Cuproptotic nanoinducer-driven proteotoxic stress potentiates cancer immunotherapy by activating the mtDNA-cGAS-STING signaling

Proteotoxic stress, caused by the accumulation of abnormal unfolded or misfolded cellular proteins, can efficiently activate inflammatory innate immune response. Initiating the mitochondrial proteotoxic stress might go forward to enable the cytosolic release of intramitochondrial DNA (mtDNA) for the immune-related mtDNA-cGAS-STING activation, which however is easily eliminated by a cell self-protection, i.e., mitophagy. In light of this, a nanoinducer (PCM) is reported to trigger mitophagy-inhibited cuproptotic proteotoxicity. Through a simple metal-phenolic coordination, PCMs reduce the original Cu2+ with the phenolic group of PEG-polyphenol-chlorin e6 (Ce6) into Cu+. Cu+ thereby performs its high binding affinity to dihydrolipoamide S-acetyltransferase (DLAT) and aggregates DLAT for cuproptotic proteotoxic stress and mitochondrial respiratory inhibition. Meanwhile, intracellular oxygen saved from the respiratory failure can be utilized by PCM-conjugated Ce6 to boost the proteotoxic stress. Next, PCM-loaded mitophagy inhibitor (Mdivi-1) protects proteotoxic products from being mitophagy-eliminated, which allows more mtDNA to be released in the cytosol and successfully stimulate the cGAS-STING signaling. In vitro and in vivo studies reveal that PCMs can upregulate the tumor-infiltrated NK cells by 24% and enhance the cytotoxic killing of effector T cells. This study proposes an anti-tumor immunotherapy through mitochondrial proteotoxicity.

Autoantibodies Against Dihydrolipoamide S-Acetyltransferase in Immune-Mediated Neuropathies

BACKGROUND AND OBJECTIVES

This study aimed to identify disease-related autoantibodies in the serum of patients with immune-mediated neuropathies including chronic inflammatory demyelinating polyneuropathy (CIDP) and to investigate the clinical characteristics of patients with these antibodies.

METHODS

Proteins extracted from mouse brain tissue were used to react with sera from patients with CIDP by western blotting (WB) to determine the presence of common bands. Positive bands were then identified by mass spectrometry and confirmed for reactivity with patient sera using enzyme-linked immunosorbent assay (ELISA) and WB. Reactivity was further confirmed by cell-based and tissue-based indirect immunofluorescence assays. The clinical characteristics of patients with candidate autoantibody-positive CIDP were analyzed, and their association with other neurologic diseases was also investigated.

RESULTS

Screening of 78 CIDP patient sera by WB revealed a positive band around 60-70 kDa identified as dihydrolipoamide S-acetyltransferase (DLAT) by immunoprecipitation and mass spectrometry. Serum immunoglobulin G (IgG) and IgM antibodies' reactivity to recombinant DLAT was confirmed using ELISA and WB. A relatively high reactivity was observed in 29 of 160 (18%) patients with CIDP, followed by patients with sensory neuropathy (6/58, 10%) and patients with MS (2/47, 4%), but not in patients with Guillain-Barré syndrome (0/27), patients with hereditary neuropathy (0/40), and healthy controls (0/26). Both the cell-based and tissue-based assays confirmed reactivity in 26 of 33 patients with CIDP. Comparing the clinical characteristics of patients with CIDP with anti-DLAT antibodies (n = 29) with those of negative cases (n = 131), a higher percentage of patients had comorbid sensory ataxia (69% vs 37%), cranial nerve disorders (24% vs 9%), and malignancy (20% vs 5%). A high DLAT expression was observed in human autopsy dorsal root ganglia, confirming the reactivity of patient serum with mouse dorsal root ganglion cells.

DISCUSSION

Reactivity to DLAT was confirmed in patient sera, mainly in patients with CIDP. DLAT is highly expressed in the dorsal root ganglion cells, and anti-DLAT antibody may serve as a biomarker for sensory-dominant neuropathies.

Cuproptosis-related gene DLAT serves as a prognostic biomarker for immunotherapy in clear cell renal cell carcinoma: multi-database and experimental verification

OBJECTIVE

Renal clear cell carcinoma (ccRCC) is the most common type of renal cancer. Here we aim to explore the prognosis and immunotherapeutic value of copper death-related gene Dihydrolipoamide S-acetyltransferase (DLAT) in ccRCC.

METHODS

The mRNA and protein expressions and methylation level of DLAT, as well as the relation of DLAT to survival prognosis, clinical characteristics, biological function, and immune microenvironment and responses in patients with ccRCC were evaluated using multiple databases. In addition, 75 paired ccRCC tissue samples and 3 kinds of cell lines were tested for experimental validation.

RESULTS

Bioinformatics analysis of multiple databases, qRT-PCR, and western blot verified that DLAT expression in ccRCC was lower than that in paracancerous tissues. Patients with low expression of DLAT had a lower survival rate, worse clinical prognosis, stronger immune cell infiltration and expression of immunosuppressive points, and higher tumor immune dysfunction and exclusion (TIDE) scores.

CONCLUSIONS

DLAT was identified as an independent prognostic factor in ccRCC and was closely related to the prognosis and immune responses of patients with ccRCC.

DLAT is a promising prognostic marker and therapeutic target for hepatocellular carcinoma: a comprehensive study based on public databases

Cuproptosis is a new mechanism of cell death that differs from previously identified regulatory cell death mechanisms. Cuproptosis induction holds promise as a new tumour treatment. Therefore, we investigated the value of cuproptosis-related genes in the management of hepatocellular carcinoma (HCC). The cuproptosis-related gene Dihydrolipoamide S-Acetyltransferase (DLAT) were significantly upregulated in liver cancer tissues. High levels of DLAT were an independent prognostic factor for shorter overallsurvival (OS) time. DLAT and its related genes were mainly involved in cell metabolism, tumor progression and immune regulation. DLAT was significantly associated with the level of immune cell infiltration and immune checkpoints in HCC. HCC with high DLAT expression was predicted to be more sensitive to sorafenib treatment. The risk prognostic signature established based on DLAT and its related genes had a good prognostic value. The cuproptosis-related gene DLAT is a promising independent prognostic marker and therapeutic target in HCC. The new prognostic signature can effectively predict the prognosis of HCC patients.

Systematic pan-cancer analysis identifies cuproptosis-related gene DLAT as an immunological and prognostic biomarker

Lipoylated dihydrolipoamide S-acetyltransferase (DLAT), the component E2 of the multi-enzyme pyruvate dehydrogenase complex, is one of the key molecules of cuproptosis. However, the prognostic value and immunological role of DLAT in pan-cancer are still unclear. Using a series of bioinformatics approaches, we studied combined data from different databases, including the Cancer Genome Atlas, Genotype Tissue-Expression, the Cancer Cell Line Encyclopedia, Human Protein Atlas, and cBioPortal to investigate the role of DLAT expression in prognosis and tumor immunity response. We also reveal the potential correlations between DLAT expression and gene alterations, DNA methylation, copy number variation (CNV), tumor mutational burden (TMB), microsatellite instability (MSI), tumor microenvironment (TME), immune infiltration levels, and various immune-related genes across different cancers. The results show that DLAT displays abnormal expression within most malignant tumors. Through gene set enrichment analysis (GSEA), we found that DLAT was significantly associated with immune-related pathways. Further, the expression of DLAT was also confirmed to be correlated with the tumor microenvironment and diverse infiltration of immune cells, especially tumor-associated macrophages (TAMs). In addition, we found that DLAT is co-expressed with genes encoding major histocompatibility complex (MHC), immunostimulators, immune inhibitors, chemokines, and chemokine receptors. Meanwhile, we demonstrate that DLAT expression is correlated with TMB in 10 cancers and MSI in 11 cancers. Our study reveals that DLAT plays an essential role in tumorigenesis and cancer immunity, which may be used to function as a prognostic biomarker and potential target for cancer immunotherapy.

R-Lipoic Acid Inhibits Mammalian Pyruvate Dehydrogenase Kinase

The four pyruvate dehydrogenase kinase (PDK) and two pyruvate dehydrogenase phosphatase (PDP) isoenzymes that are present in mammalian tissues regulate activity of the pyruvate dehydrogenase complex (PDC) by phosphorylation/dephosphorylation of its pyruvate dehydrogenase (E1) component. The effect of lipoic acids on the activity of PDKs and PDPs was investigated in purified proteins system. R-lipoic acid, S-lipoic acid and R-dihydrolipoic acid did not significantly affect activities of PDPs and at the same time inhibited PDKs to different extents (PDK1>PDK4 approximately PDK2>PDK3 for R-LA). Since lipoic acids inhibited PDKs activity both when reconstituted in PDC and in the presence of E1 alone, dissociation of PDK from the lipoyl domains of dihydrolipoamide acetyltransferase in the presence of lipoic acids is not a likely explanation for inhibition. The activity of PDK1 towards phosphorylation sites 1, 2 and 3 of E1 was decreased to the same extent in the presence of R-lipoic acid, thus excluding protection of the E1 active site by lipoic acid from phosphorylation. R-lipoic acid inhibited autophosphorylation of PDK2 indicating that it exerted its effect on PDKs directly. Inhibition of PDK1 by R-lipoic acid was not altered by ADP but was decreased in the presence of pyruvate which itself inhibits PDKs. An inhibitory effect of lipoic acid on PDKs would result in less phosphorylation of E1 and hence increased PDC activity. This finding provides a possible mechanism for a glucose (and lactate) lowering effect of R-lipoic acid in diabetic subjects.

