The Role of Sphingosine Kinase 2 in Alcoholic Liver Disease
Authors: Kwong Eric K. , Li Xiaojiaoyang , Liu Runping , Hylemon Phillip B. , Zhou Huiping 
Published: November 2023

Introduction

Alcoholic liver disease (ALD) is one of the most common liver diseases worldwide. The pathology of ALD is a combination of endoplasmic reticulum (ER) stress, oxidative stress, inflammation and the dysregulation of hepatic lipid genes. However, the specific mechanism(s) for disease progression is poorly understood. No FDA approved therapy is available for any stage of ALD and liver transplantation remains the only curative option for alcoholic liver cirrhosis. Recent studies have revealed a novel role for sphingosine kinase 2 (SphK2) in mitigating hepatic steatosis in response to ER stress. Consistent with this notion, our previous work demonstrated that SphK2 knockout mice fed a high fat diet exhibited increased hepatic steatosis and inflammation. Studies have also reported that alcohol induces ER stress in intestinal epithelial cells and increases gut permeability. Gut‐derived bacterial endotoxin further causes inflammation, dysregulation of hepatic metabolism and ultimately liver injury. However, the role of SphK2 in ethanol induced liver injury, and the molecular mechanisms by which ethanol causes ALD remain unclear and is the focus of this study.

Methods

Rat primary hepatocytes (RPH) and cultured mouse RAW264.7 macrophages were used for in vitro study. RPH were treated with different concentrations of alcohol (0–200 mM) for various time periods (0–24 h). RAW264.7 cells were treated with alcohol (0–100 mM) for 48 h followed by treatment with or without 100 ng/mL lipopolysaccharide (LPS) for 30 min. For in vivo studies, Gao‐binge ALD mouse model was used. Both C57BL/6J wild type and SphK2 knockout mice (8‐week old, male) were fed a Lieber‐DeCarli ethanol diet for 10 d followed by gavage of a single dose of ethanol (5 g/kg). At the end of treatment, mice were sacrificed and blood and liver tissues were harvested. Liver functional enzymes and total bile acid levels were measured using commercial kits. Liver tissues were processed for histological analysis. The mRNA and protein expression levels were measured using real‐time RT‐PCR and Western blot analysis, respectively.

Results

Alcohol induced ER stress and increased expression of SphK2 in RPH. LPS and alcohol also induced ER stress and upregulated the expression of SphK2 and inflammatory mediators, such as IL‐6, IL‐1β, TNF‐α, MCP‐1, COX‐2 in RAW264.7 cells. In addition, SphK2 knockout mice were more susceptible to alcohol‐induced hepatic steatosis and liver injury than wild type control.

Conclusion

Alcohol and LPS markedly induced ER stress and SphK2 expression in hepatocytes and macrophages. SphK2 knockout mice developed more severe hepatic steatosis and liver injury when challenged with alcohol. Our results suggest that SphK2 plays an important role in regulating hepatic lipid metabolism and inflammatory response and may represent a potential target for developing novel therapeutics for ALD.

Support or Funding Information

This work was supported by National Institutes of Health Grant R01 DK104893 (to HZ and PBH), R01DK‐057543‐11 (to PBH and HZ), VA Merit Award I01BX001390 (to HZ); Massey Cancer Center pilot grant (to HZ and PBH).