. This partially purified preparation of the growth-inhibitory activity, termed FGR-s, contained two major polypeptides (Mrs, 10,000 and 13,000). Using FGR-s as the immunogen, we have carried out in vitro immunization of rat splenocytes and have generated hybridoma lines, each secreting an antibody directed against components of the FGR-s preparation. One such monoclonal antibody, designated 2A4, specifically bound the Mr 13,000 polypeptide of FGR-s. Antibody 2A4 also neutralized the growth-inhibitory effect of FGR-s in a concentration-dependent fashion. These results strongly suggest that the Mr 13,000 polypeptide carries growth-inhibitory activity.

Density-dependent inhibition of growth: inhibitory diffusible factors from 3T3- and Rous sarcoma virus (RSV)-transformed 3T3 cells

We recently fractionated, from the culture medium of 3T3 cells, a thermolabile inhibitory diffusible factor (IDFN) with a molecular weight of about 40,000 daltons, which decreased nucleic acids synthesis of stimulated target 3T3 cells. In the present publication the inhibitory activities of IDFN (produced by 3T3 cells) and IDFT (produced by RSV-transformed 3T3 [3T3 SRA/H] cells) on 3T3 and 3T3 SRA/H cells have been compared. The inhibitory activity of IDFN decreased (by a mean of 57%) when it was tested on transformed instead of 3T3 cells. On the other hand, IDFT was able to decrease 14C-inosine incorporation in target 3T3 cells. However, the inhibitory activity of IDFT decreased (by mean 50%) when tested on 3T3 SRA/H instead of 3T3 cells. Therefore, transformed cells produced an inhibitory factor but were less sensitive than 3T3 cells to its inhibitory activity. The inhibitory activity of IDFT on 3T3 SRA/H cells was only 20% of the inhibitory activity of IDFN on 3T3 cells. This appreciable difference is of particular interest, since it could explain the release of density-dependent inhibition of growth (DDI) in transformed 3T3 SRA/H cells. Furthermore, it provides more evidence for the hypothesis that, in 3T3 cells, DDI of growth is due to the release of an inhibitory molecule into the medium, and that IDFN is in fact, the inhibitory molecule involved in this phenomenon.

Neuroblastoma membranes inhibit isoproterenol-stimulated rise of cAMP in glioma cells

C6 glioma cells respond to beta-adrenergic agonists (isoproterenol) with a transient rise in intracellular cyclic adenosine monophosphate level. This beta-responsiveness of C6 cells is inhibited by the presence of a plasma membrane fraction, which has a five- to six-fold purification of membrane markers, showed a greater inhibition of beta-responsiveness in C6 cells than any other subcellular fractions of B104 cells. The inhibitory effector(s) is apparently associated with integral membrane structure(s) since ionic extraction and treatment with chelating agents did not remove the effect from the particulate membrane fraction. The effector is probably proteinaceous in nature as judged by its susceptibility to inactivation by heat and protease treatment. The data indicate that neither adenylate cyclase nor phosphodiesterase enzyme is likely to be directly involved in mediating the beta-nonresponsiveness of C6 cells.

Purified plasma membranes inhibit polypeptide growth factor-induced DNA synthesis in subconfluent 3T3 cells

Plasma membranes derived from NR-6 cells, a variant line of Swiss mouse 3T3 cells that does not have cell surface receptors for epidermal growth factor (EGF), inhibited EGF-induced stimulation of DNA synthesis by 50% in serum-starved, subconfluent 3T3 cells. Membranes derived from SV3T3 cells were much less effective in inhibiting EGF-induced DNA synthesis. This inhibition on DNA synthesis by NR-6 membranes was not a direct effect of membranes on EGF, nor could it be overcome by high concentrations of EGF. NR-6 membranes were most effective when added 3 h before EGF addition and had little effect when added 2 h or more after EGF. NR-6 membranes also reduced the stimulation of DNA synthesis induced by platelet-derived growth factor or fibroblast growth factor in serum-starved 3T3 cells. These findings indicate that membrane-membrane interactions between nontransformed cells may diminish their ability to proliferate in response to serum polypeptide growth factors.

Reciprocal modulation of growth and differentiated functions of mature rat hepatocytes in primary culture by cell--cell contact and cell membranes

In primary monolayer cultures of rat mature hepatocytes, many metabolic functions as well as cell growth are regulated by cell density. There are two types of regulatory response of these functions to change of cell density. Growth-related functions, such as DNA synthesis, induction of glucose-6-phosphate dehydrogenase, 2-aminoisobutyric acid transport, synthesis of cellular protein, and cholesterogenesis, are stimulated by low cell density. In contrast, functions related to hepatocyte-specific characters, such as the inductions of tyrosine aminotransferase, serine dehydratase, and malic enzyme and synthesis of triglycerides, are stimulated by high cell density. The reciprocal responses of these cellular activities to cell density were mimicked by addition of plasma membranes purified from adult rat liver to hepatocytes cultured at low cell density. The modulator activity was heat labile and trypsin sensitive. The activity was also found in plasma membranes from kidney, brain, and erythrocytes, although the specific activities of these preparations seemed to be different. These results suggest that the reciprocal regulations of cell growth and hepatocyte-specific functions are mediated by some surface components via cell-cell contact.