What makes an autoantigen an autoantigen?

Multiple classes of proteins are modified to tailor them for specific physiological roles. The nature of these posttranslational modifications of proteins, as well as the relationships between them including those of the immune system proteins themselves, and immune system responses are reviewed. Aspects of protein posttranslational modification and their relationship to the pathogenesis of several autoimmune diseases and primary biliary cirrhosis are highlighted.

Phylogenetic and immunological definition of four lipoylated proteins from Novosphingobium aromaticivorans, implications for primary biliary cirrhosis

Novosphingobium aromaticivorans, a unique ubiquitous bacterium that metabolizes xenobiotics and activates environmental estrogens, has been suggested as a pathogenic factor in the development of primary biliary cirrhosis (PBC). To define the molecular basis of PBC sera reactivity, we investigated the characteristic of the bacterial antigens involved. We cloned and sequenced four genes from N. aromaticivorans coding for immunoreactive proteins, arbitrarily named Novo 1 through Novo 4. We subsequently analyzed these proteins for their homology to known mitochondrial proteins and defined their reactivity using monoclonal antibodies (mAbs), rabbit anti-lipoic acid antibody, and PBC/control sera. Moreover, we studied their phylogenetic relation with the known PBC autoantigens. Novo proteins have an extraordinary degree of amino acid homology with all of the major human mitochondrial autoantigens PDC-E2 (Novo 1 and 2), OGDC-E2 (Novo 3), and BCOADC-E2 (Novo 4). Moreover, Novo 1-4 contain a lipoylated domain, are recognized by AMA-positive sera, and react with specific mAbs to mitochondrial antigens. Interestingly, the phylogenetic relation of the proteins emphasizes the conservation of the lipoylated domain. In conclusion, our data provide a high degree of confidence that N. aromaticivorans may potentiate the breakdown of self tolerance in genetically susceptible individuals.

Primary biliary cirrhosis is characterized by IgG3 antibodies cross-reactive with the major mitochondrial autoepitope and its Lactobacillus mimic

The serological hallmark of primary biliary cirrhosis (PBC) is the presence of pyruvate dehydrogenase complex E2 subunit (PDC-E2) antimitochondrial antibodies (AMAs). Anti-PDC-E2 antibodies cross-react specifically with mycobacterial hsp65, and we have demonstrated that the motif SxGDL[ILV]AE shared by PDC-E2(212-226) and hsp's is a cross-reactive target. Having found that this same motif is present only in beta-galactosidase of Lactobacillus delbrueckii (BGAL LACDE), we hypothesized that this homology would also lead to cross-reactivity. The mimics were tested via ELISA for reactivity and competitive cross-reactivity using sera from 100 AMA-positive and 23 AMA-negative PBC patients and 190 controls. An Escherichia coli (ECOLI) PDC-E2 mimic that has been pathogenetically linked to PBC but lacks this motif has been also tested. Anti-BGAL(266-280) LACDE antibodies were restricted to AMA-positive patients (54 of 95, 57%) and belonged to immunoglobulin (Ig) G3. Of the 190 controls, 22 (12%; P < .001) had anti-BGAL(266-280) antibodies, mainly of the IgG4 subclass. ECOLI PDC-E2 reactivity was virtually absent. BGAL(266-280)/PDC-E2(212-226) reactivity of the IgG3 isotype was found in 52 (52%) AMA-positive PBC patients but in only 1 of the controls (P < .001). LACDE BGAL(266-280)/PDC-E2(212-226) reactivity was due to cross-reactivity as confirmed via competition ELISA. Antibody affinity for BGAL(266-280) was greater than for PDC-E2 mimics. Preincubation of a multireactive serum with BGAL(266-280) reduced the inhibition of enzymatic activity by 40%, while marginal effect (12%) or no effect (2%) was observed in human or ECOLI PDC-E2 mimics. In conclusion, IgG3 antibodies to BGAL LACDE cross-react with the major mitochondrial autoepitope and are characteristic of PBC.

Clinical and genetic spectrum of pyruvate dehydrogenase deficiency: Dihydrolipoamide acetyltransferase (E2) deficiency

Pyruvate dehydrogenase deficiency is a major cause of primary lactic acidosis and neurological dysfunction in infancy and early childhood. Most cases are caused by mutations in the X-linked gene for the E1alpha subunit of the complex. Mutations in DLAT, the gene encoding dihydrolipoamide acetyltransferase, the E2 core component of the complex, have not been described previously. We report two unrelated patients with pyruvate dehydrogenase deficiency caused by defects in the E2 subunit. Both patients are less severely affected than typical patients with E1alpha mutations and both have survived well into childhood. Episodic dystonia was the major neurological manifestation, with other more common features of pyruvate dehydrogenase deficiency, such as hypotonia and ataxia, being less prominent. The patients had neuroradiological evidence of discrete lesions restricted to the globus pallidus, and both are homozygous for different mutations in the DLAT gene. The clinical presentation and neuroradiological findings are not typical of pyruvate dehydrogenase deficiency and extend the clinical and mutational spectrum of this condition.

Chemical xenobiotics and mitochondrial autoantigens in primary biliary cirrhosis: identification of antibodies against a common environmental, cosmetic, and food additive, 2-octynoic acid

Emerging evidence has suggested environmental factors as causative agents in the pathogenesis of primary biliary cirrhosis (PBC). We have hypothesized that in PBC the lipoyl domain of the immunodominant E2 component of pyruvate dehydrogenase (PDC-E2) is replaced by a chemical xenobiotic mimic, which is sufficient to break self-tolerance. To address this hypothesis, based upon our quantitative structure-activity relationship data, a total of 107 potential xenobiotic mimics were coupled to the lysine residue of the immunodominant 15 amino acid peptide of the PDC-E2 inner lipoyl domain and spotted on microarray slides. Sera from patients with PBC (n = 47), primary sclerosing cholangitis (n = 15), and healthy volunteers (n = 20) were assayed for Ig reactivity. PBC sera were subsequently absorbed with native lipoylated PDC-E2 peptide or a xenobiotically modified PDC-E2 peptide, and the remaining reactivity analyzed. Of the 107 xenobiotics, 33 had a significantly higher IgG reactivity against PBC sera compared with control sera. In addition, 9 of those 33 compounds were more reactive than the native lipoylated peptide. Following absorption, 8 of the 9 compounds demonstrated cross-reactivity with lipoic acid. One compound, 2-octynoic acid, was unique in both its quantitative structure-activity relationship analysis and reactivity. PBC patient sera demonstrated high Ig reactivity against 2-octynoic acid-PDC-E2 peptide. Not only does 2-octynoic acid have the potential to modify PDC-E2 in vivo but importantly it was/is widely used in the environment including perfumes, lipstick, and many common food flavorings.

Autoimmune epitopes: autoepitopes

The identity of reactants for autoantibodies has been successively refined from whole cellular organelles (immunofluorescence), identified molecules (immunoblot; gene expression libraries), epitope regions (truncated cDNAs; peptide scanning) to contact residues, as described here. Most autoantibodies react with conformational epitopes, in which amino acids distant in the linear sequence come into contiguity by protein folding. Identification of contact sites with the antibody paratope requires particular technologies, crystallography, or antibody screening of phage-displayed random peptide libraries. The latter is illustrated by our studies on the autoepitope for anti-PDC-E2 (AMA) in primary biliary cirrhosis (PBC), anti-GAD65 in type 1 diabetes, and anti-C1 of type II collagen in collagen-induced arthritis. More precise definition of the structure of conformational autoepitopes could (a) clarify controversial aspects of autoimmunity including epitope mimicry, epitope spreading, and molecular spatial relationships between B and T cell autoepitopes, and (b) impact on novel diagnostic and therapeutic (vaccine) molecules.

