Induction of GLUT-1 mRNA in response to inhibition of oxidative phosphorylation: role of increased [Ca2+]i.
Exposure of Clone 9 cells (a rat liver cell line expressing only the GLUT-1 isoform) to 5 mM azide or to 3 microM ionomycin for 12 h results in 3.7 +/- 0.3- and 4.9 +/- 0.4-fold increases in GLUT-1 mRNA content, respectively, suggesting the hypothesis that a rise in cytosolic free calcium concentration ([Ca2+]i) mediates the induction of GLUT-1 mRNA by azide. Five lines of evidence were employed to test this hypothesis. 1) Exposure of cells to 0-3 microM of ionomycin increased [Ca2+]i from 83 +/- 9 to 504 +/- 20 nM (half-maximal effect at 0.1 microM ionomycin), whereas half-maximal increase in GLUT-1 mRNA occurred at 1 microM ionomycin, with the increase in the mRNA being negligible at [Ca2+]i below 400 nM. Exposure of cells to 5 mM azide, however, increased [Ca2+]i to maximal value of 174 +/- 22 nM at 15 s, suggesting that the magnitude of the increase in [Ca2+]i by azide may not be adequate for the response. 2) The increase in GLUT-1 mRNA content by azide was fully preserved in cells preloaded with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). 3) GLUT-1 mRNA content increased within 30 min of exposure to ionomycin, whereas the mRNA increased after a "delay" period of 2 h in cells exposed to 5 mM azide. 4) A brief (2-min) rise in [Ca2+]i by ionomycin was sufficient to increase GLUT-1 mRNA content, whereas continuous exposure to azide for > 1 h was necessary for a subsequent induction of the mRNA. 5) Treatment with ionomycin, A-23187, and thapsigargin caused larger increases in glucose-regulated protein 78 and 94 and in 70-kDa heat shock protein mRNAs than in GLUT-1 mRNA, whereas treatment with azide resulted in greater induction of GLUT-1 mRNA. These results strongly suggest that, whereas increased [Ca2+]i enhances GLUT-1 mRNA expression and azide increases [Ca2+]i, the rise in [Ca2+]i does not mediate the induction of GLUT-1 mRNA in response to inhibition of oxidative phosphorylation.
Mitani
Y
Y
Department of Medicine, Case Western Reserve University, Cleveland, Ohio 44106-4951, USA.
Dubyak
G R
GR
Ismail-Beigi
F
F
D000255:Adenosine Triphosphate / Q000378:metabolism; D001386:Azides / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000494:pharmacology; D002999:Clone Cells; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D000091342:Endoplasmic Reticulum Chaperone BiP; D051272:Glucose Transporter Type 1; D018840:HSP70 Heat-Shock Proteins / Q000494:pharmacology; D006360:Heat-Shock Proteins; D007425:Intracellular Membranes / Q000378:metabolism*; D015759:Ionomycin / Q000494:pharmacology; D008565:Membrane Proteins / Q000494:pharmacology; D018832:Molecular Chaperones / Q000494:pharmacology; D009004:Monosaccharide Transport Proteins / Q000235:genetics*; D009994:Osmolar Concentration; D010085:Oxidative Phosphorylation; D012333:RNA, Messenger / Q000378:metabolism*; D013997:Time Factors
D001386:Azides; D002352:Carrier Proteins; D000091342:Endoplasmic Reticulum Chaperone BiP; D051272:Glucose Transporter Type 1; D018840:HSP70 Heat-Shock Proteins; D006360:Heat-Shock Proteins; D008565:Membrane Proteins; D018832:Molecular Chaperones; D009004:Monosaccharide Transport Proteins; D012333:RNA, Messenger; C018551:glucose-regulated proteins; D004533:Egtazic Acid; D015759:Ionomycin; D000255:Adenosine Triphosphate; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium