NUCLEAR FACTOR KB-INDEPENDENT CYTOPROTECTIVE PATHWAYS ORIGINATING AT TUMOR-NECROSIS-FACTOR RECEPTOR-ASSOCIATED FACTOR-2

Most normal and neoplastic cell types are resistant to tumor necrosis factor TNF cytotoxicity unless cotreated with protein or RNA synthesis inhibitors, such as cycloheximide and actinomycin D. Cellular resista nce to TNF requires TNF receptor-associated factor 2 (TRAF2), which ha s been hypothesized to act mainly by mediating activation of the trans cription factors nuclear factor kappa B (NF kappa B) and activator pro tein 1 (AP1). NF kappa B was proposed to switch on transcription of ye t unidentified anti-apoptotic genes. To test the possible existence of NF kappa B-independent cytoprotective pathways, we systematically com pared selective trans-dominant inhibitors of the NF kappa B pathway wi th inhibitors of TRAF2 signaling for their effect on TNF cytotoxicity. Blockade of TRAF2 function(s) by signaling-deficient oligomerization partners or by molecules affecting TRAF2 recruitment to the TNF recept or 1 complex completely abrogated the cytoprotective response. Convers ely, sensitization to TNF cytotoxicity induced by a selective NF kappa B blockade affected only a fraction of TNF-treated cells in an appare ntly stochastic manner. No cytoprotective role for c-Jun amino-termina l kinases/stress-activated protein kinases (JNKs/SAPKs), which are act ivated by TRAF2 and contribute to stimulation of activator protein 1 a ctivity, could be demonstrated in the cellular systems tested. Althoug h required for cytoprotection, TRAF2 is not sufficient to protect cell s from TNF + cycloheximide cytotoxicity when overexpressed in transfec ted cells, thus indicating an essential role of additional TNF recepto r 1 complex components in the cytoprotective response. Our results ind icate that TNF-induced cytoprotection is a complex function requiring the integration of multiple signal transduction pathways.