Correspondence Arun J. Sanyal, Department of Internal Medicine, Division of Gastroenterology, Hepatology and Nutrition, Virginia Commonwealth University Medical Center, MCV Box 980341 Richmond, VA 23298-0341, USA Tel: +1 804 828 6314 Fax: +1 804 828 2992 e-mail: firstname.lastname@example.org
Hepatitis C is a common cause of chronic viral infection of the liver. It is associated with insulin resistance and the development of type 2 diabetes mellitus. It is also associated with the development of hepatic steatosis. The presence of hepatic steatosis is associated with an increased risk of having hepatic fibrosis. This is also associated with the severity of insulin resistance. These findings are specifically germane for those with genotype1 infection. Genotype 3 infection independently causes steatosis and successful treatment of the virus is followed by resolution of steatosis. In genotype 1 infection, the presence of hepatic steatosis is also a risk factor for failure to respond to pegylated interferon and ribavirin therapy. Unfortunately efforts to treat insulin resistance prior to antiviral therapy have not been very successful. Newer efforts focused on the role of specific micro RNAs in mediating the metabolic effects of hepatitis C virus infection may provide to ameliorate the metabolic risks of HCV infection.
Hepatitis C virus (HCV) infection is a common cause of chronic liver disease and affects about 2% of the general population. In specific segments of the population, e.g. incarcerated individuals, etc., the prevalence may be as high as 20–30%. The factors affecting the progression of liver disease in HCV-infected individuals have been elucidated and include increasing age at infection, concomitant alcohol consumption and male gender (1). The histological parameters associated with disease progression include the severity of inflammation, the stage of fibrosis and the presence of hepatic steatosis (2). In this paper, we will focus on the relationship between HCV and fatty liver disease.
Fatty liver diseases are a spectrum of conditions characterized by increased macrovesicular steatosis. The histological spectrum of fatty liver diseases extends from a fatty liver to steatohepatitis. Steatohepatitis is defined by the presence of steatosis, inflammation and cytological ballooning with or without pericellular fibrosis (3). Typically, these findings are seen in a centrilobular distribution. The entire spectrum of fatty liver disease can be found in individuals who consume alcohol in amounts that are injurious to the liver. This is usually considered to be approximately 60 g or more per day in those without concomitant liver diseases. In patients with HCV infection, even 30 g of alcohol can damage the liver. When fatty liver disease develops in the absence of pathological alcohol consumption, it is called non-alcoholic fatty liver disease (NAFLD). While it is generally assumed that the presence of hepatic steatosis in a patient with HCV reflects underlying NAFLD, it is important to be aware that alcohol consumption may also play an important role.
Relationship between hepatitis C virus and hepatic steatosis
The prevalence of hepatic steatosis in HCV infection varies from 20 to 30% (4, 5). In those with genotype 3 infection, the prevalence of steatosis is much higher and is directly linked to HCV-mediated alterations in hepatic lipid metabolism. In contrast with genotype 1 infection, the presence of hepatic steatosis is a sign of underlying insulin resistance and features of the metabolic syndrome (4). Specifically, the risk factors for steatosis in this population include increased BMI, type 2 diabetes and a full-blown metabolic syndrome (6). In patients with HCV liver disease, insulin resistance is associated with increasing age, male gender, genotype 1 and 4 infection, advanced fibrosis and steatosis >30% (7). Thus, in patients with HCV infection, insulin resistance is related to the severity of steatosis and the severity of steatosis is associated with insulin resistance.
Interestingly, it has been reported that the severity of fibrosis was associated with a longer duration of infection (>15 years), a higher body mass index, advanced steatosis and menopause (8). Menopausal women receiving hormone replacement therapy presented with a lower stage of fibrosis. These results support the hypothesis that oestrogens play a protective role in the progression of fibrosis. Steatosis may be implicated in the progression of fibrosis after menopause.
