Abstract
Metabolic dysfunction-associated steatotic liver disease (MASLD) is becoming a major health problem worldwide. Liver regeneration is crucial for restoring liver function, and is regulated by extraordinary complex process, involving numerous factors under both physiologic and pathologic conditions. Sphingosine-1-phosphate (S1P), a bioactive sphingolipid synthesized by sphingosine kinase 1 (SphK1), plays an important role in liver function through S1P receptors (S1PRs)-expressing cells. In this study, we investigated the effect of lipid overload on hepatocyte proliferation in a mouse hepatic steatosis model induced by feeding a methionine- and choline-deficient (MCD) diet. After 50% partial hepatectomy (PHx), liver tissues were sampled at various timepoints and then analyzed by immunohistochemistry, oil Red-O staining, quantitative-polymerase chain reaction (qPCR), and flow cytometry. In mice fed the MCD-diet, significantly exacerbated hepatic steatosis and accelerated liver regeneration were observed. After PHx, hepatocyte proliferation peaked at 48 and 36 hr in the liver of chow- and MCD-diet fed mice, respectively. By contrast, increased expression of S1PR2 was observed in hepatic neutrophils and macrophages of MCD-diet fed mice. Flow cytometry and qPCR experiments demonstrated that levels of HGF and FGF2 released by neutrophils and macrophages were significantly higher in MCD-diet fed mice. In conclusion, hepatic lipid overload recruits Kupffer cells and neutrophils that release HGF and FGF2 via SphK1/S1PR2 activation to accelerate hepatocyte proliferation.