Caldomycin, a new guanidopolyamine produced by a novel agmatine homocoupling enzyme involved in homospermidine biosynthesis

An extreme thermophilic bacterium, Thermus thermophilus produces more than 20 unusual polyamines, but their biosynthetic pathways, including homospermidine, are not yet fully understood. Two types of homospermidine synthases have been identified in plants and bacteria, which use spermidine and putrescine or two molecules of putrescine as substrates. However, homospermidine synthases with such substrate specificity have not been identified in T. thermophilus. Here we identified a novel agmatine homocoupling enzyme that is involved in homospermidine biosynthesis in T. thermophilus. The reaction mechanism is different from that of a previously described homospermidine synthase, and involves conjugation of two molecules of agmatine, which produces a diamidino derivative of homospermidine (caldomycin) as an immediate precursor of homospermidine. We conclude that there is a homospermidine biosynthetic pathway from agmatine via caldomycin synthase followed by ureohydrolase in T. thermophilus. Furthermore, it is shown that caldomycin is a novel compound existing in nature.

Inhibition of Polyamine Catabolism Reduces Cellular Senescence

The aging of the global population has necessitated the identification of effective anti-aging technologies based on scientific evidence. Polyamines (putrescine, spermidine, and spermine) are essential for cell growth and function. Age-related reductions in polyamine levels have been shown to be associated with reduced cognitive and physical functions. We have previously found that the expression of spermine oxidase (SMOX) increases with age; however, the relationship between SMOX expression and cellular senescence remains unclear. Therefore, we investigated the relationship between increased SMOX expression and cellular senescence using human-liver-derived HepG2 cells. Intracellular spermine levels decreased and spermidine levels increased with the serial passaging of cells (aged cells), and aged cells showed increased expression of SMOX. The levels of acrolein-conjugated protein, which is produced during spermine degradation, also increases. Senescence-associated β-gal activity was increased in aged cells, and the increase was suppressed by MDL72527, an inhibitor of acetylpolyamine oxidase (AcPAO) and SMOX, both of which are enzymes that catalyze polyamine degradation. DNA damage accumulated in aged cells and MDL72527 reduced DNA damage. These results suggest that the SMOX-mediated degradation of spermine plays an important role in cellular senescence. Our results demonstrate that cellular senescence can be controlled by inhibiting spermine degradation using a polyamine-catabolizing enzyme inhibitor.

Polyamines produced by an extreme thermophile are essential for cell growth at high temperature

An extreme thermophile, Thermus thermophilus grows at an optimum temperature of around 70 oC and produces 16 different polyamines including long-chain and branched-chain polyamines. We found that the composition of polyamines in the thermophile cells changes with culture temperature. Long-chain and branched-chain polyamines (unusual polyamines) were increased in the cells grown at high temperature such as 80 oC, but they were minor components in the cells grown at relatively lower temperature such as 60 oC. The effects of polyamines on cell growth were studied using T. thermophilus HB8 ΔspeA deficient in arginine decarboxylase. Cell growth of this mutant strain was significantly decreased at 70 oC. This mutant strain cannot produce polyamines and grows poorly at 75 oC. It was also determined whether polyamines are directly involved in protecting DNA from DNA double-strand breaks induced by heat. Polyamines protected DNA against double-strand breaks. Therefore, polyamines play essential roles in cell growth at extremely high temperature through maintaining a functional conformation of DNA against DNA double-strand breaks and depurination.

Identification of a novel acetylated form of branched-chain polyamine from a hyperthermophilic archaeon Thermococcus kodakarensis

Long/branched-chain polyamines are unique polycations found in thermophiles. The hyperthermophilic archaeon Thermococcus kodakarensis contains spermidine and a branched-chain polyamine, N⁴-bis(aminopropyl)spermidine, as major polyamines. The metabolic pathways associated with branched-chain polyamines remain unknown. Here, we used gas chromatography and liquid chromatography-tandem mass spectrometry analyses to identify a new acetylated polyamine, N⁴-bis(aminopropyl)-N¹-acetylspermidine, from T. kodakarensis; this polyamine was not found in other micro-organisms. The amounts of branched-chain polyamine and its acetylated form increased with temperature, indicating that branched-chain polyamines are important for growth at higher temperatures. The amount of quaternary acetylated polyamine produced was associated with the amount of N⁴-bis(aminopropyl)spermidine in the cell. The ratio of acetylated to non-acetylated forms was higher in the stationary phase than in the logarithmic growth phase under high-temperature stress condition.

Identification and characterization of natural microbial products that alter the free d-aspartate content of mammalian cells

Mammalian cells possess the molecular apparatus necessary to take up, degrade, synthesize, and release free d-aspartate, which plays an important role in physiological functions within the body. Here, biologically active microbial compounds and pre-existing drugs were screened for their ability to alter the intracellular d-aspartate level in mammalian cells, and several candidate compounds were identified. Detailed analytical studies suggested that two of these compounds, mithramycin A and geldanamycin, suppress the biosynthesis of d-aspartate in cells. Further studies suggested that these compounds act at distinct sites within the cell. These compounds may advance our current understanding of biosynthesis of d-aspartate in mammals, a whole picture of which remains to be disclosed.

The role of protein L-isoaspartyl/D-aspartyl O-methyltransferase (PIMT) in intracellular signal transduction

Under physiological conditions, L-aspartyl (L-Asp) and L-asparaginyl residues in proteins are spontaneously isomerized or racemized to D-aspartyl (D-Asp) or D,L-isoaspartyl (D,L-isoAsp) residue. These atypical Asp residues can interfere with protein activity and lead to disruption of cellular function. Protein L-isoaspartyl/D-aspartyl O-methyltransferase (PIMT) is a repair enzyme that initiates the conversion of L-isoAsp (or D-Asp) residues to L-Asp residues. PIMT-Deficient mice exhibit accumulation of L-isoAsp in several tissues and die from progressive epileptic seizures at a mean age of 42 days. However, the biological roles of PIMT are still largely unknown. To further our understanding of the function of this protein, we developed an assay to measure PIMT activity in cell lysates. Additionally, we generated PIMT-knockdown cells by stable transfection of HEK293 cells with PIMT small interfering (si) RNA. Northern blotting and immunoblot analysis revealed that PIMT mRNA and protein levels were significantly decreased in the knockdown cells. In addition, significant levels of proteins that contained isoAsp residues accumulated in these cells, and immunoblot analysis revealed that Raf-1, MEK, and ERK were hyperphosphorylated upon EGF stimulation compared to control cells. These results indicate that the ability to repair atypical Asp residues is important for normal MAP kinase signaling.

