Noncanonical function of sphingosine kinase 2 (SPHK2) sustains hepatic triglyceride homeostasis

Hepatic steatosis, caused by lipid homeostasis disruption, is implicated in the pathogenesis of metabolic dysfunction-associated steatotic liver disease (MASLD). Sphingosine kinase 2 (SPHK2), a pivotal modulator of sphingolipid catabolism, participates in hepatic lipid accumulation. Non-enzymatic regulation of SPHK2 is gaining more attention. We aimed to elucidate the non-canonical mechanism by which SPHK2 mediates hepatic steatosis independent of its enzymatic activity.

By analysing single-cell transcriptome profile of mouse hepatocytes with MASLD confirms that SPHK2 is involved in the progression of hepatic steatosis. Gain or loss of function of SPHK2 was performed using mice with hepatocyte-specific overexpression or knockout Sphk2. Integrated multi-omics approach and immunoprecipitation-mass spectrometry were employed to elucidate the role and molecular mechanism of SPHK2 in steatotic livers. Adeno-associated virus 8 (AAV8) vectors were used to overexpress a catalytically inactive form of SPHK2 in mice.

Hepatocyte SPHK2 was significantly downregulated in steatotic livers from patients and mice with MASLD. Hepatocyte knockout of Sphk2 promotes diet-induced hepatic steatosis by inhibiting adipose triglyceride lipase/patatin like domain 2, triacylglycerol lipase (ATGL)-mediated triglyceride hydrolysis, whereas hepatocyte overexpression of SPHK2 attenuated high-fat diet (HFD)-induced hepatic steatosis by stabilizing ATGL. SPHK2 bound to ATGL, inhibiting COP1-mediated ATGL ubiquitination. Furthermore, kruppel-like factor 10 (KLF10) was identified as a transcription repressor responsible for downregulation of SPHK2 in hepatocytes. Over overexpression of the non-catalytic function of SPHK2 prevented hepatic steatosis in diet-induced mice.

SPHK2 attenuates hepatic steatosis by enzymatic-independently inhibiting ATGL ubiquitination and promoting triglyceride hydrolysis, indicating its therapeutic potential for MASLD.