DNA Synthesis In Human Fibroblasts Research Articles

Inability of anti-epidermal growth factor receptor monoclonal antibody to block “autocrine” growth stimulation in transforming growth factor-secreting melanoma cells.

Mouse monoclonal antibodies to the human epidermal growth factor (EGF) receptor were raised by immunizing with plasma membrane vesicles prepared from A431 cells. This paper describes the characterization of one of the IgG anti-receptor monoclonal antibodies generated and its use to probe the role of transforming growth factor (TGF) in the autonomous growth of a melanoma cell line in culture. This antibody blocks: 1) the binding of 125I-EGF to the A431 EGF receptor; 2) the EGF stimulation of the EGF-dependent protein kinase in vitro; and 3) human fibroblast DNA synthesis and proliferation in culture. It can precipitate the EGF receptor from metabolically labeled A431 cells and human fibroblasts and these receptors have indistinguishable peptide maps. No EGF receptor could be detected by immunoprecipitation after fibroblasts were treated with EGF or conditioned medium from the melanoma cells which secrete EGF-like TGF (alpha TGF). The antibody itself did not down-regulate the receptor but could block down-regulation caused by EGF and alpha TGF. Despite its ability to block EGF-stimulated growth and down-regulation in fibroblasts, the antibody was unable to block the growth and soft agar colony formation of alpha TGF-secreting melanoma cells, nor could the antibody detect EGF receptor in these cells under the conditions developed to prevent down-regulation and lysosomal degradation of the EGF receptor. These studies suggest that these melanoma cells do not have the intact EGF receptor and that the secretion of alpha TGF by these cells plays no role in their growth in culture. The absence of receptor cannot be explained by down-regulation by secreted alpha TGF.

Antibodies to the epidermal growth factor receptor block the biological activities of sarcoma growth factor.

The role of the epidermal growth factor (EGF) receptor system in mediating the biological activities of sarcoma growth factor (SGF) has been assessed by using specific anti-EGF receptor antibodies. There are two classes of anti-EGF receptor antibodies, those that block binding of 125I-labeled EGF (125I-EGF) and those that do not block binding but do interact with a portion of the EGF receptor on the surface of intact cells. Antisera of both types have been assayed for their capacity to affect the biological activities of SGF. The antisera that block 125I-EGF binding to its receptor block the induction of DNA synthesis in human fibroblasts by either EGF or SGF but not by other polypeptide mitogens. Titration of the anti-EGF receptor antiserum indicates the presence of one population of antibody that blocks the site of both EGF and SGF action. Antisera to the EGF receptor that block 125I-EGF binding also inhibited the SGF-dependent anchorage-independent growth of normal cells in soft agar. The antisera to the EGF receptor that does not block 125I-EGF binding or EGF activity did not inhibit any of the biological activities of SGF. The results suggest that occupation of the EGF receptor is required for both the mitogenic and colony-forming activity of SGF.

Insulin stimulation of somatomedin A production in monolayer cultures of rat hepatocytes.

The effect of insulin on the generation of somatomedin by a monolayer culture of rat hepatocytes was investigated. Release of somatomedin into the medium of cultured hepatocytes was observed for at least three days, as determined by a radioreceptor assay for somatomedin A. The addition of 1-100 micrograms/ml insulin to the medium caused dose-related increases in somatomedin A in the conditioned medium for up to three days. The conditioned medium also stimulated DNA synthesis in human fibroblasts. To investigate hormone degradation by hepatocytes, labelled somatomedin A or insulin was added to the medium and the degradation was examined by trichloroacetic acid (TCA) precipitation, and gel chromatography. Although [125I]somatomedin A was slightly degraded even after 24 h of incubation, [125I]insulin was degraded within 6 h and the TCA precipitation at 24 h was 15% only. This degradation of hormones was inhibited by 1 mM bacitracin. Insulin stimulation of somatomedin production was significantly increased to 131.2% as compared with cultures without bacitracin. The molecular size of the somatomedin released into the medium was greater than that of purified somatomedin, suggesting that this somatomedin is bound to carrier proteins.