Structure and function of carnitine acyltransferases

Carnitine acyltransferases catalyze the exchange of acyl groups between carnitine and coenzyme A (CoA). These enzymes include carnitine acetyltransferase (CrAT), carnitine octanoyltransferase (CrOT), and carnitine palmitoyltransferases (CPTs). CPT-I and CPT-II are crucial for the beta-oxidation of long-chain fatty acids in the mitochondria by enabling their transport across the mitochondrial membrane. The activity of CPT-I is inhibited by malonyl-CoA, a crucial regulatory mechanism for fatty acid oxidation. Mutation or dysregulation of the CPT enzymes has been linked to many serious, even fatal human diseases, and these enzymes are promising targets for the development of therapeutic agents against type 2 diabetes and obesity. We have determined the crystal structures of murine CrAT, alone and in complex with its substrate carnitine or CoA. The structure contains two domains. Surprisingly, these two domains share the same backbone fold, which is also similar to that of chloramphenicol acetyltransferase and dihydrolipoyl transacetylase. The active site is located at the interface between the two domains, in a tunnel that extends through the center of the enzyme. Carnitine and CoA are bound in this tunnel, on opposite sides of the catalytic His343 residue. The structural information provides a molecular basis for understanding the catalysis by carnitine acyltransferases and for designing their inhibitors. In addition, our structural information suggests that the substrate carnitine may assist the catalysis by stabilizing the oxyanion in the reaction intermediate.

Fate of Immortalized Human Neuronal Progenitor Cells Transplanted in Rat Spinal Cord

BACKGROUND

Replacement of neurons and glia by transplantation has been proposed as a therapy for neurodegenerative diseases, including amyotrophic lateral sclerosis. This strategy requires using human motor neuronal progenitor cells or xenografts of animal cells, but there is little evidence that xenografted neuronal cells can survive in spinal cord despite immunosuppression.

OBJECTIVE

To clarify the mechanisms responsible for the death of xenografted neurons in spinal cord.

METHODS

Cells from an immortalized, neuronally committed, human embryonic spinal cord-derived cell line (HSP1) that expresses motor neuronal properties in vitro were transplanted into adult rat spinal cord. The rats were killed at intervals up to 8 weeks and serial sections through the graft sites were processed for immunofluorescence using primary antibodies against human nuclear and mitochondrial antigens, microtubule-associated protein 2, TUJ1, CD5, natural killer cells, and activated microglia-macrophages, caspase-3 and caspase-9.

RESULTS

Grafted cells did not migrate and underwent partial differentiation along a neuronal pathway. They were rejected after 4 weeks despite cyclosporine immunosuppression. Cells died by apoptosis via the cytochrome c/caspase-9/caspase-3 pathway. The host response included natural killer cells and activated microglia-macrophages but few T cells.

CONCLUSIONS

Intraspinal neuronal xenotransplantation failed because of apoptotic cell death. Neither T cells nor the spinal cord environment, which favors gliogenesis, are likely to have been responsible, but natural killer cells may have been involved.

Amino-terminal residues 1-45 of the Escherichia coli pyruvate dehydrogenase complex E1 subunit interact with the E2 subunit and are required for activity of the complex but not for reductive acetylation of the E2 subunit

While N-terminal amino acids 1-55 are not seen in the structure of the Escherichia coli pyruvate dehydrogenase complex E1 subunit (PDHc-E1), mass spectrometric analysis indicated that this amino-terminal region of PDHc-E1 was protected by PDHc-E2. Hence, five deletion constructs of PDHc-E1 were created, Delta6-15, Delta16-25, Delta26-35, Delta36-45, and Delta46-55, along with single-site substitutions at Asp7, Asp9, Pro10, Ile11, Glu12, Thr13, Arg14, and Asp15. The decarboxylation of pyruvate and the ability of PDHc-E1 to dimerize are not affected by any of the deletions or substitutions. While Delta46-55 and the Pro10Ala, Ile11Ala, and Thr13Ala variants could form a complex with PDHc-E2, and produced NADH in the overall assay, Delta16-25, Delta26-35, and Delta36-45 and the Asp7Ala, Asp9Ala, Glu12Gln, Glu12Asp, Arg14Ala, and Asp15Ala variants failed in both respects. Remarkably, all constructs of PDHc-E1 from E. coli, as well as PDHc-E1 from Mycobacterium tuberculosis, could carry out reductive acetylation of the E. coli lipoyl domain, but only constructs of the E. coli PDHc-E1 could reductively acetylate E. coli PDHc-E2. It was concluded that there are at least two loci of interaction between the PDHc-E1 and PDHc-E2 subunits: (1) the thiamin diphosphate-bound substrate on PDHc-E1 and the lipoylamide of PDHc-E2, as reflected by the ability to reductively acetylate the latter; and (2) amino terminal residues 1-45 of PDHc-E1 with regions of PDHc-E2 (so far undefined for the E. coli complex), as reflected by the overall activity of the entire complex. These studies add important information regarding recognition within this multienzyme complex class with an alpha(2) E1 assembly.

Disease-specific cross-reactivity between mimicking peptides of heat shock protein of Mycobacterium gordonae and dominant epitope of E2 subunit of pyruvate dehydrogenase is common in Spanish but not British patients with primary biliary cirrhosis

Previous studies on Spanish patients with Primary Biliary Cirrhosis (PBC) have shown extensive, disease-specific cross-reactivity between the 65-kDa heat shock protein (hsp65) of Mycobacterium gordonae and pyruvate dehydrogenase complex-E2 (PDC-E2), the major target of anti-mitochondrial antibody (AMA). Studies on a British population were unable to substantiate these findings. Having found that there is an excellent and almost unique match between the PDC-E2 autoepitope and a sequence in mycobacterial hsp65s, we tested the corresponding peptides by ELISA for cross-reactivity using sera from 90 PBC patients, 40 Spanish and 50 British, and 84 pathological controls. Reactivity to the MYCGO hsp65(90-104)/human PDC-E2(212-226)pair was present in 19 (47.5%) Spanish PBC patients and in 2 (4%) of the 50 British. Reactivity was not seen in any of the controls. Simultaneous reactivity to mimics was due to cross-reactivity as confirmed by inhibition studies. Three dimensional modelling predicts mycobacterial hsp65(90-104)to be exposed on the surface of the protein. The affinity of anti-hsp65(90-104)antibody was higher than that of anti-PDC-E2(212-226). Hsp65(90-104)is a target of disease-specific cross-reactivity to PDC-E2(212-226). The geographical confinement of this phenomenon is probably the result of complex genetic, environmental and immunological interaction.

Pyruvate dehydrogenase kinase isoform 2 activity limited and further inhibited by slowing down the rate of dissociation of ADP

Pyruvate dehydrogenase kinase 2 (PDK2) activity is enhanced by the dihydrolipoyl acetyltransferase core (E2 60mer) that binds PDK2 and a large number of its pyruvate dehydrogenase (E1) substrate. With E2-activated PDK2, K(+) at approximately 90 mM and Cl(-) at approximately 60 mM decreased the K(m) of PDK2 for ATP and competitive K(i) for ADP by approximately 3-fold and enhanced pyruvate inhibition. Comparing PDK2 catalysis +/- E2, E2 increased the K(m) of PDK2 for ATP by nearly 8-fold (from 5 to 39 microM), increased k(cat) by approximately 4-fold, and decreased the requirement for E1 by at least 400-fold. ATP binding, measured by a cold-trapping technique, occurred at two active sites with a K(d) of 5 microM, which equals the K(m) and K(d) of PDK2 for ATP measured in the absence of E2. During E2-aided catalysis, PDK2 had approximately 3 times more ADP than ATP bound at its active site, and the pyruvate analogue, dichloroacetate, led to 16-fold more ADP than ATP being bound (no added ADP). Pyruvate functioned as an uncompetitive inhibitor versus ATP, and inclusion of ADP transformed pyruvate inhibition to noncompetitive. At high pyruvate levels, pyruvate was a partial inhibitor but also induced substrate inhibition at high ATP levels. Our results indicate that, at physiological salt levels, ADP dissociation is a limiting step in E2-activated PDK2 catalysis, that PDK2.[ADP or ATP].pyruvate complexes form, and that PDK2.ATP.pyruvate.E1 reacts with PDK2.ADP.pyruvate accumulating.

Pyruvate dehydrogenase kinase isoform 2 activity stimulated by speeding up the rate of dissociation of ADP

Pyruvate dehydrogenase kinase 2 (PDK2) activity is stimulated by NADH and NADH plus acetyl-CoA via the reduction and reductive acetylation of the lipoyl groups of the dihydrolipoyl acetyltransferase (E2) component. Elevated K(+) and Cl(-) were needed for significant stimulation. Stimulation substantially increased both k(cat) and the K(m) for ATP; the fractional stimulation increased with the level of ATP. With an E2 structure lacking the pyruvate dehydrogenase (E1) binding domain, stimulation of PDK2 was retained, the K(m) for E1 decreased, and the equilibrium dissociation constant for ATP increased but remained much lower than the K(m) for ATP. Stimulation of PDK2 activity greatly reduced the fraction of bound ADP. These results fit an ordered reaction mechanism with ATP binding before E1 and stimulation increasing the rate of dissociation of ADP. Conversion of all of the lipoyl groups in the E2 60mer to the oxidized form (E2(ox)) greatly reduced k(cat) and the K(m) of PDK2 for ATP. Retention over an extended period of time of a low portion of reduced lipoyl groups maintains E2 in a state that supported much higher PDK2 activity than short-term (5 min) reduction of a large portion of lipoyl groups of E2(ox), but reduction of E2(ox) produced a larger fold stimulation. Reduction and to a greater extent reductive acetylation increased PDK2 binding to E2; conversion to E2(ox) did not significantly hinder binding. We suggest that passing even limited reducing equivalents among lipoyl groups maintains E2 lipoyl domains in a conformation that aids kinase function.

