Phosphotyrosine-containing lactate dehydrogenase is restricted to the nuclei of PC12 pheochromocytoma cells

PIM kinases phosphorylate lactate dehydrogenase A at serine 161 and suppress its nuclear ubiquitination

Lactate dehydrogenase A (LDHA) is a glycolytic enzyme catalysing the reversible conversion of pyruvate to lactate. It has been implicated as a substrate for PIM kinases, yet the relevant target sites and functional consequences of phosphorylation have remained unknown. Here, we show that all three PIM family members can phosphorylate LDHA at serine 161. When we investigated the physiological consequences of this phosphorylation in PC3 prostate cancer and MCF7 breast cancer cells, we noticed that it suppressed ubiquitin-mediated degradation of nuclear LDHA and promoted interactions between LDHA and 14-3-3 proteins. By contrast, in CRISPR/Cas9-edited knock-out cells lacking all three PIM family members, ubiquitination of nuclear LDHA was dramatically increased followed by its decreased expression. Our data suggest that PIM kinases support nuclear LDHA expression and activities by promoting phosphorylation-dependent interactions of LDHA with 14-3-3ε, which shields nuclear LDHA from ubiquitin-mediated degradation.

The Biochemical and Clinical Perspectives of Lactate Dehydrogenase: An Enzyme of Active Metabolism

BACKGROUND

Lactate dehydrogenase (LDH) is a group of oxidoreductase isoenzymes catalyzing the reversible reaction between pyruvate and lactate. The five isoforms of this enzyme, formed from two subunits, vary in isoelectric points and these isoforms have different substrate affinity, inhibition constants and electrophoretic mobility. These diverse biochemical properties play a key role in its cellular, tissue and organ specificity. Though LDH is predominantly present in the cytoplasm, it has a multi-organellar location as well.

OBJECTIVE

The primary objective of this review article is to provide an update in parallel, the previous and recent biochemical views and its clinical significance in different diseases.

METHODS

With the help of certain inhibitors, its active site three-dimensional view, reactions mechanisms and metabolic pathways have been sorted out to a greater extent. Overexpression of LDH in different cancers plays a principal role in anaerobic cellular metabolism, hence several inhibitors have been designed to employ as novel anticancer agents.

DISCUSSION

LDH performs a very important role in overall body metabolism and some signals can induce isoenzyme switching under certain circumstances, ensuring that the tissues consistently maintain adequate ATP supply. This enzyme also experiences some posttranslational modifications, to have diversified metabolic roles. Different toxicological and pathological complications damage various organs, which ultimately result in leakage of this enzyme in serum. Hence, unusual LDH isoform level in serum serves as a significant biomarker of different diseases.

CONCLUSION

LDH is an important diagnostic biomarker for some common diseases like cancer, thyroid disorders, tuberculosis, etc. In general, LDH plays a key role in the clinical diagnosis of various common and rare diseases, as this enzyme has a prominent role in active metabolism.

Structural and functional regulation of lactate dehydrogenase-A in cancer

Dysregulated metabolism is one of the hallmarks of cancer. Under normal physiological conditions, ATP is primarily generated by oxidative phosphorylation. Cancers commonly undergo a dramatic shift toward glycolysis, despite the presence of oxygen. This phenomenon is known as the Warburg effect, and requires the activity of LDHA. LDHA converts pyruvate to lactate in the final step of glycolysis and is often upregulated in cancer. LDHA inhibitors present a promising therapeutic option, as LDHA blockade leads to apoptosis in cancer cells. Despite this, existing LDHA inhibitors have shown limited clinical efficacy. Here, we review recent progress in LDHA structure, function and regulation as well as strategies to target this critical enzyme.

Mitochondrial Retrograde Signalling and Metabolic Alterations in the Tumour Microenvironment

This review explores the molecular mechanisms that may be responsible for mitochondrial retrograde signalling related metabolic reprogramming in cancer and host cells in the tumour microenvironment and provides a summary of recent updates with regard to the functional modulation of diverse cells in the tumour microenvironment.

