Phorbol esters enhance attachment of NIH/3T3 cells to laminin and type IV collagen substrates

Fibroblast Migration Is Regulated by Myristoylated Alanine-Rich C-Kinase Substrate (MARCKS) Protein

Myristoylated alanine-rich C-kinase substrate (MARCKS) is a ubiquitously expressed substrate of protein kinase C (PKC) that is involved in reorganization of the actin cytoskeleton. We hypothesized that MARCKS is involved in regulation of fibroblast migration and addressed this hypothesis by utilizing a unique reagent developed in this laboratory, the MANS peptide. The MANS peptide is a myristoylated cell permeable peptide corresponding to the first 24-amino acids of MARCKS that inhibits MARCKS function. Treatment of NIH-3T3 fibroblasts with the MANS peptide attenuated cell migration in scratch wounding assays, while a myristoylated, missense control peptide (RNS) had no effect. Neither MANS nor RNS peptide treatment altered NIH-3T3 cell proliferation within the parameters of the scratch assay. MANS peptide treatment also resulted in inhibited NIH-3T3 chemotaxis towards the chemoattractant platelet-derived growth factor-BB (PDGF-BB), with no effect observed with RNS treatment. Live cell imaging of PDGF-BB induced chemotaxis demonstrated that MANS peptide treatment resulted in weak chemotactic fidelity compared to RNS treated cells. MANS and RNS peptides did not affect PDGF-BB induced phosphorylation of MARCKS or phosphoinositide 3-kinase (PI3K) signaling, as measured by Akt phosphorylation. Further, no difference in cell migration was observed in NIH-3T3 fibroblasts that were transfected with MARCKS siRNAs with or without MANS peptide treatment. Genetic structure-function analysis revealed that MANS peptide-mediated attenuation of NIH-3T3 cell migration does not require the presence of the myristic acid moiety on the amino-terminus. Expression of either MANS or unmyristoylated MANS (UMANS) C-terminal EGFP fusion proteins resulted in similar levels of attenuated cell migration as observed with MANS peptide treatment. These data demonstrate that MARCKS regulates cell migration and suggests that MARCKS-mediated regulation of fibroblast migration involves the MARCKS amino-terminus. Further, this data demonstrates that MANS peptide treatment inhibits MARCKS function during fibroblast migration and that MANS mediated inhibition occurs independent of myristoylation.

Exfoliative cytologic evaluation of primary cultured lung carcinomas

This paper describes a tissue culture and exfoliative cell culture system that enables one to (1) evaluate the adequacy of primary lung carcinoma cultures for cytogenetic analysis, and (2) predict the likelihood of viable cells and type of differentiation present in the primary lung tumor cultures used for cytogenetics. Primary lung carcinomas were established from explant outgrowths and maintained in serum supplemented or serum free media on plastic or basement membrane associated protein coated dishes in order to obtain cells for karyotypic analysis (Miura et al., 1990). The media from these cultures that would ordinarily have been discarded was aspirated at each media change and used to prepare cytocentrifuge cytology preparations. Papanicolaou stained cells from the preparations were evaluated by cytotechnologists in order to assess (1) the cellularity and presence of cancer cells in the sample, (2) cytology preparations of cell and explant outgrowth cultures from primary lung tumors are reliable method for screening and evaluating the suitability of primary lung carcinoma cultures for cytogenetic analysis.