Sidechain biology and the immunogenicity of PDC-E2, the major autoantigen of primary biliary cirrhosis

The E2 component of mitochondrial pyruvate dehydrogenase complex (PDC-E2) is the immunodominant autoantigen of primary biliary cirrhosis. Whereas lipoylation of PDC-E2 is essential for enzymatic activity and predominates under normal conditions, other biochemical systems exist that also target the lysine residue, including acylation of fatty acids or xenobiotics and ubiquitinylation. More importantly, the immunogenicity can be affected by derivatization of the lysine residue, as the recognition of lipoylated PDC-E2 by patient autoantibodies is enhanced compared with octanoylated PDC-E2. Furthermore, our laboratory has shown that various xenobiotic modifications of a peptide representing the immunodominant region of PDC-E2 are immunoreactive against patient sera. The only purported regulatory system that prevents the accumulation of potentially autoreactive PDC-E2 is glutathionylation, in which the lysine-lipoic acid moiety is further modified with glutathione during apoptosis. Interestingly, this system is found in several cell lines, including HeLa, Jurkat, and Caco-2 cells, but not in cholangiocytes and salivary gland epithelial cells, both of which are targets for destruction in primary biliary cirrhosis. Hence, the failure of this or other regulatory system(s) may overwhelm the immune system with immunogenic PDC-E2 that can initiate the breakdown of tolerance in a genetically susceptible individual. In this review the authors survey the data available on the biochemical life of PDC-E2, with particular emphasis on the lysine residue and its known interactions with machinery involved in various posttranslational modifications.

A molecular switch and proton wire synchronize the active sites in thiamine enzymes

Thiamine diphosphate (ThDP) is used as a cofactor in many key metabolic enzymes. We present evidence that the ThDPs in the two active sites of the E1 (EC 1.2.4.1) component of the pyruvate dehydrogenase complex communicate over a distance of 20 angstroms by reversibly shuttling a proton through an acidic tunnel in the protein. This "proton wire" permits the co-factors to serve reciprocally as general acid/base in catalysis and to switch the conformation of crucial active-site peptide loops. This synchronizes the progression of chemical events and can account for the oligomeric organization, conformational asymmetry, and "ping-pong" kinetic properties of E1 and other thiamine-dependent enzymes.

Paradoxical downregulation of the glucose oxidation pathway despite enhanced flux in severe heart failure

Free fatty acid (FFA) oxidation is depressed in severe heart failure due to reduced activity of mitochondrial fatty acid oxidation enzymes. It is unknown whether the concomitant enhancement in cardiac glucose use is a consequence of reduced FFA oxidation, or also due to potentiation of the carbohydrate oxidative pathway. FFA and glucose oxidation rates were measured in vivo in 9 normal dogs and 9 dogs with pacing-induced heart failure by infusing (3)H-oleate and (14)C-glucose. FFA oxidation was lower (39 +/- 9 vs. 73 +/- 5 nmol min(-1) g(-1)), while glucose oxidation was higher (42 +/- 8 vs. 17 +/- 6 nmol min(-1) g(-1)) in failing compared to normal hearts (P < 0.05). At the end of the in vivo experiment, clamp-frozen biopsies were harvested from the left ventricle. Messenger RNAs encoding for proteins involved in both glucose and fatty acid metabolism, and for citrate synthase, were significantly reduced. Protein expression of GLUT-1 and GLUT-4, and GLUT-4 translocation to the sarcolemma showed no significant differences between the two groups despite a significant reduction in mRNAs with heart failure. GAPDH mRNA, protein expression, and activity were all reduced. The E2 subunit of pyruvate dehydrogenase was decreased both at the mRNA and protein level, with no effect on either fractional or maximal activity. In conclusion, we found either no changes or moderate downregulation of key enzymes of the carbohydrate metabolism in failing hearts, which suggests that the increase in glucose oxidation in vivo was principally due to impaired FFA oxidation and that the maximal myocardial capacity to obtain energy from substrate is globally depressed.

[Prediction and identification of autoepitopes of PDC-E2 specific CD8+ CTL in primary biliary cirrhosis patients]

OBJECTIVE

To identify autoepitopes of E2 subunit of pyruvate dehydrogenase complex (PDC-E2) specific CD8+ CTL in primary biliary cirrhosis (PBC) patients.

METHODS

An online database SYFPEITHI was applied to predict HLA-A*0201 restricted epitopes which located in PDC-E2 30-50 aa and 150-190 aa where B-cell epitopes clustered with CD4+ T-cell epitopes. T2 cell line reconstitution and stabilization assay, induction of specific CTL lines from peripheral blood mononuclear cells (PBMCs) of patients with PBC and cytotoxicity of peptides-induced CTL were performed to screen the epitopes from those candidates.

RESULTS

Five potential epitopes were predicted by database. Of the 5 candidates, two peptides 159-167 aa and 165-174 aa, with highly binding activity to HLA-A*0201 molecules, could stimulate PBMCs from most HLA-A*0201 positive PBC patients to proliferate and peptide-induced CTL lines showed specific cytotoxicity.

CONCLUSION

Peptides of KLSEGDLLA (159-167 aa) and LLAEIETDKA (165-174 aa) in the inner lipoyl domain of PDC-E2 are HLA-A*0201 restricted CD8+ CTL immunodominant epitopes in PBC.

Extensive homology between the major immunodominant mitochondrial antigen in primary biliary cirrhosis and Helicobacter pylori does not lead to immunological cross-reactivity

BACKGROUND

Primary biliary cirrhosis (PBC) is an immune-mediated chronic cholestatic disease characterized by the presence of antibodies directed predominantly against the E2 subunit of the pyruvate dehydrogenase complex (PDC-E2). What provokes tolerance breakdown in PBC remains to be established, though there is evidence to indicate that microbes may induce anti-mitochondrial antibodies (AMA) through a mechanism of molecular mimicry.

METHODS

Having found that urease beta (UREB)(22-36) antigen of Helicobacter pylori (HELPY) shares extensive (87%) similarity with PDC-E2(212-226), the major mitochondrial autoepitope, it was hypothesized that this would also lead to cross-reactivity. The UREB/PDC-E2 mimics were thus constructed and tested by ELISA in 112 PBC patients and 114 controls.

RESULTS

Reactivity to PDC-E2(212-226) was found in 104 patients but to UREB(22-36) in only 2. In these two patients, the double reactivity was not cross-reactive. The lack of surface antibody accessibility to UREB(22-36), as demonstrated through three-dimensional model prediction analysis, may explain this unexpected finding. There was some speculation on whether HELPY UREB(22-36) might act as a cross-reactive CD4 T-cell epitope. All seven PBC patients, tested in a standard proliferation assay against PDC-E2(212-226), gave a positive response. All seven were unresponsive to HELPY UREB(22-36). The pattern of reactivity to HELPY antigens by immunoblot was similar between anti-PDC-E2-positive and negative PBC cases, as well as between PBC patients and controls.

CONCLUSION

Contrary to common belief, extensive sequence homology (molecular mimicry) between self and microbe does not necessarily result in cross-reactivity. It is therefore likely that, when present, cross-reactivity between self and microbes is of biological importance.

Xenobiotic-induced loss of tolerance in rabbits to the mitochondrial autoantigen of primary biliary cirrhosis is reversible

Previous work has demonstrated that immunization of rabbits with the xenobiotic 6-bromohexanoate coupled to BSA breaks tolerance and induces autoantibodies to mitochondria in rabbits. Such immunized rabbits develop high-titer Abs to pyruvate dehydrogenase complex (PDC)-E2, the major autoantigen of primary biliary cirrhosis. In efforts to map the fine specificity of these autoantibodies, rabbits were immunized biweekly with 6-bromohexanoate-BSA and screened for reactivity using a unique xenobiotic-peptide-agarose microarray platform with an emphasis on identifying potential structures that mimic the molecular image formed by the association of lipoic acid with the immunodominant PDC-E2 peptide. Essentially, a total of 23 xenobiotics and lipoic acid were coupled to the 12-mer peptide backbones, PDC, a mutant PDC, and albumin. As expected, we succeeded in breaking tolerance using this small organic molecule coupled to BSA. However, unlike multiple experimental methods of breaking tolerance, we report in this study that, following continued immunization, the rabbits recover tolerance. With repeated immunization, the response to the rPDC-E2 protein increased with a gradual reduction in autoantibodies against the lipoic acid-peptide, i.e., the primary tolerance-breaking autoantigen. Detailed analysis of this system may provide strategies on how to restore tolerance in patients with autoimmune disease.