Sharing of biological effect and receptors between guinea pig insulin and platelet-derived growth factor.

Insulins from the hystricomorphs (guinea pig, porcupine, coypu, and casiragua) at high concentration stimulate DNA synthesis in human fibroblasts to a greater level than other mammalian insulins or insulin-like growth factors (IGFs). 125I-Labeled guinea pig insulin binds to a specific receptor and this binding is competed for by hystricomorph insulins but not by porcine insulin or IGFs. Fetal bovine serum also inhibits the binding of 125I-labeled guinea pig insulin and is more potent than fetal bovine plasma, in concordance with their relative potencies for growth stimulation in human fibroblasts. Of several other known growth factors tested, only platelet-derived growth factor (PDGF) inhibits binding of 125I-labeled guinea pig insulin. Four preparations of PDGF that vary in purity and potency for the stimulation of DNA synthesis in human fibroblasts over a 1,000-fold range compete with binding of 125I-labeled guinea pig insulin in proportion to their biological potencies. The purest preparation of PDGF is able to inhibit binding of 125I-labeled guinea pig insulin by 50% at 15 ng/ml (0.25 nM). Biologically, guinea pig insulin, like PDGF, exhibits a synergistic effect with plasma in initiating DNA synthesis in human fibroblasts; this effect is not observed with other mammalian insulins or IGFs. Thus, hystricomorph insulins appear to be mediating their growth-promoting effect through a different receptor and mechanism than other mammalian insulins or IGFs. Further, hystricomorph insulins may be sharing the mechanism of action for their growth effects with PDGF, perhaps suggesting some relationship between these peptides from very different sources.

A nonmitogenic analogue of epidermal growth factor induces early responses mediated by epidermal growth factor.

Cyanogen bromide-cleaved epidermal growth factor (CNBr-EGF) binds to EGF receptors with reduced affinity compared to the native hormone but fails to induce DNA synthesis. However, at similar receptor occupancy, CNBr-EGF is as potent as EGF in activating early cell responses to the hormone. The phosphorylation of membrane proteins, the stimulation of Na+-K+-ATPase as reflected by the ouabain-sensitive uptake of 86Rb of fibroblasts, changes in the organization of microfilaments and in cell-morphology, and the activation of the enzyme ornithine-decarboxylase are all induced by CNBr-EGF as well as EGF Our results are consistent with the notion that EGF-induced phosphorylation could act as a "second messenger" for the action of various EGF-induced responses such as activation of Na+-K+-ATPase, changes in the cytoskeleton and cell morphology, and the activation of the enzyme ornithine decarboxylase. However, the stimulation of phosphorylation of membrane proteins and other early responses are either not required or necessary but insufficient for the induction of DNA synthesis. Suboptimal concentrations of EGF together with CNBr-EGF stimulate DNA synthesis in human fibroblasts. Other growth factors such as insulin, fibroblast growth factor, and prostaglandin F2 alpha, which potentiate the mitogenic response of EGF, do not effect the response to CNBr-EGF. This suggests that the restoration of the mitogenic properties of CNBr-EGF by suboptimal doses of EGF occurs at the level of EGF receptors or during their processing.

The relationship of serum fibronectin and cell shape to thrombin-induced inhibition of DNA synthesis in human fibroblasts.