Post-translational processing targets functionally diverse proteins in Mycoplasma hyopneumoniae

Mycoplasma hyopneumoniae is a genome-reduced, cell wall-less, bacterial pathogen with a predicted coding capacity of less than 700 proteins and is one of the smallest self-replicating pathogens. The cell surface of M. hyopneumoniae is extensively modified by processing events that target the P97 and P102 adhesin families. Here, we present analyses of the proteome of M. hyopneumoniae-type strain J using protein-centric approaches (one- and two-dimensional GeLC–MS/MS) that enabled us to focus on global processing events in this species. While these approaches only identified 52% of the predicted proteome (347 proteins), our analyses identified 35 surface-associated proteins with widely divergent functions that were targets of unusual endoproteolytic processing events, including cell adhesins, lipoproteins and proteins with canonical functions in the cytosol that moonlight on the cell surface. Affinity chromatography assays that separately used heparin, fibronectin, actin and host epithelial cell surface proteins as bait recovered cleavage products derived from these processed proteins, suggesting these fragments interact directly with the bait proteins and display previously unrecognized adhesive functions. We hypothesize that protein processing is underestimated as a post-translational modification in genome-reduced bacteria and prokaryotes more broadly, and represents an important mechanism for creating cell surface protein diversity.

Metabolic Enzymes Moonlighting in the Nucleus: Metabolic Regulation of Gene Transcription

During evolution, cells acquired the ability to sense and adapt to varying environmental conditions, particularly in terms of fuel supply. Adaptation to fuel availability is crucial for major cell decisions and requires metabolic alterations and differential gene expression that are often epigenetically driven. A new mechanistic link between metabolic flux and regulation of gene expression is through moonlighting of metabolic enzymes in the nucleus. This facilitates delivery of membrane-impermeable or unstable metabolites to the nucleus, including key substrates for epigenetic mechanisms such as acetyl-CoA which is used in histone acetylation. This metabolism-epigenetics axis facilitates adaptation to a changing environment in normal (e.g., development, stem cell differentiation) and disease states (e.g., cancer), providing a potential novel therapeutic target.

Comparative enzymology-new insights from studies of an "old" enzyme, lactate dehydrogenase

Comparative enzymology explores the molecular mechanisms that alter the properties of enzymes to best fit and adapt them to the biotic demands and abiotic stresses that affect the cellular environment in which these protein catalysts function. For many years, comparative enzymology was primarily concerned with analyzing enzyme functional properties (e.g. substrate affinities, allosteric effectors, responses to temperature or pH, stabilizers, denaturants, etc.) in order to determine how enzyme properties were optimized to function under changing conditions. More recently it became apparent that posttranslational modifications of enzymes play a huge role in metabolic regulation. At first, such modifications appeared to target just crucial regulatory enzymes but recent work is showing that many dehydrogenases are also targets of posttranslational modification leading to substantial changes in enzyme properties. The present article focuses in particular on lactate dehydrogenase (LDH) showing that stress-induced changes in enzyme properties can be linked with reversible posttranslational modifications; e.g. changes in the phosphorylation state of LDH occur in response to dehydration stress in frogs and anoxia exposure of turtles and snails. Furthermore, these studies show that LDH is also a target of other posttranslational modifications including acetylation, methylation and ubiquitination that change in response to anoxia or dehydration stress. Selected new methods for exploring posttranslational modifications of dehydrogenases are discussed and new challenges for the future of comparative enzymology are presented that will help to achieve a deeper understanding of biochemical adaptation through enzyme regulation.

The Regulation and Function of Lactate Dehydrogenase A: Therapeutic Potential in Brain Tumor

There are over 120 types of brain tumor and approximately 45% of primary brain tumors are gliomas, of which glioblastoma multiforme (GBM) is the most common and aggressive with a median survival rate of 14 months. Despite progress in our knowledge, current therapies are unable to effectively combat primary brain tumors and patient survival remains poor. Tumor metabolism is important to consider in therapeutic approaches and is the focus of numerous research investigations. Lactate dehydrogenase A (LDHA) is a cytosolic enzyme, predominantly involved in anaerobic and aerobic glycolysis (the Warburg effect); however, it has multiple additional functions in non‐neoplastic and neoplastic tissues, which are not commonly known or discussed. This review summarizes what is currently known about the function of LDHA and identifies areas that would benefit from further exploration. The current knowledge of the role of LDHA in the brain and its potential as a therapeutic target for brain tumors will also be highlighted. The Warburg effect appears to be universal in tumors, including primary brain tumors, and LDHA (because of its involvement with this process) has been identified as a potential therapeutic target. Currently, there are, however, no suitable LDHA inhibitors available for tumor therapies in the clinic.

