Nucleotide sequence and glucocorticoid regulation of the mRNAs for the isoenzymes of rat aspartate aminotransferase

Effects of Bardoxolone Methyl on Hepatic Enzymes in Patients with Type 2 Diabetes Mellitus and Stage 4 CKD

In a multinational placebo‐controlled phase III clinical trial in 2,185 patients with type 2 diabetes mellitus and stage 4 chronic kidney disease, treatment with the Nrf2 activator bardoxolone methyl increased estimated glomerular filtration rate, a measure of kidney function, but also resulted in increases in serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), and gamma glutamyl transferase. These increases in liver enzyme level(s) were maximal after 4 weeks of treatment and reversible, trending back toward baseline through week 48. Total bilirubin concentrations did not increase, and no cases met Hy’s Law criteria, although two subjects had ALT concentrations that exceeded 10 × the upper limit of the population reference range leading to discontinuation of treatment. Animal and cell culture experiments suggested that the increases in ALT and AST induced by bardoxolone methyl may be related to its pharmacological activity. Bardoxolone methyl significantly induced the mRNA expression of ALT and AST isoforms in cultured cells. Expression of ALT and AST isoforms in liver and kidney also positively correlated with Nrf2 status in mice. Overall, these data suggest that the increases in ALT and AST observed clinically were, at least in part, related to the pharmacological induction of aminotransferases via Nrf2 activation, rather than to any intrinsic form of hepatotoxicity.

Cysteine Aminotransferase (CAT): A Pivotal Sponsor in Metabolic Remodeling and an Ally of 3-Mercaptopyruvate Sulfurtransferase (MST) in Cancer

Metabolic remodeling is a critical skill of malignant cells, allowing their survival and spread. The metabolic dynamics and adaptation capacity of cancer cells allow them to escape from damaging stimuli, including breakage or cross-links in DNA strands and increased reactive oxygen species (ROS) levels, promoting resistance to currently available therapies, such as alkylating or oxidative agents. Therefore, it is essential to understand how metabolic pathways and the corresponding enzymatic systems can impact on tumor behavior. Cysteine aminotransferase (CAT) per se, as well as a component of the CAT: 3-mercaptopyruvate sulfurtransferase (MST) axis, is pivotal for this metabolic rewiring, constituting a central mechanism in amino acid metabolism and fulfilling the metabolic needs of cancer cells, thereby supplying other different pathways. In this review, we explore the current state-of-art on CAT function and its role on cancer cell metabolic rewiring as MST partner, and its relevance in cancer cells' fitness.

Highly multiplexed and reproducible ion-current-based strategy for large-scale quantitative proteomics and the application to protein expression dynamics induced by methylprednisolone in 60 rats

A proteome-level time-series study of drug effects (i.e., pharmacodynamics) is critical for understanding mechanisms of action and systems pharmacology, but is challenging, because of the requirement of a proteomics method for reliable quantification of many biological samples. Here, we describe a highly reproducible strategy, enabling a global, large-scale investigation of the expression dynamics of corticosteroid-regulated proteins in livers from adrenalectomized rats over 11 time points after drug dosing (0.5–66 h, N = 5/point). The analytical advances include (i) exhaustive tissue extraction with a Polytron/sonication procedure in a detergent cocktail buffer, and a cleanup/digestion procedure providing very consistent protein yields (relative standard deviation (RSD%) of 2.7%–6.4%) and peptide recoveries (4.1–9.0%) across the 60 animals; (ii) an ultrahigh-pressure nano-LC setup with substantially improved temperature stabilization, pump-noise suppression, and programmed interface cleaning, enabling excellent reproducibility for continuous analyses of numerous samples; (iii) separation on a 100-cm-long column (2-μm particles) with high reproducibility for days to enable both in-depth profiling and accurate peptide ion-current match; and (iv) well-controlled ion-current-based quantification. To obtain high-quality quantitative data necessary to describe the 11 time-points protein expression temporal profiles, strict criteria were used to define “quantifiable proteins”. A total of 323 drug-responsive proteins were revealed with confidence, and the time profiles of these proteins provided new insights into the diverse temporal changes of biological cascades associated with hepatic metabolism, response to hormone stimuli, gluconeogenesis, inflammatory responses, and protein translation processes. Most profile changes persisted well after the drug was eliminated. The developed strategy can also be broadly applied in preclinical and clinical research, where the analysis of numerous biological replicates is crucial.

