Altered cellular localisation and expression, together with unconventional protein trafficking, of prion protein, PrPC, in type 1 diabetes

AIMS/HYPOTHESIS

Normal cellular prion protein (PrP^C) is a conserved mammalian glycoprotein found on the outer plasma membrane leaflet through a glycophosphatidylinositol anchor. Although PrP^C is expressed by a wide range of tissues throughout the body, the complete repertoire of its functions has not been fully determined. The misfolded pathogenic isoform PrP^Sc (the scrapie form of PrP) is a causative agent of neurodegenerative prion diseases. The aim of this study is to evaluate PrP^C localisation, expression and trafficking in pancreases from organ donors with and without type 1 diabetes and to infer PrP^C function through studies on interacting protein partners.

METHODS

In order to evaluate localisation and trafficking of PrP^C in the human pancreas, 12 non-diabetic, 12 type 1 diabetic and 12 autoantibody-positive organ donor tissue samples were analysed using immunofluorescence analysis. Furthermore, total RNA was isolated from 29 non-diabetic, 29 type 1 diabetic and 24 autoantibody-positive donors to estimate PrP^C expression in the human pancreas. Additionally, we performed PrP^C-specific immunoblot analysis on total pancreatic protein from non-diabetic and type 1 diabetic organ donors to test whether changes in PrP^C mRNA levels leads to a concomitant increase in PrP^C protein levels in human pancreases.

RESULTS

In non-diabetic and type 1 diabetic pancreases (the latter displaying both insulin-positive [INS(+)] and -negative [INS(-)] islets), we found PrP^C in islets co-registering with beta cells in all INS(+) islets and, strikingly, unexpected activation of PrP^C in alpha cells within diabetic INS(-) islets. We found PrP^C localised to the plasma membrane and endoplasmic reticulum (ER) but not the Golgi, defining two cellular pools and an unconventional protein trafficking mechanism bypassing the Golgi. We demonstrate PrP^C co-registration with established protein partners, neural cell adhesion molecule 1 (NCAM1) and stress-inducible phosphoprotein 1 (STI1; encoded by STIP1) on the plasma membrane and ER, respectively, linking PrP^C function with cyto-protection, signalling, differentiation and morphogenesis. We demonstrate that both PRNP (encoding PrP^C) and STIP1 gene expression are dramatically altered in type 1 diabetic and autoantibody-positive pancreases.

CONCLUSIONS/INTERPRETATION

As the first study to address PrP^C expression in non-diabetic and type 1 diabetic human pancreas, we provide new insights for PrP^C in the pathogenesis of type 1 diabetes. We evaluated the cell-type specific expression of PrP^C in the human pancreas and discovered possible connections with potential interacting proteins that we speculate might address mechanisms relevant to the role of PrP^C in the human pancreas.

Monogenic Diabetes and Integrated Stress Response Genes Display Altered Gene Expression in Type 1 Diabetes

Type 1 diabetes (T1D) has a multifactorial autoimmune etiology, involving environmental prompts and polygenic predisposition. We hypothesized that pancreata from individuals with and at risk for T1D would exhibit dysregulated expression of genes associated with monogenic forms of diabetes caused by nonredundant single-gene mutations. Using a "monogenetic transcriptomic strategy," we measured the expression of these genes in human T1D, autoantibody-positive (autoantibody+), and control pancreas tissues with real-time quantitative PCR in accordance with the Minimum Information for Publication of Quantitative Real-Time PCR Experiments (MIQE) guidelines. Gene and protein expression was visualized in situ with use of immunofluorescence, RNAscope, and confocal microscopy. Two dozen monogenic diabetes genes showed altered expression in human pancreata from individuals with T1D versus unaffected control subjects. Six of these genes also saw dysregulation in pancreata from autoantibody+ individuals at increased risk for T1D. As a subset of these genes are related to cellular stress responses, we measured integrated stress response (ISR) genes and identified 20 with altered expression in T1D pancreata, including three of the four eIF2α-dependent kinases. Equally intriguing, we observed significant repression of the three arms of the ISR in autoantibody+ pancreata. Collectively, these efforts suggest monogenic diabetes and ISR genes are dysregulated early in the T1D disease process and likely contribute to the disorder's pathogenesis.

Temporal Analysis of Amylase Expression in Control, Autoantibody-Positive, and Type 1 Diabetes Pancreatic Tissues

Within the human pancreas, exocrine and endocrine cells control secretion of digestive enzymes and production of hormones to maintain metabolic homeostasis, respectively. While the vast majority of type 1 diabetes research efforts have focused on endocrine function and autoimmunity, recent studies identified a series of unique features (e.g., reduced weight and volume, increased density of leukocytes) within the exocrine pancreas in this disease, but the mechanisms underlying these aberrancies are unknown. Therefore, we histologically assessed amylase, insulin, glucagon, lipase, and/or trypsinogen in 78 organ donor pancreata from birth through adulthood in control subjects and those at various stages of type 1 diabetes. While amylase-positive (AMY⁺) acinar cells were detectable in pancreata from all study groups, tissues from individuals >2 years of age contained clusters of acinar cells devoid of amylase (AMY^-). A majority of these AMY^- cell clusters localized proximal to islets (i.e., peri-islet). Additionally, most AMY^- clusters were positive for the exocrine enzymes lipase and trypsinogen. Interestingly, type 1 diabetes pancreata displayed significant reductions in the frequency of these AMY^- cell clusters. These results support a contribution of the islet-acinar axis in pancreatic development and underscore a potential role for the exocrine pancreas in the pathogenesis of type 1 diabetes.

A Map of Human Type 1 Diabetes Progression by Imaging Mass Cytometry

Type 1 diabetes (T1D) results from the autoimmune destruction of insulin-producing β cells. A comprehensive picture of the changes during T1D development is lacking due to limited sample availability, inability to sample longitudinally, and the paucity of technologies enabling comprehensive tissue profiling. Here, we analyzed 1,581 islets from 12 human donors, including eight with T1D, using imaging mass cytometry (IMC). IMC enabled simultaneous measurement of 35 biomarkers with single-cell and spatial resolution. We performed pseudotime analysis of islets through T1D progression from snapshot data to reconstruct the evolution of β cell loss and insulitis. Our analyses revealed that β cell destruction is preceded by a β cell marker loss and by recruitment of cytotoxic and helper T cells. The approaches described herein demonstrate the value of IMC for improving our understanding of T1D pathogenesis, and our data lay the foundation for hypothesis generation and follow-on experiments.

C-terminal proline deletions in KCNC3 cause delayed channel inactivation and an adult-onset progressive SCA13 with spasticity

Mutations in the potassium channel gene KCNC3 (Kv3.3) cause the autosomal dominant neurological disease, spinocerebellar ataxia 13 (SCA13). In this study, we expand the genotype-phenotype repertoire of SCA13 by describing the novel KCNC3 deletion p.Pro583_Pro585del highlighting the allelic heterogeneity observed in SCA13 patients. We characterize adult-onset, progressive clinical symptoms of two afflicted kindred and introduce the symptom of profound spasticity not previously associated with the SCA13 phenotype. We also present molecular and electrophysiological characterizations of the mutant protein in mammalian cell culture. Mechanistically, the p.Pro583_Pro585del protein showed normal membrane trafficking with an altered electrophysiological profile, including slower inactivation and decreased sensitivity to the inactivation-accelerating effects of the actin depolymerizer latrunculin B. Taken together, our results highlight the clinical importance of the intracellular C-terminal portion of Kv3.3 and its association with ion channel function.

Persistence of Pancreatic Insulin mRNA Expression and Proinsulin Protein in Type 1 Diabetes Pancreata

The canonical notion that type 1 diabetes (T1D) results following a complete destruction of β cells has recently been questioned as small amounts of C-peptide are detectable in patients with long-standing disease. We analyzed protein and gene expression levels for proinsulin, insulin, C-peptide, and islet amyloid polypeptide within pancreatic tissues from T1D, autoantibody positive (Ab+), and control organs. Insulin and C-peptide levels were low to undetectable in extracts from the T1D cohort; however, proinsulin and INS mRNA were detected in the majority of T1D pancreata. Interestingly, heterogeneous nuclear RNA (hnRNA) for insulin and INS-IGF2, both originating from the INS promoter, were essentially undetectable in T1D pancreata, arguing for a silent INS promoter. Expression of PCSK1, a convertase responsible for proinsulin processing, was reduced in T1D pancreata, supportive of persistent proinsulin. These data implicate the existence of β cells enriched for inefficient insulin/C-peptide production in T1D patients, potentially less susceptible to autoimmune destruction.