Nuclear lactate dehydrogenase modulates histone modification in human hepatocytes

It is becoming increasingly apparent that the nucleus harbors metabolic enzymes that affect genetic transforming events. Here, we describe a nuclear isoform of lactate dehydrogenase (nLDH) and its ability to orchestrate histone deacetylation by controlling the availability of nicotinamide adenine dinucleotide (NAD(+)), a key ingredient of the sirtuin-1 (SIRT1) deacetylase system. There was an increase in the expression of nLDH concomitant with the presence of hydrogen peroxide (H2O2) in the culture medium. Under oxidative stress, the NAD(+) generated by nLDH resulted in the enhanced deacetylation of histones compared to the control hepatocytes despite no discernable change in the levels of SIRT1. There appeared to be an intimate association between nLDH and SIRT1 as these two enzymes co-immunoprecipitated. The ability of nLDH to regulate epigenetic modifications by manipulating NAD(+) reveals an intricate link between metabolism and the processing of genetic information.

Purification and Properties of White Muscle Lactate Dehydrogenase from the Anoxia-Tolerant Turtle, the Red-Eared Slider, Trachemys scripta elegans

Lactate dehydrogenase (LDH; E.C. 1.1.1.27) is a crucial enzyme involved in energy metabolism in muscle, facilitating the production of ATP via glycolysis during oxygen deprivation by recycling NAD⁺. The present study investigated purified LDH from the muscle of 20 h anoxic and normoxic T. s. elegans, and LDH from anoxic muscle showed a significantly lower (47%) K_m for L-lactate and a higher V_max value than the normoxic form. Several lines of evidence indicated that LDH was converted to a low phosphate form under anoxia: (a) stimulation of endogenously present protein phosphatases decreased the K_m of L-lactate of control LDH to anoxic levels, whereas (b) stimulation of kinases increased the K_m of L-lactate of anoxic LDH to normoxic levels, and (c) dot blot analysis shows significantly less serine (78%) and threonine (58%) phosphorylation in anoxic muscle LDH as compared to normoxic LDH. The physiological consequence of anoxia-induced LDH dephosphorylation appears to be an increase in LDH activity to promote the reduction of pyruvate in muscle tissue, converting the glycolytic end product to lactate to maintain a prolonged glycolytic flux under energy-stressed anoxic conditions.

The multi-functional roles of GNMT in toxicology and cancer

Although glycine N-methyltransferase (GNMT) has been discovered for five decades, its function was not elucidated until recently. In this review, we discuss the multiple roles of GNMT in toxicology and cancer. Besides catalyzing the production of methylglycine (sarcosine) in one carbon metabolism pathway, GNMT was found to be able to bind a number of polycyclic aromatic hydrocarbons and inhibit DNA adducts formation. Moreover, GNMT exerts protective effects against the cytotoxicity and carcinogenicity of benzo(a)pyrene and aflatoxin B(1) in vitro and in vivo. Occupational study showed that workers who had genotypes with higher GNMT promoter activity may have lower content of oxidative damaged DNA products in their urine. In terms of cancer, recent studies using GNMT knockout mouse models demonstrated that GNMT deficiency has high penetrance in inducing the development of steatohepatitis and hepatocellular carcinoma. In terms of the mechanism, besides dysregulation of epigenetic modification, insights have been provided by recent identification of two novel proteins interacting with GNMT-DEPTOR and NPC2. These studies suggest that GNMT not only is involved in mTOR signaling pathway, but also plays an important role in the intracellular trafficking of cholesterol. The implication of these findings to the preventive medicine and translational research will be discussed.

