Elsevier

Journal of Hepatology

Volume 71, Issue 5, November 2019, Pages 1012-1021
Journal of Hepatology

Crosstalk between adipose tissue insulin resistance and liver macrophages in non-alcoholic fatty liver disease

https://doi.org/10.1016/j.jhep.2019.06.031Get rights and content

Highlights

  • Insulin resistance (IR) plays a pivotal role in the onset and progression of NAFLD.

  • Adipose tissue IR seems to be the main metabolic determinant of the presence and degree of liver fibrosis in NAFLD.

  • sCD163 is a specific macrophage activation marker that increases according to the degree of hepatic fibrosis in NAFLD.

  • This study suggests a link between activation of resident macrophages in the liver and alterations in adipose tissue.

Background & Aims

The pathogenesis of non-alcoholic fatty liver disease (NAFLD) and steatohepatitis (NASH) is likely due to the interaction between a deranged metabolic milieu and local mediators of hepatic inflammation and fibrosis. We undertook this study to elucidate the interplay between macrophage activation, insulin resistance (IR) in target organs/tissues and hepatic damage.

Methods

In 40 non-diabetic patients with biopsy-proven NAFLD we assessed: i) endogenous glucose production (EGP), glucose clearance and indexes of IR in the adipose tissue (Adipo-IR and Lipo-IR) and in the liver (Hep-IR) by tracer infusion ([6,6-2H2]glucose and [2H5]glycerol); ii) macrophage activity (by soluble sCD163) and iii) hepatic expression of CD163 (hCD163).

Results

We found that sCD163 levels paralleled both the plasma free fatty acid (FFA) levels and lipolysis from adipose tissue. Consistently, sCD163 significantly correlated with adipose tissue IR (Adipo-IR: r = 0.32, p = 0.042; Lipo-IR: r = 0.39, p = 0.012). At multiple regression analysis, sCD163 levels were associated with FFA levels (rp = 0.35, p = 0.026). In vitro exposure of human monocyte-derived macrophages to palmitate enhanced sCD163 secretion. Conversely, sCD163 did not correlate with EGP or with Hep-IR. In the liver, hCD163 positively correlated with sCD163 (r = 0.58, p = 0.007) and the degree of steatosis (r = 0.34, p = 0.048), but not with EGP or Hep-IR (r = −0.27 and r = 0.11, respectively, p >0.10, both).

Conclusions

Our findings suggest a link between deranged metabolism in the adipose tissue and activation of hepatic macrophages in patients with NAFLD, possibly in response to FFA overflow and independent of obesity and diabetes. Conversely, our findings do not support a link between activated hepatic macrophages and glucose metabolism (EGP or Hep-IR). The relationship between adipose tissue IR and hepatic macrophages should be considered to define therapeutic targets for NAFLD.

Lay summary

The pathogenesis of non-alcoholic fatty liver disease (NAFLD) and steatohepatitis (NASH) is likely due to the interaction between a deranged metabolic milieu and local mediators of hepatic inflammation and fibrosis in the insulin resistant state. This study provides in vivo support for a possible link between deranged metabolism in the adipose tissue and activation of hepatic macrophages in patients with NAFLD, most likely in response to free fatty acid overflow and independent of obesity and diabetes.

Introduction

The recent epidemic of chronic liver disease is related to the burden of non-alcoholic fatty liver disease (NAFLD), paralleling the worldwide increase of obesity.1 NAFLD is a complex condition related to metabolic derangements in insulin resistance (IR), but in a subset of patients the liver becomes the target of multiple hits leading to non-alcoholic steatohepatitis (NASH), the histological phenotype that may progressively lead to the development of liver fibrosis, cirrhosis and possibly hepatocellular carcinoma.[1], [2] Understanding the biological and environmental factors that drive the progression to NASH and beyond in some individuals is fundamental to the development of robust methods for diagnosis, risk stratification and therapy.2