Ischemic preconditioning exaggerates cardiac damage in PKC-δ null mice

Ischemic preconditioning confers cardiac protection during subsequent ischemia-reperfusion, in which protein kinase C (PKC) is believed to play an essential role, but controversial data exist concerning the PKC-delta isoform. In an accompanying study (26), we described metabolic changes in PKC-delta knockout mice. We now wanted to explore their effect on early preconditioning. Both PKC-delta(-/-) and PKC-delta(+/+) mice underwent three cycles of 5-min left descending artery occlusion/5-min reperfusion, followed by 30-min occlusion and 2-h reperfusion. Unexpectedly, preconditioning exaggerated ischemia-reperfusion injury in PKC-delta(-/-) mice. Whereas ischemic preconditioning increased superoxide anion production in PKC-delta(+/+) hearts, no increase in reactive oxygen species was observed in PKC-delta(-/-) hearts. Proteomic analysis of preconditioned PKC-delta(+/+) hearts revealed profound changes in enzymes related to energy metabolism, e.g., NADH dehydrogenase and ATP synthase, with partial fragmentation of these mitochondrial enzymes and of the E(2) component of the pyruvate dehydrogenase complex. Interestingly, fragmentation of mitochondrial enzymes was not observed in PKC-delta(-/-) hearts. High-resolution NMR analysis of cardiac metabolites demonstrated a similar rise of phosphocreatine in PKC-delta(+/+) and PKC-delta(-/-) hearts, but the preconditioning-induced increase in phosphocholine, alanine, carnitine, and glycine was restricted to PKC-delta(+/+) hearts, whereas lactate concentrations were higher in PKC-delta(-/-) hearts. Taken together, our results suggest that reactive oxygen species generated during ischemic preconditioning might alter mitochondrial metabolism by oxidizing key mitochondrial enzymes and that metabolic adaptation to preconditioning is impaired in PKC-delta(-/-) hearts.

[Analytic study of dot blotting for the detection of anti-Jo-1, anti-M2, anti-ribosomes and anti-LKM]

The Cyto-Dot 4 HM043 kit commercialised by BMD, has replaced the Cyto-Dot HM010 kit that allowed three auto-antibodies detection (anti-Jo-1, anti-M2 and anti-ribosomal protein). Detection of anti-LKM1 auto-antibody was added. These four auto-antibodies have in common only the intracytoplasmic localisation of their respective antigen. The aim of our study was to evaluate this new kit using 104 sera and to compare our results with reference techniques (indirect immunofluorescence IF for anti-M2, anti-ribosomal protein and anti-LKM1, double immunodiffusion ID for anti-Jo-1 and anti-LKM1, western blotting WB for anti-M2) and with Cyto-Dot HM010. The one hundred and four sera were divided into five groups: Group I (n = 12) with anti-Jo-1 detected by ID; Group II (n = 28) with 26 anti-M2 positive by IF and WB, 2 anti-M2 positive only by WB; Group III (n = 10) with anti-ribosomal protein detected by IF 5 of which precipitated by ID; Group IV (n = 32) with anti-LKM1 by IF and ID divided into 18 AIH2 and 14 HCV; Group V (n = 22) consisting of 14 healthy individuals and 8 patients with hypergammaglobulinemia. Results of this study are similar to those of Cyto-Dot HM010 for the three auto-antibodies already in use. Cyto-Dot 4 is a very good anti-LKM1 confirmation method as it is ID.

Crystal Structure of PapA5, a Phthiocerol Dimycocerosyl Transferase from Mycobacterium tuberculosis

Polyketide-associated protein A5 (PapA5) is an acyltransferase that is involved in production of phthiocerol and phthiodiolone dimycocerosate esters, a class of virulence-enhancing lipids produced by Mycobacterium tuberculosis. Structural analysis of PapA5 at 2.75-A resolution reveals a two-domain structure that shares unexpected similarity to structures of chloramphenicol acetyltransferase, dihydrolipoyl transacetylase, carnitine acetyltransferase, and VibH, a non-ribosomal peptide synthesis condensation enzyme. The PapA5 active site includes conserved histidine and aspartic acid residues that are critical to PapA5 acyltransferase activity. PapA5 catalyzes acyl transfer reactions on model substrates that contain long aliphatic carbon chains, and two hydrophobic channels were observed linking the PapA5 surface to the active site with properties consistent with these biochemical activities and substrate preferences. An additional alpha helix not observed in other acyltransferase structures blocks the putative entrance into the PapA5 active site, indicating that conformational changes may be associated with PapA5 activity. PapA5 represents the first structure solved for a protein involved in polyketide synthesis in Mycobacteria.

Immunochemical Identification of Coenzyme Q0-Dihydrolipoamide Adducts in the E2 Components of the α-Ketoglutarate and Pyruvate Dehydrogenase Complexes Partially Explains the Cellular Toxicity of Coenzyme Q0

Coenzyme Q(0) (Q(0)), a strong electrophile, is toxic to insulin-producing cells. Q(0) was incubated with rat and human pancreatic islets and INS-1 insulinoma cells, and its attachment to cellular proteins was studied with Western analysis using antiserum raised against the benzoquinone ring structure of ubiquinone (anti-Q). Q(0) covalently bonded to two proteins, one of 50 kDa and another of 70 kDa. Both proteins were found to be mitochondrial in human and rat islet cells and in many rat organs. Mitochondria were incubated with Q(0), and affinity-purified anti-Q was used to immunoprecipitate the 50-kDa protein. Amino acid sequencing identified it as dihydrolipoamide succinyltransferase, the E2 component of the alpha-ketoglutarate dehydrogenase complex (KDC). Western analysis also showed that Q bonds to the E2 components of the purified KDC and (0)the pyruvate dehydrogenase complex (PDC). Dihydrolipoamide acetyltransferase, the E2 of the PDC, has a molecular mass of 70 kDa, and the 70-kDa protein was inferred to be this enzyme. Q(0) was found to bond only to proteins containing dihydrolipoate, and in preparations of mitochondria, thiol reducing agents facilitated the attachment of Q(0), but oxidizing agents prevented it, suggesting that Q(0) bonds to thiols of dihydrolipoamide. Incubation of human or pig PDC with Q(0) followed by matrix-assisted laser desorption ionization time-of-flight and liquid chromatography/electrospray ionization mass spectrometry analyses of chymotrypsin-digested peptides of PDC E2 confirmed that Q(0) bonds to the dihydrolipoamide in these proteins. In mitochondria, coenzymes Q(1) and Q(2) did not bond to the 50-kDa protein but competed with the bonding of Q(0) to this protein. The prevention by Q(1) of characteristics the bonding of Q(0) to KDC E2, as well as other of the Q(0) effect, are reminiscent of the action of Q(0) on the mitochondrial permeability transition pore described previously (Fontaine, E., Ichas, F., and Bernardi, P. (1998) J. Biol. Chem. 273, 25734-25740).

Use of fusion proteins and procaryotic display systems for delivery of HIV-1 antigens: development of novel vaccines for HIV-1 infection

Two non-pathogenic scaffolds (represented by the filamentous bacteriophage fd and the dihydrolipoyl acetyltransferase E2 protein of the Bacillus stearothermophilus pyruvate dehydrogenase (PDH) complex) able to deliver human immunodeficiency virus (HIV)-1 antigenic determinants, were designed in our laboratories and investigated in controlled assay conditions. Based on a modification of the phage display technology, we developed an innovative concept for a safe and inexpensive vaccine in which conserved antigenic determinants of HIV-1 reverse transcriptase (RTase) were inserted into the N-terminal region of the major pVIII coat protein of bacteriophagefd virions. Analogously, we developed another antigen delivery system based on the E2 component from the PDH complex and capable of displaying large intact proteins on the surface of an icosahedral lattice. Our data show that both of these systems can deliver B and T epitopes to their respective presentation compartments in target cells and trigger a humoral response as well as a potent helper and cytolytic response in vitro and in vivo.

Interactions of the peripheral subunit-binding domain of the dihydrolipoyl acetyltransferase component in the assembly of the pyruvate dehydrogenase multienzyme complex of Bacillus stearothermophilus

The enzymes pyruvate decarboxylase (E1) and dihydrolipoyl dehydrogenase (E3) bind tightly but in a mutually exclusive manner to the peripheral subunit-binding domain (PSBD) of dihydrolipoyl acetyltransferase in the pyruvate dehydrogenase multienzyme complex of Bacillus stearothermophilus. The use of directed mutagenesis, surface plasmon resonance detection and isothermal titration microcalorimetry revealed that several positively charged residues of the PSBD, most notably Arg135, play an important part in the interaction with both E1 and E3, whereas Met131 makes a significant contribution to the binding of E1 only. This indicates that the binding sites for E1 and E3 on the PSBD are overlapping but probably significantly different, and that additional hydrophobic interactions may be involved in binding E1 compared with E3. Arg135 of the PSBD was also replaced with cysteine (R135C), which was then modified chemically by alkylation with increasingly large aliphatic groups (R135C -methyl, -ethyl, -propyl and -butyl). The pattern of changes in the values of DeltaG degrees, DeltaH degrees and TDeltaS degrees that were found to accompany the interaction with the variant PSBDs differed between E1 and E3 despite the similarities in the free energies of their binding to the wild-type. The importance of a positive charge on the side-chain at position 135 for the interaction of the PSBD with E3 and E1 was apparent, although lysine was found to be an imperfect substitute for arginine. The results offer further evidence of entropy-enthalpy compensation ('thermodynamic homeostasis') - a feature of systems involving a multiplicity of weak interactions.

