Systems Biology of Human Atherosclerosis

Systems biology describes a holistic and integrative approach to understand physiology and pathology. The “omic” disciplines include genomics, transcriptomics, proteomics, and metabolic profiling (metabonomics and metabolomics). By adopting a stance, which is opposing (yet complimentary) to conventional research techniques, systems biology offers an overview by assessing the “net” biological effect imposed by a disease or nondisease state. There are a number of different organizational levels to be understood, from DNA to protein, metabolites, cells, organs and organisms, even beyond this to an organism’s context. Systems biology relies on the existence of “nodes” and “edges.” Nodes are the constituent part of the system being studied (eg, proteins in the proteome), while the edges are the way these constituents interact. In future, it will be increasingly important to collaborate, collating data from multiple studies to improve data sets, making them freely available and undertaking integrative analyses.

Periprocedural outcomes after surgical revascularization and stenting for postradiotherapy carotid stenosis

BACKGROUND

Treatment of head and neck malignancy commonly involves radiotherapy, which is associated with the development of carotid artery stenosis. There is little evidence to guide clinicians on how to intervene in significant postradiotherapy carotid stenosis. This systematic review collated data pertaining to perioperative outcomes of carotid artery surgery and carotid stenting in postradiotherapy carotid stenosis to aid the clinical decision-making process.

METHODS

A systematic review of the literature, adhering to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2009 guidelines, was performed. We screened 575 articles related to carotid artery surgery or stenting in postradiotherapy carotid stenosis, from which 21 studies were included for quantitative analysis. The primary outcome was stroke or death ≤ 30 days of the procedure. Secondary outcomes included cranial nerve injury, restenosis, stroke, and death at >30 days.

RESULTS

Nine publications recorded 211 surgical procedures in 179 patients. In symptomatic patients, the 30-day mortality rate was 2.6% and the stroke or death rate was 2.7%. In asymptomatic patients, the 30-day mortality rate was 0% and the stroke or death rate was 1.1%. Permanent cranial nerve palsy was experienced by 0.6% of patients. Twelve publications recorded 510 carotid artery stenting procedures in 482 patients. In symptomatic patients, the 30-day mortality rate was 5.1%, and the stroke or death rate was 5.1%. In asymptomatic patients, the 30-day mortality rate was 1.4%, and the stroke or death rate was 2.1%. There was no statistically significant difference in 30-day stroke or death rate between surgical revascularization and carotid artery stenting in all (odds ratio [OR], 0.54; 95% confidence interval [CI] 0.17-1.70; P = .43), symptomatic (OR, 0.52; 95% CI, 0.14-1.98; P = .38), or asymptomatic patients (OR, 0.55; 95% CI, 0.06-5.42; P = .99).

CONCLUSIONS

The published outcomes from high-volume centers demonstrate that surgical revascularization and stenting are both technically feasible in postradiotherapy carotid stenosis and have similar safety profiles to nonirradiated necks. Radiation should therefore not be considered a contraindication to surgical intervention.

The utility of collaborative biobanks for cardiovascular research

Differences between animal and human atherosclerosis have led to the requirement for clinical data, imaging information and biological material from large numbers of patients and healthy persons. Where such "biobanks" exist, they have been fruitful sources for genomewide association, diagnostic accuracy, ethnicity, and risk stratification cohort studies. In addition once established, they attract funding for future projects. Biobanks require a network of medical contributors, secure storage facilities, bioinformatics expertise, database managers, and ethical working practices to function optimally. There is the opportunity for collaboration between individual biobanks to further amplify the advantages afforded.

Protein degradation in mitochondria: implications for oxidative stress, aging and disease: a novel etiological classification of mitochondrial proteolytic disorders

The mitochondrial genome encodes just a small number of subunits of the respiratory chain. All the other mitochondrial proteins are encoded in the nucleus and produced in the cytosol. Various enzymes participate in the activation and intramitochondrial transport of imported proteins. To finally take their place in the various mitochondrial compartments, the targeting signals of imported proteins have to be cleaved by mitochondrial processing peptidases. Mitochondria must also be able to eliminate peptides that are internally synthesized in excess, as well as those that are improperly assembled, and those with abnormal conformation caused by mutation or oxidative damage. Damaged mitochondrial proteins can be removed in two ways: either through lysosomal autophagy, that can account for at most 25-30% of the biochemically estimated rates of average mitochondrial catabolism; or through an intramitochondrial proteinolytic pathway. Mitochondrial proteases have been extensively studied in yeast, but evidence in recent years has demonstrated the existence of similar systems in mammalian cells, and has pointed to the possible importance of mitochondrial proteolytic enzymes in human diseases and ageing. A number of mitochondrial diseases have been identified whose mechanisms involve proteolytic dysfunction. Similar mechanisms probably play a role in diminished resistance to oxidative stress, and in the aging process. In this paper we review current knowledge of mammalian mitochondrial proteolysis, under normal conditions and in several disease states, and we propose an etiological classification of human diseases characterized by a decline or loss of function of mitochondrial proteolytic enzymes.

Chronic overexpression of the calcineurin inhibitory gene DSCR1 (Adapt78) is associated with Alzheimer's disease

The DSCR1 (Adapt78) gene was independently discovered as a resident of the "Down syndrome candidate region"and as an "adaptive response"shock or stress gene that is transiently induced during oxidative stress. Recently the DSCR1 (Adapt78) gene product was discovered to be an inhibitor of the serine/threonine phosphatase, calcineurin, and its signaling pathways. We hypothesized that DSCR1 (Adapt78) might also be involved in the development of Alzheimer's disease. To address this question we first studied DSCR1 (Adapt78) in multiple human tissues and found significant expression in brain, spinal cord, kidney, liver, mammary gland, skeletal muscle, and heart. Within the brain DSCR1 (Adapt78) is predominantly expressed in neurons within the cerebral cortex, hippocampus, substantia nigra, thalamus, and medulla oblongata. When we compared DSCR1 (Adapt78) mRNA expression in post-mortem brain samples from Alzheimer's disease patients and individuals who had died with no Alzheimer's diagnosis, we found that DSCR1 (Adapt78) mRNA levels were about twice as high in age-matched Alzheimer's patients as in controls. DSCR1 (Adapt78) mRNA levels were actually three times higher in patients with extensive neurofibrillary tangles (a hallmark of Alzheimer's disease) than in controls. In comparison, post-mortem brain samples from Down syndrome patients (who suffer Alzheimer's symptoms) also exhibited DSCR1 (Adapt78) mRNA levels two to three times higher than controls. Using a cell culture model we discovered that the amyloid beta(1-42) peptide, which is a major component of senile plaques in Alzheimer's, can directly induce increased expression of DSCR1 (Adapt78). Our findings associate DSCR1 (Adapt78) with such major hallmarks of Alzheimer's disease as amyloid protein, senile plaques, and neurofibrillary tangles.

Protein oxidation and 20S proteasome-dependent proteolysis in mammalian cells

The generation of reactive oxygen species is an inevitable aspect of aerobic life. In addition to being exposed to free radicals in the environment, aerobic organisms must also deal with oxygen radicals generated as byproducts of a number of physiological mechanisms for example, by the mitochondrial and endoplasmic reticulum electron transport chains, and by cells of the immune system. Although most organisms are equipped with several lines of defense against oxidative stress, these defensive mechanisms are not 100% effective, and oxidatively modified forms of proteins accumulate during aging, and in many pathological conditions. Oxidatively modified proteins can form large aggregates due to covalent cross-linking or increased surface hydrophobicity. Unless repaired or removed from cells, these oxidized proteins are often toxic and can threaten cell viability. Mammalian cells exhibit only limited direct repair mechanisms, and oxidatively damaged proteins appear to undergo selective proteolysis, primarily by the major cytosolic proteinase, the proteasome. Interestingly, it appears that the 20S 'core' proteasome conducts the recognition and elimination of oxidized proteins in an ATP-independent and ubiquitin-independent pathway.

Mechanism of the formation and proteolytic release of H2O2-induced dityrosine and tyrosine oxidation products in hemoglobin and red blood cells

Oxyhemoglobin exposed to a continuous flux of H(2)O(2) underwent oxidative modifications, including limited release of fluorescent fragmentation products. The main fragments formed were identified as oxidation products of tyrosine, including dopamine, dopamine quinone, and dihydroxyindol. Further release of these oxidation products plus dityrosine was only seen after proteolytic degradation of the oxidatively modified hemoprotein. A possible mechanism is proposed to explain the formation of these oxidation products that includes cyclization, decarboxylation, and further oxidation of the intermediates. Release of dityrosine is proposed as a useful technique for evaluating selective proteolysis after an oxidative stress, because dityrosine is metabolically stable, and it is only released after enzymatic hydrolysis of the oxidatively modified protein. The measurement can be accomplished by high performance liquid chromatography with fluorescence detection or by high efficiency thin layer chromatography. Comparable results, in terms of dityrosine release, were obtained using red blood cells of different sources after exposing them to a flux of H(2)O(2). Furthermore, dityrosine has been reported to occur in a wide variety of oxidatively modified proteins. These observations suggest that dityrosine formation and release can be used as a highly specific marker for protein oxidation and selective proteolysis.

