Current topics in cellular regulation (Curr Top Cell Regul )
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- Other titlesCurrent topics in cellular regulation, Cellular regulation
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ABSTRACT: Antioxidants are substances that delay or prevent the oxidation of cellular oxidizable substrates. The various antioxidants exert their effect by scavenging superoxide or by activating a battery of detoxifying/defensive proteins. In this chapter, we have focused on the mechanisms by which antioxidants induce gene expression. Many xenobiotics (e.g., beta-naphthoflavone) activate genes similar to those activated by antioxidants. The promoters of these genes contain a common cis-element, termed the antioxidant response element (ARE), which contains two TRE (TPA response element) or TRE-like elements followed by GC box. Mutational studies have identified GTGAC***GC as the core of the ARE sequence. Many transcription factors, including Nrf, Jun, Fos, Fra, Maf, YABP, ARE-BP1, Ah (aromatic hydrocarbon) receptor, and estrogen receptor bind to the ARE from the various genes. Among these factors, Nrf-Jun heterodimers positively regulate ARE-mediated expression and induction of genes in response to antioxidants and xenobiotics. This Nrf-Jun heterodimerization and binding to the ARE requires unknown cytosolic factors. The mechanism of signal transduction from antioxidants and xenobiotics includes several steps: (1) Antioxidants and xenobiotics undergo metabolism to generate superoxide and related reactive species, leading to the generation of a signal to activate expression of detoxifying/defensive genes. (2) The generation of superoxide and related reactive species is followed by activation of yet to be identified cytosolic factors by unknown mechanism(s). (3). Activated cytosolic factors catalyze modification of Nrf and/or Jun proteins, which bind to the ARE in promoters of the various detoxifying/defensive genes. (4) The transcription of genes encoding detoxifying/defensive proteins is increased. The unknown cytosolic factors are significant molecules because they represent the oxidative sensors within the cells. Identification of the cytosolic factors will be of considerable importance in the field of antioxidants and gene regulation research. Future studies will also be required to completely understand the molecular mechanism of signal transduction from antioxidants and xenobiotics to Nrf-Jun. In addition to the Nrf-Jun pathway, mammalian cells also contain other pathways that activate gene expression in response to oxidative stress. These include NF-KB-, HIF-1-, Mac-1-, and SRF-mediated pathways. It is expected that collectively these pathways increase transcription of more than four dozen genes to protect cells against oxidative stress.Current topics in cellular regulation 02/2000; 36:201-16.
- Current topics in cellular regulation 02/2000; 36:237-95.
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ABSTRACT: The activity of NF-kappa B is critically involved in the inflammatory activation of endothelial cells and their adhesiveness and also appears to regulate apoptosis in SMC by coordinating antiapoptotic programs. The activity of NF-kappa B has been revealed within human atheromas or following angioplasty but not in undiseased arteries. Hence, the inhibition of NF-kappa B mobilization by antioxidative or anti-inflammatory agents or by adenoviral I kappa B alpha overexpression, as reviewed herein, may act in concert to suppress endothelial activation and to induce SMC apoptosis. This synergistic concept may be a vasoprotective approach to prevent atherogenesis and restenosis by attenuating inflammatory reactions and SMC proliferation in nascent and progressing atherosclerotic lesions, as well as in developing neointima formations following angioplasty.Current topics in cellular regulation 02/2000; 36:217-35.
- Current topics in cellular regulation 02/2000; 36:133-50.
Article: Thiol-based antioxidants.[Show abstract] [Hide abstract]
ABSTRACT: The thiol redox status of intracellular and extracellular compartments is critical in the determination of protein structure, regulation of enzyme activity, and control of transcription factor activity and binding. Thiol antioxidants act through a variety of mechanisms, including (1) as components of the general thiol/disulfide redox buffer, (2) as metal chelators, (3) as radical quenchers, (4) as substrates for specific redox reactions (GSH), and (5) as specific reductants of individual protein disulfate bonds (thioredoxin). The composition and redox status of the available thiols in a given compartment is highly variable and must play a part in determining the metabolic activity of each compartment. It is generally beneficial to increase the availability of specific antioxidants under conditions of oxidant stress. Cells have devised a number of mechanisms to promote increased intracellular levels of thiols such as GSH and thioredoxin in response to a wide variety of stresses. Exogenous thiols have been used successfully to increase cell and tissue thiol levels in cell cultures, in animal models, and in humans. Increased levels of GSH and other thiols have been associated with increased tolerance to oxidant stresses in all of these systems and in some cases, with disease prevention or treatment in humans. A wide variety of thiol-related compounds have been used for these purposes. These include thiols such as GSH and its derivatives, cysteine and NAC, dithiols such as lipoic acid, which is reduced to the thiol form intracellularly, and "prothiol" compounds such as OTC, which are enzymatically converted to free thiols within the cell. In choosing a thiol for a specific function (e.g., protection of lung from oxidant exposure or protection of organs from ischemia reperfusion injury), the global effects must also be considered. For example, large increases in free thiols in the circulation are associated with toxic effects. These effects may be the result of thiyl radical-mediated reactions but could also be due to destabilizing effects of increases in thiol/disulfide ratios in the plasma, which normally is in a more oxidized state than intracellular compartments. Changes in the thiol redox gradient across cells could also adversely affect any transport or cell signaling processes, which are dependent on formation and rupture of disulfide linkages in membrane proteins. Therapeutic thiol administration has been shown to have great potential, and its efficacy should be increased by selecting compounds and methods of delivery that will minimize perturbations in the thiol status of regions external to the targeted areas.Current topics in cellular regulation 02/2000; 36:151-80.
