Amino acid depletion activates TonEBP and sodium-coupled inositol transport.
The expression of the osmosensitive sodium/myo-inositol cotransporter (SMIT) is regulated by multiple tonicity-responsive enhancers (TonEs) in the 5'-flanking region of the gene. In response to hypertonicity, the nuclear abundance of the transcription factor TonE-binding protein (TonEBP) is increased, and the transcription of the SMIT gene is induced. Transport system A for neutral amino acids, another osmosensitive mechanism, is progressively stimulated if amino acid substrates are not present in the extracellular compartment. Under this condition, as in hypertonicity, cells shrink and mitogen-activated protein kinases are activated. We demonstrate here that a clear-cut nuclear redistribution of TonEBP, followed by SMIT expression increase and inositol transport activation, is observed after incubation of cultured human fibroblasts in Earle's balanced salts (EBSS), an isotonic, amino acid-free saline. EBSS-induced SMIT stimulation is prevented by substrates of system A, although these compounds do not compete with inositol for transport through SMIT. We conclude that the incubation in isotonic, amino acid-free saline triggers an osmotic stimulus and elicits TonEBP-dependent responses like hypertonic treatment.
Franchi-Gazzola
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R
Dipartimento di Medicina Sperimentale, Sezione di Patologia Generale e Clinica, Università degli Studi di Parma, Via Volturna, 39, 43100 Parma, Italy. ovidio.bussolati@unipr.it
Visigalli
R
R
Dall'Asta
V
V
Sala
R
R
Woo
S K
SK
Kwon
H M
HM
Gazzola
G C
GC
Bussolati
O
O
D000293:Adolescent; D000596:Amino Acids / Q000494:pharmacology*; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002352:Carrier Proteins / Q000378:metabolism*; D002467:Cell Nucleus / Q000378:metabolism; D048429:Cell Size / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D005347:Fibroblasts / Q000166:cytology / Q000378:metabolism; D006360:Heat-Shock Proteins / Q000378:metabolism*; D006801:Humans; D007294:Inositol / Q000378:metabolism; D007552:Isotonic Solutions / Q000494:pharmacology; D008297:Male; D008565:Membrane Proteins; D009997:Osmotic Pressure; D012462:Saline Solution, Hypertonic / Q000494:pharmacology; D012867:Skin / Q000166:cytology* / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism; D027981:Symporters; D015534:Trans-Activators / Q000378:metabolism*; D014157:Transcription Factors
D000596:Amino Acids; D002352:Carrier Proteins; D006360:Heat-Shock Proteins; D007552:Isotonic Solutions; D008565:Membrane Proteins; C118125:NFAT5 protein, human; D012462:Saline Solution, Hypertonic; D027981:Symporters; D015534:Trans-Activators; D014157:Transcription Factors; C073884:SLC5A3 protein, human; D007294:Inositol; D012964:Sodium