Induction of metallothionein and heme oxygenase in rats after inhalation of endotoxin

The Physiological Roles of Extracellular Metallothionein

Metallothionein (MT) is a low-molecular-weight protein with a number of roles to play in cellular homeostasis. MT is synthesized as a consequence of a variety of cellular stressors, and has been found in both intracellular compartments and in extracellular spaces. The intracellular pool of this cysteine-rich protein can act as a reservoir of essential heavy metals, as a scavenger of reactive oxygen and nitrogen species, as an antagonist of toxic metals and organic molecules, and as a regulator of transcription factor activity. The presence of MT outside of cells due to the Influence of stressors suggests that this protein may make important contributions as a “danger signal” that influences the management of responses to cellular damage. While conventional wisdom has held that extracellular MT is the result of cell death or leakage from stressed cells, there are numerous examples of selective release of proteins by nontraditional mechanisms, including stress response proteins. This suggests that MT may similarly be selectively released, and that the pool of extracellular MT represents an important regulator of various cellular functions. For example, extracellular MT has effects both on the severity of autoimmune disease, and on the development of adaptive immune functions. Extracellular MT may operate as a chemotactic factor that governs the trafficking of inflammatory cells that move to resolve damaged tissues, as a counter to extracellular oxidant-mediated damage, and as a signal that influences the functional behavior of wounded cells. A thorough understanding of the mechanisms of MT release from cells, the conditions under which MT is released to the extracellular environment, and the ways in which MT Interacts with sensitive cells may both illuminate our understanding of an important control mechanism that operates in stressful conditions, and should indicate new opportunities for therapeutic management via the manipulation of this pool of extracellular MT.

The two faces of metallothionein in carcinogenesis: photoprotection against UVR-induced cancer and promotion of tumour survival

Metallothionein is a multi-functional protein that protects the host against toxic heavy metals. Under stressful situations it can protect against oxidative damage, contribute to tissue repair, modulate immune responses and limit inflammatory processes. Recently, metallothionein's role in ultraviolet radiation (UVR)-induced injury has been investigated. These studies have shown that when metallothionein is upregulated following exposure to UVR, it can protect against UVR-induced damage and the subsequent development of skin cancer. We propose that this initial protection is achieved through its anti-oxidant role resulting in reduced oxidative stress, reduced apoptosis, reduced NFkappaB activation and enhanced repair of DNA damage. However, once UVR-induced neoplasia has occurred, the cancer cells can hijack metallothionein's protective functions, resulting in increased tumour progression and malignancy. These two discordant sets of attributes are context-dependent, and represent the two faces of metallothionein.

Differential pulmonary and cardiac effects of pulmonary exposure to a panel of particulate matter-associated metals

Biological mechanisms underlying the association between particulate matter (PM) exposure and increased cardiovascular health effects are under investigation. Water-soluble metals reaching systemic circulation following pulmonary exposure are likely exerting a direct effect. However, it is unclear whether specific PM-associated metals may be driving this. We hypothesized that exposure to equimolar amounts of five individual PM-associated metals would cause differential pulmonary and cardiac effects. We exposed male WKY rats (14 weeks old) via a single intratracheal instillation (IT) to saline or 1 micromol/kg body weight of zinc, nickel, vanadium, copper, or iron in sulfate form. Responses were analyzed 4, 24, 48, or 96 h after exposure. Pulmonary effects were assessed by bronchoalveolar lavage fluid levels of total cells, macrophages, neutrophils, protein, albumin, and activities of lactate dehydrogenase, gamma-glutamyl transferase, and n-acetyl glucosaminidase. Copper induced earlier pulmonary injury/inflammation, while zinc and nickel produced later effects. Vanadium or iron exposure induced minimal pulmonary injury/inflammation. Zinc, nickel, or copper increased serum cholesterol, red blood cells, and white blood cells at different time points. IT of nickel and copper increased expression of metallothionein-1 (MT-1) in the lung. Zinc, nickel, vanadium, and iron increased hepatic MT-1 expression. No significant changes in zinc transporter-1 (ZnT-1) expression were noted in the lung or liver; however, zinc increased cardiac ZnT-1 at 24 h, indicating a possible zinc-specific cardiac effect. Nickel exposure induced an increase in cardiac ferritin 96 h after IT. This data set demonstrating metal-specific cardiotoxicity is important in linking metal-enriched anthropogenic PM sources with adverse health effects.

