Editorial

 

Iron, hepatitis C virus and hepatic steatosis

 

Nonalcoholic steatohepatitis (NASH) is a clinico-pathologic condition characterized by histological features of alcoholic liver disease that occurs in patients who do not consume significant amounts of alcohol (1). At present, NASH is considered part of a broad spectrum of nonalcoholic fatty liver disease (NAFLD) that also includes pure fatty liver (hepatic steatosis), hepatic steatosis with lobular inflammation, ballooning degeneration, sinusoidal fibrosis or Mallory body-like material (NASH) and cirrhosis of the liver (2-4). NAFLD is an emerging worldwide common problem that represents the most frequent histological finding in patients with unexplained abnormalities of the liver tests. In some Western countries, the prevalence of NAFLD in the general population is approximately 20% and the prevalence of NASH ranges between 1.2 and 4.8% (5). NASH has been found to be associated with a large number of metabolic, surgical and toxic conditions. However, the main risk factors associated with NASH include obesity, type 2 diabetes mellitus, dyslipidemia and other conditions characterized by insulin resistance and hyperinsulinemia (6). In the present issue of the Spanish Journal of Gastroenterology, Fernández-Salazar et al. (7) published a study including 53 patients with chronic hepatitis C (CHC) in which they find hepatic steatosis in 50% of them, and that factors independently associated with the presence of steatosis were iron overload and hepatitic C virus (HCV) genotype 3.

The role of iron deposition in the pathogenesis of NAFLD has raised general interest (8). Moriand et al. (9) first associated primary hepatic iron overload with the clinical features of insulin resistance. In this respect, Facchini et al. showed improvement in insulin sensitivity with the use of venesection in patients with NAFLD (10). Moreover, serum ferritin levels are increased in 43 to 62% of patients with NAFLD, and some authors have found increased prevalence of the C282Y mutation of the HFE gene (11-13). Furthermore, as iron promotes oxidative stress, it was considered a pathogenic factor in NASH. This role was supported by it association with hepatic fibrosis (13). However, the relationship between serum ferritin levels, iron stores, HFE gene mutations and NASH are a controversial area. As mentioned above, increased serum ferritin concentrations are found in a large proportion of patients with NAFLD (14,15). In our experience, hyperferritinemia was found in 37% of NASH patients, whereas increases in serum transferrin saturation were seen less frequently, in 3% of patients (16). Although George et al. (13) and Bonkovsky et al. (11) reported increased iron stores in a significant proportion of NASH patients, other investigators have failed to observe significant hepatic iron accumulation in patients with NAFLD (12,17,18). In our series of patients with NASH, we found hepatic siderosis of only grade 1 or 2 in 16.5% of cases. Finally, Fernández-Salazar et al. (7) report grade 2 or 3 stainable iron in 19.2% of patients with liver steatosis; although all of them suffered HCV infection and 34.5% consumed alcohol. Ladero et al. (19) found that hepatic iron is high in 11% patients with CHC. Few studies have quantified hepatic iron content using biochemical methods, and most of them failed to demonstrate a significant iron accumulation in patients with NAFLD when alcoholism was excluded (20,21). Most authors consider 20 g as the upper limit of acceptable daily ethanol consumption. Moreover, these studies did not find any relationship between hepatic iron concentration and fibrosis. This lack of relationship can be ascribed to the fact that iron burden is largely below the fibrogenic threshold (22). Concerning HFE gene mutations, Chitturi et al. (12) and George et al. (13), both from Australia, and Bonkovsky et al. (11), in the United States, concluded that prevalence of the C282Y mutation is increased in the NASH population. In contrast with these results, other investigators in the United States and other countries have failed to confirm these observations (17,18,21). In our own series of patients with NASH (16), prevalence of HFE mutations was not significantly increased (C282Y +/-, 1,6%; H63D +/+, 5%; H63D +/-, 30%).

In conclusion, although serum ferritin levels are frequently increased in patients with NAFLD, this finding is not expression of an enhanced hepatic iron overload. In these patients, hyperferritinemia in the presence of normal transferrin saturation has to be ascribed to inflammation, liver cell necrosis, alcohol abuse, insulin resistance, dietary factors or mutations in the ferritin gene (23,24). Thus, hyperferritinemia has been described in patients without iron overload in hereditary hyperferritinemia (25-27), a condition associated with cataracts caused by point mutations in the iron responsible element of the ferritin gene. Moreover, increased serum ferritin levels in the presence of normal transferrin saturation may also occur in the so called "insulin resistance hepatic iron overload" (28). Patients with this syndrome share one or more features of the metabolic syndrome, have histological features of NAFLD, and mild iron overload (9,29). In fact, hyperferritinemia have also been reported in patients with diabetes mellitus (30) and in subjects with a complex syndrome attributed to insulin resistance and characterized by the concentration of different metabolic abnormalities (31). Thus, the increased serum ferritin levels found in NAFLD may be simply an expression of the metabolic derangement caused by the insulin resistance (21,32). In a large number of NASH patients, hyperferritinemia may be due to dietary factors. Fargion et al. (18) demonstrated that serum ferritin levels returned to normal values in the large majority of NAFLD patients when they were put on a low-fat and hypocaloric diet. In addition to the insulin resistance, in NAFLD patients, hyperferritinemia may be caused by the hepatic damage, because activation of inflammatory cytokines would increase transcription of ferritin gene in macrophages (21). The likelihood of this mechanism is increased when NAFLD is associated with CHC, as is the case of the patients described by Fernández-Salazar et al. (7). An association between iron and viral hepatitis was first observed by Blumberg et al. (33), but many other authors have noted elevations in the serum ferritin levels in patients with CHC (34-36). Most of these patients have not elevated hepatic iron concentrations or when elevated, they are mild and not sufficient to be hepatotoxic (35).