The prevailing notion of NASH pathogenesis is that a deranged metabolic milieu specifically interacts with local mediators of hepatic inflammation and fibrosis, but the nature of these interactions has not been fully elucidated.[3], [4] It is generally believed that adipose tissue IR plays a pivotal role in the onset and progression of NAFLD.[5], [6] Briefly, weight gain leads to expansion of adipose tissue and recruitment of macrophages through the secretion of various chemo- and cytokines.7 Inflamed and dysfunctional adipose tissue actively releases free fatty acids (FFAs) into the bloodstream, promotes lipotoxicity in the liver, muscle and pancreas,3 and contributes to systemic inflammation. In the normal liver, resident macrophages or Kupffer cells (KCs) play important regulatory roles through crosstalk with the different cell types and particularly with hepatocytes.8 The pro-inflammatory polarization of hepatic macrophages is considered a hallmark of progressive disease, in the livers of patients with NASH, and an attractive therapeutic target as recently reviewed.8 Hepatic lipid accumulation facilitates pro-inflammatory KC polarization, possibly as a consequence of FFA excess, or signals from surrounding steatotic hepatocytes, such as histidine-rich glycoprotein, extracellular vesicles or damage-associated molecular patterns.8 More recently, data derived from animal models and in vitro studies suggest that both pro-inflammatory KCs and recruited hepatic macrophages (Ly6Chi) contribute to decreased hepatic insulin sensitivity by inhibiting insulin signaling and activating hepatic glucose production.9 However, most data available have been derived from mouse models, which are not fully representative of human NASH, since they reflect certain aspects of the pathogenesis and rarely incorporate the full spectrum of etiology-specific mechanisms.8

Soluble CD163 (sCD163) is the ecto-domain of the hemoglobin-haptoglobin scavenger receptor which is exclusively expressed on macrophages and monocytes. It is shed to the circulation during macrophage activation by metalloprotease activity (e.g. tumor necrosis factor-alpha converting enzyme (TACE/ADAM17)).[10], [11] CD163-positive macrophages are highly expressed in human adipose tissue from obese individuals, while sCD163 levels are associated with hepatic inflammation and fibrosis in patients with NAFLD[12], [13] and decrease after successful life-style intervention and bariatric surgery.[14], [15], [16]

Against this background, we carried out this study to further elucidate the complex interplay between IR in target organs/tissues, macrophage activation and hepatic damage in a well-characterized cohort of non-diabetic patients with biopsy-proven NAFLD.

Section snippets

Study participants

The study was performed in 40 patients with NAFLD, selected from consecutive patients who had a liver biopsy for the evaluation of suspected NAFLD between June 2010 and June 2013 in the Liver Unit of the University of Torino, according to the absence of type 2 diabetes mellitus (T2DM) and morbid obesity (body mass index [BMI] >35); the absence of the 2 latter conditions in our patients unveils the independent contribution of NAFLD to metabolic and inflammatory alterations. Other etiologies of

Characteristics of the study patients

The anthropometric, biochemical, and histological characteristics of the study participants (n = 40) are reported in Table 1. Overall, only one-third of patients (35%) were obese according to BMI, but central obesity was found in more than half (60%) and it was the most common feature of the metabolic syndrome (MetS), followed by hypertension (38%), low HDL-Chol (35%), hyperglycemia (23%) and hypertriglyceridemia (10%). Nevertheless, only 32% of patients met the criteria for the diagnosis of

Discussion

The present study indicates that macrophage activation in the liver of patients with NAFLD, possibly in addition to macrophage activation in the adipose tissue, stems from impaired lipid metabolism in the setting of IR. While the role of adipose tissue inflammation in the onset of IR is not under discussion, it is likely that the overflow of FFAs to the liver is one of the main metabolic sources of activation of resident hepatic macrophages in patients with NAFLD and may be one of the

Financial support

This work was funded by FP7/2007-2013 under grant agreement n.HEALTH-F2-2009-241762, project FLIP and by Horizon 2020 under grant agreement n. 634413, project EPoS for EB and AG. HG received funding from The NOVO Nordisk Foundation, The Danish Strategic Research Council (10-092797) and ‘‘Savværksejer Jeppe Juhl og hustru Ovita Juhls mindelegat’’.

Conflict of interest

The authors declare no conflicts of interest that pertain to this work.

Please refer to the accompanying ICMJE disclosure forms for further details.

Authors’ contributions

CR provided data collection, statistical analysis, interpreted data and drafted the manuscript, KK, RY, SE, MM and MS provided the acquisition of data, interpreted data and critically reviewed the manuscript, CB, MG and FS provided the acquisition of data, HJM, MLA, HV, AG and JG interpreted data and critically reviewed the manuscript, EB and HG led the development of the study concept and design, interpreted data, drafted and finalized the manuscript.

Acknowledgement

Professor Stephen Hamilton-Dutoit, Aarhus University Hospital, is acknowledged for CD163 IHC staining o liver biopsies.

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