Amino aid deprivation induces synthesis of four proteins in chick embryo cells

Low-frequency electromagnetic fields induce a stress effect upon higher plants, as evident by the universal stress signal, alanine

15N NMR analysis reveals alanine production in Duckweed plants exposed to low intensity sinusoidally varying magnetic fields (SVMF) at 60 and 100Hz, and fed by 15N-labeled ammonium chloride. Alanine does not accumulate in the absence of SVMF. Addition of vitamin C, a radical scavenger, reduced alanine production by 82%, indicating the roll of free radicals in the process. Alanine accumulation in plants and animals in response to exposure to a variety of stress conditions, including SVMF, is a general phenomenon. It is proposed that alanine is a universal first stress signal expressed by cells.

Single amino acid (arginine) deprivation induces G1 arrest associated with inhibition of cdk4 expression in cultured human diploid fibroblasts

Withdrawal of a single amino acid (arginine) from freely cycling early passage primary human fibroblasts caused a halt to proliferation, characterized by an accumulation of cells in the G1 phase of the cell cycle. This arrest was accompanied by the suppression of cyclin D1- and cyclin E-associated kinase activities and the appearance of hypophosphorylated retinoblastoma protein. Arginine-deprived cells remained viable for in excess of 4 days and could be made to synchronously reenter the cell cycle by restoration of the amino acid, with kinetics characteristic of exit from a quiescent state. Stimulation of cells arrested by serum withdrawal did not result in S-phase entry when arginine was omitted from the culture medium. Although cyclin D1 accumulated on normal schedule, cdk4, which increased following restimulation in amino acid-replete medium, was not induced when arginine was absent. These results suggest that arginine deprivation-in common with other "suboptimal" conditions-inhibits the passage of normal human cells through the restriction point and implicate cdk4 as the key regulatory element in amino acid-sensitive cell cycle control.

Amino acid transport systems modulate human tumor cell growth and invasion: a working hypothesis

Interactions between the extracellular matrix (ECM) and the neoplastic cells they envelop are thought to play a fundamental role in those cells' ability to invade, one of the key events in the metastatic cascade. Cellular transport of amino acids, in turn, is known to be mediated by functionally distinct membrane transport systems and is modulated by substrate bioavailability in the microenvironment. We postulate that certain advantages enjoyed by a neoplastic cell population over their normal counterparts (for example, increased proliferating capability and invasiveness across ECM barriers) are linked to changes in the cells' differential control of amino acid transport (aaT) via host ECM-tumor cell generated signals. Our studies suggest that active transport of neutral amino acids modulates a cells' functional behavior among phenotypically distinct human transformed cell types, irrespective of whether they are categorized as a sarcoma, melanoma, or carcinoma. We present preliminary laboratory evidence which has lead us to formulate a series of working hypotheses as follows: 1. aaT systems operating in both non-transformed and transformed human cells exhibit differential transport kinetics; 2. adaptive regulation of certain amino acids via cell-specific aaT systems alters a cell's ability to invade human ECM; and 3. aaT induction involves changes both at the cellular and molecular levels. This report, therefore, provides experimental support, and suggests a possible mechanism, to explain how neutral amino acids, acting as nutrient signalling factors (along with other biologic elements) within the cell milieu, have the capability of regulating the phenotypic nature of human neoplastic cells.

Translational activation of the non-AUG-initiated c-myc 1 protein at high cell densities due to methionine deprivation

c-myc belongs to a small, yet growing, group of eukaryotic mRNAs that initiate translation inefficiently from a non-AUG codon upstream from a more efficient AUG codon. We have examined the translational regulation of non-AUG-initiated c-myc 1 and AUG-initiated c-myc 2 protein synthesis in avian and mouse cells during proliferation. As lymphoid, erythroid, and embryo fibroblast cells approached high densities in culture, there was a sustained 5- to 10-fold induction in the synthesis of c-myc 1 protein to levels greater than or equal to c-myc 2 protein synthesis. Treatment with conditioned/depleted media from high-density cells was able to reproduce this activation in low-density cells within 5 hr. Additional studies with the conditioned/depleted media revealed that amino acid availability, specifically methionine deprivation, was responsible for this unique translational control. Our results describe a specific and dramatic regulation of dual translational initiation. Furthermore, these results represent a novel translational activation of a specific gene in higher eukaryotes in response to nutrient deprivation.