Glutathiolation of the proteasome is enhanced by proteolytic inhibitors

The proteasome inhibitors lactacystin, clastro lactacystin beta-lactone, or tri-leucine vinyl sulfone (NLVS), in the presence of [(35)S]cysteine/methionine, caused increased incorporation of (35)S into cellular proteins, even when protein synthesis was inhibited by cycloheximide. This effect was blocked by incubation with the glutathione synthesis inhibitor buthionine sulfoximine. Proteasome inhibitors also enhanced total glutathione levels, increased reduced/oxidized glutathione ratio (GSH/GSSG) and upregulated gamma-glutamylcysteine synthetase (rate-limiting in glutathione synthesis). Micromolar concentrations of GSH, GSSG, or cysteine stimulated the chymotrypsin-like activity of purified 20S proteasome, but millimolar GSH or GSSG was inhibitory. Interestingly, GSH did not affect 20S proteasome's trypsin-like activity. Enhanced proteasome glutathiolation was verified when purified preparations of the 20S core enzyme complex were incubated with [(35)S]GSH after pre-incubation with any of the inhibitors. NLVS, lactacystin or clastro lactacystin beta-lactone may promote structural modification of the 20S core proteasome, with increased exposure of cysteine residues, which are prone to S-thiolation. Three main conclusions can be drawn from the present work. First, proteasome inhibitors alter cellular glutathione metabolism. Second, proteasome glutathiolation is enhanced by inhibitors but still occurs in their absence, at physiological GSH and GSSG levels. Third, proteasome glutathiolation seems to be a previously unknown mechanism of proteasome regulation in vivo.

Regulatory mechanisms controlling expression of the DAN/TIR mannoprotein genes during anaerobic remodeling of the cell wall in Saccharomyces cerevisiae

The DAN/TIR genes of Saccharomyces cerevisiae encode homologous mannoproteins, some of which are essential for anaerobic growth. Expression of these genes is induced during anaerobiosis and in some cases during cold shock. We show that several heme-responsive mechanisms combine to regulate DAN/TIR gene expression. The first mechanism employs two repression factors, Mox1 and Mox2, and an activation factor, Mox4 (for mannoprotein regulation by oxygen). The genes encoding these proteins were identified by selecting for recessive mutants with altered regulation of a dan1::ura3 fusion. MOX4 is identical to UPC2, encoding a binucleate zinc cluster protein controlling expression of an anaerobic sterol transport system. Mox4/Upc2 is required for expression of all the DAN/TIR genes. It appears to act through a consensus sequence termed the AR1 site, as does Mox2. The noninducible mox4Delta allele was epistatic to the constitutive mox1 and mox2 mutations, suggesting that Mox1 and Mox2 modulate activation by Mox4 in a heme-dependent fashion. Mutations in a putative repression domain in Mox4 caused constitutive expression of the DAN/TIR genes, indicating a role for this domain in heme repression. MOX4 expression is induced both in anaerobic and cold-shocked cells, so heme may also regulate DAN/TIR expression through inhibition of expression of MOX4. Indeed, ectopic expression of MOX4 in aerobic cells resulted in partially constitutive expression of DAN1. Heme also regulates expression of some of the DAN/TIR genes through the Rox7 repressor, which also controls expression of the hypoxic gene ANB1. In addition Rox1, another heme-responsive repressor, and the global repressors Tup1 and Ssn6 are also required for full aerobic repression of these genes.

Degradation of oxidized proteins by the 20S proteasome

Oxidatively modified proteins are continuously produced in cells by reactive oxygen and nitrogen species generated as a consequence of aerobic metabolism. During periods of oxidative stress, protein oxidation is significantly increased and may become a threat to cell survival. In eucaryotic cells the proteasome has been shown (by purification of enzymatic activity, by immunoprecipitation, and by antisense oligonucleotide studies) to selectively recognize and degrade mildly oxidized proteins in the cytosol, nucleus, and endoplasmic reticulum, thus minimizing their cytotoxicity. From in vitro studies it is evident that the 20S proteasome complex actively recognizes and degrades oxidized proteins, but the 26S proteasome, even in the presence of ATP and a reconstituted functional ubiquitinylating system, is not very effective. Furthermore, relatively mild oxidative stress rapidly (but reversibly) inactivates both the ubiquitin activating/conjugating system and 26S proteasome activity in intact cells, but does not affect 20S proteasome activity. Since mild oxidative stress actually increases proteasome-dependent proteolysis (of oxidized protein substrates) the 20S 'core' proteasome complex would appear to be responsible. Finally, new experiments indicate that conditional mutational inactivation of the E1 ubiquitin-activating enzyme does not affect the degradation of oxidized proteins, further strengthening the hypothesis that oxidatively modified proteins are degraded in an ATP-independent, and ubiquitin-independent, manner by the 20S proteasome. More severe oxidative stress causes extensive protein oxidation, directly generating protein fragments, and cross-linked and aggregated proteins, that become progressively resistant to proteolytic digestion. In fact these aggregated, cross-linked, oxidized proteins actually bind to the 20S proteasome and act as irreversible inhibitors. It is proposed that aging, and various degenerative diseases, involve increased oxidative stress (largely from damaged and electron 'leaky' mitochondria), and elevated levels of protein oxidation, cross-linking, and aggregation. Since these products of severe oxidative stress inhibit the 20S proteasome, they cause a vicious cycle of progressively worsening accumulation of cytotoxic protein oxidation products.

Induction and repression of DAN1 and the family of anaerobic mannoprotein genes in Saccharomyces cerevisiae occurs through a complex array of regulatory sites

The DAN/TIR mannoprotein genes of Saccharomyces cerevisiae (DAN1, DAN2, DAN3, DAN4, TIR1, TIR2, TIR3 and TIR4) are expressed in anaerobic cells while the predominant cell wall proteins Cwp1 and Cwp2 are down-regulated. Elements involved in activation and repression of the DAN/TIR genes were defined in this study, using the DAN1 promoter as a model. Nested deletions in a DAN1/lacZ reporter pinpointed regions carrying activation and repression elements. Inspection revealed two consensus sequences subsequently shown to be independent anaerobic response elements (AR1, consensus TCGTTYAG; AR2, consensus AAAAATTGTTGA). AR1 is found in all of the DAN/TIR promoters; AR2 is found in DAN1, DAN2 and DAN3. A 120 bp segment carrying two copies of AR1 preferentially activated transcription of lacZ under anaerobic conditions. A fusion of three synthetic copies of AR1 to MEL1 was also expressed anaerobically. Mutations in either AR1 site within the 120 bp segment caused a drastic loss of expression, indicating that both are necessary for activation and implying cooperativity between adjacent transcriptional activation complexes. A single AR2 site carried on a 46 bp fragment from the DAN1 promoter activated lacZ transcription under anaerobic conditions, as did a 26 bp synthetic AR2 fragment fused to MEL1. Nucleotide substitutions within the AR2 sequence eliminated the activity of the 46 bp segment. Ablation of the AR2 sequences in the full promoter caused a partial reduction of expression. The presence of the ATTGTT core (recognized by HMG proteins) in the AR2 sequence suggests that an HMG protein may activate through AR2. One region was implicated in aerobic repression of DAN1. It contains sites for the heme-induced Mot3 and Rox1 repressors.

Skin and oral fibroblasts exhibit phenotypic differences in extracellular matrix reorganization and matrix metalloproteinase activity

BACKGROUND

Oral mucosal wounds are characterized by rapid re-epithelialization and remodelling. In vitro, oral mucosal fibroblasts exhibit a fetal phenotype with increased extracellular matrix reorganizational ability, migration and experimental wound repopulation when compared with skin fibroblasts.

OBJECTIVES

To investigate whether phenotypic differences in the expression and production of matrix metalloproteinase (MMP) -2 and tissue inhibitors of metalloproteinases (TIMPs) could play an important part in mediating these in vitro differences.

METHODS

Skin and oral mucosal fibroblast MMP-2, TIMP-1 and TIMP-2 mRNA expression and protein production were studied in three-dimensional collagen lattices using quantitative competitive reverse transcriptase-polymerase chain reaction (QCRT-PCR), enzyme-linked immunosorbent assay (ELISA), zymography and reverse zymography.

RESULTS

Oral mucosal fibroblasts exhibited increased levels of the 62-kDa active form of MMP-2 and lattice contraction when compared with skin fibroblasts. Oral mucosal and skin fibroblast MMP-2 gene expression and synthesis of the 72-kDa pro-MMP-2 was similar as assessed by QCRT-PCR, zymography and ELISA. Differential MMP-2 activation was, however, related to phenotypic differences in TIMP activity between the skin and oral mucosal fibroblasts, as assessed by reverse zymography.

CONCLUSIONS

These studies propose a mechanism by which fibroblast phenotype may contribute directly to the observed preferential remodelling of oral wounds.

