Nonspecific nature of the stimulus to DNA synthesis in cultures of chick embryo cells

A Survey of Metal-induced Mutagenicity in Vitro and in Vivo

A survey of the literature shows that organic and inorganic compounds of 53 metals have been assayed for genotoxic effects in vitro and in vivo. It is found that there are great variations in the response obtained with different test systems and that a wide range of compounds of the different metals is positive in at least one of the short-term tests. Some of the variation observed could be due to differences in uptake mechanisms. This effect plus the wide variation in the quantity and quality of the data prevents any direct comparison of in vitro activity with in vivo potency of the various metallic species.

Separation methods for isolation of human polymorphonuclear leukocytes affect their motile activity

Five commonly used methods for the isolation of human polymorphonuclear leukocytes (PMNLs) from blood and their subsequent effect on cell motile activity were compared. Although all methods (isolation from blood clots, hemolysis of erythrocytes in hypotonic solutions, and sedimentation with the use of Percoll, Ficoll 400 or Dextran T 500 solutions) preserved cell viability, they demonstrated different effects on cell spreading and the speed of spontaneous cell movement. The highest motile activity was shown by PMNLs separated from blood clots and Percoll solutions. In the presence of formylated peptides, N-FMLP-mediated movement was markedly stimulated in PMNLs separated by all five methods, but cells isolated with the use of Dextran T 500 or Ficoll 400 were relatively slower than those isolated with other methods. This suggests that the cells had preserved the sensitivity of their receptors specific for ligands stimulating chemokinesis (chemotaxis) after all five methods of separation. Immunofluorescence observations showed that PMNLs isolated in the presence of FITC-Dextran exhibited polysaccharide-coated surfaces with receptor proteins extended above that coat - an observation which explains why cell coating with polysaccharides does not disturb cell phenotyping with flow cytometry and FACS methods.

The logic of the Membrane, Magnesium, Mitosis (MMM) model for the regulation of animal cell proliferation

The addition of animal serum or specific protein growth factors to quiescent, serum-deprived vertebrate cells in culture activates a wide variety of biochemical responses within minutes, which are followed in 5-10h by the initiation of DNA synthesis and then by mitosis. The quintessential early and continuing activation step for the increase in DNA synthesis is the increased initiation rate of protein synthesis, which must be continuously maintained throughout the G1 phase for advancement into S. The aggregate of biochemical reactions to growth factors is called the coordinate response, to indicate that many related and unrelated processes are orchestrated to repetitively reproduce cells. It is, however, crucial to recognize that the coordinate response can be induced for one or more rounds of replication by a variety of non-specific and quasi-specific membrane effectors. The logic of considering this framework of events in growth control implied that a single multi-target second messenger plays a central role in coordinating the events of the overall response. The same reasoning suggested that free Mg(2+) is the unifying regulatory element in that response which includes protein kinase pathways, and that the cytoplasmic activity of Mg(2+) increases with the binding of growth factors to their receptors in the cell membrane, or of less specific perturbations of the membrane. Experimental support of this conclusion is presented here and is represented in the MMM model of cell proliferation control.

Central roles of Mg2+ and MgATP2- in the regulation of protein synthesis and cell proliferation: significance for neoplastic transformation

Growth factors are polypeptides that combine with specific membrane receptors on animal cells to stimulate proliferation, but they also stimulate glucose transport, uridine phosphorylation, intermediary metabolism, protein synthesis, and other processes of the coordinate response. There are a variety of nonspecific surface action treatments which stimulate the same set of reactions as the growth factors do, of which protein synthesis is most directly related to the onset of DNA synthesis. Mg(2+) is required for a very wide range of cellular reactions, including all phosphoryl transfers, and its deprivation inhibits all components of the coordinate response that have so far been tested. Growth factors raise the level of free Mg(2+) closer to the optimum for the initiation of protein synthesis. The resulting increase in protein synthesis accelerates progression through G1 to the onset of DNA synthesis and mitosis. None of the other 3 major cellular cations are similarly involved in growth regulation, although internal pH may play an auxiliary role. Almost 10(5) externally bound divalent cations are displaced from membranes for every attached insulin molecule, implying a conformational membrane change that releases enough Mg(2+) from the internal surface of the plasma membrane to account for the increase in free cytosolic Mg(2+). It is proposed that mTOR, the central control point for protein synthesis of the PI 3-K kinase cascade stimulated by insulin, is regulated by MgATP(2-) which varies directly with cytosolic Mg(2+). Other elements of the coordinate response to growth factors such as the increased transport of glucose and phosphorylation of uridine are also dependent upon an increase of Mg(2+). Deprivation of Mg(2+) in neoplastically transformed cultures normalizes their appearance and growth behavior and raises their abnormally low Ca(2+) concentration. Tight packing of the transformed cells at very high saturation density confers the same normalizing effects, which are retained for a few days after subculture at low density. The results suggest that the activity of Mg(2+) within the cell is a central regulator of normal cell growth, and the loss of its membrane-mediated control can account for the neoplastic phenotype.