Fibrinogen induces chemotactic activity in endothelial cells

Chemotaxis of blood monocytes into the vessel wall together with the change of the relative content of the extracellular matrix (ECM) proteins at sites of predilection is an early cellular marker of atherogenesis. To examine the influence of ECM proteins on secretion of chemoattractants by endothelial cells (EC), porcine EC were seeded on gelatin (G), fibronectin (Fn) and fibrinogen (Fg). After 24 h cells seeded on G and Fn showed the histiotypic 'cobblestone'-morphology whereas cells seeded on Fg did not. Chemotactic activity for monocytes in supernatants from cells seeded on Fg was more than two-fold higher compared with G and was independent of soluble Fn or Fg in the supernatant. Quantification of monocyte chemoattracting protein-1, PDGF-AB and IL-8 in EC supernatants showed that Fg led to a significant increase in secretion of all three proteins compared with cells cultured on G. Preincubation of porcine EC with the tripeptide arginine-glycine-aspartic acid, as inhibitor of binding of Fg to integrin receptors, but not with the control tripeptide arginine-glycine-glutamic acid showed a decrease in chemotactic activity for cells cultured on Fg but not on Fn or G. Inhibition of protein kinase C (PKC) activity in EC by GF109203 resulted in a decrease of fibrinogen-induced chemotactic activity. Also the tyrosine-kinase inhibitor herbimycin inhibited fibrinogen mediated secretion of chemokines. The role of the PKC pathway for matrix mediated signal transduction is further corroborated by Fg-dependent induction of the PKC isoform delta. These data indicate an integrin-dependent signal transduction pathway leading to induction of chemotactic activity by the ECM protein fibrinogen. This mechanism may contribute to induction of chemokines in early atherosclerotic lesions.

Annexin V inhibits the 12-O-tetradecanoylphorbol-13-acetate-induced activation of Ras/extracellular signal-regulated kinase (ERK) signaling pathway upstream of Shc in MCF-7 cells

Annexin V is a Ca2+-dependent phospholipid binding protein. Although it has been shown to inhibit protein kinase C (PKC) in cell-free systems, its role in the intact cell is unclear. A stable MCF-7 human breast cancer cell overexpression system was established to investigate the function of annexin V. In these cells, 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced phosphorylation and kinase activity of ERK1/2 were suppressed. Morphological changes induced by TPA were reduced by annexin V overexpression as well as by the pan-PKC inhibitor, bisindolylmaleimide I, and by the mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK) inhibitor, PD98059. TPA-induced MEK1/2 and Raf-1 phosphorylation were reduced in these cells. The TPA-enhanced active Ras, and its association with Raf-1, were reduced. TPA treatment of MCF-7 cells caused an increased association of Shc with Grb2. However, this increased association was prevented in the annexin V-overexpressors. p21WAF/CIP1 is responsible for inhibition of cell cycle progression in MCF-7 cells. TPA induced the expression of p21WAF/CIP1 to a greater extent in MCF-7 parent and control plasmid cells than in annexin V overexpressors. PD98059 inhibited this increase, suggesting that TPA upregulation of p21WAF/CIP1 occurs via the MEK pathway, and that annexin V overexpression blunts it. This work shows that annexin V overexpression suppresses the TPA-induced Ras/ERK signaling by inhibiting at/or upstream of Shc, possibly through the inhibition of PKCs. Oncogene (2000).

Heparin binding protein-44 (HBP-44)/receptor-associated protein (RAP)mediates cell-substratum adhesion of mouse NIH/3T3 cells through its binding to low density lipoprotein (LDL) receptor-related protein (LRP)

The low density lipoprotein receptor-related protein (LRP) is a multifunctional endocytic receptor with the ability to bind and endocytose several structurally and functionally distinct ligands. The 39 kDa receptor-associated protein (RAP) is an endoplasmic reticulum (ER) resident protein, which is believed to function intracellularly as a molecular chaperone for LRP and to regulate its ligand binding activity along the secretory pathway. Mouse heparin binding protein-44 (HBP-44) is a homologue of human RAP. Using a recombinant form of HBP-44 expressed in Escherichia coli cells as a highly specific ligand for LRP, we demonstrated that HBP-44 coated on cell culture plates mediates the cell-substratum adhesion of mouse 3T3 fibroblasts in a dose-dependent manner, with 50% attachment at the concentration of 0.2 micrograms/ml. Ligand blot analysis with HBP-44 of whole cell extracts and the materials precipitated by anti-LRP antibodies revealed that the receptor for HBP-44 on NIH/3T3 cells was LRP. The results suggest that LRP serves as a cell adhesion receptor in some cells.

