PEPCK, Glyceroneogenesis and fatty acid cycling

One of the very under appreciated physiological roles of PEPCK involves fatty acid cycling.  The majority of fat present in the body is in one of two forms, either as a single long chain or a group of three independent chains connected to a glycerol backbone.  Single chains are referred to as free fatty acids and the group is called a triacylglyceride or simply a triglyceride.  In general, free fatty acids are used in muscle and other tissues for fuel, and triglyerides are the stored form of fat.  Both the liver and adipose (fat storing) tissues convert free fatty acids to triglycerides and vice versa. One of the determinants of the ratio of free fatty acids to triglycerides is the cellular concentration of glycerol-glyceride to form a backbone for triglycerides.

Under normally fed conditions, glyerol-glyceride is produced from glycolysis.  Glycerol-glyceride is converted to glyerol-3-phosphate which then has fatty acids attached.  During fasting conditions, however there is a reduction in the amount of glycerol-glyceride that is produced from glucose via glycolysis.  If Glycerol-glyceride cannot be made from glucose, it is then make by a truncated form of gluconeogenic pathway called Glyceroneogenesis (glycerogenesis).  The role of PEPCK is the same in Glyceroneogenesis as it is in gluconeogenesis-  to convert oxaloacetate to PEP.  PEP is then converted to dihydroxyacetone and then glycerol-3-phosphate where it can form the backbone of a triglyceride.  So, increasing the activity of PEPCK will increase the incorporation of FFA in to TG, and shift the cellular balance between free fatty acids and triglycerides towards triglycerides.  This principle is the same in both liver and adipose tissue.  However, there is some interesting physiology based on differential hormonal regulation of PEPCK in these tissues.

Insulin down regulates PEPCK expression in both liver and adipose tissue, and other hormones such as glucagon up regulates the enzyme.  The difference between the tissues is how they respond to glucocorticoids.  The liver will increase PEPCK expression in response to cortisol and dexamethasone, where adipose tissue will decrease it.  Physiologically, this means that glucocoritcoids have opposite effects on the fatty acid triglyceride ratio in liver and adipose tissue.  They will increase triglycerides in liver, and increase FA in adipose.

The ability of PEPCK to regulate the balance between free fatty acids and triglycerides may be important clinically.  Thiazolidinediones are new class of pharmaceuticals used to treat diabetes are and are considered to be insulin sensitizers.  The mechanism of action of these compounds is to activate peroxisome proliferator-activated receptors (PPAR), which cause in increase in the transcription and expression of PEPCK in adipose tissue.  As noted above, increasing the enzymatic activity of PEPCK increases the incorporation of free fatty acids in to triglycerides in adipocytes.  This not only reduces the cellular concentration of free fatty acids in fat tissue, but it reduces the amount of free fatty acids released in to the blood.  There is strong evidence that part of the insulin insensitivity of diabetics is due to high levels of free fatty acids circulating in the blood.  Reducing circulating fatty acids leads to an improvement in insulin sensitivity.  Thiazolidinediones have many other bedsides PEPCK transcription that are also important for their clinical effects and are currently marketed as vandia (rosiglitazone) and actos (pioglitazone). 

Effects of free fatty acids per se on glucose production, gluconeogenesis, and glycogenolysis. Staehr P, Hother-Nielsen O, Landau BR, Chandramouli V, Holst JJ, Beck-Nielsen H. Diabetes. 2003 Feb;52(2):260-7. [PDF]

Glyceroneogenesis revisited. Hanson RW, Reshef L. Biochimie. 2003 Dec;85(12):1199-205.

Glyceroneogenesis and the triglyceride/fatty acid cycle. Reshef L, Olswang Y, Cassuto H, Blum B, Croniger CM, Kalhan SC, Tilghman SM, Hanson RW. J Biol Chem. 2003 Aug 15;278(33):30413-6  [PDF]

The effect of pioglitazone on peroxisome proliferator-activated receptor-gamma target genes related to lipid storage in vivo.Bogacka I, Xie H, Bray GA, Smith SR.  Diabetes Care. 2004 Jul;27(7):1660-7. [PDF]

Thiazolidinediones block fatty acid release by inducing glyceroneogenesis in fat cells. Tordjman J, Chauvet G, Quette J, Beale EG, Forest C, Antoine B. J Biol Chem. 2003 May 23;278(21):18785-90. [PDF]

The effect of pioglitazone on peroxisome proliferator-activated receptor-gamma target genes related to lipid storage in vivo. Bogacka I, Xie H, Bray GA, Smith SR. Diabetes Care. 2004 Jul;27(7):1660-7. [PDF]

PPAR-gamma and epilepsy

Ameliorative effect of pioglitazone on seizure responses in genetically epilepsy-susceptible EL mice.

