A small specific‐sized hyaluronic acid ameliorates alcoholic liver disease by targeting a small RNA: New hope for therapy?

Potential conflict of interest: Nothing to report. See Article On Page 602 Alcoholic liver disease (ALD) is a major type of chronic liver disease worldwide ranging from simple fatty liver to more severe forms of liver injury, including alcoholic steatohepatitis, cirrhosis, and hepatocellular carcinoma. So far, there are no Food and Drug Administration–approved drugs for the treatment of ALD. Increased inflammation due to overactivation of Kupffer cells is an important mechanism contributing to the pathogenesis of ALD and has been actively investigated as a therapeutic target for the treatment of ALD. Many potential targets against inflammation have been identified in preclinical studies of ALD,1 but none of them have been approved to be effective in the clinic for the treatment of patients with ALD yet. In this issue of hepatology, Saikia et al.2 add two more new targets including a small specific‐sized hyaluronic acid (HA) 35 (HA35) and microRNA (miRNA) 181b‐3p that may have therapeutic potential for the treatment of ALD. By screening a series of small specific‐sized HA fragments including HA7, HA12, HA59, HA35, and HA74 for their immunoregulatory effects on Kupffer cells from ethanol‐fed rats, the authors identified that only small specific‐sized HA of ∼35 kDa (HA35) completely normalized the overactivated Kupffer cells from ethanol‐fed rats in vitro and in vivo. Furthermore, oral supplementation with HA35 ameliorated ethanol‐induced liver injury and inflammation in mice. These findings suggest that HA35 is a potent negative regulator to control Kupffer cell overactivation in ethanol‐fed animals. HA, also called hyaluronan, is an anionic, nonsulfated glycosaminoglycan that is distributed in all bodily tissues and fluids and is a key component of the extracellular matrix. HA is known to play an important role in the control of cell proliferation and migration, but its role in modulating inflammation is complicated.3 Dependent on the size and matrix structure of the HA molecules and interaction with various types of HA binding proteins and receptors, HA can act either as an anti‐inflammatory mediator or as a proinflammatory factor in response to tissue damage.3 The study by Saikia et al.2 clearly demonstrated that HA35, but not several other small‐sized HAs they tested, acts as a potent anti‐inflammatory mediator to ameliorate ethanol‐induced overactivation of Kupffer cells in vivo and in vitro. Further mechanistic studies demonstrated that the inhibitory effect of HA35 on hepatic inflammation is mediated by restoring the miRNA‐181b‐3p expression in Kupffer cells that is down‐regulated by ethanol. MiRNAs are endogenous small noncoding RNAs (∼22 nucleotides) that work at the posttranscriptional level to regulate gene expression through complementary base pairing with the 3′ untranslated region of target mRNA. Over the last decade, a large number of miRNAs have been demonstrated to play a critical role in the pathogenesis of various types of liver diseases including ALD, and many of them exert their functions in cell‐specific manners (Fig. 1).4 For example, miR‐122 is mainly expressed in hepatocytes, playing a crucial role in lipid metabolism and liver carcinogenesis. MiR‐155 is highly expressed in macrophages/Kupffer cells and is further increased in isolated Kupffer cells after alcohol feeding, playing a proinflammatory role in promoting alcoholic liver injury.5 In contrast, miR‐223, which is highly expressed in neutrophils, is a critical inhibitor to limit overactivation of neutrophils in ethanol‐induced or drug‐induced hepatotoxicity.6 Moreover, Blaya et al.8 have demonstrated that miR‐182 is the most highly up‐regulated miRNA in alcoholic hepatitis and mainly expressed in ductular reaction cells, playing an important role in promoting bile acid accumulation and inflammation. The study by Saikia et al.2 identified miR‐181b‐3p as a critical negative regulator for Toll‐like receptor 4 signaling in Kupffer cells. Using next‐generation sequencing analysis, the authors demonstrated that 30 miRNAs were markedly down‐regulated by more than 2‐fold in Kupffer cells from ethanol‐fed rats compared to those from pair‐fed animals, and three miRNAs from these 30 down‐regulated miRNAs were restored by HA35 treatment. Among these three miRNAs, miR‐181b‐3p was the most highly down‐regulated by ethanol. Further studies from this article demonstrated that importin α5, a protein involved in the activation of nuclear factor κB signaling, is a downstream target of miR‐181b‐3p and that miR‐181b‐3p inhibited importin α5 expression, thereby attenuating nuclear factor κB signaling activation. Ethanol feeding down‐regulated miR‐181b‐3p expression and subsequently up‐regulated importin α5 and nuclear factor κB signaling activation. Interestingly, all of these effects were normalized by HA35 treatment.Figure 1: MiRNAs play important roles in the control of ALD development and progression in cell‐specific manners. For example, miR‐181b‐3p acts as an anti‐inflammatory mediator to inhibit Kupffer cell activation, whereas miR‐155 promotes Kupffer cell activation. Ethanol down‐regulates miR‐181b‐3p in Kupffer cells, thereby inducing Kupffer cell activation. Treatment of HA35 normalizes ethanol‐induced miR‐181b‐3p down‐regulation in Kupffer cells and subsequently ameliorates ethanol‐induced liver injury and inflammation.As discussed above, many miRNAs including miR‐181b‐3p play important roles in the pathogenesis of ALD. The obvious question is whether miRNAs can be used as therapeutic targets for the treatment of ALD. To our knowledge, there have been no reports on miRNA‐based therapy in ALD. However, miRNA‐based therapeutic approaches have been tested in other types of liver diseases. For example, the miR‐122 inhibitor miravirsen was tested in a phase 2a study for the treatment of hepatitis C virus, which showed that five weekly treatments with miravirsen resulted in a dose‐dependent and prolonged reduction in hepatitis C virus RNA levels with no evidence of viral resistance.9 In addition, Stelma et al.10 indicated that a single dose of RG‐101, which is an N‐acetylgalactosamine‐conjugated anti‐miR‐122 oligonucleotide, caused a significant viral load reduction in hepatitis C virus patients, which was associated with normalization of natural killer cell proportions and reduction of natural killer cell activation receptor expression without eliciting systemic immune response. All of the above clinical results strongly highlighted that miRNAs can be considered as novel and prospective therapeutic targets for the treatment of liver diseases. Given an important role of miR‐181b‐3p in the control of Kupffer cell activation and HA35 restoration of miR‐181b‐3p as demonstrated by Saikia et al.,2 HA35, miR‐181b‐3p, or their combination may have therapeutic potential for the treatment of ALD. However, before testing these targets in clinical studies, more experiments should be performed to examine the effects of miR‐181b‐3p and HA35 on other types of immune cells such as neutrophils that also play important roles in the pathogenesis of ALD. Author names in bold designate shared co‐first authorship.