Determination of queuosine modification system deficiencies in cultured human cells

Queuine Micronutrient Deficiency Promotes Warburg Metabolism and Reversal of the Mitochondrial ATP Synthase in Hela Cells

Queuine is a eukaryotic micronutrient, derived exclusively from eubacteria. It is incorporated into both cytosolic and mitochondrial transfer RNA to generate a queuosine nucleotide at position 34 of the anticodon loop. The transfer RNA of primary tumors has been shown to be hypomodified with respect to queuosine, with decreased levels correlating with disease progression and poor patient survival. Here, we assess the impact of queuine deficiency on mitochondrial bioenergetics and substrate metabolism in HeLa cells. Queuine depletion is shown to promote a Warburg type metabolism, characterized by increased aerobic glycolysis and glutaminolysis, concomitant with increased ammonia and lactate production and elevated levels of lactate dehydrogenase activity but in the absence of significant changes to proliferation. In intact cells, queuine deficiency caused an increased rate of mitochondrial proton leak and a decreased rate of ATP synthesis, correlating with an observed reduction in cellular ATP levels. Data from permeabilized cells demonstrated that the activity of individual complexes of the mitochondrial electron transport chain were not affected by the micronutrient. Notably, in queuine free cells that had been adapted to grow in galactose medium, the re-introduction of glucose permitted the mitochondrial F₁F_O-ATP synthase to operate in the reverse direction, acting to hyperpolarize the mitochondrial membrane potential; a commonly observed but poorly understood cancer trait. Together, our data suggest that queuosine hypomodification is a deliberate and advantageous adaptation of cancer cells to facilitate the metabolic switch between oxidative phosphorylation and aerobic glycolysis.

Lost in Translation: Defects in Transfer RNA Modifications and Neurological Disorders

Transfer RNAs (tRNAs) are key molecules participating in protein synthesis. To augment their functionality they undergo extensive post-transcriptional modifications and, as such, are subject to regulation at multiple levels including transcription, transcript processing, localization and ribonucleoside base modification. Post-transcriptional enzyme-catalyzed modification of tRNA occurs at a number of base and sugar positions and influences specific anticodon-codon interactions and regulates translation, its efficiency and fidelity. This phenomenon of nucleoside modification is most remarkable and results in a rich structural diversity of tRNA of which over 100 modified nucleosides have been characterized. Most often these hypermodified nucleosides are found in the wobble position of tRNAs, where they play a direct role in codon recognition as well as in maintaining translational efficiency and fidelity, etc. Several recent studies have pointed to a link between defects in tRNA modifications and human diseases including neurological disorders. Therefore, defects in tRNA modifications in humans need intensive characterization at the enzymatic and mechanistic level in order to pave the way to understand how lack of such modifications are associated with neurological disorders with the ultimate goal of gaining insights into therapeutic interventions.

The queuine micronutrient: charting a course from microbe to man

Micronutrients from the diet and gut microbiota are essential to human health and wellbeing. Arguably, among the most intriguing and enigmatic of these micronutrients is queuine, an elaborate 7-deazaguanine derivative made exclusively by eubacteria and salvaged by animal, plant and fungal species. In eubacteria and eukaryotes, queuine is found as the sugar nucleotide queuosine within the anticodon loop of transfer RNA isoacceptors for the amino acids tyrosine, asparagine, aspartic acid and histidine. The physiological requirement for the ancient queuine molecule and queuosine modified transfer RNA has been the subject of varied scientific interrogations for over four decades, establishing relationships to development, proliferation, metabolism, cancer, and tyrosine biosynthesis in eukaryotes and to invasion and proliferation in pathogenic bacteria, in addition to ribosomal frameshifting in viruses. These varied effects may be rationalized by an important, if ill-defined, contribution to protein translation or may manifest from other presently unidentified mechanisms. This article will examine the current understanding of queuine uptake, tRNA incorporation and salvage by eukaryotic organisms and consider some of the physiological consequence arising from deficiency in this elusive and lesser-recognized micronutrient.

Modify or die?--RNA modification defects in metazoans

Chemical RNA modifications are present in all kingdoms of life and many of these post-transcriptional modifications are conserved throughout evolution. However, most of the research has been performed on single cell organisms, whereas little is known about how RNA modifications contribute to the development of metazoans. In recent years, the identification of RNA modification genes in genome wide association studies (GWAS) has sparked new interest in previously neglected genes. In this review, we summarize recent findings that connect RNA modification defects and phenotypes in higher eukaryotes. Furthermore, we discuss the implications of aberrant tRNA modification in various human diseases including metabolic defects, mitochondrial dysfunctions, neurological disorders, and cancer. As the molecular mechanisms of these diseases are being elucidated, we will gain first insights into the functions of RNA modifications in higher eukaryotes and finally understand their roles during development.