Role of the active site residues arginine-216 and arginine-237 in the substrate specificity of mammalian D-aspartate oxidase

D-aspartate oxidase (DDO) and D-amino acid oxidase (DAO) are flavin adenine dinucleotide-containing flavoproteins that catalyze the oxidative deamination of D-amino acids. Unlike DAO, which acts on several neutral and basic D-amino acids, DDO is highly specific for acidic D-amino acids. Based on molecular modeling and simulated annealing docking analyses, a recombinant mouse DDO carrying two substitutions (Arg-216 to Leu and Arg-237 to Tyr) was generated (R216L-R237Y variant). This variant and two previously constructed single-point mutants of mouse DDO (R216L and R237Y variants) were characterized to investigate the role of Arg-216 and Arg-237 in the substrate specificity of mouse DDO. The R216L-R237Y and R216L variants acquired a broad specificity for several neutral and basic D-amino acids, and showed a considerable decrease in activity against acidic D-amino acids. The R237Y variant, however, did not show any additional specificity for neutral or basic D-amino acids and its activity against acidic D-amino acids was greatly reduced. The kinetic properties of these variants indicated that the Arg-216 residue is important for the catalytic activity and substrate specificity of mouse DDO. However, Arg-237 is, apparently, only marginally involved in substrate recognition, but is important for catalytic activity. Notably, the substrate specificity of the R216L-R237Y variant differed significantly from that of the R216L variant, suggesting that Arg-237 has subsidiary effects on substrate specificity. Additional experiments using several DDO and DAO inhibitors also suggested the involvement of Arg-216 in the substrate specificity and catalytic activity of mouse DDO and that Arg-237 is possibly involved in substrate recognition by this enzyme. Collectively, these results indicate that Arg-216 and Arg-237 play crucial and subsidiary role(s), respectively, in the substrate specificity of mouse DDO.

High-performance liquid chromatographic method to measure protein L-isoaspartyl/D-aspartyl o-methyltransferase activity in cell lysates

Protein L-isoaspartyl/D-aspartyl o-methyltransferase (PIMT) is a widely expressed protein repair enzyme that restores isomerized aspartyl residues to their normal configuration. Current methods for measuring PIMT activity have limited sensitivity or require radioactivity. We have developed a highly sensitive new assay method to measure PIMT activity in cell lysates. As a substrate, we used a fluorescently labeled delta sleep-inducing peptide (DSIP) that contains an isoaspartyl residue: 7-nitro-2,1,3-benzoxadiazole (NBD)-DSIP(isoAsp). The PIMT-catalyzed transfer of a methyl group onto this substrate can be detected with a simple high-performance liquid chromatography (HPLC) procedure. After the enzyme reaction, the methylated form of the peptide is stable and can be reproducibly separated from the unmethylated form in an acidic solvent and fluorometrically detected by HPLC. The limit of detection was estimated to be approximately 1 pmol of NBD-DSIP(isoAsp) (signal/noise ratio [S/N]=3), and the quantitation limit of the activity was approximately 18 microg of total cell lysate from HEK293 cells (10.7 pmol/min/mg protein). This assay method is sensitive enough to detect PIMT activity in biological samples without the use of radioisotopes, offering significant advantages over previously reported methods.

Cloning and functional characterization of Arabidopsis thaliana D-amino acid aminotransferase--D-aspartate behavior during germination

The understanding of D-amino acid metabolism in higher plants lags far behind that in mammals, for which the biological functions of these unique amino acids have already been elucidated. In this article, we report on the biochemical behavior of D-amino acids (particularly D-Asp) and relevant metabolic enzymes in Arabidopsis thaliana. During germination and growth of the plant, a transient increase in D-Asp levels was observed, suggesting that D-Asp is synthesized in the plant. Administration of D-Asp suppressed growth, although the inhibitory mechanism responsible for this remains to be clarified. Exogenous D-Asp was efficiently incorporated and metabolized, and was converted to other D-amino acids (D-Glu and D-Ala). We then studied the related metabolic enzymes, and consequently cloned and characterized A. thaliana D-amino acid aminotransferase, which is presumably involved in the metabolism of D-Asp in the plant by catalyzing transamination between D-amino acids. This is the first report of cDNA cloning and functional characterization of a D-amino acid aminotransferase in eukaryotes. The results presented here provide important information for understanding the significance of D-amino acids in the metabolism of higher plants.

Hyperactive mutants of mouse D-aspartate oxidase: mutagenesis of the active site residue serine 308

The role of Ser-308 of murine D-aspartate oxidase (mDASPO), particularly its side chain hydroxyl group, was investigated through the use of site-specific mutational analysis of Ser-308. Recombinant mDASPO carrying a substitution of Gly, Ala, or Tyr for Ser-308 was generated, and fused to either His (His-mDASPO), or glutathione S-transferase, His, and S (GHS-mDASPO) at its N-terminus. Wild-type His-mDASPO or GHS-mDASPO or their mutant derivatives were expressed in Escherichia coli and purified by affinity chromatography. All purified recombinant proteins had functional DASPO activity. The Gly-308 and Ala-308 mutants had significantly higher catalytic efficiency towards D-Asp and N-methyl-D-Asp, and a higher affinity for flavin adenine dinucleotide (FAD) compared to the wild-type enzyme. The Tyr-308 mutant had lower catalytic efficiency and binding capacity. These results suggest that the side chain hydroxyl group of a critical residue of mDASPO, Ser-308, down-regulates enzymatic activity, substrate binding, and FAD binding. This study provides information on the active site of DASPO that will considerably enhance our understanding of the biological significance of this enzyme.