Inorganics and hormesis

The article is a comprehensive review of the occurrence of hormetic dose-response relationships induced by inorganic agents, including toxic agents, of significant environmental and public health interest (e.g., arsenic, cadmium, lead, mercury, selenium, and zinc). Hormetic responses occurred in a wide range of biological models (i.e., plants, invertebrate and vertebrate animals) for a large and diverse array of endpoints. Particular attention was given to providing an assessment of the quantitative features of the dose-response relationships and underlying mechanisms that could account for the biphasic nature of the hormetic response. These findings indicate that hormetic responses commonly occur in appropriately designed experiments and are highly generalizeable with respect to biological model responses. The hormetic dose response should be seen as a reliable feature of the dose response for inorganic agents and will have an important impact on the estimated effects of such agents on environmental and human receptors.

A general classification of U-shaped dose-response relationships in toxicology and their mechanistic foundations

The development of a comprehensive database of chemical hormetic responses (i.e., U- or inverted U-shaped dose-response relationships) using objective a priori study design, statistical and study replication criteria has recently been reported. An assessment of this database reveals the existence of a wide range of hormetic dose-response relationships including those demonstrating a direct stimulation or an overcompensation response to a disruption of homeostasis. These two broad types of hormetic responses are affected by temporal factors and display unique patterns of dose-range stimulation, magnitude of stimulatory response and relationship of the maximum stimulatory response to the NOAEL. A general classification of U-shaped dose-response relationships is proposed to provide a more organized framework to evaluate the highly distinctive and diverse hormetic responses within the context of establishing underlying biological mechanisms and exploring risk assessment implications.

Serum-stimulated cell cycle progression and stress protein synthesis in C3H10T1/2 fibroblasts treated with sodium arsenite

In this work, we demonstrated that a nonlethal dose of arsenite administered to quiescent C3H10T1/2 fibroblasts can enhance the mitogenic effect of suboptimal concentrations of serum. The mitogenic effect was dependent on the serum concentration and on the time interval between the administration of arsenite and that of serum. This suggests that mitogen sensitivity changes in time after arsenite treatment. It is shown that the concentrations of arsenite that enhance the mitogenic effect of serum also increase the mRNA levels of c-fos, HSP68, and HSP84 and induce the specific synthesis of Heat Shock Proteins (HSPs). The physiological significance of this phenomenon is most likely to counteract the long-term toxic effect of arsenite by early induction of compensation for cell loss.

Very low cadmium concentrations stimulate DNA synthesis and cell growth

Uptake of cadmium into cultured cells and its effects on cell growth and DNA synthesis are measured over a range of Cd concentrations of seven orders of magnitude. Cd uptake is found to be proportional to the external Cd concentration and to incubation time over a very broad range of concentrations. At least 200 mmol cadmium per kg dry weight of cells can be accumulated in this way, leading to exhaustion of the major intracellular Cd binding sites before cell death. On the other hand, very low cadmium concentrations down to 100 pM stimulate cell growth and DNA synthesis significantly. Stimulation is found in all three mammalian cell types examined: namely L6J1, a rat permanent myoblast cell line, LLC-PK1 porcine renal epithelial cells, and a primary rat chondrocyte culture. Cd acts as a cofactor with serum in L6J1 cultures, but is stimulatory only in serum-free cultures of chondrocytes. Stimulation occurs at Cd concentrations too low to result in a measurable induction of metallothionein. This might implicate the action of response amplifiers in the chain of events leading to Cd-stimulated DNA replication and cell growth.

