Roles of Kupffer Cells in Alcoholic Liver Disease:

Peripheral vascular disease: preclinical models and emerging therapeutic targeting of the vascular endothelial growth factor ligand-receptor system

Introduction: Vascular endothelial growth factor (VEGF)-A is a sought therapeutic target for PAD treatment because of its potent role in angiogenesis. However, no therapeutic benefit was achieved in VEGF-A clinical trials, suggesting that our understanding of VEGF-A biology and ischemic angiogenic processes needs development. Alternate splicing in VEGF-A produces pro- and anti-angiogenic VEGF-A isoforms; the only difference being a 6-amino acid switch in the C-terminus of the final 8th exon of the gene. This finding has changed our understanding of VEGF-A biology and may explain the lack of benefit in VEGF-A clinical trials. It presents new therapeutic opportunities for peripheral arterial disease (PAD) treatment.Areas covered: Literature search was conducted to include: 1) predicted mechanism by which the anti-angiogenic VEGF-A isoform would inhibit angiogenesis, 2) unexpected mechanism of action, and 3) how this mechanism revealed novel signaling pathways that may enhance future therapeutics in PAD.Expert opinion: Inhibiting a specific anti-angiogenic VEGF-A isoform in ischemic muscle promotes perfusion recovery in preclinical PAD. Additional efforts focused on the production of these isoforms, and the pathways altered by modulating different VEGF receptor-ligand interactions, and how this new data may allow bedside progress offers new approaches to PAD are discussed.I.

Lipid peroxidation derived reactive aldehydes in alcoholic liver disease

Lipid peroxidation is a known consequence of oxidative stress and is thought to play a key role in numerous disease pathologies, including alcoholic liver disease (ALD). The overaccumulation of lipid peroxidation products during chronic alcohol consumption results in pathogenic lesions on protein, DNA, and lipids throughout the cell. Molecular adducts due to secondary end products of lipid peroxidation impact a host of biochemical processes, including inflammation, antioxidant defense, and metabolism. The aggregate burden of lipid peroxidation which occurs due to chronic alcohol metabolism, including downstream signaling events, contributes to the development and progression of ALD. In this current opinion we highlight recent studies and approaches relating cellular mechanisms of lipid peroxidation to the pathogenesis of alcoholic liver disease.

Nrf2 and antioxidant defense against CYP2E1 toxicity

The transcription factor Nrf2 regulates the expression of important cytoprotective enzymes. Induction of cytochrome P450 2E1(CYP2E1) is one of the central pathways by which ethanol generates oxidative stress. CYP2E1 can be induced by ethanol and several low molecular weight chemicals such as pyrazole. The chapter discusses biochemical and toxicological effects of CYP2E1 and the effects of Nrf2 in modulating these actions of CYP2E1.Besides ethanol, CYP2E1 metabolizes and activates many other important toxicological compounds. One approach to try to understand basic effects and actions of CYP2E1 was to establish HepG2 cell lines that constitutively express human CYP2E1. Ethanol, polyunsaturated fatty acids and iron were toxic to the HepG2 cells which express CYP2E1 (E47 cells) but not control C34HepG2 cells which do not express CYP2E1.Toxicity was associated with enhanced oxidant stress and could be prevented by antioxidants and potentiated if glutathione (GSH) was removed. The E47 cells had higher GSH levels and a Twofold increase in catalase, cytosolic and microsomal glutathione transferase, and heme oxygenase-1 (HO-1) than control HepG2 cells due to activation of their respective genes. These activations were prevented by antioxidants, suggesting that reactive oxygen species (ROS) generated by CYP2E1 were responsible for the up-regulation of these antioxidant genes. This upregulation of antioxidant genes may reflect an adaptive mechanism to remove CYP2E1-derived oxidants. Increases in Nrf2 protein and mRNA were observed in livers of chronic alcohol-fed mice or rats and of pyrazole-treated rats or mice, conditions known to elevate CYP2E1. E47 cells showed increased Nrf2 mRNA and protein expression compared with control HepG2 C34 cells. Upregulation of antioxidant genes in E47 cells is dependent on Nrf2 and is prevented by siRNA-Nrf2. Blocking Nrf2 by siRNA-Nrf2 decreases GSH and increases ROS and lipid peroxidation, resulting in decreased mitochondrial membrane potential and loss of cell viability of E47 cells but not C34 cells. Nrf2 is activated and levels of Nrf2 protein and mRNA are increased when CYP2E1 is elevated. These results suggest that Nrf2 plays a key role in the adaptive response against increased oxidative stress caused by CYP2E1 in the HepG2 cells.

Effect of alcohol exposure on hepatic superoxide generation and hepcidin expression

AIM: To understand the role of mitochondrial-produced superoxide (O₂^•-) in the regulation of iron-regulatory hormone, hepcidin by alcohol in the liver.