AbstractIn a previous report it was shown that inhibited DNA synthesis and altered morphology resulted when human fibroblasts (HF) were plated in 3% fetal calf serum (FCS) medium preincubated with thrombin (Hall and Ganguly, 1980a). This was in contrast to the stimulatory effects of this enzyme when added to cells several hours after subculture. Those observations suggested that thrombin may act upon serum components of the growth medium necessary for initial culture establishment following cell plating. In this report, the relationship of serum fibronectin (FN) to this thrombin‐mediated inhibitory phenomena was investigated. It was found that the development of altered morphology and inhibited DNA synthesis could be completely prevented by the addition of this glycoprotein to medium preincubated with thrombin. Cell shape and DNA synthesis appeared to be closely related and both parameters showed a dose‐dependent sensitivity to added fibronectin. To further investigate this, a technique was developed in which cell shape could be selectively varied and DNA synthesis measured in the absence of serum or thrombin. These studies indicated that cell shape was closely related to DNA synthesis and morphologies identical to that seen in thrombin‐treated medium were produced. As observed in the thrombin system, normal cellular appearance and DNA synthesis could be restored by the addition of fibronectin. The results of this work suggest that thrombin acts upon medium components necessary for normal morphological development, possibly fibronectin, in cells following subculture. Inhibited DNA synthesis and growth seem to arise as a direct consequence of this effect.

Effect of 2-deoxy-D-glucose on cytomegalovirus-induced DNA synthesis in human fibroblasts.

2-Deoxy-D-glucose (dGlc) was found to selectively inhibit virus DNA synthesis in human embryonic lung cells infected with human cytomegalovirus (HCMV). The effective concentration of dGlc was approx. 10-fold higher in culture medium containing glucose instead of sodium pyruvate. This inhibitory action of dGlc was fully reversible following replacement of the inhibitor medium by fresh medium after a 48 h treatment of infected cells. Virus DNA synthesis could be selectively inhibited by addition of dGlc even after initiation of HCMV DNA replication. In contrast, virus DNA synthesis in herpes simplex virus-infected cells was insensitive to dGlc. The drug was found to deplete HCMV-infected cells of uridine triphosphate and caused a progressive reduction of uridine incorporation into RNA. To substantiate a possible interference by dGlc with the expression and/or function of virus-induced, chromatin-associated factors essential for virus DNA replication, DNA synthesis of chromatin preparations from dGlc-treated, HCMV-infected cells was analysed. In contrast to preparations of untreated or phosphonoacetic acid (PAA)-treated, HCMV-infected cells, those of dGlc-treated cells lacked significant in vitro DNA-synthesizing activity; virus DNA was not synthesized by these preparations. Tunicamycin in the presence of low concentrations of dimethyl sulphoxide was also found to be effective in abolishing HCMV-induced DNA replication. It is thus suggested that dGlc interferes with the function of an 'early' chromatin-associated glycoprotein essential for virus DNA synthesis.

The mutagenicity of isoniazid in salmonella and its effects on dna repair and synthesis in human fibroblasts

A commercial sample of the tuberculostatic drug isoniazid (INH) was found to have a weak mutagenic activity towards Salmonella typhimurium strains TA100 and TA1535. The addition of a rat or mouse liver homogenate to the test system decreased the mutagenic effect of INH. Hydrazine, an impurity of the INH sample, was also weakly mutagenic in strains TA100 and TA1535, but not to the extent that could account for the mutagenicity of the INH sample. An inhibition of DNA synthesis in human fibroblasts was observed for INH, this effect being potentiated by the addition of manganese to the test system. There was no induction of unscheduled DNA synthesis in human cells by INH except in the presence of manganese.

A simplified method for measuring scheduled and unscheduled DNA synthesis in human fibroblasts

A simplified method for the rapid measurement of scheduled and unscheduled DNA synthesis in cultures of human fibroblasts is described. The use of cultures in which DNA synthesis is blocked by hydroxyurea (HU) together with those which are not blocked allows an estimation of the effect of chemicals on both types of DNA synthesis. This facilitates the interpretation of results and the selection of an optimum range of doses. Some results obtained with carcinogens and non-carcinogens are presented of the compounds so far tested. The methodology here described compares well in its validity with other more difficult and time consuming methods.