Induction of bovine bronchial epithelial cell filopodia by tetradecanoyl phorbol acetate, calcium ionophore, and lysophosphatidic acid

The morphological responses of primary bovine bronchial epithelial cells (BBECs) cultured in serum-free medium to protein activators have been examined. When attached to type I collagen-coated tissue culture dishes, the cells responded to tetradecanoyl phorbol acetate (TPA), calcium ionophore A23187, and lysophosphatidic acid (LPA) by extruding filopodia. In contrast, no morphological changes were elicited by exposures to either epinephrine or dibutyryl-cAMP. Formation of filopodia was accompanied by actin filament reorganization as demonstrated by staining with labeled phalloidin. Exposures to varied TPA concentrations for 2 h showed maximal stimulation of filopodial extrusions at 10 nM TPA with half-maximal stimulation at 1 nM. Time-course measurements with 10 nM TPA showed filopodia formation within 30 min of exposure, with 85% of the BBECs being filopodia positive after 5 h. Filopodia induction in 20-30% of the cells could be achieved by 1-100 microM LPA concentrations. BBECs acquired increasing resistance to TPA-induced filopodia during the initial 5 days in culture; however, responsiveness to TPA was regenerated by mild treatment with trypsin. Inclusion of fibronectin or vitronectin into the attachment matrix had no effects on the rates or extent of TPA-induced filopodia formation.

Phorbol ester-induced scattering of HT-29 human intestinal cancer cells is associated with down-modulation of E-cadherin

The effects of tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) on the growth characteristics of the colon cancer cell line HT-29 M6 were studied. TPA induced the scattering of proliferative HT-29 M6 cells: in the presence of the phorbol ester, HT-29 M6 colonies scattered and the cells acquired a flatter aspect with diminished cell-cell contacts. This effect of TPA required a persistent activation of PK-C and was accompanied by a slight decrease (30%) in the growth rate. Modifications by TPA of two scattering associated properties of these cells were also detected: TPA decreased cell-to-cell aggregation and enhanced the cellular attachment to matrix substrata (collagen, laminin). The decrease in cell-to-cell adhesion was correlated with a loss of cellular E-cadherin as evidenced by immunofluorescence or immunoblotting with a specific monoclonal antibody. Cell scattering was dependent on the extracellular concentration of Ca2+; an increase from 1.6 to 10 mM in the concentration of this ion completely blocked the morphological effects of TPA as well as its action on cell aggregation. This high concentration of Ca2+ also prevented the down modulation of E-cadherin as determined by immunofluorescence. However, the TPA-induced increase in cell attachment to the matrix was not affected by high calcium. These findings support the importance of altered cell-cell adhesion in the process of scattering and provide a good system for the study of down modulation of E-cadherin, a protein involved in the control of cell growth, differentiation and invasion of epithelial cells.

Adhesiveness and proliferation of epithelial cells are differentially modulated by activation and inhibition of protein kinase C in a substratum-dependent manner

