Dichloroacetate stabilizes the mutant E1alpha subunit in pyruvate dehydrogenase deficiency

Pyruvate Dehydrogenase Kinase Inhibition by Dichloroacetate in Melanoma Cells Unveils Metabolic Vulnerabilities

Melanoma is characterized by high glucose uptake, partially mediated through elevated pyruvate dehydrogenase kinase (PDK), making PDK a potential treatment target in melanoma. We aimed to reduce glucose uptake in melanoma cell lines through PDK inhibitors dichloroacetate (DCA) and AZD7545 and through PDK knockdown, to inhibit cell growth and potentially unveil metabolic co-vulnerabilities resulting from PDK inhibition. MeWo cells were most sensitive to DCA, while SK-MEL-2 was the least sensitive, with IC₅₀ values ranging from 13.3 to 27.0 mM. DCA strongly reduced PDH phosphorylation and increased the oxygen consumption rate:extracellular acidification rate (OCR:ECAR) ratio up to 6-fold. Knockdown of single PDK isoforms had similar effects on PDH phosphorylation and OCR:ECAR ratio as DCA but did not influence sensitivity to DCA. Growth inhibition by DCA was synergistic with the glutaminase inhibitor CB-839 (2- to 5-fold sensitization) and with diclofenac, known to inhibit monocarboxylate transporters (MCTs) (3- to 8-fold sensitization). CB-839 did not affect the OCR:ECAR response to DCA, whereas diclofenac strongly inhibited ECAR and further increased the OCR:ECAR ratio. We conclude that in melanoma cell lines, DCA reduces proliferation through reprogramming of cellular metabolism and synergizes with other metabolically targeted drugs.

Potential role of stress-induced gluconeogenesis in disease aggravation and mortality in pyruvate dehydrogenase deficiency: A case-based hypothesis

Pyruvate dehydrogenase (PDH) deficiency is an inherited metabolic disorder caused by a defect in any subunit of the pyruvate dehydrogenase complex (PDHC), which has an essential role in glucose metabolism. The causes of disease progression in PDH deficiency are not fully understood yet. Based on repeated observations of a patient with PDH deficiency at our center, we hypothesized that stress-induced gluconeogenesis contributes to rapid exacerbation of the disease. This link has not been established previously.

Pyruvate dehydrogenase complex deficiency and its relationship with epilepsy frequency--An overview

The pyruvate dehydrogenase complex (PDHc) is a member of a family of multienzyme complexes that provides the link between glycolysis and the tricarboxylic acid (TCA) cycle by catalyzing the physiologically irreversible decarboxylation of various 2-oxoacid substrates to their corresponding acyl-CoA derivatives, NADH and CO2. PDHc deficiency is a metabolic disorder commonly associated with lactic acidosis, progressive neurological and neuromuscular degeneration that vary with age and gender. In this review, we aim to discuss the relationship between occurrence of epilepsy and PDHc deficiency associated with the pyruvate dehydrogenase complex (E1α subunit (PDHA1) and E1β subunit (PDHB)) and PDH phosphatase (PDP) deficiency. PDHc plays a crucial role in the aerobic carbohydrate metabolism and regulates the use of carbohydrate as the source of oxidative energy. In severe PDHc deficiency, the energy deficit impairs brain development in utero resulting in physiological and structural changes in the brain that contributes to the subsequent onset of epileptogenesis. Epileptogenesis in PDHc deficiency is linked to energy failure and abnormal neurotransmitter metabolism that progressively alters neuronal excitability. This metabolic blockage might be restricted via inclusion of ketogenic diet that is broken up by β-oxidation and directly converting it to acetyl-CoA, and thereby improving the patient's health condition. Genetic counseling is essential as PDHA1 deficiency is X-linked. The demonstration of the X-chromosome localization of PDHA1 resolved a number of questions concerning the variable phenotype displayed by patients with E1 deficiency. Most patients show a broad range of neurological abnormalities, with the severity showing some dependence on the nature of the mutation in the Elα gene, while PDHB and PDH phosphatase (PDP) deficiencies are of autosomal recessive inheritance. However, in females, the disorder is further complicated by the pattern of X-chromosome inactivation, i.e., unfavorable lyonization. Furthermore research should focus on epileptogenic animal models; this might pave a new way toward identification of the pathophysiology of this challenging disorder.

