Transmembrane TNF-alpha mediates "forward" and "reverse" signaling, inducing cell death or survival via the NF-kappaB pathway in Raji Burkitt lymphoma cells.
Interestingly, some lymphoma cells, expressing high levels of transmembrane (tm)TNF-alpha, are resistant to secretory (s)TNF-alpha-induced necrosis but sensitive to tmTNF-alpha-mediated apoptosis. As tmTNF-alpha mediates "forward" as well as "reverse" signaling, we hypothesize that a balanced signaling between forward and reverse directions may play a critical role in determining the fate of cells bearing tmTNF-alpha. Using Raji cells as a model, we first added exogenous tmTNF-alpha on fixed, transfected NIH3T3 cells onto Raji cells to examine tmTNF-alpha forward signaling and its effects, showing that constitutive NF-kappaB activity and cellular inhibitor-of-apoptosis protein 1 transcription were down-regulated, paralleled with Raji cell death. As Raji cells express tmTNF-alpha, an inhibition of their tmTNF-alpha expression by antisense oligonucleotide caused down-regulation of NF-kappaB activity. Conversely, increasing tmTNF-alpha expression by suppressing expression of TNF-alpha-converting enzyme that cleaves tmTNF-alpha led to an enhanced activation of NF-kappaB, indicating that tmTNF-alpha, but not sTNF-alpha, contributes to constitutive NF-kappaB activation. We next transfected Raji cells with a mutant tmTNF-alpha lacking the intracellular domain to competitively suppress reverse signaling via tmTNF-alpha; as expected, constitutive NF-kappaB activity was decreased. In contrast, treating Raji cells with sTNFR2 to stimulate reverse signaling via tmTNF-alpha enhanced NF-kappaB activation. We conclude that tmTNF-alpha, when highly expressed on tumor cells and acting as a receptor, promotes NF-kappaB activation through reverse signaling, which is helpful to maintain tumor cell survival. On the contrary, tmTNF-alpha, when acting as a ligand, inhibits NF-kappaB activity through forward signaling, which is inclined to induce tumor cell death.
Department of Immunology, Tongji Medical College, Huazhong University of Science and Technology, 13 Hangkong Rd., Wuhan, Hubei, 430030, PR China.
Hailong
H
Zhang
Dan
D
Yan
Xu
X
Shi
Huifang
H
Liang
Yan
Y
Pang
Nalin
N
Qin
Hui
H
Chen
Jing
J
Wang
Bingjiao
B
Yin
Xiaodan
X
Jiang
Wei
W
Feng
Wenjie
W
Zhang
Muxiang
M
Zhou
Zhuoya
Z
Li
D017209:Apoptosis / Q000187:drug effects*; D002051:Burkitt Lymphoma / Q000276:immunology / Q000378:metabolism / Q000473:pathology*; D002462:Cell Membrane / Q000502:physiology*; D049109:Cell Proliferation / Q000187:drug effects*; D004797:Enzyme-Linked Immunosorbent Assay; D005434:Flow Cytometry; D006801:Humans; D051034:Inhibitor of Apoptosis Proteins / Q000235:genetics / Q000378:metabolism; D008156:Luciferases / Q000378:metabolism; D016328:NF-kappa B / Q000235:genetics / Q000378:metabolism*; D016376:Oligonucleotides, Antisense / Q000494:pharmacology; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D047889:Receptors, Tumor Necrosis Factor, Type II / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction / Q000187:drug effects*; D014162:Transfection; D014407:Tumor Cells, Cultured; D014409:Tumor Necrosis Factor-alpha / Q000037:antagonists & inhibitors / Q000235:genetics / Q000494:pharmacology*
D051034:Inhibitor of Apoptosis Proteins; D016328:NF-kappa B; D016376:Oligonucleotides, Antisense; D012333:RNA, Messenger; D047889:Receptors, Tumor Necrosis Factor, Type II; D014409:Tumor Necrosis Factor-alpha; D008156:Luciferases