An 18,000 molecular weight polypeptide induces early events and stimulates DNA synthesis in cultured cells

Insulin-induced mitogenesis associated with transformation by the SV40 large T antigen

Simian virus 40 (SV40) large T antigen-transformed cells typically show a markedly reduced serum requirement for growth and the inability to growth arrest and differentiate. An SV40 large T antigen-transformed 3T3 T cell line, CSV3-1, that can growth arrest and differentiate into adipocytes with high efficiency has, however, recently been described (Scott et al: Proc. Natl. Acad. Sci. U.S.A. 86:1652-1656, 1989; Estervig et al: J. Virol. 63:2718-2725, 1989; J. Cell. Physiol. 142:552-558, 1990). The results of the current studies using these cells show that whereas quiescent 3T3 T cells show no mitogenic response to insulin, quiescent CSV3-1 cells show a highly significant insulin-induced mitogenic responsiveness in the absence of other added growth factors. Maximum mitogenesis was observed at an insulin concentration of 1 microgram/ml, which induced 40-70% of the cells to undergo DNA synthesis within 48 hours. The half maximum response was achieved with 1-10 ng/ml of insulin. Insulin's mitogenic effect on CSV3-1 cells was evident under several different culture conditions that induce quiescence and was not mediated by any detectable autocrine growth factors that might make CSV3-1 cells competent to respond to insulin. In CSV3-1 cells insulin appears to act on its own receptor rather than on the IGF-1 receptor, because at comparable dosages IGF-1 is 10- to 100-fold less effective than insulin. Insulin also is shown to be a mitogen for another SV40-transformed cell line, CSV3-35, which can be growth arrested; in contrast insulin has no mitogenic effect on two control cell lines that are stably transfected with pSV2neo, a plasmid containing SV40 early promoter/enhancer but lacking large T antigen gene: These results suggest a significant relationship between SV40 T antigen-associated transformation and the expression of mitogenic responsiveness to insulin.

Autocrine growth factor and spontaneous transformation of rat liver epithelial cells

Liver epithelial cells from a young rat were cultured and passaged for a long period until they showed a malignant transformation (tumorigenicity in athymic mice), to know when an autocrine growth factor is produced in the course of a spontaneous malignant transformation of normal cells in vitro. The cells initially showed a cellular transformation and an increased growth rate, then produced an autocrine growth factor after Passage 55, and finally the cells after Passage 105 showed a malignant transformation (tumorigenicity in vivo). It is suggested that in addition to an autocrine growth factor, some other factors might be related to a malignant transformation (tumorigenicity in vivo).

Effects of serum, cycloheximide and actinomycin D on protein secretion by quiescent mouse embryo fibroblasts

Quiescent secondary cultures of Swiss mouse embryo fibroblasts secrete several proteins in response to the addition of 20% fetal calf serum (FCS). Of these proteins, a polypeptide of molecular weight (Mr) 48,000 (48 K) was identified in the medium within an hour of mitogenic stimuli. In the next hour an additional protein of Mr26,000 (26 K) appeared in the medium. These two proteins were absent in the conditioned medium of quiescent cells. A third protein of molecular weight 45,000 (45 K) was found in small quantities in the conditioned medium of quiescent cells but a 2-3 fold increase in the level of this protein was observed in the medium of stimulated cells. The level of the serum-induced 45 K protein was much higher in the medium of cells that were treated with cycloheximide (CH) and FCS than that found in the medium of cells treated with FCS alone. A 40,000 dalton protein was found to be a quiescence specific protein which was observed in large amounts in the medium of quiescent cells; the level of this protein gradually declined in the conditioned medium as the cells entered into the proliferative phase. Actinomycin D specifically inhibited the level of the 45 K secreted protein and a 29 K intracellular protein when added along with CH. In contrast to the inhibition of the synthesis of mitogen induced proteins, actinomycin D super-induced the intracellular and extracellular levels of the matrix proteins fibronectin and procollagens.

Characterization of Na+ transport in normal human fibroblasts and neoplastic H.Ep.2 cells and the role of inhibitin

Na+ transport was characterized in normal human fibroblasts and neoplastic H.Ep.2 cells in order to investigate the role of the endogenous peptidic factor 'inhibitin' that is secreted by a variety of neoplastic cells (including H.Ep.2) and inhibits Na+/Na+ exchange in human erythrocytes. Although active (Na+,K+-ATPase mediated) Na+ fluxes were similar in the two cell types, H.Ep.2 cells maintained higher intracellular Na+ concentration (26 mM) compared to fibroblasts (12 mM). An analysis of passive Na+ fluxes showed a difference in the handling of Na+ via ouabain and bumetanide-insensitive transport between the two cell types: H.Ep.2 cells achieved net Na+ influx via an amiloride-sensitive pathway that was only demonstrated in fibroblasts when 10% fetal calf serum (FCS) was present. Kinetic studies were undertaken to investigate the interaction between Na+ flux via Na+/H+ and Na+/Na+ exchanges. For this purpose, an outwardly directed Na+ gradient was created by loading the cells with Na+ (Nai greater than 100 mM) to activate the reverse functioning of Na+/H+ exchange (i.e., Na+out H+in). The rates of ouabain- and bumetanide-insensitive Na+ efflux were measured over a range of extracellular Na+ concentrations (Na+o 14-140 mM). In the presence of 10% FCS, the two cell types showed different responses: in fibroblasts the Na+ efflux rate showed an inverse correlation with extracellular Na+ concentration, while H.Ep.2 cells significantly increased their rate of Na+ efflux as extracellular Na+ concentration increased. So although the thermodynamic force would direct net Na+ efflux when Na+i greater than Na+o, H.Ep.2 cells were under kinetic control to perform Na+/Na+ exchange. When exogenous inhibitin was tested on fibroblasts, the steady-state intracellular Na+ concentration increased from 14 to 19 mM (p less than 0.01). In Na+-loaded fibroblasts, serum-stimulated Na+ efflux was partially inhibitin sensitive and the maximal inhibitory effect was seen when extracellular Na+ concentration was 14 mM and presumably the Na+/H+ exchanger operating in the reverse mode. This study demonstrated that, in contrast to fibroblasts, H.Ep.2 cells have a modified Na+/H+ exchange system whereby it acts in the Na+in H+out mode without exogenous growth factor activation and resists functioning in the reversed mode. It is proposed that inhibitin is the endogenous modifier of this transport system in H.Ep.2 cells with the result that H.Ep.2 cells maintain a higher concentration of intracellular Na+ compared to fibroblasts.