In the present study, we have examined the regulation of attachment, onset of proliferation and the subsequent growth, in vitro, of chick retinal pigmented epithelial (RPE) cells as a function of the nature of the substratum and of either the activation or inhibition of protein kinase C (PKC). The RPE cells have an adhesive preference for protein carpets which contain laminin. This preference disappears gradually with time in culture. The adhesion of RPE cells to fibronectin is shown to be a receptor-mediated process which involves the RGD recognition signal. This study also demonstrates that a PKC activator, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), affects RPE cell adhesion in a substratum-dependent manner. Exposure of RPE cells to TPA lowers the cell attachment efficacy to ECM protein substrata but does not affect cell attachment to plastic. The onset of cell proliferation is accelerated by TPA on all of the substrata tested. The minimal duration of an effective TPA pulse exerting a long-lasting influence on RPE cell proliferation is between 1.5 and 3.5 hr. Stimulation of cell proliferation by TPA in long-term cultures is independent of the nature of the growth substratum. The acceleration of the onset of cell proliferation by TPA is sensitive to 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7), an inhibitor of conventional PKC, and thus appears to be dependent on the activation of conventional PKC. H7 also affects cell-cell contacts, causing an alteration in the shape ("squaring") of RPE cells packed into large colonies. Conversely, the effects of TPA on both the attachment and the long-term proliferation of RPE cells are not dependent a conventional PKC isotype, since H7 cannot abolish the influence of TPA on either process. We conclude that the effect of TPA on long-term proliferation of RPE cells is either dependent on a novel PKC isotype or independent of PKC.

Cellular interactions with the extracellular matrix are coupled to diverse transmembrane signaling pathways

Extracellular matrix (ECM) glycoproteins such as laminin, fibronectin, or collagen IV play a major role in cell behavior regulation. The molecular mechanisms taking place at the interface between the ECM and the cell surface are now rather well defined; however, very little is known about intracellular signals induced by these interactions. In order to get insights into the transduction pathways involved in cell-ECM interactions we have investigated the effects of several intracellular kinase inhibitors. Calmodulin-dependent kinase inhibitors, W-7 and sphingosine, have negative effects on cell-matrix interactions. They inhibit adhesion of several cell lines to laminin (IC50 = 4-10 microM), fibronectin and collagen IV (IC50 = 7-25 microM). The effects are immediate, reversible, and also cell specific, certain combinations of cell line-substrate being irresponsive to these inhibitors. In contrast, two inhibitors, H-7 and staurosporine, for which protein kinase C is a common target, increase two- to fourfold the attachment of HT1080, OVCAR-4, and B16F10 cells to laminin but not to fibronectin. Another inhibitor, HA-1004, known to inhibit protein kinase A at low concentrations, has an activating effect only at high concentration (> 200 microM) when it becomes an inhibitor of protein kinase C. These inhibitors are without effect on RuGli and Saos-2 cell adhesion on the three substrates. Altogether these results suggest that calmodulin-dependent kinases and protein kinase C could be separately involved in ECM-induced cellular responses. However, the effects of kinase inhibitors are substrate-specific and cell type-specific, suggesting that the intracellular signals induced by the extracellular matrix vary with the nature of integrin involved in signal transmission.

Protein kinase C affects microfilaments, bone resorption, and [Ca2+]o sensing in cultured osteoclasts

The effects of protein kinase C (PKC) in the control of osteoclast activity are still unknown. We investigated the role of the enzyme in the control of microfilament organization, podosome assembly, bone resorption, and extracellular Ca2+ sensing in chicken and rabbit osteoclasts treated with agents known to affect PKC activity. Cells were treated for 20 min with a PKC activator [phorbol 12-myristate 13-acetate (PMA)], a PKC inhibitor (staurosporine), a protein kinase A (PKA) inhibitor (H-9), a guanosine 3',5'-cyclic monophosphate-dependent protein kinase-PKA-PKC inhibitor (H-7), or with the inactive phorbol, 4 alpha-phorbol, to examine microfilaments by decoration with rhodamine-phalloidin. In PMA-treated osteoclasts, the number of microfilament-containing adhesion structures (podosomes) per cell decreased. However, enlarged microfilamentous cores in podosomes and stress fiber-like filaments, otherwise absent in controls, appeared. Whereas H-7 induced increase of the number of podosomes, staurosporine, H-9, and 4 alpha-phorbol failed to change microfilament organization. Chicken osteoclasts received also long-term treatment with the agents in the presence of [3H]proline-prelabeled chicken or rat bone particles to measure bone resorption. PMA, as well as staurosporine and H-7, stimulated the resorbing activity, whereas cells were insensitive to H-9 and 4 alpha-phorbol. Measurement of cytosolic free calcium concentration in PMA-treated fura-2-loaded single osteoclasts demonstrated a synergistic effect of PKC activation on the inhibitory extracellular calcium concentration-sensing mechanism, which was, by contrast, blocked by H-7, staurosporine, and H-9 and was insensitive to 4 alpha-phorbol. These results indicate that PKC regulates osteoclast activity inducing both morphological and functional modifications.

