Steatohepatitis/Metabolic Liver Disease
Relationship between disease severity, hyperinsulinemia, and impaired insulin clearance in patients with nonalcoholic steatohepatitis
Potential conflict of interest: Nothing to report.
This work was supported by the Burroughs Wellcome Fund (to K.C.), the American Diabetes Association (1-08-CR-08; to K. C.), and a VA Merit Award (1 I01 CX000167-01; to K. C.).
Hyperinsulinemia is believed to play a key role in the pathogenesis of nonalcoholic steatohepatitis (NASH) and associated cardiovascular risk. However, the relative contribution of insulin clearance to hyperinsulinemia and its relationship to liver histology have not been carefully evaluated before. To examine this, we enrolled 190 patients (32 without nonalcoholic fatty liver disease [NAFLD], 36 with simple steatosis [SS], and 122 with biopsy-proven NASH). Insulin secretion and hepatic insulin clearance were estimated by means of an oral glucose tolerance test, whereas peripheral insulin sensitivity and whole-body insulin clearance were measured during a euglycemic insulin clamp. A liver biopsy was performed to assess histology (grade/stage). Patients with NASH had similar hepatic insulin sensitivity, compared to patients with SS, but more severe adipose tissue insulin resistance and worse hyperinsulinemia. Patients with SS and NASH had a similar ∼30% reduction (P < 0.01) in hepatic insulin clearance, when compared to patients without NAFLD. Reduced hepatic insulin clearance was not associated with severity of inflammation, ballooning, and fibrosis. In contrast, worse histological inflammation and ballooning (but not steatosis or fibrosis) were associated with a progressive reduction in whole-body insulin clearance (P < 0.001 for trend). There was no significant difference in insulin secretion between patients with SS versus NASH. Conclusion: Decreased hepatic insulin clearance develops with a mild increase in liver fat (LFAT) accumulation. It appears to be largely driven by hepatic steatosis, whereas steatohepatitis is more closely associated with reduced whole-body insulin clearance. Hyperinsulinemia in NAFLD correlated strongly with impaired insulin clearance, but not with insulin secretion. Strategies that reduce LFAT and improve insulin clearance hold the potential to revert the unfavorable effects of hyperinsulinemia in these patients. (Hepatology 2014;59:2178–2187)
The liver is the main site where insulin is cleared from the plasma. It is estimated that it metabolizes approximately 50% of endogenously secreted insulin during its presystemic first pass. Advanced liver disease (cirrhosis) has been associated with low insulin clearance and hyperinsulinemia. This implies that hepatic histological damage may affect the liver's ability to clear insulin.
Nonalcoholic fatty liver disease (NAFLD) usually develops within an insulin-resistant environment and is therefore strongly related to metabolic syndrome (MetS) and type 2 diabetes mellitus (T2DM).[3-6] Some studies have shown that the amount of liver fat (LFAT) is associated with a reduction in insulin clearance in both people with and without T2DM. However, although liver triglyceride (TG) accumulation and impaired insulin clearance may appear to be causally associated, they could just represent the common final pathway of liver insulin resistance (IR). Because insulin clearance was shown to be decreased in people with obesity, high visceral adiposity, and T2DM, it has been suggested that it could represent a compensatory response to induce peripheral hyperinsulinemia in patients with IR. Yet, when insulin clearance was measured in patients with and without T2DM, results have been rather inconsistent, with some studies showing lower insulin clearance in patients with T2DM[7, 11] and others showing no difference in insulin clearance.[9, 12] In addition, studies do not usually distinguish whole-body (largely muscle and kidney) versus hepatic insulin clearance, making data interpretation difficult. The degree of LFAT accumulation may link reduced insulin clearance with obesity and T2DM and help explain the inconsistencies of previous works that have not quantified LFAT in the setting of IR.[13, 14] However, whether LFAT per se is responsible for reduction of insulin clearance in insulin-resistant states remains to be elucidated. No previous study has carefully assessed the role of insulin clearance in patients with biopsy-proven nonalcoholic steatohepatitis (NASH), and decreases in insulin clearance could have simply mirrored more advanced steatohepatitis.
The aim of this study was to assess the role of histological damage in patients with biopsy-proven NASH in whole-body and hepatic insulin clearance and their final influence on hyperinsulinemia.
Plasma insulin concentration depends on a tightly regulated balance between insulin secretion and its clearance. Although much consideration has been given to the importance of insulin secretion and IR to the development of hyperinsulinemia and eventually T2DM, insulin clearance has attracted much less attention as an important determinant of plasma insulin concentration. This is an important topic because hyperinsulinemia has been linked, in epidemiological studies, to cardiovascular disease[23, 24] and hepatocellular carcinoma and both are the leading causes of death in NASH. Previous studies on the role of hepatic steatosis on insulin clearance have been small, usually without simultaneous measurements of LFAT and insulin clearance or relied exclusively on imaging techniques that did not explore the role of steatohepatitis severity.[2, 7] Without a liver biopsy, it is not possible to separate the relative contribution of LFAT per se versus steatohepatitis on insulin clearance. In addition, LFAT imaging may be a poor surrogate of the severity of NAFLD because it is well established that advanced liver disease in NASH is associated with a reduction in LFAT. In our study, we have tried to overcome the above-described limitations by being the first to study a large cohort of patients with NAFLD and biopsy-proven NASH. Finally, another unique aspect of this study is differentiating whole-body from hepatic insulin clearance when assessing the effect of liver histology in insulin clearance. Previous studies assessing insulin clearance in patients with NAFLD relied only on a single measurement of insulin clearance (mainly whole-body) and did not distinguish between hepatic and extrahepatic (mainly renal and skeletal muscle) insulin clearance. This is important to examine because it offers a more comprehensive view of insulin and glucose metabolism with implications for treatment.