Ubiquitin-conjugating enzyme Cdc34 mediates cadmium resistance in budding yeast through ubiquitination of the transcription factor Met4

Overexpression of the ubiquitin-conjugating enzyme Cdc34 conferred strong cadmium resistance on budding yeast. Proteasome activity, which is involved in the degradation of ubiquitinated proteins, was not essential for the acquisition of resistance to cadmium. The overexpression of Cdc34 accelerated the ubiquitination of the transcription factor Met4 and reduced expression of MET25 gene, which is a target of Met4. A MET25-disrupted strain of yeast was more resistant to cadmium than was the wild-type strain, but overexpression of Cdc34 in the MET25-disrupted cells did not affect sensitivity to cadmium. Met25 is an enzyme that catalyzes the synthesis of homocysteine from sulfide (S(2-)) and O-acetylhomocysteine and we detected the increased production of S(2-) upon overexpression of Cdc34. Our results suggest that overexpression of Cdc34 inactivates Met4 and interferes with expression of the MET25, with subsequent production of CdS, which has low toxicity, and, thus, a decrease in the cadmium toxicity.

Endocytic Ark/Prk kinases play a critical role in adriamycin resistance in both yeast and mammalian cells

To elucidate the mechanism of acquired resistance to Adriamycin, we searched for genes that, when overexpressed, render Saccharomyces cerevisiae resistant to Adriamycin. We identified AKL1, a gene of which the function is unknown but is considered, nonetheless, to be a member of the Ark/Prk kinase family, which is involved in the regulation of endocytosis, on the basis of its deduced amino acid sequence. Among tested members of the Ark/Prk kinase family (Ark1, Prk1, and Akl1), overexpressed Prk1 also conferred Adriamycin resistance on yeast cells. Prk1 is known to dissociate the Sla1/Pan1/End3 complex, which is involved in endocytosis, by phosphorylating Sla1 and Pan1 in the complex. We showed that Akl1 promotes phosphorylation of Pan1 in this complex and reduces the endocytic ability of the cell, as does Prk1. Sla1- and End3-defective yeast cells were also resistant to Adriamycin and overexpression of Akl1 in these defective cells did not increase the degree of Adriamycin resistance, suggesting that Akl1 might reduce Adriamycin toxicity by reducing the endocytic ability of cells via a mechanism that involves the Sla1/Pan1/End3 complex and the phosphorylation of Pan1. We also found that HEK293 cells that overexpressed AAK1, a member of the human Ark/Prk family, were Adriamycin resistant. Our findings suggest that endocytosis might be involved in the mechanism of Adriamycin toxicity in yeast and human cells.

Caenorhabditis eleganshas two genes encoding functionald-aspartate oxidases

Four cDNA clones that were annotated in the database as encoding d-amino acid oxidase (DAAO) or d-aspartate oxidase (DASPO) were isolated by RT-PCR from Caenorhabditis elegans RNA. The proteins (Y69Ap, C47Ap, F18Ep, and F20Hp) encoded by the cloned cDNAs were expressed in Escherichia coli as recombinant proteins with an N-terminal His-tag. All proteins except F20Hp were recovered in the soluble fractions. The recombinant Y69Ap has functional DAAO activity, as it can deaminate neutral and basic d-amino acids, whereas the recombinants C47Ap and F18Ep have functional DASPO activities, as they can deaminate acidic d-amino acids. Additional experiments using purified recombinant proteins revealed that Y69Ap deaminates d-Arg more efficiently than d-Ala and d-Met, and that C47Ap and F18Ep show distinct kinetic properties against d-Asp, d-Glu, and N-methyl-d-Asp. This is the first time that cDNA cloning of invertebrate DAAO and DASPO genes has been reported. In addition, our study reveals for the first time that C. elegans has at least two genes encoding functional DASPOs and one gene encoding DAAO, although it had previously been thought that organisms only bear one copy each of these genes. The two C. elegans DASPOs differ in their substrate specificities and possibly also in their subcellular localization.

Molecular cloning of a cDNA encoding mouse D-aspartate oxidase and functional characterization of its recombinant proteins by site-directed mutagenesis

The cDNA encoding D-aspartate oxidase (DASPO) was cloned from mouse kidney RNA by RT-PCR. Sequence analysis showed that it contained a 1023-bp open reading frame encoding a protein of 341 amino acid residues. The protein was expressed in Escherichia coli with or without an N-terminal His-tag and had functional DASPO activity that was highly specific for D-aspartate and N-methyl-D-aspartate. To investigate the roles of the Arg-216 and Arg-237 residues of the mouse DASPO (mDASPO), we generated clones with several single amino acid substitutions of these residues in an N-terminally His-tagged mDASPO. These substitutions significantly reduced the activity of the recombinant enzyme against acidic D-amino acids and did not confer any additional specificity to other amino acids. These results suggest that the Arg-216 and Arg-237 residues of mDASPO are catalytically important for full enzyme activity.

Cytoplasmic localization and efflux of endogenous d-aspartate in pheochromocytoma 12 cells

In our previous reports [Z. Long, H. Homma, J.-A. Lee, T. Fukushima, T. Santa, T. Iwatsubo, R. Yamada, K. Imai, FEBS Lett. 434 (1998) 231-235; Z. Long, M. Sekine, M. Adachi, T. Furuchi, K. Imai, N. Nimura, H. Homma, Arch. Biochem. Biophys. 404 (2002) 92-97], we demonstrated for the first time that D-aspartate (D-Asp) is actually synthesized in cultured mammalian cells such as PC12, MPT1, and GH3 cells. After its synthesis, this unique amino acid is spontaneously and continuously released into the extracellular space during cell culture. In the current study, we characterized two different types of D-Asp efflux in PC12 cells. One is a spontaneous and continuous form of release of cytoplasmic origin that does not involve exocytotic efflux of vesicular origin. Endogenous D-Asp is predominantly localized to the cytoplasm of cells, and this form of D-Asp release presents a striking contrast to exocytotic, quantal discharge of vesicular dopamine. The other form of efflux is also of cytoplasmic origin and occurs through volume-sensitive organic anion channels that are opened upon hyposmotic stimuli. Interestingly, this latter form of efflux is potentiated by acetylcholine stimulation.