Mouse genetics suggests cell-context dependency for Myc-regulated metabolic enzymes during tumorigenesis

c-Myc (hereafter called Myc) belongs to a family of transcription factors that regulates cell growth, cell proliferation, and differentiation. Myc initiates the transcription of a large cast of genes involved in cell growth by stimulating metabolism and protein synthesis. Some of these, like those involved in glycolysis, may be part of the Warburg effect, which is defined as increased glucose uptake and lactate production in the presence of adequate oxygen supply. In this study, we have taken a mouse-genetics approach to challenge the role of select Myc-regulated metabolic enzymes in tumorigenesis in vivo. By breeding λ-Myc transgenic mice, Apc(Min) mice, and p53 knockout mice with mouse models carrying inactivating alleles of Lactate dehydrogenase A (Ldha), 3-Phosphoglycerate dehydrogenase (Phgdh) and Serine hydroxymethyltransferase 1 (Shmt1), we obtained offspring that were monitored for tumor development. Very surprisingly, we found that these genes are dispensable for tumorigenesis in these genetic settings. However, experiments in fibroblasts and colon carcinoma cells expressing oncogenic Ras show that these cells are sensitive to Ldha knockdown. Our genetic models reveal cell context dependency and a remarkable ability of tumor cells to adapt to alterations in critical metabolic pathways. Thus, to achieve clinical success, it will be of importance to correctly stratify patients and to find synthetic lethal combinations of inhibitors targeting metabolic enzymes.

Apoptosis induced clustering of IP(3)R1 in nuclei of non-differentiated PC12 cells

Inositol 1,4,5-trisphosphate (IP(3)) receptors are emerging as key sites for regulation by pro- and anti-apoptotic factors. Induction of apoptosis for 3 h increased mRNA and protein levels of type 1 IP(3) receptors in non-differentiated (ND), but not in differentiated (D) PC12 cells. Inhibitors of the IP(3) R's calcium release-2-aminoethoxydiphenyl borate (2-APB) and xestospongin-completely prevented Bax and caspase-3 mRNA increase after treatment with the apoptosis inducer set (AIK), and this reinforces the importance of IP(3) R1 in the apoptosis of ND PC12 cells. Apoptosis induction not only increases the IP(3) R1 protein, but it also causes formation of IP(3) R1 clusters in the nucleus which most likely result from fusion of the nucleoplasmic reticulum and/or IP(3) R1 translocation to the nucleus. This is quite similar to the observations noted after overexpression of IP(3) R1 in PC12 cells. The amount of IP(3) induced calcium release was higher in control than in AIK-treated cells. From our results we propose that after the apoptosis induction the amount of intranuclear calcium decreased dramatically due to the increase of calcium permeability of the nuclear calcium store vesicles. Therefore, increase of the calcium permeability may result from IP(3) receptors translocation to nuclei that can boost the calcium transport through IP(3) receptors.

Malondialdehyde inhibits an AMPK-mediated nuclear translocation and repression activity of ALDH2 in transcription

Aging process results from deleterious damages by reactive oxygen species, in particular, various metabolic aldehydes. Aldehyde dehydrogenase 2 (ALDH2) is one of metabolic enzymes detoxifying various aldehydes under oxidative conditions. AMP-activated protein kinase (AMPK) plays a key role in controlling metabolic process. However, little was known about the relationship of ALDH2 with AMPK under oxidative conditions. Here, we, by using MDA-specific monoclonal antibody, screened the tissues of young and old rats for MDA-modified proteins and identified an ALDH2 as a prominent MDA-modified protein band in the old rat kidney tissue. ALDH2 associates with AMPK and is phosphorylated by AMPK. In addition, AICAR, an activator of AMP-activated protein kinase, induces the nuclear translocation of ALDH2. ALDH2 in nucleus is involved in general transcription repression by association with histone deacetylases. Furthermore, MDA modification inhibited the translocation of ALDH2 and the association with AMPK, and ultimately led to de-repression of transcription in the reporter system analysis. In this study, we have demonstrated that ALDH2 acts as a transcriptional repressor in response to AMPK activation, and MDA modifies ALDH2 and inhibits repressive activity of ALDH2 in general transcription. We thus suggest that increasing amount of MDA during aging process may interrupt the nuclear function of ALDH2, modulated by AMPK.

