Characterization of the transcriptional regulatory region of the human dihydrolipoamide dehydrogenase gene

Hepatitis B Virus Core Promoter Double Mutations (A1762T, G1764A) Are Associated with Lower Levels of Serum Dihydrolipoyl Dehydrogenase

OBJECTIVES

The aim of this study was to identify serum proteins with differential concentrations between hepatocellular carcinoma (HCC) patients and HBsAg asymptomatic carriers among individuals infected with hepatitis B virus (HBV) with basal core promoter (BCP) double mutations (A1762T, G1764A).

METHODS

iTRAQ and liquid chromatography-tandem mass spectrometry were used to identify differentially expressed protein, and an ELISA test was used for the validation test.

RESULTS

The total number of proteins identified was 1,125, of which 239 showed statistically significant differences in their expression. The relative concentrations of serum dihydrolipoyl dehydrogenase (DLD), which showed the most significant correlation with liver diseases and infection, were significantly lower in HCC patients than asymptomatic HBsAg carriers and individuals negative for HBsAg. However, only the difference between HCC patients with BCP double mutations and HBsAg-negative individuals could be confirmed by ELISA. Meanwhile, we found that the concentrations of serum DLD in those infected with HBV with BCP double mutations were significantly lower than in individuals with the wild-type BCP. However, the difference in the concentrations of serum DLD between individuals with wild-type BCP and those negative for HBsAg was not significant.

CONCLUSIONS

HBV with BCP double mutations are associated with lower concentrations of serum DLD.

Mitochondrial Dihydrolipoamide Dehydrogenase is Upregulated in Response to Intermittent Hypoxic Preconditioning

Intermittent hypoxia preconditioning (IHP) has been shown to protect neurons against ischemic stroke injury. Studying how proteins respond to IHP may identify targets that can help fight stroke. The objective of the present study was to investigate whether mitochondrial dihydrolipoamide dehydrogenase (DLDH) would respond to IHP and if so, whether such a response could be linked to neuroprotection in ischemic stroke injury. To do this, we subjected male rats to IHP for 20 days and measured the content and activity of DLDH as well as the three α-keto acid dehydrogenase complexes that contain DLDH. We also measured mitochondrial electron transport chain enzyme activities. Results show that DLDH content was indeed upregulated by IHP and this upregulation did not alter the activities of the three α-keto acid dehydrogenase complexes. Results also show that the activities of the five mitochondrial complexes (I-V) were not altered either by IHP. To investigate whether IHP-induced DLDH upregulation is linked to neuroprotection against ischemic stroke injury, we subjected both DLDH deficient mouse and DLDH transgenic mouse to stroke surgery followed by measurement of brain infarction volume. Results indicate that while mouse deficient in DLDH had exacerbated brain injury after stroke, mouse overexpressing human DLDH also showed increased brain injury after stroke. Therefore, the physiological significance of IHP-induced DLDH upregulation remains to be further investigated.

Impaired TCA cycle flux in mitochondria in skeletal muscle from type 2 diabetic subjects: marker or maker of the diabetic phenotype?

The diabetic phenotype is complex, requiring elucidation of key initiating defects. Recent research has shown that diabetic myotubes express a primary reduced tricarboxylic acid (TCA) cycle flux. A reduced TCA cycle flux has also been shown both in insulin resistant offspring of T2D patients and exercising T2D patients in vivo. This review will discuss the latest advances in the understanding of the molecular mechanisms regulating the TCA cycle with focus on possible underlying mechanism which could explain the impaired TCA flux in insulin resistant human skeletal muscle in type 2 diabetes. A reduced TCA is both a marker and a maker of the diabetic phenotype.

