Mitochondrial dysfunction in hepatitis C

HCV proteins increase expression of heme oxygenase-1 (HO-1) and decrease expression of Bach1 in human hepatoma cells

BACKGROUND/AIMS

Hepatitis C infection induces hepatic oxidative stress. Heme oxygenase (HO), the rate-controlling enzyme of heme catabolism, plays a key role as a protector against oxidative, and other stresses. Other recent work has implicated Bach1, a heme binding protein that represses gene expression, in the regulation of HO-1 gene expression.

METHODS

We investigated the effects of HCV polyprotein expression on expression of HO-1 and Bach1 genes in human hepatoma cells (Huh-7 cells).

RESULTS

HO-1 was up-regulated in the cell line expressing HCV proteins from core up to the aminoterminal domain of NS3. Addition of increasing concentrations of N-acetylcysteine (NAC) led to down-regulation of HO-1 in cells expressing HCV proteins. In contrast, Bach1 was significantly down-regulated in these cells. Sodium arsenite, a strong inducer of oxidative stress and HO-1, reduced Bach1 expression in wild type Huh-7 cells, and NAC partially abrogated this decrease.

CONCLUSIONS

Huh-7 cells expressing HCV proteins show significant up-regulation of the HO-1 gene, and reciprocal down-regulation of the Bach1 gene. Exogenous oxidative stressors and anti-oxidants can modulate expression of these genes. These and other results suggest a key role of down-regulation of Bach1 and up-regulation of HO-1 in diminishing cytotoxic effects of HCV proteins in human hepatocytes.

Mechanisms of liver injury. III. Role of glutathione redox status in liver injury

GSH is the most abundant redox molecule in cells and thus the most important determinant of cellular redox status. Thiols in proteins can undergo a wide range of reversible redox modifications (e.g., S-glutathionylation, S-nitrosylation, and disulfide formation) during times of increased exposure to reactive oxygen and nitrogen species, which can affect protein activity. These reversible thiol modifications regulated by GSH may be nanoswitches to turn on and off proteins, similar to phosphorylation, in cells. In the cytoplasm, an altered redox state can activate (e.g., MAPKs and NF-E2-related factor-2) and inhibit (e.g., phosphatases and caspases) proteins, whereas in the nucleus, redox alterations can inhibit DNA binding of transcription factors (e.g., NF-kappaB and activator protein-1). The consequences include the promotion of expression of antioxidant genes and alterations of hepatocyte survival as well as the balance between necrotic versus apoptotic cell death. Therefore, the understanding of the redox regulation of proteins may have important clinical ramifications in understanding the pathogenesis of liver diseases.

Mitochondrial dysfunction in hepatitis C virus infection

The mechanisms of liver injury in chronic hepatitis C virus (HCV) infection are poorly understood though HCV induces a state of hepatic oxidative stress that is more pronounced than that present in many other inflammatory diseases. This mini-review will focus on recent findings revealing an unexpected role of mitochondria in providing a central role in the innate immunity and in addition will illustrate the application of stably transfected human-derived cell lines, inducibly expressing the entire HCV open reading frame for in vitro studies on mitochondria. Results obtained by a comparative analysis of the respiratory chain complexes activities along with mitochondrial morpho-functional confocal microscopy imaging show a detrimental effect of HCV proteins on the cell oxidative metabolism with specific inhibition of complex I activity, decrease of mtDeltaPsi, increased production of reactive oxygen species. A possible de-regulation of calcium recycling between the endoplasmic reticulum and the mitochondrial network is discussed to provide new insights in the pathogenesis of hepatitis C.

Hepatitis C virus and oxidative stress: a dangerous liaison

Chronic hepatitis C infection persists in more than 170 million people worldwide and is one of the major causes of hepatic failure and liver transplantation. Recent studies have demonstrated that hepatitis C virus (HCV)-derived proteins have the capacity to generate substantial oxidative stress within the hepatocyte. Subsequently, oxidative stress has been identified as a significant mechanistic pathway culminating in the development of hepatic cirrhosis, liver failure and liver cancer. In vitro HCV-induced oxidative stress has been demonstrated to activate cellular signaling pathways involved in both inflammatory and fibrogenic responses. In addition, oxidative stress has also been shown to play a role in HCV replication. Preliminary data from small clinical trials have implicated oxidative stress as a risk factor for liver fibrosis and increased HCV viral load. Although conclusive evidence from large-scale clinical trials is lacking, it is possible that antioxidant co-therapies may improve hepatic inflammation, retard fibrosis progression and improve the ability to achieve a sustained viral response with standard antiviral therapy.

Clinical implications of oxidative stress and antioxidant therapy

Oxidative stress occurs when there is an imbalance between generation of reactive oxygen species and inadequate antioxidant defense systems. Oxidative stress can cause cell damage either directly or through altering signaling pathways. Oxidative stress is a unifying mechanism of injury in many types of disease processes, including gastrointestinal diseases. For example, in alcoholic liver disease, reactive oxygen species have been detected through direct spin-trapping techniques and through indirect markers, such as products of lipid peroxidation. A host of antioxidants have protected against liver injury in animal models of alcoholic liver disease. Similarly, in inflammatory bowel disease, oxidative stress has been postulated to play a role in disease initiation and progression, and antioxidant therapy, such as green tea polyphenols and gene therapy with superoxide dismutase, has a markedly attenuated disease. Downregulation of specific detoxification genes may play a role in the pathogenesis of inflammatory bowel disease, especially in ulcerative colitis. Oxidative stress is postulated to play a sustaining role in acute and chronic pancreatitis. Antioxidant supplementation has been used with some success in the treatment of chronic pancreatitis. This review covers recent findings related to oxidative stress in liver disease, inflammatory bowel disease, and pancreatitis.

The Role of Hepatitis C Virus (HCV) in Mitochondrial Dna Damage in HIV/HCV-Coinfected Individuals

Oxidative stress accompanying hepatitis C virus (HCV) infection seems to result in mitochondrial (mt) dysfunction. In HIV/HCV-coinfected individuals, HCV-related mt damage could be further enhanced and clinical manifestations of mt damage may appear, particularly following exposure to some antiretroviral drugs. Furthermore, when HCV medications are used together with certain antiretrovirals, the risk of developing mt adverse events may be particularly frequent, such as development of pancreatitis when ribavirin and didanosine are coadministered. The management of HIV/HCV-coinfected individuals needs to consider the high risk of mitochondria-associated toxicities in this population, which may significantly influence treatment decisions and therapeutic modalities.