Insufficiency of transformation by simian virus 40, polyomavirus, EJ-ras, or v-myc oncogenes for conversion of ethanolamine-responsive mammary cells to ethanolamine-nonresponsive cells

Normal mammary epithelial cells (ethanolamine responsive) require ethanolamine to enable them to grow in defined culture medium because they cannot synthesize de novo a sufficient amount of phosphatidylethanolamine. Mammary tumor cells which retain properties of the normal tissue are also likely to be ethanolamine responsive, whereas dedifferentiated, highly tumorigenic mammary tumor cells are ethanolamine nonresponsive. The nonresponsive tumor cells are able to synthesize the necessary amount of phosphatidylethanolamine to sustain growth. Therefore, the progression of malignancy seems to convert ethanolamine-responsive mammary cells to ethanolamine-nonresponsive ones. In an attempt to prove the above assumption and to understand the mechanism responsible for the conversion during the progression of malignant transformation, mammary tumor cell line 64-24, which is typically ethanolamine responsive, was transfected with simian virus 40, polyomavirus, EJ-ras, or v-myc oncogenes, and the resulting transfectants were examined for their growth response to ethanolamine. Many of the transfectants exhibited typical transformed phenotypes; however, none of the transfectants converted to ethanolamine-nonresponsive cells. Some of the SV40 and polyomavirus transformants were able to grow in the absence of ethanolamine, although they grew better in the presence of ethanolamine, unlike typical ethanolamine-nonresponsive cells. These cells could grow in the absence of ethanolamine, even though their membrane phospholipid was phosphatidylethanolamine deficient. The present study indicates that the expression of any one of the four oncogenes tested, which allows the cells to exhibit transformed phenotypes in 64-24 cells, is not sufficient for the conversion of ethanolamine-responsive cells to -nonresponsive cells.

Regulation of cell growth by interferon

Interferons can regulate growth and differentiation in a wide range of cell types. These mechanisms are currently being examined. Interferons inhibit the growth of tumour cells and are thus potential anti-cancer agents. They can also inhibit normal cell growth in vitro, and stimulate tumour cell growth in vitro. They may also be involved in some autoimmune diseases. This review examines the effect of interferons on cell proliferation, function, and growth, focusing primarily on in vitro cell systems.

Augmentation of fibroblast proliferation by bleomycin

Bleomycin-induced lung disease is characterized by fibroblast proliferation and pulmonary fibrosis. In these studies, we demonstrate that fibroblasts are relatively resistant to clinically relevant amounts (below 10(-4) U/ml) of bleomycin and that these levels of bleomycin augment fibroblast proliferation in response to various fibroblast growth factors. These observations suggest that one mechanism by which bleomycin causes pulmonary fibrosis is augmentation of fibroblast proliferation.

Early signals in the mitogenic response

Polypeptide growth factors, regulatory peptides, and a variety of pharmacological agents acting alone or synergistically induce mitogenesis in cultured fibroblasts. The early signals in the membrane, cytosol, and nucleus promoted by these extracellular factors, together with their mitogenic effectiveness, are integrated in a unified hypothesis for the regulation of fibroblast growth.

High-affinity receptors for peptides of the bombesin family in Swiss 3T3 cells

Gastrin-releasing peptide (GRP) labeled with 125I at tyrosine-15 (125I-GRP) binds to intact quiescent Swiss 3T3 cells in a specific and saturable manner. Scatchard analysis indicates the presence of a single class of high-affinity binding sites of Kd = 0.5 X 10(-9) M and a value for the number of sites per cell of about 100,000. 125I-GRP binding was not inhibited by other mitogens for these cells, and cell lines that are mitogenically unresponsive to GRP do not exhibit specific GRP binding. Structure-activity relationships show a close parallel between the ability of a range of GRP-related peptides to both inhibit GRP binding and to stimulate mitogenesis. Further, GRP binding is selectively blocked in a competitive fashion by a novel bombesin antagonist, [D-Arg1, D-Pro2, D-Trp7,9, Leu11] substance P. In addition, this compound selectively inhibits GRP and bombesin-induced mitogenesis. These results demonstrate that the mitogenic response of Swiss 3T3 cells to peptides of the bombesin family is mediated by a class of receptors distinct from those of other mitogens for these cells.

Diacylglycerol treatment rapidly decreases the affinity of the epidermal growth factor receptors of Swiss 3T3 cells

The synthetic diacylglycerol 1-oleoyl-2-acetyl glycerol (OAG) and phorbol esters activate protein kinase C in intact cells. We report here that OAG inhibits the binding of 125I-labelled epidermal growth factor (125I-EGF) to Swiss 3T3 cells. The inhibition was detected as early as 1 min after treatment at 37 degrees C and persisted for at least 120 min. The effect of OAG was reversed upon removal of this diacylglycerol. Detailed Scatchard analysis of 125I-EGF binding to Swiss 3T3 cells at 4 degrees C after a 1 h incubation with a saturating dose of OAG at 37 degrees C, demonstrates that this OAG pretreatment does not change the apparent number of EGF receptors but causes a marked decrease in their apparent affinity for the ligand. Prolonged treatment (40 h) of the cells with phorbol dibutyrate (PBt2) which causes a marked decrease in the number of phorbol ester binding sites and in the activity of protein kinase C, prevented the inhibition of 125I-EGF binding by both PBt2 and OAG. The results support the possibility that protein kinase C plays a role in the transmodulation of the EGF receptor in intact cells.

