By Timothy J. King, Ph.D., Vice President, Research
The role of oxidative stress and inflammation in liver injury and liver dysfunction as well as the potential for antioxidants to ameliorate liver disease has been established in both animal models and human trials [Sanchez-Valle et al. 2012; Pacana and Sanyal 2012]. Studies utilizing rodent models of liver disease have also demonstrated the efficacy of astaxanthin (ASTX) in ameliorating liver injury [Kang et al. 2001; Curek et al. 2010]. Recently, Shen et al. extended these observations by evaluating the effects of 20, 40 and 80mg/kg/day of ASTX in mouse models of surgically induced (bile duct ligation, BDL) and chemically induced (carbon tetrachloride, CCL4) liver injury. In the chemically induced study (CCL4), seven mice/group were treated either with vehicle, CCL4 or CCL4+ASTX (3 doses) for 8 weeks. In the surgically induced model (BDL), 3 groups of ten mice/group were either untreated, BDL or BDL+ASTX (80mg/kg) for 2 weeks. In these models, both BDL and CCL4 treatment induce severe and acute liver injury leading to histological fibrosis.
Here, Shen et al. observed amelioration of liver fibrosis following ASTX administration in both the CCL4 and BDL models. In both models, ASTX treatment significantly reduced pathologically elevated liver enzyme levels (ALT and AST). In addition, histological analysis of liver tissue revealed significant reductions in hepatocytic necrosis, liver lobule damage and pericellular bridging fibrosis in ASTX groups. Staining for collagen levels supported ASTX reduction of total collagen and area/thickness of collage fiber bundles that are associated with liver fibrosis. Evaluation of the BDL study revealed a similar amelioration of liver histopathology in ASTX treated groups.
A key step in liver fibrosis pathology is the activation of hepatic stellate cells (HSCs). HSCs are the primary liver cell involved in fibrogenesis and, when activated by increased levels of inflammatory cytokines such as TNF-a and TGFb1, express extracellular matrix (ECM) products including; collagen type 1, a-smooth muscle actin and b-platelet derived growth factor receptor. Indeed, increased NF-kB signaling resulting in elevated TNF-a expression has been observed in animal models and humans with liver fibrosis [Sunami et al. 2012]. Importantly, ASTX has been shown to significantly decrease TNF-a levels in humans and animals as well as robustly reduce NF-kB signaling both in vitro and in vivo [Suzuki et al. 2006; Lee et al. 2003]. In further support of ASTX capacity to lessen liver fibrosis, Shen et al. observed decreased HSC activation measured as a reduction in HSC secreted proteins in both animal models and in cell culture. In addition, when Shen et al. evaluated CCL4 induced animals treated with ASTX, they observed decreased TGFb/smad and NF-kB signaling compared to untreated animals. This in vivo observation was again supported with cell culture studies. NF-kB driven TGF-b1 expression regulates levels of matrix metalloproteinase-2 (MMP2) and tissue inhibitor of matrix metalloproteinase-1 (TIMP1). While the contribution of MMPs to liver fibrosis (either HSC stimulatory or apoptosis-inducing) is still unclear, the balance between the MMP2/TIMP1 ratio is thought to be important in fibrosis progression [Han 2006]. Here, ASTX treatment resulted in increased MMP2 levels and maintenance of a MMP2/TIMP1 ratio favoring prevention of fibrosis.
Additionally, TGF-b1 has been shown to regulate autophagy (a process for removing damaged organelles and recycling cellular material) that also stimulates HSC activation and liver fibrogenesis [Kim et al. 2012]. Here, ASTX treatment of CCL4 induced animals resulted in decreased autophagy in HSCs measured as reduced expression of LC3 and beclin-1 mRNA/protein levels. Additionally, ASTX administration decreased areas of positive liver cells with fewer lysosomes and autophagosomes evident. The observed decrease in HSC autophagy following ASTX treatment may in part explain the overall ASTX-induced reduction in liver fibrosis.
In total, these results support the potential role of ASTX in reducing liver fibrosis using two commonly employed models of disease and joins several published studies in underscoring the potential for ASTX to ameliorate liver-associated oxidative stress, inflammation and the resulting pathology of liver disease.
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