Glycine cleavage enzyme complex: Molecular cloning and expression of the H-protein cDNA from cultured human skin fibroblasts

The human H-protein is one of four essential components (H-, L-, P-, and T-proteins) of the mammalian glycine cleavage enzyme complex and its function is involved in the pathogenesis and diagnosis of glycine encephalopathy. A transcript corresponding to the glycine cleavage H-protein functional gene was isolated from cultured human skin fibroblasts along with a transcript for a putative processed pseudogene on chromosome 2q33.3. Sequence analysis of the fibroblast H-protein functional gene transcript showed complete identity to that reported from human liver. The H-protein cDNA was subsequently cloned with a hexahistidine affinity tag in the Pichia pastoris plasmid vector pPICZαA and recombined into the yeast genome downstream of the alcohol oxidase promoter for methanol-induced expression. The recombinant H-protein was secreted into the culture medium and purified to homogeneity using a one-step nickel-nitrilotriacetic acid resin column. Approximately 4 mg of homogeneous H-protein was obtained from 1 L of culture medium. Since the attachment of a lipoic acid prosthetic group is required for H-protein function, we have expressed and purified E. coli lipoate protein ligase and succeeded in lipoylating H-protein, converting the apo-H-protein to the functional holo-H-protein. A lipoamide dehydrogenase assay was performed to confirm that the apo-H-protein was inactive, whereas the holo-H-protein was approximately 2.3-fold more active than free lipoic acid as a hydrogen donor in driving the reaction. The availability of copious amounts of human recombinant H-protein by using Pichia pastoris expression and affinity purification will facilitate the elucidation of the structure and function of the H-protein and its relationship to the P-, T-, and L-proteins in the glycine cleavage enzyme complex. In view of the fact that there is no detectable glycine cleavage enzyme activity in human skin fibroblasts, we speculate that a plausible function of the H-protein is to interact with the L-protein, which is also part of the l-ketoglutarate dehydrogenase complex present in fibroblasts.

Maple syrup urine disease: mutation analysis in Turkish patients

Maple syrup urine disease (MSUD), the most frequently occurring organic acidaemia in Turkey, is caused by a deficiency of the activity of branched-chain keto acid dehydrogenase enzyme (BCKAD) complex. Mutation analysis of the E1alpha, E1beta, and E2 genes of the BCKAD complex in 12 Turkish MSUD patients yielded three disease-specific mutations and a polymorphism in the E1alpha gene, none in the E1beta gene and one mutation in the E2 gene. Among them, three missense mutations (Q80E, C213Y, T106M) and the F280F polymorphism occurring in the E1alpha gene and the splice site mutation (IVS3 - 1G>A) in the E2 gene were novel. Three of the missense mutations and the splicing mutation occurred homozygously and caused classical MSUD. One patient carried the splicing mutation homozygously and the T106M mutation in the heterozygous state; this patient is the first case having simultaneously two different mutations in two different genes in the BCKAD complex. IVS3 - IG>A splicing mutation detected on the E2 gene causes deletion of the first 14 bp of exon 3 in the mutant mRNA extending between 190 and 204 nt. The deletion spans the cleavage point between mitochondrial targeting and lipoyl-bearing site of the E2 protein.

Human dihydrolipoamide dehydrogenase gene transcription is mediated by cAMP-response element-like site and TACGAC direct repeat

Dihydrolipoamide dehydrogenase is a common component of four multienzyme complexes which are involved in oxidation of carbohydrates, lipids and amino acids. To better understand the regulation of human DLD gene expression, we have analyzed the proximal promoter region of this gene. DNase I footprinting analysis of the promoter region (-322 to +47 bp) revealed four major protein-binding domains (termed P1-P4). Nested deletions and site-specific mutations of approximately 100 bp proximal promoter region identified two elements, TACGAC direct repeat sequence and cAMP-response element (CRE)-like site, which are localized in the P2 and P1 domains, respectively, and mediate basal transcription of the DLD gene. Electrophoretic mobility supershift assays showed that the CRE-like site is associated with CRE binding protein. Interestingly, when DLD promoter constructs (-1.8 kb to +47 bp and -78 to +47 bp) fused with the chloramphenicol acetyltransferase (CAT) reporter gene were transiently transfected into human HepG2 cells either in the presence or absence of 0.5 mM 8-Br-cAMP, the levels of CAT expression remained unaffected. In addition, endogenous DLD mRNA levels in HepG2 cells also remained unaffected by treatment with 0.5 mM 8-Br-cAMP. These results indicate that the CRE binding protein is essential for basal transcription of the human DLD promoter, but does not confer cAMP-dependent gene regulation.