Diacylglycerol stimulates DNA synthesis and cell division in mouse 3T3 cells: role of Ca2+-sensitive phospholipid-dependent protein kinase

The synthetic diacylglycerol 1-oleoyl-2-acetylglycerol competes directly with [3H]phorbol 12,13-dibutyrate for common binding sites in monolayer cultures of Swiss 3T3 cells and rapidly stimulates the phosphorylation of a Mr 80,000 cellular protein that has recently been shown to reflect the activation of protein kinase C in intact cells. Thus, this diacylglycerol provided a useful tool to determine whether exogenously added diacylglycerols can mimic the potent tumor promoter phorbol ester in eliciting DNA synthesis and cell division in quiescent cells. We found that OAG acts synergistically with insulin and other growth factors to stimulate reinitiation of cell proliferation, and several lines of evidence indicate that OAG shares with phorbol esters a common pathway of mitogenic action via stimulation of protein kinase C activity in intact 3T3 cells. The findings support the hypothesis that diacylglycerols represent endogenous analogs of phorbol esters and raise the possibility that diacylglycerols generated in the plasma membrane could act as a mitogenic signal for quiescent cells.

The c-sis gene encodes a precursor of the B chain of platelet-derived growth factor

The relationship between platelet-derived growth factor (PDGF) and the proto-oncogene c-sis has been determined by amino acid sequence analysis of PDGF and nucleotide sequence analysis of c-sis genomic clones. The nucleotide sequences of five regions of the human c-sis gene which are homologous to sequences of the transforming region (v-sis) of simian sarcoma virus (SSV) were determined. By alignment of the c-sis and v-sis nucleotide sequences the predicted amino acid sequence of a polypeptide homologous to the putative transforming protein p28sis of SSV was deduced. Both predicted sequences use the same termination codon and additional coding sequences may lie 5' to the homologous regions. Amino acid sequence analysis of the PDGF B chain shows identity to the amino acid sequence predicted from the c-sis sequences over 109 amino acid residues. Polymorphism may exist at two amino acid residues. These results suggest that c-sis encodes a polypeptide precursor of the B chain. A partial amino acid sequence of the PDGF A chain is also described. This chain is 60% homologous to the B chain and cannot be encoded by that part of c-sis which has been sequenced but could be encoded by sequences which lie 5' to the five regions of v-sis homology in c-sis, or at a separate locus.

Evidence for a transforming growth factor(s) in the serum-free conditioned medium of SV40-transformed 3T3 fibroblasts

Unlike untransformed 3T3 fibroblasts, Simian Virus 40-transformed 3T3 cells (SV3T3) do not require the presence of exogenous polypeptide mitogens in order to become competent for the initiation of DNA synthesis. As an explanation for this state of perpetual competence, growth activity for 3T3 cells was found in the serum-free conditioned medium of SV3T3 cells. This activity, which is abolished by incubation with pepsin, but withstands to a fair degree denaturants (8 M urea and 4 M guanidinium HCl) and a reducing agent (65 mM dithioerythritol), enhances the growth of Swiss and BALB/c 3T3 cells in 1% (v/v) calf serum medium and ensures the complete survival of Swiss 3T3 cells for several days in the total absence of serum. The growth activity appears to be derived from the SV3T3 cells, since it is not found in the non-conditioned medium (and the insulin and transferrin components of this serum-free medium have only weak growth activity alone) and the chances of it being due to residual serum growth factors are slight because of the method of growing the cells and the collection of the conditioned medium. Moreover, in a separate experiment to test the possible involvement of cell attachment factors, rat fibronectin was found not to affect 3T3 growth in the standard growth assay. The conditioned medium growth activity apparently has the biological capabilities of transforming growth factors. After concentration by ammonium sulfate precipitation, it can stimulate DNA synthesis in confluent, quiescent Swiss 3T3 cells, in both the absence and presence of 10% depleted calf serum. It also permits the growth of large colonies of Swiss 3T3 cells in soft agar. As part of a preliminary characterization of the conditioned medium growth activity, gel filtration in 0.15 M NaCl, 0.001 M HCl resolves the activity into high (mol. wt. greater than or equal to 100,000) and low (mol. wt. 5,000-6,000) molecular weight factors. Since re-chromatography of the high molecular weight factor, which may still be in an aggregated form, does not generate the smaller, the latter is not a dissociation product of the former. The precise relationship of these SV3T3-conditioned medium growth factors to other growth factors, including known transforming growth factors, has not yet been determined.

Close similarity of epidermal growth factor receptor and v-erb-B oncogene protein sequences

Each of six peptides derived from the human epidermal growth factor (EGF) receptor very closely matches a part of the deduced sequence of the v-erb-B transforming protein of avian erythroblastosis virus (AEV). In all, the peptides contain 83 amino acid residues, 74 of which are shared with v-erb-B. The AEV progenitor may have acquired the cellular gene sequences of a truncated EGF receptor (or closely related protein) lacking the external EGF-binding domain but retaining the transmembrane domain and a domain involved in stimulating cell proliferation. Transformation of cells by AEV may result, in part, from the inappropriate acquisition of a truncated EGF receptor from the c-erb-B gene.

Molecular aspects of growth factor action: receptors and intracellular signals

An increasing number of factors have been described which influence the proliferative responses of normal and neoplastic cells. A review of such moieties, their occurrence and possible mechanisms of their action and interaction is presented together with a discussion of their potential importance in the genesis and progression of neoplasms.