Cadmium-enhanced gene expression in suspension-culture cells of tobacco

The effects of various concentrations of cadmium on Nicotiana tabacum L. cv. Xanthi suspension cells were examined. Surprisingly, certain concentrations of Cd (100-150 μM) stimulated growth of cell cultures considerably, whereas all other concentrations were inhibitory. Synthesis of DNA was severly affected in a dose-dependent manner by Cd concentrations of 250 μM and higher. In contrast, RNA and protein synthesis were similarly stimulated by 100 μM Cd, thus indicating that enhancement of RNA synthesis was the primary cause for the observed stimulation of cell culture growth. The transient expression of a chimeric chloramphenicol-acetyltransferase gene was similarly affected by Cd. When the effects of other heavy metals (Cu, Zn, Pb, Co, Mn, Al) on these cellular processes were investigated, only Zn showed a comparable stimulation of RNA and protein synthesis, although a tenfold higher concentration of Zn compared with Cd was required. As Zn and Cd are chemically very similar, these results are discussed in view of the well-known role of Zn in the regulation of transcription.

Cadmium produces a delayed mitogenic response and modulates the EGF response in quiescent NRK cells

Recent studies have shown that cadmium, at subtoxic levels, may induce a response characteristic of that elicited by a type of growth factor that supports the anchorage independent growth of cells that are not fully transformed. That is, Cd++ was found to replace transforming growth factor beta in supporting soft agar growth of NRK-49F cells. To test the extent to which Cd++ further mimics transforming growth factor beta in its effects and to establish response patterns that suggest possible molecular mechanisms of action, we have determined the effects of Cd++ and/or epidermal growth factor (EGF) on DNA synthesis in quiescent NRK-49F cells. We found that subtoxic doses of Cd++ modulate EGF-induced DNA synthesis in a dose-dependent fashion. Although Cd++ effects on early (16-24 hr) EGF-induced DNA synthesis are primarily inhibitory, later effects involve stimulation as well. Subtoxic doses of Cd++ did not stimulate DNA synthesis in quiescent cells within 24 hr of addition. At later times (40 or 64 hr), however, an increase in DNA synthesis of up to threefold was induced by 0.25 microM Cd++. This pattern of mitogenic response, involving inhibition of early growth-factor induced DNA synthesis and stimulation of late DNA synthesis, is consistent with that reported to be effected in some instances by transforming growth factor beta. Because a defined pattern of gene expression also is associated with the mitogenic responses to transforming growth factor beta, future studies at the molecular level can definitively test the degree to which Cd++ and transforming growth factor beta effects are common.

Ionic changes associated with lead stimulation of DNA synthesis in Balb/c3T3 cells

Lead at slightly subtoxic concentrations markedly stimulated the rate of DNA synthesis in cultured animal cells. This stimulation was closely correlated with formation of a precipitate that was adsorbed and taken up by the cells under certain medium conditions. Data suggest that a precipitate-induced perturbation of the surface membrane leads to intracellular changes responsible for stimulation of DNA synthesis. Maximum stimulation of(3)H-thymidine incorporation by optimum concentrations of lead is delayed about 8 h compared to that in serum stimulation. In cells stimulated significantly by lead, but not in unstimu-lated cells, a reproducible rise of about 13% in intracellular magnesium occurred over a 24 h period, with an 8 h lag in the increase compared to that observed in serum stimulation. In view of the increases in intracellular magnesium consistently associated with and preceding stimulation of DNA synthesis by several different mitogens including serum and insulin, the present time-coordinated positive correlation between magnesium and DNA synthesis provides evidence for the primary involvement of this divalent cation in growth stimulation produced by lead.

Stimulation of DNA synthesis in kidney epithelial cells in culture by potassium

The hypothesis that the K+ concentration of extracellular fluid is a determinant of renal DNA synthesis was examined in quiescent, high-density cultures of monkey kidney epithelial cells of the BSC-1 line. The addition of KCl to the medium increased the number of cells engaged in DNA synthesis in a concentration-dependent manner. The capacity of K+ to stimulate DNA synthesis in a greater number of cells was additive with exogenous NaCl and calf serum and was associated with an increment in the steady-state cell K+ content. Studies with other monovalent cations indicated that the stimulatory effect of K+ on DNA synthesis was not mediated by increments in the chloride concentration or osmotic pressure of the medium. The addition of K+ to confluent cultures was associated with a concentration-dependent increase in cell multiplication. The commitment of cells to increased multiplication required exposure of the culture to added KCl for longer than 3 but not more than 6 h. Addition of KCl to cultures of mouse fibroblasts did not alter DNA synthesis, multiplication, or cell K+ content. These observations indicate that increased availability of K+ in the extracellular fluid can stimulate DNA synthesis in kidney epithelial cells in culture.