Protein oxidation and degradation during cellular senescence of human BJ fibroblasts: part I--effects of proliferative senescence

Oxidized and cross-linked proteins tend to accumulate in aging cells. Declining activity of proteolytic enzymes, particularly the proteasome, has been proposed as a possible explanation for this phenomenon, and direct inhibition of the proteasome by oxidized and cross-linked proteins has been demonstrated in vitro. We have further examined this hypothesis during both proliferative senescence (this paper) and postmitotic senescence (see the accompanying paper, ref 1 ) of human BJ fibroblasts. During proliferative senescence, we found a marked decline in all proteasome activities (trypsin-like activity, chymotrypsin-like activity, and peptidyl-glutamyl-hydrolyzing activity) and in lysosomal cathepsin activity. Despite the loss of proteasome activity, there was no concomitant change in cellular levels of actual proteasome protein (immunoassays) or in the steady-state levels of mRNAs for essential proteasome subunits. The decline in proteasome activities and lysosomal cathepsin activities was accompanied by dramatic increases in the accumulation of oxidized and cross-linked proteins. Furthermore, as proliferation stage increased, cells exhibited a decreasing ability to degrade the oxidatively damaged proteins generated by an acute, experimentally applied oxidative stress. Thus, oxidized and cross-linked proteins accumulated rapidly in cells of higher proliferation stages. Our data are consistent with the hypothesis that proteasome is progressively inhibited by small accumulations of oxidized and cross-linked proteins during proliferative senescence until late proliferation stages, when so much proteasome activity has been lost that oxidized proteins accumulate at ever-increasing rates. Lysosomes attempt to deal with the accumulating oxidized and cross-linked proteins, but declining lysosomal cathepsin activity apparently limits their effectiveness. This hypothesis, which may explain the progressive intracellular accumulation of oxidized and cross-linked proteins in aging, is further explored during postmitotic senescence in the accompanying paper (1).

Protein oxidation and degradation during cellular senescence of human BJ fibroblasts: part II--aging of nondividing cells

Oxidized/cross-linked intracellular protein materials, known as ceroid pigment, age pigment, or lipofuscin, accumulate in postmitotic tissues. It is unclear, however, whether diminishing proteolytic capacities play a role in the accumulation of such oxidized intracellular proteins. Previous studies revealed that the proteasome is responsible for the degradation of most oxidized soluble cytoplasmic and nuclear proteins and, we propose, for the prevention of such damage accumulations. The present investigation was undertaken to test the changes in protein turnover, proteasome activity, lysosome activity, and protein oxidation status during the aging of nondividing cells. Since the companion paper shows that both proteasome activity and the overall protein turnover decline during proliferative senescence whereas the accumulation of oxidized proteins increases significantly, we decided to use the same human BJ fibroblasts, this time at confluency, at different PD levels (including those that are essentially postmitotic) to investigate the same parameters under conditions where cells do not divide. We find that the activity of the cytosolic proteasome declines dramatically during senescence of nondividing BJ fibroblasts. The peptidyl-glutamyl-hydrolyzing activity was particularly affected. This decline in proteasome activity was accompanied by a decrease in the overall turnover of short-lived (radiolabeled) proteins in the nondividing BJ fibroblasts. On the other hand, no decrease in the actual cellular proteasome content, as judged by immunoblots, was found. The decline in the activity of the proteasome was also accompanied by an increased accumulation of oxidized proteins, especially of oxidized and cross-linked material. Unlike the loss of lysosomal function seen in our accompanying studies of proliferative senescence (1), however, the present study of hyperoxic senescence in nondividing cells actually revealed marked increases in lysosomal cathepsin activity in all but the very 'oldest' postmitotic cells. Our comparative studies of proliferating (1) and nonproliferating (this paper) human BJ fibroblasts reveal a good correlation between the accumulation of oxidized/cross-linked proteins and the decline in proteasome activity and overall cellular protein turnover during in vitro senescence, which may predict a causal relationship during actual cellular aging.

4-Hydroxynonenal-modified amyloid-beta peptide inhibits the proteasome: possible importance in Alzheimer's disease

The amyloid beta-peptide (Abeta) is a 4-kDa species derived from the amyloid precursor protein, which accumulates in the brains of patients with Alzheimer's disease. Although we lack full understanding of the etiology and pathogenesis of selective neuron death, considerable data do imply roles for both the toxic Abeta and increased oxidative stress. Another significant observation is the accumulation of abnormal, ubiquitin-conjugated proteins in affected neurons, suggesting dysfunction of the proteasome proteolytic system in these cells. Recent reports have indicated that Abeta can bind and inhibit the proteasome, the major cytoslic protease for degrading damaged and ubiquitin-conjugated proteins. Earlier results from our laboratory showed that moderately oxidized proteins are preferentially recognized and degraded by the proteasome; however, severely oxidized proteins cannot be easily degraded and, instead, inhibit the proteasome. We hypothesized that oxidatively modified Abeta might have a stronger (or weaker) inhibitory effect on the proteasome than does native Abeta. We therefore also investigated the proteasome inhibitory action of Abeta1-40 (a peptide comprising the first 40 residues of Abeta) modified by the intracellular oxidant hydrogen peroxide, and by the lipid peroxidation product 4-hydroxynonenal (HNE). H2O2 modification of Abeta1-40 generates a progressively poorer inhibitor of the purified human 20S proteasome. In contrast, HNE modification of Abeta1-40 generates a progressively more selective and efficient inhibitor of the degradation of fluorogenic peptides and oxidized protein substrates by human 20S proteasome. This interaction may contribute to certain pathological manifestations of Alzheimer's disease.

Oxidative stress, antioxidant defenses, and damage removal, repair, and replacement systems

Oxidative stress is an unavoidable consequence of life in an oxygen-rich atmosphere. Oxygen radicals and other activated oxygen species are generated as by-products of aerobic metabolism and exposure to various natural and synthetic toxicants. The "Oxygen Paradox" is that oxygen is dangerous to the very life-forms for which it has become an essential component of energy production. The first defense against oxygen toxicity is the sharp gradient of oxygen tension, seen in all mammals, from the environmental level of 20% to a tissue concentration of only 3-4% oxygen. These relatively low tissue levels of oxygen prevent most oxidative damage from ever occurring. Cells, tissues, organs, and organisms utilize multiple layers of antioxidant defenses and damage removal, and replacement or repair systems in order to cope with the remaining stress and damage that oxygen engenders. The enzymes comprising many of these protective systems are inducible under conditions of oxidative stress adaptation, in which the expression of over 40 mammalian genes is upregulated. Mitotic cells have the additional defensive ability of entering a transient growth-arrested state (in the first stages of adaptation) in which DNA is protected by histone proteins, energy is conserved by diminished expression of nonessential genes, and the expression of shock and stress proteins is greatly increased. Failure to fully cope with an oxidative stress can switch mitotic cells into a permanent growth-arrested, senescence-like state in which they may survive for long periods. Faced with even more severe oxidative stress, or the declining protective enzymes and adaptive capacity associated with aging, cells may "sacrifice themselves" by apoptosis, which protects surrounding healthy tissue from further damage. Only under the most severe oxidative stress conditions will cells undergo a necrotic death, which exposes surrounding tissues to the further vicissitudes of an inflammatory immune response. This remarkable array of systems for defense; damage removal, replacement, and repair; adaptation; growth modulation; and apoptosis make it possible for us to enjoy life in an oxygen-rich environment.

Proteasome inhibition by lipofuscin/ceroid during postmitotic aging of fibroblasts

We have studied the effects of hyperoxia and of cell loading with artificial lipofuscin or ceroid pigment on the postmitotic aging of human lung fibroblast cell cultures. Normobaric hyperoxia (40% oxygen) caused an irreversible senescence-like growth arrest after about 4 wk and shortened postmitotic life span from 1-1/2 years down to 3 months. During the first 8 wk of hyperoxia-induced 'aging', overall protein degradation (breakdown of [(35)S]methionine metabolically radiolabeled cell proteins) increased somewhat, but by 12 wk and thereafter overall proteolysis was significantly depressed. In contrast, protein synthesis rates were unaffected by 12 wk of hyperoxia. Lysosomal cathepsin-specific activity (using the fluorogenic substrate z-FR-MCA) and cytoplasmic proteasome-specific activity (measured with suc-LLVY-MCA) both declined by 80% or more over 12 wk. Hyperoxia also caused a remarkable increase in lipofuscin/ceroid formation and accumulation over 12 wk, as judged by both fluorescence measurements and FACscan methods. To test whether the association between lipofuscin/ceroid accumulation and decreased proteolysis might be causal, we next exposed cells to lipofuscin/ceroid loading under normoxic conditions. Lipofuscin/ceroid-loaded cells indeed exhibited a gradual decrease in overall protein degradation over 4 wk of treatment, whereas protein synthesis was unaffected. Proteasome specific activity decreased by 25% over this period, which is important since proteasome is normally responsible for degrading oxidized cell proteins. In contrast, an apparent increase in lysosomal cathepsin activity was actually caused by a large increase in the number of lysosomes per cell. To test whether lipofuscin/ceroid could in fact directly inhibit proteasome activity, thus causing oxidized proteins to accumulate, we incubated purified proteasome with lipofuscin/ceroid preparations in vitro. We found that proteasome is directly inhibited by lipofuscin/ceroid. Our results indicate that an accumulation of oxidized proteins (and lipids) such as lipofuscin/ceroid may actually cause further increases in damage accumulation during aging by inhibiting the proteasome.