Defining insulin resistance and its relationship with hepatitis C virus
Insulin resistance is defined as an impaired ability to clear glucose from the circulation at a given level of circulating insulin. The insulin resistance syndrome, on the other hand, is a much more complex entity and is associated with numerous metabolic disturbances, which include several lipid metabolic pathways. It is also associated with the activation of the innate immune system, resulting in a systemic pro-inflammatory, profibrotic state and an increased tendency for atherogenesis. This distinction between how insulin resistance is defined and the insulin resistance (metabolic) syndrome may help explain why attempts to improve glucose disposal capacity do not reliably affect other disease phenotypes associated with the insulin resistance syndrome.
Non-alcoholic fatty liver disease is associated with insulin resistance, both biochemically by impaired glucose disposal as well as in relation to the metabolic syndrome (9). In subjects with HCV, even genotype 3 infection is associated with impaired glucose disposal (10). However, the greatest impairment in glucose disposal is found in non-genotype 3 infection. This is mainly because of a decrease in glucose oxidation corresponding to the changes seen in NAFLD and non-alcoholic steatohepatitis (NASH). However, during exposure to insulin, e.g. during a euglycaemic hyperinsulinaemic clamp, subjects with NAFLD have impaired suppression of circulating free fatty acids and glycerol. These products of lipolysis mark the effects of insulin on peripheral lipolysis (9). In contrast, subjects with HCV do not show any significant changes in peripheral levels of free fatty acids and glycerol. This suggests that the insulin resistance in HCV infection is not mainly peripheral (11). Nevertheless, there is no suppression of lipid oxidation in subjects with HCV (11). Both NAFLD and HCV are associated with increased gluconeogenic drive and hepatic insulin resistance shown by the impaired suppression of hepatic glucose output by insulin.
A large body of literature supports the notion of chronic inflammatory states producing insulin resistance and vice versa. It is well known that obesity and insulin resistance are directly related to chronic inflammation associated with atherosclerosis. This notion is now used to explain numerous chronic inflammatory conditions such as periodontal disease, glomerulonephritis and rheumatoid arthritis. It is therefore not surprising that chronic hepatitis C is associated with evidence of insulin resistance. These data are further supported by results suggesting that patients with hepatitis C have an increased risk of diabetes. Older age, obesity, a family history of diabetes and the degree of liver fibrosis are correlated to the development of diabetes in patients with HCV (12–14).
Insulin resistance promotes fibrosis in chronic fibro-inflammatory states. This may be mediated by the modulation of local pro-inflammatory and fibrogenic cytokine expressions as well as directly by circulating adipokines. A direct correlation has been found between the degree of insulin resistance and the degree of fibrosis in subjects with hepatitis C (15). Also, necroinflammation has been linked to the progression of fibrosis (16). The driving force for the progression of fibrosis might be insulin resistance rather than steatosis.
Interestingly, in genotype 1 chronic hepatitis C patients, a higher visceral adiposity index was independently associated with both steatosis and necroinflammatory activity and has a direct correlation with viral load (17).
As expected, adiponectin levels are inversely related to the degree of insulin resistance and fibrosis. The precise mechanisms underlying these effects remain to be defined.
Insulin resistance also facilitates neoplastic transformation. A link between obesity and numerous adenocarcinomas, e.g. breast, pancreas, colon and oesophagus is well established. Hyperinsulinaemia and increased levels of insulin growth factors have been shown to promote cell proliferation (18, 19), and IL-6, another important adipokine, also promotes cell proliferation in chronic inflammatory conditions. The specific cytokines and mechanisms that promote neoplasia during insulin resistance have not yet been fully defined. Subjects with HCV and diabetes have a higher risk of developing hepatocellular carcinoma (HCC) (20, 21). Thus, the same mechanisms of oncogenesis operating at other sites during insulin resistance probably also influence the genesis of HCC in patients with HCV and insulin resistance.