Characterization and regulation of monocarboxylate cotransporters Slc16a7 and Slc16a3 in preimplantation mouse embryos

Concurrent with compaction, preimplantation mouse embryos switch from the high pyruvate consumption that prevailed during cleavage stages to glucose consumption against a constant background of pyruvate uptake. However, zygotes exposed to and subsequently deprived of glucose can form blastocysts by increasing pyruvate uptake. This metabolic switch requires cleavage-stage exposure to glucose and is one aspect of metabolic differentiation that normally occurs in vivo. Monocarboxylates, such as pyruvate and lactate, are transported across membranes via the SLC16 family of H(+)-monocarboxylate cotransporter (MCT) proteins. Thus, the increase in pyruvate uptake in embryos developing without glucose must involve changes in activity and localization of MCT. In mouse embryos, continued expression of Slc16a1 (MCT1) requires glucose supply. Messenger RNA for Slc17a7 (MCT2) and Slc16a3 (MCT4) has been detected in mouse preimplantation embryos; however, protein function, localization, and regulation of expression at the basis of these net pyruvate uptake changes remain unclear. The expression and localization of SLC16A7 and SLC16A3 have therefore been examined to clarify their respective roles in embryos derived from the reproductive tract and cultured under varied conditions. SLC16A3 appears localized to the plasma membrane until the morula stage and also maintains a nuclear distribution throughout preimplantation development. However, continued Slc16a3 mRNA expression is dependent on prior exposure to glucose. SLC16A7 localizes to apical cortical regions with punctate, vesicular expression throughout blastomeres, partially colocalizing in peroxisomes with peroxisomal catalase (CAT). In contrast to SLC16A3 and SLC16A1, SLC16A7 and CAT demonstrate upregulation in the absence of glucose. These striking differences between the two isoforms in expression localization and regulation suggest unique roles for each in monocarboxylate transport and pH regulation during preimplantation development, and implicate peroxisomal SLC16A7 as an important redox regulator in the absence of glucose.

Distribution of lactate dehydrogenase in healthy and degenerative canine stifle joint cartilage

In dogs, degenerative joint diseases (DJD) have been shown to be associated with increased lactate dehydrogenase (LDH) activity in the synovial fluid. The goal of this study was to examine healthy and degenerative stifle joints in order to clarify the origin of LDH in synovial fluid. In order to assess the distribution of LDH, cartilage samples from healthy and degenerative knee joints were investigated by means of light and transmission electron microscopy in conjunction with immunolabeling and enzyme cytochemistry. Morphological analysis confirmed DJD. All techniques used corroborated the presence of LDH in chondrocytes and in the interterritorial matrix of healthy and degenerative stifle joints. Although enzymatic activity of LDH was clearly demonstrated in the territorial matrix by means of the tetrazolium-formazan reaction, immunolabeling for LDH was missing in this region. With respect to the distribution of LDH in the interterritorial matrix, a striking decrease from superficial to deeper layers was present in healthy dogs but was missing in affected joints. These results support the contention that LDH in synovial fluid of degenerative joints originates from cartilage. Therefore, we suggest that (1) LDH is transferred from chondrocytes to ECM in both healthy dogs and dogs with degenerative joint disease and that (2) in degenerative joints, LDH is released from chondrocytes and the ECM into synovial fluid through abrasion of cartilage as well as through enhanced diffusion as a result of increased water content and degradation of collagen.

A novel adaptation of aldolase regulates virulence in Streptococcus pyogenes

Regulation of virulence factor expression is critical for pathogenic microorganisms that must sense and adapt to a dynamic host environment; yet, the signal transduction pathways that enable this process are generally poorly understood. Here, we identify LacD.1 as a global regulator of virulence factor expression in the versatile human pathogen, Streptococcus pyogenes. LacD.1 is derived from a class I tagatose-1,6-bisphosphate aldolase homologous to those involved in lactose and galactose metabolism in related prokaryotes. However, regulation of transcription by LacD.1 is not dependent on this enzymatic activity or the canonical catabolite repression pathway, but likely does require substrate recognition. Our results suggest that LacD.1 has been adapted as a metabolic sensor, and raise the possibility that regulation of gene expression by metabolic enzymes may be a novel mechanism by which Gram-positive bacteria, including S. pyogenes, coordinate multiple environmental cues, allowing essential transcription programs to be coupled with perceived nutritional status.