A study to the fibroblast-populated collagen lattic es

OBJECTIVE: To prepare fibroblast-populated col lagen lattices quickly and economically, and to observe the shapes and distribut ion of the fibroblasts and measure FPCLs contraction successfully. METHODS: Three different volumes of FPCLs were prepared with 24 -well plates and the best volume was chosen. RESULTS: The shapes and distribution of fibroblasts of three vo lumes of FPCLs of 24-well plates were all visible clearly; the shapes of FPCLs of plate 1 were circular dish-like and their contraction was measured. Because of plate 2 with folded edge the contraction could not be measured and plate 3 w ith thickest structure was found to have no change of contraction. CONCLUSIONS: The volume (2xDMEM: 0.16 ml) of plate 1 was t he best to be chosen.

Defective extracellular matrix reorganization by chronic wound fibroblasts is associated with alterations in TIMP-1, TIMP-2, and MMP-2 activity

Chronic leg wounds are characterized by defective remodeling of the extracellular matrix, failure of reepithelialization, and prolonged inflammation. The hypothesis that this defective extracellular matrix remodeling is associated with phenotypic differences in the activity of the matrix metalloproteinases and tissue inhibitors of metalloproteinases was studied in chronic wound and patient-matched normal fibroblasts in three-dimensional collagen lattice systems. Chronic wound fibroblasts exhibited no differences in morphology or proliferation (p > 0.1) compared with patient-matched uninvolved dermal fibroblasts. The ability of chronic wound fibroblasts to reorganize extracellular matrix was significantly impaired, however, in comparison to the uninvolved dermal fibroblasts (p < 0.01). This difference in extracellular matrix reorganization was not related to differences in proliferation within the collagen lattices (p > 0.05) or attachment to type I collagen (p > 0.1). Marked differences were evident in matrix metalloproteinase-2 activity between chronic wound and patient-matched normal fibroblasts. Whereas levels of pro-matrix metalloproteinase-2 were similar between the two fibroblast populations (p > 0.1), the chronic wound fibroblasts exhibited significantly decreased levels of the 62 kDa active form of matrix metalloproteinase-2 (p < 0.01). Reverse zymography and enzyme-linked immunosorbent assay demonstrated that the decreased matrix metalloproteinase-2 activity was associated with increased production of tissue inhibitors of metalloproteinase-1 and -2 by the chronic wound fibroblasts (p < 0.05). Increased production of tissue inhibitors of metalloproteinases in chronic wound fibroblasts was also reflected in decreased levels of matrix metalloproteinase-1 (p < 0.005). These data suggest that the impaired ability of chronic wound fibroblasts to reorganize extracellular matrix in vitro is related to decreased levels of active matrix metalloproteinase-2 and matrix metalloproteinase-1 resulting from increased production of tissue inhibitors of metalloproteinase-1 and -2 by chronic wound fibroblasts. These findings provide a mechanism to explain the impaired cellular responses and extracellular matrix reorganization observed in chronic leg wounds in vivo.

Proteasome inhibition by lipofuscin/ceroid during postmitotic aging of fibroblasts

We have studied the effects of hyperoxia and of cell loading with artificial lipofuscin or ceroid pigment on the postmitotic aging of human lung fibroblast cell cultures. Normobaric hyperoxia (40% oxygen) caused an irreversible senescence-like growth arrest after about 4 wk and shortened postmitotic life span from 1-1/2 years down to 3 months. During the first 8 wk of hyperoxia-induced 'aging', overall protein degradation (breakdown of [(35)S]methionine metabolically radiolabeled cell proteins) increased somewhat, but by 12 wk and thereafter overall proteolysis was significantly depressed. In contrast, protein synthesis rates were unaffected by 12 wk of hyperoxia. Lysosomal cathepsin-specific activity (using the fluorogenic substrate z-FR-MCA) and cytoplasmic proteasome-specific activity (measured with suc-LLVY-MCA) both declined by 80% or more over 12 wk. Hyperoxia also caused a remarkable increase in lipofuscin/ceroid formation and accumulation over 12 wk, as judged by both fluorescence measurements and FACscan methods. To test whether the association between lipofuscin/ceroid accumulation and decreased proteolysis might be causal, we next exposed cells to lipofuscin/ceroid loading under normoxic conditions. Lipofuscin/ceroid-loaded cells indeed exhibited a gradual decrease in overall protein degradation over 4 wk of treatment, whereas protein synthesis was unaffected. Proteasome specific activity decreased by 25% over this period, which is important since proteasome is normally responsible for degrading oxidized cell proteins. In contrast, an apparent increase in lysosomal cathepsin activity was actually caused by a large increase in the number of lysosomes per cell. To test whether lipofuscin/ceroid could in fact directly inhibit proteasome activity, thus causing oxidized proteins to accumulate, we incubated purified proteasome with lipofuscin/ceroid preparations in vitro. We found that proteasome is directly inhibited by lipofuscin/ceroid. Our results indicate that an accumulation of oxidized proteins (and lipids) such as lipofuscin/ceroid may actually cause further increases in damage accumulation during aging by inhibiting the proteasome.

Mitochondrial free radical generation, oxidative stress, and aging

Mitochondria have been described as "the powerhouses of the cell" because they link the energy-releasing activities of electron transport and proton pumping with the energy conserving process of oxidative phosphorylation, to harness the value of foods in the form of ATP. Such energetic processes are not without dangers, however, and the electron transport chain has proved to be somewhat "leaky." Such side reactions of the mitochondrial electron transport chain with molecular oxygen directly generate the superoxide anion radical (O2*-), which dismutates to form hydrogen peroxide (H2O2), which can further react to form the hydroxyl radical (HO*). In addition to these toxic electron transport chain reactions of the inner mitochondrial membrane, the mitochondrial outer membrane enzyme monoamine oxidase catalyzes the oxidative deamination of biogenic amines and is a quantitatively large source of H2O2 that contributes to an increase in the steady state concentrations of reactive species within both the mitochondrial matrix and cytosol. In this article we review the mitochondrial rates of production and steady state levels of these reactive oxygen species. Reactive oxygen species generated by mitochondria, or from other sites within or outside the cell, cause damage to mitochondrial components and initiate degradative processes. Such toxic reactions contribute significantly to the aging process and form the central dogma of "The Free Radical Theory of Aging." In this article we review current understandings of mitochondrial DNA, RNA, and protein modifications by oxidative stress and the enzymatic removal of oxidatively damaged products by nucleases and proteases. The possible contributions of mitochondrial oxidative polynucleotide and protein turnover to apoptosis and aging are explored.

Polynucleotide degradation during early stage response to oxidative stress is specific to mitochondria

Oxidative stress is known to modulate RNA expression in both prokaryotic and eukaryotic cells. We have previously determined that a preferential and calcium-dependent downregulation of mitochondrial RNA occurs in HA-1 hamster fibroblasts in response to hydrogen peroxide, and that this is accompanied by the degradation of mitochondrial genomic DNA. Here we extended these studies to determine whether downregulation is specific to transcripts derived from mitochondrial-encoded genes; to determine whether genomic DNA degradation occurs in the nucleus; and to compare overall polynucleotide stress response with cellular growth arrest and apoptosis. We observed that nuclear genome-encoded mRNAs whose protein products are targeted for the electron transport chain of mitochondria were not degraded. Furthermore, early stage degradation of genomic DNA, assessed within the first 5 h of peroxide exposure, was specific to mitochondria, as nuclear genomic DNA was not degraded under the same treatment conditions. These differential degradations occurred under conditions where extensive growth-arrest and moderate apoptosis were observed, and were accompanied by significant induction of the growth arrest mRNAs gadd45, gadd153, and adapt15/gadd7. Combined, these results indicate that there is a general degradation of mitochondrial- but not nuclear-polynucleotides during early stage response of HA-1 fibroblasts to oxidative stress.