Overexpression of Bop3 confers resistance to methylmercury in Saccharomyces cerevisiae through interaction with other proteins such as Fkh1, Rts1, and Msn2

We found that overexpression of Bop3, a protein of unknown function, confers resistance to methylmercury in Saccharomyces cerevisiae. Bmh2, Fkh1, and Rts1 are proteins that have been previously shown to bind Bop3 by the two-hybrid method. Overexpression of Bmh2 and the homologous protein Bmh1 confers resistance to methylmercury in yeast, but overexpression of either Fkh1 or Rts1 has a minimal effect. However, the increased level of resistance to methylmercury produced by overexpression of Bop3 was smaller in Fhk1-deleted yeast as compared with that of the wild-type strain. In contrast, the degree of resistance was significantly elevated in Rts1-deleted yeast. Msn2 and Msn4 were previously reported as proteins that bind to Bmh1 and Bmh2. Overexpression of Msn2 conferred a much greater sensitivity to methylmercury in yeast, while deletion of the corresponding gene lowered the degree of resistance to methylmercury induced by overexpression of Bop3. These results suggest that multiple proteins are involved in minimizing the toxicity of methylmercury induced by overexpression of Bop3.

A novel L-glutamate transporter inhibitor reveals endogenous D-aspartate homeostasis in rat pheochromocytoma MPT1 cells

We previously reported for the first time that D-aspartate (D-Asp) is biosynthesized by cultured mammalian cells such as pheochromocytoma (PC)12 cells and its subclone MPT1 (FEBS Lett. 434 (1998) 231, Arch. Biochem. Biophys. 404 (2002) 92). We speculated that D-Asp levels in the intra- and extracellular spaces of the cultured cells are maintained in a dynamic state of homeostasis. To test this here, we utilized a novel and potent L-Glu transporter inhibitor, TFB-TBOA. This inhibitor proved to be a genuine nontransportable blocker of the transporter even during long periods of culture. Use of this inhibitor with MPT1 cells confirmed that D-Asp levels are in a dynamic steady state where it is constantly released into the extracellular space by a yet undefined mechanism as well as being constantly and intensively taken up by the cells via the L-Glu transporter. We estimated the rate with which D-Asp is constitutively released from MPT1 cells is approx. 3.8 pmol/h/1x10(5) cells.

d-Glutamic Acid-Induced Muscle Contraction in the Silkworm, Bombyx mori

Agonists for muscle contraction in silkworms were screened by injecting test solutions into the hemolymph of decapitated silkworm larvae. Kainic acid, a glutamate receptor agonist, and D-glutamic acid induced muscle contractions, and D-aspartic acid was partially effective, whereas NMDA and AMPA, representative mammalian glutamate receptor agonists, did not induce contraction. L-Glutamic acid inhibited the kainic acid or D-glutamic acid-induced contraction. Amino acid analysis revealed that 3% of the total glutamic acid in the silkworm hemolymph is D-glutamic acid. These results suggest that d-glutamic acid acts physiologically as an agonist for muscle contraction in silkworms, and that L-glutamic acid functions as an inhibitor.

Free D-Aspartate in Mammals

D-Aspartate (D-Asp) is an endogenous amino acid present in nervous and endocrine tissues in mammals. A high concentration of D-Asp is observed in embryos, which disappears in nervous tissues after delivery, but increases temporarily in endocrine glands, particularly in the pituitary, pineal and adrenal glands at the specific stages. In the pineal gland, D-Asp that is apparently derived from other tissues suppresses melatonin secretion from parenchymal cells. Additionally, D-Asp levels increase in the testis just before birth and during maturation. The amino acid is presumed to be synthesized by the pituitary gland and testis. In the testis, D-Asp produced inside the seminiferous tubules acts on Leydig cells following release to enhance testosterone synthesis by activating the expression of Steroidogenic Acute Regulatory protein. Mammalian cells appear to contain all the molecular components required to regulate D-Asp homeostasis, as they can synthesize, release, take up, and degrade the amino acid. These findings collectively indicate that D-Asp is a novel type of messenger in the mammalian body.

Functions of yeast helicase Ssl2p that are essential for viability are also involved in protection from the toxicity of adriamycin

We have found that, in the yeast Saccharomyces cerevisiae, overexpression of the DNA helicase Ssl2p confers resistance to adriamycin. Ssl2p is involved, as a subunit of the basic transcription factor TFIIH, in the initiation of transcription and in nucleotide-excision repair (NER), and this helicase is essential for the survival of yeast cells. An examination of the relationship between the known functions of Ssl2p and adriamycin resistance indicated that overexpression of Ssl2p caused little or no increase in the rate of RNA synthesis and in NER. The absence of any involvement of NER in adriamycin resistance was supported by the finding that yeast cells that overexpressed the mutant form of Ssl2p that lacked the carboxy-terminal region, which is necessary for NER, remained resistant to adriamycin. When we examined the effects of overexpression in yeast of other mutant forms of Ssl2p with various deletions, we found that, of the 843 amino acids of Ssl2p, the entire amino acid sequence from position 81 to position 750 was necessary for adriamycin resistance. This region is identical to the region of Ssl2p that is necessary for the survival of yeast cells. Although this region contains helicase motifs, the overexpression of other yeast helicases, such as Rad3 and Sgs1, had little or no effect on adriamycin resistance, indicating that a mere increase in the intracellular level of helicases does not result in adriamycin resistance. Our results suggest that the functions of Ssl2p that are essential for yeast survival are also required for protection against adriamycin toxicity.

l-Glutamate in the extracellular space regulates endogenous d-aspartate homeostasis in rat pheochromocytoma MPT1 cells

In previous studies [FEBS Lett. 434 (1998) 231, Arch. Biochem. Biophys. 404 (2002) 92], we demonstrated for the first time that D-aspartate (D-Asp) is synthesized in cultured mammalian cell lines, such as pheochromocytoma 12 (PC12) and its subclone, MPT1. Our current focus is analysis of the dynamics of D-Asp homeostasis in these cells. In this communication, we show that L-glutamate (Glu) and L-Glu transporter substrates in the extracellular space regulate the homeostasis of endogenous D-Asp in MPT1 cells. D-Asp is apparently in dynamic homeostasis, whereby endogenous D-Asp is constantly released into the extracellular space by an undefined mechanism, and continuously and intensively taken up into cells by an L-Glu transporter. Under these conditions, L-Glu and its transporter substrates in the medium may competitively inhibit the uptake of D-Asp via the transporter, resulting in accumulation of the amino acid in the extracellular space. We additionally demonstrate that DL-TBOA, a well-established L-Glu transporter inhibitor, is taken up by the transporter during long time intervals, but not on a short time-scale.

