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Altered signaling of TNFalpha-TNFR1 and SODD/BAG4 is responsible for radioresistance in human HT-R15 cells.

Radioresistance in pre-irradiated human HT-R15 cells is associated with changes of the TNFR1-dependent death pathway. HT-R15 cells are characterized by elevated protein levels of TNFR1 and TNF and by increased sensitivity to exogenous TNFalpha compared to their parental HT-29 cells. Alterations are also observed downstream in the signaling cascade, such as the activation of NF-kappaB or the overexpression of the death domain kinase RIP and reduced caspase 8 activity. However, these changes seem to be a consequence of defective upstream TNFalpha signaling rather than the primary cause of cellular resistance in HT-R15 cells. Of major importance for resistance in HT-R15 cells is the silencer of death domain, SODD/BAG4. Following gamma-irradiation, the membrane-associated 49 kDa SODD decreases in the parental but not in the resistant cells, whereas after TNFalpha treatment, SODD expression declines only in the resistant cells. A 42 kDa cytoplasmic SODD protein is detected, which is elevated only in the resistant cells. This SODD protein is not involved in the regulation of cell survival after radiation or TNFalpha treatment but rather in altered TNFR1 shedding. Inhibition of TNFR1 release by the metalloprotease inhibitor BB-2516 results in a significant increase of the 42 kDa SODD protein without affecting cell survival in sensitive or resistant HT cells. Moreover, TNFR1 release into the culture medium is augmented in the resistant cells. These results suggest that defective TNFalpha signaling and/or altered silencing by SODD/BAG4 in HT-R15 cells are involved in the radiation resistance of HT-R15 cells and also affect the paracrine functions of TNFR1. Resistance is circumvented by TNFalpha treatment, independent of cytoplasmic TNFalpha/TNFR1 functions.