Multifaceted roles of glycolytic enzymes

Although glycolysis is a biochemical pathway that evolved under ancient anaerobic terrestrial conditions, recent studies have provided evidence that some glycolytic enzymes are more complicated, multifaceted proteins rather than simple components of the glycolytic pathway. These glycolytic enzymes have acquired additional non-glycolytic functions in transcriptional regulation [hexokinase (HK)-2, lactate dehydrogenase A, glyceraldehyde-3-phosphate dehydrogenase (GAPD) and enolase 1], stimulation of cell motility (glucose-6-phosphate isomerase) and the regulation of apoptosis (glucokinase, HK and GAPD). The existence of multifaceted roles of glycolytic proteins suggests that links between metabolic sensors and transcription are established directly through enzymes that participate in metabolism. These roles further underscore the need to consider the non-enzymatic functions of enzymes in proteomic studies of cells and tissues.

Modulation of DNA polymerases alpha, delta and epsilon by lactate dehydrogenase and 3-phosphoglycerate kinase

Literature documents that glycolytic enzymes (among them lactate dehydrogenase and 3-phosphoglycerate kinase) can reside in nuclei of mammalian cells and exert functions in DNA replication, transcription and DNA repair, in addition to their role as catalysts in the cytoplasm. Transfer of glycolytic enzymes to cell nuclei requires modification, for example phosphorylation. We studied the effects of phosphorylated lactate dehydrogenase and 3-phosphoglycerate kinase on (i) UV-induced DNA repair, using permeabilized human fibroblasts, and (ii) in vitro DNA synthesis catalyzed by purified DNA polymerases alpha, delta, and epsilon from proliferating rat liver. (i) Phosphorylated lactate dehydrogenase stimulated UV-induced DNA repair synthesis in normal fibroblasts in a dose-dependent manner; the unphosphorylated enzyme slightly inhibited. In repair-deficient xeroderma pigmentosum fibroblasts reparative synthesis was not enhanced whether lactate dehydrogenase was phosphorylated or not, indicating that reparative DNA synthesis must be possible in order to be stimulated. (ii) Activity of purified DNA polymerases alpha, delta, and epsilon was differentially stimulated or inhibited, according to the phosphorylation status of lactate dehydrogenase. DNA polymerases were also modulated by 3-phosphoglycerate kinase, depending on the primer-templates used which were gapped DNA (mimicking a repair mode of DNA synthesis) or single-stranded M13 DNA (representing the replicative mode of DNA synthesis). Since glycolytic enzymes in cell nuclei retain binding ability for their cofactors, cytoplasmic substrates and inhibitors, a regulatory linkage might exist between the energy state of a cell and its replicative and reparative functions.

Inhibition of platelet-derived growth factor BB-induced expression of glyceraldehyde-3-phosphate dehydrogenase by sodium butyrate in rat vascular smooth muscle cells

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a key regulatory enzyme of glycolysis, which exists in nuclei and functions as a DNA-binding protein as well as a nuclear protein, appears to be modulated by cellular activities. Exposure of quiescent rat smooth muscle cells (SMCs) to platelet-derived growth factor BB (PDGF-BB), which stimulates SMCs proliferation, caused a time-dependent increase in mRNA for GAPDH and its catalytic activity. Treatment of quiescent SMCs with sodium butyrate (SB), which is shown to inhibit PDGF-BB-induced SMC proliferation, caused a time- and concentration-dependent decrease in PDGF-BB-induced GAPDH mRNA expression and its catalytic activity. Nuclear run-on studies revealed that the PDGF-BB-induced rate of GAPDH gene transcription was reduced by about 50% in the presence of 5 mmol/L SB. The protein synthesis inhibitor, cycloheximide, failed to abolish the SB-inhibited PDGF-BB-induced rate of transcription of GAPDH, suggesting that SB is not dependent on ongoing protein synthesis to exert its effects on PDGF-BB-induced GAPDH transcription. Furthermore, measurement of GAPDH mRNA stability at various times after the inhibition of transcription with actinomycin D indicated that 5 mmol/L SB has no significant effect on the half-life of PDGF-BB-induced mRNA. The reduction in PDGF-BB-induced GAPDH expression by SB is probably caused by a cycloheximide-insensitive transcriptional mechanism. Thus, the inhibition of PDGF-BB-induced expression of GAPDH by SB suggests a link between SMC proliferation, energy consumption, and GAPDH gene upregulation.

