Laminin increases the release of type IV collagenase from malignant cells

We have studied the effect of laminin on type IV collagenolytic activity elaborated by malignant cells in culture. Laminin (at concentrations of 4-8 micrograms/ml) added to serum-free culture supernatants of subconfluent A2058 human melanoma cells significantly increased the release of the type IV collagenolytic activity (200-300%). The induction of type IV collagenase was more pronounced (580%) using a fragment of laminin which binds to the cell surface laminin receptor. A monoclonal antibody against the human laminin receptor blocked the effect of laminin on type IV collagenase, suggesting that occupation of the laminin receptor may be necessary for the effect. Increase in the type IV collagenolytic activity mediated by laminin was also demonstrated in two other malignant cell lines, HT fibrosarcoma (168%) and mouse melanoma (B16-F10) (271%). The increase in type IV collagenase was found to be specific for laminin because another cell-binding matrix protein, fibronectin, did not have any effect, and epidermal growth factor and transferrin actually decreased the type IV collagenase in human melanoma culture medium (epidermal growth factor, 50% at 20 ng/ml; and transferrin, 20% at 10 micrograms/ml). These studies suggest that tumor cell binding to laminin, which comprises the first step of basement membrane invasion, will induce the second step, namely the collagenolytic dissolution of the basement membrane.     

Modulation of glucocorticoid receptor activity by cyclic nucleotides and its implications on the regulation of human skin fibroblast growth and protein synthesis

The modulation of glucocorticoid receptor activity by cyclic nucleotides was studied in cultured human skin fibroblasts. The receptors appeared to be activated in the presence of dibutyryl-cAMP and inactivated by dibutyryl-cGMP. Significantly, the cGMP content of the fibroblasts increased during cell growth, with a concomitant decrease in the glucocorticoid receptor activity, while when the cells reached early confluency the decrease in cGMP content was accompanied by an increase in cAMP and increased activity of the glucocorticoid receptors. In addition, cortisol induced (2'-5')oligoadenylate synthetase in these cells and raised the cellular (2'-5')oligoadenylate concentrations. This resulted in a decrease in both DNA and protein synthesis activity in the cells, a response which correlated with the (2'-5')oligoadenylate concentration. The combination of cortisol and dibutyryl-cAMP had a synergetic stimulatory effect on the (2'-5')oligoadenylate concentration and a synergetic inhibitory effect on protein synthesis. In conclusion, it is demonstrated here that cyclic nucleotides can modulate glucocorticoid receptor activity in cultured human skin fibroblasts, and thus these compounds may indirectly affect cellular metabolism by regulating the cellular responses to glucocorticoids.     

Tumor-promoting phorbol esters and cell proliferation stimulate secretion of basement membrane (type IV) collagen-degrading metalloproteinase by human fibroblasts

The secretion of a type IV collagen-specific proteinase is stimulated in cultured human skin fibroblasts by the phorbol ester tumor promoter 12-O-tetradecanoyl phorbol 13-acetate (TPA) and during cell proliferation. Exposure of the cells at the late log phase of growth to 10(-9) to 10(-6) M TPA resulted in the secretion of type IV collagenase activity to the medium, this effect being reversible. Incubation of intact type IV procollagen with TPA-induced fibroblast medium protein produced six peptides, four of which corresponded in size to the fragments produced by a type IV collagen-specific collagenase (Fessler, L., Duncan, K., Fessler, J., Salo, T., and Tryggvason (1984) J. Biol. Chem. 259, 9783-9789). The TPA-induced type IV collagen-degrading enzyme could be activated by trypsin, was inhibited by EDTA, but was not affected by soybean trypsin inhibitor, N-ethylmaleimide, aprotinin, or cysteine. Therefore, in human skin fibroblasts, TPA can induce a type IV collagen-specific, metal-dependent collagenase as was previously described in some invasive tumor cells. Furthermore, another metalloprotease is apparently secreted under the same conditions of TPA exposure. The production of metal-dependent, type IV collagen-degrading activity was also studied at different stages of cellular proliferation. In early log phase, a significant amount of enzyme activity was observed in the control cell medium; this activity disappeared during both late log and stationary growth phases. This activity could be markedly increased by the addition of 10(-8) M TPA to the culture medium. The production of matrix-degrading proteinases is therefore likely to be associated with rapid cell proliferation in both transformed and untransformed cells.     