Synthesis of DNA in human fibroblasts treated with T-2 toxin and HT-2 toxin (the trichothecene metabolites of Fusarium species) and the effects of hydroxyurea

(1) 3α-Hydroxy-4β,15-diacetoxy-8α-(3-methylbutyryloxy)-12,13-epoxytrichothec-9-ene (T-2 toxin) and HT-2 toxin both inhibited the incorporation of tritiated thymidine into the DNA of human fibroblasts; the inhibition was dose-related. 1. (2) The presence of hydroxyurea (HU) did not affect unscheduled DNA synthesis in human fibroblasts damaged by T-2 toxin (0.006 μg/ml to 20.0 μg/ml), or by HT-2 toxin (0.032 μg/ml to 100.0 μg/ml) as with cycloheximide. 2. (3) In the presence of the rat-liver microsomal fraction, S-9 and of hydroxyurea, there was an increase in unscheduled DNA synthesis in human fibroblasts exposed to 100 μg/ml of HT-2 toxin. 3. (4) Thus in the presence of the microsomal S-9 fraction and of hydroxyurea the effect of HT-2 toxin (at 100 μg/ml) appeared to resemble that of benzo(a)pyrene: further studies are needed to clarify the rôle of its derivatives in the mechanism of action of T-2 toxin.

Effects of [formula omitted] lysozyme on human fibroblasts

The purified lysozyme excreted by Staphylococcus aureus strains promotes elongation and spreading on plastic surfaces and stimulates DNA synthesis of human fibroblasts (WI38). This enzyme also raises twofold the saturation density level of cultures of these cells. It is suggested that the primitive and main effect of lysozyme on fibroblasts is the triggering of morphogenesis.

Stimulation of DNA synthesis in human fibroblasts by a human nonsuppressible insulin-like protein (NSILP)

When nonsuppressible insulin-like protein (NSILP) isolated and purified from human serum was added at concentrations of 5 and 50 ug/ml to cultures of human dermal fibroblasts, both cell proliferation and DNA synthesis were enhanced. However, NSILP, 50 ug/ml, had no effect on glucose uptake. In contrast, insulin, 40 ng/ml (1.0 mU/ml), had no effect on cell proliferation or DNA synthesis, but stimulated glucose uptake. These observations suggest that human NSILP may play an important role in tissue repair or growth by enhancing fibroblast proliferation, but not a significant glucoregulatory role.

Stimulation of DNA synthesis in human fibroblasts by thrombin

Earlier work showed that thrombin stimulates proliferation of human fibroblasts in serumfree medium. This work demonstrates (1) that thrombin has to be prensent during most or all of the G1 period to ensure maximal DNA synthesis, (2) that DNA synthesis increases about three hours later after thrombin than after serum treatment, (3) that both thrombin and serum activate transport of uridine, D--2-deoxy-glucose and putrescine, (4) that thrombin is able to increase 3H-thymidine incorporation also in SV40 transformed human fibroblasts, in HeLa cells and in two continuous monkey cell lines.

A cell-free assay measuring repair DNA synthesis in human fibroblasts.

Osmotic disruption of confluent cultured human fibroblasts that have been irradiated or exposed to chemical carcinogens allows the specific measurement of repair DNA synthesis using dTTP as a precursor. Fibroblasts similarly prepared from various xeroderma pigmentosum cell lines show the deficiencies of UV-induced DNA synthesis predicted from in vivo studies, while giving normal responses to methyl methanesulfonate. A pyrimidine-dimer-specific enzyme, T4 endonuclease V, stimulated the rate of UV-induced repair synthesis with normal and xeroderma pigmentosum cell lines. This system should prove useful for identifying agents that induce DNA repair, and cells that respond abnormally to such induction. It should also be applicable to an in vitro complementation assay with repair-defective cells and proteins obtained from repair-proficient cells. Finally, by using actively growing fibroblasts and thymidine in the system, DNA replication can be measured and studied in vitro.

Purified human somatomedin A and rat multiplication stimulating activity. Mitogens for cultured fibroblasts that cross-react with the same growth peptide receptors.