The early effects of short-term dexamethasone administration on hepatic and serum alanine aminotransferase in the rat

Dexamethasone (DEXA) administration has been associated with serum alanine aminotransferase (ALT) elevations that may result from enhanced ALT expression. The aim of our current study was to compare liver vs. serum ALT activity and to examine the onset of any hepatocellular changes. Groups of 4 male Sprague-Dawley rats were administered a single dose of DEXA or corn oil at 12, 16, and 24 h prior to euthanasia or once-daily for 2, 3, or 4 days. All (nonfasted) rats were necropsied together on Day 5. While DEXA incrementally increased liver ALT activity in the 1-, 2-, 3-, and 4-day treatment groups (maximal, 3.7-fold), liver aspartate aminotransferase (AST) never exceeded 1.4-fold over control. Significant hepatic glycogen elevations were detected after DEXA treatment, which correlated with microscopic observations. Serum ALT, AST, sorbitol dehydrogenase, and glutamate dehydrogenase (GLDH) increased after 2, 3, and 4 days of DEXA dosing (1.3-10.3-fold). DEXA-related necropsy findings included pale livers consistent with glycogen deposition. The relative percent liver to body weight was elevated in all DEXA-treated rats. Hepatocellular necrosis was observed in 1/4 rats at 12 h, 2/4 rats at 2 days, 4/4 rats at 3 days, and 3/4 rats at 4 days. DEXA treatment <2 days failed to produce consistent evidence of hepatic injury, as detected by serum biomarkers and pathology assessment. However, early DEXA treatment did correlate with apparent ALT induction. Ultimately, this may explain some early asymptomatic serum ALT elevations seen clinically.

Zonation of multidrug resistance-associated protein 2 in rat liver after induction with dexamethasone

BACKGROUND AND AIM

The present study was aimed to evaluate the hepatic zonation of multidrug resistance-associated protein 2 (mrp2), an important drug transporter, and its potential changes during the induction of its expression by known inducer, dexamethasone (DEX).

METHODS

The hepatic expression of mrp2 was studied by immunohistochemistry with consequent quantification by measurement of integral optical densities of mrp2 staining in the periportal and perivenous areas of the liver acinus in control and DEX-pretreated rats (1 mg/kg daily per os for 4 days). Overall changes in mrp2 expression and function produced by DEX were monitored using Western blotting and an in vivo clearance study of endogenous-conjugated bilirubin, a mrp2 substrate.

RESULTS

In the control animals, a quantitative image analysis revealed the primary periportal localization of mrp2 within the liver acinus with the expression of mrp2 being 16.7-fold of that in the perivenous area. After DEX pretreatment, the expression of mrp2 increased, especially in the perivenous hepatocytes. The overall expression of mrp2 increased 3.2-fold in comparison with the control group. This observation was confirmed by Western blotting, which showed a 1.3-fold increase in the mrp2 protein after DEX pretreatment. The functional consequences of the induced mrp2 protein in the livers of the DEX-pretreated rats were demonstrated by the increased biliary excretion of conjugated bilirubin.

CONCLUSION

In conclusion, these results indicate the zonation of mrp2 protein expression primarily to periportal hepatocytes. The induction by DEX produced spatially disproportional changes with an increase in the mrp2 protein being most prominent in the perivenous hepatocytes.

Pharmacodynamic Modeling of Acute and Chronic Effects of Methylprednisolone on Hepatic Urea Cycle Genes in Rats

Corticosteroids (CS) regulate many enzymes at both mRNA and protein levels. This study used microarrays to broadly assess regulation of various genes related to the greater urea cycle and employs pharmacokinetic/pharmacodynamic (PK/PD) modeling to quantitatively analyze and compare the temporal profiles of these genes during acute and chronic exposure to methylprednisolone (MPL). One group of adrenalectomized male Wistar rats received an intravenous bolus dose (50 mg/kg) of MPL, whereas a second group received MPL by a subcutaneous infusion (Alzet osmotic pumps) at a rate of 0.3 mg/kg/hr for seven days. The rats were sacrificed at various time points over 72 hours (acute) or 168 hours (chronic) and livers were harvested. Total RNA was extracted and Affymetrix® gene chips (RGU34A for acute and RAE 230A for chronic) were used to identify genes regulated by CS. Besides five primary urea cycle enzymes, many other genes related to the urea cycle showed substantial changes in mRNA expression. Some genes that were simply up- or down-regulated after acute MPL showed complex biphasic patterns upon chronic infusion indicating involvement of secondary regulation. For the simplest patterns, indirect response models were used to describe the nuclear steroid-bound receptor mediated increase or decrease in gene transcription (e.g. tyrosine aminotransferase, glucocorticoid receptor). For the biphasic profiles, involvement of a secondary biosignal was assumed (e.g. ornithine decarboxylase, CCAAT/enhancer binding protein) and more complex models were derived. Microarrays were used successfully to explore CS effects on various urea cycle enzyme genes. PD models presented in this report describe testable hypotheses regarding molecular mechanisms and quantitatively characterize the direct or indirect regulation of various genes by CS.