Dissimilar cyclic nucleotide phosphodiesterase activities in subcellular fractions from normal and SV40-transformed WI-38 fibroblasts.

Broken cell preparations of WI-38 and SV40-transformed WI-38 (VA13) fibroblasts were used to compare the cyclic nucleotide phosphodiesterase activities of the two cell strains. The bulk of the cAMP or cGMP phosphodiesterase activity of WI-38 and VA13 homogenates was found in the 100,000 x g fibroblast supernatant fractions. WI-38 and VA13 soluble phosphodiesterase activities showed anomalous kinetic behavior with either cAMP or cGMP as the substrate. At low substrate concentrations, e.g., 0.1 muM, WI-38 supernatant fractions hydrolyzed cGMP much more rapidly than cAMP. At high substrate concentrations, e.g., 100muM, the same enzyme preparations degraded cAMP more than twice as fast as cGMP. In contrast, VA13 soluble phosphodiesterase activity catalyzed the hydrolysis of a wide range of cAMP and cGMP concentrations at similar rates. Phosphodiesterase activity in WI-38 supernatant fractions was generally more sensitive than that of the comparable VA13 enzyme activity to inhibition by MIX and papaverine. The cAMP phosphodiesterase activity of both WI-38 and VA13 supernatant preparations was decreased by cGMP in a concentration-dependent manner. cAMP was an effective inhibitor of cGMP hydrolysis by VA13 soluble phosphodiesterase activity. Yet, the cGMP phosphodiesterase activity of WI-38 supernatant fractions was only slightly reduced in the presence of cAMP. DEAE-cellulose chromatography of WI-38 and VA13 supernatant preparations revealed two major peaks of phosphodiesterase activity for each cell type. WI-38 peak I showed much greater activity with 1muM cGMP than with 1muM cAMP and appeared to be composed of two different phosphodiesterase activities. WI-38 peak Ia included phosphodiesterase activity which could be stimulated by boiled, dialyzed fibroblast homogenates while WI-38 peak Ib coincided with column fractions which contained most of the cyclic GMP hydrolytic activity. VA13 peak I phosphodiesterase activity was eluted from DEAE cellulose columns at the same ionic strength as WI-38 peak Ia and hydrolyzed these two substrates at nearly identical rates. This enzyme activity was also increased in the presence of boiled, dialyzed fibroblast preparations. Peak II phosphodiesterase activities from both WI-38 and VA13 fibroblasts were relatively specific for cAMP as the substrate. Phosphodiesterase activity with the properties of WI-38 peak Ib was not isolated from VA13 supernatant fractions. These results suggested that the dissimilar patterns of cAMP accumulation in WI-38 and VA13 cultures may be at least partially related to different phosphodiesterase activities in the normal and the transformed fibroblasts.     

Plasminogen activator and collagenase production by cultured capillary endothelial cells

Cultured bovine capillary endothelial (BCE) cells produce low levels of collagenolytic activity and significant amounts of the serine protease plasminogen activator (PA). When grown in the presence of nanomolar quantities of the tumor promoter 12-O-tetradecanoyl phorbol-13-acetate (TPA), BCE cells produced 5-15 times more collagenolytic activity and 2-10 times more PA than untreated cells. The enhanced production of these enzymes was dependent on the dose of TPA used, with maximal response at 10(-7) to 10(-8) M. Phorbol didecanoate (PDD), an analog of TPA which is an active tumor promoter, also increased protease production. 4-O-methyl-TPA and 4α-PDD, two analogs of TPA which are inactive as tumor promoters, had no effect on protease production. Increased PA and collagenase activities were detected within 7.5 and 19 h, respectively, after the addition of TPA. The TPA-stimulated BCE cells synthesized a urokinase-type PA and a typical vertebrate collagenase. BCE cells were compared with bovine aortic endothelial (BAE) cells and bovine embryonic skin (BES) fibroblasts with respect to their production of protease in response to TPA. Under normal growth conditions, low levels of collagenolyic activity were detected in the culture fluids from BCE, BAE, and BES cells. BCE cells produced 5-13 times the basal levels of collagenolytic activity in response to TPA, whereas BAE cells and BES fibroblasts showed a minimal response to TPA. Both BCE and BAE cells exhibited relatively high basal levels of PA, the production of which was stimulated approximately threefold by the addition of TPA. The observation that BCE cells and not BAE cells produced high levels of both PA and collagenase activities in response to TPA demonstrates a significant difference between these two types of endothelial cells and suggests that the enhanced detectable activities are a property unique to bovine capillary and microvessel and endothelial cells.     