Our findings suggest that even a mild increase in liver steatosis (i.e., grade 1) significantly impairs hepatic and whole-body insulin clearance, without further worsening as LFAT increases to 34%-65% (grade 2) and only minimally the hepatic insulin clearance when LFAT is ≥66% (grade 3). This implies that metabolic alterations in patients with NAFLD occur at a low threshold for LFAT. The clinical implication is that an early diagnosis of NAFLD, and eventual treatment, may be important to avoid severe chronic hyperinsulinemia. Chronically elevated plasma insulin levels may drive hepatic lipogenesis and very-low-density lipoprotein oversecretion in patients with NAFLD.[15, 27] Whereas the role of hyperinsulinemia in the pathogenesis of CVD remains controversial, the association between NAFLD and CVD cannot be ignored.[23, 24, 28] Thus, whereas a causal relationship between chronic hyperinsulinemia and CVD needs further validation, amelioration of LFAT and hyperinsulinemia, at the current time, appear desirable. Consistent with this view, hepatic insulin clearance and LFAT were significantly correlated (P < 0.001). However, the presence of inflammation and ballooning was not associated with worse hepatic insulin clearance, compared to simple steatosis (Fig. 2A-C), indicating that LFAT accumulation affects hepatic insulin clearance independently of inflammation, ballooning, or fibrosis.
When the role of NASH on whole-body insulin clearance was assessed, we found that patients with NASH had a lower whole-body insulin clearance than patients with simple steatosis (Fig. 1A). Of note, this difference occurred regardless of patients being well matched for frequent confounders, such as age, BMI, TBF, and prevalence of diabetes. When we divided these patients according to their grades and stages of NAFLD, severity of inflammation and ballooning was associated with a reduction in whole-body insulin clearance (Fig. 3B and 3C). One can only speculate whether liver histology (inflammation and ballooning) may be the driver for the impairment in whole-body insulin clearance or, on the contrary, adipose tissue or muscle IR, both tightly correlated with whole-body insulin clearance (both P < 0.001), are indeed affecting the liver. For the former, several inflammatory biomarkers have been reported to be elevated in NASH and believed to contribute to subclinical inflammation and IR. For the latter, IR in adipose tissue promotes an overflow of plasma FFA to the liver, triggering inflammation and lipotoxicity. Compensatory hyperinsulinemia from muscle IR can also promote hepatocyte lipogenesis, TG accumulation, inflammation, and apoptosis in patients with NASH.
Of note, fibrosis played no role in hepatic or whole-body insulin clearance impairment. This implies that hyperinsulinemia and impaired insulin clearance observed in cirrhosis may not be related to the presence of fibrosis itself, but rather to a different mechanism related to severe IR. Activation of inflammatory cells in the liver has been shown to induce IR, and this can partly explain hyperinsulinemia observed in patients with cirrhosis. Moreover, the relationship between hyperglycemia, hyperinsulinemia, and/or advanced glycation endproducts with hepatic stellate cells and their promotion of fibrosis is complex and not well understood.
One potential limitation of this study is that the prevalence of T2DM was slightly higher in patients with liver disease (no NAFLD, 31%; SS, 47%; NASH, 58%). Because the presence of T2DM could be associated with a decrease in insulin clearance, it could have been acting as a confounding factor. However, in order to overcome this issue, we repeated the analyses excluding patients with T2DM. When only patients without T2DM were considered, results were similar to those of the entire population, with a similarly reduced hepatic insulin clearance in patients with SS and NASH and a further reduction on whole-body insulin clearance in patients with NASH, when compared to patients with SS. These results suggest that differences in insulin clearance among the three groups are independent of the presence of T2DM.
In summary, we have shown that patients with NASH have impaired insulin clearance, both at the hepatic and whole-body levels. Decreased hepatic insulin clearance develops even after a relatively mild increase in liver TG accumulation and is independent of the severity of liver ballooning, inflammation, or fibrosis. However, steatohepatitis is associated with a further reduction in whole-body insulin clearance. Whether severity of NASH has a direct role in impairing whole-body insulin clearance, or they are both the common final pathway of severe IR, remains to be elucidated. Impaired insulin clearance, rather than changes in insulin secretion, lead to hyperinsulinemia in NAFLD. Strategies that ameliorate hepatic steatosis hold the potential to revert the unfavorable effects of hyperinsulinemia in these patients, such as dyslipidemia and CVD.