Platelet-derived growth factor is structurally related to the putative transforming protein p28sis of simian sarcoma virus

A partial amino acid sequence of human platelet-derived growth factor, the major mitogen in serum for cells of mesenchymal origin, has been determined. A region of 104 contiguous amino acids shows virtual identity with the predicted sequence of p28sis, the putative transforming protein of simian sarcoma virus (SSV). This similarity suggests a mechanism for transformation by SSV and other agents, involving expression of growth factors.

Interplay of cyclic AMP and microtubules in modulating the initiation of DNA synthesis in 3T3 cells

The results presented here show that disruption of the microtubule network acts synergistically with cAMP-elevating agents to stimulate the entry into DNA synthesis of 3T3 cells. Antimicrotubule agents and increased cAMP levels require an additional growth-promoting factor for inducing initiation of DNA synthesis; such requirement can be furnished by insulin, vasopressin, epidermal growth factor, platelet-derived growth factor, or fibroblast-derived growth factor. The involvement of the microtubules is indicated by the fact that enhancement of the DNA synthetic response was demonstrated with the chemically diverse agents colchicine, nocodazole, vinblastine, or demecolcine, all of which elicited the response in a dose-dependent manner. We verified that colchicine and nocodazole, at the doses used in this study, induced microtubule disassembly in the absence as well as in the presence of cAMP-elevating agents as judged by measurement of [3H]colchicine binding of total and pelletable tubulin. The involvement of cAMP was revealed by increasing its endogenous production by cholera toxin or by treatment with 8BrcAMP. The enhancing effects of antimicrotubule drugs and cAMP-elevating agents could be demonstrated by incorporation of [3H]thymidine into acid-insoluble material, autoradiography of labeled nuclei, or flow cytofluorometric analysis. The addition of antimicrotubule drugs does not increase the intracellular level of cAMP nor does addition of cAMP-elevating agents promote disassembly of microtubules (as judged by measuring [3H]colchicine binding of total and pelletable tubulin) in 3T3 cells. In view of these findings and the striking synergistic effects between these agents in stimulating DNA synthesis in the presence of a peptide growth factor, we conclude that increased cAMP levels and a disrupted microtubule network regulate independent pathways involved in proliferative response.

Bombesin stimulation of DNA synthesis and cell division in cultures of Swiss 3T3 cells

Bombesin is shown to be a potent mitogen for Swiss 3T3 cells. At nanomolar concentrations the peptide markedly enhances the ability of fresh serum to stimulate DNA synthesis in confluent and quiescent cultures of these cells. In the presence of a low concentration (3.5%) of serum, bombesin stimulates 3T3 cell proliferation. In serum-free medium, bombesin induces DNA synthesis in the absence of any other added growth factor; half-maximal effect is obtained at 1 nM. The mitogenic effect of bombesin is dependent on dose and time, is mimicked by litorin, and is markedly potentiated by insulin, colchicine, platelet-derived growth factor, and fibroblast-derived growth factor. These mitogens increase the maximal response elicited by bombesin and decrease the bombesin concentration required to produce half-maximal effect (from 1 nM to 0.3 nM). In contrast, vasopressin, phorbol esters, or cAMP increasing agents fail to enhance the maximal level of DNA synthesis induced by bombesin. Bombesin and litorin may provide useful model peptides for studies on the mechanism(s) by which extracellular ligands control cell proliferation.

Vasopressin induces selective desensitization of its mitogenic response in Swiss 3T3 cells

Prior incubation of quiescent cultures of Swiss 3T3 cells with vasopressin leads to loss of mitogenic stimulation on its subsequent addition in the presence of a synergistic growth factor. This desensitization is selective for vasopressin, requires prolonged incubation (half-maximal desensitization after 12 hr of treatment) for its induction, and is reversed after a 48-hr incubation in the absence of vasopressin. It is elicited by concentrations of vasopressin, and several analogues, similar to those required for stimulation of DNA synthesis. Inhibition of 125I-labeled epidermal growth factor binding and stimulation of 86Rb+ uptake by vasopressin are also selectively decreased in the refractory cells. The vasopressin receptors that mediate mitogenesis in Swiss 3T3 cells are of the pressor type, not coupled to adenylate cyclase. These cells bind [3H]vasopressin in a specific and saturable (Kd = 1 X 10(-8) M) manner. The receptors are down-regulated after prolonged vasopressin treatment; however, this cannot provide a complete explanation of desensitization because cells that are completely refractory to vasopressin retain 60% of their [3H]vasopressin binding sites. Vasopressin refractoriness must therefore occur partly at a post-receptor locus.

Production of growth factors by type 5 adenovirus transformed rat embryo cells

Transformation of Sprague-Dawley rat embryo (RE) cells and a cloned Fischer rat embryo cell line (CREF) with wild-type (Ad5) or a temperature-sensitive DNA-minus mutant (H5ts125) of type 5 adenovirus results in a reduction in binding of epidermal growth factor (EGF) to cell surface receptors. A reduction in EGF binding is also seen in a Syrian hamster embryo cell line transformed by a hexon mutant of Ad5. In contrast, a human embryonic kidney cell line (293) transformed by sheared Ad5 DNA or transfected clones of KB cells expressing the E1 transforming region of Ad5 do not show a decrease in receptor binding. When cocultivated, the adenovirus transformed rat cells were able to induce the growth in agar of normal CREF cells. Medium from Ad5 transformed RE cells stimulated the growth in agar of CREF cells and also inhibited [125I]-EGF binding in CREF cells. When fractionated by gel filtration, two peaks of [125I]-EGF inhibiting activities were obtained with apparent molecular weights of 35,000 and 16,000. These results provide the first evidence that cells transformed by an adenovirus can produce a growth factor(s) that inhibits EGF-receptor binding and induces anchorage-independent growth of normal cells.

