Hepatic glutaminase gene expression in the tumor-bearing rat

The role of skeletal muscle in liver glutathione metabolism during acetaminophen overdose

Marked alterations in systemic glutamate-glutamine metabolism characterize the catabolic state, in which there is an increased breakdown and decreased synthesis of skeletal muscle protein. Among these alterations are a greatly increased net release of glutamine (Gln) from skeletal muscle into blood plasma and a dramatic depletion of intramuscular Gln. Understanding the catabolic state is important because a number of pathological conditions with very different etiologies are characterized by its presence; these include major surgery, sepsis, trauma, and some cancers. Acetaminophen (APAP) overdose is also accompanied by dramatic changes in systemic glutamate-glutamine metabolism including large drops in liver glutathione (for which glutamate is a precursor) and plasma Gln. We have constructed a mathematical model of glutamate and glutamine metabolism in rat which includes liver, blood plasma and skeletal muscle. We show that for the normal rat, the model solutions fit experimental data including the diurnal variation in liver glutathione (GSH). We show that for the rat chronically dosed with dexamethasone (an artificial glucocorticoid which induces a catabolic state) the model can be used to explain empirically observed facts such as the linear decline in intramuscular Gln and the drop in plasma glutamine. We show that for the Wistar rat undergoing APAP overdose the model reproduces the experimentally observed rebound of liver GSH to normal levels by the 24-h mark. We show that this rebound is achieved in part by the action of the cystine-glutamate antiporter, an amino acid transporter not normally expressed in liver but induced under conditions of oxidative stress. Finally, we explain why supplementation with Gln, a Glu precursor, assists in the preservation of liver GSH during APAP overdose despite the fact that under normal conditions only Cys is rate-limiting for GSH formation.

Mechanisms governing the expression of the enzymes of glutamine metabolism--glutaminase and glutamine synthetase

Whether on the scale of a single cell, organ or organism, glutamine homeostasis is to a large extent determined by the activities of glutaminase (GA, EC 3.5.1.2) and glutamine synthetase (GS, EC 6.3.1.2), the two enzymes that are the focus of this report. GA and GS each provide examples of regulation of gene expression at many different levels. In the case of GA, two different genes (hepatic- and kidney-type GA) encode isoforms of this enzyme. The expression of hepatic GA mRNA is increased during starvation, diabetes and high protein diet through a mechanism involving increased gene transcription. In contrast, the expression of kidney GA mRNA is increased post-transcriptionally by a mechanism that increases mRNA stability during acidosis. We found recently that several isoforms of rat and human kidney-type GA are formed by tissue-specific alternative RNA splicing. Although the implications of this post-transcriptional processing mechanism for GA activity are not yet clear, it allows for the expression of different GA isoforms in different tissues and may limit the expression of GA activity in muscle tissues by diverting primary RNA transcripts to a spliceform that produces a nonfunctional translation product. The expression of GS enzyme is also regulated by both transcriptional and post-transcriptional mechanisms. For example, the GS gene is transcriptionally activated by glucocorticoid hormones in a tissue-specific fashion. This hormonal response allows GS mRNA levels to increase in selected organs during catabolic states. However, the ultimate level of GS enzyme expression is further governed by a post-transcriptional mechanism regulating GS protein stability. In a unique form of product feedback, GS protein turnover is increased by glutamine. This mechanism appears to provide a means to index the production of glutamine to its intracellular concentration and, therefore, to its systemic demand. Herein, we also provide experimental evidence that GS protein turnover is dependent upon the activity of the 26S proteosome.

Tumor-influenced amino acid transport activities in zonal-enriched hepatocyte populations

Cancer influences hepatic amino acid metabolism in the host. To further investigate this relationship, the effects of an implanted fibrosarcoma on specific amino acid transport activities were measured in periportal (PP)- and perivenous (PV)-enriched rat hepatocyte populations. Na(+)-dependent glutamate transport rates were eightfold higher in PV than in PP preparations but were relatively unaffected during tumor growth. System N-mediated glutamine uptake was 75% higher in PV than in PP preparations and was stimulated up to twofold in both regions by tumor burdens of 9 +/- 4% of carcass weight compared with hepatocytes from pair-fed control animals. Excessive tumor burdens (26 +/- 7%) resulted in hypophagia, loss of PV-enriched system N activities, and reduced transporter stimulation. Conversely, saturable arginine uptake was enhanced fourfold in PP preparations and was induced twofold only after excessive tumor burden. These data suggest that hepatic amino acid transporters are differentially influenced by cancer in a spatial and temporal manner, and they represent the first report of reciprocal zonal enrichment of system N and saturable arginine uptake in the mammalian liver.

Glutamine enhances gut glutathione production

BACKGROUND

The gastrointestinal tract is recognized as having important metabolic functions. This study examined gut glutathione (GSH) extraction and the effect of supplemental oral glutamine (GLN) on gut GSH fractional release.

METHODS

Healthy female Fisher-344 rats weighing approximately 150 to 200 g were pair-fed chow and supplemented by gavage with 1 g/kg/d GLN or an isonitrogenous amount of Freamine (McGaw, St. Louis, MO). Rats were sacrificed at 6 weeks. Arterial and portal blood was assayed for GLN and GSH content. The gut GLN and GSH extractions were calculated.

RESULTS

The gut GLN fractional uptake was increased by approximately 50%, and there was a near threefold increase in gut GSH fractional release in the GLN-supplemented group.

CONCLUSIONS

The discovery of gut's role as a major producer of GSH may give insight into why feeding via the gut rather than by the venous route is so important. Supplemental oral GLN further enhances GLN extraction as well as GSH fractional release in the gut.