Brain Research 2006 Aug 2;1102(1):175-8.
Okada K, Yamashita U, Tsuji S.

Pioglitazone, a peroxisome proliferator-activated receptor-gamma agonist, delayed the development of seizure responses and mildly shortened the duration of convulsion of genetically epileptic EL mice. mRNA levels of IL-1beta, IL-6 and TNF-alpha before seizure and mRNA levels of IL-6 and TNF-alpha after seizure were decreased in the brains of the mice with pioglitazone. These results suggest that pioglitazone may have ameliorative effects on epileptic seizure responses partly through the reduction of inflammatory responses in the brain.

Neuronal peroxisome proliferator-activated receptor gamma signaling: regulation by mood-stabilizer valproate.

J Mol Neurosci. 2008 Jun;35(2):225-34.
Lan MJ, Yuan P, Chen G, Manji HK.

Laboratory of Molecular Pathophysiology, National Institute of Mental Health, National Institutes of Health, Bethesda, MD, USA.
Valproate (Depakote) remains an effective medication for the prevention and treatment of seizures in epilepsy and of mood symptoms in bipolar disorder. Both of these disorders are severe and debilitating, and both warrant further medication options as well as a better understanding of the side effects associated with their current treatments. Although a number of molecular and cellular processes have been found to be altered by valproate, the medication's therapeutic mechanism has not been fully elucidated. In this paper, peroxisome proliferator-activated receptor (PPAR) signaling was examined to determine valproate's effects on this transcriptional regulatory system in neuronal tissue. PPAR signaling has been found to affect a number of biochemical processes, including lipid metabolism, cellular differentiation, insulin sensitivity, and cell survival. When primary neuronal cultures were treated with valproate, a significant decrease in PPARgamma signaling was observed. This effect was demonstrated through a change in nuclear quantities of PPARgamma receptor and decreased DNA binding of the receptor. Valproate also caused gene expression changes and a change to the peroxisome biochemistry consistent with a decrease of PPARgamma signaling. These biochemical changes may have functional consequences for either valproate's therapeutic mechanism or for its neurological side effects and merit further investigation.

Characteristics of glutamine metabolism in human precision-cut kidney slices: a 13C-NMR study

Biochem J. 2005 May 1; 387(Pt 3): 825–834.

Anne Vittorelli, Catherine Gauthier, Christian Michoudet, Guy Martin, and Gabriel Baverel

The metabolism of glutamine, a physiological substrate of the human kidney, plays a major role in systemic acid–base homoeostasis. Not only because of the limited availability of human renal tissue but also in part due to the lack of adequate cellular models, the mechanisms regulating the renal metabolism of this amino acid in humans have been poorly characterized. Therefore given the renewed interest in their use, human precision-cut renal cortical slices were incubated in Krebs–Henseleit medium (118 mM NaCl, 4.7 mM KCl, 1.18 mM KH2PO4, 1.18 mM MgSO4·7H2O, 24.9 mM NaHCO3 and 2.5 mM CaCl2·2H2O) with 2 mM unlabelled or 13C-labelled glutamine residues. After incubation, substrate utilization and product formation were measured by enzymatic and NMR spectroscopic methods. Glutamate accumulation tended to plateau but glutamine removal and ammonia, alanine and lactate production as well as flux through GLDH (glutamate dehydrogenase) increased to various extents with time for up to 4 h of incubation indicating the metabolic viability of the slices. Valproate, a stimulator of renal glutamine metabolism, markedly and in a dose-dependent fashion increased ammonia production. With [3-13C]glutamine as a substrate, and in the absence and presence of valproate, [13C]glutamate, [13C]alanine and [13C]lactate accounted for 81 and 96%, 34 and 63%, 30 and 46% of the glutamate, alanine and lactate accumulations measured enzymatically respectively. The slices also metabolized glutamine and retained their reactivity to valproate during incubations lasting for up to 48 h. These results demonstrate that, although endogenous metabolism substantially operates in the presence of glutamine, human precision-cut renal cortical slices are metabolically viable and strongly respond to the ammoniagenic effect of valproate. Thus, this experimental model is suitable for metabolic and pharmaco-toxicological studies

Copyright 2011 Steve Kriegler