The alpha-ketoglutarate dehydrogenase complex

The alpha-ketoglutarate dehydrogenase complex (KGDHC) is an important mitochondrial constituent, and deficiency of KGDHC is associated with a number of neurological disorders. KGDHC is composed of three proteins, each encoded on a different and well-characterized gene. The sequences of the human proteins are known. The organization of the proteins into a large, ordered multienzyme complex (a "metabolon") has been well studied in prokaryotic and eukaryotic species. KGDHC catalyzes a critical step in the Krebs tricarboxylic acid cycle, which is also a step in the metabolism of the potentially excitotoxic neurotransmitter glutamate. A number of metabolites modify the activity of KGDHC, including inactivation by 4-hydroxynonenal and other reactive oxygen species (ROS). In human brain, the activity of KGDHC is lower than that of any other enzyme of energy metabolism, including phosphofructokinase, aconitase, and the electron transport complexes. Deficiencies of KGDHC are likely to impair brain energy metabolism and therefore brain function, and lead to manifestations of brain disease. In general, the clinical manifestations of KGDHC deficiency relate to the severity of the deficiency. Several such disorders have been recognized: infantile lactic acidosis, psychomotor retardation in childhood, intermittent neuropsychiatric disease with ataxia and other motor manifestations, Friedreich's and other spinocerebellar ataxias, Parkinson's disease, and Alzheimer's disease (AD). A KGDHC gene has been associated with the first two and last two of these disorders. KGDHC is not uniformly distributed in human brain, and the neurons that appear selectively vulnerable in human temporal cortex in AD are enriched in KGDHC. We hypothesize that variations in KGDHC that are not deleterious during reproductive life become deleterious with aging, perhaps by predisposing this mitochondrial metabolon to oxidative damage.

The alpha-ketoglutarate dehydrogenase complex in neurodegeneration

Altered energy metabolism is characteristic of many neurodegenerative disorders. Reductions in the key mitochondrial enzyme complex, the alpha-ketoglutarate dehydrogenase complex (KGDHC), occur in a number of neurodegenerative disorders including Alzheimer's Disease (AD). The reductions in KGDHC activity may be responsible for the decreases in brain metabolism, which occur in these disorders. KGDHC can be inactivated by several mechanisms, including the actions of free radicals (Reactive Oxygen Species, ROS). Other studies have associated specific forms of one of the genes encoding KGDHC (namely the DLST gene) with AD, Parkinson's disease, as well as other neurodegenerative diseases. Reductions in KGDHC activity can be plausibly linked to several aspects of brain dysfunction and neuropathology in a number of neurodegenerative diseases. Further studies are needed to assess mechanisms underlying the sensitivity of KGDHC to oxidative stress and the relation of KGDHC deficiency to selective vulnerability in neurodegenerative diseases.

Lipoamide dehydrogenase deficiency due to a novel mutation in the interface domain

An infant with a neurodegenerative disorder accompanied by lactic acidemia is described. In muscle homogenate, the activity of lipoamide dehydrogenase (LAD), the third catalytic subunit of pyruvate dehydrogenase complex (PDHc), alpha-ketoglutarate dehydrogenase complex (KGDHc), and branched-chain keto acid dehydrogenase complex was reduced to 15% of the control. The activity of PDHc was undetectable and the activity of KGDHc was 2% of the control mean. The immunoreactive LAD protein was reduced to about 10% of the control. Direct sequencing of LAD cDNA revealed only one mutation, substituting Asp for Val at position 479 of the precursor form. The mutation resides within the interface domain and likely perturbs stable dimerization. The phenotypic heterogeneity in LAD deficiency is not directly correlated with the residual LAD activity but rather with its impact on the multienzymatic complex activity.

