Biosynthesis and processing of mitochondrial glutaminase in HTC hepatoma cells

Glutaminases

Mammalian glutaminases catalyze the stoichiometric conversion of L-glutamine to L-glutamate and ammonium ions. In brain, glutaminase is considered the prevailing pathway for synthesis of the neurotransmitter pool of glutamate. Besides neurotransmission, the products of glutaminase reaction also fulfill crucial roles in energy and metabolic homeostasis in mammalian brain. In the last years, new functional roles for brain glutaminases are being uncovered by using functional genomic and proteomic approaches. Glutaminases may act as multifunctional proteins able to perform different tasks: the discovery of multiple transcript variants in neurons and glial cells, novel extramitochondrial localizations, and isoform-specific proteininteracting partners strongly support possible moonlighting functions for these proteins. In this chapter, we present a critical account of essential works on brain glutaminase 80 years after its discovery. We will highlight the impact of recent findings and thoughts in the context of the glutamate/glutamine brain homeostasis.

Circadian clock gene expression regulates cancer cell growth through glutaminase

Glutamine is an essential amino acid for malignant tumor cells. Glutaminase that metabolizes glutamine reaches a maximum expression in tumors immediately before the maximum proliferation rate. Tumor cells grow at different rates during the day. We postulated that the activity of glutaminase in tumor cells is subject to the regulation of circadian clock gene. We measured glutaminase by western blot analysis and circadian clock gene expression by real-time polymerase chain reaction in the liver and tumor cells at six equispaced time points of the day in individual mice of a 12/12 h light/dark schedule. The results showed that the tumor-bearing mice, under normal diurnal conditions, are circadianly entrained, as reflected by the normal host locomotor activity rhythms and rhythmic liver clock gene expression. The tumors within these mice are also circadianly organized, as reflected by circadian clock gene (Bmal1) expression. What is most remarkable is that kidney-type glutaminase also showed circadian rhythms in the same pattern with tumor circadian clock gene expression in liver cancer xenograft model, indicating that conditionally inhibiting glutaminase activity may provide a new target for cancer therapy.

Co-expression of glutaminase K and L isoenzymes in human tumour cells

The pattern of expression of glutaminase isoenzymes in tumour cells has been investigated to clarify its role in the malignant transformation and the prospect of its use as a clinically relevant factor. Using leukaemia cells from medullar blood of human patients and several established human cancer cell lines, we have developed a competitive RT (reverse transcriptase)-PCR assay to quantify simultaneously K-type (kidney-type) and L-type (liver-type) glutaminase mRNAs. Co-expression of both transcripts and higher amounts of L-type mRNA were always found in all cancer cell types analysed. However, mature lymphocytes from the medullar blood of a patient suffering aplasia did not express the K-type transcript and showed a 15-fold increase of L-type transcript. Co-expression was also confirmed at the protein level using isoform-specific antibodies; nevertheless, it did not correlate with the relative abundance of glutaminase transcripts and strong K-type protein signals were detected. On the other hand, marked differences were found with regard to glutamate inhibition and phosphate activation of tumour glutaminase activity. Taken together, the protein data suggest that K isoform would account for the majority of glutaminase activity in these human tumour cells. The results confirm that simultaneous expression of both isoenzymes in human cancer cells is a more frequent event than previously thought. Furthermore, the present work and other previous data suggest that K isoform is up-regulated with increased rates of proliferation, whereas prevalence of the L isoform seems to be related with resting or quiescent cell states.

Glutamine metabolism: Role in acid-base balance*

The intent of this review is to provide a broad overview of the interorgan metabolism of glutamine and to discuss in more detail its role in acid-base balance. Muscle, adipose tissue, and the lungs are the primary sites of glutamine synthesis and release. During normal acid-base balance, the small intestine and the liver are the major sites of glutamine utilization. The periportal hepatocytes catabolize glutamine and convert ammonium and bicarbonate ions to urea. In contrast, the perivenous hepatocytes are capable of synthesizing glutamine. During metabolic acidosis, the kidney becomes the major site of glutamine extraction and catabolism. This process generates ammonium ions that are excreted in the urine to facilitate the excretion of acids and bicarbonate ions that are transported to the blood to partially compensate the acidosis. The increased renal extraction of glutamine is balanced by an increased release from muscle and liver and by a decreased utilization in the intestine. During chronic acidosis, this adaptation is sustained, in part, by increased renal expression of genes that encode various transport proteins and key enzymes of glutamine metabolism. The increased levels of phosphoenolpyruvate carboxykinase result from increased transcription, while the increase in glutaminase and glutamate dehydrogenase activities result from stabilization of their respective mRNAs. Where feasible, this review draws upon data obtained from studies in humans. Studies conducted in model animals are discussed where available data from humans is either lacking or not firmly established. Because there are quantitative differences in tissue utilization and synthesis of glutamine in different mammals, the review will focus more on common principles than on quantification.

