Effects of losartan and l-serine in a mouse liver fibrosis model

Abstract

Hepatic fibrosis is a common liver disease caused by excessive collagen deposition in the liver. Since liver transplantation is the only current treatment for cirrhosis with worsened fibrosis, a new strategy to develop anti-fibrosis drugs with no adverse effects is necessary. In recent studies, amino acids have been applied as a type of therapy in various fields. l-serine plays a major role in antioxidant production via the maintenance of nicotinamide adenine dinucleotide phosphate hydride production in the mitochondria. l-serine may reduce fibrotic lesions in a mouse model of chronic liver injury. This study used 27 six-week-old C57BL/6 mice and injected them three times a week for eight weeks with carbon tetrachloride (CCl₄) (1.5 mg/kg, 10% v/v CCl₄ in olive oil) to create a hepatic fibrosis mouse model. The mice, which weighed approximately 20–30 g, were randomly classified into four groups: 1) the olive oil group, which received intraperitoneal injection of olive oil (1.5 mg/kg, 3 times per week for 8 weeks); 2) the CCl4-only group; 3) the CCl4 + losartan (10 mg/kg, PO, 5 days on, weekend off for 8 weeks) group; and 4) the CCl4 + l-serine (100 g/L, free access for 8 weeks) group. Hematoxylin and eosin staining and Masson's trichrome staining showed reduced inflammatory cell deposition and collagen deposition in the liver tissue in the l-serine supplemented group. l-serine was found to reduce the spread of hepatic fibrosis and has potential use in clinical settings. Based on these histopathological observations, l-serine is a potential anti-fibrosis drug.

Introduction

The liver is the largest organ in the human body and plays several important roles, such as maintaining blood glucose and cholesterol levels [1]. The liver produces approximately 90% of the body's protein and detoxifies drugs and alcohol that enter the body. Liver regeneration is a normal response to liver damage [1], and the liver is the only organ with the ability to restore itself to its original size and regenerate in a time-limited manner [1]. Chronic liver regeneration occurs in response to persistent inflammation caused by chronic liver disease [[1], [2], [3]].

Hepatic fibrosis, which is the final state of several chronic liver diseases, has a high mortality rate and affects more than 70 million people worldwide [3]. Hepatic fibrosis is characterized by collagen accumulation in liver [4,5], and although its pathophysiology has been extensively studied in the past decade, treatment for this condition remains unclear [4]. Liver damage can be caused by a variety of factors, such as viruses, alcohol consumption, and obesity. As the presence of these factors increases in the body, the levels of reactive oxygen species (ROS) in the liver also increase [6,7]. ROS production plays an important role in causing liver damage and initiating the development of hepatic fibrosis [6,7]. Hepatic stellate cell (HSC) and Kupffer cell activation generates various cytokines and free radicals. Activated myofibroblasts are alpha-smooth muscle actin-positive cells that produce extracellular matrix proteins [2,8,9]. Activated HSCs therefore produce extracellular matrices in which hepatocytes fill in the spaces created by lesions and maintain the liver's shape [9,]. Transforming growth factor beta (TGF-β) is responsible for the activation-transdifferentiation of quiescent HSCs to a myofibroblast phenotype, which results in a persistent inflammatory response [11,12]. Therefore, TGF-β signaling participates in different stages of disease progression, from the initial liver injury to fibrosis and cirrhosis. Liver transplantation is currently the only treatment for cirrhosis [13], and new strategies are required to prevent hepatic fibrosis.

The CCl₄-induced hepatic fibrosis model is a highly reproducible model that has previously been used for drug screening [13,14]. This model is the most commonly used hepatic fibrosis induction model in the world [13,15,16].

l-serine, one of the two forms of serine, has a major effect on the synthesis of the antioxidants glutathione and S-adenosylmethionine in the liver [17,18]. l-serine is biosynthesized in 3-phosphoglycerate and serves as a precursor for the synthesis of the amino acids glycine and cysteine in the mitochondria [19]. l-serine levels in hepatocytes have been reported to increase through the intake of l-serine [17]. l-serine has been employed in various fields of treatment [20], and previous studies have shown that l-serine promotes l-serine-dependent homocysteine metabolism, thereby improving alcoholic fatty liver disease (Sim et al., 2015a). The long-term administration of l-serine reduces orexigenic peptide expression; consequently, body weight, oxidative stress, and inflammation can all be reduced during aging in mice by modulating the sirtuin 1/nuclear factor kappa B pathway. This mechanism demonstrates that l-serine can protect neurons from oxidative stress-mediated apoptosis by contributing to intracellular antioxidant glutathione synthesis and maintaining balanced mitochondrial fusion and division [21]. In an alcohol-induced fatty liver mouse model, l-serine reduced the triglyceride and neutral lipid accumulation in the liver [2,3]. Various examples indicate that l-serine is closely related to the reduction of ROS, thereby highlighting the antioxidant effects of l-serine [1]. This study aimed to investigate whether l-serine could reduce hepatic fibrosis in a hepatic fibrosis mouse model.