The study of Fernández-Salazar et al. (7) also shows that presence of hepatic steatosis in CHC is associated with HCV genotype 3 infection. Steatosis of the liver and Mallory body-like material within hepatocytes are frequent histopathological features in CHC and have been proposed as histological markers of HCV infection (37-39). Many factors, including alcohol abuse, obesity, diabetes, and drugs may account for the fat accumulation in these patients. However, fatty liver can be found in patients with CHC in whom these risk factors had been excluded (40). Therefore, it has been suggested that hepatic steatosis may be due to a direct cytopathic effect of HCV (40). Nevertheless, because fatty liver disease and HCV infection are frequent in Western countries, concurrence of fatty infiltration and CHC is likely. In a prospective study that included 98 consecutive patients with CHC, we found that risk factors for NASH [elevated body mass index (BMI), serum triglyceride and glucose, frequency of diabetes mellitus, and metabolic syndrome] were more frequent in the presence than in absence of NASH lesions. In this study, in which patients who consumed alcohol were excluded, BMI and HCV genotype 3, but not serum iron or serum ferritin levels, were the only factors associated with hepatic steatosis. These results concur with those recently reported by Patton et al. (41) and suggest that, in addition to overweight, HCV infection, particularly with genotype 3, may play an important role in the pathogenesis of steatosis in these patients. As a matter of fact, degree of hepatic steatosis has been correlated with viral replication in patients infected with HCV genotype 3 (42,43) and with the amount of HCV core protein expression in the liver (44). In this respect, HCV core protein can induce steatosis in transfected cells and transgenic mice (45-47). Furthermore, while a number of authors have observed an improvement of hepatic steatosis after the eradication of HCV infection by successful antiviral therapy (41-43,48), occurrence of steatosis after orthotopic liver transplantation has been associated with HCV reinfection (49).

Mechanisms by which HCV might induce hepatic steatosis are uncertain. It has been hypothesized that microsomal triglyceride transfer protein (MTP) may play an important role in the pathogenesis of NASH. This protein transfers triglycerides to apolipoprotein B, producing very low-density lipoprotein and removing lipids from the liver cells. Reduced MTP activity results in an impaired secretion of lipids from the liver and hepatic steatosis. Congenital abetalipoproteinemia, a disease caused by mutations in the MTP gene, is characterized by marked hepatic steatosis (50). In this respect, Charlton et al. (51) reported that synthesis of apolipoprotein B is decreased in patients with NASH and Namikawa et al. (52) and Bernard et al. (53), respectively, demonstrated MTP mutations in patients with NASH or type 2 diabetes. Interestingly, MTP gene expression is down regulated by insulin in liver cells (54). Likewise, in patients infected with HCV, particularly with genotype 3, hepatic steatosis has been found to be associated with hypobetalipoproteinemia (55,56). Furthermore, animal models of viral-related steatosis have shown that HCV core protein decreases MTP activity, impairs very low-density lipoprotein secretion and induces hepatic steatosis (47).

Although the HCV genotype 3 infection should be considered mainly as an etiological factor for steatosis, our study demonstrated that only BMI, but not HCV RNA viral load or genotype, was independently associated with NASH-related lesions (ballooning degeneration, Mallory body-like material, pericellular fibrosis) in patients with HCV infection. Although pathogenesis of NASH is not well understood, available evidence suggests that NASH development requires a "double hit". While the "first hit" involves hepatic steatosis, the "second hit" includes oxidative stress resulting in lipid peroxidation, the production of malondialdehyde, 4-hydroxynonenal, proinflammatory cytokines, stellate cells activation and fibrogenesis (57,58). Mitochondrial dysfunction might play a central role in the induction of this stress (59). We have recently shown that the activity of the mitochondrial respiratory chain is decreased in patients with NASH (60), and that this dysfunction correlated with the BMI. This correlation may be ascribed to the fact that adipose tissue is a major source of tumor necrosis factor alpha (61). This cytokine induces mitochondrial abnormalities, reduces the activity of the mitochondrial respiratory chain (62) and has been implicated in the pathogenesis of NASH (63). Thus, our study suggests that, in patients with CHC, overweight might be the "second hit" necessary for the progression of hepatic steatosis to NASH.

J. A. Solís Herruzo and P. Solís-Muñoz

Service of Digestive Diseases. University Hospital 12 de Octubre. Madrid, Spain

 

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