Isolation, characterization and expression of a human brain mitochondrial glutaminase cDNA

Various cDNAs that encode overlapping portions of the full-length human brain glutaminase (GA) cDNA were cloned and sequenced. The overall nucleotide sequence of hGA has a very high degree of identity with that of the rat kidney-type GA cDNA (77.4%) and the known portion of the cDNA that encodes the 5.0-kb porcine GA mRNA (81.1%). The identity is even more remarkable at the amino acid level, particularly in the C-terminal half where the three proteins share a 99.7% sequence identity. The hGA cDNA encodes a 73,427-Da protein that contains an N-terminal mitochondrial targeting signal and retains the primary proteolytic cleavage site characterized for the cytosolic precursor of the rat renal mitochondrial glutaminase. The entire coding region was assembled through the use of unique restriction sites and cloned into a baculovirus. Sf9 cells infected with the recombinant virus express high levels of properly processed and active glutaminase. Thus, expression of the isolated hGA cDNA should provide a means to purify large amounts of the mitochondrial glutaminase, a protein that catalyzes a key reaction in the metabolism of glutamine and the synthesis of important excitatory and inhibitory neurotransmitters.

Molecular cloning, sequencing and expression studies of the human breast cancer cell glutaminase

Phosphate-activated glutaminase (GA) is overexpressed in certain types of tumour but its exact role in tumour cell growth and proliferation is unknown. Here we describe the isolation of a full-length cDNA clone of human breast cancer ZR75 cells, by a combination of lambdagt10 cDNA library screening and the rapid amplification of cDNA ends ('RACE') technique. The cDNA of human GA is 2408 nt with a 1806-base open reading frame encoding a 602-residue protein with a predicted molecular mass of 66309 Da. The deduced amino acid sequence contains a putative mitochondrial import presequence of 14 residues at the N-terminal end. Heterologous expression and purification in Escherichia coli yielded a product of the expected molecular size that was recognized by using antibodies against the recombinant human GA. Sequence analyses showed that human GA was highly similar to the rat liver enzyme. Northern gel analysis revealed that the gene is present in human liver, brain and pancreas, in which a major transcript of 2.4 kb was demonstrated, but not in kidney, heart, skeletal muscle, lung or placenta. These results strongly suggest that the first human GA cloned, the GA from ZR-75 breast cancer cells, and presumably those from human liver and brain, are liver-type isoenzymes, in sharp contrast with the present view that considers the kidney type as the isoform expressed in all tissues with GA activity, with the exception of postnatal liver.

In vitro characterization of the mitochondrial processing and the potential function of the 68-kDa subunit of renal glutaminase

Rat renal mitochondrial glutaminase (GA) is initially synthesized in primary cultures of proximal tubule cells as a 74-kDa precursor and is processed via a 72-kDa intermediate to generate a heterotetrameric enzyme which contains three 66-kDa subunits and one 68-kDa subunit (Perera, S. Y., Chen, T. C., and Curthoys, N. P. (1990) J. Biol. Chem. 265, 17764-17770). The two mature subunits may be derived by either of two possible mechanisms: 1) alternative proteolytic processing or 2) initial synthesis of the 66-kDa subunit followed by its covalent modification to generate the 68-kDa subunit. An in vitro system was utilized to further characterize this unique processing pathway and to investigate the potential function of the 68-kDa subunit. In vitro transcription and translation of the GA cDNA yields a single 74-kDa precursor. Upon incubation with isolated rat liver mitochondria, the precursor is translocated into the mitochondria and processed via a 72-kDa intermediate to yield a 3:1 ratio of the 66- and 68-kDa subunits, respectively. The kinetics of the in vitro processing reaction also closely approximate the kinetics observed in cultured cells. Mitochondrial processing is blocked by o-phenanthroline, an inhibitor of the matrix processing peptidase (MPP). The 72-amino acid presequence of the 66-kDa subunit contains a large proportion of basic amino acids. Two-dimensional gel electrophoresis of mature GA established that the 68-kDa subunit is slightly more basic than the 66-kDa subunit. In addition, incubation of the 74-kDa precursor with purified MPP yields equimolar amounts of the two mature peptides. A cDNA construct, p delta GA, was created which lacks the nucleotides that encode the amino acid residues 32 through 72 of GA. When transcribed and translated in vitro, p delta GA yields a 70-kDa precursor. This precursor is processed by mitochondria to a single mature subunit with a M of 66 kDa. This observation suggests that the 68-kDa subunit is not produced by covalent modification of the 66-kDa subunit and further supports the conclusion that the two mature subunits of GA are produced by alternative processing reactions which can be catalyzed by MPP. However, the yield of products obtained in intact mitochondria may be determined by some unidentified accessory factor. Submitochondrial fractionation of imported GA and delta GA precursors suggest that the 68-kDa subunit may function to retain the mature GA within the mitochondrial matrix.