Expression of gelatinases A and B and their endogenous regulators in immortal and transformed fibroblasts

Matrix metalloproteinases (MMP) play a critical role in tumor invasion and metastasis. The goal of this study was to elucidate peculiarities of expression of gelatinases A and B (MMP-2 and MMP-9), membrane type MMP (MT1-MMP) and tissue inhibitor of MMP (TIMP-2) in immortal (IF) and transformed fibroblasts (TF). The study was carried out using embryo rat fibroblasts, sequentially immortalized with the polyomavirus LT gene and transformed with the E7 gene of human papillomavirus (HPV-16). Papillomaviruses type16 and 18 are the etiological factor for cervical cancer. A primary fibroblast (PF) culture of Fisher rats was used as control. Analysis of TF and IF included determination of MMP-2 and MMP-9 activity by hydrolysis of the specific substrate, radioactive collagen type IV; analysis of MMP spectra by a zymographic assay, and estimation of the mRNA expression by RT-PCR. It was found that: (1) collagenolytic activity of MMP was increased only in TF and it depended on the degree of cell tumorigenicity; (2) the study of MMP spectra revealed the presence of MMP-9 only in TF, whereas MMP-2 was found in IF as well; (3) the mRNA expression of MMP-9, MT1-MMP and TIMP-2 increased in all TF while the MMP-2 expression increased in TF only after TF cell selection on rats; (4) the collagenolytic activity as well as the mRNA expression of MMP-2 and MMP-9 and endogenous regulators (MT1-MMP and TIMP-2) did not change in immortalized fibroblasts compared to the PF culture. The data obtained indicate changes in the ratio enzyme/activator/inhibitor and also suggest a significant increase in the TF destructive potential. MMP-9 is supposed to be a marker of fibroblasts transformed by E7 HPV16 gene in a cell culture.     

Heterogeneity of fibroblast response in host-tumor cell-cell interactions in metastatic tumors

The spread and invasion of tumor cells into host tissues are associated with the release of elevated levels of collagenolytic activity of both host and tumor cell origins. However, the mechanisms of regulation of the enzyme activity is still unresolved. Histological examination of human and animal tumors revealed morphological changes in stromal fibroblasts and mast cells at the tumor periphery. Numerous mast cells appeared at microfoci along the tumor: host tissue junction and mast cell degranulation were associated with collagenolysis. In vitro studies, using rat mammary adenocarcinoma and human lung adenocarcinoma cells, showed that both tumor cells and host fibroblasts participate in matrix degradation. Tumor-associated stromal fibroblasts released higher levels of enzyme activity than normal fibroblasts and were more responsive to stimulation by tumor-conditioned media and soluble mast cell products. Host fibroblasts appear to be heterogeneous populations of responsive and nonresponsive subpopulations based on their response to tumor- or mast-cell-mediated stimulation of collagenase release. Fibroblast subpopulations were obtained by density fractionation of serum-deprived, synchronized confluent fibroblasts on discontinuous Percoll gradient. Density-fractionated fibroblast subpopulations differed in their response to stimulation by mast cell products and tumor-cell-conditioned media. The stimulatory activity of tumor-cell-conditioned media also varied as a function of the metastatic potential of the tumor cells. The data suggest that cellular interactions between tumor cells and select subpopulations of host fibroblasts at the tumor periphery play a key role in host tissue degradation. However, heterogeneity of stromal fibroblasts may determine the site and extent of the tissue damage at foci of tumor invasion.     