L-lactate dehydrogenase A4- and A3B isoforms are bona fide peroxisomal enzymes in rat liver. Evidence for involvement in intraperoxisomal NADH reoxidation

The subcellular localization of l-lactate dehydrogenase (LDH) in rat hepatocytes has been studied by analytical subcellular fractionation combined with the immunodetection of LDH in isolated subcellular fractions and liver sections by immunoblotting and immunoelectron microscopy. The results clearly demonstrate the presence of LDH in the matrix of peroxisomes in addition to the cytosol. Both cytosolic and peroxisomal LDH subunits have the same molecular mass (35.0 kDa) and show comparable cross-reactivity with an anti-cytosolic LDH antibody. As revealed by activity staining or immunoblotting after isoelectric focussing, both intracellular compartments contain the same liver-specific LDH-isoforms (LDH-A4 > LDH-A3B) with the peroxisomes comprising relatively more LDH-A3B than the cytosol. Selective KCl extraction as well as resistance to proteinase K and immunoelectron microscopy revealed that at least 80% of the LDH activity measured in highly purified peroxisomal fractions is due to LDH as a bona fide peroxisomal matrix enzyme. In combination with the data of cell fractionation, this implies that at least 0.5% of the total LDH activity in hepatocytes is present in peroxisomes. Since no other enzymes of the glycolytic pathway (such as phosphoglucomutase, phosphoglucoisomerase, and glyceraldehyde-3-phosphate dehydrogenase) were found in highly purified peroxisomal fractions, it does not seem that LDH in peroxisomes participates in glycolysis. Instead, the marked elevation of LDH in peroxisomes of rats treated with the hypolipidemic drug bezafibrate, concomitantly to the induction of the peroxisomal beta-oxidation enzymes, strongly suggests that intraperoxisomal LDH may be involved in the reoxidation of NADH generated by the beta-oxidation pathway. The interaction of LDH and the peroxisomal palmitoyl-CoA beta-oxidation system could be verified in a modified beta-oxidation assay by adding increasing amounts of pyruvate to the standard assay mixture and recording the change of NADH production rates. A dose-dependent decrease of NADH produced was simulated with the lowest NADH value found at maximal LDH activity. The addition of oxamic acid, a specific inhibitor of LDH, to the system or inhibition of LDH by high pyruvate levels (up to 20 mm) restored the NADH values to control levels. A direct effect of pyruvate on palmitoyl-CoA oxidase and enoyl-CoA hydratase was excluded by measuring those enzymes individually in separate assays. An LDH-based shuttle across the peroxisomal membrane should provide an efficient system to regulate intraperoxisomal NAD+/NADH levels and maintain the flux of fatty acids through the peroxisomal beta-oxidation spiral.

Expression of mitochondrial genes and DNA-repair-related nuclear genes is altered in xeroderma pigmentosum fibroblasts