Nitric oxide enhances the manganese superoxide dismutase-dependent suppression of proliferation in HT-1080 fibrosarcoma cells

The overexpression of manganese superoxide dismutase (MnSOD), an enzyme that catalyzes the removal of superoxide (O2*-) from the mitochondria, has been shown to be closely associated with tumor regression in vivo and loss of the malignant phenotype in vitro. To investigate the mechanism by which MnSOD overexpression mediates this reversal, we have established 29 independent, clonal MnSOD-overexpressing HT-1080 fibrosarcoma cells. MnSOD activity is inversely correlated with cell proliferation in our cell lines. Incubating cells in 3% oxygen can prevent the inhibition of cellular proliferation mediated by MnSOD, suggesting that oxygen is a prerequisite component of the MnSOD-dependent proliferative inhibition. Confocal laser microscopy was used in combination with the oxidant-sensitive fluorescent dyes dihydrorhodamine-123, dihydroethidium, and 2',7'-dichlorodihydrofluorescein diacetate to determine the oxidizing capacity of the MnSOD-overexpressing cells. When compared with parental or control cell lines, there was a significant decrease in the rate of oxidation of the fluorophores in the MnSOD-overexpressing cell lines. Thus, an increase in the oxidizing capacity of the cells does not appear to mediate the inhibition of proliferation associated with MnSOD overexpression. Superoxide dismutase has also been shown to enhance the cytotoxic activity of NO* toward tumor cells. In this study, we have shown that MnSOD overexpression enhances the cytostatic action of the NO* donors, sodium nitroprusside, 3-morpholinosydnonomine, and (Z)-1-[2-aminethyl)-N-(2-ammonioethyl)amino]diazen-1-+ ++ium-1,2-diolate in a dose-dependent manner. In addition, the NO* toxicity is blocked by oxyhemoglobin, a NO* scavenger. Our findings suggest that NO* may play a role in the reversal of tumorigenicity associated with MnSOD overexpression.

adapt78, a stress-inducible mRNA, is related to the glucose-regulated protein family of genes

We have recently reported a new oxidant- and calcium-inducible mRNA, adapt78, from hamster HA-1 cells. The adapt78 mRNA is induced in HA-1 cells under conditions where a protective adaptive response is observed and contains a translatable open reading frame whose protein product shows strong homology to a human sequence. Computer analysis of the predicted Adapt78 protein sequence also revealed a stretch of amino acids homologous to a portion of the glucose-regulated protein78 (Grp78). Based on this homology, we tested the hypothesis that adapt78 may be a new member of the grp gene family. Toward this, we assessed the modulation of adapt78 mRNA by stress agents known to induce grp78. In HA-1 cells, adapt78 mRNA was induced by the calcium ionophore A23187, 2-deoxyglucose, brefeldin A, tunicamycin, thapsigargin, and cyclopiazonic acid, with thapsigargin being the most potent inducer (7.3-fold). As expected, grp78 mRNA was also induced by these agents in our model system. In contrast, heat shock treatment produced little if any modulation of either grp78 or adapt78. Differences were also observed, as adapt78 mRNA but not grp78 mRNA was induced by 160 microM hydrogen peroxide, and adapt78 demonstrated earlier induction kinetics for certain agents compared with grp78. adapt78 mRNA was also found to be induced in several different human cell lines. A23187 had the strongest effect on adapt78 mRNA levels in human cells, inducing greater than 20-fold in all human cell cultures tested. Furthermore, in vitro transcription translation of human adapt78 cDNA produced an Adapt78 protein product. We conclude that adapt78 may be a new member of the grp family of genes and may represent an early response grp that complements the actions of grp78 and grp94.

The broad spectrum of responses to oxidants in proliferating cells: a new paradigm for oxidative stress

Proliferating mammalian cells exhibit a broad spectrum of responses to oxidative stress, depending on the stress level encountered. Very low levels of hydrogen peroxide, e.g., 3 to 15 microM, or 0.1 to 0.5 micromol/10(7) cells, cause a significant mitogenic response, 25% to 45 % growth stimulation. Greater concentrations of H2O2, 120 to 150 microM, or 2 to 5 micromol/10(7) cells, cause a temporary growth arrest that appears to protect cells from excess energy use and DNA damage. After 4-6 h of temporary growth arrest, many cells will exhibit up to a 40-fold transient adaptive response in which genes for oxidant protection and damage repair are preferentially expressed. After 18 h of H2O2 adaptation (including the 4-6 h of temporary growth arrest) cells exhibit maximal protection against oxidative stress. The H2O2 originally added is metabolized within 30-40 min, and if no more is added the cells will gradually de-adapt, so that by 36 h after the initial H2O2 stimulus they have returned to their original level of H2O2 sensitivity. At H2O2 concentrations of 250 to 400 microM, or 9 to 14 micromol/10(7) cells, mammalian fibroblasts are not able to adapt but instead enter a permanently growth-arrested state in which they appear to perform most normal cell functions but never divide again. This state of permanent growth arrest has often been confused with cell death in toxicity studies relying solely on cell proliferation assays as measures of viability. If the oxidative stress level is further increased to 0.5 to 1.0 mM H2O2, or 15 to 30 micromol/10(7) cells, apoptosis results. This oxidative stress-induced apoptosis involves nuclear condensation, loss of mitochondrial transmembrane potential, degradation/down-regulation of mitochondrial mRNAs and rRNAs, and degradation/laddering of both nuclear and mitochondrial DNA. At very high H2O2 concentrations of 5.0 to 10.0 mM, or 150 to 300 micromol/10(7) cells and above, cell membranes disintegrate, proteins and nucleic acids denature, and necrosis swiftly follows. Cultured cells grown in 20% oxygen are essentially preadapted or preselected to survive under conditions of oxidative stress. If cells are instead grown in 3% oxygen, much closer to physiological cellular levels, they are more sensitive to an oxidative challenge but exhibit far less accumulated oxidant damage. This broad spectrum of cellular responses to oxidant stress, depending on the amount of oxidant applied and the concentration of oxygen in the cell culture system, provides for a new paradigm of cellular oxidative stress responses.

Poly-ADP ribose polymerase activates nuclear proteasome to degrade oxidatively damaged histones

The 20S proteasome has been shown to be largely responsible for the degradation of oxidatively modified proteins in the cytoplasm. Nuclear proteins are also subject to oxidation, and the nucleus of mammalian cells contains proteasome. In human beings, tumor cells frequently are subjected to oxidation as a consequence of antitumor chemotherapy, and K562 human myelogenous leukemia cells have a higher nuclear proteasome activity than do nonmalignant cells. Adaptation to oxidative stress appears to be one element in the development of long-term resistance to many chemotherapeutic drugs and the mechanisms of inducible tumor resistance to oxidation are of obvious importance. After hydrogen peroxide treatment of K562 cells, degradation of the model proteasome peptide substrate suc-LLVY-MCA and degradation of oxidized histones in nuclei increases significantly within minutes. Both increased proteolytic susceptibility of the histone substrates (caused by modification by oxidation) and activation of the proteasome enzyme complex occur independently during oxidative stress. This rapid up-regulation of 20S proteasome activity is accompanied by, and depends on, poly-ADP ribosylation of the proteasome, as shown by inhibitor experiments, 14C-ADP ribose incorporation assays, immunoblotting, in vitro reconstitution experiments, and immunoprecipitation of (activated) proteasome with anti-poly-ADP ribose polymerase antibodies. The poly-ADP ribosylation-mediated activated nuclear 20S proteasome is able to remove oxidatively damaged histones more efficiently and therefore is proposed as an oxidant-stimulatable defense or repair system of the nucleus in K562 leukemia cells.

Influence of DNA binding on the degradation of oxidized histones by the 20S proteasome

The 20S proteasome is localized in the cytosol and nuclei of mammalian cells. Previous work has shown that the cytosolic 20S proteasome is largely responsible for the selective recognition and degradation of oxidatively damaged cytosolic proteins. Since nuclear proteins are also susceptible to oxidative damage (e.g., from metabolic free radical production, ionizing radiation, xenobiotics, chemotherapy) we investigated the degradation of oxidatively damaged histones, in the presence and in the absence of DNA, by the 20S proteasome. We find that both soluble histones and DNA-bound histones are susceptible to selective proteolytic degradation by the 20S proteasome following mild oxidative damage. In contrast, more severe oxidative damage actually decreases the proteolytic susceptibility of histones. Soluble H1 showed the highest basal and maximal absolute proteolytic rates. Histone fraction H4 exhibited the greatest relative increase in proteolytic susceptibility following oxidation, almost 14-fold, and this occurred at a peroxide exposure of 5 mM. At the other end of the spectrum, histone H2A exhibited a maximal proteolytic response to H2O2 of only 6-fold, and this required an H2O2 exposure of 15 mM. An oxidation of reconstituted linear DNA plasmid-histone complex makes up to 95% of the histones bound to DNA susceptible to degradation, whereas undamaged protein-DNA complexes are not substrates for the proteasome. Severe oxidation by high concentrations of H2O2 appears to decreases the proteolytic susceptibility of histones due to the formation of cross-linked histone-DNA aggregates which appear to inhibit the proteasome. We conclude that the degradation of nuclear proteins is highly selective and requires prior damage of the substrate protein, such as that caused by oxidation.