Increased adhesion of fibroblasts from patients with scleroderma to extracellular matrix components: in vitro modulation by IFN-gamma but not by TGF-beta

A characteristic feature of systemic scleroderma is fibrosis of the skin and eventually of internal organs resulting from an overproduction of collagen and other connective tissue components by the resident fibroblasts. The balance between the cells and the amount of the surrounding extracellular matrix is then altered. Because cellular metabolism depends to a large extent on cellular contacts and communications with connective tissue molecules, we have therefore investigated the interactions with extracellular matrix components of fibroblasts obtained from skin of patients affected with scleroderma. In comparison to fibroblasts from healthy skin, all fibroblasts from scleroderma patients had an increased adhesion capacity to collagens I, IV, VI, fibronectin, and laminin. In addition, whereas adhesion of control fibroblasts was stimulated by a pre-treatment with transforming growth factor-beta, adhesion patterns of scleroderma fibroblasts remained unchanged. However, pre-incubation of the cells with interferon-gamma decreased the adhesion of both scleroderma and control fibroblasts.

Sphingosine inhibits attachment of murine Lewis lung carcinoma cells to laminin and type IV collagen

The effect of sphingosine (SPH) on the adhesive properties of Lewis lung carcinoma (3LL) cells was investigated using plastic precoated with the extracellular matrix proteins, laminin, fibronectin, or type IV collagen. Treatment of 3LL cells with SPH (0.5-10 microM) resulted in a dose-dependent decrease in the ability to bind to laminin and type IV collagen but had little or no effect on attachment to fibronectin. Phorbol 12-myristate 13-acetate (PMA) selectively enhanced attachment of 3LL cells to laminin and collagen. The inhibitory effect of SPH on attachment to both proteins was competitively antagonized by PMA. These results suggest that SPH acts as a negative effector for cell attachment to laminin and collagen, and that the cell attachment process to both proteins might be regulated in part by protein kinase C.

Retinoids induce tissue transglutaminase in NIH-3T3 cells

We report that all-trans and 13-cis-retinoic acid as well as the synthetic compound CH-55 enhance tissue transglutaminase activity as they increase NIH-3T3 cell adhesiveness. The 4-hydroxyphenylretinamide (4-HPR) with low activity in inducing attachment, lectin binding and growth inhibition also fails to induce transglutaminase. Thyroxine (Thy), a compound with a response element common to RA, is inactive. The tumor promoter 12-tetradecanoyl-phorbol-13-acetate (TPA), which increases adhesiveness with different kinetics than RA, failed to enhance tranglutaminase. We conclude that retinoids with biological activity in inducing adhesion, inhibition of growth and increase of lectin binding, are also active in inducing transglutaminase activity.

Effects of a tumour promoter, 12-0-tetradecanoyl-phorbol-13-acetate (TPA), on expression of differentiated phenotype in the chick retinal pigmented epithelial cells and on their interactions with the native basement membrane and with artificial substrata