A novel deletion creating a new terminal exon of the dihydrolipoyl transacylase gene is a founder mutation of Filipino maple syrup urine disease

Maple syrup urine disease (MSUD) is a rare, autosomal-recessive disorder of branched-chain amino-acid metabolism. In the Philippines, many MSUD cases have been diagnosed clinically. Here, molecular analysis of the dihydrolipoyl transacylase (E2) gene was done in 13 unrelated families from the Philippines. A novel deletion spanning 4.1 kb of intron 10 and 601 bp of exon 11, caused by non-homologous recombination between an L1 repeat in intron 10 and an Alu repeat in exon 11, was found in 8 out of 13 families, with 5 of them being homozygous for the mutation, implicating it as a founder mutation of Filipino MSUD. The resulting mutant E2 mRNA contains a 239-bp insertion after exon 10, thereby producing a new terminal exon. Large-scale population screening of the deletion revealed that one carrier of the mutation was identified in 100 normal Filipinos. These findings suggest that a limited number of mutations might underlie MSUD in the Filipino population, potentially facilitating prenatal diagnosis and carrier detection of MSUD in this group.

Microbial mimics are major targets of crossreactivity with human pyruvate dehydrogenase in primary biliary cirrhosis

BACKGROUND/AIMS

Previous studies on patients with primary biliary cirrhosis (PBC) have shown extensive cross-reactivity between the dominant B- and T-cell epitopes of human pyruvate dehydrogenase complex-E2 (PDC-E2), and microbial mimics. Such observations have suggested microbial infection as having a role in the induction of anti-mitochondrial antibodies, through a mechanism of molecular mimicry. However the biological significance of these cross-reactivities is questionable, because PDC-E2 is so highly conserved among various species.

METHODS

Interrogating protein databases, ten non-PDC-E2 microbial sequences with high degree of similarity to PDC-E2(212-226) were found in Escherichia coli (6), Helicobacter pylori, Pseudomonas aeruginosa, Cytomegalovirus, and Haemophilus influenzae. We report on a study testing reactivity and competitive cross-reactivity against these respective peptides, and in some cases the parent protein, using sera from 55 patients with PBC, compared to reactivity of 190 pathological and 28 healthy controls.

RESULTS

Cross-reactivity to E. coli mimics was commonly seen in PBC, and in a subset of pathological controls except where there was no evidence of urinary tract infection and correlated with anti-mitochondrial reactivity.

CONCLUSIONS

E. coli/PDC-E2 cross-reactive immunity characterizes primary biliary cirrhosis; the large number of E. coli immunogenic mimics may account for the dominance of the major PDC-E2 autoepitope.

Optimal determination of particle orientation, absolute hand, and contrast loss in single-particle electron cryomicroscopy

A computational procedure is described for assigning the absolute hand of the structure of a protein or assembly determined by single-particle electron microscopy. The procedure requires a pair of micrographs of the same particle field recorded at two tilt angles of a single tilt-axis specimen holder together with the three-dimensional map whose hand is being determined. For orientations determined from particles on one micrograph using the map, the agreement (average phase residual) between particle images on the second micrograph and map projections is determined for all possible choices of tilt angle and axis. Whether the agreement is better at the known tilt angle and axis of the microscope or its inverse indicates whether the map is of correct or incorrect hand. An increased discrimination of correct from incorrect hand (free hand difference), as well as accurate identification of the known values for the tilt angle and axis, can be used as targets for rapidly optimizing the search or refinement procedures used to determine particle orientations. Optimized refinement reduces the tendency for the model to match noise in a single image, thus improving the accuracy of the orientation determination and therefore the quality of the resulting map. The hand determination and refinement optimization procedure is applied to image pairs of the dihydrolipoyl acetyltransferase (E2) catalytic core of the pyruvate dehydrogenase complex from Bacillus stearothermophilus taken by low-dose electron cryomicroscopy. Structure factor amplitudes of a three-dimensional map of the E2 catalytic core obtained by averaging untilted images of 3667 icosahedral particles are compared to a scattering reference using a Guinier plot. A noise-dependent structure factor weight is derived and used in conjunction with a temperature factor (B=-1000A(2)) to restore high-resolution contrast without amplifying noise and to visualize molecular features to 8.7A resolution, according to a new objective criterion for resolution assessment proposed here.

Organization of the cores of the mammalian pyruvate dehydrogenase complex formed by E2 and E2 plus the E3-binding protein and their capacities to bind the E1 and E3 components

The subunits of the dihydrolipoyl acetyltransferase (E2) component of mammalian pyruvate dehydrogenase complex can form a 60-mer via association of the C-terminal I domain of E2 at the vertices of a dodecahedron. Exterior to this inner core structure, E2 has a pyruvate dehydrogenase component (E1)-binding domain followed by two lipoyl domains, all connected by mobile linker regions. The assembled core structure of mammalian pyruvate dehydrogenase complex also includes the dihydrolipoyl dehydrogenase (E3)-binding protein (E3BP) that binds the I domain of E2 by its C-terminal I' domain. E3BP similarly has linker regions connecting an E3-binding domain and a lipoyl domain. The composition of E2.E3BP was thought to be 60 E2 plus approximately 12 E3BP. We have prepared homogenous human components. E2 and E2.E3BP have s(20,w) values of 36 S and 31.8 S, respectively. Equilibrium sedimentation and small angle x-ray scattering studies indicate that E2.E3BP has lower total mass than E2, and small angle x-ray scattering showed that E3 binds to E2.E3BP outside the central dodecahedron. In the presence of saturating levels of E1, E2 bound approximately 60 E1 and maximally sedimented 64.4 +/- 1.5 S faster than E2, whereas E1-saturated E2.E3BP maximally sedimented 49.5 +/- 1.4 S faster than E2.E3BP. Based on the impact on sedimentation rates by bound E1, we estimate fewer E1 (approximately 12) were bound by E2.E3BP than by E2. The findings of a smaller E2.E3BP mass and a lower capacity to bind E1 support the smaller E3BP substituting for E2 subunits rather than adding to the 60-mer. We describe a substitution model in which 12 I' domains of E3BP replace 12 I domains of E2 by forming 6 dimer edges that are symmetrically located in the dodecahedron structure. Twelve E3 dimers were bound per E248.E3BP12 mass, which is consistent with this model.

Listeria intracellular growth and virulence require host-derived lipoic acid

Listeria monocytogenes is a Gram-positive intracytosolic pathogen that causes severe disease in pregnant and immunocompromised individuals. We found that L. monocytogenes lacking the lipoate protein ligase LplA1 was defective for growth specifically in the host cytosol and was less virulent in animals by a factor of 300. A major target for LplA1, the E2 subunit of pyruvate dehydrogenase (PDH), lacked a critical lipoyl modification when the DeltalplA1 strain was grown intracellularly, which suggests that abortive growth was due to loss of PDH function. Thus, the use of host-derived lipoic acid may be a critical process for in vivo replication of bacterial pathogens.

Distinct costimulation dependent and independent autoreactive T-cell clones in primary biliary cirrhosis

BACKGROUND & AIMS

Previous work has suggested that CD4+ CD28- or costimulation-independent T cells are increased in autoimmune diseases. In this study, we compared frequency and qualitative characteristics of autoreactive costimulation-independent or CD4+ CD28- T cells in primary biliary cirrhosis (PBC) by taking advantage of the well-defined immunodominant autoepitope of the E2 component of pyruvate dehydrogenase (PDC-E2).

METHODS

We determined the frequency of costimulation-independent autoreactive T cells that respond to PDC-E2 163-176 and the frequency of CD4+ CD28- T cells. Finally, we determined the role of biliary epithelial cells (BEC) as both an antigen-presenting cell or, alternatively, as a target cell for T-cell-mediated cytotoxicity.

RESULTS

The precursor frequency of costimulation-independent CD4+ T cells that respond to PDC-E2 163-176 and the frequency of CD4+ CD28- T cells were dramatically elevated in PBC. Furthermore, 2 types of T-cell clones that respond to PDC-E2 163-176 emerged from this study. One type was costimulation dependent and the other costimulation independent. Both types of clones lyse BEC in a similar effector target (E/T) ratio distribution. However, BEC did not help the proliferation of any T-cell clones. Furthermore, costimulation-independent T-cell clones do not become anergic by BEC.

