Role of hepatic carbonic anhydrase in de novo lipogenesis.
The role of carbonic anhydrase in de novo lipid synthesis was examined by measuring [1-14C]acetate incorporation into total lipids, fatty acids and non-saponifiable lipids in freshly isolated rat hepatocytes. Two carbonic anhydrase inhibitors, trifluoromethylsulphonamide (TFMS) and ethoxozolamide (ETZ) decreased incorporation of 14C into total lipids. Both fatty acid and non-saponifiable lipid components of the total lipid were inhibited to approximately the same extent by 100 microM TFMS (29 +/- 0.3% and 35 +/- 0.3% of control respectively in replicate studies). However, neither drug significantly affected ATP concentrations or the transport activity of Na+/K(+)-ATPase, two measures of cell viability. To establish the site of this inhibition, water-soluble 14C-labelled metabolites from perchloric acid extracts of the radiolabelled cells were separated by ion-exchange chromatography. TFMS inhibited 14C incorporation into citrate, malate, alpha-oxoglutarate and fumarate, but had no effect on incorporation of 14C into acetoacetate. Since ATP citrate-lyase, the cytosolic enzyme that catalyses the conversion of citrate into acetyl-CoA, catalyses an early rate-limiting step in fatty acid synthesis, levels of cytosolic citrate may be rate controlling for de novo fatty acid and sterol synthesis. Indeed citrate concentrations were significantly reduced to 37 +/- 6% of control in hepatocytes incubated with 100 microM TFMS for 30 min. TFMS also inhibited the incorporation of 14C from [1-14C]pyruvate into malate, citrate and glutamate, but not into lactate. This supports the hypothesis that TFMS inhibits pyruvate carboxylation, i.e. since all of the 14C from [1-14C]pyruvate converted into citric acid cycle intermediates must come via pyruvate carboxylase (i.e. rather than pyruvate dehydrogenase). Our findings indicate a role for carbonic anhydrase in hepatic de novo lipogenesis at the level of pyruvate carboxylation.
Lynch
C J
CJ
Department of Cellular and Molecular Physiology, College of Medicine, Milton S. Hershey Medical Center, Pennsylvania State University, Hershey 17033, USA.
Fox
H
H
Hazen
S A
SA
Stanley
B A
BA
Dodgson
S
S
Lanoue
K F
KF
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002257:Carbonic Anhydrase Inhibitors / Q000494:pharmacology; D002256:Carbonic Anhydrases / Q000378:metabolism*; D005016:Ethoxzolamide / Q000494:pharmacology; D008055:Lipids / Q000096:biosynthesis*; D008099:Liver / Q000166:cytology / Q000201:enzymology* / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013449:Sulfonamides / Q000494:pharmacology
D002257:Carbonic Anhydrase Inhibitors; D008055:Lipids; D013449:Sulfonamides; C095186:trifluoromethylsulfonamide; D000255:Adenosine Triphosphate; D002256:Carbonic Anhydrases; D000254:Sodium-Potassium-Exchanging ATPase; D005016:Ethoxzolamide