MondoA deficiency enhances sprint performance in mice

MondoA is a basic helix-loop-helix (bHLH)/leucine zipper (ZIP) transcription factor that is expressed predominantly in skeletal muscle. Studies in vitro suggest that the Max-like protein X (MondoA:Mlx) heterodimer senses the intracellular energy status and directly targets the promoter region of thioredoxin interacting protein (Txnip) and possibly glycolytic enzymes. We generated MondoA-inactivated (MondoA-/-) mice by gene targeting. MondoA-/- mice had normal body weight at birth, exhibited normal growth and appeared to be healthy. However, they exhibited unique metabolic characteristics. MondoA-/- mice built up serum lactate and alanine levels and utilized fatty acids for fuel during exercise. Gene expression and promoter analysis suggested that MondoA functionally represses peroxisome-proliferator-activated receptor γ co-activator-1α (PGC-1α)-mediated activation of pyruvate dehydrogenase kinase 4 (PDK-4) transcription. PDK4 normally down-regulates the activity of pyruvate dehydrogenase, an enzyme complex that catalyses the decarboxylation of pyruvate to acetyl-CoA for entry into the Krebs cycle; in the absence of MondoA, pyruvate is diverted towards lactate and alanine, both products of glycolysis. Dynamic testing revealed that MondoA-/- mice excel in sprinting as their skeletal muscles display an enhanced glycolytic capacity. Our studies uncover a hitherto unappreciated function of MondoA in fuel selection in vivo. Lack of MondoA results in enhanced exercise capacity with sprinting.

Phenylbutyrate increases pyruvate dehydrogenase complex activity in cells harboring a variety of defects

Objective

Deficiency of pyruvate dehydrogenase complex (PDHC) is the most common genetic disorder leading to lactic acidosis. PDHC deficiency is genetically heterogenous and most patients have defects in the X-linked E1-α gene but defects in the other components of the complex encoded by PDHB, PDHX, DLAT, DLD genes or in the regulatory enzyme encoded by PDP1 have also been found. Phenylbutyrate enhances PDHC enzymatic activity in vitro and in vivo by increasing the proportion of unphosphorylated enzyme through inhibition of pyruvate dehydrogenase kinases and thus, has potential for therapy of patients with PDHC deficiency. In the present study, we investigated response to phenylbutyrate of multiple cell lines harboring all known gene defects resulting in PDHC deficiency.

Methods

Fibroblasts of patients with PDHC deficiency were studied for their enzyme activity at baseline and following phenylbutyrate incubation. Drug responses were correlated with genotypes and protein levels by Western blotting.

Results

Large deletions affecting PDHA1 that result in lack of detectable protein were unresponsive to phenylbutyrate, whereas increased PDHC activity was detected in most fibroblasts harboring PDHA1 missense mutations. Mutations affecting the R349-α residue were directed to proteasome degradation and were consistently unresponsive to short-time drug incubation but longer incubation resulted in increased levels of enzyme activity and protein that may be due to an additional effect of phenylbutyrate as a molecular chaperone.

Interpretation

PDHC enzyme activity was enhanced by phenylbutyrate in cells harboring missense mutations in PDHB, PDHX, DLAT, DLD, and PDP1 genes. In the prospect of a clinical trial, the results of this study may allow prediction of in vivo response in patients with PDHC deficiency harboring a wide spectrum of molecular defects.

SIRT5-mediated lysine desuccinylation impacts diverse metabolic pathways

Protein function is regulated by diverse posttranslational modifications. The mitochondrial sirtuin SIRT5 removes malonyl and succinyl moieties from target lysines. The spectrum of protein substrates subject to these modifications is unknown. We report systematic profiling of the mammalian succinylome, identifying 2,565 succinylation sites on 779 proteins. Most of these do not overlap with acetylation sites, suggesting differential regulation of succinylation and acetylation. Our analysis reveals potential impacts of lysine succinylation on enzymes involved in mitochondrial metabolism; e.g., amino acid degradation, the tricarboxylic acid cycle (TCA) cycle, and fatty acid metabolism. Lysine succinylation is also present on cytosolic and nuclear proteins; indeed, we show that a substantial fraction of SIRT5 is extramitochondrial. SIRT5 represses biochemical activity of, and cellular respiration through, two protein complexes identified in our analysis, pyruvate dehydrogenase complex and succinate dehydrogenase. Our data reveal widespread roles for lysine succinylation in regulating metabolism and potentially other cellular functions.

Pyruvate dehydrogenase complex deficiency caused by ubiquitination and proteasome-mediated degradation of the E1 subunit