Cytosolic aspartate aminotransferase, a new partner in adipocyte glyceroneogenesis and an atypical target of thiazolidinedione

We show that cytosolic aspartate aminotransferase (cAspAT) is involved in adipocyte glyceroneogenesis, a regulated pathway that controls fatty acid homeostasis by promoting glycerol 3-phosphate formation for fatty acid re-esterification during fasting. cAspAT activity, as well as the incorporation of [(14)C]aspartate into the neutral lipid fraction of 3T3-F442A adipocytes was stimulated by the thiazolidinedione (TZD) rosiglitazone. Conversely, the ratio of fatty acid to glycerol released into the medium decreased. Regulation of cAspAT gene expression was specific to differentiated adipocytes and did not require any peroxisome proliferator-activated receptor gamma (PPARgamma)/retinoid X receptor-alpha direct binding. Nevertheless, PPARgamma is indirectly necessary for both cAspAT basal expression and TZD responsiveness because they are, respectively, diminished and abolished by ectopic overexpression of a dominant negative PPARgamma. The cAspAT TZD-responsive site was restricted to a single AGGACA hexanucleotide located at -381 to -376 bp whose mutation impaired the specific RORalpha binding. RORalpha ectopic expression activated the cAspAT gene transcription in absence of rosiglitazone, and its protein amount in nuclear extracts is 1.8-fold increased by rosiglitazone treatment of adipocytes. Finally, the amounts of RORalpha and cAspAT mRNAs were similarly increased by TZD treatment of human adipose tissue explants, confirming coordinated regulation. Our data identify cAspAT as a new member of glyceroneogenesis, transcriptionally regulated by TZD via the control of RORalpha expression by PPARgamma in adipocytes.

Molecular characterization and expression of the gene encoding aspartate aminotransferase from the Pacific oyster Crassostrea gigas exposed to environmental stressors

A partial cDNA encoding cytosolic aspartate aminotransferase (AST) (EC 2.6.1.1) was isolated from a Crassostrea gigas digestive gland library. This sequence was used to design specific primers to amplify the AST genomic sequence. We obtained a complete gene, 5054 bp in length, encoding cytosolic AST and containing a 404 amino acid open reading frame. Phylogenetic analysis showed that C. gigas AST sequence constitutes a branch distinct from homologous sequences from other invertebrate groups. We also investigated AST mRNA expression in different tissues of oysters exposed to hydrocarbons, pesticides, hypoxia and hypo-salinity stress. The results showed that AST expression responds to hydrocarbon exposure, hypoxia and salinity stress, but not to pesticide exposure in an organ and time-specific manner. Use of AST as a potential molecular biomarker for monitoring of disturbed ecosystems is discussed.

Molecular identification and expression of two non-P450 enzymes, monoamine oxidase A and flavin-containing monooxygenase 2, involved in phase I of xenobiotic biotransformation in the Pacific oyster, Crassostrea gigas

Marine bivalve metabolism can be perturbed by hydrocarbon and pesticide pollution in coastal ecosystems. In this study, in the Pacific oyster, Crassostrea gigas, full-length cDNAs encoding two non-P450 phase I enzymes, flavin-containing monooxygenase 2 (FMO-2) and monamine oxidase A (MAO A), were characterized. Both sequences contained the co-factor fixation motifs characteristic of their respective enzyme families. Using reverse transcription polymerase chain reaction (RT-PCR), the messenger RNA (mRNA) transcription levels of these two enzymes in tissues of oysters exposed, under experimental conditions, to hydrocarbons and two pesticide treatments were investigated. The pesticide treatments were exposure to either glyphosate or to a mixture composed of atrazine, diuron and isoproturon. The results showed a strong differential expression of FMO-2 and MAO A that was both tissue-specific as well as time- and treatment-dependent. It was also clearly demonstrated that the transcription levels of MAO A (generally considered a constitutive enzyme without external regulation) were induced by hydrocarbons and pesticides in digestive gland and inhibited by pesticides in gill tissue. Furthermore, the transcription levels of FMO-2 and MAO A mRNA in digestive gland might be useful as a marker of hydrocarbon or pesticide exposure in monitoring programs.

