Inhibition of sphingosine kinase-2 suppresses inflammation and attenuates graft injury after liver transplantation in rats

Inflammation mediates/promotes graft injury after liver transplantation (LT). This study investigated the roles of sphingosine kinase-2 (SK2) in inflammation after LT. Liver grafts were stored in UW solution with and without ABC294640 (100 μM), a selective inhibitor of SK2, before implantation. Hepatic sphingosine-1-phosphate (S1P) levels increased ~4-fold after LT, which was blunted by 40% by ABC294640. Hepatic toll-like receptor-4 (TLR4) expression and nuclear factor-κB (NF-κB) p65 subunit phosphorylation elevated substantially after transplantation. The pro-inflammatory cytokines/chemokines tumor necrosis factor-α, interleukin-1β and C-X-C motif chemokine 10 mRNAs increased 5.9-fold, 6.1-fold and 16-fold, respectively following transplantation, while intrahepatic adhesion molecule-1 increased 5.7-fold and monocytes/macrophage and neutrophil infiltration and expansion of residential macrophage population increased 7.8-13.4 fold, indicating enhanced inflammation. CD4+ T cell infiltration and interferon-γ production also increased. ABC294640 blunted TLR4 expression by 60%, NF-κB activation by 84%, proinflammatory cytokine/chemokine production by 45-72%, adhesion molecule expression by 54% and infiltration of monocytes/macrophages and neutrophils by 62-67%. ABC294640 also largely blocked CD4+ T cell infiltration and interferon-γ production. Focal necrosis and apoptosis occurred after transplantation with serum alanine aminotransferase (ALT) reaching ~6000 U/L and serum total bilirubin elevating to ~1.5 mg/dL. Inhibition of SK2 by ABC294640 blunted necrosis by 57%, apoptosis by 74%, ALT release by ~68%, and hyperbilirubinemia by 74%. Most importantly, ABC294640 also increased survival from ~25% to ~85%. In conclusion, SK2 plays an important role in hepatic inflammation responses and graft injury after cold storage/transplantation and represents a new therapeutic target for liver graft failure.