Mitogenic proteins of the 3T3 plasma membrane

A mitogenic factor from 3T3 plasma membranes has been identified and partially characterized. The factor appears to be a peripheral membrane protein that can be released by mild trypsin, chymotrypsin, or plasmin treatment. This component is sensitive to heat and acid, and has a molecular weight in the range of 150,000-200,000 daltons as determined by gel filtration. A similar mitogenic activity has also been found on the membranes of both SV-40-transformed 3T3 cells and human fibroblasts. The factor appears to be distinct from all previously described mitogenic components.

Growth stimulation of rous sarcoma virus-transformed BHK cells by biotin and serum lipids

Biotin or a serum lipid extract stimulated proliferation of G1 arrested Rous sarcoma virus-transformed BHK cells in modified Eagle's MEM (BM). The cells could be maintained continuously in BM plus biotin (BMB), but not in BM plus serum lipid extract (BM X L). Avidin inhibited growth stimulation when added to BMB, but did not inhibit growth when added to BM X L. 14C-acetate incorporation into total cellular lipids was stimulated in BMB, but not in BM. Thin-layer chromatography of the labeled cellular lipid extract indicated that relatively large amounts of 14C-acetate were incorporated into phosphatidylserine and little into the other major phospholipids. In the neutral lipids, the largest amount of incorporation was in cholesterol. G1 arrested cells multiplied rapidly in BM supplemented with dialyzed serum (BM X DS), but they did not multiply in BM with delipidized serum (BM X DLS). The addition of biotin or serum lipid extract to BM X DLS stimulated growth. Growth stimulation in BM X DLS by biotin was inhibited by avidin, but avidin had no effect on growth stimulation by serum lipid extract. Biotin stimulated additional multiplication in BM X DS and avidin inhibited this additional growth stimulation. These results suggest that growth stimulation requires lipids supplied by serum lipids or by de novo synthesis stimulated by biotin. In the absence of serum, the stimulation of the synthesis of growth factor(s) by biotin are also required for continuous multiplication.

Na+/H+ antiport in Swiss 3T3 cells: mitogenic stimulation leads to cytoplasmic alkalinization

Addition of Na+ to Na+-depleted Swiss 3T3 cells causes a rapid and dramatic increase in intracellular pH, as monitored by uptake of the weak acid 5,5-dimethyloxazolidine-2,4-dione. The effect of Na+ is concentration dependent (half-maximal effect at 38 mM); this cation can be replaced by Li+ but not by K+ or the choline ion. Amiloride prevents the Na+-induced increase in intracellular pH and also blocks the entry of Na+ into 3T3 cells; the half-maximal concentrations of amiloride for inhibiting the two processes are similar (40 microM). Increase in extracellular pH caused an increase in the initial rate of Na+ influx that was of sufficient magnitude to stimulate the activity of the Na+/K+ pump in quiescent 3T3 cells. Taken together, these findings suggest the presence of a functional Na+/H+ antiport in Swiss 3T3 cells. Addition of the potent mitogenic combination platelet-derived growth factor, vasopressin, and insulin to quiescent Swiss 3T3 cells increased the intracellular pH from 7.21 +/- 0.07 to 7.36 +/- 0.09 in 10 independent experiments (P less than 0.001). This combination of growth factors also stimulated Na+ entry and ouabain-sensitive Rb+ uptake. The data support the hypothesis that early changes in ion fluxes play a role in signaling mitogenesis in 3T3 cells.

Intracellular K+ and the mitogenic response of 3T3 cells to peptide factors in serum-free medium

The stimulation of DNA synthesis in cultures of Swiss 3T3 cells by vasopressin, epidermal growth factor, and insulin added in serum-free medium is strikingly dependent on the intracellular K+ content or concentration. The relationship between these parameters is sigmoid; DNA synthesis commences only when the intracellular K+ increases above a certain threshold level (0.56 mumol/mg of protein; 90 mM). Addition of K+ to K+- depleted cultures reverses the block on DNA synthesis after a lag period of at least 8 hr. The sigmoid dependence of DNA synthesis on intracellular K+ is generated in early G1 phase rather than at the G1/S boundary. The effects of K+ on the G1-S transition are, at least in part, exerted through its control of protein synthesis. In serum-free medium, the K+ content is close to the threshold required for allowing a mitogenic response. The findings suggest that a small change in the intracellular K+ level can influence the ability of these cells to initiate DNA synthesis in serum-free medium.

Cyclic AMP stimulation of Na-K pump activity in quiescent swiss 3T3 cells

Recently, we have found that an increase in the intracellular level of cAMP acts as a mitogenic signal for Swiss 3T3 cells (Rozengurt et al., Proc. Natl. Acad, Sci. USA, 78:4392, 1981). The results presented in this paper demonstrate that addition of cAMP-elevating agents to confluent and quiescent cultures of Swiss 3T# causes a marked increase in the rate of 86Rb+ uptake but has no effect on the rate of cation efflux. The stimulation of ion uptake is mediated by the Na-K pump as shown by the ouabain sensitivity of the 86Rb+ fluxes. The increase in Na-K pump activity occurs whether cAMP is generated endogenously by stimulation of adenylate cyclase activity by cholera toxin, adenosine agonists, or PGE1 or added exogenously as 8BrcAMP. The stimulatory effect of these compounds on 86Rb+ uptake is potentiated by inhibitors of cyclic nucleotide phosphodiesterase activity. Cholera toxin stimulates the Na-K pump in a dose-dependent manner; half-maximal effect is achieved at 0.7 ng/ml. The stimulation of ouabain-sensitive 86Rb+ uptake by cAMP-elevating agents reaches a maximum after 2-3 h of incubation. This contrasts with the rapid (within minutes) stimulation of the Na-K pump caused by serum and other mitogenic agents. Further, cAMP-elevating agents fail to increase Na+ influx into 3T3 cells whereas serum causes a marked increase in Na+ influx, under identical experimental conditions. These findings suggest that the stimulation of Na-K pump activity caused by increased cAMP levels contrasts mechanistically with the rapid control of pump activity by serum which is primarily mediated by increased Na+ entry into the cells.

