Elsevier

Journal of Hepatology

Volume 57, Issue 5, November 2012, Pages 1076-1082
Journal of Hepatology

Research Article
Translocator protein (18 kDa), a potential molecular imaging biomarker for non-invasively distinguishing non-alcoholic fatty liver disease

https://doi.org/10.1016/j.jhep.2012.07.002Get rights and content

Background & Aims

Mitochondrial dysfunction is responsible for liver damage and disease progression in non-alcoholic fatty liver disease (NAFLD). Translocator protein (18 kDa) (TSPO), a mitochondrial transmembrane protein, plays important roles in modulating mitochondrial function. This study explored whether TSPO can be used as an imaging biomarker of non-invasive diagnosis and staging of NAFLD, monitored using positron emission tomography (PET) with a TSPO radioligand [18F]FEDAC.

Methods

PET with [18F]FEDAC, non-enhanced computerized tomography (CT), autoradiography, histopathology, and gene analysis were performed to evaluate and quantify TSPO levels and NAFLD progression in methionine and choline-deficient diet-fed mice. Correlations were analyzed between uptake ratio of radioactivity and NAFLD activity score (NAS) in the liver.

Results

Uptake of [18F]FEDAC obviously increased with disease progression from simple steatosis to non-alcoholic steatohepatitis (NASH) (p <0.01). A close correlation was identified between [18F]FEDAC uptake ratio and NAS in the liver (Pearson’s r = 0.922, p = 0.000). Specific binding of [18F]FEDAC to TSPO in the NAFLD livers was assessed in competition studies with the unlabelled TSPO-selective ligand PK11195. Autoradiography and histopathology confirmed the PET imaging results. Further, the mRNA levels of the functional macromolecular signaling complex composed of TSPO were obviously higher compared to controls.

Conclusions

TSPO expression increases in NAFLD and closely correlates with NAFLD progression. TSPO as a specific molecular imaging biomarker may open a novel avenue for non-invasive, reliable, and quantitative diagnosis and staging of NAFLD.

Introduction

Non-alcoholic fatty liver disease (NAFLD) is becoming a public health concern worldwide. It represents a wide spectrum of conditions, ranging from simple steatosis, which generally follows a benign clinical course, to non-alcoholic steatohepatitis (NASH), which progresses to fibrosis in 30–40% of patients and to cirrhosis in 10–15% of patients [1], [2]. Due to the limitations of liver biopsy and currently available non-invasive imaging techniques, new non-invasive biomarkers, that accurately distinguish NASH from NAFLD and allow staging NAFLD, have become an urgent requirement in hepatology [3], [4]. Researchers have tried to identify serum biomarkers on the basis of the current knowledge of the pathophysiological mechanisms of NAFLD, including markers of reactive oxygen species (ROS), inflammation, apoptosis, and fibrosis. However, the results of these studies are controversial [4], suggesting that histological evaluation of NASH may be difficult on the basis of serum biomarker measurements only. Unlike blood tests and histological testing, imaging biomarkers applied to positron emission tomography (PET) technology enable direct, quantitative, and multispatial visualization of physiological and cellular processes at multiple time points and at the whole organism level.

Translocator protein (18 kDa) (TSPO) (also known as peripheral-type benzodiazepine receptor, PBR), a nucleus-encoded mitochondrial target transmembrane protein, has been indicated as an active participant in the modulation of mitochondrial function [5], [6]. PET with radiolabeled TSPO probes has allowed non-invasive and reliable investigation of TSPO in neuropathological damages of experimental animals and humans [7], [8], [9], [10], [11], [12]. Mitochondrial dysfunction plays a key role in the physiopathology of NASH irrespective of the initial cause. Ultrastructural abnormalities and impaired mitochondrial function occur in the liver of patients with NASH, as well as in animal models of NASH [13], [14]. Numerous studies have considered the close relationship between TSPO and mitochondrial dysfunction in conditions such as cardiovascular disease, ischemia, and reperfusion injury [15], [16]. However, to date, no study has examined TSPO expression in the liver during NAFLD progression. Thus, TSPO needs to be explored as a novel imaging marker for non-invasive diagnosis and staging of NAFLD.

N-Benzyl-N-methyl-2-[7,8-dihydro-7-(2-[18F]fluoroethyl)-8-oxo-2-phenyl-9H-purin-9-yl]acetamide([18F]FEDAC) is a specific PET probe for TSPO imaging, developed by our research group [7], [9]. Here, TSPO expression and NAFLD progression were evaluated and quantified using [18F]FEDAC-PET, computerized tomography (CT), histology, and gene analysis in methionine and choline-deficient (MCD) diet-fed mice, one of the most commonly used NASH animal models with severe oxidative stress and hepatocellular injury [17], [18]. We demonstrated the feasibility of TSPO as an imaging biomarker for the diagnosis and staging of NAFLD.

Section snippets

Animals and dietary treatments

All animal experiment protocols were approved by the Animal Ethics Committee of the National Institute of Radiological Sciences and were carried out according to the recommendations of the Committee for the Care and Use of Laboratory Animals, National Institute of Radiological Sciences. Male C57BL/6 mice, aged 6–8 weeks, were purchased from Shizuoka Laboratory Animal Center (Shizuoka, Japan) and maintained in temperature- and light-controlled chambers. The mice were studied at 2, 4, and 8 weeks

Physiological characteristics and liver pathology

Mice fed the MCD diet had lower weights than those fed the normal diet (p <0.05, Table 1). They also had a slightly lower liver weight, but the difference was not statistically significant. When the liver weight was expressed as a percent of the body weight, no difference was found between MCD-fed mice and controls throughout the course of dietary feeding (p >0.05, Table 1). Further, serum ALT and AST levels were significantly greater in MCD-fed mice compared to controls (p <0.05, Table 1).

Discussion

To our knowledge, this is the first study to directly investigate TSPO involved in the modulation of mitochondrial function in MCD diet-fed NAFLD models using PET with a radiolabeled TSPO ligand. TSPO expression increased with NAFLD progression and showed heterogeneous distribution in the liver. The PET signals of [18F]FEDAC-TSPO corresponded to the pathological features observed by histology and CT. Strikingly, the uptake ratio of radioactivity exhibited statistically significant differences

Financial support

This study was supported by research grants from the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant-in-Aid 24790543).

Conflict of interest

The authors who have taken part in this study declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.

Acknowledgements

The authors are grateful to Dr. M. Higuchi for the NP155 antibody and Dr. A. Tsuji for his advice on CT scanning. We thank the staff of the National Institute of Radiological Sciences for their support with cyclotron operation, radioisotope production, radiosynthesis, and animal experiments.

References (28)

  • P. Sorrentino et al.

    Liver iron excess in patients with hepatocellular carcinoma developed on non-alcoholic steato-hepatitis

    J Hepatol

    (2009)
  • J. Liu et al.

    Protein–protein interactions mediate mitochondrial cholesterol transport and steroid biosynthesis

    J Biol Chem

    (2006)
  • N. Bhala et al.

    The natural history of nonalcoholic fatty liver disease with advanced fibrosis or cirrhosis: an international collaborative study

    Hepatology

    (2011)
  • C. Soderberg et al.

    Decreased survival of subjects with elevated liver function tests during a 28-year follow-up

    Hepatology

    (2009)
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