Congenital deficiencies of the human pyruvate dehydrogenase (PDH) complex are considered to be due to loss of function mutations in one of the component enzymes. Here we describe a case of PDH deficiency associated with the PDH E1beta subunit (PDHB) gene. The clinical phenotype of the patient was consistent with reported cases of PDH deficiency. Cultured skin fibroblasts demonstrated a 55% reduction in PDH activity and markedly decreased immunoreactivity for PDHB protein, compared with healthy controls. Surprisingly, nucleotide sequence analyses of cDNAs corresponding to the patient PDH E1alpha (PDHA1) and PDHB genes revealed no pathological mutations. Moreover, the relative expression level of PDHB mRNA and the rates of transcription and translation of the PDHB gene were normal. However, PDC activity could be restored in cells from this patient following treatment with MG132, a specific proteasome inhibitor, and normal levels of E1beta could be detected in MG132-treated cells. Similar results were obtained following treatment with Tyr-phostin 23 (Tyr23), a specific inhibitor of epidermal growth factor receptor-protein-tyrosine kinase (EGFR-PTK), which also restored E1beta protein levels to those in cells from healthy subjects or from patients with PDHA1 deficiency. The index patient's cells contained a high basal level of EGFR-PTK activity that correlated with the high level of ubiquitination of cellular proteins, although the total EGFR protein levels were similar to those in cells from Elalpha-deficient subjects and healthy subjects. These data indicate that PDH deficiency in our patient involves a post-translational modification in which EGFR-PTK-mediated tyrosine phosphorylation of the E1beta protein leads to enhanced ubiquitination followed by proteasome-mediated degradation. They also provide a novel mechanism accounting for congenital deficiency of the PDH complex and perhaps other inborn errors of metabolism.

Therapeutic potential of dichloroacetate for pyruvate dehydrogenase complex deficiency

We reviewed the use of oral dichloroacetate (DCA) in the treatment of children with congenital lactic acidosis caused by mutations in the pyruvate dehydrogenase complex (PDC). The case histories of 46 subjects were analyzed with regard to diagnosis, clinical presentation and response to DCA. DCA decreased blood and cerebrospinal fluid lactate concentrations, and was generally well tolerated. DCA may be particularly effective in children with PDC deficiency by stimulating residual enzyme activity and, consequently, cellular energy metabolism. A controlled trial is needed to determine the definitive role of DCA in the management of this devastating disease.

Controlled clinical trial of dichloroacetate for treatment of congenital lactic acidosis in children

OBJECTIVE

Open-label studies indicate that oral dichloroacetate (DCA) may be effective in treating patients with congenital lactic acidosis. We tested this hypothesis by conducting the first double-blind, randomized, control trial of DCA in this disease.

METHODS

Forty-three patients who ranged in age from 0.9 to 19 years were enrolled. All patients had persistent or intermittent hyperlactatemia, and most had severe psychomotor delay. Eleven patients had pyruvate dehydrogenase deficiency, 25 patients had 1 or more defects in enzymes of the respiratory chain, and 7 patients had a mutation in mitochondrial DNA. Patients were preconditioned on placebo for 6 months and then were randomly assigned to receive an additional 6 months of placebo or DCA, at a dose of 12.5 mg/kg every 12 hours. The primary outcome results were (1) a Global Assessment of Treatment Efficacy, which incorporated tests of neuromuscular and behavioral function and quality of life; (2) linear growth; (3) blood lactate concentration in the fasted state and after a carbohydrate meal; (4) frequency and severity of intercurrent illnesses and hospitalizations; and (5) safety, including tests of liver and peripheral nerve function.

OUTCOME

There were no significant differences in Global Assessment of Treatment Efficacy scores, linear growth, or the frequency or severity of intercurrent illnesses. DCA significantly decreased the rise in blood lactate caused by carbohydrate feeding. Chronic DCA administration was associated with a fall in plasma clearance of the drug and with a rise in the urinary excretion of the tyrosine catabolite maleylacetone and the heme precursor delta-aminolevulinate.

CONCLUSIONS

In this highly heterogeneous population of children with congenital lactic acidosis, oral DCA for 6 months was well tolerated and blunted the postprandial increase in circulating lactate. However, it did not improve neurologic or other measures of clinical outcome.

Rapid rates of newly synthesized mitochondrial protein degradation are significantly affected by the generation of mitochondrial free radicals

Exposure of biological material to high levels of free radicals causes extensive cellular damage. Reactive oxygen species (ROS) generated by mitochondria have been associated with a variety of diseases and aging. We investigated the effect of low-level mitochondrial ROS production on newly synthesized mitochondrial proteins which are potentially vulnerable to mitochondrial ROS due to their location and unfolded state. We show that elevated mitochondrial ROS increases the degradation of newly synthesized mitochondrial proteins with some proteins more sensitive than others. In the long term reduced assembly of mitochondrial complexes would affect mitochondrial function and may trigger a vicious cycle of mitochondrial ROS production.

