Hepatoprotective effect of pyrroloquinoline quinone against alcoholic liver injury through activating Nrf2-mediated antioxidant and inhibiting TLR4-mediated inflammation responses

Highlights

• PQQ alleviates acute alcoholic liver injury in mice.

• PQQ against acute alcoholic liver injury by activating Nrf2-mediated signaling in vivo.

• PQQ against acute alcoholic liver injury by inhibiting TLR4/NF-κB signaling in vivo.

Abstract

The present study was aimed at investigating the hepatoprotective effect of pyrroloquinoline quinone (PQQ) against acute alcoholic liver injury in mice. Acute alcoholic liver injury model was established in mice, and they were administrated with PQQ to investigate its hepatoprotective effect. Our results shows that PQQ can significantly ameliorate acute alcoholic liver injury by decreasing the hepatic marker enzymes, including serum alanine transaminase (ALT) and aspartate transaminase (AST), and increasing the levels of alcohol dehydrogenase (ADH), aldehyde dehydrogenase (ALDH), superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and catalase (CAT) in the liver. And PQQ can also significantly reduce the content of hepatic triglyceride (TG) and malondialdehyde (MDA). Moreover, PQQ attenuated alcohol-induced oxidative damage by activating NF-E2-related factor 2 (Nrf2)-mediated signaling pathway, and inhibiting Toll-like receptor 4 (TLR4)-mediated nuclear factor-kappa B (NF-κB) signaling pathway. Our findings have elucidated the liver protection mechanism of PQQ, which would encourage the further exploitation of PQQ as a hepatoprotective functional food.

Introduction

Consumption of alcohol has become an irreplaceable part of the modern-day life [1,2]. Along with the increase in alcohol consumption, the incidence of alcoholic liver diseases (ALD) is also rising every year [3]. After viral hepatitis, alcohol has become the second-most reason for liver diseases [4,5]. ALD is a progressive disease, and early treatment can help its amelioration. Also, ALD can further progress to alcoholic hepatitis, alcoholic liver fibrosis or even cirrhosis [6]. However, little progress has been made in the clinical control of ALD. Therefore, developing new hepatoprotective agents is of prime importance for treating acute alcoholic liver injury (AALI).

During ethanol metabolism, different oxidative and inflammatory intermediates are formed, which cause acute or chronic alcoholic liver injury [7,8]. Ethanol metabolism increases NADH level, which in turn activates microsome ethanol oxidation system (MEOS) and NADPH oxidase (NOX), producing several reactive oxygen species (ROS) [6,7,9]. Activation of NOX in liver Kupffer cells produces ROSs, which contribute to alcoholic liver injury. During alcohol metabolism, some antioxidant pathways are triggered for preventing the oxidative stress (OS) damage, such as NF-E2-related factor 2 (Nrf2) signal pathway [10,11]. Endogenous antioxidants can protect from alcohol-induced damage of liver, such as superoxide dismutase (SOD) and glutathione (GSH) [4,12]. Experimental studies have confirmed that anti-oxidative and anti-inflammatory genes are expressed via the Nrf2 signaling pathway in mammalian cells [10,13]. Furthermore, alcohol intake can also impair intestinal barrier function and increase inflammatory lipopolysaccharide (LPS) level [14], which in turn activates nuclear factor-kappa B (NF-κB) by binding to CD14 receptor, releases endogenous inflammatory factors or other induced hepatocyte necrosis and apoptosis, and aggravates hepatocyte injury [10,15]. Meanwhile, alcohol intake can also promote the expression of pro-inflammatory enzymes, and thereby increase the levels of inflammatory cytokines [16,17]. CD14, a key glycoprotein receptor, binds with many microbial ligands, accelerates the expression of immune-inflammatory mediators, and thus bolsters the host's natural defense response. After being activated by Toll-like receptor 4 (TLR4) oligomerization, CD14 transduces the signal into the cell via triggering a series of signal cascades through both MyD88-dependent and non-MyD88 (TRIF/IRF-3)-dependent pathways [18].

Nowadays, many natural antioxidants have been investigated for preventing ALD, including anthocyanins [19], melanin [20], genistein and puerarin [21]. Pyrroloquinoline quinone (PQQ) is a novel anionic and water-soluble redox substance, which was first identified as the redox cofactor for methanol dehydrogenase [22]. Previous studies have demonstrated that PQQ possesses antioxidative, hypolipidemic, antiproliferative, neuroprotective, growth-stimulating, and free radical scavenging properties [[23], [24], [25]]. Human milk is a major source of PQQ and can be an interesting dietary therapy for preventing oxidative stress caused by maternal obesity [25], or improving mitochondrial dysfunction and inflammation that is caused by many toxic lipids [[26], [27], [28]]. However, little information is available about the effects of PQQ on AALI.

In the present research, the protective effect and possible mechanism of PQQ on the AALI were explored by investigating the oxidative stress and inflammatory markers in mice. The antioxidant and inflammatory signaling pathways were also selected for elucidating the potential molecular mechanisms of PQQ on AALI. Our study would elucidate the role of PQQ supplementation in alleviating AALI.