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ABSTRACT: In contrast to the conventional notion that reactive oxygen is mostly a trigger for oxidative damage of biological structures, now we know that low physiologically relevant concentrations of ROS can regulate a variety of key molecular mechanisms that may be linked with important cell functions (Fig. 4). Redox-based regulation of gene expression has emerged as a fundamental regulatory mechanism in cell biology. Several proteins, with apparent redox-sensing activity, have been described. Electron flow through side-chain functional CH2-SH groups of conserved cysteinyl residues in these proteins account for the redox-sensing properties. Protein thiol groups with high thiol-disulfide oxidation potentials are likely to be redox-sensitive. The ubiquitous endogenous thiols thioredoxin and glutathione are of central importance in redox signaling. Signals are transduced from the cell surface to the nucleus through phosphorylation and dephosphorylation chain reactions of cellular proteins at tyrosine and serine/threonine. Protein phosphorylation, one of the most fundamental mediators of cell signaling, is redox-sensitive. DNA-binding proteins are involved in the regulation of cellular processes such as replication, recombination, viral integration and transcription. Several studies show that the interaction of certain transcription regulatory proteins with their respective cognate DNA sites is also redox-regulated. Changes in the concentration of Ca2+i control a wide variety of cellular functions, including transcription and gene expression; Ca(2+)-driven protein phosphorylation and proteolytic processing of proteins are two major intracellular events that are implicated in signal transduction from the cell surface to the nucleus. Intracellular calcium homeostasis is regulated by the redox state of cellular thiols, and it is evident that cell calcium may play a critical role in the activation of the redox-sensitive transcription factor NF-kappa B. Among the several thiol agents tested for their efficacy in modulating cellular redox status, N-acetyl-L-cysteine and alpha-lipoic acid hold most promise for human use. A strong therapeutic potential of strategies that would modulate the cellular thioredoxin system has been also evident.Current topics in cellular regulation 02/2000; 36:1-30.
- Current topics in cellular regulation 02/2000; 36:117-32.
- Current topics in cellular regulation 02/2000; 36:31-75.
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ABSTRACT: The HO-1 isoenzyme is an early stress response gene regulated by many forms of oxidative stress. The HO-2 isoenzyme is predominantly a constitutive enzyme, which may serve to sequester heme as well as degrade it. All HO enzyme activity results in the degradation of heme and the production of antioxidant bile pigments, which would favor an antioxidant role for the enzyme. In fact, in oxidative stress in vitro, HO-1 is protective (91-94) but within a narrow threshold of overexpression (93,94) in some models, since iron released in the HO reaction may obviate any cytoprotective effect (Fig. 3). So far, HO-2 appears to be beneficial in oxygen toxicity in vivo, but the consequences of HO-2 overexpression have not yet been tested. It will be important to better define the role of each HO isoenzyme in oxidative stress so as to determine whether enhancing these complex systems could alleviate some of the cellular changes seen as a result of oxidative injury. Furthermore, prior to considering therapeutic maneuvers to enhance HO, a complete understanding of the physiologic consequences of HO-1 induction and associated reactions, in each particular setting, will be crucial.Current topics in cellular regulation 02/2000; 36:181-99.
Article: Regulation of glutathione synthesis.Current topics in cellular regulation 02/2000; 36:95-116.
- Current topics in cellular regulation 02/2000; 36:77-94.
- Current topics in cellular regulation 02/1997; 35:179-92.
