Abdominal subcutaneous adipose tissue insulin resistance and lipolysis in patients with non-alcoholic steatohepatitis

Abstract

Background: Systemic insulin resistance (IR) is a primary feature in non-alcoholic steatohepatitis (NASH), however, there remain limited data on tissue-specific insulin sensitivity in vivo.

Methods: We examined tissue-specific (adipose, muscle and liver) insulin sensitivity and inflammation in 16 European Caucasian patients with biopsy-confirmed NASH and in 15 healthy controls. All underwent a two-step hyperinsulinaemic euglycaemic clamp incorporating stable isotope measurements of carbohydrate and lipid metabolism with concomitant subcutaneous adipose tissue (SAT) microdialysis.

Results: Hepatic and muscle insulin sensitivity were decreased in patients with NASH compared with controls, as demonstrated by reduced suppression of hepatic glucose production and glucose disposal (Gd) rates following insulin infusion. In addition, rates of lipolysis were higher in NASH patients with impaired insulin-mediated suppression of free fatty acid levels. At a tissue specific level, abdominal SAT in patients with NASH was severely insulin resistant, requiring >sixfold more insulin to cause ½-maximal suppression of glycerol release when compared with healthy controls. Furthermore, patients with NASH had significantly higher circulating levels of pro-inflammatory adipocytokines than controls.

Conclusion: NASH patients have profound IR in the liver, muscle and in particular adipose tissues. This study represents the first in vivo description of dysfunctional SAT in patients with NASH.

Keywords: adipose tissue; fatty liver; insulin sensitivity; lipolysis; steatohepatitis.

Figure 1

Subjects with non-alcoholic steatohepatitis (NASH) have significant systemic, muscle and hepatic insulin resistance (IR). Circulating glucose (A) and insulin (B) concentrations during the two-step hyperinsulinaemic euglycaemic clamp. The degree muscle and hepatic insulin sensitivity was determined by glucose disposal (C) and suppression of hepatic glucose production (D), respectively. White bar = controls, black bar = NASH. ^&p < 0.05, ^&&p < 0.01, ^&&&p < 0.001 versus controls.

Figure 2

Subjects with non-alcoholic steatohepatitis (NASH) have significant global adipose tissue insulin resistance (IR). (A) Circulating non-esterified fatty acid (NEFA) concentrations at basal and hyperinsulinaemic phases of euglycaemic clamp. (B) As a marker of global adipose tissue insulin resistance, the concentration of circulating insulin concentrations (pmol/l) causing 1/2-maximal suppression of circulating NEFA was calculated. White bar = controls, black bar = NASH. ^&p < 0.05, ^&&&p < 0.001 versus controls. ⁺⁺⁺⁺p < 0.0001 versus basal phase. NS, non-significant.

Figure 3

Non-alcoholic steatohepatitis (NASH) is associated with significant abdominal SAT IR. (A) SAT interstitial fluid concentrations of glycerol during the 2-step hyperinsulinaemic euglycaemic clamp. (B) To determine the rate of lipolysis in subcutaneous adipose tissue (SAT) under basal and hyperinsulinaemic conditions area under the curve (AUC) analysis was performed using the trapezoidal method for interstitial glycerol release. Broken line/white bar = controls, solid line/black bar = NASH. ^&&&&p < 0.0001 versus controls; ⁺⁺⁺p < 0.001, ⁺⁺⁺⁺p < 0.0001 versus basal phase. NS, non-significant.

Figure 4

Subjects with non-alcoholic steatohepatitis (NASH) have a disproportionate higher degree of insulin resistance (IR) in subcutaneous adipose tissue (SAT) (sixfold vs. controls) compared with whole-body adipose tissue (threefold vs. controls). Line graph representing the concentrations of circulating non-esterified fatty acid (NEFA) (whole-body lipolysis) and interstitial fluid glycerol (SAT-specific lipolysis) in basal, low-dose and high-dose insulin phases of the euglycaemic clamp. Black lines = NASH (mean ± s.e.), Grey line = control. Sold line = glycerol levels, broken line = NEFA levels.

Figure 5

Subjects with non-alcoholic steatohepatitis (NASH) have significantly lower levels of fasting adiponectin (A) and higher levels of fasting pro-inflammatory adipocytkines [(B) leptin, (C) high sensitivity C-reactive protein (hs-CRP), (D) tumour necrosis factor alpha (TNF-α), (E) interleukin-6 (IL-6) and (F) chemokine ligand-2 (CCL-2)/MCP-1]. ^&p < 0.05, ^&&p < 0.01, ^&&&p < 0.001, ^&&&&p < 0.0001 versus controls.