A KCNC3 mutation causes a neurodevelopmental, non-progressive SCA13 subtype associated with dominant negative effects and aberrant EGFR trafficking

The autosomal dominant spinocerebellar ataxias (SCAs) are a diverse group of neurological disorders anchored by the phenotypes of motor incoordination and cerebellar atrophy. Disease heterogeneity is appreciated through varying comorbidities: dysarthria, dysphagia, oculomotor and/or retinal abnormalities, motor neuron pathology, epilepsy, cognitive impairment, autonomic dysfunction, and psychiatric manifestations. Our study focuses on SCA13, which is caused by several allelic variants in the voltage-gated potassium channel KCNC3 (Kv3.3). We detail the clinical phenotype of four SCA13 kindreds that confirm causation of the KCNC3R423H allele. The heralding features demonstrate congenital onset with non-progressive, neurodevelopmental cerebellar hypoplasia and lifetime improvement in motor and cognitive function that implicate compensatory neural mechanisms. Targeted expression of human KCNC3R423H in Drosophila triggers aberrant wing veins, maldeveloped eyes, and fused ommatidia consistent with the neurodevelopmental presentation of patients. Furthermore, human KCNC3R423H expression in mammalian cells results in altered glycosylation and aberrant retention of the channel in anterograde and/or endosomal vesicles. Confirmation of the absence of plasma membrane targeting was based on the loss of current conductance in cells expressing the mutant channel. Mechanistically, genetic studies in Drosophila, along with cellular and biophysical studies in mammalian systems, demonstrate the dominant negative effect exerted by the mutant on the wild-type (WT) protein, which explains dominant inheritance. We demonstrate that ocular co-expression of KCNC3R423H with Drosophila epidermal growth factor receptor (dEgfr) results in striking rescue of the eye phenotype, whereas KCNC3R423H expression in mammalian cells results in aberrant intracellular retention of human epidermal growth factor receptor (EGFR). Together, these results indicate that the neurodevelopmental consequences of KCNC3R423H may be mediated through indirect effects on EGFR signaling in the developing cerebellum. Our results therefore confirm the KCNC3R423H allele as causative for SCA13, through a dominant negative effect on KCNC3WT and links with EGFR that account for dominant inheritance, congenital onset, and disease pathology.

Endothelin-1-mediated vasoconstriction alters cerebral gene expression in iron homeostasis and eicosanoid metabolism

Endothelins are potent vasoconstrictors and signaling molecules. Their effects are broad, impacting processes ranging from neurovascular and cardiovascular health to cell migration and survival. In stroke, traumatic brain injury or subarachnoid hemorrhage, endothelin-1 (ET-1) is induced resulting in cerebral vasospasm, ischemia, reperfusion and the activation of various pathways. Given the central role that ET-1 plays in these patients and to identify the downstream molecular events specific to transient vasoconstriction, we studied the consequences of ET-1-mediated vasoconstriction of the middle cerebral artery in a rat model. Our observations demonstrate that ET-1 can lead to increases in gene expression, including genes associated with the inflammatory response (Ifnb, Il6, Tnf) and oxidative stress (Hif1a, Myc, Sod2). We also observed inductions (>2 fold) of genes involved in eicosanoid biosynthesis (Pla2g4a, Pla2g4b, Ptgs2, Ptgis, Alox12, Alox15), heme metabolism (Hpx, Hmox1, Prdx1) and iron homeostasis (Hamp, Tf). Our findings demonstrate that mRNA levels for the hormone hepcidin (Hamp) are induced in the brain in response to ET-1, providing a novel target in the treatment of multiple conditions. These changes on the ipsilateral side were also accompanied by corresponding changes in a subset of genes in the contralateral hemisphere. Understanding ET-1-mediated events at the molecular level may lead to better treatments for neurological diseases and provide significant impact on neurological function, morbidity and mortality.

A TEAD1/p65 complex regulates the eutherian-conserved MnSOD intronic enhancer, eRNA transcription and the innate immune response

Manganese superoxide dismutase (MnSOD), a critical anti-oxidant enzyme, detoxifies the mitochondrial-derived reactive oxygen species, superoxide, elicited through normal respiration or the inflammatory response. Proinflammatory stimuli induce MnSOD gene expression through a eutherian-conserved, intronic enhancer element. We identified two prototypic enhancer binding proteins, TEAD1 and p65, that when co-expressed induce MnSOD expression comparable to pro-inflammatory stimuli. TEAD1 causes the nuclear sequestration of p65 leading to a novel TEAD1/p65 complex that associates with the intronic enhancer and is necessary for cytokine induction of MnSOD. Unlike typical NF-κB-responsive genes, the induction of MnSOD does not involve p50. Beyond MnSOD, the TEAD1/p65 complex regulates a subset of genes controlling the innate immune response that were previously viewed as solely NF-κB-dependent. We also identified an enhancer-derived RNA (eRNA) that is induced by either proinflammatory stimuli or the TEAD1/p65 complex, potentially linking the intronic enhancer to intra- and interchromosomal gene regulation through the inducible eRNA.

Characterization of a mechanism to inhibit ovarian follicle activation

OBJECTIVE

To demonstrate that a small molecule can induce the transcription factor Foxo3 in the ovary and lead to inhibition of follicle activation.

DESIGN

Cell culture, organ culture, and animal studies.

SETTING

University-based laboratory.

ANIMAL(S)

23 female C57BL/6 mice.

INTERVENTION(S)

Human ovary cells and mouse ovaries in culture treated with 2-deoxyglucose (2-DG) to mimic glucose deprivation, and mice intraperitoneally injected with 100 mg/kg, 300 mg/kg, or 600 mg/kg 2-DG daily for 2 weeks.

MAIN OUTCOME MEASURE(S)

In cell and organ culture, Foxo3 expression analyzed by quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR); in treated animals, expression of genes regulated by nutrient deprivation (Foxo3, ATF4, GRP78, CHOP, ASNS, c-Myc) measured in brain, kidney, and ovary by qRT-PCR; and ovarian follicles histologically classified and counted.

RESULT(S)

Foxo3 expression is induced by 2-DG at both the mRNA and protein level in human ovarian cell culture, possibly through ATF4-dependent gene regulation. Foxo3 expression is also induced by 2-DG in ovarian organ culture. Treatment of mice with 100 mg/kg 2-DG resulted in a 2.6 fold induction of Foxo3 in the ovary and a 58% decrease in type 3a primary follicles.

CONCLUSION(S)

Expression of Foxo3 is induced by nutrient deprivation in cell culture, organ culture, and in vivo. In mice, 2-DG treatment results in an inhibition of primordial follicle activation. These data indicate that Foxo3 induction by 2-DG may be useful for fertility preservation.

Conservation of the PTEN catalytic motif in the bacterial undecaprenyl pyrophosphate phosphatase, BacA/UppP

Isoprenoid lipid carriers are essential in protein glycosylation and bacterial cell envelope biosynthesis. The enzymes involved in their metabolism (synthases, kinases and phosphatases) are therefore critical to cell viability. In this review, we focus on two broad groups of isoprenoid pyrophosphate phosphatases. One group, containing phosphatidic acid phosphatase motifs, includes the eukaryotic dolichyl pyrophosphate phosphatases and proposed recycling bacterial undecaprenol pyrophosphate phosphatases, PgpB, YbjB and YeiU/LpxT. The second group comprises the bacterial undecaprenol pyrophosphate phosphatase, BacA/UppP, responsible for initial formation of undecaprenyl phosphate, which we predict contains a tyrosine phosphate phosphatase motif resembling that of the tumour suppressor, phosphatase and tensin homologue (PTEN). Based on protein sequence alignments across species and 2D structure predictions, we propose catalytic and lipid recognition motifs unique to BacA/UppP enzymes. The verification of our proposed active-site residues would provide new strategies for the development of substrate-specific inhibitors which mimic both the lipid and pyrophosphate moieties, leading to the development of novel antimicrobial agents.