Differential hybridization was used to detect repair defects in xeroderma pigmentosum (XP) that are not amenable to current analyses. cDNA libraries were constructed from cytoplasmic RNA of normal and XP fibroblast strains (complementation groups A and D) and analyzed for differential gene expression. More than 40,000 lambda gt10 cDNA clones were differentially screened with in vitro transcripts made from cDNA in the pBluescript vector. Six differential clones were detected in the libraries of the XP group A and D strains which caused stronger or weaker signals when probed with transcripts from XP strains than with those from the normal strains. Two clones coded for mitochondrial genes: mitochondrial 16 S rRNA and ATPase 6L. Overexpression of mitochondrial genes in XP may indicate that functions of the ATP-generating system are impaired since such functions are intensified whenever they become insufficient, for example as a consequence of DNA damage. It is tempting to assume that abnormal mitochondria are one of the causes for the neurological malfunctions in XP. Furthermore, densitometric analysis of Northern blots revealed that mRNA of lactate dehydrogenase, chain M, was less abundant in four XP group A strains (extent of reduction: 70%) and in two XP group D strains (extent of reduction: 58%). Enzyme activity was also diminished. In addition, mRNA of the gene for glyceraldehyde-3-phosphate dehydrogenase was less expressed in the same XP group A and D fibroblast strains investigated (reduction in both complementation groups: 50%). Both glycolytic enzymes have nuclear functions apart from their role in sugar metabolism. Lactate dehydrogenase, chain M, is identical to a helix-destabilizing protein; it is closely associated with chromatin and unfolded DNA, suggesting a role in DNA synthesis and transcription. The 37-kDa subunit of glyceraldehyde-3-phosphate dehydrogenase is involved in transcription and was shown to be identical to uracil-DNA glycosylase, a base-excision repair enzyme. We presume that the nuclear functions of these glycolytic enzymes may be thwarted in the XP strains investigated and may account for malfunctions in XP, particularly for neurological disturbances.

Glycolytic enzymes as DNA binding proteins

1. Numerous studies have demonstrated the presence of at least four glycolytic enzymes in the nuclear compartment of several cell systems. 2. These include, lactate dehydrogenase, phosphoglycerate kinase, aldolase and glyceraldehyde-3-phosphate dehydrogenase. 3. In some cases the glycolytic enzymes found in the nuclei were a modified form from that found in the cytoplasmic counterpart. 4. In all four cases, the nuclear form of these glycolytic enzymes has been reported to bind DNA. 5. Although none of these enzymes interact with a specific target DNA sequence, their association with DNA may play a role in transcription and replication of DNA through general stabilization of the nuclear matrix or chromatin structure. 6. The present review aims to summarize the current understanding of this phenomenon and to examine the role of the DNA-binding activities of the glycolytic enzymes in cell growth and differentiation.

In vitro phosphorylation and hormone binding activation of the synthetic wild type human estradiol receptor

A tyrosine kinase purified from calf uterus activates the hormone binding of endogenous estradiol receptor (ER) predephosphorylated and preinactivated by a nuclear phosphotyrosine phosphatase. The kinase also activates and phosphorylates the human estradiol receptor HEO synthesized in vitro, which differs from the wild type receptor HEGO because a glycine is replaced by a valine at position 400. Moreover, the kinase activates and phosphorylates a deletion mutant of HEO which consists almost exclusively of the hormone binding domain. Using HEGO and HEO in parallel and measuring both binding activation and phosphorylation of ER we now observe that the wild type receptor is a good kinase substrate, slightly better than HEO. Furthermore, HEGO like the calf uterus receptor in the presence of estradiol, stimulates the kinase. From present findings it appears that ER and uterus tyrosine kinase are functionally associated and that this association is abolished by glycine to valine substitution at position 400 of ER.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-resistant, flat-cell PC12 variants having a partial loss of transformed phenotype

We have cloned and characterized two variants of PC12 cells. MPT1 cells were selected by their resistance to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and variant 2068 was isolated nonselectively as a large, flat-cell variant commonly occurring in PC12 cultures. Variant 2068 cells also exhibit resistance to MPTP. Karyotype analysis revealed that these variants are true derivatives of wild-type PC12 cells; however, each variant is tetraploid, whereas the wild-type parent is diploid. The two variants contain an altered level and composition of lactate dehydrogenase isoenzymes, which could account for a previously described difference in lactate metabolism. Both variants exhibit a partial loss of transformed phenotype in culture in that they are nonrefractile, grow in monolayers, and fail to multiply in soft agar. We suggest that this alteration in transformed phenotype may result in altered mitochondria and lactate dehydrogenase and thus account for their resistance to MPTP. Compared with wild-type PC12 cells, MPT1 cells have a decreased level of fos mRNA and an increased level of myc mRNA; the latter results from an increased level of transcription of exon 1 of the myc gene. Studies with hybrid cells obtained by fusing MPT1 cells with wild-type-like cells show that most, but not all, of the parameters of the MPT1 phenotype predominate.