Oxidative stress causes a general, calcium-dependent degradation of mitochondrial polynucleotides

Oxidative stress has many effects on biological cells, including the modulation of gene expression. Reactive oxygen species are known to up-regulate and down-regulate RNA expression in prokaryotic and eukaryotic cells. We have previously reported that a preferential and calcium-dependent down-regulation of mitochondrial RNAs occurs when HA-1 hamster fibroblasts are exposed to hydrogen peroxide. Here we extend these studies to determine whether this down-regulation is specific to mitochondria RNA or involves general polynucleotide degradation. Degradation and associated decreases in the levels of 16S mitochondrial rRNA following exposure of cells to 400 microM hydrogen peroxide were found to be dependent on calcium at 2 and 5 h. Degradation of mitochondrial genomic DNA was also observed following peroxide exposure, and occurred at similar time points as for mitochondrial RNA degradation. As with mitochondrial RNA degradation, this mitochondrial genomic DNA degradation was dependent on calcium. These results indicate that there is a general, calcium-dependent degradation of mitochondrial polynucleotides following exposure of HA-1 fibroblasts to oxidative stress, and suggest that a dramatic shut-down in mitochondrial biosynthesis is an early-stage response to oxidative stress.

Comparative resistance of the 20S and 26S proteasome to oxidative stress

Oxidatively modified ferritin is selectively recognized and degraded by the 20S proteasome. Concentrations of hydrogen peroxide (H2O2) higher than 10 micromol/mg of protein are able to prevent proteolytic degradation. Exposure of the protease to high amounts of oxidants (H2O2, peroxynitrite and hypochlorite) inhibits the enzymic activity of the 20S proteasome towards the fluorogenic peptide succinyl-leucine-leucine-valine-tyrosine-methylcoumarylamide (Suc-LLVY-MCA), as well as the proteolytic degradation of normal and oxidant-treated ferritin. Fifty per cent inhibition of the degradation of the protein substrates was achieved using 40 micromol of H2O2/mg of proteasome. No change in the composition of the enzyme was revealed by electrophoretic analysis up to concentrations of 120 micromol of H2O2/mg of proteasome. In further experiments, it was found that the 26S proteasome, the ATP- and ubiquitin-dependent form of the proteasomal system, is much more susceptible to oxidative stress. Whereas degradation of the fluorogenic peptide, Suc-LLVY-MCA, by the 20S proteasome was inhibited by 50% with 12 micromol of H2O2/mg, 3 micromol of H2O2/mg was enough to inhibit ATP-stimulated degradation by the 26S proteasome by 50%. This loss in activity could be followed by the loss of band intensity in the non-denaturing gel. Therefore we concluded that the 20S proteasome was more resistant to oxidative stress than the ATP- and ubiquitin-dependent 26S proteasome. Furthermore, we investigated the activity of both proteases in K562 cells after H2O2 treatment. Lysates from K562 cells are able to degrade oxidized ferritin at a higher rate than non-oxidized ferritin, in an ATP-independent manner. This effect could be followed even after treatment of the cells with H2O2 up to a concentration of 2mM. The lactacystin-sensitive ATP-stimulated degradation of the fluorogenic peptide Suc-LLVY-MCA declined, after treatment of the cells with 1mM H2O2, to the same level as that obtained without ATP stimulation. Therefore, we conclude that the regulation of the 20S proteasome by various regulators takes place during oxidative stress. This provides further evidence for the role of the 20S proteasome in the secondary antioxidative defences of mammalian cells.

Peroxynitrite increases the degradation of aconitase and other cellular proteins by proteasome

We report that exposure of aconitase to moderate concentrations of peroxynitrite, 3-morpholinosydnonimine (SIN-1; a superoxide- and nitric oxide-liberating substance), or hydrogen peroxide, inhibits the enzyme and enhances susceptibility to proteolytic digestion by the isolated 20 S proteasome. Exposure to more severe levels of oxidative stress, from these same agents, causes further inhibition of the enzymatic activity of aconitase but actually decreases its proteolytic breakdown by proteasome. It should be noted that the superoxide and nitric oxide liberated by SIN-1 decomposition react to form a steady flux of peroxynitrite. S-Nitroso-N-acetylpenicillamine, a compound that liberates nitric oxide alone, causes only a small loss of aconitase activity (25% or less) and has no effect on the proteolytic susceptibility of the enzyme. Proteasome also seems to be the main protease in cell lysates that can degrade aconitase after it has been oxidatively modified by exposure to peroxynitrite, SIN-1, or hydrogen peroxide. Using cell lysates isolated from K562 cells treated for several days with an antisense oligodeoxynucleotide to the initiation codon region of the C2 subunit of proteasome (a treatment which diminishes proteasome activity by 50-60%), the enhanced degradation of moderately damaged aconitase was essentially abolished. Other model proteins as well as complex mixtures of proteins, such as cell lysates, also exhibit enhanced proteolytic susceptibility after moderate SIN-1 treatment. Therefore we conclude that peroxynitrite reacts readily with proteins and that mild modification by peroxynitrite results in selective recognition and degradation by proteasome.

The DAN1 gene of S. cerevisiae is regulated in parallel with the hypoxic genes, but by a different mechanism

The DAN1 gene is expressed under anaerobic conditions in yeast and completely repressed during aerobic growth. The function of the gene is unknown, and genetic disruption had no effect on fitness which could be detected, even upon prolonged anaerobic growth. Expression of DAN1 was constitutive in a heme-deficient strain, indicating that heme participates in repression. Expression was blocked by heme in anaerobic medium, suggesting that heme acts as a negative co-effector rather than through its metabolic functions, i.e., in the production of a co-effector. Expression of DAN1 was regulated in parallel with the hypoxic gene ANB1, showing identical kinetics of induction and dose response to heme. However, unlike ANB1, DAN1 is not regulated by the repressor of the hypoxic regulon, ROX1, as shown by observation of normal aerobic repression of DAN1 in a strain carrying a deletion of ROX1. These results indicate the existence of a parallel regulatory system which produces an identical response to oxygen by a different mechanism than that controlling the hypoxic regulon.

Degradation of oxidized proteins in mammalian cells

Protein oxidation in vivo is a natural consequence of aerobic life. Oxygen radicals and other activated oxygen species generated as by-products of cellular metabolism or from environmental sources cause modifications to the amino acids of proteins that generally result in loss of protein function/enzymatic activity. Oxidatively modified proteins can undergo direct chemical fragmentation or can form large aggregates due to covalent cross-linking reactions and increased surface hydrophobicity. Mammalian cells exhibit only limited direct repair mechanisms and most oxidized proteins undergo selective proteolysis. The proteasome appears to be largely responsible for the degradation of soluble intracellular proteins. In most cells, oxidized proteins are cleaved in an ATP-and ubiquitin-independent pathway by the 20 S "core" proteasome. The proteasome complex recognizes hydrophobic amino acid residues, aromatic residues, and bulky aliphatic residues that are exposed during the oxidative rearrangement of secondary and tertiary protein structure: increased surface hydrophobicity is a feature common to all oxidized proteins so far tested. The recognition of such (normally shielded) hydrophobic residues is the suggested mechanism by which proteasome catalyzes the selective removal of oxidatively modified cell proteins. By minimizing protein aggregation and cross-linking and by removing potentially toxic protein fragments, proteasome plays a key role in the overall antioxidant defenses that minimize the ravages of aging and disease.

Hamster adapt78 mRNA is a Down syndrome critical region homologue that is inducible by oxidative stress

We are using the technique of mRNA differential display to identify RNAs that may be important in protecting cells against the damaging effects of oxidative stress. For these studies, we utilize a so-called "adaptive response" model system in which hamster HA-1 cells respond to a minimally toxic "pretreatment" dose of hydrogen peroxide by synthesizing RNAs and proteins that protect them against subsequent exposure to a highly cytotoxic concentration of hydrogen peroxide. Using this approach, we have recently reported several novel RNAs whose levels are increased under conditions of adaptive response. Here we report a new RNA, designated adapt78, whose steady-state level is significantly induced by a pretreatment dose of hydrogen peroxide. adapt78 mRNA was calculated to be 2.35 kb in size and inducible by the standard pretreatment dose of 4 micromol H2O2/10(7) cells. It was induced as early as 90 min after peroxide exposure and maximally at 5 h. Induction was strongly dependent upon calcium. Cloning and sequencing revealed a large predicted open reading frame of 197 amino acids. In vitro transcription and translation generated a protein of 25,000 Da. GenBank homology analysis revealed that much of adapt78 is strongly homologous to a sequence that has been mapped to the Down syndrome critical region (Fuentes et al., Hum. Mol. Genet. 4, 1935-1944, 1995). However, both the 5' and the 3' ends of adapt78 show no homology to any previously reported complete sequence. adapt78 represents a new oxidant-inducible RNA and marker of cellular oxidative stress and may provide new insight into our understanding of oxidant-related disorders and neural degeneration.

Modulation of a cardiogenic shock inducible RNA by chemical stress: adapt73/PigHep3

BACKGROUND

Understanding how mammalian cells respond to stress is important in the study, detection, and therapy of stress-related disorders. We have been studying cellular stress response in hamster HA-1 cells by using an adaptive response model. HA-1 cells respond to a minimally toxic "pretreatment" dose of hydrogen peroxide by synthesizing RNAs and proteins that protect them against subsequent exposure to a higher cytotoxic concentration of peroxide. The purpose of our studies is to identify and partially characterize any mRNA whose steady state level is significantly modulated during adaptation.