Chick retinal pigmented epithelial (RPE) cells grown in vitro on basement membrane matrices from the Engelbreth-Holm-Swarm tumour (BM-matrigel) do not spread, and they maintain their differentiated phenotype, most notably the heavy pigmentation. Maintenance of the differentiated phenotype by RPE cells on BM-matrigel is promoted not only by the biochemical composition of the gel but also by its mechanical properties, i.e., its low rigidity prevents cell spreading. In this report, RPE cells on BM-matrigel were treated with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) to promote the transformed phenotype and diminish cell traction. In contrast to most cell types TPA treatment induced RPE cells to increase their spread area. TPA promoted RPE cell spreading on BM-matrigel and changed the spatial organization of actin and actin-associated proteins in the cytoskeleton-ECM linkage complexes, uncoupling actin from its extracellular counterpart. TPA did not affect other components of the cytoskeleton in RPE cells. TPA also affected labile adhesions i.e., focal contacts and adherens junctions in statu nascendi, but preformed, stable adherens junctions were resistant to TPA. TPA enhanced proliferation, blocked melanogenesis and thus inhibited differentiation of RPE cells grown on either artificial substrata or their natural basement membrane.

Alterations in integrin receptor expression on chemically transformed human cells: specific enhancement of laminin and collagen receptor complexes

The abilities of malignant tumor cells to bind and migrate through basement membranes are important steps in invasion and metastasis. Malignant tumor cells would therefore be expected to express receptors on their surfaces for basement membrane and stromal components, such as collagens, laminin, and fibronectin, although the pattern of expression of these receptors on the malignant cells may be different from that on their normal progenitors. We report here that chemically transformed tumorigenic human cells express an altered pattern of integrin receptors on their cell surfaces as compared with their untransformed nontumorigenic counterparts. Specifically, N-methyl-N'-nitro-N-nitrosoguanidine transformation of HOS cells into highly tumorigenic cells results in a significant specific increase in the expression of (in descending order of level of cell surface expression) the integrins alpha 6/beta 1, alpha 2/beta 1, and alpha 1/beta 1, which are receptors for laminin, collagens, and collagen type IV and laminin, respectively. The level of expression of two fibronectin receptor integrins, alpha 5/beta 1 and alpha 3/beta 1, are, however, unaltered, whereas the level of expression of vitronectin receptor integrin, alpha v/beta 3, is drastically reduced on the transformed cells. Consistent with the increased expression of laminin and collagen receptors and the decreased expression of vitronectin receptors on the transformed cells, these cells attached three- to fivefold more strongly to laminin and collagen but attached very poorly to vitronectin. The MNNG-HOS cells were also found to have a greater potential for invasion through reconstituted basement membrane, matrigel, the major components of which are laminin and type IV collagen. The invasion of both the HOS and MNNG-HOS cells was inhibited 45-50% by a polyclonal anti-fibronectin receptor antibody. However, although the invasion of HOS cells could be inhibited up to 75% by an anti-alpha 6 monoclonal antibody, a similar concentration of this antibody had no effect on the alpha 6-overproducing MNNG-HOS cells. A fivefold higher concentration of this antibody did result in partial inhibition of MNNG-HOS invasion. These data indicate a critical role for the alpha 6/beta 1 laminin receptor in the invasion of these cells through basement membranes and demonstrate that chemical transformation of nontumorigenic human cells to highly tumorigenic cells is associated with an altered pattern of integrin expression which may play a direct role in the increased capacity of these cells to bind and invade through basement membranes.

Endothelial monolayer integrity. Perturbation of F-actin filaments and the dense peripheral band-vinculin network

The role of the actin microfilaments in maintaining the integrity of the monolayer and activating endothelial repair processes is not well understood. This study was designed to characterize the prominent changes in F-actin distribution in endothelial cells that are associated with shape changes in the cells after perturbation of a confluent monolayer. F-actin was localized by using rhodamine phalloidin and fluorescence microscopy. The dense peripheral band (DPB) and vinculin cell-cell junctions were co-localized by using double fluorescence and immunofluorescence microscopy. Thrombin and 12-o-tetradecanoyl-myristyl-13-acetate (TPA) caused loss of the DPB and an increase in the central microfilament bundles, while agents that caused rounding of the cells (including plasmin, trypsin, and chymotrypsin) did not cause loss of the DPB although large gaps were formed between cells. The thrombin and TPA effects were rapid and reversible and were associated with an accompanying loss of vinculin cell-cell plaques. The mechanisms of the effects were not studied. It was postulated that thrombin and TPA were activating endothelial repair processes.

