Serum‐mediated regulation of amino acid transport in cultured chick embryo fibroblasts

Abstract

When chicken serum was added to serum-deprived quiescent cultures of chick embryo fibroblasts the activity of amino acid transport by means of the A system, as measured by a-aminoisobutyric acid and L-proline uptake after discrimination of the contribution of interacting systems, increased with time of exposure to serum between 30 and 120 minutes (remaining constantly high thereafter). Under the same conditions, DNA synthesis, as measured by thymidine incorporation, increased abruptly six to eight hours after the addition of serum. Serum-mediated increases of transport activity by the A system have also been detected with glycine, L-alanine and L-serine. Transport activities of systems ASC, L and Ly+ did not change appreciably (or decreased slightly) after the addition of serum. The stimulation of amino acid transport was apparently proportional to the length of exposure to serum; its rate declined progressively with time after withdrawal of serum from the culture medium. Kinetic analysis indicated that stimulation of the activity of transport system A by serum occurred through a mechanism affecting V, , , rather than K,; stimulation was prevented by inhibitors of protein synthesis. Our results indicate that the A transport system is the only system which is regulated by serum in cultured avian fibroblasts.Remarkably, the A transport system appears to be the target on which widely different factors and conditions converge to regulate amino acid transport in eukaryotic cells. Each system acts on a discrete group of amino acids with rather extensive overlap of substrate reactivity (Christensen, '69, '73). Therefore, the characterization of a specific regulatory effect on the transport process re-J. CELL. PHYSIOL., 93: 425-434.quires the formal identification of the system(s) involved (Christensen, '76).The purpose of the present study was to characterize the serum-dependent control mechanism of amino acid transport in cultured avian fibroblasts. The results to be presented indicate that changes in transport activity for amino acids occur earlier than the onset of DNA synthesis in cells committed by serum to enter into the S phase of the cell cycle. These changes consist of increases in activity of only one discrete transport system for amino acids; the A system. They require continuous exposure to serum, are likely to reflect an acceleration of the maximal transport velocity and are abolished by inhibitors of protein synthesis.

Growth regulation and amino acid transport in epithelial cells: Influence of culture conditions and transformation on A, ASC, and l transport activities

“…This result supports the suggestion of Tramacere et al (1977) that the A system may be a"target" upon which different conditions of regulation converge. As a focal point of regulation, the specific response of the A system in these cells to environmental change may alter the intracellular concentration of specific amino acids, an event which could induce or contribute to a change in cell behavior.…”

The effect of transport system A and N amino acids and of nerve and epidermal growth factors on the induction of ornithine decarboxylase activity

“…Tramacere et al (1977) have proposed that these systems, especially system A, may be the focus of converging regulatory conditions. In spite of these correlative data, a causal relationship between transport activity and growth control was not demonstrated.…”

Effect of hyperosmolarity on the activity of amino acid transport system L in avian fibroblasts

“…In this paper we present evidence that in hyperosmolaritytreated cells the apparent stimulation of System L activity reflects the higher internal level of exchangeable amino acids that, in turn, is the result of the increased activity of System A. Cells Chick embryo fibroblasts were obtained from 9-to 11-day-old embryos by the method of Rubin (1973) as described previously (Tramacere et al, 1977. All experiments were performed with secondary cultures seeded with 2.5 x lo5 cells in 9-cm2 wells of disposable multiwell trays (Costar) and grown in 3 ml of growth medium in a humidified COz-atmosphere incubator at 37°C.…”

“…Amino acid transport System A is a Na+-dependent system with reactivity for a group of neutral amino acids with a short, polar, or linear side chain (Guidotti et al, 1978). This system is considered the target on which widely different factors and environmental conditions converge to regulate amino acid transport in eukaryotic cells (Tramacere et al, 1977). Both amino acid transport systems…”