Opposite regulation of the rat and human cytosolic aspartate aminotransferase genes by fibrates

Fenofibrate, a peroxisome proliferator-activated receptor alpha (PPARalpha) activator, increases the expression of the cytosolic aspartate aminotransferase (cAspAT) gene in human liver cells, which may partially explain the increase of this enzyme in the serum of individuals undergoing fenofibrate treatment. Conversely, in rodents, fenofibrate represses the expression of the cAspAT gene. We compared the mechanisms of fenofibrate action in human and rat hepatoma cells. Transfection assays of the wild-type and mutated rat promoters in Fao and H4IIEC3 cells established a critical role for sequences similar to nuclear receptor responsive elements in the -404/-366 bp region. Nuclear proteins bound to these sequences and the amounts of protein bound were decreased by fenofibrate treatment, probably accounting for the decreased gene expression. Pharmacological studies confirmed the involvement of PPARalpha. However, this receptor did not bind directly to these sequences. The human promoter was cloned and the regulatory region localized between -2663/-706 bp. Co-transfection assays suggested that, in humans, the PPARalpha was also involved in the increase in expression of the cAspAT gene due to fibrates, without the presence of a canonical PPAR responsive element.

Analysis of alterations in gene expression after amplification of recombinant genes in CHO cells

Dihydrofolate reductase (DHFR) based amplification of recombinant genes using increasing concentrations of methotrexate (MTX) is a common method to establish CHO cell lines producing high amounts of the desired protein. Once, cell lines with highly amplified target genes and good expression rates are isolated, further characterization of their transcriptional pattern is intended to clarify the question what other factors are elevated, as a prerequisite or consequence of recombinant protein production. In order to define genes which are upregulated in a cell line that shows high production rates, we have investigated alterations in gene expression which occur beside amplification of the recombinant genes. For this purpose, the suppression subtractive hybridization method was used to create a cDNA library enriched for differentially expressed sequences in the recombinant antibody producing CHO cell line versus the original counterpart. Differential expression was confirmed by Northern blotting and Northern ELISA. In addition to the expected recombinant genes, we have identified 5 transcripts which are upregulated in the recombinant cell line. One sequence has not been found in existing data bases, the others revealed to be genes involved in protein synthesis and regulation of transcription. Furthermore, an alternatively spliced, non-functional form of the DHFR mRNA was detected, suggesting a dramatic increase of the selection pressure exerted by MTX.

Divergent Hsc70 binding properties of mitochondrial and cytosolic aspartate aminotransferase. Implications for their segregation to different cellular compartments

Cytosolic Hsc70 discriminates between the homologous mitochondrial and cytosolic isozymes of aspartate aminotransferase, binding exclusively the mitochondrial form. By screening a library of synthetic peptides spanning the sequence of the mitochondrial enzyme, we have identified binding sites in this polypeptide that interact with Hsc70. These potential binding sites are scattered over the entire sequence and map to secondary structure elements, particularly the alpha-helix, that are partly exposed on the surface of the native protein. Several peptides corresponding to analogous positions in the cytosolic enzyme sequence do not bind to Hsc70. Phylogenetic analyses suggest that Hsc70 binding sequences have diverged as a consequence of biochemical specialization ensuring differential interaction of each isozyme with the cellular machinery in charge of protein folding and translocation.

Identification of an adipocyte-specific negative glucose response region in the cytosolic aspartate aminotransferase gene

Cytosolic aspartate aminotransferase (cAspAT) participates in gluconeogenesis in the liver and is expected to exert a glyceroneogenic function in the adipose tissue when the supply of glucose is limited. Here we demonstrate that adipose cAspAT messenger RNA (mRNA) is increased when rats are fed a low carbohydrate diet. In the 3T3-F442A, BFC-1 adipocyte cell lines and differentiated adipocytes in primary culture, a 24 h glucose deprivation induces approximately a 4-fold increase in cytosolic AspAT (cAspAT) mRNA, whereas mitochondrial AspAT mRNA remains unchanged. cAspAT activity is also increased in a weaker but reproducible manner. Addition of glucose within a physiological range of concentrations reverses the increase of cAspAT mRNA in 8 h (EC50 = 1.25 g/liter). Such a regulation requires protein synthesis and is specific for adipocytes differentiated in culture. It does not occur in Fao or H4IIE hepatoma cells, in C2 muscle cells, or in 293 kidney cells. 2-deoxyglucose mimicks glucose, while 3-orthomethyl-glucose has no effect, suggesting that glucose-6-phosphate is the effector. cAspAT mRNA stability is not affected by glucose deprivation. To ascertain the transcriptional nature of the glucose effect, we have stably transfected 3T3-F442A adipoblasts with constructs containing the chloramphenicol acetyltransferase reporter gene under the control of either 5'-deletions of the cAspAT gene promoter or internal fragments in an heterologous context. We demonstrate that a glucose response element(s) is present in the region between -1838 and -1702 bp relative to the translation start site. In this region, three DNA sequences bind nuclear proteins from adipocytes as shown by footprinting experiments. Our results indicate that cAspAT gene transcription is repressed by glucose selectively in adipocytes.