Pulmonary fibroblasts mobilize the membrane-tethered matrix metalloprotease, MT1-MMP, to destructively remodel and invade interstitial type I collagen barriers

In acute and chronic lung disease, widespread disruption of tissue architecture underlies compromised pulmonary function. Pulmonary fibroblasts have been implicated as critical effectors of tissue-destructive extracellular matrix (ECM) remodeling by mobilizing a spectrum of proteolytic enzymes. Although efforts to date have focused on the catabolism of type I collagen, the predominant component of the lung interstitial matrix, the key collagenolytic enzymes employed by pulmonary fibroblasts remain unidentified. Herein, membrane type-1 matrix metalloprotease (MT1-MMP) is identified as the dominant and direct-acting protease responsible for the type I collagenolytic activity mediated by both mouse and human pulmonary fibroblasts. Furthermore, MT1-MMP is shown to be essential for pulmonary fibroblast migration within three-dimensional (3-D) hydrogels of cross-linked type I collagen that recapitulate ECM barriers encountered in the in vivo environment. Together, these findings demonstrate that MT1-MMP serves as a key effector of type I collagenolytic activity in pulmonary fibroblasts and earmark this pericellular collagenase as a potential target for therapeutic intervention.     

Dexamethasone-resistant cystic fibrosis fibroblasts show cross-resistance to sex steroids

Diploid skin fibroblasts derived from individuals with the autosomal recessive disease, cystic fibrosis (CF), were shown previously to be significantly more resistant to the cytotoxicity of dexamethasone, a glucocorticoid hormone, than were normal human fibroblasts. Here cystic fibrosis fibroblasts are also shown to be more resistant than normal human fibroblasts to the cytotoxic effects of the sex hormones, 17 beta-estradiol, dihydrotestosterone and progesterone. Since cells are believed to contain different receptors for each of the steroid hormones, it is not probable than the resistance of CF cells to these hormones results from a receptor deficiency. This was shown by the fact that CF cells were found to exhibit the same receptor activity as normal cells for 3-H-dexamethasone. Furthermore, neither normal human nor CF fibroblasts could be demonstrated to contain detectable receptor activity for 3H-17 beta-estradiol. In addition, the studies of fibroblast killing by hormones led to the further interesting observation that normal human diploid fibroblasts, regardless of the sex of the tissue donor, are sensitive to killing by each of the sex hormones. These findings suggest that the cytotoxic effects of the steroid hormones may be observed independently of the specific hormone receptors. The studies reported here thus suggest that the resistance of CF cells to the different steroid hormones is probably the result of a defect in a pathway in cellular steroid hormone metabolism other than that involving receptors.     

Transient expression of type IV collagenolytic metalloproteinase by human mononuclear phagocytes

A type IV collagenolytic metalloproteinase secreted by human monocytes/macrophages has been isolated and characterized. Monocytes isolated from peripheral blood and cultured in vitro exhibited a high type IV collagenolytic activity during the first and second day, but such activity declined markedly over subsequent days. Type IV collagenolytic activity was also transiently elaborated by macrophages isolated from (a) bronchioalveolar lavage of patients with pulmonary sarcoidosis, (b) primary human colostrum, and (c) peritoneal lavage of a patient with peritonitis. In contrast, macrophages isolated from the bronchioalveolar lavage of normal individuals, or from noninflammatory peritoneal fluids, failed to exhibit type IV collagenolytic activity. A type IV collagenolytic neutral proteinase was purified from macrophages isolated from inflammatory peritoneal fluid. The proteinase has a mass of 67 kDa on gel electrophoresis and is not altered in its migration under reducing conditions. It produces a characteristic 1/4-3/4 cleavage of type IV collagen, and its activity is abolished by treatment with EDTA but not phenylmethanesulfonyl fluoride. The isoelectric pH of the proteinase is 5.2 as judged by two-dimensional gel electrophoresis. The amino acid composition of the proteinase was notable for a high content of serine, glutamic acid, glycine, and alanine and no detectable hydroxyproline, cysteine, or methionine residues. The carbohydrate content of the proteinase was 11.2%, and galactose was the most abundant monosaccharide (8.7%) released following acid hydrolysis, followed by glucose (1.3%), mannose (1.2%), and trace amounts of fucose and galactosamine. Such a type IV collagenolytic protease may play an important role during the traversal of the vascular basement membrane by extravasating monocytes. The biochemical characteristics and biologic function of the macrophage proteinase may be similar or identical to the type IV collagenolytic proteinase identified in metastatic tumor cells.     