Aluminum ions stimulate DNA synthesis in quiescent cultures of Swiss 3T3 and 3T6 cells

Micromolar concentrations of AI3+ are shown to be strongly mitogenic for quiescent cultures of Swiss 3T3 and 3T6 cells. AI3+ caused a striking shift in the dose-response curve for the effect of fetal bovine serum on 3H-thymidine incorporation. In the absence of serum the mitogenic effect of aluminum was greatly potentiated by insulin or cholera toxin, but not epidermal growth factor or 12-0-tetradecanoyl-phorbol-13-acetate. The stimulation of DNA synthesis was maximal by 15-20 microM AI3+ X AI3+ at 100 microM had no inhibitory effect on DNA synthesis. AI3+ had no significant effect on cellular cyclic adenosine monophosphate in the presence or absence of insulin or an inhibitor of cyclic nucleotide phosphodiesterases.

Cell growth and net Na+ flux are inhibited by a protein produced by kidney epithelial cells in culture

Proliferation of confluent kidney epithelial cell cultures (BSC-1 line) is inhibited by a protein (Mr approximately equal to 24,000) that is secreted by the cells. The mechanism of action of this growth inhibitor was sought by studying its effect on net Na+ flux because increased availability of Na+ in the culture medium had been shown to stimulate cell growth. The increase in cell Na+ content observed during stimulation of the growth after a medium change was attenuated in the presence of the purified inhibitor. Inhibition of both cell Na+ accumulation and growth in the presence of the protein was reversed completely by addition of NaCl to the medium. These results suggest that control of net Na+ flux and growth in kidney epithelial cells could be mediated, at least in part, by a secreted cellular protein.

Vanadium ions stimulate DNA synthesis in Swiss mouse 3T3 and 3T6 cells

Vanadyl sulfate and sodium orthovanadate in the concentration range between 5 and 50 microM are shown to be mitogenic for quiescent cultures of Swiss mouse 3T3 and 3T6 cells. The compounds caused a striking shift in the dose-response for the effect of serum on [3H]thymidine incorporation and DNA synthesis. In the absence of serum the effect of vanadium was greatly potentiated by insulin. Vanadium ions produced no more than additive increases in [3H]thymidine incorporation when combined with epidermal growth factor, cholera toxin, or phorbol 12-myristate 13-acetate. Both vanadium compounds stimulated ouabain-inhibitable 86Rb+ uptake, indicating that the vanadium ions increase, rather than inhibit, Na+/K+ pump activity in the intact cell. Neither vanadium compound had any effect on cellular cAMP under a variety of different conditions. The mitogenic effect of the vanadium compounds was similar to that of colchicine. Taxol, which stabilizes cytoplasmic microtubules, prevented the stimulation of DNA synthesis by vanadium.

Induction of growth in kidney epithelial cells in culture by Na+

The role of Na+ in the regulation of cell growth was examined in quiescent, high-density cultures of kidney epithelial cells of the BSC-1 line. The addition of NaCl to the medium increased the number of cells initiating DNA synthesis in a concentration-dependent manner after serum stimulation. In the presence of added NaCl, cells in confluent cultures grew to high density at an increased rate, whereas growth in sparse cultures was retarded. These results suggest that, in the presence of serum, Na+ can act as a mediator of the events that initiate cell proliferation.

Effects of depletion of K+, Na+, or Ca2+ on DNA synthesis and cell cation content in chick embryo fibroblasts

Decreasing the K+ concentration of the medium from 5 mM to 0.59 mM decreased the K+ content of chick embryo fibroblasts to 22% of control values and increased the Na+ content to 820% of control values. The alteration of monovalent cation content occurred within two hours but had no effect on the rate of DNA synthesis, as measured by 3H-thymidine incorporation, for at least 16 hours. By decreasing the Na+ concentration in the medium, a 50% reduction in cellular Na+ could be obtained with no effect on thymidine incorporation. Since these changes in cellular Na+ or K+ are much larger than any known to occur under physiological conditions but have no effect on thymidine incorporation, we conclude that Na+ and K+ do not play a critical role in determining multiplication rate. Addition of 1.8 mEGTA to cells in media containing 1.7 mM Ca2+ and 0.8 mM Mg2+ inhibited thymidine incorporation and sharply decreased cellular K+ and increased cellular Na+ content. However, there was no reduction in total cellular Ca2+ levels. Likewise, decreasing the Ca2+ concentration of the medium below 0.01 mM inhibited thymidine incorporation, decreased cellular K+ and Mg2+, and increased cellular Na+ but did not affect total cellular Ca2+ levels. Inhibition of DNA synthesis, therefore, could not be correlated with changes in cellular Ca2+ levels.