Overexpression of Ssl2p confers resistance to adriamycin and actinomycin D in Saccharomyces cerevisiae

Adriamycin is one of the most active anticancer drugs but the development of resistance to this drug hampers its efficacy. In an effort to identify novel genes that confer resistance to adriamycin, we introduced a yeast genomic library into Saccharomyces cerevisiae and selected transformants that grew in the presence of a normally toxic concentration of adriamycin. Detailed examination of a plasmid recovered from these transformants revealed that overexpression of the gene for Ssl2p rendered yeast cells resistant to adriamycin. Ssl2p is a protein that is involved in the initiation of transcription and in DNA repair. Overexpression of Ssl2p did not confer resistance to aclarubicin, an anthracycline anticancer drug, which, like adriamycin, is intercalated into DNA. Both adriamycin and aclarubicin inhibit topoisomerase II and, thus, topoisomerase II might not be a major factor in the acquired resistance to adriamycin that results from overexpression of Ssl2p. We tested several other compounds but the only one to which Ssl2p-overexpressing cells were cross-resistant was actinomycin D. Mammalian cells that overexpress P-glycoprotein, which is a transmembrane protein that is involved in the efflux of certain drugs, are resistant to both adriamycin and actinomycin D but not to aclarubicin. However, overexpression of Ssl2p had little or no effect on the intracellular accumulation of adriamycin. Our results suggest that a novel mechanism might be involved in the sensitivity of yeast to both adriamycin and actinomycin D.

A novel role for Bsd2 in the resistance of yeast to adriamycin

In a search for undiscovered mechanisms of resistance to adriamycin, we screened a genomic library derived from Saccharomyces cerevisiae for genes related to adriamycin resistance. To our surprise, we found that overexpression of BSD2 rendered yeast cells resistant to adriamycin. Downregulation of the metal transporters Smf1 and Smf2 is the only activity of Bsd2 reported to date, and Bsd2 deficiency increases intracellular levels of Smf1 and Smf2. SMF2-disrupted cells exhibited significantly greater resistance to adriamycin, whereas the resistance of SMF1-disrupted cells was only slightly improved. The sensitivity of the SMF1- and SMF2-disrupted yeast cell line overexpressing BSD2 was almost the same as that of the BSD2-overexpressing parental yeast cell. Thus the overexpression of BSD2 and the disruption of SMF1 and SMF2 might be involved in the same mechanism that confers resistance to adriamycin. Although both SMF1- and SMF2-disrupted cells were very sensitive to EGTA, overexpression of BSD2 had little or no effect on sensitivity to EGTA. However, a partial decrease in the intracellular level of FLAG-Smf2 was observed by overexpression of BSD2. Thus, the resistance to adriamycin acquired by overexpression of BSD2 might be partially explained by down-regulation of Smf2, but in addition to Smf2, other as of yet unidentified targets of Bsd2 must also be responsible for the resistance.

Copper(II) protects yeast against the toxicity of cisplatin independently of the induction of metallothionein and the inhibition of platinum uptake

We have made the unexpected discovery that copper sulfate protects Saccharomyces cerevisiae from the toxic effects of cisplatin. Addition of copper to the culture medium of yeast cells at concentrations above 0.1 microM significantly reduced the toxicity of cisplatin. Since a high-affinity copper transporter, Ctr1, has been reported to play a major role in the uptake of cisplatin, we examined the effects of copper on the cellular uptake of cisplatin. We found that the cellular concentration of platinum was not significantly affected by treatment of cells with 1 microM copper. It is known that mammalian metallothionein is induced by copper and is involved in acquired resistance to cisplatin. Copper significantly increased the level of mRNA for yeast metallothionein at a concentration that has effectively reduced the toxicity of cisplatin. However, the toxicity of cisplatin in cells with a disrupted gene for ACE1, a factor that regulates transcription of the yeast gene for metallothionein, was also significantly reduced by treatment with copper. These results suggest that copper protects yeast cells from cisplatin toxicity independently of induction of the synthesis of metallothionein and of the inhibition of platinum uptake. Since copper is one of the trace elements that are essential for cell function and since a relatively low concentration of copper (0.1 microM) significantly reduced cisplatin toxicity, it is possible that copper might play an important role in the expression of cisplatin toxicity.

A novel chiral thiol reagent for automated precolumn derivatization and high-performance liquid chromatographic enantioseparation of amino acids and its application to the aspartate racemase assay

A novel optically active thiol compound, N-(tert-butylthiocarbamoyl)-L-cysteine ethyl ester (BTCC), is synthesized as a chiral derivatization reagent. This compound and o-phthalaldehyde react with amino acid enantiomers to produce fluorescent diastereomers that are readily separable on a reverse-phase column by HPLC. Enantioseparation of acidic amino acids in particular is markedly improved using BTCC. In this study, the HPLC method for enantioseparation with the novel compound is applied to the aspartate (Asp) racemase assay. Derivatized D-Asp is eluted before the L-Asp derivative. Consequently, a small amount of D-Asp produced by the activity of racemase on a large quantity of L-Asp substrate may be quantified accurately, even at very low activity. Since the derivatization reaction proceeds rapidly at room temperature, a fully automated system is established for derivatization and sample injection. The automated method is practical and successfully applied to the archaeal Asp racemase assay. We presume that the procedure is additionally applicable to the enantioseparation of other amino acids, amino alcohols, and catecholamines.

