Control of the uptake of amino acids by serum chick embryo cells, untransformed or transformed rous sarcoma virus

Regulation of amino acid transport in chicken embryo fibroblasts by purified multiplication-stimulating activity (MSA)

Enhanced amino acid transport is observed when quiescent cultures of chicken embryo fibroblasts are stimulated to proliferate by the addition of purified multiplication-stimulating activity (MSA). This increase in amino acid transport is an early event occurring prior to the onset of DNA synthesis in stimulated cells. Results indicate that the changes in transport activity, as measured by alpha-aminoisobutyric acid (AIB) uptake, are due to stimulation of only the Na+-dependent A transport system. There is little or no change in the activities of transport systems ASC, L, or Ly+ upon exposure to MSA. A kinetic analysis shows this increased activity is due to a change in Vmax while Km remains unaltered. Continuous exposure to the stimulus is required to maintain the increased level of transport activity and the presence of inhibitors of RNA and protein synthesis significantly inhibits the response. Results also indicate that a similar specific increase in the A transport system is initiated when RSV tsNY68 infected cells are shifted to the permissive temperature. It appears that the A system of mediation is emerging as a strategic regulatory site for cell function.

Oxocarboxylic acids, pyridine nucleotide-linked oxidoreductases and serum factors in regulation of cell proliferation

When serum is made rate-limiting for clonal multiplication of human diploid fibroblasts, the presence of a 2-oxocarboxylic acid in the medium becomes essential. The requirement is independent of the 20 amino acids and glucose. Glyoxylic, pyruvic, 2-oxoglutaric, and oxalacetic acids are most effective. The types of 2-oxocarboxylic acids that support multiplication are oxidized substrates for several, pyridine nucleotide-linked intracellular oxidoreductases. The requirement is not satisfied by carboxylic acids, oxidized substrates for oxidoreductases that are not lniked to pyridine nucleotides, or by nonspecific electron acceptors. The quantitative requirement for 2-oxocarboxylic acids in cell multiplication is markedly affected by the concentration of serum proteins in the medium. Therefore, 2-oxocarboxylic acid metabolism may be related to the mechanism by which serum growth factors regulate cell multiplication.

Role of the membrane potential in serum-stimulated uptake of amino acid in a diploid human fibroblast

The Na+-dependent accumulation of alpha-aminoisobutyric acid (AIB), measured in normal growing and quiescent (serum-deprived) HSWP cells (human diploid fibroblast), was found to be twofold higher (AIB/in/AIBout = 20-25) under the normal growing conditions. Serum stimulation of quiescent cells increases their AIB concentrating capacity by approximately 70% within 1 hr. These observations suggest that the driving forces for AIB accumulation may be reversibly influenced by the serum concentration of the growth medium. Addition of valinomycin (Val) to cells preequilibrated with AIB causes an enhanced accumulation of AIB, suggesting that the membrane potential can serve as a driving force for AIB accumulation. After preequilibration with AIB in 6 mM K+, transfer to 94 mM K+ with Val results in a marked and rapid net loss of AIB. The effect of Val on the accumulation of AIB is greatest in quiescent cells, with the intracellular AIB concentrations reaching those seen both in Val-stimulated normal cells and in Val-stimulated serum-stimulated cells. By adjusting [K+]0, in the presence of Val, the membrane potential of growing cells can be matched to that of quiescent cells or vice versa. When this is done, the two accumulate AIB to the same extent. Hence the AIB accumulating capacity is characteristic of the membrane potential rather than of the growth state. In summary, these data suggest that the accumulation of AIB in HSWP cells is influenced by changes in membrane potential and that a serum-associated membrane hyperpolarization could be responsible for the increased capacity for AIB accumulation in serum-stimulated cells.

A comparison of the ability of normal liver, a premalignant liver, a solid hepatoma and the Zajdela ascitic hepatoma, to take up amino acids in vitro

The net total uptake of several amino acids at low (0.8-3.1 mumoles/liter) as well as high (800-1200 mumoles/liter) extracellular concentrations, by normal rat liver, a premalignant liver, a solid hepatoma, and the Zajdela ascitic hepatoma cells, has been compared under conditions in which protein synthesis continues. At low amino acid concentrations, the initial (3 min) total uptake of the various amino acids in the Zajdela cells, was 3-10 (average 7) times more, and the intracellular concentration of the labeled amino acids taken up 14-45 (average 31) times more, than in normal liver. At the high amino acid concentrations, the total uptake in the Zajdela cells, at 60-120 min was 2-5 (average 3.5) times more, and the intracellular concentration of the amino acids taken up 8-18 (average 13) times more, than in normal liver; the corresponding values for the premalignant liver and the solid hepatoma were in between those for normal liver and the Zajdela cells. Further, the rate of the total uptake of amino acids, their intracellular concentration, the proportion of the amino acid taken up utilized for protein synthesis, the rate of incorporation of the amino acid taken up into protein, and the cellular growth rate, seemed to be correlated in the four cell/tissue preparations studied. In most cases, the rate of the net uptake fell drastically with time, the uptake virtually stopping after 90-180 min, probably due to lack of serum in the incubation medium.

Uptake of amino acids and thymidine during the first cell cycle of synchronized hamster cells

The net total uptake of four amino acids (valine, leucine, lysine and methionine) used at concentrations required for growth, and of thymidine at tracer concentrations, has been studied during the first cell cycle of an asparagine-dependent strain of transformed BHK cells synchronized by asparagine starvation. The rate of the total uptake of the amino acids, the free pool of the amino acids taken up, and the rate of their incorporation into protein at the end of the first cell cycle were, on the average, 12-fold that at the beginning of the cell cycle. The increase in these parameters during the cell cycle was not linear. The uptake of thymidine started before the onset of DNA synthesis and proceeded linearly beyond the peak of the S phase. The rate of accumulation of thymidine into the acid-soluble fraction also increased during the S phase, apart from a tendency to plateau off at the peak of this phase. It reached a second plateau towards the end of the cell cycle, and then declined slightly. Evidence is presented which suggests that the total quantity of protein synthesized during the cell cycle is more than the newly synthesized protein present in the cells at the end of the cell cycle; this indicated degradation and/or secretion of a substantial proportion of the newly synthesized protein. The total protein synthesized at different time points in the cell cycle appeared to contain different proportions of the amino acids used.