Investigations into the molecular mechanisms that regulate whether tumour necrosis factor (TNF) induces cell survival or cell death
TNFR2 can activate NR-kB independently of TNFR1 although to a lesser degree. It was also demonstrated that caspases and NF-kB do not interact directly as caspase inhibition could not prevent NF-kB activation. The interaction between caspases and MAP kinases in HeLa-TNFR2 cells was also investigated. It was shown that JNK activation in response to TNF was attenuated following caspase inhibition while p42/44 MAP kinase and p38 MAP kinase activation were unaffected by caspase inhibition (Littlejohn et al, 2002). Further to these studies TNF receptor associated cytoplasmic adaptor proteins, in particular TRAF1, TRAF2 and RIP, were investigated with a view to determining the molecular switch between life and death in HeLa-TNFR2 cells. It was established using immunoprecipitation that TNFR2 could associate with RIP, as could TRAF1 and TRAF2. However, on the whole this technique gave limited information and further experiments are required to determine more conclusively the interactions taking place between the adaptor proteins, TNFR1 and TNFR2. In order to investigate the molecular switch more effectively, TF-1 cells were used. It was already known that TF-1 cells proliferate in response to TNF it cultured in the absence of GMCSF however, if cultured in the presence of GMCSF then TNF stimulation induces apoptotic cell death. This model of TNF induced life versus death demonstrated that caspase mediated RIP cleavage only occurs in the death scenario. RIP degradation is accompanied by reduced NF-kB activation suggesting that NF-kB activation is indeed a protective mechanism. Although many different avenues have been investigated the main conclusions that can be drawn from these studies are that TNF can regulate NF-kB via TNFR1 and TNFR2 and that competent NF-kB activation is required to protect cancer cells from TNF induced death. Perhaps if it was possible to inhibit NF-kB then TNF could be used effectively as a cancer killing agent.