A distal enhancer controls cytokine-dependent human cPLA2α gene expression

Specific control of group IVA cytosolic phospholipase A2 (cPLA2α or PLA2G4A) expression modulates arachidonic acid production, thus tightly regulating the downstream effects of pro- and anti-inflammatory eicosanoids. The significance of this pathway in human disease is apparent in a range of pathologies from inflammation to tumorigenesis. While much of the regulation of cPLA2α has focused on posttranslational phosphorylation of the protein, studies on transcriptional regulation of this gene have focused only on proximal promoter regions. We have identified a DNase I hypersensitive site encompassing a 5' distal enhancer element containing a highly conserved consensus AP-1 site involved in transcriptional activation of cPLA2α by interleukin (IL)-1β. Chromatin immunoprecipitation (ChIP), knockdown, knockout, and overexpression analyses have shown that c-Jun acts both in a negative and positive regulatory role. Transcriptional activation of cPLA2α occurs through the phosphorylation of c-Jun in conjunction with increased association of C/EBPβ with the distal novel enhancer. The association of C/EBPβ with the transcriptional activation complex does not require an obvious DNA binding site. These data provide new and important contributions to the understanding of cPLA2α regulation at the transcriptional level, with implications for eicosanoid metabolism, cellular signaling, and disease pathogenesis.

Effect of allergy and inflammation on eicosanoid gene expression in CFTR deficiency

BACKGROUND

Allergic bronchopulmonary aspergillosis (ABPA) is a complicating factor in cystic fibrosis (CF), affecting 2-15% of patients. We hypothesized that sensitization/challenge of CFTR(-/-) mice with an Aspergillus fumigatus (Af) extract will affect eicosanoid pathway gene expression, impacting ABPA and CF.

METHODS

FABP-hCFTR(+/-)-CFTR(-/-) mice were sensitized/challenged with an Af extract and gene expression of lung mRNA was evaluated for >40 genes, with correlative data in human CF (IB3.1) and CFTR-corrected (S9) bronchoepithelial cell lines.

RESULTS

Pla2g4c, Pla2g2c, Pla2g2d and Pla2g5 were induced in response to Af in CFTR(-/-) mice. Interestingly, PLA2G2D was induced by LPS, IL-2, IL-6, IL-13, and Af only in CFTR-deficient human IB3.1 cells. Prostanoid gene expression was relatively constant, however, several 12/15-lipoxygenase genes were induced in response to Af. Numerous cytokines also caused differential expression of ALOX15 only in IB3.1 cells.

CONCLUSIONS

The distinct regulation of PLA2G4C, PLA2G2D and ALOX15 genes in Aspergillus sensitization and/or cystic fibrosis could provide new insights into diagnosis and treatment of ABPA and CF.

cPLA(2)α gene activation by IL-1β is dependent on an upstream kinase pathway, enzymatic activation and downstream 15-lipoxygenase activity: a positive feedback loop

Cytosolic phospholipase A(2)α (cPLA(2)α) is the most widely studied member of the Group IV PLA(2) family. The enzyme is Ca(2+)-dependent with specificity for phospholipid substrates containing arachidonic acid. As the pinnacle of the arachidonic acid pathway, cPLA(2)α has a primary role in the biosynthesis of a diverse family of eicosanoid metabolites, with potent physiological, inflammatory and pathological consequences. cPLA(2)α activity is regulated by pro-inflammatory stimuli through pathways involving increased intracellular Ca(2+) levels, phosphorylation coupled to increased enzymatic activity and de novo gene transcription. This study addresses the signal transduction pathways for protein phosphorylation and gene induction following IL-1β stimulation in human fetal lung fibroblasts. Our results utilizing both inhibitors and kinase-deficient cells demonstrate that cPLA(2)α is phosphorylated within 10min of IL-1β treatment, an event requiring p38 MAPK as well as the upstream kinase, MKK3/MKK6. Inhibition of p38 MAPK also blocks the phosphorylation of a downstream, nuclear kinase, MSK-1. Our results further demonstrate that the activities of both cPLA(2)α and a downstream lipoxygenase (15-LOX2) are required for IL-1β-dependent induction of cPLA(2)α mRNA expression. Overall, these data support an MKK3/MKK6→p38 MAPK→MSK-1→cPLA(2)α→15-LOX2-dependent, positive feedback loop where a protein's enzymatic activity is required to regulate its own gene induction by a pro-inflammatory stimulus.

Distinct functions of CCAAT enhancer-binding protein isoforms in the regulation of manganese superoxide dismutase during interleukin-1beta stimulation

The mitochondrial antioxidant enzyme manganese superoxide dismutase (Mn-SOD) is crucial in maintaining cellular and organismal homeostasis. Mn-SOD expression is tightly regulated in a manner that synchronizes its cytoprotective functions during inflammatory challenges. Induction of Mn-SOD gene expression by the proinflammatory cytokine IL-1beta is mediated through a complex intronic enhancer element. To identify and characterize the transcription factors required for Mn-SOD enhancer function, a yeast one-hybrid assay was utilized, and two CCAAT enhancer-binding protein (C/EBP) members, C/EBP beta and C/EBP delta, were identified. These two transcription factors responded to IL-1beta treatment with distinct expression profiles, different temporal yet inducible interactions with the endogenous Mn-SOD enhancer, and also opposite effects on Mn-SOD transcription. C/EBP beta is expressed as three isoforms, LAP* (liver-activating protein), LAP, and LIP (liver-inhibitory protein). Our functional analysis demonstrated that only the full-length C/EBP beta/LAP* served as a true activator for Mn-SOD, whereas LAP, LIP, and C/EBP delta functioned as potential repressors. Finally, our systematic mutagenesis of the unique N-terminal 21 amino acids further solidified the importance of LAP* in the induction of Mn-SOD and emphasized the crucial role of this isoform. Our data demonstrating the physiological relevance of the N-terminal peptide also provide a rationale for revisiting the role of LAP* in the regulation of other genes and in pathways such as lipogenesis and development.

Metabolic regulation of manganese superoxide dismutase expression via essential amino acid deprivation

Organisms respond to available nutrient levels by rapidly adjusting metabolic flux, in part through changes in gene expression. A consequence of adaptations in metabolic rate is the production of mitochondria-derived reactive oxygen species. Therefore, we hypothesized that nutrient sensing could regulate the synthesis of the primary defense of the cell against superoxide radicals, manganese superoxide dismutase. Our data establish a novel nutrient-sensing pathway for manganese superoxide dismutase expression mediated through essential amino acid depletion concurrent with an increase in cellular viability. Most relevantly, our results are divergent from current mechanisms governing amino acid-dependent gene regulation. This pathway requires the presence of glutamine, signaling via the tricarboxylic acid cycle/electron transport chain, an intact mitochondrial membrane potential, and the activity of both the MEK/ERK and mammalian target of rapamycin kinases. Our results provide evidence for convergence of metabolic cues with nutrient control of antioxidant gene regulation, revealing a potential signaling strategy that impacts free radical-mediated mutations with implications in cancer and aging.

Engineering and characterization of human manganese superoxide dismutase mutants with high activity and low product inhibition

Human manganese superoxide dismutase is a mitochondrial metalloenzyme that is involved in protecting aerobic organisms against superoxide toxicity, and has been implicated in slowing tumor growth. Unfortunately, this enzyme exhibits strong product inhibition, which limits its potential biomedical applications. Previous efforts to alleviate human manganese superoxide dismutase product inhibition utilized rational protein design and site-directed mutagenesis. These efforts led to variants of human manganese superoxide dismutase at residue 143 with dramatically reduced product inhibition, but also reduced catalytic activity and efficiency. Here, we report the use of a directed evolution approach to engineer two variants of the Q143A human manganese superoxide dismutase mutant enzyme with improved catalytic activity and efficiency. Two separate activity-restoring mutations were found--C140S and N73S--that increase the catalytic efficiency of the parent Q143A human manganese superoxide dismutase enzyme by up to five-fold while maintaining low product inhibition. Interestingly, C140S is a context-dependent mutation, and the C140S-Q143A human manganese superoxide dismutase did not follow Michaelis-Menten kinetics. The re-engineered human manganese superoxide dismutase mutants should be useful for biomedical applications, and our kinetic and structural studies also provide new insights into the structure-function relationships of human manganese superoxide dismutase.