METHODS

HA-1 cells were exposed to a pretreatment dose of hydrogen peroxide and RNA extracted. The differential display technique was used to identify modulated mRNAs. The effects of calcium ionophore A23187 and cis (II)-platinum on the modulation of mRNA from HA-1 cells and A23187 on the modulation of mRNAs from human IMR-90 cells were also determined.

RESULTS

One of the RNAs induced by a pretreatment concentration of hydrogen peroxide was designated adapt73. The size of the induced adapt73 RNA was determined to be 2.1 kb. Induction of adapt73 was maximal 5 hours after peroxide treatment, but elevated levels were still obvious at 10 hours. This induction was not specific to oxidative stress, because other stress agents including as (II)-platinum and especially calcium ionophore A23187 also induced adapt73 mRNA levels. Partial sequencing of adapt73 and a subsequent GenBank homology search revealed extensive homology to a novel RNA from pig, designated PigHep3, that was identified as a cardiogenic shock response gene from liver in pigs that were undergoing resuscitation after circulatory shock. Homology to a completely sequenced but uncharacterized human homolog was also found. Using a partially sequenced expressed sequence tag (EST) human clone to adapt73, we probed Northern blots containing RNA from IMR-90 human fibroblasts treated with A23187. A strongly induced human adapt73 mRNA homolog was observed, almost identical in size to its hamster homolog. In vitro transcription and translation of the human EST clone revealed a translatable Adapt73 protein product.

CONCLUSIONS

These data indicate that adapt73/PigHep3 RNA can be induced by multiple chemical stress, that these inductions occur under protective or adaptive response conditions, that there is an inducible human homolog to adapt73, and suggest that adapt73 may be an important physiologic mediator of organ and cellular shock response in mammals.

16S mitochondrial ribosomal RNA degradation is associated with apoptosis

The use of mitochondrial RNA as an indicator of apoptosis was investigated. Exposure of HA-1 fibroblastic cells to 10 micromol H(2)O(2) per 10(7) cells induced nuclear fragmentation, cell shrinkage, and internucleosomal DNA fragmentation, all characteristics of apoptosis. RNA extracted from control and apoptotic cultures, and analyzed by Northern blot hybridization, revealed a significant increase in the degradation of mitochondrial 16S ribosomal RNA (rRNA) that was associated with apoptosis. Conversely, minimal, if any, degradation of glyceraldehyde-3-phosphate dehydrogenase or actin mRNAs was observed. Similar results were obtained for HA-1 cells treated with the protein kinase inhibitor staurosporine, and for HT-2 T-lymphocytes induced to undergo apoptosis by interleukin-2 withdrawal. In addition, 16S rRNA degradation was an early event that was discernable well before chromatin condensation in hydrogen peroxide-treated HA-1 cells. These observations suggest that degradation of mitochondrial 16S ribosomal RNA is a new marker of mammalian cell apoptosis.

Down-regulation of mammalian mitochondrial RNAs during oxidative stress

We have identified an RNA species that appears to be induced by oxidative stress in hamster HA-1 fibroblasts using the differential display technique, but instead is found to be degraded when evaluated by Northern blot hybridization. Cloning and subsequent sequencing identified the partially degraded RNA as 16S ribosomal RNA (rRNA), a major component of mitochondrial ribosomes. Degradation, and associated decreases in the levels of the mature- and precursor-species of 16S rRNA, appear to be dependent upon calcium, but not cytoplasmic protein synthesis nor nuclear transcription. Other decreased mitochondrial RNAs were also identified, including 12S rRNA, NADH dehydrogenase subunit 6, ATPase subunit 6, and cytochrome oxidase subunits I and III. A significant part of many, if not all, of these RNA decreases was due to degradation. As compared with 16S rRNA, significantly less degradation was observed for cytoplasmic 28S/18S rRNAs, even at very high peroxide concentration. Analysis of 21 cytoplasmic mRNAs revealed little or no decrease in mature band signal in response to peroxide, and several cytoplasmic mRNAs were actually up-regulated. Thus, a preferential down-regulation of mitochondrial RNAs occurs in HA-1 fibroblasts in response to hydrogen peroxide. Subcellular fractionation analysis, using 16S rRNA degradation as a gauge, indicates that this down-regulation is specific to mitochondria. The down-regulation of mitochondrial RNAs may represent a general mechanism by which cells protect themselves against oxidative stress.

Breakdown of oxidized proteins as a part of secondary antioxidant defenses in mammalian cells

The degradation of oxidized proteins is an essential part of antioxidant defenses against free radical attack. Selective degradation of oxidatively damaged proteins allows proteolytic systems to function directly in the removal of useless cellular debris and therefore prevent the accumulation of potentially toxic fragments or large aggregates of cross-linked proteins. The degradation of oxidized proteins in dividing mammalian cells after hydrogen peroxide treatment has been demonstrated. Cells are able to increase proteolysis rates after treatment with moderate levels of oxidants. The role of proteasome in the removal of oxidized proteins has been demonstrated by treatment of cells with an anti-sense oligodesoxynucleotide to the proteasome C2 subunit gene. This treatment decreases the proteasome content of the cells and prevents increased proteolysis rates after hydrogen peroxide treatment. Thus proteasome clearly plays a role in the removal of oxidized proteins. As part of antioxidant defenses the proteasome provides a second line of defense against the numerous radicals and oxidants which contact cells during their lifetime. The degradation of oxidatively damaged proteins enables-cells to recover from a moderate oxidant attack.

adapt33, a novel oxidant-inducible RNA from hamster HA-1 cells

We have recently chosen to undertake a comprehensive evaluation of the modulation of gene expression by oxidative stress, using mRNA as a marker. Our model system is HA-1 hamster fibroblasts, using conditions under which we observe an adaptive response. Under these conditions, the HA-1 cells respond to a minimally toxic "pretreatment" dose of hydrogen peroxide by synthesizing RNAs and proteins that protect them against subsequent exposure to a highly cytotoxic concentration of hydrogen peroxide. Using the differential display technique to screen for modulated RNAs, we have recently reported an RNA species, adapt15/gadd7, whose steady-state level is significantly induced by a pretreatment dose of hydrogen peroxide (D. R. Crawford, G. P. Schools, S. L. Salmon, and K. J. A. Davies (1996) Arch. Biochem. Biophys. 325, 256-264). Here we report a second induced mRNA, designated adapt33. Two homologous adapt33 mRNAs were revealed by Northern blot hybridization. Both of these species were inducible by hydrogen peroxide, and they were sized at 1.46 and 0.99 kb. These inductions appeared to be dependent upon calcium, occurred as early as 90 min, and were maximal at 5 h. Cell fractionation revealed that a significant proportion of adapt33 RNA is associated with active translation. adapt33 is a novel sequence, as determined by cloning, sequencing, and GenBank analysis. adapt33 represents a new oxidant-inducible RNA and marker of cellular oxidative stress and a potential aid in the study, detection, and possible therapy of oxidant-related disorders.

Manganese superoxide dismutase modulates interleukin-1alpha levels in HT-1080 fibrosarcoma cells

Reactive oxygen species of mitochondrial origin have been implicated in regulating the expression of several tumor necrosis factor (TNF)-induced genes. Manganese superoxide dismutase (Mn-SOD) is one of many genes, but only antioxidant enzyme, induced in response to tumor necrosis factor. Mn-SOD is a nuclear-encoded mitochondrial matrix protein and serves a protective function by detoxifying superoxide. To address the role of superoxide in regulating gene expression in response to TNF, we have constitutively overexpressed Mn-SOD in a human fibrosarcoma cell line and asked what effect this has on the expression of a number of TNF-responsive genes using reverse transcription-polymerase chain reaction. Of the TNF-induced transcripts analyzed, only interleukin-1alpha (IL-1alpha) was modulated in response to Mn-SOD overexpression. In all cases of Mn-SOD overexpression, IL-1alpha protein and mRNA levels were lowered constitutively and in response to TNF when compared to the parental and mock-transfected cell lines. The induction of IL-1alpha by TNF can also be decreased by growth in 3% oxygen as compared to growth in 21% O2; in addition, growth in low oxygen lowers the basal level of IL-1alpha protein. The effect of Mn-SOD overexpression on IL-1alpha expression can be overcome by treatment with the protein kinase C activator, phorbol 12-myristate 13-acetate. Mn-SOD overexpression and low oxygen alter IL-1alpha mRNA levels by decreasing the stability of the IL-1alpha mRNA. These findings indicate that both Mn-SOD and O2 may regulate the levels of a cellular oxidant involved in both basal and TNF-induced IL-1alpha expression, presumably superoxide.