Automated column-switching high-performance liquid chromatography system for quantifying N-methyl-D- and -L-aspartate

The occurrence and biological significance of the D-amino acids, N-methyl-D-aspartate (NMDA) and N-methyl-L-aspartate (NMLA), have been recently studied in a variety of living organisms. In this study, we established a highly sensitive and reliable fluorometric HPLC system for determining levels of N-methyl-aspartate (NMA). The system comprises fluorescent derivatization of NMA with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F) and two chromatographic steps: one that separates NMA from other primary amino acids in reverse-phase mode and another that enantioseparates NMDA and NMLA in a normal-phase mode. These two steps are linked by an automated column-switching system. A simple pretreatment step with o-phthalaldehyde to remove primary amino acids that can interfere with sensitivity is also described. The detection limit for NMDA is as low as 5fmol and the correlation between peak heights and concentrations between 5fmol and 1pmol is satisfactory (r=0.999). Following sample preparation and separation using the column-switching HPLC system, more than 80% of NMDA was recovered from rat liver homogenates spiked with NMDA. This method was employed to determine the levels of NMDA in tissues from bivalves and the results obtained were consistent with the values reported previously.

Cell density inversely regulates D- and L-aspartate levels in rat pheochromocytoma MPT1 cells

In a previous report (FEBS Lett. 434 (1998) 231), we demonstrated for the first time that D-aspartate (D-Asp) is synthesized in rat pheochromocytoma 12 (PC12) cells. This unique amino acid is believed to act as a novel messenger in mammalian cell regulation. However, the dynamics of D-Asp homeostasis in mammalian cells is yet to be elucidated. In this communication, we demonstrate that D-Asp is also synthesized in MPT1 cells (a subclone of PC12 cells) and that the D- and L-Asp levels in cells are regulated by cell density of the culture. Our data show that D-Asp levels increase, while in contrast, L-Asp levels decrease as a function of increased cell density. Conversely, in PC12 cells, which do not express the glutamate transporter involved in the incorporation of D- and L-Asp into cells, L-Asp levels decrease upon cell density increase while D-Asp concentrations remain almost unchanged. The results indicate that the biochemical behaviors of D- and L-Asp in mammalian cells are distinct and that the cellular levels of these stereoisomers appear to be under different control mechanisms.

A ubiquitin-proteasome system is responsible for the protection of yeast and human cells against methylmercury

The mechanism responsible for the toxic effects of methylmercury (MeHg), an important environmental pollutant, is poorly understood. We have identified a gene, CDC34, that confers resistance to MeHg in Saccharomyces cerevisiae by screening a yeast genomic DNA library. CDC34 encodes a ubiquitin-conjugating enzyme, Cdc34, which is involved in ubiquitin-dependent proteolysis. Overexpression of Cdc34 results in significant resistance to MeHg both in yeast and human cells, and it increases the cellular level of ubiquitinated proteins. The ubiquitin-conjugating activity of Cdc34 is essential for the Cdc34-mediated resistance to MeHg, and the protective effect of the overexpression of Cdc34 is depressed by inhibition of proteasome activity. Our results support the hypothesis that MeHg induces the cellular accumulation of a certain protein(s) that causes cell damage and that this protein(s) is degraded after its ubiquitination in proteasomes.

Overexpression of the ubiquitin-conjugating enzyme Cdc34 confers resistance to methylmercury in Saccharomyces cerevisiae

A search was made for genes that confer resistance to methylmercury in yeast using a genomic DNA library derived from Saccharomyces cerevisiae. The genomic library was introduced into yeast and transformants that grew in the presence of a normally toxic concentration of methylmercury were selected. We sequenced the genomic DNA fragment in the plasmid from the clone with the highest resistance to methylmercury and analyzed the sequence for presence of an open reading frame that might confer resistance to methylmercury. We identified a gene, CDC34 (also known as UBC3), that increased resistance to methylmercury when overexpressed in yeast. CDC34 encodes a ubiquitin-conjugating enzyme; such proteins play important roles in the selective targeting of proteins for degradation. Overexpression of UBC4 and of UBC7, two other genes for ubiquitin-conjugating enzymes, also conferred resistance to methylmercury. Yeast strains transformed with the CDC34 gene were resistant not only to methylmercury but also to mercuric chloride and p-chloromercuribenzoate. To our knowledge, this is the first demonstration that overexpression of genes for ubiquitin-conjugating enzymes confers resistance to xenobiotics. Our results suggest that ubiquitination system might be involved in protection against the toxicity of mercury compounds, such as methylmercury, in eukaryotic cells.

Investigation of intracellular factors involved in methylmercury toxicity

Methylmercury is a known pollutant that causes severe central nervous system disorders. It is capable of passing through the blood-brain barrier and accumulates in cerebral cells. However, little is known regarding the mechanism of its toxicity at the molecular level. Using yeast cells, we searched for the genes involved in the expression of methylmercury toxicity, and found that genes encoding L-glutamine.D-fructose-6-phosphate amidotransferase (GFAT) and ubiquitin transferase (Ubc3) confer methylmercury resistance on the cells. It has also been shown that GFAT is the target molecule of methylmercury in yeast cells. These findings provide important clues about the mechanism underlying methylmercury toxicity in mammals.

Two nuclear proteins, Cin5 and Ydr259c, confer resistance to cisplatin in Saccharomyces cerevisiae

In an attempt to identify genes that can confer resistance to cisplatin, we introduced a yeast genomic library into Saccharomyces cerevisiae and selected for transformants that grew in the presence of a normally toxic concentration of cisplatin. Plasmids were rescued from the transformants and were analyzed for the presence of individual open reading frames that conferred resistance to cisplatin. We isolated two genes, CIN5 and YDR259c, that increased resistance to cisplatin when overexpressed in Saccharomyces cerevisiae. These genes encoded two proteins, Cin5 and Ydr259c, that were homologous to yAP-1, a basic leucine zipper transcriptional factor that is known to mediate cellular resistance to various toxic agents. The two proteins exhibited stronger homology to each other (33.2% identity, 49.2% similarity) than to all other gene products in S. cerevisiae. Overexpression of each of these proteins also conferred resistance to two DNA-alkylating agents, methylmethanesulfonate and mitomycin C. An experiment with fusion proteins with green fluorescent protein revealed that Cin5 and Ydr259c were localized constitutively in the nuclei of yeast cells. Our results suggest that Cin5 and Ydr259c might be involved in pleiotropic drug-resistance and might protect yeast against the toxicity of cisplatin and other alkylating agents via a single mechanism. These two nuclear proteins might act as transcriptional factors, regulating the expression of certain genes that confer resistance to DNA-alkylating agents.

