Transmembrane TNF-α mediates “forward” and “reverse” signaling, inducing cell death or survival via the NF-κB pathway in Raji Burkitt lymphoma cells

Interestingly, some lymphoma cells, expressing high levels of transmembrane (tm)TNF-α, are resistant to secretory (s)TNF-α-induced necrosis but sensitive to tmTNF-α-mediated apoptosis. As tmTNF-α 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-α. Using Raji cells as a model, we first added exogenous tmTNF-α on fixed, transfected NIH3T3 cells onto Raji cells to examine tmTNF-α forward signaling and its effects, showing that constitutive NF-κB activity and cellular inhibitor-of-apoptosis protein 1 transcription were down-regulated, paralleled with Raji cell death. As Raji cells express tmTNF-α, an inhibition of their tmTNF-α expression by antisense oligonucleotide caused down-regulation of NF-κB activity. Conversely, increasing tmTNF-α expression by suppressing expression of TNF-α-converting enzyme that cleaves tmTNF-α led to an enhanced activation of NF-κB, indicating that tmTNF-α, but not sTNF-α, contributes to constitutive NF-κB activation. We next transfected Raji cells with a mutant tmTNF-α lacking the intracellular domain to competitively suppress reverse signaling via tmTNF-α; as expected, constitutive NF-κB activity was decreased. In contrast, treating Raji cells with sTNFR2 to stimulate reverse signaling via tmTNF-α ehanced NF-κB activation. We conclude that tmTNF-α, when highly expressed on tumor cells and acting as a receptor, promotes NF-κB activation through reverse signaling, which is helpful to maintain tumor cell survival. On the contrary, tmTNF-α, when acting as a ligand, inhibits NF-κB activity through forward signaling, which is inclined to induce tumor cell death.