Conformation of aspartate aminotransferase isozymes folding under different conditions probed by limited proteolysis

The partially homologous mitochondrial (mAAT) and cytosolic (cAAT) aspartate aminotransferase have nearly identical three-dimensional structures but differ in their folding rates in cell-free extracts and in their affinity for binding to molecular chaperones. In its native state, each isozyme is protease-resistant. Using limited proteolysis as an index of their conformational states, we have characterized these proteins (a) during the early stages of spontaneous refolding; (b) as species trapped in stable complexes with the chaperonin GroEL; or (c) as newly translated polypeptides in cell-free extracts. Treatment of the refolding proteins with trypsin generates reproducible patterns of large proteolytic fragments that are consistent with the formation of defined folding domains soon after initiating refolding. Binding to GroEL affords considerable protection to both isozymes against proteolysis. The tryptic fragments are similar in size for both isozymes, suggesting a common distribution of compact and flexible regions in their folding intermediates. cAAT synthesized in cell-free extracts becomes protease-resistant almost instantaneously, whereas trypsin digestion of the mAAT translation product produces a pattern of fragments qualitatively akin to that observed with the protein refolding in buffer. Analysis of the large tryptic peptides obtained with the GroEL-bound proteins reveals that the cleavage sites are located in analogous regions of the N-terminal portion of each isozyme. These results suggest that (a) binding to GroEL does not cause unfolding of AAT, at least to an extent detectable by proteolysis; (b) the compact folding domains identified in AAT bound to GroEL (or in mAAT fresh translation product) are already present at the early stages of refolding of the proteins in buffer alone; and (c) the two isozymes seem to bind in a similar fashion to GroEL, with the more compact C-terminal portion completely protected and the more flexible N-terminal first 100 residues still partially accessible to proteolysis.

Opposite behavior of two isozymes when refolding in the presence of non-ionic detergents

GroEL has a greater affinity for the mitochondrial isozyme (mAAT) of aspartate aminotransferase than for its cytosolic counterpart (cAAT) (Mattingly JR Jr, Iriarte A, Martinez-Carrion M, 1995, J Biol Chem 270:1138-1148), two proteins that share a high degree of sequence similarity and an almost identical spatial structure. The effect of detergents on the refolding of these large, dimeric isozymes parallels this difference in behavior. The presence of non-ionic detergents such as Triton X-100 or lubrol at concentrations above their critical micelle concentration (CMC) interferes with reactivation of mAAT unfolded in guanidinium chloride but increases the yield of cAAT refolding at low temperatures. The inhibitory effect of detergents on the reactivation of mAAT decreases progressively as the addition of detergents is delayed after starting the refolding reaction. The rate of disappearance of the species with affinity for binding detergents coincides with the slowest of the two rate-limiting steps detected in the refolding pathway of mAAT. Limited proteolysis studies indicate that the overall structure of the detergent-bound mAAT resembles that of the protein in a complex with GroEL. The mAAT folding intermediates trapped in the presence of detergents can resume reactivation either upon dilution of the detergent below its CMC or by adding beta-cyclodextrin. Thus, isolation of otherwise transient productive folding intermediates for further characterization is possible through the use of detergents.

Aminotransferase variants as probes for the role of the N-terminal region of a mature protein in mitochondrial precursor import and processing