Effects of D-threo-PDMP, an inhibitor of glucosylceramide synthetase, on expression of cell surface glycolipid antigen and binding to adhesive proteins by B16 melanoma cells

Incubating B16 melanoma cells with an inhibitor of glucosylceramide (GlcCer) synthetase, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-threo-PDMP), led to a considerable decrease in the levels of GlcCer and lactosylceramide (LacCer). The content of ganglioside GM3 was little affected, but the ability to bind a monoclonal antibody against the ganglioside (M2590) was greatly reduced, suggesting that the reduction in the simple glycolipids led to encryption of the membrane antigen. This interpretation is supported by the observation that permeabilization of the treated cells with Triton X-100 restored immunological reactivity. Specificity of the PDMP effect was shown by its lack of effect on the reactivity of two other surface antigens to anti-melanoma monoclonal antibodies M562 and M622, and of the major histocompatibility antigens to anti-H-2KbDb monoclonal antibody. The ability of the treated cells to attach to laminin or type IV collagen was lost but that to fibronectin was not. The effects of the enzyme inhibitor were counteracted by including GlcCer in the culture medium. This indicates that the lipid was absorbed by the cells and utilized like endogenously-formed GlcCer. Cells preattached to laminin or collagen could be induced to round up by addition of inhibitor. In contrast, L-threo-PDMP (which does not block the synthesis of GlcCer) had no effect on the immunologic reactivity of GM3 or the adhesion properties of the cells. However, it did produce considerable accumulation of LacCer. These data suggest that the simple glycolipid, GlcCer, is an essential factor for antigenic expression of the more complex glycolipids on cell surfaces and that there is a close association and interaction between glycolipids and adhesive receptors on the cell surface.

Phorbol ester modulation of integrin-mediated cell adhesion: a postreceptor event

Chinese hamster ovary (CHO) suspension culture cells adhere readily to substrata coated with extracellular matrix proteins such as fibronectin, vitronectin, or laminin. In the case of fibronectin, it is known that adhesion is mediated by an integrin-type, cell surface fibronectin receptor (FnR). We demonstrate here that treatment of CHO cells with submicromolar concentrations of phorbol ester produces a remarkable increase in the ability of these cells to adhere to fibronectin. Both the rate of adhesion and the efficiency of adhesion are enhanced about four- to fivefold. Further, phorbol ester treatment renders the fibronectin-mediated adhesion process less sensitive to inhibitors, including GRGDSP peptide and PB1, a monoclonal anti-FnR antibody. By contrast, nonspecific adhesion processes, for example cell attachment to substrata coated with polylysine or concanavalin A, are not affected by phorbol ester treatment. Thus integrin-mediated adhesion is modulated by phorbol esters, but nonspecific adhesion is not. Neither the number of cell surface FnRs nor the receptor affinity, as measured by 125I-fibronectin and 125I-anti-FnR antibody binding, is altered by phorbol ester treatment. Thus, the effect of phorbol ester on cell adhesion seems to occur at a step subsequent to initial ligand-receptor binding events. Since phorbol ester is a potent activator of protein kinase C, we examined phosphorylation patterns in control and phorbol-treated cells. In immunoprecipitates of lysates from suspension culture cells, there was no evidence of phorbol ester-stimulated phosphorylation of FnR or of talin, a protein thought to interact with FnR. These results suggest that phorbol ester effects on fibronectin-dependent adhesion are not due to phosphorylation of the FnR itself but rather may be due to postreceptor events, possibly the phosphorylation of cytoskeletal proteins involved in integrin-mediated adhesion.