TNF-alpha-induced sphingosine 1-phosphate inhibits apoptosis through a phosphatidylinositol 3-kinase/Akt pathway in human hepatocytes.
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.
Osawa
Y
Y
First Department of Internal Medicine and Department of Biochemistry, Gifu University School of Medicine, Gifu, Japan.
Banno
Y
Y
Nagaki
M
M
Brenner
D A
DA
Naiki
T
T
Nozawa
Y
Y
Nakashima
S
S
Moriwaki
H
H
D000256:Adenoviridae / Q000235:genetics; D000276:Adjuvants, Immunologic / Q000037:antagonists & inhibitors / Q000494:pharmacology; D017209:Apoptosis / Q000235:genetics / Q000276:immunology*; D020169:Caspases / Q000378:metabolism; D002460:Cell Line; D053938:DNA Fragmentation / Q000276:immunology; D004789:Enzyme Activation / Q000235:genetics / Q000276:immunology; D053222:Fas Ligand Protein; D022781:Hepatocytes / Q000166:cytology / Q000201:enzymology* / Q000276:immunology / Q000821:virology; D006801:Humans; D020672:I-kappa B Proteins / Q000235:genetics; D008024:Ligands; D008246:Lysophospholipids; D008562:Membrane Glycoproteins / Q000037:antagonists & inhibitors / Q000494:pharmacology; D016328:NF-kappa B / Q000037:antagonists & inhibitors / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000502:physiology*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000037:antagonists & inhibitors / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000502:physiology*; D011518:Proto-Oncogene Proteins / Q000037:antagonists & inhibitors / Q000502:physiology*; D051057:Proto-Oncogene Proteins c-akt; D015398:Signal Transduction / Q000235:genetics / Q000276:immunology; D013110:Sphingosine / Q000031:analogs & derivatives* / Q000096:biosynthesis* / Q000276:immunology* / Q000378:metabolism / Q000494:pharmacology; D014407:Tumor Cells, Cultured; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology*; D019014:fas Receptor / Q000378:metabolism; D000099157:Sphingosine Kinase
D000276:Adjuvants, Immunologic; D020169:Caspases; D053222:Fas Ligand Protein; D020672:I-kappa B Proteins; D008024:Ligands; D008246:Lysophospholipids; D008562:Membrane Glycoproteins; D016328:NF-kappa B; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D017853:Phosphotransferases (Alcohol Group Acceptor); D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D013110:Sphingosine; D014409:Tumor Necrosis Factor-alpha; D019014:fas Receptor; D000099157:Sphingosine Kinase; C505661:FASLG protein, human; C505658:Fasl protein, mouse; C505662:Faslg protein, rat; C060506:sphingosine 1-phosphate; C494918:AKT1 protein, human; C061800:N,N-dimethylsphingosine; C005682:safingol