Queuosine modification of tRNA: its divergent role in cellular machinery

tRNAs possess a high content of modified nucleosides, which display an incredible structural variety. These modified nucleosides are conserved in their sequence and have important roles in tRNA functions. Most often, hypermodified nucleosides are found in the wobble position of tRNAs, which play a direct role in maintaining translational efficiency and fidelity, codon recognition, etc. One of such hypermodified base is queuine, which is a base analogue of guanine, found in the first anticodon position of specific tRNAs (tyrosine, histidine, aspartate and asparagine tRNAs). These tRNAs of the ‘Q-family’ originally contain guanine in the first position of anticodon, which is post-transcriptionally modified with queuine by an irreversible insertion during maturation. Queuine is ubiquitously present throughout the living system from prokaryotes to eukaryotes, including plants. Prokaryotes can synthesize queuine de novo by a complex biosynthetic pathway, whereas eukaryotes are unable to synthesize either the precursor or queuine. They utilize salvage system and acquire queuine as a nutrient factor from their diet or from intestinal microflora. The tRNAs of the Q-family are completely modified in terminally differentiated somatic cells. However, hypomodification of Q-tRNA (queuosine-modified tRNA) is closely associated with cell proliferation and malignancy. The precise mechanisms of queuine- and Q-tRNA-mediated action are still a mystery. Direct or indirect evidence suggests that queuine or Q-tRNA participates in many cellular functions, such as inhibition of cell proliferation, control of aerobic and anaerobic metabolism, bacterial virulence, etc. The role of Q-tRNA modification in cellular machinery and the signalling pathways involved therein is the focus of this review.

Queuine promotes antioxidant defence system by activating cellular antioxidant enzyme activities in cancer

Constant generation of ROS (reactive oxygen species) during normal cellular metabolism of an organism is generally balanced by a similar rate of consumption by antioxidants. Imbalance between ROS production and antioxidant defence results in an increased level of ROS, causing oxidative stress, which leads to promotion of malignancy. Queuine is a hyper-modified base analogue of guanine, found at the first anticodon position of the Q-family of tRNAs. These tRNAs are completely modified with respect to queuosine in terminally differentiated somatic cells; however, hypo-modification of Q-tRNAs is closely associated with cell proliferation. Q-tRNA modification is essential for normal development, differentiation and cellular function. Queuine is a nutrient factor for eukaryotes. It is found to promote the cellular antioxidant defence system and inhibit tumorigenesis. The activities of antioxidant enzymes such as catalase, superoxide dismutase, glutathione peroxidase and glutathione reductase are found to be low in the DLAT (Dalton's lymphoma ascites transplanted) mouse liver compared with normal mouse liver. However, exogenous administration of queuine to the DLAT cancerous mouse improves the activities of antioxidant enzymes. These results suggest that queuine promotes the antioxidant defence system by increasing antioxidant enzyme activities and in turn inhibits oxidative stress and tumorigenesis.

Possible involvement of queuine in regulation of cell proliferation

An increase in cell number is one of the most prominent characteristics of cancer cells. This may be caused by an increase in cell proliferation or decrease in cell death. Queuine is one of the modified base which is found at first anticodon position of specific tRNAs. It is ubiquitously present throughout the living system except mycoplasma and yeast. The tRNAs of Q-family are completely modified to Q-tRNAs in terminally differentiated somatic cells, however hypomodification of Q-tRNA is closely associated with cell proliferation and malignancy. Queuine participates at various cellular functions such as regulation of cell proliferation, cell signaling and alteration in the expression of growth associated proto-oncogenes. Like other proto-oncogenes bcl2 is known to involve in cell survival by inhibiting apoptosis. Queuine or Q-tRNA is suggested to inhibit cell proliferation but the mechanism of regulation of cell proliferation by queuine or Q-tRNA is not well understood. Therefore, in the present study regulation in cell proliferation by queuine in vivo and in vitro as well as the expression of cell death regulatory protein Bcl2 are investigated. For this DLAT cancerous mouse, U87 cell line and HepG2 cell line are treated with different concentrations of queuine and the effect of queuine on cell proliferation and apoptosis are studied. The results indicate that queuine down regulates cell proliferation and expression of Bcl2 protein, suggesting that queuine promotes cell death and participates in the regulation of cell proliferation.