Of the two homologous isozymes of aspartate aminotransferase that are also nearly identical in their folded structures, only the mitochondrial form (mAAT) is synthesized as a precursor (pmAAT). After its in vitro synthesis in rabbit reticulocyte lysate, it can also be efficiently imported into isolated rat liver mitochondria, where it is processed to its native form by removal of the N-terminal presequence. The homologous cytosolic isoenzyme (cAAT) is not imported into mitochondria, even after fusion of the mitochondrial presequence from pmAAT to its N-terminal end. Substitution of the 30-residue N-terminal peptide of the mature portion of pmAAT with the corresponding sequence from the homologous, import-incompetent cytosolic isozyme (pcmAAT) does not prevent import but reduces substantially its processing in the matrix. A detectable amount of the pcmAAT chimera is found associated with the inner mitochondrial membrane. Single and double substitution mutants of Trp-5 and Trp-6 at the N-terminal end of the mature protein are imported into mitochondria with efficiency similar to that of wild type. However, replacement of Trp-5 with proline, or of both tryptophans with either alanine (W5A/W6A mutant) or valine and alanine (W5V/W6A mutant), allows import but interferes with the correct processing of the imported protein despite the presence of an intact cleavage site for the processing peptidase. Similar cleavage results were obtained using newly synthesized proteins and mitochondrial matrix extracts. These results indicate that translocation and processing for a precursor are independent events and that sequences C-terminal to the cleavage site are indeed important for the correct maturation of pmAAT in the matrix, probably because of their contribution to the conformation and flexibility of the peptide region surrounding the cleavage site required for efficient processing. The same region from the mature component of the protein may play a role in the commitment of the passenger protein to complete its translocation into the matrix.

Contribution of a nuclear factor 1 binding site to the glucocorticoid regulation of the cytosolic aspartate aminotransferase gene promoter

Two regions of the cAspAT gene promoter mediate the glucocorticoid regulation of this gene in the Fao hepatoma cell line. The proximal region was localized by deletion studies and stable transfections in the Fao cells to the sequence -553/-398. This region includes the glucocorticoid-responsive element (GRE) A sequence, which consists of two overlapping GREs and which can mediate the glucocorticoid regulation of a heterologous promoter. DNase I footprinting studies have shown that a site 80 base pairs upstream of the GRE A was protected by liver and brain nuclear extracts (site P8). The binding was displaced by an excess of an oligonucleotide containing a typical NF1 binding site and by NF1-specific antibodies. In electrophoretic mobility shift assay using the P8 oligonucleotide as a probe, several complexes were formed. Most complexes were common to liver and brain but were less abundant when testis extracts were used. At least one complex was specific to the liver. All complexes, with the exception of two, were competed for by the NF1 oligonucleotide. Furthermore, the sequence of the P8 site showed a 7/9-base pair homology with a typical NF1 site. A mutation of the P8 site, which prevents the binding of NF1-like proteins to it, considerably decreases the regulation of the cAspAT promoter fragment by glucocorticoids. Surprisingly, the basal activity of the mutant promoter was increased 2-fold. Thus, the regulation of the cAspAT gene promoter is mediated by a regulatory unit comprising the GRE A and a NF1 binding site.

Biliary electrolytes and enzymes in patients with and without gallstones

pH, osmolarity, various electrolytes, nine enzymes, and bile acid were determined in hepatic and gallbladder biles from 108 and 100 patients, respectively, relating to various types of gallstones. The pH, osmolarity, and electrolytes were essentially identical in all groups of patients except for slightly higher Ca and Mg in the hepatic bile in patients with muddy pigment stones. The gallbladder bile contained much higher inorganic cations yet remained isosmotic as a result of their sequestration into bile acid micelles. Excluding extremely high values, the activities of nine enzymes in the bile showed only minor differences among four groups of patients except for a high beta-glucuronidase activity in the hepatic bile in patients with muddy pigment stones. The biliary baseline activities of various enzymes and the relation to their serum levels were determined by their sources and subcellular localization in the hepatocytes. We concluded that biliary electrolytes and enzymes were basically similar in patients with and without gallstones except for higher levels of Ca, Mg, and beta-glucuronidase in hepatic bile in patients with muddy pigment stones.

Acinar zonation of the hormonal regulation of cytosolic aspartate aminotransferase in the liver

The zonation of the expression and regulation of the cytosolic aspartate aminotransferase (cAspAT) mRNAs in the liver acinus was investigated in diabetic and/or adrenalectomized rats. Dexamethasone increased cAspAT activity two- to threefold alone and up to sixfold in combination with streptozotocin-induced diabetes. Northern blot analysis showed that the cAspAT mRNAs were increased by those treatments; the effect of streptozotocin was reversed by the administration of insulin. In situ hybridization experiments showed that basal cAspAT mRNAs were uniformly distributed within the liver acinus. However, cAspAT mRNAs were induced by glucocorticoids specifically in the periportal zone and by streptozotocin in a larger area including the periportal and intermediary zone. The alpha 2u-globulin mRNAs which are specifically expressed in the perivenous hepatocytes are also induced by glucocorticoids in this zone, suggesting that the specific regulation of the cAspAT gene by glucocorticoids in the periportal zone is not due to the absence of functional glucocorticoid receptors in the other zones. We conclude that the regulation of the cAspAT housekeeping gene is zone specific in the liver. Furthermore, this zonation depends on the gene and on the type of hormonal or pharmacological treatment.