Viral mechanisms inducing insulin resistance
In addition to inflammation, HCV proteins also play a role in the development of insulin resistance and oxidative stress, the two key pathways in the pathogenesis of NAFLD. These have been reviewed recently in detail (22). The virus is composed of three structural proteins (core, E1 and E2) and six non-structural proteins (NS3-NS5B) (22). The proteins are arranged in the following order: H2N-Core-E1-E2-p7-NS2-NS3-NS4A-NS4B-NS5A-NS5B-COOH. NS5A localizes to the endoplasmic reticulum (ER) and affects the ER–nucleus pathway by activating nuclear factor-κ B. Similarly, NS3 localizes to the ER and produces ROS through the activation of NADPH oxidase. In addition, the HCV core protein localizes to the mitochondrial membrane, inducing mitochondrial dysfunction. The HCV virus, through both direct and indirect pathways, also affects the insulin signalling pathways, thus promoting insulin resistance at a cellular level.
Mechanisms of steatosis in hepatitis C virus genotype 3
Hepatic steatosis is a common non-specific histological feature found in >50% of individuals infected with HCV. Genotype 3a has been shown to by more strongly correlated to steatosis than other genotypes (23, 24). Obesity is also a well-known risk factor in the development of steatosis and fibrosis in HCV-infected patients (25, 26).
Genotype 3 is the only subtype shown to be correlated to a higher grade of steatosis independent of other host-related factors such as the presence of NAFLD (27). The severity of steatosis in these patients is directly related to the HCV RNA viral load. Indeed, steatosis often resolves with the loss of viraemia after antiviral treatment (28–30). It has been suggested that HCV genotype 3 may also cause steatosis by interfering with triglyceride secretion.
Mechanisms of steatosis in hepatitis C virus genotype 1
Steatosis in genotype 1 infection is thought to be because of a metabolic disturbance from the activation of pro-inflammatory mechanisms as well as underlying obesity and insulin resistance. The degree of steatosis in this genotype is independent of the HCV viral load. Antiviral therapy alone does not improve steatosis in these patients. Similar data have been obtained for genotype 4 infection while little data are available for genotype 2 (31).
The HCV virus has been shown to cause insulin resistance whatever the genotype through direct effects on viral, pro-inflammatory cytokines and cytokine signalling suppressors (15, 32). Impaired glucose tolerance and marked insulin resistance were demonstrated in a recent study of HCV core gene transgenic mice (33). Furthermore, these same transgenic mice were also shown to have an increased level of tumour necrosis factor á, which was previously found to induce insulin resistance both in vivo and in vitro, via the inhibition of tyrosine phosphorylation of IRS-1 and IRS-2 (34, 35).
Hepatitis C virus entry into hepatocytes may be mediated by the low-density lipoprotein (LDL) receptor and the HCV core protein may interact with apoA2, which is a major component of high-density lipoproteins. This interaction can lead to hepatocellular steatosis by inhibiting microsomal triglyceride transfer protein activity (36). Another non-structural protein NS5A has been found to interact with apoA1 and apoA2, which can lead to altered cholesterol trafficking (37, 38). Recently, there has been considerable interest in the role of micro-RNAs (miRNA) in the genesis of both fatty liver and HCV replication. In particular, mir122, the most abundant liver miRNA, has been shown to affect the development of steatosis by increasing lipogenesis and by enhancing HCV virus replication (39–41).
Effect of concomitant steatosis and hepatitis C virus infection on liver-related outcomes
Steatosis secondary to the metabolic syndrome (genotype 1, NAFLD) or the HCV virus (genotype 3) worsens the process, leading to advanced fibrosis in HCV patients. Subjects with HCV are also more likely to develop type 2 diabetes mellitus, a major risk factor for developing NAFLD and the progression to cirrhosis. The risk of HCC is increased in the presence of cirrhosis and HCV is considered to be a principal cause of the current epidemic of this cancer in the Western world. Patients with HCV and type 2 diabetes are particularly at risk of developing HCC.