Selective inhibition of DNA synthesis by a protein released from spleen cells

Spleen cells cultured release proteins into the culture medium, some of which shows both binding activity to single stranded DNA (ss-DNA) and inhibitory activity to 3H-thymidine incorporation into cells. The inhibitory protein of DNA synthesis was purified to near homogeneity by ammonium sulfate precipitation, ss-DNA-cellulose affinity chromatography, and gel filtration chromatography. The molecular weight of the major band of this protein, as estimated by sodium dodecyl-sulfate gel electrophoresis, was approximately 14,000. The effect of the protein was compared with various cell lines and primary cell culture, and it was found that the protein strongly inhibited DNA synthesis of HeLa, L, and HEp2 cells, but only inhibited to a minor extent synthesis of Ehrlich cells and primary culture of mouse embryo.

Transforming growth factor(s) production enables cells to grow in the absence of serum: an autocrine system

Kirsten murine sarcoma virus (KiMSV)-transformed rat-1, normal rat kidney (NRK), and BALB/c 3T3 cells are capable of continual growth in a serum-free medium supplemented with transferrin and insulin but with no exogenous mitogenic growth factors. Cells transformed by a mutant of KiMSV that is temperature sensitive for the maintenance of transformation grow in this medium at the permissive temperature only. At the nonpermissive temperature, growth is dependent upon the presence of serum-free conditioned medium from the transformed cells. Normal rat-1 cells are also dependent upon factors from the transformed cells for growth in this serum-free/mitogen-free medium. The serum-derived growth factors, epidermal growth factor, and fibroblast growth factor have no effect on the transformed cells, although epidermal growth factor can replace transforming growth factors produced by KiMSV-transformed cells for the growth of rat-1 cells. Growth of the transformed cells in serum-free medium at clonal densities is dependent upon the presence of conditioned medium collected from the same cells grown to high densities. These results show that (i) growth in serum-free/mitogen-free medium is a general property of KiMSV-transformed cells and (ii) growth of the transformed cells in this medium is dependent upon the presence of growth factors known to be produced by the cells, and they provide support for the hypothesis that serum-free growth of KiMSV-transformed cells is dependent upon ectopically produced growth factors.

Regulation of passive potassium transport of normal and transformed 3T3 mouse cell cultures by external calcium concentration and temperature

Regulation of passive potassium ion transport by the external calcium concentration and temperature was studied on cell cultures of 3T3 mouse cells and their DNA-virus transformed derivatives. Upon lowering of external calcium concentration, passive potassium efflux generally exhibits a sharp increase at about 0.1 mM. The fraction of calcium-regulated potassium efflux is largely independent of temperature in the cases of the transformed cells, but shows a sharp increase for 3T3 cells upon increasing temperature above 32 degrees C. In the same range of temperature, the 3T3 cells exhibit the phenomenon of high-temperature inactivation of the residual potassium efflux at 1 mM external calcium. At comparable cellular growth densities, the transformed cell lines do not show high-temperature inactivation of "residual" potassium efflux. These results are consistent with the notion of a decisive role of the internal K+ concentration in the cell-density dependent regulation of cell proliferation. In particular, the growth-inhibiting effect of lowering the external Ca2+ concentrations is considered as largely due to a rise of passive K+ efflux and a subsequent decrease of internal K+ concentration. The experimental data on the Ca2+ dependence of passive K+ flux are quantitatively described by a theoretical model based on the constant field relations including negative surface charges on the external face of the membrane, which cooperatively bind Ca2+ ions and may concomitantly undergo a lateral redistribution. The present evidence is consistent with acidic phospholipids as representing these negative surface charges.

Stimulation of DNA synthesis by transient exposure of cell cultures to TPA or polypeptide mitogens: induction of competence or incomplete removal?

A transient exposure of cell cultures to 12-0-tetradecanoyl-phorbol-13-acetate (TPA) is sufficient to stimulate DNA synthesis during a subsequent incubation in TPA-free medium. We show that (1) a substantial fraction of TPA remains bound to cultures following a transient exposure to TPA and thorough washing, (2) the ability of TPA to induce DNA synthesis is a function of the amount of TPA bound to cell cultures irrespective of whether it is incubated continuously with cultures or transiently exposed to cultures under various conditions, and that (3) a transient exposure of cultures to phorbol-12-13-dibuytrate (PDB), a mitogenic phorbol ester which binds reversibly to cell cultures, does not stimulate DNA synthesis during a subsequent incubation in PDB-free medium. Therefore the persisting effects of TPA are due to it binding to cultures in a manner resistant to washing and not due to the induction of a stable cellular change prerequisite for mitogenesis. Further, we show that certain combinations of polypeptide growth factors induce DNA synthesis in the absence of any such stable cellular change. Evidence is also presented that the persisting effects on DNA synthesis following transient exposure of cultures to other polypeptide growth factors (e.g., platelet-derived growth factor) reflect tenacious binding rather than induction of a lasting biological event.