Degradation of oxidized proteins in K562 human hematopoietic cells by proteasome

Exposure to various forms of oxidative stress (H2O2 and O2.-) significantly increased the intracellular degradation of both "short-lived" and "long-lived" cellular proteins in the human hematopoietic cell line K562. Oxidatively modified hemoglobin and superoxide dismutase used as purified proteolytic substrates were also selectively degraded by K562 cell lysates, but exposure of these protein substrates to very high hydrogen peroxide concentrations actually decreased their proteolytic susceptibility. Our studies found little or no change in the overall capacity of cells and cell lysates to degrade "foreign" oxidized proteins after treatment of K562 cells with hydrogen peroxide or paraquat, a finding supported by proteasome Western blots and unchanged capacity of cell lysates to degrade the fluorogenic peptide succinyl-leucine-leucine-valine-tyrosine-4-methylcoumarin-7-amide. Six days of daily treatment of K562 cells with an antisense oligodeoxynucleotide directed against the initiation codon region of the human proteasome C2 subunit gene dramatically depressed hydrogen peroxide-induced degradation of metabolically radiolabeled intracellular proteins. The actual amount of proteasome in antisense-treated K562 cells was also severely depressed, as revealed by Western blots and by measurements of the degradation of the fluorogenic peptide succinyl-leucine-leucine-valine-tyrosine-4-methylcoumarin-7-amide. The degradation of oxidatively modified foreign protein substrates was also markedly depressed in lysates prepared from K562 cells treated with the proteasome C2 antisense dideoxynucleotide. The inhibitor profile for the degradation of H2O2-modified hemoglobin by K562 cell lysates was consistent with a major role for the ATP-independent 20 S "core" proteasome complex. We conclude that proteasome, probably the 20 S core proteasome complex, is primarily responsible for the selective degradation of oxidatively damaged proteins in human hematopoietic cells. Since "oxidative marking" of cellular proteins by lipoxygenase has been proposed as an important step in red blood cell maturation, it is important to determine which protease or proteases could recognize and degrade such modified substrates. Our results provide evidence that proteasome can, indeed, conduct such selective degradation and appears to be the major cellular protease capable of fulfilling such a role in maturation.

A comparison of the ability of intra-oral and extra-oral fibroblasts to stimulate extracellular matrix reorganization in a model of wound contraction

Intra-oral wounds, like wounds in children, demonstrate privileged healing when compared with adult wounds at extra-oral sites. This study investigated whether this preferential healing is related to an increased ability of oral mucosal fibroblasts to reorganize extracellular matrix (ECM) when compared with their dermal counterparts. ECM reorganization was investigated by means of a fibroblast-populated collagen lattice (FPCL) system. The effect of donor age was also investigated in this system. Differences in ECM reorganization and FPCL contraction were evident: FPCL contraction was more rapid by oral mucosal fibroblasts than dermal fibroblasts (p < 0.01). FPCL contraction was also greater in child (donor < 10 years) than adult (donor > 18 years) oral mucosal fibroblasts (p < 0.01). These differences were not related to phenotypic differences in cell viability (p > 0.5), DNA synthesis (p > 0.05), and cell number (p > 0.5) within the FPCLs, or cellular attachment to collagen (p > 0.07). FPCL contraction was not stimulated by the addition of conditioned medium from oral mucosal or dermal fibroblasts (p > 0.05). These data show that the significantly increased ability of oral mucosal fibroblasts to reorganize ECM in vitro, when compared with dermal fibroblasts, represents a distinct phenotypic contractile difference, rather than differences in their production of soluble mediators or cell attachment to ECM.

Oxidant-inducible adapt 15 RNA is associated with growth arrest- and DNA damage-inducible gadd153 and gadd45

We have recently described a novel RNA, designated adapt15, which is strongly induced by a pretreatment concentration of hydrogen peroxide in hamster HA-1 fibroblasts under conditions where a protective "adaptive response" occurs. adapt15 may therefore be involved in protecting cells against the damaging effects of oxidative stress. Since two other known sequences, gadd45 and gadd153m were also found to be induced under our pretreatment conditions and are known to be growth arrest and DNA damage inducible, we decided to also assess the possible association of adapt15 with growth arrest and DNA damage. We found that, like gadd45 and gadd153, the levels of adapt15 RNA were low during proliferation, but high during density saturation- and low serum (G0)-growth arrests. Exposure of HA-1 cells to DNA-damaging agents revealed significant induction of adapt15 RNA by methylmeth- anesulfonate and cis-platinum but not X-irradiation. Near identical responses were also observed for gadd45 and gadd153 RNAs, suggesting coordinate regulation of adapt15, gadd45, and gadd153. All three RNAs were also increased relative to control following heat shock, a nongenotoxic treatment. Finally, the induction of adapt15 by hydrogen peroxide was strongly dependent upon calcium, a hallmark of gadd153 induction. The coordinate inductions of adapt15, gadd45, and gadd153 by multiple agents, and their induction by an adaptive- but not by a nonadaptive-response level of hydrogen peroxide, suggest these RNAs may act in concert to protect cells against the damaging effects of oxidative stress.

An investigation of the interaction between alcohol and fibroblasts in wound healing

This study investigated the effect of clinical concentrations of alcohol on fibroblast function (proliferation and ECM synthesis) in vitro. Basal and TGF-beta-induced collagen synthesis was assayed in confluent cultures in serum-free medium at 48 h with a commercial collagen assay system. At concentrations of alcohol > 5%, fibroblast proliferation was significantly inhibited. Although noninhibitory, subclinical concentrations of alcohol failed to inhibit basal collagen synthesis (P > 0.1), they significantly decreased TGF-beta-induced collagen synthesis (P < 0.03). These data support the notion that the local, as well as the systemic, effects of alcohol are important in mediating delayed healing in alcoholic patients.

Hydrogen peroxide induces the expression of adapt15, a novel RNA associated with polysomes in hamster HA-1 cells

In mammalian cells, hydrogen peroxide is known to induce the expression of several genes thought to be important in protection against oxidative stress. Using mRNA differential display, we have identified a novel RNA in hamster HA1 cells that is strongly induced by hydrogen peroxide under conditions where a protective adaptive response occurs. This induction was observed between 1 and 18 h after peroxide treatment, and at an H202 concentration that caused little or no cytotoxicity. Northern blot analysis revealed that this major inducible RNA species, termed adapt15, is 950 bases in size. Two versions of this RNA were found by sequence analysis, differing only by a short trinucleotide stretch. Despite polyadenylation, no large open reading frame was observed. Fractionation studies, however, indicate that adapt15 RNA is primarily located in the cytoplasm, and a significant percentage of it is associated with active translation. adapt15 RNA may act at the level of translation to protect cells against the damaging effect of oxidative stress.

Oxidative stress induces the levels of a MafG homolog in hamster HA-1 cells

The Maf family encodes nuclear proteins that recognize AP-1-like response elements. MafB, MafK, MafF, and MafG, are all able to heterodimerize with each other, c-fos, and erythroid cell-specific transcription factor NF-E2, to affect the transcription of target genes. Using the technique of differential display, we recently identified a new oxidant-inducible mRNA, designated adapt66, in HA-1 hamster fibroblasts. Cloning, partial sequencing, and GenBank analysis of adapt66 revealed strong homology to chicken mafG, a newly identified member of the maf oncogene family. The mafG homolog/adapt66 mRNA induction appeared to be dependent upon calcium; occurred as early as 90 minutes following exposure of HA-1 cells to hydrogen peroxide; and peaked between 5 and 10 hours after peroxide treatment. It has previously been demonstrated that several cellular transcription factors, including Fos, can be induced by oxidative stress. The induction of the DNA binding sequence mafG homolog/adapt66 by hydrogen peroxide, and it's known interaction with c-fos, may represent important mechanisms by which oxidative stress can modulate gene expression.

Seroepidemiology of herpes virus infections among dental personnel

OBJECTIVES

To examine the possible occupational hazard of infection with human herpes viruses among dental personnel.

METHODS

Sera from 81 preclinical dental students, 53 clinical dental students and 103 qualified dental surgeons were tested for antibodies to herpes simplex virus type 1 (HSV-1), cytomegalovirus (CMV), Epstein-Barr virus (EBV) and human herpes virus type 6 (HHV-6). The same number of control subjects, matched individually for age (+/- 1 year), sex and social class, was also examined. Antibodies were detected by ELISA for HSV-1, latex agglutination for CMV, indirect immunofluorescence with P3HR1 cells for EBV and indirect immunofluorescence with infected JJhan cells for HHV-6. Each participant also completed a questionnaire to permit correlation of demographic data and risk factors with serological results.

RESULTS

No significant difference in seroprevalence was detected between any of the dental groups and their respective controls. There was a significantly higher prevalence of antibodies to EBV among clinical students (P = 0.02) and qualified dentists (P = 0.0003) than among preclinical students. These significant increases were not mirrored in the three corresponding control groups.

CONCLUSION

The results suggest a possible occupational risk of infection with EBV in dentists. There was no evidence for a significant risk of occupational infection with HSV-1, CMV or HHV-6.