We have compared the mitogenic activity and reactivity in receptor binding assays of somatomedin A and multiplication‐stimulating activity (MSA), two acid‐soluble polypeptides of molecular weight 7–10 000 that possess weak intrinsic insulin‐like metabolic activity. Somatomedin A was purified to homogeneity from human plasma, and MSA was purified to homogeneity from the conditioned culture media of a cell line, BRL 3A, derived from rat liver. The two peptides stimulated DNA synthesis in chick embryo fibroblasts 3–4‐fold and have superimposable dose‐response curves. Addition of either peptide to chick embryo fibroblasts plated in serum‐free medium increased the cell number by 150% in 4 days, compared with an increase of 25% in control cultures receiving no additions. The two peptides also stimulated DNA synthesis in human fibroblasts in culture 3–4‐fold.Specific receptors for MSA and/or somatomedin A could be demonstrated in intact cells or membranes from chick embryo fibroblasts, human fibroblasts, human placenta, rat liver, and the BRL 3A2 cell line, a subclone of the line that produces MSA. Unlabeled MSA and somatomedin A inhibited the binding of 125I‐labeled MSA and 125I‐labeled somatomedin A to each of these receptors with comparable potency. In chick embryo fibroblasts, human fibroblasts, and human placental membranes, the binding of both radioactive ligands also was inhibited by insulin, consistent with the interpretation that 125I‐labeled MSA and 125I‐labeled somatomedin A were binding to the same receptor. By contrast, in the BRL 3A2 cell line, insulin inhibited the binding of 125I‐labeled somatomedin A, but not the binding of 125I‐labeled MSA, suggesting that the two labeled peptides were binding to different receptors in this cell line. Moreover, 125I‐labeled MSA, but not 125I‐labeled somatomedin A, bound specifically to rat liver plasma membranes. These results indicate that human somatomedin A and rat MSA are closely related, but not identical, peptides. They have similar mitogenic activity for cultured fibroblasts, and cross‐react with specific cell receptors with comparable potency. The two radioactively labeled peptides, however, can be distinguished by certain cell surface receptors for somatomedin‐like peptides.

Modulation of serum-stimulated DNA synthesis in cultured human fibroblasts by cAMP

Density-dependent inhibition of growth of cultured human fibroblasts was associated with a 3- to 4-fold rise in the intracellular concentration of cyclic AMP (cAMP). Serum lowered cAMP levels in 2–5 min, with the low levels persisting for several hours. When quiescent fibroblast cultures were treated with 10% serum, the incorporation of [ 3H]TdR into DNA increased after a 10–16 h lag, reaching a peak by 20–24 h. Dibutyryl cyclic AMP (db-cAMP), when present throughout serum treatment, produced a dose-dependent inhibition of [ 3H]TdR incorporation. Half-maximal inhibition was seen with 0.1 mM db-cAMP. When db-cAMP or another cyclic nucleotide phosphodiesterase inhibitor, l-methyl-3-isobutylxanthine (SC-2964), was added together with serum to maintain elevated cAMP levels and after 4 h was replaced with fresh serum-containing medium, the wave of DNA synthesis induced by serum was not delayed. This implied that stimulation by serum could occur without an initial decrease in cAMP concentration. In contrast, db-cAMP added 8 h later than serum and not removed, inhibited [ 3H]TdR incorporation at the peak to the same extent as db-cAMP added together with serum. The inhibition decreased progressively when db-cAMP was added more than 8 h after serum. These results suggested that a cAMP-sensitive step occurred approx. 8 h after the addition of serum in mid-G1 of the cell cycle. Results obtained using fibroblasts synchronized at the G1/S boundary with hydroxyurea or exposed to db-cAMP for 24 h suggested that db-cAMP also inhibited TdR incorporation at the G1/S interphase or during S phase. Thus, whereas reduced cAMP concentrations did not appear to serve as an initial trigger for serum-stimulated DNA synthesis in human fibroblasts, db-cAMP and SC-2964, presumably by elevating cAMP levels, appeared to act in mid-G1 and possibly at the G1/S boundary or within S phase to inhibit thymidine incorporation.