The alleviating effect of sphingosine kinases 2 inhibitor K145 on nonalcoholic fatty liver

Sphingosine kinase 2 and p62 regulation are determinants of sexual dimorphism in hepatocellular carcinoma

OBJECTIVE

Hepatocellular carcinoma (HCC) is the third leading cause of cancer mortality, and its incidence is increasing due to endemic obesity. HCC is sexually dimorphic in both humans and rodents with higher incidence in males, although the mechanisms contributing to these correlations remain unclear. Here, we examined the role of sphingosine kinase 2 (SphK2), the enzyme that regulates the balance of bioactive sphingolipid metabolites, sphingosine-1-phosphate (S1P) and ceramide, in gender specific MASH-driven HCC.

METHODS

Male and female mice were fed a high fat diet with sugar water, a clinically relevant model that recapitulates MASH-driven HCC in humans followed by physiological, biochemical cellular and molecular analyses. In addition, correlations with increased risk of HCC recurrence were determined in patients.

RESULTS

Here, we report that deletion of SphK2 protects both male and female mice from Western diet-induced weight gain and metabolic dysfunction without affecting hepatic lipid accumulation or fibrosis. However, SphK2 deficiency decreases chronic diet-induced hepatocyte proliferation in males but increases it in females. Remarkably, SphK2 deficiency reverses the sexual dimorphism of HCC, as SphK2-/- male mice are protected whereas the females develop liver cancer. Only in male mice, chronic western diet induced accumulation of the autophagy receptor p62 and its downstream mediators, the antioxidant response target NQO1, and the oncogene c-Myc. SphK2 deletion repressed these known drivers of HCC development. Moreover, high p62 expression correlates with poor survival in male HCC patients but not in females. In hepatocytes, lipotoxicity-induced p62 accumulation is regulated by sex hormones and prevented by SphK2 deletion. Importantly, high SphK2 expression in male but not female HCC patients is associated with a more aggressive HCC differentiation status and increased risk of cancer recurrence.

CONCLUSIONS

This work identifies SphK2 as a potential regulator of HCC sexual dimorphism and suggests SphK2 inhibitors now in clinical trials could have opposing, gender-specific effects in patients.

Integrative roles of sphingosine kinase in liver pathophysiology

Bioactive sphingolipids and enzymes that metabolize sphingolipid-related substances have been considered as critical messengers in various signaling pathways. One such enzyme is the crucial lipid kinase, sphingosine kinase (SphK), which mediates the conversion of sphingosine to the potent signaling substance, sphingosine-1-phosphate. Several studies have demonstrated that SphK metabolism is strictly regulated to maintain the homeostatic balance of cells. Here, we summarize the role of SphK in the course of liver disease and illustrate its effects on both physiological and pathological conditions of the liver. SphK has been implicated in a variety of liver diseases, such as steatosis, liver fibrosis, hepatocellular carcinoma, and hepatic failure. This study may advance the understanding of the cellular and molecular foundations of liver disease and establish therapeutic approaches via SphK modulation.

CerS5 deficiency promotes liver fibrosis development in non-alcoholic fatty liver disease

BACKGROUND

Hepatocyte lipotoxicity mediated by sphingolipids was considered one of important factors in NAFLD development. Knocking out key enzymes for sphingolipids synthesis, such as DES-1, SPHK1 and CerS6, could reduce hepatocyte lipotoxicity and improve NAFLD progression. Previous studies showed that roles of CerS5 and CerS6 in sphingolipids metabolism were similar, but the role of CerS5 was controversial in NAFLD development. This study aimed to clarify the role and mechanism of CerS5 in NAFLD development.

METHODS

Hepatocyte conditional CerS5 knockout (CerS5 CKO) and wild type (WT) mice were fed with standard control diet (SC) and choline-deficient, l-amino acid-defined, high-fat diet (CDAHFD) and then divided into four groups: CerS5 CKO-SC, CerS5 CKO-CDAHFD, WT-SC and WT-CDAHFD. RT-PCR, IHC and WB were used to analyze the expression of inflammatory, fibrosis and bile acids (BA) metabolism factors. RNA-seq was used to analyze differences of transcriptional levels of liver molecules among the four groups. Metabolomics was used to measured differences of hepatic BAs among the four groups.

RESULTS

Hepatocyte specific knockout of CerS5 did not increase or reduce the severity of 8-weeks CDAHFD induced hepatic steatosis and inflammation, but significantly worsened the progression of liver fibrosis in these mice. At the molecular level, hepatocyte specific knockout of CerS5 did not increase or reduce expression of hepatic inflammatory factors: CD68, F4/80 and MCP-1, but increased expression of hepatic fibrosis factors: α-SMA, COL1α and TGF-β in mice fed with CDAHFD. Transcriptome analysis showed that hepatocyte specific knockout of CerS5 significantly decreased the expression of hepatic cyp27a1, and decreased expression of cyp27a1 was further validated by RT-PCR and WB. Considering that cyp27a1 was a key enzyme in the alternative pathway of BA synthesis, we further found that hepatic BA pools in CerS5 CKO mice were more conducive to the progression of liver fibrosis, which were characterized by elevated hydrophobic 12α-OH BAs and decreased hydrophilic non-12α-OH BAs.

CONCLUSION

CerS5 played an important role in the progression of NAFLD related fibrosis, and hepatocyte specific knockout of CerS5 accelerated the progression of NAFLD related fibrosis, which was possibly due to the inhibition of BA synthesis alternative pathway by knocking out hepatocyte CerS5.

Lipid alterations in chronic liver disease and liver cancer

Lipids are a complex and diverse group of molecules with crucial roles in many physiological processes, as well as in the onset, progression, and maintenance of cancers. Fatty acids and cholesterol are the building blocks of lipids, orchestrating these crucial metabolic processes. In the liver, lipid alterations are prevalent as a cause and consequence of chronic hepatitis B and C virus infections, alcoholic hepatitis, and non-alcoholic fatty liver disease and steatohepatitis. Recent developments in lipidomics have also revealed that dynamic changes in triacylglycerols, phospholipids, sphingolipids, ceramides, fatty acids, and cholesterol are involved in the development and progression of primary liver cancer. Accordingly, the transcriptional landscape of lipid metabolism suggests a carcinogenic role of increasing fatty acids and sterol synthesis. However, limited mechanistic insights into the complex nature of the hepatic lipidome have so far hindered the development of effective therapies.