A zebrafish model for pyruvate dehydrogenase deficiency: rescue of neurological dysfunction and embryonic lethality using a ketogenic diet

Defects in the pyruvate dehydrogenase (PDH) complex result in severe neurological dysfunction, congenital lactic acidosis, growth retardation, and early death. Current treatments for PDH deficiency are administered postnatally and are generally unsuccessful. Because many patients with this disease are born with irreversible defects, a model system for the development of effective pre- and postnatal therapies would be of great value. In a behavioral genetic screen aimed to identify zebrafish with visual function defects, we previously isolated two alleles of the recessive lethal mutant no optokinetic response a (noa). Here we report that noa is deficient for dihydrolipoamide S-acetyltransferase (Dlat), the PDH E2 subunit, and exhibits phenotypes similar to human patients with PDH deficiency. To rescue the deficiency, we added ketogenic substrates to the water in which the embryos develop. This treatment successfully restored vision, promoted feeding behavior, reduced lactic acidosis, and increased survival. Our study demonstrates an approach for establishing effective therapies for PDH deficiency and other congenital diseases that affect early embryonic development.

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.

Gene therapy for pyruvate dehydrogenase E1alpha deficiency using recombinant adeno-associated virus 2 (rAAV2) vectors

To determine the feasibility of gene transfer to correct defects in the E1alpha subunit of the pyruvate dehydrogenase (PDH) complex (PDC), we constructed rAAV vectors that expressed PDH E1alpha, either alone or with a green fluorescent protein tag, from a hybrid cytomegalovirus (CMV) enhancer/chicken beta-actin (CB) promoter. These vectors were functional in vitro, as judged by increased expression of mRNA in vector-transduced deficient cell lines and correction of the biochemical defect in PDH activity in these cells. Approximately 30% of wild-type levels of PDH activity were restored under conditions with which only about 15% of cells were transduced. These same vectors were then used in vivo to transduce neurons within the rat striatum. Gene transfer, expression, and translocation into mitochondria were observed, without any obvious untoward effects. In vivo vector-mediated PDH expression persisted for at least 1 year after injection, indicating the stability of gene transfer. These studies provide the basis for future efforts to develop a recombinant AAV (rAAV)-based gene therapy approach for the correction of PDC deficiency.

Mitochondrial Disease

Mitochondrial diseases are disorders of energy metabolism that include defects of pyruvate metabolism, Krebs cycle, respiratory chain (RC), and fatty acid oxidation (FAO). Treatment of pyruvate metabolism, Krebs cycle, and RC disorders is, in general, disappointing. Therapeutic approaches consist of electron acceptors, enzyme activators, vitamins, coenzymes, free-radical scavengers, dietary measures, and supportive therapy. These treatment assumptions are based on current understanding of the pathophysiology, on anecdotal clinical reports, and on a few controlled clinical trials, which have not been encouraging. Although it is difficult to perform clinical trials in these conditions due to their rarity and genotypic and phenotypic heterogeneity, there is a great need for well-performed double-blind placebo- controlled clinical trials with comparable groups of patients and with sufficient follow-up periods. Treatment options for FAO disorders are, in general, satisfactory and are mainly based on diet, lifestyle recommendations, and administration of L-carnitine and, in some cases, riboflavin. Special conditions that involve primary deficiencies of L-carnitine, coenzyme Q(10), and cofactor- and vitamin-responsive enzyme defects must be systematically considered, because supplementation with these substances may be curative or produce dramatic improvements. While awaiting more specific therapies for mitochondrial disorders, it is useful to reach a consensus regarding the management of these patients. The expected outcome is a slowing of the disease process and stabilization of the clinical syndrome. More definitive treatments hopefully will follow in the near future.

ATP synthesis in lipoamide dehydrogenase deficiency

Lipoamide dehydrogenase deficiency is an inborn error of several metabolic pathways, including pyruvate metabolism, Krebs cycle, and branched-chain amino acid degradation. The clinical course is variable, ranging from infantile neurodegenerative disease to recurrent episodes of liver failure or myoglobinuria starting later in life. In contrast, residual enzymatic activity in muscle tissue spans over a narrow range. Despite the recent elucidation of the underlying molecular pathology in most patients, relationships between the genotype and the biochemical and clinical phenotype remain unclear. In order to find a suitable assay for the prediction of clinical outcome and assessment of treatment, we have evaluated enzymatic activities and energetic states in fibroblasts from lipoamide dehydrogenase-deficient patients representing three different phenotypes and genotypes. Direct relationships between clinical parameters such as age of onset and disease severity and biochemical characteristics, including lipoamide dehydrogenase activity, pyruvate dehydrogenase complex activity, and ATP production ratio in fibroblasts, were identified. Clinical parameters were not reflected by lactate/pyruvate ratio. ATP production rate was in direct relationship with the severity of the neurological involvement; the patient with reduced ATP synthesis to 30% of the control mean had a severe neurodegenerative disease, whereas ATP synthesis values above 45% were associated with a more favorable course. Incubation of the patients' fibroblasts with dichloroacetate coupled with thiamin resulted in slight but significant improvement of the cell energetic state.