The contribution of endoplasmic reticulum stress to liver diseases

Hepatocellular carcinoma in non-alcoholic steatohepatitis: Growing evidence of an epidemic?

The incidence of hepatocellular carcinoma in non-viral-related chronic liver disease has gradually increased in Japan. Obesity and diabetes mellitus type 2 have been established as a significant risk factor for hepatocellular carcinoma (HCC) by epidemiologic observations and experimental studies. The risks of these factors for HCC are likely conferred by two factors: the increased risk for development of non-alcoholic steatohepatitis (NASH) and the carcinogenic potential of themselves. Hepatocellular carcinoma in NASH is difficult to evaluate because histological diagnosis is required for diagnosis of NASH, which can lead selection bias. Furthermore, end-stage NASH is in effect "burned-out" NASH, for which the diagnosis of NASH cannot be made any more. At all events, previous studies on the etiology of Japanese HCC showed that non-alcoholic fatty liver disease accounts for 1-5% of all HCC (male predominant, median age 72 years). They have high prevalences of obesity and/or diabetes mellitus type 2 and 10-75% of the HCC arose from non-cirrhotic livers. HCC in NASH may be of multicentric origin, similar to HCC based on viral hepatitis. Regular screening for HCC is extremely important especially in cirrhotic NASH patients and recurrence should be warned. In western and Asian countries, the prevalence of non-alcoholic fatty liver disease in the general population is increasing dramatically. Therefore, there is an urgent need to elucidate pathogenesis and clinical features of HCC in NASH. In this review we summarize current concepts for HCC in NASH.

Ah receptor- and Nrf2-gene battery members: modulators of quinone-mediated oxidative and endoplasmic reticulum stress

Quinones are ubiquitously present in mammals and their environment. They are involved in physiologic functions such as electron transport but are also toxic compounds. In particular, quinone-quinol redox cycles may lead to oxidative stress, and arylating quinones have been demonstrated to activate endoplasmic reticulum (ER) stress. To detoxify quinones coordinately regulated Ah receptor and Nrf2 gene batteries evolved. Two pathways are emphasized: (i) glutathione S-transferases, and (ii) NAD(P)H:quinone oxidoreductases NQO1 and NQO2 acting together with UDP-glucuronosyltransferases and sulfotransferases. Coupling between these enzymes may prevent oxidative and ER stress in a tissue-dependent manner, as discussed using benzo[a]pyrene detoxification in enterocytes, catecholestrogen metabolism in breast tissue and endometrium, and aminochromes in neurones and astrocytes. Possible consequences of chronic ER stress such as apoptosis and inflammation as well as therapeutic possibilities of modulating Ah receptor and Nrf2 are discussed. In conclusion, tight coupling of Ah receptor- and Nrf2-regulated enzymes may prevent quinone-mediated oxidative and ER stress.

Time-dependent changes in lipid metabolism in mice with methionine choline deficiency-induced fatty liver disease

Methionine and choline-deficient diet (MCD)-induced fatty liver is one of the best-studied animal models of fatty liver disease. The present study was performed to clarify the relative contributions of individual lipid metabolic pathways to the pathogenesis of MCD-induced fatty liver. Hepatic lipogenesis mediated by the sterol regulatory element-binding protein (SREBP-1c) was increased at 1 week, but not at 6 weeks, of MCD feeding. On the other hand, (14)C-palmitate oxidation did not change at 1 week, but significantly decreased at 6 weeks. This decrease was associated with increased expression of fatty acid translocase, a key enzyme involved in fatty acid uptake. Expression of endoplasmic reticulum stress markers was increased in mice given MCD for both 1 and 6 weeks. These findings suggest the presence of time-dependent differences in lipid metabolism in MCD-induced fatty liver disease: SREBP-1c-mediated lipogenesis is important in the early stages of fatty liver disease, whereas increased fatty acid uptake and decreased fatty acid oxidation become more important in the later stages.

Mechanisms of alcohol-induced endoplasmic reticulum stress and organ injuries

Alcohol is readily distributed throughout the body in the blood stream and crosses biological membranes, which affect virtually all biological processes inside the cell. Excessive alcohol consumption induces numerous pathological stress responses, part of which is endoplasmic reticulum (ER) stress response. ER stress, a condition under which unfolded/misfolded protein accumulates in the ER, contributes to alcoholic disorders of major organs such as liver, pancreas, heart, and brain. Potential mechanisms that trigger the alcoholic ER stress response are directly or indirectly related to alcohol metabolism, which includes toxic acetaldehyde and homocysteine, oxidative stress, perturbations of calcium or iron homeostasis, alterations of S-adenosylmethionine to S-adenosylhomocysteine ratio, and abnormal epigenetic modifications. Interruption of the ER stress triggers is anticipated to have therapeutic benefits for alcoholic disorders.

Vitamin-dependent methionine metabolism and alcoholic liver disease

Emerging evidence indicates that ethanol-induced alterations in hepatic methionine metabolism play a central role in the pathogenesis of alcoholic liver disease (ALD). Because malnutrition is a universal clinical finding in this disease and hepatic methionine metabolism is dependent upon dietary folate and vitamins B-6 and B-12, ALD can be considered an induced nutritional disorder that is conditioned by alcohol abuse. The present review describes the etiologies of these 3 vitamin deficiencies in ALD and how they interact with chronic ethanol exposure to alter hepatic methionine metabolism. Subsequent sections focus on molecular mechanisms for the interactions of aberrant methionine metabolism with ethanol in the pathogenesis of ALD, in particular the role of S-adenosylmethionine (SAM) in regulating the epigenetic expressions of genes relevant to pathways of liver injury. The review will conclude with descriptions of studies on the efficacy of SAM in the treatment of ALD and with discussion of potentially fruitful future avenues of research.