Structural Mobility in Human Manganese Superoxide Dismutase

Human manganese superoxide dismutase (MnSOD) is a homotetramer of 22 kDa subunits, a dimer of dimers containing dimeric and tetrameric interfaces. We have investigated conformational mobility at these interfaces by measuring amide hydrogen/deuterium (H/D) exchange kinetics and 19F NMR spectra, both being excellent methods for analyzing local environments. Human MnSOD was prepared in which all nine tyrosine residues in each subunit are replaced with 3-fluorotyrosine. The 19F NMR spectrum of this enzyme showed five sharp resonances that have been assigned by site-specific mutagenesis by replacing each 3-fluorotyrosine with phenylalanine; four 19F resonances not observed are near the paramagnetic manganese and extensively broadened. The temperature dependence of the line widths and chemical shifts of the 19F resonances were used to estimate conformational mobility. 3-Fluorotyrosine 169 at the dimeric interface showed little conformational mobility and 3-fluorotyrosine 45 at the tetrameric interface showed much greater mobility by these measures. In complementary studies, H/D exchange mass spectrometry was used to measure backbone dynamics in human MnSOD. Using this approach, amide hydrogen exchange kinetics were measured for regions comprising 78% of the MnSOD backbone. Peptides containing Tyr45 at the tetrameric interface displayed rapid exchange of hydrogen with deuterium while peptides containing Tyr169 in the dimeric interface only displayed moderate exchange. Taken together, these studies show that residues at the dimeric interface, such as Tyr169, have significantly less conformational freedom or mobility than do residues at the tetrameric interface, such as Tyr45. This is discussed in terms of the role in catalysis of residues at the dimeric interface.

Enhanced IgE allergic response to Aspergillus fumigatus in CFTR-/- mice

To gain insight into aberrant cytokine regulation in cystic fibrosis (CF), we compared the phenotypic manifestations of allergen challenge in gut-corrected CFTR-deficient mice with background-matched C57Bl6 (B6) mice. Aspergillus fumigatus (Af) antigen was used to mimic allergic bronchopulmonary aspergillosis, a peculiar hyper-IgE syndrome with a high prevalence in CF patients. CFTR-/-, C57BL/6 and FVB/NJ mice were sensitized with Af antigen by serial intraperitoneal injections. Control mice were mock sensitized with PBS. Challenges were performed by inhalation of Af antigen aerosol. After Af antigen challenge, histologic analysis showed goblet cell hyperplasia and lymphocytic infiltration in both strains. However, total serum IgE levels were markedly elevated in CF mice. Sensitized CF mice showed a five-fold greater IgE response to sensitization as compared with B6- and FVB-sensitized controls. Additional littermate controls to fully normalize for B6-FVB admixture in the strain background confirmed the role of CFTR mutation in the hyper-IgE syndrome. Cytokine mRNA levels of IL-5 and GM-CSF in the bronchoalveolar lavage (BAL) fluid, and BAL cell differentials indicated that CFTR mutation caused a shift from an IL-5-predominant to an IL-4-predominant cytokine profile. This system models a very specific type of airway inflammation in CF and could provide insights into pathogenesis and treatment of the disease.

Hydrogen bonding in human manganese superoxide dismutase containing 3-fluorotyrosine

Incorporation of 3-fluorotyrosine and site-specific mutagenesis has been utilized with Fourier transform infrared (FTIR) spectroscopy and x-ray crystallography to elucidate active-site structure and the role of an active-site residue Tyr34 in human manganese superoxide dismutase (MnSOD). Calculated harmonic frequencies at the B3LYP/6-31G** level of theory for L-tyrosine and its 3-fluorine substituted analog are compared to experimental frequencies for vibrational mode assignments. Each of the nine tyrosine residues in each of the four subunits of the homotetramer of human MnSOD was replaced with 3-fluorotyrosine. The crystal structures of the unfluorinated and fluorinated wild-type MnSOD are nearly superimposable with the root mean-square deviation for 198 alpha-carbon atoms at 0.3 A. The FTIR data show distinct vibrational modes arising from 3-fluorotyrosine in MnSOD. Comparison of spectra for wild-type and Y34F MnSOD showed that the phenolic hydroxyl of Tyr34 is hydrogen bonded, acting as a proton donor in the active site. Comparison with crystal structures demonstrates that the hydroxyl of Tyr34 is a hydrogen bond donor to an adjacent water molecule; this confirms the participation of Tyr34 in a network of residues and water molecules that extends from the active site to the adjacent subunit.

Cellular localization and enzymatic activity of cathepsin B after spinal cord injury in the rat

Mechanical spinal cord injury (SCI) initiates a cascade of pathochemical and pathophysiological events, collectively known as the secondary injury. There has been a long-standing interest in understanding the activation and involvement of proteases in this secondary injury process. Several proteases including the calpains, caspases and matrix metalloproteinases are activated by perturbations to the spinal cord and have been linked to cell death following SCI and in other models of CNS disease and insult. Cathepsin B (Cath B), a potent lysosomal protease, has also been implicated in the pathology of CNS diseases including brain tumors, Alzheimer's disease, amyotrophic lateral sclerosis and stroke. Previously, we reported significant increases in Cath B mRNA and protein expression following contusion-SCI. This characterization of Cath B continues with the experiments reported herein, which were designed to examine Cath B enzymatic activity and cellular localization following contusion-SCI in the rat. Cath B enzymatic activity was significantly increased in the injury epicenter at 5 and 7 days post-injury and was highly correlated with increases in the active forms of the Cath B protein reported earlier. Furthermore, the immunohistochemical analyses revealed that the post-injury increases in expression and enzymatic activity at the injury epicenter were due to the presence of a large and diverse population of inflammatory cells. However, in areas adjacent to the injury epicenter, it appears that parenchymal neurons may also contribute to these increases. Our findings coupled with the documented role of Cath B in other CNS pathologies make this potent protease an attractive candidate for involvement in the tissue destruction associated with the secondary injury cascade following SCI.

Ultrafine carbon particles induce interleukin-8 gene transcription and p38 MAPK activation in normal human bronchial epithelial cells

Epidemiological studies suggest that ultrafine particles contribute to particulate matter-induced adverse health effects. Interleukin (IL)-8 is an important proinflammatory cytokine in the human lung that is induced in respiratory cells exposed to a variety of environmental insults, including ambient air ultrafine particles. In this study, we examined the effect of a model ultrafine particle on IL-8 expression and the cellular mechanisms responsible for this event. Here, we report that carbonaceous ultrafine particles consisting of synthetic elemental carbon particles (UfCP) markedly increase the expression of IL-8 mRNA and protein in normal human bronchial epithelial (NHBE) cells. IL-8 promoter activity was increased by UfCP exposure in NHBE cells, indicating UfCP-induced IL-8 expression is transcriptionally regulated. IL-8 expression in NHBE is known to be regulated by nuclear factor (NF)-κB activation. However, UfCP did not induce inhibitory factor κBα degradation, NF-κB-DNA binding, or NF-κB-dependent promoter activity in NHBE cells, indicating that UfCP induces IL-8 expression through a mechanism that is independent of NF-κB activation. Additionally, we observed that UfCP exposure induces the phosphorylation and activation of p38 mitogen-activated protein kinase (MAPK) in a biphasic manner and that the inhibition of p38 MAPK activity can block IL-8 mRNA expression induced by UfCP in NHBE cells. These results demonstrate that UfCP-induced expression of IL-8 involves a transcriptional mechanism and activation of p38 MAPK in NHBE cells.

Role of Hydrogen Bonding in the Active Site of Human Manganese Superoxide Dismutase†,‡

The side chain of Gln143, a conserved residue in manganese superoxide dismutase (MnSOD), forms a hydrogen bond with the manganese-bound solvent and is critical in maintaining catalytic activity. The side chains of Tyr34 and Trp123 form hydrogen bonds with the carboxamide of Gln143. We have replaced Tyr34 and Trp123 with Phe in single and double mutants of human MnSOD and measured their catalytic activity by stopped-flow spectrophotometry and pulse radiolysis. The replacements of these side chains inhibited steps in the catalysis as much as 50-fold; in addition, they altered the gating between catalysis and formation of a peroxide complex to yield a more product-inhibited enzyme. The replacement of both Tyr34 and Trp123 in a double mutant showed that these two residues interact cooperatively in maintaining catalytic activity. The crystal structure of Y34F/W123F human MnSOD at 1.95 A resolution suggests that this effect is not related to a conformational change in the side chain of Gln143, which does not change orientation in Y34F/W123F, but rather to more subtle electronic effects due to the loss of hydrogen bonding to the carboxamide side chain of Gln143. Wild-type MnSOD containing Trp123 and Tyr34 has approximately the same thermal stability compared with mutants containing Phe at these positions, suggesting the hydrogen bonds formed by these residues have functional rather than structural roles.