Article: Aging and regulation of apoptosis.[Show abstract] [Hide abstract]
ABSTRACT: When Lockshin and Zakeri discussed the relevance of apoptosis to aging, the common view was that apoptosis had primarily a negative impact on aging by destroying essential and often irreplaceable cells. That view has now changed to one that acknowledges that there are two general ways in which apoptosis can play a role in aging: (1) elimination of damaged and presumably dysfunctional cells (e.g., fibroblasts, hepatocytes) which can then be replaced by cell proliferation, thereby maintaining homeostasis and elimination of essential postmitotic cells (e.g., neurons) which cannot be replaced, thereby leading to pathology. Evidence exists in two systems (fibroblasts and thymocytes/lymphocytes) that there are age-related decreases in the potential for apoptosis, although the molecular bases for these decreases appear to differ (Table II). Fibroblasts (and neurons?) lose the ability to downregulate bcl-2 in response to an apoptotic signal; thus, apoptosis is blocked even though an initiating signal has been received. In contrast, thymocytes/lymphocytes lack the ability to initiate the signal due to downregulation of the cell surface receptor Fas. There is limited information available for other tissue types, and nothing is known about why and how these age-related changes occur. An interesting observation, but not necessarily a critical one, is that the frequency of upregulation of the bcl-2 gene due to chromosome translocation increases with age. The role of apoptosis in regulating cell number is also a promising area of research. The studies on liver damage and neoplastic lesions suggest an extremely important role for apoptosis in controlling cancer. This may be particularly important in the prostate, where hypertrophy and cancer are a virtual certainty with ever-increasing age. It is not known whether the ability to undergo apoptosis declines in the prostate with increasing age, but it appears likely that it does. One problem in answering questions about the actual regulation of apoptosis is the lack of a quantitative assay. Apoptosis appears to be either "on" or "off" in cells, while the basic cell-killing machinery may often be present, but in an inactive form. Most assays for apoptosis are microscopic rather than kinetic, and the rate-limiting step may be at the level of the initiating signal. Thus, if CR, which extends the life span of rodents, does upregulate apoptosis, it is not clear how to quantify the magnitude of this effect or what should be quantified. The best one can do is to measure the frequency of occurrence of apoptotic bodies. This is essentially a pool size assay which provides little knowledge about how rapidly cells are leaving and entering the pool. Nevertheless, the results currently available do suggest that apoptosis is a process which may be important in aging, at least in some tissues, and the mechanism of its regulation needs to be understood. Although a variety of tumor suppressor gene and oncogene products are known to be involved in signal transduction associated with apoptosis, it remains to be shown which of these, if any, are actually involved in "on-off" switches for apoptosis and which might regulate the intrinsic rate of apoptosis. As Driscoll has already pointed out: "regulation and execution of cell death is an absolutely critical process that interfaces with nearly every aspect of life. Future investigation of the links of cell death to cellular aging and the aging of organisms should be an exciting enterprise."Current topics in cellular regulation 02/1997; 35:107-21.
- Current topics in cellular regulation 02/1997; 35:163-77.
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ABSTRACT: We have identified novel signal transduction cascades in activating NF-kappa B, as well as its pathogenetic roles in various disease processes. By applying the basic knowledge obtained through these studies, we hope to find new therapeutic measures against currently incurable diseases such as hematogenic cancer cell metastasis, rheumatoid arthritis, and AIDS. We also propose a novel strategy in screening effective inhibitors against transcription factors. Elucidation of the cis-regulatory element for expression of pathogenetic genes and identification of the responsible transcription factor will not only facilitate the study of pathogenesis but will also promote the development of effective therapy. Recognition of control mechanisms of the NF-kappa B activation pathway has explained the therapeutic efficacy of various compounds with different pharmacologic actions. A similar strategy may be applicable for other inducible transcription factors. From the medical point of view, one of the purposes of these approaches is to find small molecular weight compounds that can be administered orally and that are effective in controlling gene expression of pathogenetic genes.Current topics in cellular regulation 02/1997; 35:149-61.
- Current topics in cellular regulation 02/1997; 35:123-48.
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ABSTRACT: Iron metabolism is regulated in cells to ensure that iron supplies are adequate and nontoxic. The expression of iron metabolism is regulated primarily by posttranscriptional mechanisms. Ferritin, eALAS, SDHb of Drosophila, and mammalian mitochondrial aconitase are translationally regulated. The TfR is regulated at the level of mRNA stability. Iron regulatory proteins are regulated either by assembly or by disassembly of an iron-sulfur cluster (IRP1) or by rapid degradation in the presence of iron (IRP2). The list of targets for IRP-mediated regulation is growing longer, and a range of possibilities for versatile regulation exists, as each IRP can bind to unique targets that differ from the consensus IRE. The reactivity of iron with oxygen and the creation of toxic by-products may be the evolutionary stimulus that produced this system of tight posttranscriptional gene regulation.Current topics in cellular regulation 02/1997; 35:1-19.
- Current topics in cellular regulation 02/1997; 35:21-68.
- Current topics in cellular regulation 02/1996; 34:101-36.
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