Dual pathways for the secretion of lysosomal cholesterol into a medium from cultured macrophages

The removal of cholesterol from macrophages is important for reversing foam cell formation. In a previous study, we demonstrated that mouse peritoneal macrophages in culture secrete significant amounts of unesterified cholesterol from the lysosomes into the medium during endocytosis and subsequent metabolism of cholesterol-containing liposomes [Furuchi, T., Aikawa, K., Arai, H., and Inoue, K. (1993) J. Biol. Chem. 268, 27345-27348]. In this study, we found that at least two distinct mechanisms are involved in this process. The efflux of unesterified cholesterol into the medium was greatly suppressed by pregnenolone, an inhibitor of lysosomal cholesterol transport, but an appreciable proportion of the unesterified cholesterol was still released into the medium. Analysis of the medium containing the secreted cholesterol by NaBr density gradient ultracentrifugation revealed that the unesterified cholesterol was distributed in two different density peaks (bottom and d =/ approximately 1.1). The d =/ approximately 1.1 peak material formed high-density lipoprotein (HDL)-like particles that were produced and secreted by the macrophages. The lipid components of these particles were phosphatidylcholine and sphingomyelin, while the sole protein component was apolipoprotein E (apo E). Treatment with pregnenolone completely abolished the production of these HDL-like particles but had little effect on the bottom fractions. These data indicate that macrophages release lysosomal cholesterol via both pregnenolone-sensitive and -insensitive pathways, and that only the cholesterol secreted through the pregnenolone-sensitive pathway is associated with endogenously synthesized apo E-containing HDL-like particles. Moreover, we found that the pregnenolone-sensitive pathway operated independently of the presence or absence of exogenous HDL, whereas secretion via the pregnenolone-insensitive pathway was greatly stimulated by exogenously added HDL.

GFAT as a target molecule of methylmercury toxicity in Saccharomyces cerevisiae

Using a genomic library constructed from Saccharomyces cerevisiae, we have identified a gene GFA1 that confers resistance to methylmercury toxicity. GFA1 encodes L-glutamine:D-fructose-6-phosphate amidotransferase (GFAT) and catalyzes synthesis of glucosamine-6-phosphate. Transformed yeast cells expressing GFA1 demonstrated resistance to methylmercury but no resistance to p-chloromercuribenzoate, a GFAT inhibitor. The cytotoxicity of methylmercury was inhibited by loading excess glucosamine 6-phosphate into yeast. Considering that GFAT is an essential cellular enzyme, our findings suggest that GFAT is the major target molecule of methylmercury in yeasts. This report is the first to identify the target molecule of methylmercury toxicity in eukaryotic cells.

Overexpression of L-glutamine:D-fructose-6-phosphate amidotransferase provides resistance to methylmercury in Saccharomyces cerevisiae

To identify novel genes that confer resistance to methylmercury (MeHg), a yeast genomic DNA library was transfected into Saccharomyces cerevisiae. Two functional plasmids were isolated from transfected yeast clones D1 and H5 that exhibited resistance to MeHg. The yeast transfected with plasmid isolated from clone H5 was several-fold more resistant than yeast transfected with plasmid from clone D1. Functional characterization of the genomic DNA fragment obtained from clone H5 determined that the GFA1 gene conferred resistance to MeHg. GFA1 was reported to encode L-glutamine:D-fructose-6-phosphate amidotransferase (GFAT) which catalyzes the synthesis of glucosamine-6-phosphate from glutamine and fructose-6-phosphate. Accumulation of mercury in yeast clone W303B/pGFA1, which contains the transfected GFA1 gene, did not differ from that in control yeast clone W303B/pYES2. The W303B/pGFA1 strain did not show resistance to mercuric chloride, zinc chloride, cadmium chloride or copper chloride, suggesting that the resistance acquired by GFA1 gene transfection might be specific to MeHg. This is the first report of a gene involved in MeHg resistance in eukaryotic cells identified by screening a DNA library.

Inhibition of cholesteryl ester formation in macrophages by azole antimycotics

Cultured macrophages take up and metabolize cholesterol-containing liposomes, resulting in massive accumulation of cholesteryl esters in the cells. Using this system, the effects of azole antimycotics on cholesteryl ester formation were studied. Incubation of mouse peritoneal macrophages with ketoconazole, miconazole, or econazole (0.1-10 microM) resulted in concentration-dependent inhibition of cholesteryl ester synthesis from endocytosed cholesterol. IC50 values (concentration resulting in 50% inhibition) were 1.4 +/- 0.1 microM, 4.1 +/- 0.2 microM, and 3.6 +/- 0.2 microM for ketoconazole, miconazole, and econazole, respectively. Complete inhibition was observed with 10 microM ketoconazole, and miconazole and econazole, each at 10 microM, caused 70 and 75% inhibition, respectively, of cholesteryl ester synthesis. The mechanism underlying the inhibition by ketoconazole was further studied. Ketoconazole did not appreciably block the uptake of liposomes or formation of triacylglycerol up to 10 microM. Interestingly, ketoconazole suppressed only 30% of 25-hydroxycholesterol-induced endogenous cholesterol esterification under conditions where esterification of endocytosed cholesterol was completely inhibited. Cytochemical studies with filipin-cholesterol staining revealed that ketoconazole induced massive accumulation of endocytosed cholesterol in macrophage phagolysosomes. These results indicate that ketoconazole inhibits cholesteryl ester formation in macrophages by blocking the intracellular transport of endocytosed cholesterol from lysosomes to the endoplasmic reticulum.

