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Nuclear factor kB-independent cytoprotective pathways originating at tumor necrosis factor receptor-associated factor
G Natoli1, A Costanzo, F Guido
1Fondazione Andrea Cesalpino, Policlinico Umberto I, University of Rome La Sapienza, Viale del Policlinico 155, 00161 Rome, Italy.
Abstract:
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 resistance to TNF requires TNF receptor-associated factor 2 (TRAF2), which has been hypothesized to act mainly by mediating activation of the transcription factors nuclear factor kB (NFkB) and activator protein 1 (AP1). NFkB was proposed to switch on transcription of yet unidentified anti-apoptotic genes. To test the possible existence of NFkB-independent cytoprotective pathways, we systematically compared selective trans-dominant inhibitors of the NFkB pathway with 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 receptor 1 complex completely abrogated the cytoprotective response. Conversely, sensitization to TNF cytotoxicity induced by a selective NFkB blockade affected only a fraction of TNF-treated cells in an apparently stochastic manner. No cytoprotective role for c-Jun amino-terminal kinases/stress-activated protein kinases (JNKs/SAPKs), which are activated by TRAF2 and contribute to stimulation of activator protein 1 activity, could be demonstrated in the cellular systems tested. Although required for cytoprotection, TRAF2 is not sufficient to protect cells from TNF + cycloheximide cytotoxicity when overexpressed in transfected cells, thus indicating an essential role of additional TNF receptor 1 complex components in the cytoprotective response. Our results indicate that TNF-induced cytoprotection is a complex function requiring the integration of multiple signal transduction pathways.
Insights
Tumor necrosis factor (TNF) resistance in cells requires TNF receptor-associated factor 2 (TRAF2). NFkB-independent pathways are not the main mechanism for TNF cytoprotection.
Area of Science:
- Cellular biology
- Immunology
- Molecular signaling
Background:
- Most cells resist tumor necrosis factor (TNF) cytotoxicity without protein/RNA synthesis inhibitors.
- Cellular resistance to TNF involves TNF receptor-associated factor 2 (TRAF2), linked to nuclear factor kB (NFkB) and activator protein 1 (AP1) activation.
- NFkB was hypothesized to induce anti-apoptotic genes, but NFkB-independent pathways' roles are unclear.
Purpose of the Study:
- To investigate NFkB-independent cytoprotective pathways against TNF cytotoxicity.
- To compare the effects of NFkB pathway inhibitors versus TRAF2 signaling inhibitors on TNF cytotoxicity.
- To elucidate the precise role of TRAF2 and other TNF receptor 1 complex components in cellular protection.
Main Methods:
- Systematic comparison of selective trans-dominant NFkB pathway inhibitors with TRAF2 signaling inhibitors.
- Assessment of TNF cytotoxicity in cells treated with various signaling pathway modulators.
- Analysis of TRAF2 function using signaling-deficient oligomerization partners and molecules affecting TRAF2 recruitment.
- Overexpression of TRAF2 in transfected cells to evaluate its sufficiency for cytoprotection.
Main Results:
- Complete abrogation of TNF cytoprotection upon blockade of TRAF2 function or its recruitment to the TNF receptor 1 complex.
- Selective NFkB blockade sensitized only a fraction of TNF-treated cells, suggesting a stochastic effect.
- No cytoprotective role was found for c-Jun amino-terminal kinases/stress-activated protein kinases (JNKs/SAPKs).
- Overexpressed TRAF2 alone was insufficient for cytoprotection, highlighting the necessity of other TNF receptor 1 complex components.
Conclusions:
- TNF-induced cytoprotection is a complex process, not solely reliant on NFkB activation.
- TRAF2 is essential but not sufficient for TNF cytoprotection, indicating the involvement of multiple signaling pathways.
- The integration of multiple signal transduction pathways is critical for TNF-induced cytoprotection.