Tumor necrosis factor α sensitizes malignant cells to chemotherapeutic drugs via the mitochondrial apoptosis pathway independently of caspase-8 and NF-κB

The Hodgkin cell line HD-MyZ is resistant to apoptosis induced by tumor necrosis factor {alpha} (TNF{alpha}). In the present work, we show that pretreatment with TNF{alpha} sensitized the cells to apoptosis induced by antineoplastic agents and ceramide. TNF{alpha} pretreatment resulted in enhanced cleavage and activity of caspase-3 upon addition of etoposide, epirubicin or ceramide. No caspase-8 activation was detectable, although caspase-8 could be activated in cell-free extracts. Inhibition of caspase-8 by z-IETD-fmk did not block the sensitizing effect of TNF{alpha}. Furthermore, exogenous ceramide, a mediator of TNF{alpha} signaling, could not substitute for TNF{alpha} in sensitization to drug-induced apoptosis. In contrast, we observed mitochondrial changes following cotreatment of cells with TNF{alpha} and drugs. Mitochondrial permeability transition, cytochrome c release and subsequent processing of caspase-9 preceded the onset of apoptosis, and were enhanced by TNF{alpha} pretreatment. Interestingly, although transcription factor NF-{kappa}B protected HD-MyZ cells from drug-induced apoptosis, TNF{alpha}-mediated sensitization was independent of NF-{kappa}B, since overexpressing a dominant-negative I{kappa}B mutant did not alter the TNF{alpha} effect. Sensitization for drug-induced apoptosis by TNF{alpha} was abrogated by Bcl-xL. Thus, the sensitizing effect of TNF{alpha} is mediated by the mitochondrial pathway and involves processing of caspase-2, -3 and -9, but appears to be independent of caspase-8 processing, Bid cleavage and NF-{kappa}B signaling. Therefore, sensitization by TNF{alpha} is mediated at least in part through different pathways, as reported for TRAIL. There, sensitization occurs through a FADD/caspase-8-dependent mechanism. Regarding TNF{alpha}, the sensitizing effect was also observed in myeloid leukemia cells. Therefore, TNF{alpha} or alternate molecules activating its pathways might be useful as sensitizers for chemotherapy in hematological malignancies.