Amino Acid Substitution at the Dimeric Interface of Human Manganese Superoxide Dismutase

The side chains of His30 and Tyr166 from adjacent subunits in the homotetramer human manganese superoxide dismutase (Mn-SOD) form a hydrogen bond across the dimer interface and participate in a hydrogen-bonded network that extends to the active site. Compared with wild-type Mn-SOD, the site-specific mutants H30N, Y166F, and the corresponding double mutant showed 10-fold decreases in steady-state constants for catalysis measured by pulse radiolysis. The observation of no additional effect upon the second mutation is an example of cooperatively interacting residues. A similar effect was observed in the thermal stability of these enzymes; the double mutant did not reduce the major unfolding transition to an extent greater than either single mutant. The crystal structures of these site-specific mutants each have unique conformational changes, but each has lost the hydrogen bond across the dimer interface, which results in a decrease in catalysis. These same mutations caused an enhancement of the dissociation of the product-inhibited complex. That is, His30 and Tyr166 in wild-type Mn-SOD act to prolong the lifetime of the inhibited complex. This would have a selective advantage in blocking a cellular overproduction of toxic H2O2.

Immunity to adeno-associated virus serotype 2 delivered transgenes imparted by genetic predisposition to autoimmunity

Adeno-associated virus (AAV) is widely considered a promising vector for therapeutic gene delivery. This promise is based on previous studies assessing AAVs safety and toxicity, ability to infect nondividing cells, elicit a limited immune response and provide long-term gene expression. However, we now find that earlier studies underappreciated the degree of AAV immunogenicity as well as the extent to which genetic background, through regulation of immune responsiveness, influences the duration of gene expression and thereby the effectiveness of AAV-mediated gene therapy. We evaluated antibody responses in 12 mouse strains to AAV serotype 2 (AAV2) and AAV2-expressed transgene products including green fluorescent protein (GFP), human alpha1-antitrypsin and murine interleukin-10. As expected, all immunocompetent mice administered AAV2 developed serologic evidence of immune responsiveness to the virus. However, a previously unidentified serologic prozone effect was observed suggesting that the concentrations of anti-AAV2 antibodies may have historically been subject to marked underestimation. Furthermore, strains with genetic predisposition to autoimmunity (eg, NOD, NZW, MRL-lpr) specifically imparted a functionally deleterious immune response to AAV-delivered transgene products. These findings suggest that more thorough studies of anti-AAV immunity should be performed, and that genetic predisposition to autoimmunity should be considered when assessing AAV efficacy and safety in humans.

Potent anti-tumor effects of an active site mutant of human manganese-superoxide dismutase. Evolutionary conservation of product inhibition

Mn-SOD serves as the primary cellular defense against oxidative damage by converting superoxide radicals (O(2)(-)) to O(2) and H(2)O(2). A unique characteristic of this mitochondrial anti-oxidant enzyme is the conservation from bacteria to man of a rapidly formed product inhibited state. Using site-directed mutagenesis, we have generated an active site mutant (H30N) of human Mn-SOD, which exhibits significantly reduced product inhibition and increased enzymatic efficiency. Overexpression of the H30N enzyme causes anti-proliferative effects in vitro and anti-tumor effects in vivo. Our results provide a teleological basis for the phylogenetically invariant nature of position His-30 and the evolutionary conservation of product inhibition. These data also provide more direct intracellular evidence for the signaling role associated with H(2)O(2).

An internal enhancer regulates heme- and cadmium-mediated induction of human heme oxygenase-1

Heme oxygenase-1 (HO-1) catalyzes the rate-limiting step in heme degradation, releasing iron, carbon monoxide, and biliverdin. Induction of HO-1 is an adaptive and beneficial response in renal and nonrenal settings of tissue injury. The purpose of this study was to characterize the regulation of the human HO-1 gene in renal proximal tubule and aortic endothelial cells in response to heme and cadmium. Evaluation of multiple human HO-1 promoter-reporter constructs up to -9.1 kb demonstrated only a partial response to heme and cadmium. In an effort to mimic endogenous stimulus-dependent levels of HO-1 induction, we evaluated the entire 12.5 kb of the human HO-1 gene, including introns and exons, in conjunction with a -4.5-kb human HO-1 promoter and observed significant heme- and cadmium-mediated induction of the reporter gene, suggesting the presence of an internal enhancer. Enhancer function was orientation independent and required a region between -3.5 and -4.5 kb of the human HO-1 promoter. Our studies identified a novel enhancer internal to the human HO-1 gene that, in conjunction with the HO-1 promoter, recapitulates heme- and cadmium-mediated induction of the endogenous HO-1 gene. Elucidation of the molecular regulation of the human HO-1 gene will allow for the development of therapeutic strategies to manipulate HO-1 gene expression in pathological states.

A cis-acting region regulates oxidized lipid-mediated induction of the human heme oxygenase-1 gene in endothelial cells

OBJECTIVE

Several proatherogenic agents including oxidized LDL and its major component, 13-hydroperoxyoctadecadienoic acid (13-HPODE), upregulate heme oxygenase-1 (HO-1). Our previous studies have demonstrated that 13-HPODE-mediated HO-1 induction occurs via transcriptional mechanisms. The purpose of this study was to evaluate the molecular regulation and identify the signaling pathways involved in 13-HPODE-mediated HO-1 induction in human aortic endothelial cells.

METHODS AND RESULTS

The half-life of HO-1 mRNA after stimulation with 13-HPODE was approximately 1.8 hours. Antioxidants such as N-acetylcysteine, iron chelation with deferoxamine mesylate, and protein kinase C inhibition with Gö6976 blocked HO-1 induction. Using promoter constructs up to 9.1 kb, no significant reporter activity was observed in response to 13-HPODE. A 13-HPODE-inducible DNase I hypersensitive site was identified that maps to a region approximately 10 to 11 kb from the transcription start site of the human HO-1 gene. Based on the DNase I analysis, a -11.6-kb human HO-1 promoter construct was generated and elicited a 2.5-fold increase in reporter activity, indicating that 13-HPODE-mediated human HO-1 induction requires, at least in part, sequences that reside between 9.1 and 11.6 kb of the human HO-1 promoter.

CONCLUSIONS

Elucidation of the molecular mechanisms which control HO-1 gene expression will allow us to develop therapeutic strategies to enhance the cytoprotective potential of HO-1 in atherosclerosis.

Catalytic and structural effects of amino acid substitution at histidine 30 in human manganese superoxide dismutase: insertion of valine C gamma into the substrate access channel

Catalysis of the disproportionation of superoxide by human manganese superoxide dismutase (MnSOD) is characterized by an initial burst of catalysis followed by a much slower region that is zero order in superoxide and due to a product inhibition by peroxide anion. We have prepared site-specific mutants with replacements at His30, the side chain of which lies along the substrate access channel and is about 5.8 A from the metal. Using pulse radiolysis to generate superoxide, we have determined that kcat/K(m) was decreased and product inhibition increased for H30V MnSOD, both by 1-2 orders of magnitude, compared with wild type, H30N, and H30Q MnSOD. These effects are not attributed to the redox potentials, which are similar for all of these variants. An investigation of the crystal structure of H30V Mn(III)SOD compared with wild type, H30Q, and H30N Mn(III)SOD showed the positions of two gamma carbons of Val30 in the active site; Cgamma1 overlaps Cgamma of His30 in wild type, and Cgamma2 extends into the substrate access channel and occupies the approximate position of a water molecule in the wild type. The data suggest that Cgamma2 of the Val side chain has significantly interrupted catalysis by this overlap into the access channel with possible overlap with the substrate-product binding site. This is supported by comparison of the crystal structure of H30V MnSOD with that of azide bound to Mn(III)SOD from Thermus thermophilus and by visible absorption spectra showing that azide binding to the metal in H30V Mn(III)SOD is abolished. Moreover, the presence of Val30 caused a 100-fold decrease in the rate constant for dissociation of the product-inhibited complex compared with wild type.