METHODS: For alcohol experiments, manganese superoxide dismutase knockout mice heterozygous for Sod2 gene expression (Sod2^+/-) and age-matched littermate control mice (LMC), expressing Sod2 gene on both alleles, were exposed to either 10% (w/v) ethanol in the drinking water or plain water (control) for 7 d. Total cellular O₂^•- levels in hepatocytes isolated from the livers of mice were measured by electron paramagnetic resonance spectroscopy. The mitochondrial-targeted, O₂^•--sensitive fluorogenic probe, MitoSOX Red and flow cytometry were utilized to measure O₂^•- in mitochondria. Gene and protein expression were determined by Taqman Real-time quantitative PCR and Western blotting, respectively.

RESULTS: Sod2^+/- mice expressed 40% less MnSOD protein (SOD2) in hepatocytes compared to LMC mice. The deletion of Sod2 allele did not alter the basal expression level of hepcidin in the liver. 10% ethanol exposure for 1 wk inhibited hepatic hepcidin mRNA expression three-fold both in Sod2^+/- and LMC mice. O₂^•- levels in hepatocytes of untreated Sod2^+/- mice were three-fold higher than in untreated LMC mice, as observed by electron paramagnetic resonance spectroscopy. O₂^•- levels in mitochondria of Sod2^+/ mice were four-fold higher than in mitochondria of untreated LMC mice, as measured by MitoSOX Red fluorescence and flow cytometry. Alcohol induced a two-fold higher increase in O₂^•- levels in hepatocytes of LMC mice than in Sod2^+/- mice compared to respective untreated counterparts. In contrast, 1 wk alcohol exposure did not alter mitochondrial O₂^•- levels in both Sod2^+/- and control mice.

CONCLUSION: Mitochondrial O₂^•- is not involved in the inhibition of liver hepcidin transcription and thereby regulation of iron metabolism by alcohol. These findings also suggest that short-term alcohol consumption significantly elevates O₂^•- levels in hepatocytes, which appears not to originate from mitochondria.

Nrf2 and antioxidant defense against CYP2E1 toxicity

The transcription factor Nrf2 regulates the expression of important cytoprotective enzymes. Induction of CYP2E1 is one of the central pathways by which ethanol generates oxidative stress. CYP2E1 can be induced by ethanol and several low molecular mass chemicals such as pyrazole. This review discusses biochemical and toxicological effects of CYP2E1 and the effects of Nrf2 in modulating these actions of CYP2E1. Besides ethanol, CYP2E1 metabolizes and activates many other toxicologic important compounds. One approach to try to understand the basic effects and actions of CYP2E1 was to establish HepG2 cell lines that constitutively express human CYP2E1. Ethanol, polyunsaturated fatty acids and iron were toxic to the HepG2 cells, which express CYP2E1 (E47 cells) but not control C34HepG2 cells, which do not express CYP2E1. Toxicity was associated with enhanced oxidant stress and could be prevented by antioxidants and potentiated if glutathione was removed. The E47 cells had higher glutathione levels and a twofold increase in catalase, cytosolic and microsomal glutathione transferase, and heme oxygenase-1 than control HepG2 cells due to activation of their respective genes. These activations were prevented by antioxidants, suggesting that reactive oxygen species generated by CYP2E1 were responsible for the upregulation of these antioxidant genes. This upregulation may reflect an adaptive mechanism to remove CYP2E1-derived oxidants. Increases in Nrf2 protein and mRNA were observed in livers of chronic alcohol-fed mice or rats and of pyrzole-treated rats or mice, conditions known to elevate CYP2E1. E47 cells showed increased Nrf2 mRNA and protein expression compared with control HepG2 C34 cells. Upregulation of antioxidant genes in E47 cells is dependent on Nrf2 and is prevented by siRNA-Nrf2. Blocking Nrf2 by siRNA-Nrf2 decreases glutathione and increases reactive oxygen species and lipid peroxidation, resulting in decreased mitochondrial membrane potential and loss of cell viability of E47 cells, but not C34 cells. Nrf2 is activated and levels of Nrf2 protein and mRNA are increased when CYP2E1 is elevated. These results suggest that Nrf2 plays a key role in the adaptive response against increased oxidative stress caused by CYP2E1 in the HepG2 cells. However, it is not clear whether Nrf2 is protective against CYP2E1 toxicity in vivo as pyrazole which elevates CYP2E1 in wild-type mice did not elevate CYP2E1 in Nrf2 knockout mice, although pyrazole produced toxicity in the Nrf2 knockout mice.

CYP2E1 potentiation of LPS and TNFα-induced hepatotoxicity by mechanisms involving enhanced oxidative and nitrosative stress, activation of MAP kinases, and mitochondrial dysfunction

The mechanisms by which alcohol causes cell injury are not clear. A major mechanism that is the focus of considerable research is the role of lipid peroxidation and oxidative stress in alcohol toxicity. Many pathways have been suggested to play a role in how alcohol induces oxidative stress. Considerable attention has been given to alcohol-elevated production of lipopolysaccharide (LPS) and TNFα and to alcohol induction of CYP2E1. These two pathways are not exclusive of each other, however, associations and interactions between them, especially in vivo, have not been extensively evaluated. We have shown that increased oxidative stress from induction of CYP2E1 in vivo sensitizes hepatocytes to LPS and TNF toxicity and that oxidants, such as peroxynitrite, activation of p38 and JNK MAP kinases, inactivation of NF-kB protective pathways and mitochondrial dysfunction are downstream mediators of this CYP2E1-LPS/TNF potentiated hepatotoxicity. This review will summarize studies showing potentiated interactions between these two risk factors in promoting liver injury and the mechanisms involved.