Cholesterol depletion of caveolae causes hyperactivation of extracellular signal-related kinase (ERK)

Previously we showed that activation of Erk in quiescent cells occurs in the caveolae fraction isolated from fibroblasts. Since the structure and function of caveolae is sensitive to the amount of cholesterol in the membrane, it might be that a direct link exists between the concentration of membrane cholesterol and mitogen-activated protein (MAP) kinase activation. We acutely lowered the cholesterol level of the caveolae fraction by incubating Rat-1 cells in the presence of either cyclodextrin or progesterone. Cholesterol-depleted caveolae had a reduced amount of several key protein components of the MAP kinase complex, including Ras, Grb2, Erk2, and Src. Incubation of these cells in the presence of epidermal growth factor (EGF) caused a rapid loss of EGF receptor from the caveolae fraction, but the usual recruitment of c-Raf was markedly inhibited. Despite the reduced amount of c-Raf and Erk2 in the cholesterol-depleted caveolae fraction, EGF caused a hyperactivation of the remaining caveolae Erk isoenzymes. This was followed by an increase in the amount of active Erk in the cytoplasm. The increased amount of activated Erk produced under these conditions was linked to a 2-fold higher level of EGF-stimulated DNA synthesis. Even cholesterol depletion by itself stimulated Erk activation and DNA synthesis. These results suggest that the MAP kinase pathway can connect the cholesterol level of caveolae membrane to the control of cell division.

Inhibition of testicular germ cell apoptosis and differentiation in mice misexpressing Bcl-2 in spermatogonia

During normal spermatogenesis, more than half of the germ cells undergo apoptosis, but the physiological significance and molecular mechanisms of this programmed cell death are largely unknown. Because Bcl-2 functions as a death repressor, we have investigated the effect of misexpressing Bcl-2 in spermatogonia in transgenic mice using the human bcl-2 cDNA under the control of the human polypeptide chain elongation factor 1alpha (EF-1alpha) promoter. In the 2-week-old transgenic testes, exogenous Bcl-2 was expressed in spermatogonia and massive accumulation of spermatogonia was observed in seminiferous tubules by 4 weeks. At this time, only a few spermatocytes were apparent, and the accumulated cells degenerated, leading to vacuolization in some seminiferous tubules by 7 weeks. In older transgenic mice, abnormal accumulation of spermatogonia and degeneration of these germ cells was still observed, but some seminiferous tubules in which the level of Bcl-2 expression was reduced recovered normal spermatogenesis. These observations indicate that spermatogonial apoptosis is part of the normal program of mammalian spermatogenesis and is regulated by a pathway affected by Bcl-2.

Structure-specific inhibition of lysosomal cholesterol transport in macrophages by various steroids

Cultured mouse peritoneal macrophages effectively take up and metabolize liposomes containing phosphatidylserine and cholesterol, resulting in massive accumulation of cholesteryl esters and triacylglycerols in their cytoplasm (Nishikawa, K., Arai, H. and Inoue, K. (1990) J. Biol. Chem. 265, 5226-5231). With this system, various steroid derivatives were assessed as to their ability to inhibit the cholesteryl ester formation from endocytosed cholesterol in macrophages. Among the steroids tested, one group of steroids having an oxo group at the C17 or C20 position, such as androstenedione, dehydroisoandrosterone, progesterone and pregnenolone, completely inhibited cholesteryl ester formation at 10 microM. Another group of steroids having a hydroxy group at the C17 position, such as testosterone and androstenediol, had a lesser effect; complete inhibition of cholesteryl ester formation was achieved with 100 microM or more. The mechanism underlying the inhibition by the former class of steroids was further studied. These steroids did not block the uptake or lysosomal hydrolysis of liposomes, nor esterification of fatty acyl chains into triacylglycerols. Moreover, dehydroisoandrosterone and pregnenolone, both of which possess a hydroxy group at the C3 position, had essentially no effect on 25-hydroxycholesterol-stimulated esterification of endogenous cellular cholesterol. On the other hand, androstenedione and progesterone, which possess an oxo group at the C3 position, had a mild inhibitory effect on the esterification of endogenous cholesterol. Upon incubation with a steroid having an oxo group at the C17 or C20 position, free cholesterol taken up by macrophages was accumulated in phagolysosomes, as judged from cytochemical study with filipin-cholesterol staining. These results indicate that a series of structurally-related steroids characterized by the presence of an oxo group at the C17 or C20 position inhibit cholesteryl ester formation in macrophages through blocking the intracellular transport of endocytosed cholesterol from lysosomes to endoplasmic reticulum.

Bafilomycin A1, a specific inhibitor of vacuolar-type H(+)-ATPase, blocks lysosomal cholesterol trafficking in macrophages

Certain steroids having an oxo group at the C-17 or C-20 position such as pregnenolone and dehydroisoandrosterone inhibit the cholesterol transport from lysosomes to other cellular sites. Taking advantage of the fact that the inhibition is reversed upon removal of the steroids, we studied the factors that control the cholesterol transport from lysosomes to other cellular sites in macrophages. Macrophages that accumulated unesterified cholesterol in their lysosomes were prepared by incubating cells with liposomes containing cholesterol and phosphatidylserine in the presence of a steroid inhibitor. These cells were chased by means of steroid washout, and then the effects of various pharmacological agents on the subsequent metabolism of cholesterol were examined. When the cells were chased in the absence of the agents, some of the cholesterol was converted to cholesteryl esters in the cells, and others were desorbed into the medium as unesterified forms, suggesting recovery of lysosomal cholesterol trafficking. Among the agents tested, bafilomycin A1, a specific inhibitor of vacuolar-type H(+)-ATPase, completely blocked both cholesterol esterification and cholesterol desorption at 10 nM. Moreover, agents that neutralize the lysosomal proton gradient, such as ammonium chloride and chloroquine, also reduced both of the processes. Fluorescent microscopic examination of bafilomycin A1-treated cells revealed extensive filipin-cholesterol staining of perinuclear lysosomes. From these data, we conclude that acidic pH is required for the efflux of cholesterol from lysosomes to other cellular sites.