Induction of glut1 mrna in response to inhibition of oxidative
phosphorylation: role of increased cytosolic free calcium.
Mitani, Yasuo, George R. Dubyak, and Faramarz Ismail-Beigi.
Department of Medicine, and of *Physiology and Biophysics, Division
of Clinical and Molecular Endocrinology, Case Western Reserve
University, Cleveland, Ohio 44106-4951
APStracts 2:0252C, 1995.
Exposure of Clone 9 cells (a rat liver cell line expressing only the
GLUT1 isoform) to 5 mM azide or to 3 [mu]M ionomycin for 12 h results
in a 3.7 +/- 0.3- and 4.9 +/- 0.4-fold increases in GLUT1 mRNA
content, respectively, suggesting the hypothesis that a rise in
cytosolic free calcium ([Ca2+]i) mediates the induction of GLUT1 mRNA
by azide. Five lines of evidence were employed to test this
hypothesis: 1) Exposure of cells to 0 to 3 [mu]M of ionomycin
increased [Ca2+]i from 83 +/- 9 to 504 +/- 20 nM (half-maximal effect
at 0.1 [mu]M ionomycin), while half-maximal increase in GLUT1 mRNA
occurred at 1 [mu]M 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
sec, suggesting that the magnitude of the increase in [Ca2+]i by
azide may not be adequate for the response; 2) The increase in GLUT1
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) GLUT1 mRNA content increased within 30 min of exposure to
ionomycin, while 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 GLUT1 mRNA content,
whereas continuous exposure to azide for > 1 h was necessary for
a subsequent induction of the mRNA; and 5) Treatment with ionomycin,
A23187, and thapsigargin caused larger increases in glucose-regulated
protein 78 and 94 and in heat shock protein 70 mRNAs than in GLUT1
mRNA, whereas treatment with azide resulted in greater induction of
GLUT1 mRNA. These results strongly suggest that while increased
[Ca2+]i enhances GLUT1 mRNA expression, and while azide increases
[Ca2+]i, the rise in [Ca2+]i does not mediate the induction of GLUT1
mRNA in response to inhibition of oxidative phosphorylation.
Received 18 May 1995; accepted in final form 23 June 1995.
APS Manuscript Number C277-5.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 18 July 1995.