This study confirms that the pathophysiology of fatty liver associated chronic hepatitis C is different in patients infected with genotype 1 and 3. The study also shows that insulin resistance in patients infected with genotype 1 is the cause rather than the consequence of hepatic steatosis and fibrosis, and suggests that increased circulating insulin is a risk factor for fibrosis through insulin resistance induced steatosis.
Although steatosis was more severe in patients infected with genotype 3, insulin resistance was associated with steatosis only in patients infected with genotype 1. Insulin resistance depends mainly on age and is a major risk factor for steatosis, independent of BMI. Steatosis was associated with more severe fibrosis, whatever the genotype, supporting the major role of steatosis, whatever its cause, in the progression of fibrosis. Indeed, while univariate analysis identified a significant link between circulating insulin level and fibrosis stage, multivariate analysis revealed that steatosis, but not insulin, was independently associated with fibrosis, suggesting an indirect effect of the circulating insulin level on fibrosis stage through a steatosis related pathway.
To clarify the intricate relationship between insulin resistance, steatosis, and fibrosis, the study was performed in two groups of patients with chronic HCV infection due to genotype 1 or genotype 3. The results of the present study confirm the cogency of this distinction as two distinct mechanisms appear to operate in HCV associated fatty liver. In genotype 1 infected patients, steatosis was linked to BMI, while in genotype 3 infected patients, steatosis was related to HCV viral load. Moreover, we took care to exclude patients with usual causes of steatosis, such as alcohol, drugs, or diabetes mellitus. In this selected population, the prevalence of significant steatosis (
10%) was still high, reaching 60% in patients infected with genotype 3. These results are in accordance with previous studies showing a strong association between steatosis and genotype 3 infection.4,5,6,7,8,9,10,11,12
Because cirrhosis is a well known cause of insulin resistance,31
patients with biopsy proven cirrhosis were also excluded.
The mechanisms of development of insulin resistance in patients with chronic HCV infection are not well understood. It has been suggested that insulin resistance may result from steatosis, as excess free fatty acids could downregulate insulin receptor substrate 1 (IRS-1) signalling.32
This concept was further supported by recent evidence that reversing hepatic steatosis may improve insulin resistance in rats with diet induced fatty liver.33
Despite the significant relationship in genotype 1 infected patients, the lack of association between steatosis and insulin resistance in genotype 3 infected patients does not support this hypothesis. Shintani et al
showed that insulin resistance preceded the occurrence of steatosis in transgenic mice expressing HCV core protein, suggesting that insulin resistance is not a consequence of hepatic steatosis in these mice.22
As expected, BMI was correlated with the degree of insulin resistance in univariate analysis.34
However, in multivariate analysis, the degree of insulin resistance depended mainly on the age of the patient. Finally, genotype 1 infected patients with fatty liver were more insulin resistant than genotype 3 infected patients, probably because they were older and had higher BMI values. It has been suggested that age associated decline in mitochondrial function could contribute to insulin resistance.35
Our results do not exclude other mechanisms. The higher prevalence of type 2 diabetes in patients with chronic hepatitis C suggests implementation of HCV infection itself.14–20
Impaired IRS-1 signalling could be a possible mechanism, as recently shown in non-obese/non-diabetic patients with chronic HCV infection.36
In a large cohort study, Hui et al
showed a genotype specific association between chronic HCV infection and insulin resistance.21
The significant link between genotype 1 related steatosis and insulin resistance in our study population tends to support this hypothesis.
As in previous studies, we found that steatosis (
10%) was common among patients with chronic hepatitis C, occurring in 34% of biopsy specimens.1–3
Our data confirm the strong correlation between the degree of steatosis and level of HCV viraemia in genotype 3 infected patients.4
In patients infected with genotype 1, multivariate analysis demonstrated that insulin resistance was a risk factor for steatosis, independent of BMI. These data confirm the existence of two distinct entities: a group of patients infected with genotype 1 that may have steatosis secondary to metabolic causes such as insulin resistance, and a second group infected with genotype 3 that may have steatosis as a direct consequence of HCV infection.
In patients with chronic hepatitis C, we and others have shown a significant relationship between the degree of steatosis and severity of fibrosis.4,6,7
In the present study, steatosis was associated with fibrosis, irrespective of viral genotype. Because steatosis was associated with insulin resistance in genotype 1 infected patients, fibrosis could be the result of hyperinsulinaemia. In fact, it has been demonstrated that high levels of insulin and glucose could promote fibrogenesis by stimulating the release of connective tissue growth factor, a fibrogenic growth factor, from hepatic stellate cells.37
In genotype 1 infected patients, fibrosis stage significantly correlated with circulating insulin levels in univariate analysis. When considering other potential risk factors of fibrosis, such as steatosis, insulin was no longer an independent risk factor. The relationship between steatosis and fibrosis could be explained by several other mechanisms, such as lipid peroxidation.38–41
According to the “two hits hypothesis”, steatosis could increase the sensitivity of hepatocytes to oxidative stress, the second hit being HCV infection itself in patients with chronic hepatitis C.39
Production of reactive oxygen species in an in vitro model expressing HCV core protein is consistent with this hypothesis.42
The second independent risk factor for fibrosis in the present study was the histological score of activity. This result agrees well with the findings of previous longitudinal studies that showed that the necroinflammatory score was predictive of the development of severe fibrosis in patients with chronic hepatitis C.43,44
In our study, neither the duration of contamination nor alcohol consumption was associated with fibrosis. The non-linear progression of fibrosis in chronic hepatitis C probably explains the former result.43,44
In most of the studies showing a significant association between alcohol and fibrosis, the cut off value was 20 g/day.5–7
In our study population, drinking less than 20 g/day, we did not find any significant effect of small amounts of alcohol on fibrosis.
In conclusion, increased circulating insulin is a risk factor for fibrosis in genotype 1 infected patients with chronic hepatitis C through insulin resistance induced steatosis. Accordingly, it may be speculated that intervention strategies to reduce insulin resistance associated with steatosis should target these patients. In the near future, metformin or peroxisome proliferator activated receptor γ agonists could be interesting therapeutic options for improving steatosis and fibrosis in HCV patients with insulin resistance.45,46