Role of oxidative stress in alcohol-induced liver injury

Reactive oxygen species (ROS) are highly reactive molecules that are naturally generated in small amounts during the body's metabolic reactions and can react with and damage complex cellular molecules such as lipids, proteins, or DNA. Acute and chronic ethanol treatments increase the production of ROS, lower cellular antioxidant levels, and enhance oxidative stress in many tissues, especially the liver. Ethanol-induced oxidative stress plays a major role in the mechanisms by which ethanol produces liver injury. Many pathways play a key role in how ethanol induces oxidative stress. This review summarizes some of the leading pathways and discusses the evidence for their contribution to alcohol-induced liver injury. Special emphasis is placed on CYP2E1, which is induced by alcohol and is reactive in metabolizing and activating many hepatotoxins, including ethanol, to reactive products, and in generating ROS.

Is the iron regulatory hormone hepcidin a risk factor for alcoholic liver disease?

Despite heavy consumption over a long period of time, only a small number of alcoholics develop alcoholic liver disease. This alludes to the possibility that other factors, besides alcohol, may be involved in the progression of the disease. Over the years, many such factors have indeed been identified, including iron. Despite being crucial for various important biological processes, iron can also be harmful due to its ability to catalyze Fenton chemistry. Alcohol and iron have been shown to interact synergistically to cause liver injury. Iron-mediated cell signaling has been reported to be involved in the pathogenesis of experimental alcoholic liver disease. Hepcidin is an iron-regulatory hormone synthesized by the liver, which plays a pivotal role in iron homeostasis. Both acute and chronic alcohol exposure suppress hepcidin expression in the liver. The sera of patients with alcoholic liver disease, particularly those exhibiting higher serum iron indices, have also been reported to display reduced prohepcidin levels. Alcohol-mediated oxidative stress is involved in the inhibition of hepcidin promoter activity and transcription in the liver. This in turn leads to an increase in intestinal iron transport and liver iron storage. Hepcidin is expressed primarily in hepatocytes. It is noteworthy that both hepatocytes and Kupffer cells are involved in the progression of alcoholic liver disease. However, the activation of Kupffer cells and TNF-α signaling has been reported not to be involved in the down-regulation of hepcidin expression by alcohol in the liver. Alcohol acts within the parenchymal cells of the liver to suppress the synthesis of hepcidin. Due to its crucial role in the regulation of body iron stores, hepcidin may act as a secondary risk factor in the progression of alcoholic liver disease. The clarification of the mechanisms by which alcohol disrupts iron homeostasis will allow for further understanding of the pathogenesis of alcoholic liver disease.

Regulation of liver hepcidin expression by alcohol in vivo does not involve Kupffer cell activation or TNF-alpha signaling

Alcohol downregulates hepcidin expression in the liver leading to an increase in intestinal iron transport and liver iron storage. We have previously demonstrated that alcohol-mediated oxidative stress is involved in the inhibition of hepcidin transcription by alcohol in vivo. Kupffer cells and TNF-alpha play a key role in alcohol-induced liver injury. The aim of this study was to define their involvement in the regulation of hepcidin expression by alcohol. Kupffer cells were inactivated or depleted by employing gadolinium chloride and liposomes containing clodronate, respectively. Rats pair fed with the alcohol-Lieber-DeCarli diet for 6 wk and mice fed with 20% ethanol in the drinking water for 1 wk were used as experimental models. Interestingly, alcohol downregulated hepcidin expression in the livers of rats and mice independent of gadolinium chloride or clodronate treatment. One week of alcohol treatment was sufficient to induce a significant increase in TNF-alpha levels and phosphorylation of NF-kappaB subunit p65. The neutralization of TNF-alpha by specific antibodies inhibited p65 phosphorylation. However, neither the neutralization of TNF-alpha nor the lack of TNF-alpha receptor expression reversed alcohol-induced suppression of liver hepcidin expression. The level of alcohol-induced ROS in the liver was also undiminished following Kupffer cell inactivation or depletion. Our results demonstrate that alcohol-induced Kupffer cell activation and TNF-alpha signaling are not involved in the suppression of liver hepcidin expression by alcohol-mediated oxidative stress in vivo. Therefore, these findings suggest that alcohol acts within hepatocytes to suppress hepcidin expression and thereby influences iron homeostasis.

Alcohol and oxidative liver injury

Acute and chronic ethanol treatment has been shown to increase the production of reactive oxygen species, lower cellular antioxidant levels, and enhance oxidative stress in many tissues, especially the liver. Ethanol-induced oxidative stress plays a major role in the mechanisms by which ethanol produces liver injury. Many pathways play a key role in how ethanol induces oxidative stress. This review summarizes some of the leading pathways and discusses the evidence for their contribution to alcohol-induced liver injury. Many of the seminal reports in this topic have been published in Hepatology , and it is fitting to review this research area for the 25th Anniversary Issue of the Journal.