Identification of two human glutaminase loci and tissue-specific expression of the two related genes.

Glutaminolysis is initiated by either of two isoforms, K- and L-types, of the enzyme phosphate-activated glutaminase. The chromosomal localization, genomic organization, and the tissue-specific expression of the genes have been investigated in the human by using isoform-specific cDNA probes. Results obtained from radiation hybrid mapping experiments assigned the K-glutaminase gene to human Chromosome (Chr) 2, and a second locus for L-glutaminase in Chr 12 was identified. Southern blot analysis with the L-cDNA probe showed hybridization to a single restriction fragment, while four to seven fragments were found to hybridize to the K-cDNA probe. The distribution of human glutaminase expression was also investigated: the L-cDNA probe detected a single band of 2.4 kb in liver, brain, and pancreas, whereas a single transcript of approximately 4.4 kb was detected in kidney, brain, heart, placenta, lung, and pancreas by using the K-cDNA probe. This work provides evidence that the human liver and kidney glutaminase isozymes are encoded by separate genes located on different chromosomes; furthermore, the expression pattern in human tissues revealed for both isoenzymes differs notably from the paradigm based upon the isoenzyme distribution in rats.

Changes in mRNAs for enzymes of glutamine metabolism in the tumor-bearing mouse.

Changes in the relative mRNA levels of phosphate-activated glutaminase (PAG) and glutamine synthetase (GS) in the liver and kidney of mice bearing a highly malignant strain of Ehrlich ascites tumor cells were determined at different days after tumor transplantation. Kidney glutaminase mRNA steadily increased, reaching maximum values at day 10 of tumor growth, while those of glutamine synthetase did not change, resulting in a sustained decrease of the GS/PAG ratio in the kidneys of tumor-bearing animals compared with controls. However, the GS/PAG ratio in the liver significantly increased, mainly due to a strong decrease in PAG, whereas GS mRNA levels remained almost unaffected. These results, combined with those previously reported on enzymatic activities and glutamine concentrations in the host-tumor system, suggest a long-term regulation of the host glutaminase enzymes in order to increase the circulating glutamine levels needed for tumor growth.

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.

Polyamine contents of human breast cancer cells treated with the cytotoxic agents chlorpheniramine and dehydrodidemnin B.

The cytotoxic agents chlorpheniramine and dehydrodidemnin B decreased the cell growth of estrogen receptor-negative human breast cancer cells MDA-MB231 and estrogen receptor-positive MCF-7, after 48 h treatment. Both agents reduced ornithine decarboxylase activity, but polyamine levels were increased in MDA-MB231 cells treated with dehydrodidemnin B. MCF-7 cells when treated with dehydrodidemnin B showed significant increases in spermidine and spermine contents. The results suggest that besides other effects, the cytotoxicity of DDB could be explained in part by the over-accumulation of spermidine and spermine.

Submitochondrial localization and membrane topography of Ehrlich ascitic tumour cell glutaminase.

The intramitocondrial localization of the phosphate-activated glutaminase from Ehrlich cells has been examined by a combination of techniques, including: mitochondria subfractionation studies, chemical modification with sulfhydryl group reagents of different permeability, enzymatic digestion in both sides of the inner mitochondrial membrane, and immunological studies. Using alkaline extraction at high ionic strength, hypoosmotic shock and freezing-thawing cycle techniques, the enzyme was found in the particulate fraction. On the contrary, glutaminase activity was labile when subfractionation was carried out by digitonin/lubrol method; Western blot analysis localized the inactive enzyme in the matrix fraction. In addition, glutaminase was fully inactivated when mitoplasts were incubated with phospholipase A2 and phospholipase C. The enzyme also showed a non-linear Arrhenius plot with a break at 24 degrees C. The membrane-impermeant thiol reagents mersalyl and p-chloromercuriphenylsulfonic acid do not inhibit glutaminase activity in freeze-thawed mitochondria and mitoplasts, but N-ethylmaleimide, which is membrane permeant, strongly inhibited the enzyme. However, mersalyl and p-chloromercuriphenylsulfonic acid were effective inhibitors when the alkylation was performed on the matrix side of mitoplasts or using detergent-solubilized enzyme. Furthermore, trypsin digestion of mitoplasts was only effective inactivating glutaminase when the proteolysis was carried out on the matrix side of the vesicles. Enzyme-linked immunosorbent assay of the soluble and membrane fractions obtained in the preparation of submitochondrial particles, revealed that most of the enzyme was solubilized, but in the inactive form. Phase separation with Triton X-114 rendered most of the protein in the aqueous phase. These results taken together discard a transmembrane localization for the protein, whereas they are consistent with anchorage of glutaminase on the matrix side of the inner mitochondrial membrane, the matrix portion of the enzyme being relevant for its function.