Cyclic AMP: a mitogenic signal for Swiss 3T3 cells

Addition of cholera toxin (100 ng/ml) to quiescent cultures of Swiss 3T3 cells acts synergistically with serum (2-4%), insulin, phorbol esters, epidermal growth factor, and fibroblast-derived growth factor to stimulate DNA synthesis. In the presence of insulin, cholera toxin caused a dose-dependent increase in cumulative [3H]thymidine incorporation into acid-insoluble material and in the intracellular cyclic AMP (cAMP) level. The dose--response curves for the two processes were similar. Furthermore, addition of 1-methyl-3-isobutylxanthine (15--500 microM) or of 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (5--100 microM), both of which are potent inhibitors of cyclic nucleotide phosphodiesterase which are potent inhibitors of cyclic nucleotide phosphodiesterase activity, stimulated DNA synthesis and increased cAMP levels in Swiss 3T3 cells. These compounds strikingly potentiated the effect of cholera toxin on DNA synthesis and on cAMP levels. When quiescent Swiss 3T3 cells were exposed to cholera toxin (100 ng/ml) and insulin at 10 micrograms/ml (4- to 7-fold increase in cAMP level) or to these agents and 1-methyl-3-isobutyl xanthine at 50 microM (35-fold increase in cAMP level), DNA synthesis began after a lag of 16 hr. These results indicate that cAMP acts as a mitogenic signal for Swiss 3T3 cells and differ from the widely held view that cyclic AMP inhibits the proliferation of fibroblast cells.

Hormonal control of immunoreactive somatomedin production by cultured human fibroblasts

Human growth hormone (hGH) is known to be a potent stimulator of somatomedin secretion in vivo. The induction of somatomedin by growth hormone has been difficult to study in vitro, however, because no organ containing a high concentration of somatomedin has been identified. Because fetal mouse explants have been shown to produce somatomedin in vitro, we have undertaken studies to determine whether postnatal human fibroblast monolayers also produce somatomedin, and if so, whether its production is regulated by other hormones. Quiescent human fibroblasts were exposed to serum-free minimum essential medium, and the medium was assayed for somatomedin concentration using a specific radioimmunoassay for somatomedin-C. A progressive rise in immunoreactive somatomedin to 0.08 U/ml per 10(5) cells per 24 h was observed over 72 h of incubation. This was an underestimation of the actual concentration of immunoreactive somatomedin since the amount measured following acid treatment was at least fourfold higher than in the untreated medium. Growth hormone stimulated immunoreactive somatomedin production in a dose-dependent manner: 5 ng hGH/ml = 0.1 U/ml per 10(5) cells; 50 ng hGH/ml = 0.25 U/ml per 10(5) cells. Platelet-derived growth factor and fibroblast growth factor were also stimulatory, but epidermal growth factor, thyroxine, or cortisol had no effect. Media that had been exposed to human fibroblasts stimulated DNA synthesis in BALB/c 3T3 fibroblasts (a cell type that does not produce somatomedin). Medium-derived immuno-reactive somatomedin eluted from Sephacryl S-200 in two major peaks (150,000 and 8,000 mol wt). The higher molecular weight peak is similar to the one observed when whole serum was used. These studies provide a model system for studying the humoral and nonhumoral factors that control the biosynthesis of somatomedin by human tissues. Since immunoreactive somatomedin production may be a rate-limiting factor for fibroblast growth, the delineation of the hormonal control of somatomedin production should lead to a better understanding of the mechanisms controlling human fibroblast growth.

Cell cycle activation and ouabain-sensitive ion movements of 3T3 and C3H-10T1/2 fibroblasts

The introduction of either PGF2 alpha-(10(-7) M) or TPA (10(-7) M) stimulated, ouabain-sensitive 86Rb+ influx at 30 min in postconfluent 3T3-4 mouse fibroblast cultures by 117% and 124%, respectively. Both TPA and PGF2 alpha at these concentrations stimulated the incorporation of 3H-TdR into DNA. TPA had the greatest stimulatory effect, which was similar to that obtained with 10% fetal calf serum. In accord with the idea that modulation of membrane processes such as Na+/K+ pump activity in fibroblasts may reflect important events related to the initiation of DNA synthesis, it was observed that in both 3T3-4 and C3H-10T1/2 cells there were parallel increases in 3H-TdR incorporation and ouabain-sensitive 86Rb+ influxes with 10(-7) M TPA, whereas PGF2 alpha stimulated a significant increase in 3H-TdR incorporation in 3T3-4 but not C3H-10T1/2 cells and only marginal increases in ouabain-sensitive 86Rb+ influx in both. Therefore, although there appears to be a close correlation between Na+/K+ pump activation and subsequent S-phase entry following TPA stimulation, a similar correlation for PGF2 alpha cannot be confirmed.

Chemical and biological properties of a growth factor from human-cultured osteosarcoma cells: resemblance with platelet-derived growth factor

A human osteosarcoma cell line, U-2 OS, cultured under serum-free conditions, was shown to produce a growth factor (osteosarcoma-derived growth factor, ODGF) for human-cultured glial cells, fibroblasts, and other cells. ODGF, collected from the spent medium of 2 OS cultures, was purified by a sequence involving heparin-Sepharose chromatography, hydrophobic chromatography, gel chromatography, and preparative gel electrophresis in SDS. Purified ODGF, at a concentration of 3 ng/ml, elicited a mitogenic response in human glial cells equivalent to 50% of that afforded by human serum at a final concentration of 1%. The preparation was estimated to be > 50% pure. The biological activity of ODGF resided in a cationic, relatively heat-resistant, reduction-susceptible protein with a molecular weight of 30,000 (by gel chromatography and SDS-gel electrophoresis). The electrophoretic behaviour of radioiodinated ODGF suggested that the protein was composed of two different polypeptide chains (about 13,000-14,00 and 16,000-17,000 daltons, respectively) linked via disulphide bonds. The molecular makeup of ODGF was thus similar to that of platelet-derived growth factor. 125I-ODGF could be precipitated by an antibody to platelet-derived growth factor, indicating that the two factors were immunologically related. Resemblance with platelet-derived growth factor was also indicated by the finding that the latter (but not, e.g., fibroblast growth factor or epidermal growth factor) competed with 125I-ODGF for binding to human-cultured glial cells.

