Transmembrane TNF-alpha mediates forward and reverse signaling, inducing cell death or survival via the NF-kappa B 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-kappa B 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-kappa B 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-kappa B, indicating that tmTNF-alpha, but not sTNF-alpha, contributes to constitutive NF-kappa B 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-kappa B activity was decreased. In contrast, treating Raji cells with sTNFR2 to stimulate reverse signaling via tmTNF-alpha ehanced NF-kappa B activation. We conclude that tmTNF-alpha, when highly expressed on tumor cells and acting as a receptor, promotes NF-kappa B activation through reverse signaling, which is helpful to maintain tumor cell survival. On the contrary, tmTNF-alpha, when acting as a ligand, inhibits NF-kappa B activity through forward signaling, which is inclined to induce tumor cell death.