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Rational design of anticancer therapies intended to stimulate apoptosis requires knowledge of the mechanisms by which apoptosis is induced, the pathways that confer resistance and how these differ between different apoptosis-inducing agents. Towards these goals, our laboratory has examined the protein, TAPP, isolated by isoelectric focusing and size fractionation from fibroblast cytosolic extracts. Incubation of fibroblasts (NHDF or HEL-299) with TAPP protects them from apoptosis induced by TNFα. Incubation with TAPP does not interfere with the binding of TNFα to NHDF. TAPP does not neutralize or proteolyze TNFα. Incubation of NHDF with TAPP prior to the addition of TNFα prevents activation of caspases 8, 9 and 3, cleavage of PARP, and disappearance of cFLIP evaluated by immunoblot. Treatment of NHDF with TAPP increases levels of p65/RelA NF-κB phosphorylated on serine 536 detected by ELISA which indicates that TAPP activates the NF-κB transcription factor. TAPP also increases both the long and short forms of the NF-κB-induced antiapoptotic protein cFLIP detected by immunoblot. Since cFLIP is a known inhibitor of caspase 8, these data support the hypothesis that TAPP increases cFLIP through NF-κB activation and this prevents TNFα-induced apoptosis by inhibiting caspase 8. Consistent with this hypothesis, TAPP is incapable of protecting NHDF from doxorubicin-induced apoptosis, an agent that induces apoptosis by a caspase 8-independent pathway. Based on these data, it is surprising that TAPP cannot protect NHDF from Fas-induced apoptosis which requires caspase 8 cleavage and is typically sensitive to inhibition by cFLIP. These investigations provide important information about a novel cellular factor which, when incubated exogenously with cells, induces resistance to apoptosis. In addition, the data indicate that certain cells may utilize pathways different from the canonical pathways of apoptosis and apoptotic resistance. (Supported by NIH R01 CA093594-02)

[Proc Amer Assoc Cancer Res, Volume 47, 2006]