Identification of a metalloprotease from Alzheimer's disease brain able to degrade the beta-amyloid precursor protein and generate amyloidogenic fragments

A 4.2-kDa polypeptide termed beta protein (A beta) accumulates in senile plaques and blood vessels in Alzheimer's disease and Down's syndrome. It is widely believed that A beta is the product of the posttranslational processing of a larger precursor protein, the beta amyloid precursor protein (APP). The proteolytic processes involved in the generation of the A beta are virtually unknown. Here the purification and characterization of a protease from Alzheimer's disease brain capable of cleaving a 10 amino acid synthetic substrate flanking the N terminus of A beta at the Met-Asp bond are described. Most importantly, the purified protease degrades human recombinant APP and generates a 15-kDa amyloidogenic fragment. The protease requires the presence of a reducing agent for its activity. Its pH optimum is around physiological pH, while the enzyme is inactive at acidic pH (below pH 5.0) and basic pH (over pH 7.6). The enzyme is inhibited by N-ethylmaleimide, (hydroxymercuri)benzoate, 1.10-phenanthroline, EDTA, and EGTA. Phenylmethanesulfonyl fluoride has no effect on its activity. This protease is devoid of caseinolytic or gelatinase activities, as well as activities against cathepsin B and cathepsin L substrates. Sequence analysis reveals high homology to the rat metallopeptidase EC 3.4.24.15, a protease involved in neuropeptide processing.

Exercise interrupts ongoing glucocorticoid-induced muscle atrophy and glutamine synthetase induction

This study was undertaken to determine whether regular endurance exercise is a deterrent to a developing state of muscle atrophy from glucocorticoids and to evaluate whether the contractile activity antagonizes the hormonal actions on glutamine synthetase, alanine aminotransferase, and cytosolic aspartate aminotransferase (cAspAT). Adult female rats were administered cortisol acetate (CA, 100 mg/kg body wt) or an equal volume of the vehicle solution for up to 15 days. Exercise (treadmill running at 31 m/min, 10% grade, 90 min/day) was introduced after 4 days of CA treatment, at which time plantaris and quadriceps muscle mass had been reduced to 90% of control levels. Running for 11 consecutive days prevented 40 mg of the 90-mg loss and 227 mg of the 808-mg loss that were subsequently observed in plantaris and quadriceps muscles, respectively, in the sedentary animals. Glutamine synthetase mRNA and enzyme activity were elevated threefold by glucocorticoid treatment in the deep quadriceps (fast-twitch red) muscles after 4 days. Initiating exercise completely interfered with the further hormonal induction (to approximately 5-fold) of this enzyme and, after 11 consecutive days of the exercise regimen, glutamine synthetase mRNA and enzyme activity were 58 and 68% of values from CA-treated sedentary animals. In vehicle-treated groups, basal levels of glutamine synthetase expression were also diminished by exercise to approximately 40% of the values in sedentary controls. Hormone treatment did not alter either aminotransferase enzyme activity but reduced cAspAT mRNA in fast-twitch red muscles by 50%. Exercise abolished the glucocorticoid effect on cAspAT mRNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Domain closure in mitochondrial aspartate aminotransferase

The subunits of the dimeric enzyme aspartate aminotransferase have two domains: one large and one small. The active site lies in a cavity that is close to both the subunit interface and the interface between the two domains. On binding the substrate the domains close together. This closure completely buries the substrate in the active site and moves two arginine side-chains so they form salt bridges with carboxylate groups of the substrate. The salt bridges hold the substrate close to the pyridoxal 5'-phosphate cofactor and in the right position and orientation for the catalysis of the transamination reaction. We describe here the structural changes that produce the domain movements and the closure of the active site. Structural changes occur at the interface between the domains and within the small domain itself. On closure, the core of the small domain rotates by 13 degrees relative to the large domain. Two other regions of the small domain, which form part of the active site, move somewhat differently. A loop, residues 39 to 49, above the active site moves about 1 A less than the core of the small domain. A helix within the small domain forms the "door" of the active site. It moves with the core of the small domain and, in addition, shifts by 1.2 A, rotates by 10 degrees, and switches its first turn from the alpha to the 3(10) conformation. This results in the helix closing the active site. The domain movements are produced by a co-ordinated series of small changes. Within one subunit the polypeptide chain passes twice between the large and small domains. One link involves a peptide in an extended conformation. The second link is in the middle of a long helix that spans both domains. At the interface this helix is kinked and, on closure, the angle of the kink changes to accommodate the movement of the small domain. The interface between the domains is formed by 15 residues in the large domain packing against 12 residues in the small domain and the manner in which these residues pack is essentially the same in the open and closed structures. Domain movements involve changes in the main-chain and side-chain torsion angles in the residues on both sides of the interface. Most of these changes are small; only a few side-chains switch to new conformations.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of human liver cytosolic aspartate aminotransferase mRNAs generated by alternative polyadenylation site selection