Identification of cell surface dipeptidylpeptidase IV in human fibroblasts.

An antigen with dipeptidylpeptidase IV activity was identified at the surface of normal human fibroblasts. Hydrophobic interaction electrophoresis in phenyl-Sepharose revealed that the enzyme contained a hydrophobic domain, while lactoperoxidase-catalysed iodination with 125I of living cells indicated that the protein was located at the cell surface. Crossed immunoelectrophoresis with specific antibodies of acid-extracted or papain-treated cells showed a shift of the dipeptidylpeptidase IV peak to a faster mobility. The molecular properties of the fibroblast enzyme were clearly different from those described for dipeptidylpeptidase IV from other tissues and species. Fibroblast dipeptidylpeptidase IV contained two different disulphide-linked subunits, of apparent Mr values 125000 and 135000 (denatured and reduced). In gel filtration, an Mr of about 400000 was observed for the unreduced molecule. The enzymic properties of fibroblast dipeptidylpeptidase IV were very similar to those of the well-characterized pig kidney enzyme. Activity towards glycyl-L-prolyl-beta-naphthylamide was inhibited 50% by 0.023 mM-di-isopropylphosphorofluoridate. L-Alanyl-L-alanyl-beta-naphthylamide was hydrolysed ten times more slowly than glycyl-L-prolyl-beta-naphthylamide.     

Fibroblast chemotaxis and prolidase activity modulation by insulin-like growth factor II and mannose 6-phosphate

Chemotactic locomotion of fibroblasts requires extensive degradation of extracellular matrix components. The degradation is provided by a variety of proteases, including lysosomal enzymes. The process is regulated by cytokines. The present study shows that mannose 6-phosphate and insulin-like growth factor II (IGF-II) enhance fibroblast chemotaxis toward platelet-derived growth factor (PDGF). It is suggested that lysosomal enzymes (bearing mannose 6-phosphate molecules) are involved in chemotactic activity of the cells. The suggestion is supported by the observation that a-mannosidase and cathepsin D inhibitor - pepstatin are very potent inhibitors of fibroblast chemotaxis. Simultaneously, mannose 6-phosphate stimulates extracellular collagen degradation. The final step in collagen degradation is catalyzed by the cytosolic enzyme - prolidase. It has been found that mannose 6-phosphate stimulates also fibroblast prolidase activity with a concomitant increase in lysosomal enzymes activity. The present study demonstrates that the prolidase activity in fibroblasts may reflect the chemotactic activity of the cells and suggests that the mechanism of cell locomotion may involve lysosomal enzyme targeting, probably through IGF-II/mannose 6-phosphate receptor.     

Effects of platelet-derived growth factor on phosphorylation of the epidermal growth factor receptor in human skin fibroblasts

Heterologous regulation of the epidermal growth factor (EGF) receptor by platelet-derived growth factor (PDGF) was studied in FS4 human skin fibroblasts. The addition of PDGF to FS4 cells inhibited high affinity binding of 125I-EGF and stimulated phosphorylation of the EGF receptor. Phosphopeptide analysis by high performance liquid chromatography revealed that PDGF treatment of cells increased phosphorylation at several distinct sites of the EGF receptor. However, PDGF did not stimulate phosphorylation of threonine 654, a residue previously shown to be phosphorylated when protein kinase C is activated. The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) also stimulated phosphorylation of the same peptides from the EGF receptor as PDGF, and, in addition, induced phosphorylation of threonine 654. TPA inhibited both high and low affinity 125I-EGF binding by these cells. PDGF treatment of cells had no effect on EGF-dependent, tyrosine-specific autophosphorylation of the receptor, whereas TPA treatment was inhibitory. TPA, but not PDGF, stimulated phosphorylation of a Mr = 80,000 protein, known to be a substrate for protein kinase C, even though PDGF appeared to mediate breakdown of phosphoinositides. These data suggest that regulation of EGF receptor function by PDGF and TPA are distinct in these cells, even though some elements of regulation are shared. The results differ from those previously reported for a human lung fibroblast isolate, indicating that cell type-specific differences may exist in metabolism of the EGF receptor.     