Roles of an Ets motif and a novel CACGAC direct repeat in transcription of the murine dihydrolipoamide dehydrogenase (Dld) gene

The 5'-flanking region of the murine dihydrolipoamide dehydrogenase (Dld) gene was characterized for its promoter activity. DNase I footprinting analysis of the promoter region (-545 bp to +41 bp) revealed six major protein-binding domains (termed P1 to P6) that were protected by NIH3T3 fibroblast nuclear extracts. Transient transfection assays, using a series of nested deletions of the 2.5 kb 5'-flanking region ligated to the chloramphenicol acetyltransferase reporter gene, identified that the -42-bp to +41-bp region, which harbours the P1, P2, and P3 domains, had minimal transcriptional activity. When the 5'-flanking region was extended from -42 bp to -82 bp, there was an approx. 5-fold increase in promoter activity. To identify further the cis elements involved in transcription of the Dld gene (-82 bp to +41 bp), a series of mutations were introduced into this region and evaluated for functional effects using transient transfection and electrophoretic mobility shift assays. Mutation or deletion of the CACGAC direct repeat, located from -61 bp to -46 bp, resulted in minimal promoter activity. Mutation of the Ets motif, located from -37 bp to -32 bp, reduced the minimal promoter activity by approx. 50%, whereas the deletion of this motif almost abolished the promoter activity. These results indicate that: (i) the Ets motif is required for the minimal promoter activity and (ii) the CACGAC direct repeat enhances promoter activity. Database searches failed to identify the direct repeat with the CACGAC motif and hence the CACGAC sequence may represent a novel motif. The requirement of both the Ets motif and the direct repeat element for optimal promoter activity represents a unique combination for gene transcription.

Primary structure of an invertebrate dihydrolipoamide dehydrogenase with phylogenetic relationship to vertebrate and bacterial disulfide oxidoreductases

Dihydrolipoamide dehydrogenase (E3) is a flavoprotein component of multi-enzyme complexes catalyzing oxidative decarboxylation of alpha-ketoacids in the Krebs' cycle. We have cloned a 2.4-kb E3 cDNA from an arthropod, Manduca sexta, that codes for 497 amino acids and translates to a 51-kDa protein in vitro. Sequences at and around the dinucleotide binding domains, disulfide active site and the C-terminal interface domain involved in substrate binding are highly conserved in Manduca E3. Phylogenetic analysis of protein sequences from the flavoprotein class of disulfide oxidoreductases family of enzymes suggests that in spite of the homologous nature of E3 and glutathione reductase (goR) in sequence and structure, E3 shares a common ancestor with mercuric reductase (merA), whereas goR is more related to trypanothione reductase (tryR) than to other members. All members, except goRs, seemed to be monophyletic. Plant goRs seemed to have arisen differently and are more closely related to tryRs than to bacterial and vertebrate goRs. Earlier speculation on the nature of origin of E3 in Pseudomonas is not supported by phylogenetic data. A possible structural relationship of Manduca E3 to other pyridine-binding proteins, such as the neurotransmitter transporters and channels, is proposed.

Characterization of the mouse dihydrolipoamide dehydrogenase (Dld) gene: genomic structure, promoter sequence, and chromosomal localization

The mouse dihydrolipoamide dehydrogenase (Dld) gene has been cloned, characterized, and mapped. This nuclear gene encodes a mitochondrial protein that is shared among several alpha-keto acid dehydrogenase complexes and the glycine cleavage system. The Dld gene is contained within an approximately 21-kb region and consists of 14 exons ranging in size from 69 to 521 nucleotides. The open reading frame codes for a preprotein of 509 amino acids with a predicted mature protein of 474 amino acids that is highly conserved among mammalian species (> 90% identical). Primer extension analyses have shown the gene to have transcription initiation sites with tissue-specific differences in relative utilization. The 5' flanking region is G-C rich and lacks a TATA box, but does contain initiator element and multiple transcription factor-binding consensus sequences. Northern blot analysis shows that the Dld mRNA in various tissues is approximately 2.4 kb in size. The Dld gene has been localized to the proximal region of chromosome 12, approximately 21 cM from the centromere.