Proliferation and morphology of chick embryo cells cultured in the presence of horse serum and hemoglobin

We have shown previously that hemoglobin greatly stimulates chick embryo cell proliferation in Eagle's minimal essential medium supplemented with horse serum. In the present study we compared the effects of horse serum plus 10 micrometers hemoglobin to those of fetal bovine serum on subcultures of chick embryo cells serially propagated at high cell densities. The cells became elongated in the presence of fetal bovine serum and their rate of proliferation progressively decreased, whereas they became polygonal in the presence of horse serum plus hemoglobin and proliferated well in successive cell passages. The polygonal cells obtained in the presence of horse serum plus hemoglobin rapidly elongated if cultured at low cell densities in the presence of fetal bovine serum, but, in contrast, elongated cells did not yield polygonal cells if cultured at low densities in the presence of horse serum plus hemoglobin. It is possible that the polygonal and elongated cells are undifferentiated cells and differentiating myogenic cells, respectively.

Cadmium-induced alterations in RNA metabolism in cultures of Chinese hamster cells sensitive to and resistant to the cytotoxic effects of cadmium

A variant population (CdR) of cultured Chinese hamster cells (line CHO) was derived that is more than 100 times as resistant to the cytotoxic effects of Cd2+ than is the parent population. The effects on RNA metabolism of exposure to sublethal concentrations of Cd2+ were studied in CHO and CdR. Exposure to 2 X 10(-7) M CdCl2 for 24 h resulted in increased polysome content (1.2 times) and increased uridine or adenosine incorporation into heterogeneous nuclear RNA (1.2-1.4 times) and messenger RNA ((1.5-1.7 times) in both populations. Measurement of ATP pool specific activity following exposure to radiolabeled adenosine showed that increased incorporation reflects increased synthesis. The equivalence of CHO and CdR in dose-response in terms of stimulated RNA synthesis and their disparity in dose-response in terms of cytotoxic effects indicate that the systems involved in conferring protection against the lethal effects of Cd2+ are not similarly involved in attenuating the effects on RNA metabolism.

Reversible regulation by magnesium of chick embryo fibroblast proliferation

The rate of 3H-thymidine incorporation and of cell proliferation in chick embryo fibroblast cultures are reduced coordinately when the [Mg2+] of the external medium is reduced below the physiological concentration of about 0.8 mM. These effects of moderately reduced [Mg2+] and the accompanying change in appearance of the cells, resemble the effects produced by lowering the [serum] of the medium. Cells subjected to severe Mg2+ deprivation, especially at low [Ca2+], die and detach from the culture dish. Cells kept at a reduced rate of proliferation for three days by moderate Mg2+ deprivation are quickly restored to rapid proliferation upon restoration of the normal [Mg2+] of the medium. The rate of proliferation of the chick embryo cells is reduced markedly by lowering [Ca2+] about 100-fold, but unlike the case of Mg2+-deprivation this can occur without significant effect on the rate of 3H-thymidine incorporation. More severe Ca2+ deprivation, which does lower the rate of 3H-thymidine incorporation, produces retraction of cells from one another and from the dish, and results in a distinctly abnormal, rounded appearance. The results lend weight to the thesis that free [Mg2+] plays a central role within the cell in the coordinate control of metabolism and growth. They also suggest that the effects produced by varying [Ca2+] in the medium are caused by changes at the external surface of the cell.

Magnesium deprivation reproduces the coordinate effects of serum removal or cortisol addition on transport and metabolism in chick embryo fibroblasts

A variety of unrelated effectors stimulate or inhibit coordinately the same array of metabolic reactions in chick embryo fibroblasts, including the uptake of 2-deoxy-D-glucose and uridine, and the incorporation of uridine and thymidine into acid insoluble material. The coordinate inhibition of these reactions by omission of serum or addition of cortisol is reproduced quantitatively by lowering the concentration of magnesium (Mg2+) in medium containing 0.2 mM Ca2+. The response times for the utilization of uridine and thymidine following the removal of addition of Mg2+ are similar to those which follow removal or addition of serum. The effect of serum on the incorporation of choline, which is not part of the coordinate response to unrelated effectors, is not reproduced by varying Mg2+ concentrations. The results support the hypothesis that the availability of Mg2+ within the cell plays a central role in the coordinate control of transport, metabolism and growth by external physiological effectors.