CONCLUSIONS

In PBC, costimulation-independent autoreactive T cells, which do not become anergic, increase and maintain the autoimmune response. In controls, although autoantigens are expressed on BEC and autoantigen-reactive T cells exist around BEC, autoantigen-reactive T cells are costimulation dependent and will become anergic and maintain peripheral tolerance.

Patients with primary biliary cirrhosis react against a ubiquitous xenobiotic-metabolizing bacterium

Infectious and environmental agents have been proposed as immunologic triggers for primary biliary cirrhosis (PBC). Recently, a ubiquitous organism that metabolizes organic compounds and estrogens, Novosphingobium aromaticivorans, has been defined. Importantly, 2 bacterial proteins have homology with the E2 component of the pyruvate dehydrogenase complex (PDC-E2). Sera from 97 patients with PBC, 46 first-degree relatives, 10 spouses, and 195 controls were studied for reactivity against N. aromaticivorans and Escherichia coli. The reactivity was defined by absorption, affinity purification, and using monoclonal antibodies to PDC-E2. Stool samples from 20 patients with PBC and 34 controls were analyzed by polymerase chain reaction (PCR) for the presence of N. aromaticivorans. Sera from 100% of anti-PDC-E2 positive (77/77), 33% of anti-BCOADC E2 positive (1/3), and 12% of antimitochondrial antibody (AMA) negative patients with PBC (2/17) reacted with titers up to 10(-6) against two known lipoylated bacterial proteins (47 and 50 kd) from N. aromaticivorans, including patients with early disease. This titer was approximately 100- to 1,000-fold higher than against E. coli and verified by absorption, use of affinity-purified sera, and monoclonal antibody reagents. Moreover, 78 of 80 AMA-positive and 5 of 17 AMA-negative patients with PBC had antibodies against 3 other N. aromaticivorans proteins. In contrast, 0 of 195 control sera reacted against N. aromaticivorans. Approximately 25% of patients and controls had N. aromaticivorans in their fecal specimens. In conclusion, based on protein homology, capacity to metabolize xenobiotics as well as modulate estrogens, its presence in feces, and specific immunologic response, we propose that N. aromaticivorans is a candidate for the induction of PBC.

[Cloning and expressing the E2 subunit of pyruvate dehydrogenase complex]

OBJECTIVES

To construct the expression vector of the pyruvate dehydrogenase complex E2 subunit gene (PDC-E2).

METHODS

The PDC-E2 gene was amplified from human lymphocytes with RT-PCR, and was cloned into pExSecI vector to induce the PDC-E2 expression. The products were identified with western blot and ELISA.

RESULTS

The expression vector pExSecI/PDC-E2 was successfully constructed. The products could be identified by the specific self-antibodies in the sera from the primary biliary cirrhosis patients.

CONCLUSION

High efficient expression vector of PDC-E2 lays the foundation for serum assay of primary biliary cirrhosis patients with prokaryotic expressing PDC-E2.

Facilitated interaction between the pyruvate dehydrogenase kinase isoform 2 and the dihydrolipoyl acetyltransferase

The dihydrolipoyl acetyltransferase (E2) has an enormous impact on pyruvate dehydrogenase kinase (PDK) phosphorylation of the pyruvate dehydrogenase (E1) component by acting as a mobile binding framework and in facilitating and mediating regulation of PDK activity. Analytical ultracentrifugation (AUC) studies established that the soluble PDK2 isoform is a stable dimer. The interaction of PDK2 with the lipoyl domains of E2 (L1, L2) and the E3-binding protein (L3) were characterized by AUC. PDK2 interacted very weakly with L2 (Kd approximately 175 microM for 2 L2/PDK2) but much tighter with dimeric glutathione S-transferase (GST)-L2 (Kd approximately 3 microM), supporting the importance of bifunctional binding. Reduction of lipoyl groups resulted in approximately 8-fold tighter binding of PDK2 to GST-L2red, which was approximately 300-fold tighter than binding of 2 L2red and also much tighter than binding by GST-L1red and GST-L3red. The E2 60-mer bound approximately 18 PDK2 dimers with a Kd similar to GST-L2. E2.E1 bound more PDK2 (approximately 27.6) than E2 with approximately 2-fold tighter affinity. Lipoate reduction fostered somewhat tighter binding at more sites by E2 and severalfold tighter binding at the majority of sites on E2.E1. ATP and ADP decreased the affinity of PDK2 for E2 by 3-5-fold and adenosine 5'-(beta,gamma-imino)triphosphate or phosphorylation of E1 similarly reduced PDK2 binding to E2.E1. Reversible bifunctional binding to L2 with the mandatory singly held transition fits the proposed "hand-over-hand" movement of a kinase dimer to access E1 without dissociating from the complex. The gain in binding interactions upon lipoate reduction likely aids reduction-engendered stimulation of PDK2 activity; loosening of binding as a result of adenine nucleotides and phosphorylation may instigate movement of lipoyl domain-held kinase to a new E1 substrate.

Proteasome is required for class I-restricted presentation by Fcgamma receptor-mediated endocytosis in primary biliary cirrhosis

There is a considerable database on the effector mechanisms for CD8 recognition of PDC-E2 in primary biliary cirrhosis (PBC). In particular, the specific roles of MHC class I, the mitochondrial autoepitope, and the liver-specific T cell precursor frequency, are defined for HLA-A2.1 patients. There is evidence for a role of MHC class I-mediated presentation of exogenous antigens, or cross-presentation, in the development of the antimitochondrial response and a contributory role of Fcgamma receptor-mediated uptake of autoantigen-autoantibody complexes for the induction of a PDC-E2 specific autoreactive CTL response. Based on this background, we examined potential intracellular pathways for processing the immunodominant mitochondrial autoantigen, PDC-E2, by dendritic cells (DC). In particular, we studied the effects of the proteasome inhibitor lactacystin and the endosomal acidification inhibitor bafilomycin on the induction of PDC-E2-specific CTL response in PBC. Importantly, our data indicate that pre-treatment with either lactacystin or bafilomycin inhibits the PDC-E2 immune complex-induced CTL response. The processing and presentation of PDC-E2 by CD8(+)T cells is mediated by proteasomes and facilitated by Fcgamma receptor-mediated endocytosis. This data reflects another layer of interaction between components of the immune system in the development of autoimmunity. Further characterization of autoantigen uptake and processing may lead to potential therapeutic intervention.

Does a betaretrovirus infection trigger primary biliary cirrhosis?

Patients with primary biliary cirrhosis develop progressive ductopenia associated with the production of antimitochondrial antibodies that react with a protein aberrantly expressed on biliary epithelial cells and peri-hepatic lymph nodes. Although no specific microbe has been identified, it is thought that an infectious agent triggers this autoimmune liver disease in genetically predisposed individuals. Previous serologic studies have provided evidence to suggest a viral association with primary biliary cirrhosis. Here we describe the identification of viral particles in biliary epithelium by electron microscopy and the cloning of exogenous retroviral nucleotide sequences from patients with primary biliary cirrhosis. The putative agent is referred to as the human betaretrovirus because it shares close homology with the murine mammary tumor virus and a human retrovirus cloned from breast cancer tissue. In vivo, we have found that the majority of patients with primary biliary cirrhosis have both RT-PCR and immunohistochemistry evidence of human betaretrovirus infection in lymph nodes. Moreover, the viral proteins colocalize to cells demonstrating aberrant autoantigen expression. In vitro, we have found that lymph node homogenates from patients with primary biliary cirrhosis can induce autoantigen expression in normal biliary epithelial cells in coculture. Normal biliary epithelial cells also develop the phenotypic manifestation of primary biliary cirrhosis when cocultivated in serial passage with supernatants containing the human betaretrovirus or the murine mammary tumor virus, providing a model to test Koch's postulates in vitro.

Conformational flexibility of pyruvate dehydrogenase complexes: a computational analysis by quantized elastic deformational model

Pyruvate dehydrogenase complex (PDC) is one of the largest multienzyme complexes known and consists of a dodecahedral E2 core to which other components are attached. We report the results of applying a new computational method, quantized elastic deformational model, to simulating the conformational fluctuations of the truncated E2 core, using low-resolution electron cryomicroscopy density maps. The motional features are well reproduced; especially, the symmetric breathing mode revealed in simulation is nearly identical with what was observed experimentally. Structural details of the motions of the trimeric building blocks, which are critical to facilitating the global expansion and contraction of the complex, were revealed. Using the low-resolution maps from electron cryomicroscopy reconstructions, the simulations showed a picture of the motional mechanism of the PDC core, which is an example without precedent of thermally activated global dynamics. Moreover, the current results support an earlier suggestion that, at low resolution and without the use of amino acid sequence and atomic coordinates, it is possible for computer simulations to provide an accurate description of protein dynamics.