Metallothionein as an anti-inflammatory mediator

The integration of knowledge concerning the regulation of MT, a highly conserved, low molecular weight, cystein-rich metalloprotein, on its proposed functions is necessary to clarify how MT affects cellular processes. MT expression is induced/enhanced in various tissues by a number of physiological mediators. The cellular accumulation of MT depends on the availability of cellular zinc derived from the diet. MT modulates the binding and exchange/transport of heavy metals such as zinc, cadmium, or copper under physiological conditions and cytoprotection from their toxicities, and the release of gaseous mediators such as hydroxyl radicals or nitric oxide. In addition, MT reportedly affects a number of cellular processes, such as gene expression, apoptosis, proliferation, and differentiation. Given the genetic approach, the apparently healthy status of MT-deficient mice argues against an essential biological role for MT; however, this molecule may be critical in cells/tissues/organs in times of stress, since MT expression is also evoked/enhanced by various stresses. In particular, because metallothionein (MT) is induced by inflammatory stress, its roles in inflammation are implied. Also, MT expression in various organs/tissues can be enhanced by inflammatory stimuli, implicating in inflammatory diseases. In this paper, we review the role of MT of various inflammatory conditions.

Role of metallothionein in antigen-related airway inflammation

Metallothionein (MT) is a protein that can be induced by inflammatory mediators and participates in cytoprotection. However, its role in antigen-related inflammation remains to be established. We determined whether intrinsic MT protects against antigen-related airway inflammation induced by ovalbumin (OVA) in MT-I/II null (MT [-/-]) mice and in corresponding wild-type (WT) mice. MT (-/-) mice and WT mice were intratracheally challenged with OVA (1 mug per body) biweekly four times. Twenty-four hours after the last OVA challenge, significant increases were shown in the numbers of total cells, eosinophils, and neutrophils in bronchoalveolar lavage fluid from MT (-/-) mice than in those from WT mice. The protein level of interleukin-1beta (IL-1beta) was significantly greater in MT (-/-) mice than in WT mice after OVA challenge. Immunohistochemical analysis showed that the formations of 8-oxy-deoxyguanosine and nitrotyrosine in the lung were more intense in MT (-/-) mice than in WT mice after OVA challenge. These results indicate that endogenous MT is a protective molecule against antigen-related airway inflammation induced by OVA, at least partly, via the suppression of enhanced lung expression of IL-1beta and via the antioxidative properties. Our findings suggest that MT may be a therapeutic target for the treatment of antigen-related airway inflammatory diseases such as bronchial asthma.

Protective role of metallothionein in acute lung injury induced by bacterial endotoxin

BACKGROUND

Metallothionein (MT) is a protein that can be induced by inflammatory mediators and participate in cytoprotection. However, its role in inflammation remains to be established. A study was undertaken to determine whether intrinsic MT protects against acute inflammatory lung injury induced by bacterial endotoxin in MT-I/II knock out (-/-) and wild type (WT) mice.

METHODS

MT (-/-) and WT mice were given vehicle or lipopolysaccharide (LPS, 125 microg/kg) intratracheally and the cellular profile of the bronchoalveolar lavage (BAL) fluid, pulmonary oedema, lung histology, expression of proinflammatory molecules, and nuclear localisation of nuclear factor-kappaB (NF-kappaB) in the lung were evaluated.

RESULTS

MT (-/-) mice were more susceptible than WT mice to lung inflammation, especially to lung oedema induced by intratracheal challenge with LPS. After LPS challenge, MT deficiency enhanced vacuolar degeneration of pulmonary endothelial cells and type I alveolar epithelial cells and caused focal loss of the basement membrane. LPS treatment caused no significant differences in the enhanced expression of proinflammatory cytokines and chemokines nor in the activation of the NF-kappaB pathway in the lung between the two genotypes. Lipid peroxide levels in the lungs were significantly higher in LPS treated MT (-/-) mice than in LPS treated WT mice.

CONCLUSIONS

Endogenous MT protects against acute lung injury related to LPS. The effects are possibly mediated by the enhancement of pulmonary endothelial and epithelial integrity, not by the inhibition of the NF-kappaB pathway.

Complementary DNA microarray analysis in acute lung injury induced by lipopolysaccharide and diesel exhaust particles

We have recently shown that diesel exhaust particles (DEP) synergistically enhance acute lung injury related to lipopoly-saccharide (LPS) in mice. The present study used cDNA microarray to elucidate the effects of DEP on the global pattern of LPS-related gene expression in the murine lung. The number of genes upregulated >/=2-fold as compared with their expression levels in the vehicle group was greater in the LPS group than in other groups, but treatment with DEP and LPS dramatically increased the number of the genes upregulated >/=6-fold. In particular, gene expression of metallothionein-1 and -2, S100 calcium-binding protein A9, lipocalin 2, and small inducible cytokine B family member 10 was higher by >/=20-fold in the DEP + LPS group than in the vehicle group. These results were concomitant with those obtained by real-time reverse transcription-polymerase chain reaction analysis in the overall trend. Our findings suggest that intense, focused expression of genes such as S100 calcium-binding protein A9, lipocalin 2, and small inducible cytokine B family member 10 relates to the synergistic aggravation of acute lung injury by LPS and DEP rather than weak, broad expression of various genes by exposure of LPS alone.