Impact of non-alcoholic fatty liver disease on response to antiviral therapy
Clinical response to antiviral therapy in subjects with steatosis
Generally, hepatic steatosis reduces the probability of achieving a sustained virological response (SVR) to pegylated interferon (peg-IFN) and ribavirin (RBV) combination therapy, especially if steatosis involves more than 33% of the liver (37). Some studies have suggested that insulin resistance and cytological ballooning are additional markers for unsuccessful antiviral therapy. Although steatosis is an important cofactor in the progression of fibrosis and necroinflammation in HCV, the exact mechanisms of its affect on the response to antiviral therapy are not well understood, but certain notions are reviewed below:
Hepatic steatosis is associated with obesity and insulin resistance, which, along with the HCV genotype, have been reported to affect the antiviral response. Insulin resistance is especially correlated to a poor response to antiviral therapy in HCV genotype 1 (42). Obesity has been reported to be a risk factor of non-response independent of the HCV genotype and the presence of cirrhosis. Obese patients have approximately an 80% lower chance of achieving SVR compared with non-obese patients (43). While the actual mechanisms of obesity in the response to antiviral therapy are not completely understood, obese HCV patients with steatosis are thought to have increased lipid droplets in hepatocytes, which can act as a functional barrier for the interaction between antiviral drugs and hepatocytes (44). Also, certain peg-IFN may be preferentially absorbed through blood capillaries or the lymphatic circulation. Since obese people are known to have a poor lymphatic circulation (45), this could result in suboptimal serum levels of peg-IFN and a reduced response to antivirals.
Obesity may also affect the antiviral response modulating the interferon (IFN) signalling pathway (Fig. 1). Normally, interferon alpha (IFN-á)-activated cellular signalling is negatively regulated by inhibitory factors such as the suppressor cytokine signalling (SOCS) family (Fig 1). A recent study showed that obese HCV genotype 1 patients had increased mRNA expression of SOCS-3 compared with normal controls (46). This association between obesity and altered SOCS may contribute to the poor antiviral response in HCV patients with concomitant NAFLD/NASH.
Future directions in managing hepatitis C virus patients with superimposed non-alcoholic fatty liver disease
Dyslipidaemia is common in NAFLD and HCV genotype 1. Cholesterol is an integral part of HCV replication and it is believed that the HCV virus enters hepatocytes via LDL receptors (47). HMG-CoA reductase is associated with an upregulation of LDL receptors (48) and its inhibitor (statins) has been shown to be capable of inhibiting HCV viral replication in vitro (49–51). The same finding was not observed in vivo at a similar conventional dose of statins (52). Ikeda and colleagues recently demonstrated in vitro that fluvastatin exhibits the strongest anti-HCV activity of the other statins, and a much loser dose is required to achieve a 50% reduction in RL activity (IC50) (44). Statins may be a potential therapeutic option for HCV infection; however, further studies are needed to define the interaction between these molecules and interferon response.
Improve the efficacy of antiviral therapy in hepatitis C virus patients with concomitant non-alcoholic steatohepatitis
The presence of hepatic steatosis in patients with HCV genotype 2 or 3 does not affect their probability of achieving SVR with combination antiviral therapy, unlike patients with HCV genotype 1. It may not be necessary to treat underlying NAFLD/NASH before antiviral therapy in the former patients. On the other hand, genotype 1 patients are at a higher risk of becoming virological non-responders and HCV treatment should be modified to increase the response rate.
While the presence of NAFLD increases the risk of antiviral treatment failure, treating NAFLD has not been shown to improve the virological response. The results of a pilot study with pioglitazone therapy and retreatment with peg-IFN and RBV were not promising. Nevertheless, there is still hope that NAFLD can be treated to improve the response to antiviral therapy in subjects with HCV. Although the optimal therapy for NAFLD remains to be defined, treatment focused on the underlying risks, i.e. obesity and insulin resistance to improve cardiovascular risks, is a reasonable therapeutic goal. Thus, obesity and the metabolic syndrome should be treated when they are present in these patients.
Hepatitis C is a common chronic liver disease worldwide. Both disease progression and virological response to antiviral therapy are greatly affected by several crucial host and viral factors, in particular, NAFLD and insulin resistance. Obesity and insulin resistance are significant public health concerns in North America. It is now clear that NAFLD is a frequent challenge in the clinical and histological management of patients with HCV as well as for treatment efficacy. The presence of concomitant HCV and NAFLD is associated with increased fibrosis and a low SVR rate to antiviral therapy.