The compartmentation of inositol phospholipids was examined by using a combination of radiolabelling approaches in intact and permeabilized 1321N1 astrocytoma cells. A ‘chase’ protocol was developed with whole cells in which phosphoinositide (PI) pools were labelled to steady state with [3H]inositol and the cellular [3H]inositol pool was then diluted selectively with non-radioactive inositol. In these cells muscarinic-receptor-stimulated phospholipase C (PLC) hydrolysed [3H]PI at approx. 1-2%/min. However, after the chase procedure the relative specific radioactivity of [3H]Ins(1,3,4)P3, a rapidly metabolized and sensitive marker of PLC activity, decreased only after more than 5 min and over a time course similar to that during which the labelling of each [3H]PtdIns, [3H]PtdInsP and [3H]PtdInsP2 declined by at least 50%. These results demonstrate a large receptor-responsive [3H]PI pool that is accessed by stimulated PLC without apparent metabolic compartmentation, despite its probable distribution between different membrane fractions. Support for this was obtained in intact cells by using an acute [3H]inositol labelling method in which increases in the specific radioactivity of [3H]inositol phosphates stimulated by carbachol occurred only in parallel with similar increases in the labelling of the bulk of cellular [3H]PI. In [3H]inositol-prelabelled cells permeabilized to deplete cytosolic proteins, carbachol and guanosine 5ʹ-[γ-thio]triphosphate stimulated the endogenous PLC to degrade only approx. 5% of [3H]PI. This was increased to approx. 30% in the presence of exogenous PtdIns transfer protein, which, at a concentration approx. 5-10% of that in 1321N1 cell cytosol, was sufficient to support PLC activity comparable with that observed in response to carbachol in whole cells. These and earlier results in 1321N1 cells suggest a model of integrated PI pools involving an obligatory role for lipid transport. Given the multifunctional capacity of PI in cellular signalling mechanisms, this model has important implications, particularly for the hypothesis that the ability of Li+ ions to influence these selectively might account for its therapeutic actions.
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Research Article|
March 15 1998
Evidence for a model of integrated inositol phospholipid pools implies an essential role for lipid transport in the maintenance of receptor-mediated phospholipase C activity in 1321N1 cells
H. Ian BATTY;
H. Ian BATTY
1
1Department of Biochemistry, Medical Sciences Institute, University of Dundee, Dundee DD1 4HN, Scotland, U.K.
1To whom correspondence should be addressed.
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A. Richard CURRIE;
A. Richard CURRIE
1Department of Biochemistry, Medical Sciences Institute, University of Dundee, Dundee DD1 4HN, Scotland, U.K.
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C. Peter DOWNES
C. Peter DOWNES
1Department of Biochemistry, Medical Sciences Institute, University of Dundee, Dundee DD1 4HN, Scotland, U.K.
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Publisher: Portland Press Ltd
Received:
September 02 1997
Revision Received:
December 01 1997
Accepted:
December 12 1997
Online ISSN: 1470-8728
Print ISSN: 0264-6021
The Biochemical Society, London © 1998
1998
Biochem J (1998) 330 (3): 1069–1077.
Article history
Received:
September 02 1997
Revision Received:
December 01 1997
Accepted:
December 12 1997
Citation
H. Ian BATTY, A. Richard CURRIE, C. Peter DOWNES; Evidence for a model of integrated inositol phospholipid pools implies an essential role for lipid transport in the maintenance of receptor-mediated phospholipase C activity in 1321N1 cells. Biochem J 15 March 1998; 330 (3): 1069–1077. doi: https://doi.org/10.1042/bj3301069
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