Induction of stress proteins in chicken embryo cells by low-level zinc contamination in amino acid-free media

It has been reported that chicken embryo cells deprived of exogenous amino acids for 4 hours synthesize stress (heat-shock) proteins. Herein, we show that amino acid deprivation is not sufficient to cause induction of stress proteins. Zinc contaminating a component of commercial cell culture medium used to prepare amino acid-free medium was an inducer in our cultures. In the absence of exogenous amino acids, the concentration of zinc ions needed for half-maximal induction of stress proteins was an order of magnitude lower than the dose required for cells in complete medium. Histidine and cystine, which have high affinities for zinc ions, were the amino acids most effective in blocking the induction of stress proteins by zinc. Problems posed by heavy metal ions in culture media and biologic fluids for searches for in vivo inducers of the cellular stress (heat shock) response are discussed.

Durable synthesis of high molecular weight heat shock proteins in G0 cells of the yeast and other eucaryotes

We report that eucaryotic cells were induced to synthesize a specific class of heat shock proteins (hsps) when they entered the resting state, G0. This finding was originally made with Saccharomyces cerevisiae cells by taking advantage of the system in which we can distinguish between G1 arrests leading to G0 and those that do not result in G0 (Iida, H., and I. Yahara, 1984, J. Cell Biol. 98:1185-1193). Similar observations were subsequently made with higher eucaryotic cells including chick embryonic fibroblasts (CEF), mouse T lymphocytes, and Drosophila GM1 cells. The induction of hsps in G0 cells was distinct from that in heat-shocked cells in two respects. First, hsps with molecular weight around 25,000 were not induced in G0 cells, whereas most, if not all, high molecular weight (HMW) hsps were commonly induced both in G0 cells and in heat-shocked cells. Second, in contrast to the transient synthesis of hsps in heat-shocked cells, G0 cells continued to synthesize hsps at the stimulated rate for a relatively long period. These results suggest the possibility that high molecular weight hsps might function in a transition from the proliferating state to G0 or in maintaining G0 in the eucaryote.

Regulation of amino acid transport in L6 myoblasts. II. Different chemical properties of transport after amino acid deprivation

The mechanism of stimulation of amino acid transport system A caused by amino acid deprivation in L6 cells was investigated. In cells loaded with alpha-aminoisobutyric acid (AIB), amino acid deprivation increased the rate of proline uptake only after the intracellular [AIB] dropped below 7 mM. Efflux of proline was not sensitive to the presence of proline in the outer medium (with or without external Na+), suggesting that efflux through system A (and possibly uptake) is not susceptible to transinhibition. Transport (stimulated uptake) into amino acid-deprived cells and that into amino acid-supplemented cells differed in several chemical properties: 1) In the former group, transport was higher at lower pH values than in the latter, and the optimum pH values were 7.5 and 7.8, respectively. 2) Unlike proline uptake in supplemented cells, uptake in deprived cells was inhibited by 50% with N-ethylmaleimide (1 mM) or by 50 microM p-chloromercuribenzoate (PCMBS). Inhibition by PCMBS was not due to collapse of the Na+ gradient. The mercurial inhibited only the deprivation-induced stimulation of transport, bringing the rate of proline uptake to the "basal" uptake level observed in amino acid-supplemented cells. Proline uptake was not stimulated by a second deprivation following treatment with PCMBS and a supplementation-deprivation cycle. However, in untreated cells, or by reversing mercaptide formation with dithiotreitol, the second deprivation stimulated transport. Deprivation at 4 degrees C did not elicit stimulation of proline uptake. Cycloheximide prevented the stimulation and decreased the rate of proline uptake in deprived cells more efficiently than in supplemented cells. Actinomycin D prevented stimulation when added at the onset of deprivation. The above data indicate that stimulation of transport by deprivation is protein synthesis-dependent and that the stimulated transport had chemical properties distinct from the "basal" transport in supplemented cells. The evidence presented is consistent with a model of activation of a finite pool of transporters upon deprivation, the chemical characteristics of which differ from those of the "basal" transport system.

Heat shock proteins and thermotolerance; a comparison of induction kinetics

The hypothesis that the expression of heat shock proteins following a preliminary hyperthermic treatment is responsible for subsequent thermotolerance to a second heat treatment is examined. CHO cells were given a 12 min, 45 degrees C pretreatment and then incubated for varying intervals at 37 degrees C. The synthesis of certain intracellular proteins was monitored as a function of time post-incubation by using 35S-methionine incorporation as determined in SDS polyacrylamide gel electrophoresis. Cell survival was concurrently measured by challenging the cells with a second heat treatment (45 degrees C/27 min). Major heat shock proteins were observed at 68 000, 89 000 and 110 000 daltons. The synthesis of these proteins was significantly reduced in the presence of cyclohexamide. The total 35S-methionine incorporation into these proteins correlated well with the induction of survival resistance (thermotolerance). An approximate exponential relationship between survival and the amount of each of these proteins may occur. These and other heat shock proteins were also present, in a significantly reduced degree, in control (non-heat shocked) cells maintained under normal culture conditions at 37 degrees C. It is possible that heat shock proteins are responsible for the phenomenon of thermotolerance.