Human cytosolic aspartate aminotransferase (cAspAT) cDNA clones have been isolated from an adult human liver cDNA library. Among the clones, two cDNAs of 1550 and 1950 base pairs, respectively, have been characterized. These two cDNAs differ only in the lengths of their 3' noncoding regions and by the presence of one or two putative polyadenylation signals AATAAA. Northern blot analysis revealed two different mRNAs of 2.1 and 1.8 kbp in several human tissues, whereas Southern blot analysis suggested the existence of a single gene for the human cAspAT. The two mRNA species result from the alternative use of two polyadenylation signals. In the liver, the relative ratio of these mRNAs varies among different species and, in humans at least, during development. The properties of the two mRNAs were compared. The half-lives of the 2.1 and 1.8 kbp mRNAs, in the HepG2 cell line, are 8 and 12 h, respectively. The two mRNAs have similar and rather short poly(A) tracts of 20-50 nucleotides. Both mRNAs are capable of directing the in vitro synthesis of the cAspAT protein. We conclude that both the 2.1 and 1.8 kbp cAspAT mRNAs are functional and exhibit similar properties.

Regulation of cytosolic aspartate aminotransferase mRNAs in the Fao rat hepatoma cell line by dexamethasone, insulin and cyclic AMP

Glucocorticoid hormones increase the activity of cytosolic aspartate aminotransferase (cAspAT) in the Fao rat hepatoma cell line. Maximal increase (6-10-fold) was observed 48 h following the addition of the glucocorticoid agonist dexamethasone at a concentration of 0.1 microM. The effect of dexamethasone was specific since it was not mimicked by sex steroids and was inhibited by the glucocorticoid antagonist RU 486. Insulin (0.1 microM) inhibited by more than 50% the induction of cAspAT by glucocorticoids. The cAMP analog, 8-bromoadenosine 3',5'-monophosphate (Br8cAMP, 0.5 mM), potentiated the effect of dexamethasone (2-3-fold) and partially relieved the inhibitory effect of insulin on the induction by dexamethasone. Both insulin and Br8-cAMP had no significant effect on basal activity. The mitochondrial isoenzyme was insensitive to the various hormonal treatments. Northern blot analysis revealed the presence of two major (2.1-kb and 1.8-kb) and one minor (4-kb) mRNA species hybridizing with a rat cAspAT probe. The regulation of these mRNAs by glucocorticoids, insulin and cAMP correlated with the variation of the cAspAT activity, suggesting that these hormones act at the pretranslational level. We compared the regulation of cAspAT mRNAs with those of tyrosine aminotransferase mRNA. Both were similarly increased by dexamethasone but the latter was also increased by cAMP even in the absence of the glucocorticoid agonist. In addition, the increase in tyrosine aminotransferase mRNA was inhibited by cycloheximide whereas the increase in cAspAT mRNAs was not. These results show that there are significant differences in the regulation of cAspAT and tyrosine aminotransferase by glucocorticoids and other hormones, although both enzymes probably contribute to the same metabolic pathway.

Chromosomal localization of human aspartate aminotransferase genes by in situ hybridization

The localization of the human genes for cytosolic and mitochondrial aspartate aminotransferase (AspAT) has been determined by chromosomal in situ hybridization with specific human cDNA probes previously characterized in our laboratory. The cytosolic AspAT gene is localized on chromosome 10 at the interface of bands q241-q251. Mitochondrial AspAT is characterized by a multigene family located on chromosomes 12 (p131-p132), 16 (q21), and 1 (p32-p33 and q25-q31). Genomic DNA from ten blood donors was digested by ten restriction enzymes, and Southern blots were hybridized with the two specific probes. Restriction fragment length polymorphism was revealed in only one case for cytosolic AspAT, with PvuII, while no polymorphism for mitochondrial AspAT was found.