Glutamine Inhibits the Lowering Effect of Glucose on the Level of Phosphoenol pyruvate Carboxykinase mRNA in Isolated Rat Hepatocytes

Abstract

The expression of phosphoenolpyruvate carboxykinase (P-pyruvate CK) was shown to be decreased by hypoosmolarity and increased by glutamine in perfused liver from fed rats [Newsome, W. P., Warskulat, U., Noe, B., Wettstein, M., Stoll, B., Gerok, W. & Haussinger, D. (1994) Biochern. J . 304, 555-5601, This work was undertaken to specify the mechanisms of glutamine action, using isolated hepatocytes from rats that had been starved for 24 h. At low concentrations (up to 5 mM), glutamine elicited a decrease in the level of P-pyruvate CK mRNA through cell swelling and, at higher concentrations, an increase in the mRNA level was observed. Experiments with combinations of glucose and glutamine or glucose and various amino acids demonstrated that glutamine counteracted the inhibitory effect of glucose on P-pyruvate CK mRNA at a transcriptional level, and strongly suggested that the amide group of glutamine was involved in this effect. The metabolism of glucose was required for the reinforcement of the apparent stimulatory effect of glutamine, as demonstrated by the use of various sugars. Glucosamine, but not mannosamine, increased the level of P-pyruvate CK mRNA, as did glucose plus glutamine. These results suggest that the pathway leading from glucosamine-6-phosphate production might be responsible, at least partly, for the effect observed on P-pyruvate CK mRNA.Keywords: hepatocyte ; mRNA ; phosphoenolpyruvate carboxykinase; glucose ; glutamine.Glutamine regulates liver metabolism by stimulating glycogen synthesis and lipogenesis from glucose [I], by stimulating protein synthesis [2], and by inhibiting ketogenesis [I] and proteolysis 13, 41. To exert this anabolic signal on glucose metabolism, glutamine activates the key enzymes of glycogen and lipid synthesis, glycogen synthase and acetyl-CoA carboxylase, respectively [5]. This activation has been reported to be dependent, at least in part, on the cell swelling that results from the sodium-dependent transport of glutamine (5, 61. Recently, we have reported that glutamine may regulate liver metabolism at the level of specific mRNAs such as that of argininosuccinate synthetase, a key enzyme of ureagenesis, by the same mechanism, i.e. cell swelling [7]. It may therefore be proposed that glutamine-induced cell swelling regulates liver metabolism at two levels: the activity of some key enzymes and the gene expression of others. However, the mRNA level of phosphoennlpyruvate carboxykinase (P-pyruvate CK) was recently reported to be decreased by cell swelling induced by hypoosmolarity and increased by glutamine in perfused rat liver [S, 91. These results strongly suggest that glutamine regulates the expression in liver of genes for key enzymes in intermediary metabolism by a mechanism other than cell swelling. In addition to its effect on the level of P-pyruvate CK mRNA, glutamine is a good substrate for gluconeogenesis, and its contribution to Correspondence fo A. Lavoinne, GBPDN, Facultt de MCdecineFux: +33 02 35 66 44 50. Abbreviations P-pyruvate CK, phosphoenolpyruvate...

Effect of hyperosmotic shock on phosphoenolpyruvate carboxykinase gene expression and gluconeogenic activity in the crab muscle

“…In this work, the increase in the PEPCK mRNA level after 24 h of the hyperosmotic stress led to the increase of the gluconeogenic and PEPCK activities as consequence of the metabolism of amino acids in muscle. In mammals it was suggested that glutamine may regulate the PEPCK gene expression through two di¡erent mechanisms: the glutamineinduced cell swelling, decreasing the mRNA level, and another unidenti¢ed mechanism, depending on its metabolism, that increases the mRNA level [7,24].…”

Glutamine‐dependent changes in gene expression and protein activity

“…Glutamine at low concentrations (up to 5 mM) elicited a decrease of PEPCK mRNA, through cell swelling but at higher concentrations, an increase in the mRNA level was observed. 31 This latter effect seems to result from glutamine metabolism by an unknown mechanism. Lavoinne et al reported that 10 mM glutamine increases the level of PEPCK mRNA in perfused rat liver.…”

Liver glyconeogenesis: a pathway to cope with postprandial amino acid excess in high-protein fed rats?

“…Glucose, in contrast to other factors, has opposing effects on PEPCK and G6PC1 gene expression, as it downregulates PEPCK (15) and upregulates G6Pase (40). Glutamine has been shown to counteract the effect of glucose on PEPCK gene expression (35), but no study has examined the effect of a physiological increase of amino acid availability on the regulation of PEPCK and G6PC1 gene expression in gluconeogenic organs. There is, however, some evidence that glucose alone is not a good precursor for hepatic glycogen synthesis in vitro in perfused rat liver (26) or in isolated hepatocytes (21,31) and that the addition of modest amounts of gluconeogenic substrates in the culture medium (lactate or amino acids) results in markedly enhanced glycogenesis (7,47).…”