Abstract
The regulation of the transport of neutral amino acids across the cell membrane by adaptive mechanisms has been studied in cultured human fibroblasts. Among the three transport systems (A, ASC, and L) individually discriminated, only the Na+-dependent System A was subject to adaptive regulation, showing enhancement of its activity when the cells were incubated under conditions of amino acid shortage (derepression phase) and decrease of its activity when the cells were exposed to a medium supplied with Site A-reactive amino acids (repression phase). Starvation-induced derepression of transport activity and its reversal by amino acid refeeding required active RNA and protein synthesis. Derepression involved an early mRNA synthesis which started within 30 min from the abrupt change in extracellular amino acid concentration and apparently lasted 90 min. The transcribed mRNA was rather stable and translatable for a few hours (presumably into transport proteins) in cells maintained in the absence of amino acids. Repression by amino acid refeeding also involved an early mRNA synthesis, the product of its translation being presumably a protein capable of causing degradation or inactivation of transport proteins. The rate of decay in transport activity of previously derepressed cells was somewhat faster in the presence of added Site A-reactive amino acids than in their absence. A model is proposed in which the concentration of Site A-reactive amino acids affects transport activity of System A by modulating transcription of mRNA species coding for transport proteins and their putative inactivators and by regulating the efficiency of transport protein inactivation at the cell membrane.
Highlights
The regulation of the transporotf neutral amino acids associated with the operation of the transportsystems and to across thecell membrane by adaptive mechanisms has the participation of trans-effects in modulating the activity of been studied in cultured human fibroblasts
Among the these systems as a function of the size of intracellular amino three transport systems (A, ASC, and L) individually discriminated, only the Na+-dependentSystem A was subject to adaptive regulation, showing enhancement of its activity when the cells were incubated under conditions of amino acid shortage and decrease of its activity when the cells were exposed to a medium supplied with Site A-reactive amino acids
Adaptive regulation, linking transport activity to the availability of pertinent amino acids in the cell environment, has been formerly described in isolated chick embryo heart cells [4]as a mechanism affecting the transport of neutral amino acids through System A(5).’The activity of the lattersystem increases when cells are exposed to conditions of amino acid shortage or starvation and decreases when cells are exposed to conditions of abundant substrate amino acids
Summary
From the Istituto di Patologia Generale, Universita di Parma, ViaGramsci 14, 43100 Parma, Italy. The regulation of the transporotf neutral amino acids associated with the operation of the transportsystems and to across thecell membrane by adaptive mechanisms has the participation of trans-effects in modulating the activity of been studied in cultured human fibroblasts. The we undertook this investigation with two aims: (a) evaluation of the basic activity of the relevant transport to explore in detail the mechanism of adaptive regulation as systems requiredminimization of the regulatory actions acting defined by its constituent derepression and repression phases on these cells as a result of their adaptationto the conditions and ( b )to add information for a comprehensive description of of cell culturing and incubation All unlabeled natural amino acids were purchased from Sigma, London, England
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