PIN*POINT analysis on the endogenous MnSOD promoter: specific demonstration of Sp1 binding in vivo

Manganese superoxide dismutase (MnSOD) is a critical antioxidant enzyme that protects against superoxide anion generated as a consequence of normal cellular respiration, as well as during the inflammatory response. By employing dimethyl sulfate in vivo footprinting, we have previously identified ten basal protein binding sites within the MnSOD promoter. On the basis of consensus sequence comparison and in vitro footprinting data, one would predict that Sp1 might occupy five of these binding sites. To address these findings in the context of the nucleoprotein environment, we first utilized chromatin immunoprecipitation (ChIP) to demonstrate the nuclear association of Sp1 with the MnSOD promoter region. To identify the precise location of Sp1 binding, we have modified the original protein position identification with nuclease tail (PIN*POINT) methodology, providing an approach to establish both the identity and binding occupancy of Sp1 in the context of the endogenous MnSOD promoter. These data, coupled with site-directed mutagenesis, demonstrate the functional importance of two of the Sp1 binding sites in the stimulus-specific regulation of MnSOD gene expression. We feel that the combination of ChIP and PIN*POINT analysis allows unequivocal identification and localization of protein/DNA interactions in vivo, specifically the demonstration of Sp1 with the MnSOD promoter.

Induction of manganese superoxide dismutase in acute spinal cord injury

Free radical-mediated mechanisms of cellular damage have been implicated in the early stages of spinal cord injury (SCI). Manganese superoxide dismutase (MnSOD) is a potent scavenger of superoxide radicals and likely serves an important cytoprotective role in preventing cellular damage after SCI. We have evaluated the expression of MnSOD to address its role during the early events of SCI using a well-established rat contusion model. Northern analysis showed a rapid induction of MnSOD mRNA between 2 and 6 h post injury. Observed time-dependent increases in MnSOD message was maximal 6 h post injury over that of MnSOD mRNA levels induced by laminectomy alone. Immunoblot and immunohistochemical analysis demonstrated increased expression of MnSOD protein 24 h after SCI with localization primarily within neurons. Interestingly, laminectomy alone also caused an induction of MnSOD gene and protein expression. To evaluate one potential mechanism of MnSOD induction, we microinjected the naive spinal cord with IL-1beta, which caused a similar fold induction of MnSOD mRNA levels by 6 h as observed with SCI, thus implicating it as a potential inducer of MnSOD during SCI. In summary, these results demonstrate that this potent cytoprotective antioxidant enzyme is rapidly and significantly induced as a consequence of SCI.

Catalytic pathway of manganese superoxide dismutase by direct observation of superoxide

Measurement of catalysis by MnSOD using direct observation of the UV absorbance of superoxide allows determination of steady-state catalytic constants. Stabilizing superoxide in aprotic solvents such as dimethyl sulfoxide permits the use of stopped-flow spectrophotometry, although significant information is lost in the 2- to 4-msec mixing time; generating superoxide by pulse radiolysis requires no mixing time. Studies show that kcat/Km for the decay of superoxide catalyzed by MnSOD proceeds at diffusion control. Investigations using solvent hydrogen isotope effects and enhancement of catalysis by exogenous proton donors show that kcat near 10(4) sec-1 contains a significant contribution from proton transfer steps. The active site of MnSOD is dominated by a hydrogen bond network comprising the manganese-bound aqueous ligand, the side chains of four residues (Gln-143, Tyr-34, His-30, and Tyr-166 from an adjacent subunit), as well as other water molecules. Interrupting this hydrogen bond network by conservative replacement of residues 30, 34, and 166 causes a 10- to 40-fold decrease in maximal velocity, interpreted as an effect on proton transport to the active site, with smaller effects on kcat/Km. Replacement of Gln-143 causes a much greater decrease in catalytic activity, by two to three orders of magnitude, and causes significant changes to the redox potential as well. During catalysis, MnSOD is inhibited by a peroxide complex of the metal in the active site, different from the inhibition of FeSOD and Cu,ZnSOD by Fenton chemistry. Site-specific mutagenesis of active-site residues alters the extent of product inhibition of MnSOD as well, indicating that this is not only a property of the metal. The replacement of Trp-161 with phenylalanine results in a variant that is completely blocked in catalysis by product inhibition.

Manganese superoxide dismutase attenuates Cisplatin-induced renal injury: importance of superoxide

Cisplatin is a potent chemotherapeutic agent that is used to treat many human malignancies. Unfortunately, in addition to side effects such as ototoxicity, anaphylaxis, and bone marrow suppression, a significant percentage of patients receiving cisplatin develop severe nephrotoxicity. Mitochondrial dysfunction that is mediated via the generation of reactive oxygen species has been implicated in the pathogenesis of cisplatin-induced renal injury. To address the mechanism, it was hypothesized that overexpression of antioxidant enzymes, such as mitochondria-localized manganese superoxide dismutase (MnSOD) or mitochondria-targeted catalase (mito-Cat), would be cytoprotective in cisplatin-induced cell injury. To this end, human MnSOD or a mito-Cat vector were stably transfected into human embryonic kidney 293 cells. Cells that overexpressed MnSOD exhibited significantly less cell rounding and detachment compared with both mito-Cat and vector controls after exposure to 20 microM cisplatin. Cell injury as assessed by DNA fragmentation and annexin V binding assays was significantly decreased in the cells that overexpressed MnSOD compared with vector alone and mito-Cat. In addition, elevated levels of MnSOD were strongly associated with increased clonogenic potential after cisplatin challenge. Thus, overexpression of MnSOD, and not catalase, protects against cisplatin-induced renal epithelial cell injury. These results demonstrate the importance of reactive oxygen species in the mechanism that underlies cisplatin-induced renal injury and specifically implicate the superoxide radical, and not hydrogen peroxide, as the mediator.

Mechanism of heme oxygenase-1 gene induction by curcumin in human renal proximal tubule cells

Heme oxygenase-1 (HO-1) catalyzes the rate-limiting step in heme degradation, releasing iron, carbon monoxide, and biliverdin. Induction of HO-1 occurs as an adaptive and protective response to several inflammatory stimuli. The transcription factor activator protein-1 (AP-1) has been implicated in the activation of the HO-1 gene. To elucidate the molecular mechanism of HO-1 induction, we examined the effects of diferuloylmethane (curcumin), an inhibitor of the transcription factor AP-1. Surprisingly, curcumin by itself was a very potent inducer of HO-1. Curcumin has anti-inflammatory, antioxidant, and renoprotective effects. To evaluate the mechanism of curcumin-mediated induction of HO-1, confluent human renal proximal tubule cells were exposed to curcumin (1-8 microM). We observed a time- and dose-dependent induction of HO-1 mRNA that was associated with increased HO-1 protein. Coincubation of curcumin with actinomycin D completely blocked the upregulation of HO-1 mRNA. Blockade of nuclear factor-kappaB (NF-kappaB) with an IkappaBalpha phosphorylation inhibitor attenuated curcumin-mediated induction of HO-1 mRNA and protein. These data demonstrate that curcumin induces HO-1 mRNA and protein in renal proximal tubule cells. HO-1 induction by curcumin is mediated, at least in part, via transcriptional mechanisms and involves the NF-kappaB pathway.

Mechanism of heme oxygenase-1 gene induction by curcumin in human renal proximal tubule cells

First published August 9, 2001; 10.1152/ajprenal.00140.2001.—Heme oxygenase-1 (HO-1) catalyzes the rate-limiting step in heme degradation, releasing iron, carbon monoxide, and biliverdin. Induction of HO-1 occurs as an adaptive and protective response to several inflammatory stimuli. The transcription factor activator protein-1 (AP-1) has been implicated in the activation of the HO-1 gene. To elucidate the molecular mechanism of HO-1 induction, we examined the effects of diferuloylmethane (curcumin), an inhibitor of the transcription factor AP-1. Surprisingly, curcumin by itself was a very potent inducer of HO-1. Curcumin has anti-inflammatory, antioxidant, and renoprotective effects. To evaluate the mechanism of curcumin-mediated induction of HO-1, confluent human renal proximal tubule cells were exposed to curcumin (1–8 μM). We observed a time- and dose-dependent induction of HO-1 mRNA that was associated with increased HO-1 protein. Coincubation of curcumin with actinomycin D completely blocked the upregulation of HO-1 mRNA. Blockade of nuclear factor-κB (NF-κB) with an IκBα phosphorylation inhibitor attenuated curcumin-mediated induction of HO-1 mRNA and protein. These data demonstrate that curcumin induces HO-1 mRNA and protein in renal proximal tubule cells. HO-1 induction by curcumin is mediated, at least in part, via transcriptional mechanisms and involves the NF-κB pathway.