Butanedione treatment reduces receptor binding of a lysosomal enzyme to cells and membranes

Treatment of the lysosomal enzyme, α-L-iduronidase, with 2,3 butanedione, an arginine modifying reagent, under conditions where enzyme activity was unaffected, reduced by 50% the internalization of the enzyme into cultured human fibroblasts. The lowered rate of internalization was a result of a reduced binding of the enzyme to cell surface receptors. The butanedione treatment of α-L-iduronidase caused a 90% reduction of binding when isolated fibroblast membranes were used as the source of receptor. This marked reduction of binding was also seen when membranes from a rat chondrosarcoma were examined. Although there is ample evidence that the receptor recognizes mannose 6-phosphate residues on the enzyme, the results suggest that other structural features, such as arginine moieties, may also be important in iduronidase binding.     

Assay for radiolabeled type IV collagen in the presence of other proteins using a specific collagenase

An enzymatic assay is described which quantitates radiolabeled type IV basement membrane collagen in the presence of large amounts of other proteins. A partially purified neutral protease is used which cleaves type IV collagen into fragments at 37°C which are not precipitated at 1.3% (final concentration) trichloroacetic acid-tannic acid. The kinetics of type IV collagen digestion by this enzyme are not significantly altered by the presence of a 10-fold excess of type III collagen. [¹⁴C]Tryptophan-labeled control proteins prepared from fibroblast cultures are not degraded significantly by this protease in the presence of 2.5 mmN-ethylmaleimide. The proportion of type IV collagen in a mixture of labeled placenta collagenous proteins was calculated after separate digestions with the type IV collagenolytic activity and bacterial collagenase: this value compared favorably with the proportion of type IV collagen estimated by gel electrophoresis.     

Fibroblast heterogeneity in collagenolytic response to cyclosporine.

To investigate the mechanism of cyclosporine (CS)-induced fibrotic gingival enlargement, the effect of CS on the collagenolytic activities of 14 different human gingival fibroblast strains derived from healthy individuals with non-inflammed gingiva was examined in vitro. There was marked heterogeneity among individuals in basal levels of collagenase activity, and there was also variation among the subpopulations derived from one strain. Fibroblasts from different individuals also varied markedly in their collagenolytic response to CS (0.1 to 0.75 micrograms/ml). In most strains, CS decreased collagenase activity, but in some, the drug caused no change or significantly increased activities. In most of the subpopulations CS significantly decreased collagenolytic activity. Two of the fibroblasts strains and the subpopulations described above were examined for the production of immunoreactive collagenase and tissue inhibitor of metalloproteinase (TIMP). The two strains made similar amounts of collagenase, but differed markedly in TIMP levels; CS affected their collagenase production differently but had similar effects on TIMP. Among the subpopulations there was variation in the production of collagenase, although none made detectable levels of TIMP; they also varied in the production of both proteins in response to CS. In two of the subpopulations and in both strains at some concentrations, the effect of CS on the relative levels of collagenase and TIMP could account for the decreased collagenase activity; i.e., the level of collagenase was unchanged or decreased, and TIMP production was unchanged or increased. This study demonstrates the variation among individuals as well as intrastrain heterogeneity of human gingival fibroblasts with regard to collagenase activity and the production of collagenase and TIMP. The heterogeneity of the collagenolytic response of different gingival fibroblast strains and their subpopulations to CS treatment may partly explain the susceptibility of only some individuals to CS-induced gingival enlargement.     

A high level of cell surface phosphatidylinositol-specific phospholipase C activity is characteristic of growth-arrested 3T3 fibroblasts but not of transformed variants.