Phorbol esters and vasopressin stimulate DNA synthesis by a common mechanism

The potent tumour promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) acts synergistically with all known mitogens to induce DNA synthesis in 3T3 cells. In contrast, the neurohypophyseal hormone vasopressin, which is mitogenic for 3T3 cells, fails to synergize with TPA to stimulate DNA synthesis, ornithine decarboxylase activity or 2-deoxyglucose uptake. Thus TPA acts through a pathway which converges with that used by vasopressin, and at least some of its biological effects occur through mechanisms normally utilized by specific hormones.

Tissue culture in nephrology: potential and limits for the study of renal disease

Kidney cells, when isolated and cultivated in vitro, retain differentiated renal properties. Glomerular epithelial and mesangial cells from animal and human kidneys express their normal ultrastructure and the ability for basement membrane biosynthesis. Mesangial cells in culture have been utilized particularly for the study of hormonal tissue receptors, of prostaglandin production, and of their contractile response to various hormonal stimuli. Cells of tubule origin have been a valuable tool for the study of transport mechanism which, as a consequence of the heterogeneity of nephron functions, can not be assessed in vivo. Ion transport and its structural basis, as well as transport regulation by hormones has been studied in established epithelial cell lines. Induction of ion transport and enzyme activities, and the control of cell proliferation and differentiation has also been succesfully evaluated in cultured epithelia derived from the kidney. Future work will attempt to prepare cell lines from defined nephron segments to study chemical and physical phenomena of renal disease.

Vasopressin rapidly stimulates Na entry and Na-K pump activity in quiescent cultures of mouse 3T3 cells

Addition of (Arg) vasopressin to quiescent cultures of Swiss 3T3 cells rapidly stimulates an ouabain-sensitive 86Rb uptake. In contrast the hormone has no significant effect on the rate of efflux of this cation from preloaded cells. The stimulation of 86Rb uptake is cycloheximide-insensitive, occurs within minutes of hormone addition and results from an increase in the Vmax of the uptake system. Vasopressin stimulates ion uptake in a concentration-dependent fashion (1--100 ng/ml); oxytocin also stimulated the Na-K pump but at significantly higher concentrations. The stimulation of the Na-K pump by vasopressin is apparently mediated by an increase in Na entry into the cells, since the hormone 1) strikingly shifts the concentration dependence on Na+ of the Na-K pump, 2) increases 22Na uptake, and 3) increases intracellular Na contents when the efflux of this ion is blocked by ouabain. Since vasopressin is a potent mitogen for Swiss 3T3 cells, the results provide further evidence in support of a possible role of monovalent ion fluxes in signalling the initiation of growth stimulation.

Differential effect of interferon on DNA synthesis, 2-deoxyglucose uptake and ornithine decarboxylase activity in 3T3 cells stimulated by polypeptide growth factors and tumor promotors

Quiescent 3T3 cells can be stimulated to enter S by defined factors. When used in combination, three polypeptide hormones (EGF, vasopressin, and insulin), or a tumor promotor and insulin, are very effective in stimulating DNA synthesis. Like serum, the defined factors also stimulate deoxyglucose uptake and induce the synthesis of ornithine decarboxylase during G1. The second stage of deoxyglucose uptake and the induction of ornithine decarboxylase are protein synthesis-dependent events. When added with the growth factors, mouse interferon inhibits the synthesis of DNA and the induction of ornithine decarboxylase but has no effect on the uptake of deoxyglucose. Kinetic experiments comparing the effect of inhibitors of translation or transcription on induction of ornithine decarboxylase with the effect of interferon suggest that interferon may affect the synthesis of enzyme by inhibiting both transcription and translation of message. The findings provide further support for the proposition that interferon exerts a differential effect on mitogen-stimulated events events which are dependent on continuous protein synthesis.

Colchicine inhibits epidermal growth factor degradation in 3T3 cells

Colchicine (2 microM) did not affect the initial rate of association of 125I-labeled epidermal growth factor (125I-EGF) to Swiss 3T3 cells but continued incubation (up to 24 hr) led to an increase in cell-associated radioactivity. The effect is also produced by Colcemid, vinblastine, and podophyllotoxin but not by lumicolchicine. Disruption of microtubules with colchicine does not alter the rate of "down regulation" of EGF receptors, suggesting the binding and internalization of the factor proceed unchanged. However, colchicine markedly decreases the rate of appearance of acid-soluble radioactivity from cells either incubated continuously with 125I-EGF for 24 hr or exposed to the radioactive peptide for only 1 or 3 hr. The results indicate that colchicine decreases the rate of degradation of internalized 125I-EGF. Because antitubulin agents enhance the mitogenic effect of EGF our results suggest that peptide degradation can be dissociated from the long-term biological effect.

Elucidation of an A and L system for amino acid transport in the human lymphoblast using a membrane filtration technique

Optimum conditions have been established for the measurement of amino acid transport by human lymphoblastoid cell lines using a membrane-filtration technique. The parameters we found to be important for the reproducibility of the method are: the types and combination of filters, the strength of the vacuum applied to the filters and the density of the cultures at the time of harvesting and during uptake and filtration. We found that bovine serum albumin added to phosphate buffered saline (PBS) glucose in which the cells are washed, resuspended and assayed is essential for the maintenance of viability, the prevention of clumping and the retention of the accumulated amino acid. Using this procedure we have characterized two transport systems for the neutral amino acids; an A and an L system, which are similar but not identical to the A and L systems characterized in rodent cell lines. These A and L systems have characteristically lower Km's and Vm's for alanine and phenylalanine, when compared to rodent cell lines. In addition, we find alpha-AIB to be a poor competitor of alanine and phenylalanine uptake.