Determination of queuosine derivatives by reverse-phase liquid chromatography for the hypomodification study of Q-bearing tRNAs from various mammal liver cells

Three queuosine derivatives (Q-derivatives) have been found at position 34 of four mammalian so-called Q-tRNAs: queuosine (Q) in tRNA(Asn) and tRNA(His), mannosyl-queuosine (manQ) in tRNA(Asp), and galactosyl-queuosine (galQ) in tRNA(Tyr). An analytical procedure based on the combined means of purified tRNA isolation from liver cells and ribonucleoside analysis by reverse-phase high performance liquid chromatography coupled with real-time UV-spectrometry (RPLC-UV) was developed for the quantitative analysis of the three Q-derivatives present in total tRNA from liver tissues and liver cell cultures. Using this analytical procedure, the rates of Q-tRNA modification were studied in total tRNAs from various mammalian hepatic cells. Our results show that the four Q-tRNAs are fully modified in liver tissues from adult mammals, regardless of the mammal species. However, a lack in the Q-modification level was observed in Q-tRNAs from newborn rat liver, as well in Q-tRNAs from normal rat liver cell cultures growing in a low queuine content medium, and from a rat hepatoma cell line. It is noteworthy that in all cases of Q-tRNA hypomodification, our analytical procedure showed that tRNA(Asp) is always the least affected by the hypomodification. The biological significance of this phenomenon is discussed.

Elevated expression level of 60-kDa subunit of tRNA-guanine transglycosylase in colon cancer

tRNA-guanine transglycosylase (TGT) is an enzyme which synthesizes a modified nucleoside, queuosine, by exchanging the base moiety of guanosine for queuine in tRNA. We have reported that the expression level of the 60-kDa subunit of TGT (TGT60kD) is elevated in leukemic cells, however, there is no other report on the expression of TGT60kD in cancer cells. The expression levels of the TGT60kD protein are elevated in four of the five colon cancer cell lines and 83% of colon cancer tissues compared with normal tissues. The expression levels of the TGT60kD protein decreased in two colon cancer cell lines, after cell differentiation was induced. A marked positive staining of cancer cells in colon tissues was observed, and the subcellular staining pattern was mainly cytosolic. These data suggest that the role of TGT60kD in colon carcinogenesis.

Queuosine modification of tRNA: a case for convergent evolution

Queuosine is a hypermodified nucleoside found in position 34, the anticodon wobble position, of four tRNA species. This modification is distributed with near uniformity across all life forms found on this planet. Yet the molecular mechanisms involved with accomplishing this ubiquitous posttranscriptional modification of tRNA are dramatically different between prokaryotic and eukaryotic organisms, which suggests that these were formed by convergent evolution of a fundamental life process essential to nearly all life forms. This minireview describes the differences between these modification systems and points to a new direction for developing research on the molecular function queuosine-modified tRNA in diverse species.

Characterization of cDNA encoding the human tRNA-guanine transglycosylase (TGT) catalytic subunit

Queuosine (Q) is a 7-deazaguanosine found in the first position of the anticodon of tRNAs that recognize NAU and NAC codons (Tyr, Asn, Asp and His). Eukaryotes synthesize Q by the base-for-base exchange of queuine (Q base) for guanine in the unmodified tRNA, a reaction catalyzed by TGT. A search of the human EST database for sequences with significant homology to the well studied TGT from Escherichia coli identified several candidates for full-length (1.3-1.4 kb) cDNA clones. Three candidate cDNA clones, available from IMAGE Consortium, LLNL, (Lennon et al., 1996, Genomics 33, 151-152) were obtained: IMAGE Clone Id Nos. 611146, 1422928, and 72154. Here we report the complete sequences of these clones. IMAGE:72154 contains an ORF encoding a 44 kDa polypeptide with high homology to bacterial TGTs and was subcloned into the mammalian expression vector pMAMneo-Cat. When this construct was transfected into the TGT-negative cell line, GC(3)/c1 (Gündüz et al., 1992, Biochim. Biophys. Acta 1139, 229-238), it restored the ability of the cells to form Q-containing tRNA. This TGT cDNA sequence is encoded in human chromosome 19 clone CTC-539A10 (GenBank accession no. AC011475), enabling determination of the exon-intron boundaries for the TGT gene. The sequence of IMAGE:611146 is 5'-truncated by 76 bp compared to that from IMAGE:72154 and, except for two differences in the 3'-non-coding region, the remainder of the sequence is identical to that of IMAGE:72154. IMAGE:1422928 is a 1390 bp chimera: the 5'-portion, bp 1-708, is identical to a genomic DNA sequence from chromosome 15 (GenBank accession no. AC067805, bp 148976-149683); the 3'-end, bp 726-1390, is identical to the 3'-end of the TGT cDNA sequence from IMAGE:611146.