Kinetic analysis of product inhibition in human manganese superoxide dismutase

Manganese superoxide dismutase (MnSOD) cycles between the Mn(II) and Mn(III) states during the catalyzed disproportionation of O(2)(*-), a catalysis that is limited at micromolar levels of superoxide by a peroxide-inhibited complex with the metal. We have investigated the role in catalysis and inhibition of the conserved residue Trp161 which forms a hydrophobic side of the active site cavity of MnSOD. Crystal structures of mutants of human MnSOD in which Trp161 was replaced with Ala or Phe showed significant conformational changes on adjacent residues near the active site, particularly Gln143 and Tyr34 which in wild-type MnSOD participate in a hydrogen bond network believed to support proton transfer during catalysis. Using pulse radiolysis and observing the UV absorbance of superoxide, we have determined rate constants for the catalytic dismutation of superoxide. In addition, the rates of formation and dissociation of the product-inhibited complex of these mutants were determined by direct observation of the characteristic visible absorption of the oxidized and inhibited states. Catalysis by W161A and W161F MnSOD was associated with a decrease of at least 100-fold in the catalytic rate of reduction of superoxide, which then promotes a competing pathway leading to product inhibition. The structural changes caused by the mutations at position 161 led to small changes, at most a 6-fold decrease, in the rate constant for formation of the inhibited complex. Solvent hydrogen isotope effects support a mechanism in which formation of this complex, presumably the peroxide dianion bound to the manganese, involves no rate-contributing proton transfer; however, the dissociation of the complex requires proton transfer to generate HO(2)(-) or H2O2.

A coding region determinant of instability regulates levels of manganese superoxide dismutase mRNA

The mitochondria-localized manganese superoxide dismutase (MnSOD), serves a key cytoprotective role against reactive oxygen species arising from a variety of cellular processes and immunological stresses. Previous data from our laboratory suggest that the regulation of the rat MnSOD gene may occur not only at the transcriptional but quite possibly at the post-transcriptional level. To verify this hypothesis, we have attempted to identify regions within the rat MnSOD cDNA that may be functionally involved in regulating the stability of the mRNA. Using a c-fos-based promoter activation system, we have identified an approximately 280-nucleotide fragment within the MnSOD mRNA coding region that, when fused to a rabbit beta-globin gene, destabilizes the normally stable beta-globin mRNA. This cis-directed destabilization phenomenon confers its effects independent of position and stimulus. Most importantly, the MnSOD coding region determinant functions when placed in the 3'-untranslated region of the beta-globin transcript, demonstrating its activity in the absence of ribosome transit. We feel that these data provide a mechanistic basis for both the basal and stimulus-dependent post-transcriptional regulation of MnSOD.

Redox properties of human manganese superoxide dismutase and active-site mutants

The redox potential of human manganese superoxide dismutase (MnSOD) has been difficult to determine because of the problem of finding suitable electron mediators. We have found that ferricyanide and pentacyanoaminoferrate can be used as electron mediators, although equilibration is very slow with a half-time near 6 h. Values of the midpoint potential were determined both by allowing enzyme and mediators to equilibrate up to 38 h and by reductive titration adding dithionite to enzyme and mediator. An overall value of the midpoint potential was found to be 393 +/- 29 mV. To elucidate the role of His30 and Tyr34 in the active site of human MnSOD, we have also measured the redox properties of the site-specific mutants His30Asn (H30N) and Tyr34Phe (Y34F) and compared them with the wild-type enzyme. Crystal structures have shown that each mutation interrupts a hydrogen bond network in the active site, and each causes a 10-fold decrease in the maximal velocity of catalysis of superoxide dismutation as compared with wild type. The present study shows that H30N and Y34F human MnSOD have very little effect, within experimental uncertainty, on the redox potential of the active-site metal. The redox potentials determined electrochemically were 365 +/- 28 mV for H30N and 435 +/- 30 mV for Y34F MnSOD. These results suggest that the role of His30 and Tyr34 is more in support of catalysis, probably proton transport, and not in the tuning of the redox potential.

Tumor necrosis factor-alpha selectively induces MnSOD expression via mitochondria-to-nucleus signaling, whereas interleukin-1beta utilizes an alternative pathway

Mitochondrial levels of the anti-oxidant enzyme, manganese superoxide dismutase (MnSOD), are dramatically elevated in response to stimulation with tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), and lipopolysaccharide (LPS). However, the precise intracellular signaling pathways responsible for this inducible expression are poorly understood. MnSOD expression in pulmonary epithelial and endothelial cells, treated with inflammatory mediators and various inhibitors, was studied by Northern analysis. The mitochondrial electron transport chain inhibitors, antimycin A and myxothiazol, selectively blocked TNF-alpha-inducible expression of MnSOD but not that of IL-1beta or LPS, indicating different signaling pathways. N-Acetylcysteine could reliably decrease inducible MnSOD expression by TNF-alpha, but not IL-1, linking reactive oxygen species (ROS) to the TNF-alpha signaling pathway. Elevated levels of arachidonic acid have been demonstrated previously to generate mitochondrial ROS. A specific cytoplasmic phospholipase A(2) inhibitor reduced stimulated MnSOD expression by TNF-alpha, but not by IL-1beta, further supporting the role of ROS. Other investigators have shown that MnSOD expression may be regulated by NF-kappaB. Our results with a specific inhibitory kappa-kinase inhibitor indicate that NF-kappaB modulates IL-1beta signaling but not the TNF-alpha pathway. Thus, we have demonstrated that although inducible MnSOD transcription may appear similar at the messenger RNA level, the intracellular signaling pathways are differentially regulated.

Cytokine-induced changes in chromatin structure and in vivo footprints in the inducible NOS promoter

Transcription of the human inducible nitric oxide synthase (iNOS) gene is regulated by inflammatory cytokines in a tissue-specific manner. To determine whether differences in cytokine-induced mRNA levels between pulmonary epithelial cells (A549) and hepatic biliary epithelial cells (AKN-1) result from different protein or DNA regulatory mechanisms, we identified cytokine-induced changes in DNase I-hypersensitive (HS) sites in 13 kb of the iNOS 5'-flanking region. Data showed both constitutive and inducible HS sites in an overlapping yet cell type-specific pattern. Using in vivo footprinting and ligation-mediated PCR to detect potential DNA or protein interactions, we examined one promoter region near -5 kb containing both constitutive and cytokine-induced HS sites. In both cell types, three in vivo footprints were present in both control and cytokine-treated cells, and each mapped within a constitutive HS site. The remaining footprint appeared only in response to cytokine treatment and mapped to an inducible HS site. These studies, performed on chromatin in situ, identify a portion of the molecular mechanisms regulating transcription of the human iNOS gene in both lung- and liver-derived epithelial cells.

Smad7-dependent regulation of heme oxygenase-1 by transforming growth factor-beta in human renal epithelial cells

Heme oxygenase-1 (HO-1), a 32-kDa microsomal enzyme, is induced as a beneficial and adaptive response in cells/tissues exposed to oxidative stress. Transforming growth factor-beta1 (TGF-beta1) is a regulatory cytokine that has been implicated in a variety of renal diseases where it promotes extracellular matrix deposition and proinflammatory events. We hypothesize that the release of TGF-beta1 via autocrine and/or paracrine pathways may induce HO-1 and serve as a protective response in renal injury. To understand the molecular mechanism of HO-1 induction by TGF-beta1, we exposed confluent human renal proximal tubule cells to TGF-beta1 and observed a significant induction of HO-1 mRNA at 4 h with a maximal induction at 8 h. This induction was accompanied by increased expression of HO-1 protein. TGF-beta1 treatment in conjunction with actinomycin D or cycloheximide demonstrated that induction of HO-1 mRNA requires de novo transcription and, in part, protein synthesis. Exposure to TGF-beta1 resulted in marked induction of Smad7 mRNA with no effect on Smad6 expression. Overexpression of Smad7, but not Smad6, inhibited TGF-beta1-mediated induction of endogenous HO-1 gene expression. We speculate that the induction of HO-1 in the kidney is an adaptive response to the inflammatory effects of TGF-beta1 and manipulations of the Smad pathway to alter HO-1 expression may serve as a potential therapeutic target.

