TNF-alpha-induced sphingosine 1-phosphate inhibits apoptosis through a phosphatidylinositol 3-kinase/Akt pathway in human hepatocytes

• Published in: The Journal of immunology (1950) 2001-07, Vol.167 (1), p.173-180
• Main Authors: Osawa, Y, Banno, Y, Nagaki, M, Brenner, D A, Naiki, T, Nozawa, Y, Nakashima, S, Moriwaki, H
• Format: Article
• Language: English
• Subjects: Adenoviridae - genetics; Adjuvants, Immunologic - antagonists & inhibitors; Adjuvants, Immunologic - pharmacology; Akt protein; Apoptosis - genetics; Apoptosis - immunology; Caspases - metabolism; Cell Line; DNA Fragmentation - immunology; Enzyme Activation - genetics; Enzyme Activation - immunology; Fas Ligand Protein; fas Receptor - metabolism; FasL protein; Hepatocytes - cytology; Hepatocytes - enzymology; Hepatocytes - immunology; Hepatocytes - virology; Humans; I-kappa B Proteins - genetics; Ligands; Lysophospholipids; Membrane Glycoproteins - antagonists & inhibitors; Membrane Glycoproteins - pharmacology; NF-kappa B - antagonists & inhibitors; NF-kappa B - metabolism; Phosphatidylinositol 3-Kinases - antagonists & inhibitors; Phosphatidylinositol 3-Kinases - physiology; Phosphotransferases (Alcohol Group Acceptor) - antagonists & inhibitors; Phosphotransferases (Alcohol Group Acceptor) - metabolism; Protein-Serine-Threonine Kinases - antagonists & inhibitors; Protein-Serine-Threonine Kinases - physiology; Proto-Oncogene Proteins - antagonists & inhibitors; Proto-Oncogene Proteins - physiology; Proto-Oncogene Proteins c-akt; Signal Transduction - genetics; Signal Transduction - immunology; Sphingosine - analogs & derivatives; Sphingosine - biosynthesis; Sphingosine - immunology; Sphingosine - metabolism; Sphingosine - pharmacology; sphingosine kinase; Tumor Cells, Cultured; Tumor Necrosis Factor-alpha - pharmacology
• ISSN: 0022-1767; 1550-6606
• DOI: 10.4049/jimmunol.167.1.173

Human hepatocytes usually are resistant to TNF-alpha cytotoxicity. In mouse or rat hepatocytes, repression of NF-kappaB activation is sufficient to induce TNF-alpha-mediated apoptosis. However, in both Huh-7 human hepatoma cells and Hc human normal hepatocytes, when infected with an adenovirus expressing a mutated form of IkappaBalpha (Ad5IkappaB), which almost completely blocks NF-kappaB activation, >80% of the cells survived 24 h after TNF-alpha stimulation. Here, we report that TNF-alpha activates other antiapoptotic factors, such as sphingosine kinase (SphK), phosphatidylinositol 3-kinase (PI3K), and Akt kinase. Pretreatment of cells with N,N-dimethylsphingosine (DMS), an inhibitor of SphK, or LY 294002, an inhibitor of PI3K that acts upstream of Akt, increased the number of apoptotic cells induced by TNF-alpha in Ad5IkappaB-infected Huh-7 and Hc cells. TNF-alpha-induced activations of PI3K and Akt were inhibited by DMS. In contrast, exogenous sphingosine 1-phosphate, a product of SphK, was found to activate Akt and partially rescued the cells from TNF-alpha-induced apoptosis. Although Akt has been reported to activate NF-kappaB, DMS and LY 294002 failed to prevent TNF-alpha-induced NF-kappaB activation, suggesting that the antiapoptotic effects of SphK and Akt are independent of NF-kappaB. Furthermore, apoptosis mediated by Fas ligand (FasL) involving Akt activation also was potentiated by DMS pretreatment in Hc cells. Sphingosine 1-phosphate administration partially protected cells from FasL-mediated apoptosis. These results indicate that not only NF-kappaB but also SphK and PI3K/Akt are involved in the signaling pathway(s) for protection of human hepatocytes from the apoptotic action of TNF-alpha and probably FasL.