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.
Zhang
Hailong
H
Department of Immunology, Tongji Medical College, Huazhong University of Science and Technology, 13 Hangkong Rd., Wuhan, Hubei, 430030, PR China.
Yan
Dan
D
Shi
Xu
X
Liang
Huifang
H
Pang
Yan
Y
Qin
Nalin
N
Chen
Hui
H
Wang
Jing
J
Yin
Bingjiao
B
Jiang
Xiaodan
X
Feng
Wei
W
Zhang
Wenjie
W
Zhou
Muxiang
M
Li
Zhuoya
Z
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