Enhanced pulmonary inflammatory response to inhaled endotoxin in pregnant rats

Evidence suggests that pregnant animals are more sensitive than nonpregnant animals to the systemic administration of endotoxin. Studies were undertaken to assess whether an enhanced sensitivity of the pulmonary system to aerosolized endotoxin might exist during pregnancy. Pregnant Sprague-Dawley female rats (17 d of gestation) or age-matched virgin female rats were exposed to air or endotoxin (lipopolysaccharide) by inhalation for 3 h. At 18 h following exposure to endotoxin, lactate dehydrogenase activity levels in bronchoalveolar lavage (BAL) fluid samples from pregnant rats were 1.5-fold greater than those from endotoxin-exposed virgin rats. BAL polymorphonuclear leukocyte (PMN) numbers were also approximately twofold greater in pregnant rats than in virgins following the inhalation of endotoxin. The increases in BAL PMNs in pregnant rats following endotoxin exposure were observed just following exposure to endotoxin as well as at 18 h following exposure. These results indicate that an increased pulmonary inflammatory response to inhaled endotoxin occurs during pregnancy in rats. Additional findings suggest that these pregnancy-linked pulmonary responses to endotoxin cannot be explained by the following potential mechanisms: changes in the inhaled dose of endotoxin, or alterations in the responsiveness of alveolar macrophages to endotoxin. To our knowledge this is the first study that has evaluated pulmonary responses to inhaled endotoxin during pregnancy. Our finding that pregnancy is associated with an increased lung inflammatory response to aerosolized endotoxin raises the possibility that there may be a generalized enhancement of pulmonary responses to inhaled toxic agents during pregnancy.

Endotoxin potentiates the hepatotoxicity of cocaine in male mice

Cocaine produces hepatotoxicity by a mechanism that remains undefined but that has been linked to its oxidative metabolism. Endotoxin (lipopolysaccharide, LPS) is also a well-known cause of hepatic damage, where exposure to non-injurious doses of LPS increases the toxicity of certain hepatotoxins. This study was conducted to investigate the possible potentiation of cocaine-mediated hepatotoxicity (CMH) by LPS. Male CF-1 mice were administered oral cocaine hydrochloride for 5 consecutive days at a dose of 20 mg/kg with and without 12 x 10(6) EU LPS/kg given intraperitoneally 4 h after the last cocaine injection. Serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were measured as markers of liver injury. Blood and liver glutathione (GSH) levels were determined, as well as the activities of glutathione peroxidase (GPx) and catalase (CAT). In addition, the activity of liver glutathione reductase (GRx) was measured. The results demonstrate that endotoxin potentiated the hepatotoxicity of cocaine. Serum ALT and AST were significantly elevated with the combined cocaine and LPS treatment versus all other treatments. While cocaine alone resulted in centrilobular necrosis, the cocaine and LPS combination produced submassive necrosis. The increased hepatic GSH content and GRx activity observed with cocaine alone were not observed with the combination treatment, rendering the liver more susceptible to oxidative stress. Moreover, there was a significant decrease in the activities of hepatic GPx and CAT, particularly with the combination treatment. In conclusion, this study demonstrates that LPS potentiates the hepatotoxicity of cocaine as revealed by an array of biochemical and morphological markers.

Induction, regulation, degradation, and biological significance of mammalian metallothioneins

MTs are small cysteine-rich metal-binding proteins found in many species and, although there are differences between them, it is of note that they have a great deal of sequence and structural homology. Mammalian MTs are 61 or 62 amino acid polypeptides containing 20 conserved cysteine residues that underpin the binding of metals. The existence of MT across species is indicative of its biological demand, while the conservation of cysteines indicates that these are undoubtedly central to the function of this protein. Four MT isoforms have been found so far, MT-1, MT-2, MT-3, and MT-4, but these also have subtypes with 17 MT genes identified in man, of which 10 are known to be functional. Different cells express different MT isoforms with varying levels of expression perhaps as a result of the different function of each isoform. Even different metals induce and bind to MTs to different extents. Over 40 years of research into MT have yielded much information on this protein, but have failed to assign to it a definitive biological role. The fact that multiple MT isoforms exist, and the great variety of substances and agents that act as inducers, further complicates the search for the biological role of MTs. This article reviews the current knowledge on the biochemistry, induction, regulation, and degradation of this protein in mammals, with a particular emphasis on human MTs. It also considers the possible biological roles of this protein, which include participation in cell proliferation and apoptosis, homeostasis of essential metals, cellular free radical scavenging, and metal detoxification.