Confluent monolayers of four contact-inhibited mouse fibroblast lines (Swiss 3T3, Balb/c 3T3, NIH 3T3, and C3H10T1/2) were found to have substantial levels of a cell surface phosphatidylinositol-specific phospholipase C (ecto-PLC). In contrast, confluent cultures of virally, chemically, or spontaneously transformed variants derived from these cell lines expressed undetectable or negligible levels of this enzyme activity. A simple and rapid assay, using lysophosphatidylinositol radio-labeled in the inositol group ([3H]-lysoPI) as the substrate was developed to provide a quantitative measure of the phospholipase C activity present at the external cell surface. For cells testing positive for ecto-PLC activity, rapid uptake of [3H]-lysoPI is accompanied by the simultaneous appearance of [3H]-inositol phosphate in the external medium. Confluent monolayers of the four mouse fibroblast lines exhibiting density-dependent growth inhibition had levels of ecto-PLC activity in the range of 50-800 pmol/min/10(6) cells (i.e., about 20-50 times greater than the activity observed for the transformed variants). The expression of ecto-PLC activity at the cell surface of the Swiss or Balb/c cells was dependent on the state of cell proliferation. Cultures which had become quiescent through attainment of confluence displayed a tenfold increased activity over that of subconfluent, growing cultures of these cells. Similarly, subconfluent Swiss 3T3 cells which had become quiescent following exposure to low serum conditions also showed increased activity. These results indicate that there may exist a correlation between the control of cell proliferation in contact-inhibited mouse fibroblasts and the expression of inositol phospholipid-specific phospholipase C activity at the external cell surface.     

Regulation of the phosphorylation state and function of the epidermal growth factor receptor by vitamin K-3

Vitamin K-3 or 12-O-tetradecanoylphorbol 13-acetate (TPA) reduced the binding of epidermal growth factor (EGF) to its receptor by more than 90% in human foreskin fibroblasts. After the equilibration of fibroblasts with [32P]orthophosphate, vitamin K-3 or TPA markedly increased the amount of 32P found in the receptor; the increase was principally due to serine and threonine phosphorylation. By the use of two-dimensional tryptic phosphopeptide mapping, using a synthetic phosphopeptide as a standard, threonine-654 was identified as one of the residues whose phosphorylation state was elevated by vitamin K-3 or TPA. Because of the large amounts of EGF receptor present on A431 human carcinoma cells, these cells were used to study further the relationship between the phosphorylation state of threonine-654, the tyrosine phosphorylation state of the receptor, and the receptor's protein tyrosine kinase activity toward exogenous substrates. Vitamin K-3 and TPA both increased the amount of phosphate on threonine-654 in A431 cells. However, whereas receptor from TPA-treated cells lacked phosphotyrosine, vitamin K-3-treated cells contained receptor with markedly elevated levels of phosphotyrosine. The addition of vitamin K-3, TPA or EGF to intact A431 cells followed by homogenization of the cells and the assay of EGF receptor protein tyrosine kinase activity by the use of a synthetic peptide substrate resulted in marked decreases in apparent receptor kinase activity. Therefore, assuming that the activity measured in the peptide assay reflects the protein tyrosine kinase activity of the receptor in the intact cell, the activity of the EGF receptor kinase cannot be deduced from the amount of phosphotyrosine associated with the receptor.     

Regulation of synthesis and activity of NAD(+)-dependent 15-hydroxy-prostaglandin dehydrogenase (15-PGDH) by dexamethasone and phorbol ester in human erythroleukemia (HEL) cells

Dexamethasone stimulated 15-PGDH activity in HEL cells in a time and concentration dependent manner. Increase in 15-PGDH activity by dexamethasone was found to be accompanied by an increase in enzyme synthesis as revealed by Western blot and [35S]methionine labeling studies. In addition to dexamethasone, other anti-inflammatory steroids also increased 15-PGDH activity in the order of their glucocorticoid activity. Among sex steroids only progesterone increased significantly 15-PGDH activity. 12-0-Tetradecanoylphorbol-13-acetate (TPA) also induced the synthesis of 15-PGDH but inhibited the enzyme activity. It appears that TPA caused a time dependent inactivation of 15-PGDH by a protein kinase C mediated mechanism.