Activation of the human asparagine synthetase gene by the amino acid response and the endoplasmic reticulum stress response pathways occurs by common genomic elements

The human asparagine synthetase (AS) gene is transcriptionally regulated by amino acid deprivation (amino acid response, AAR) and the endoplasmic reticulum stress response (ERSR), also known as the unfolded protein response pathway. The results reported here document the novel observation that induction of the AS gene by the AAR and ERSR pathways occurs via the same set of genomic elements. Data supporting this conclusion include transient transfection of AS promoter/reporter gene constructs that illustrate that the transcriptional control elements used by both pathways are contained with nucleotides -111 to -34 of the AS promoter. In vivo footprinting analysis of this region identified six specific protein-binding sites. Within two of these sites, altered footprinting was observed following amino acid or glucose deprivation, but the patterns were identical for both the AAR and the ERSR pathway. Site-directed mutation of individual nucleotides within these two binding sites confirmed their importance for regulated transcription, and none of the mutations resulted in loss of response of only one pathway. Neither of these two sites corresponds to a recently identified ERSR cis-element, nor do they contain consensus sequences for known transcription factors. Collectively, the data document that there are at least two independent transcriptional mechanisms for gene activation by the ERSR pathway, one of which terminates at the same genomic elements used by the AAR pathway.

Multiple replacements of glutamine 143 in human manganese superoxide dismutase: effects on structure, stability, and catalysis

Glutamine 143 in human manganese superoxide dismutase (MnSOD) forms a hydrogen bond with the manganese-bound solvent molecule and is investigated by replacement using site-specific mutagenesis. Crystal structures showed that the replacement of Gln 143 with Ala made no significant change in the overall structure of the mutant enzyme. Two new water molecules in Q143A MnSOD were situated in positions nearly identical with the Oepsilon1 and Nepsilon2 of the replaced Gln 143 side chain and maintained a hydrogen-bonded network connecting the manganese-bound solvent molecule to other residues in the active site. However, their presence could not sustain the stability and activity of the enzyme; the main unfolding transition of Q143A was decreased 16 degrees C and its catalysis decreased 250-fold to k(cat)/K(m) = 3 x 10(6) M(-)(1) s(-)(1), as determined by stopped-flow spectrophotometry and pulse radiolysis. The mutant Q143A MnSOD and other mutants at position 143 showed very low levels of product inhibition and favored Mn(II)SOD in the resting state, whereas the wild type showed strong product inhibition and favored Mn(III)SOD. However, these differences did not affect the rate constant for dissociation of the product-inhibited complex in Q143A MnSOD which was determined from a characteristic absorbance at 420 nm and was comparable in magnitude ( approximately 100 s(-)(1)) to that of the wild-type enzyme. Hence, Gln 143, which is necessary for maximal activity in superoxide dismutation, appears to have no role in stabilization and dissociation of the product-inhibited complex.

Renal response to tissue injury: lessons from heme oxygenase-1 GeneAblation and expression

Heme oxygenase-1 (HO-1) is a microsomal enzyme involved in the degradation of heme, resulting in the generation of biliverdin, iron, and carbon monoxide. Recent attention has focused on the biologic effects of product(s) of this enzymatic reaction that have important antioxidant, anti-inflammatory, and cytoprotective functions. Induction of HO-1 occurs as an adaptive and beneficial response to a wide variety of oxidant stimuli, including heme, hydrogen peroxide, cytokines, growth factors, heavy metals, nitric oxide, and oxidized LDL. HO-1 has been implicated in several clinically relevant disease states, including transplant rejection, hypertension, acute renal injury, atherosclerosis, and others. Previous studies indicate a protective role for HO-1 in heme and non-heme-mediated models of acute renal injury using chemical inducers and inhibitors of HO-1. Studies in HO-1 knockout mice further corroborate these observations, highlighting the important role of HO-1 in the pathophysiology of acute renal injury. Expression of HO-1 has been linked to prolonged xenograft survival and is important in transplant rejection as well. More recently, the first known case of human HO-1 deficiency was reported with several phenotypical similarities to the mouse HO-1 knockout. The role of HO-1 has extended far beyond its initial description as an enzyme involved in heme degradation to being an important mediator in modulating adaptive and protective responses not only in renal injury, but in other organ systems as well.

Heme oxygenase-1 gene ablation or expression modulates cisplatin-induced renal tubular apoptosis

Heme oxygenase-1 (HO-1) is a 32-kDa microsomal enzyme that catalyzes the conversion of heme to biliverdin, releasing iron and carbon monoxide. Induction of HO-1 occurs as a protective response in cells/tissues exposed to a wide variety of oxidant stimuli. The chemotherapeutic effects of cis-diamminedichloroplatinum(II) (cisplatin), a commonly used anticancer drug, are limited by significant nephrotoxicity, which is characterized by varying degrees of renal tubular apoptosis and necrosis. The purpose of this study was to evaluate the functional significance of HO-1 expression in cisplatin-induced renal injury. Our studies demonstrate that transgenic mice deficient in HO-1 (-/-), develop more severe renal failure and have significantly greater renal injury compared with wild-type (+/+) mice treated with cisplatin. In vitro studies in human renal proximal tubule cells demonstrate that hemin, an inducer of HO-1, significantly attenuated cisplatin-induced apoptosis and necrosis, whereas inhibition of HO-1 enzyme activity reversed the cytoprotective effect. Overexpression of HO-1 resulted in a significant reduction in cisplatin-induced cytotoxicity. These studies provide a basis for future studies using targeted gene expression of HO-1 as a therapeutic and preventive modality in high-risk settings of acute renal failure.

Transcriptional regulation and structural organization of the human cytosolic phospholipase A(2) gene

Cytokines are established regulators of the arachidonic acid cascade in lung cells. The levels of various arachidonic metabolites distinguish the normal and pathogenic states of the human lung. Arachidonyl-selective cytosolic phospholipase A(2) (cPLA(2)) is ubiquitously present in human lung and is most likely the rate-limiting step in eicosanoid generation. We therefore studied the regulation of this pivotal gene in human lung fibroblasts and epithelial cells by proinflammatory cytokines. We demonstrate a dose- and time-dependent induction of human cPLA(2) mRNA by interleukin-1beta, tumor necrosis factor-alpha, and interferon-gamma as well as the abrogation of this induction by glucocorticoids. Nuclear runoff studies demonstrate that de novo transcription of the cPLA(2) gene is required for cytokine induction. We have characterized the human cPLA(2) gene, which is encoded by 18 exons and spans in excess of 137 kb. Deletion analysis of a 3.4-kb fragment of the human promoter identified two regions responsible for basal expression of the cPLA(2) gene. Conversely, a CA-dinucleotide repeat in the proximal promoter appears to repress overall promoter activity. Understanding the molecular mechanisms associated with cytokine-dependent expression of the cPLA(2) gene should provide further insight into regulating the level of proinflammatory mediators in pulmonary diseases.

Cytokine-inducible enhancer with promoter activity in both the rat and human manganese-superoxide dismutase genes

Diverse pro-inflammatory mediators regulate transcription of the gene (MnSOD) encoding the mitochondrial anti-oxidant protein manganese-superoxide dismutase. Understanding the regulation of this gene is crucial to comprehending its role in cytoprotection. In transfected lung epithelial cells, a human-growth-hormone reporter gene system was utilized to identify a potential enhancer in the MnSOD genomic fragment previously shown to contain multiple DNase-I-hypersensitive sites. Northern analysis demonstrated a 10-20-fold increase in response to pro-inflammatory mediators. Inclusion of the MnSOD genomic fragment in reporter constructs was necessary to mimic these stimulus-dependent endogenous levels. The inducible enhancer element was localized to a 260 bp fragment in intron 2, coinciding with a previously defined DNase-I-hypersensitive site. This element functions in an orientation- and position-independent manner as well as with the heterologous thymidine kinase promoter. In addition, we have demonstrated that a homologous sequence within the human MnSOD gene exhibits identical enhancer activity. A novel characteristic of the rat and human enhancer elements involves the ability to promote cytokine-inducible transcription in the absence of a classical promoter.