Immunization with a xenobiotic 6-bromohexanoate bovine serum albumin conjugate induces antimitochondrial antibodies

The E2 subunit of pyruvate dehydrogenase complex (PDC-E2) is the major autoantigen recognized by antimitochondrial Abs (AMA) in primary biliary cirrhosis (PBC). Recently, we replaced the lipoic acid moiety of PDC-E2 with a battery of synthetic structures designed to mimic a xenobiotically modified lipoyl hapten on a 12-aa peptide that was found within the immunodominant autoepitope of PDC-E2 and demonstrated that AMA in PBC reacted against several organic modified mimotopes as well as, or sometimes significantly better than, the native lipoyl domain. Based on this data, we immunized rabbits with one such xenobiotic organic compound, 6-bromohexanoate, coupled to BSA. One hundred percent of immunized rabbits developed AMA that have each and every characteristic of human AMAs with reactivity against PDC-E2, E2 subunit of branched chain 2-oxo-acid dehydrogenase, and E2 subunit of 2-oxoglutarate dehydrogenase complex. The rabbit AMA also inhibited enzymatic function of PDC-E2 and, importantly, binds to peptide sequences not present in the xenobiotic carrier immunogen. In contrast, BSA-immunized controls did not produce such activity. Our observation that animals immunized with a xenobiotic BSA complex produce autoantibodies that react not only with the xenobiotic, but also with mitochondrial autoantigens recognized by autoimmune PBC sera, suggests that environmental xenobiotic agents can be a risk factor for the induction of PBC.

Differential effect of DCA treatment on the pyruvate dehydrogenase complex in patients with severe PDHC deficiency

Dichloroacetate (DCA) is a structural analog of pyruvate that has been recommended for the treatment of primary lactic acidemia, particularly in patients with pyruvate dehydrogenase (PDHC) deficiency. Recent reports have demonstrated that the response to DCA may depend on the type of molecular abnormality. In this study, we investigated the response to DCA in various PDHC-deficient cell lines and tried to determine the mechanism involved. The effect of chronic 3-d DCA treatment on PDHC activity was assessed in two PDHC-deficient cell lines, each with a different point mutation in the E1alpha subunit gene (R378C and R88C), and one cell line in which an 8-bp tandem repeat was deleted (W383 del). Only two (R378C and R88C) of the three PDHC-deficient cell lines with very low levels of PDHC activity and unstable polypeptides were sensitive to chronic DCA treatment. In these cell lines, DCA treatment resulted in an increase in PDHC activity by 125 and 70%, respectively, with concomitant increases of 121 and 130% in steady-state levels of immunoreactive E1alpha. DCA treatment reduced the turnover of the E1alpha subunit in R378C and R88C mutant cells with no significant effect on the E1beta subunit. Chronic DCA treatment significantly improved the metabolic function of PDHC in digitonin-permeabilized R378C and R88C fibroblasts. The occurrence of DCA-sensitive mutations suggests that DCA treatment is potentially useful as an adjuvant to ketogenic and vitamin treatment in PDHC-deficient patients.

Correlation between histopathological findings of the liver and IgA class antibodies to 2-oxo-acid dehydrogenase complex in primary biliary cirrhosis

Although anti-mitochondrial antibody (AMA) is the characteristic serological feature of primary biliary cirrhosis (PBC), its pathogenetic role remains unclear. We tested sera from 72 Japanese patients with histologically confirmed PBC for AMA by indirect immunofluorescence, anti-pyruvate dehydrogenase complex (PDC) by enzyme inhibition assay, immunoglobulin (Ig) G class anti-PDC by ELISA, and IgG, IgM, and IgA class anti-2-oxo-acid dehydrogenase complex (2-OADC) by immunoblotting. Of the 72 sera, 60 (83%), 50 (69%), 42 (58%), and 71 (99%) were positive for AMA by immunofluorescence, enzyme inhibition assay, ELISA, and immunoblotting, respectively. There was no significant correlation between histological stages and AMA by immunofluorescence, PDC inhibitory antibodies by enzyme inhibition assay, IgG class anti-PDC antibodies by ELISA, or IgG and IgM class anti-2-OADC by immunoblotting. IgA class anti-2-OADC by immunoblotting was more frequent in stages 2-4 than in stage 1 (P = 0.0083). Of the IgA class anti-2-OADC, anti-PDC-E2 (74 kDa) and anti-E3BP (52 kDa) were more frequent in stages 2-4 than in stage 1 (P = 0.0253 and 0.0042, respectively). Further examination of histopathological findings in 53 of 72 liver biopsy specimens showed that IgA class anti-PDC-E2 and IgA class anti-E3BP were associated with bile duct loss, and IgA class anti-PDC-E2 was also associated with interface hepatitis and atypical ductular proliferation. IgA is known to be secreted into the bile through biliary epithelial cells, implying that IgA class anti-PDC-E2 and E3BP may have a specific pathogenetic role during their transport into the bile by binding to their target antigen(s) in biliary epithelial cells, and this may be followed by dysfunction and finally destruction of biliary epithelial cells. Our present results suggest that these autoantibodies against 2-OADC detected by immunoblotting may be associated with the pathogenesis and pathologic progression of PBC.

Promiscuous T cells selected by Escherichia coli: OGDC-E2 in primary biliary cirrhosis

The etiology of primary biliary cirrhosis (PBC) remains enigmatic. One theory that has attracted attention proposes that PBC is induced via molecular mimicry with Escherichia coli. If molecular mimicry is responsible for the immunogenic response in PBC, then T cell clones specific for E. coli antigens should stimulate and be cross-reactive with peptides specific for the human immunodominant autoepitopes. To address this issue, we developed T cell clones specific for E. coli OGDC-E2 peptide. Importantly, we demonstrate the presence of T cell clones specific for E. coli OGDC-E2 that react promiscuously with the human mitochondrial equivalents. Indeed, there was a significant increase in the liver derived T cell precursor frequency of such reactivity and such liver clones were only found in patients with PBC. In conclusion, these data suggest that PBC is a multi-hit disease involving a genetic predisposition, a mucosal response, and activation of promiscuous T cells; such activation may occur either directly from bacterial antigens, or indirectly through chemically-modified bacterial antigens. Dissection of the mechanisms involved will lead not only to understanding the immunogenetic basis of PBC, but likely its pathogenic etiology.

Evaluation of newly developed ELISA using "MESACUP-2 test mitochondrial M2" kit for the diagnosis of primary biliary cirrhosis

OBJECTIVES

An enzyme-linked immunosorbent assay (ELISA) using MESACUP-2 Test Mitochondria M2 kit (new-M2 ELISA) has recently become commercially available. The aim of this study was to evaluate the clinical utility of this newly developed ELISA for the diagnosis of primary biliary cirrhosis (PBC).

DESIGN AND METHODS

We tested the immunoreactivity of sera from 82 Japanese PBC patients to the 2-oxo-acid dehydrogenase complex (2-OADC) enzymes by indirect immunofluorescence, enzyme inhibition assay using commercially available TRACE Enzymatic Mitochondrial Antibody (M2) Assay (EMA) kit, commercial ELISAs using MESACUP Mitochondria M2 kit (old-M2 ELISA) and new-M2 ELISA, and immunoblotting on bovine heart mitochondria.

RESULTS

Each test gave the following positive results; antimitochondrial antibodies (AMA) by immunofluorescence in 71 (87%) out of the 82 sera, enzymatic inhibitory antibody to pyruvate dehydrogenase complex (PDC) by EMA in 61 (74%), immunoglobulin (Ig) G class anti-PDC antibody by old-M2 ELISA in 55 (67%), IgG/M/A class anti-E2 subunit of PDC (PDC-E2)/anti-E2 subunit of branched chain oxo-acid dehydrogenase complex (BCOADC-E2)/anti-E2 subunit of 2-oxoglutarate dehydrogenase complex (OGDC-E2) antibodies by new-M2 ELISA in 73 (89%), and IgG, IgM, or IgA class antibodies against at least one of the 2-OADC enzymes by immunoblotting in 82 (100%). Fifty-three of the 82 sera (65%) were all positive by these five assays. Of the 18 sera that were positive by new-M2 ELISA but negative by old-M2 ELISA, 12 were theoretically interpretable. Of the 11 sera that were negative for AMA by immunofluorescence but positive for at least one of anti-2-OADC enzymes by immunoblotting, four (36%) were positive by new-M2 ELISA, whereas only two and one sera were positive by EMA and old-M2 ELISA, respectively.

CONCLUSIONS

Our results indicated that the sensitivity of the newly developed new-M2 ELISA was higher than that of EMA and old-M2 ELISA, and comparable with that of immunofluorescence. However, it is still unclear whether the new-M2 ELISA could replace the conventional immunofluorescence testing for routine assay requests because six (7%) sera showed discrepant results between these two assays.