Key Concepts:

Figure 1. Space-filling model of the homotrimeric tumour necrosis factor (TNF). The separate subunits are depicted in blue, light red and brown. Additionally, the marked spheres belong to the side-chain atoms of amino acids which are critical for TNF-receptor binding in the binding cleft which is formed by two of the three subunits: the residues shown in red (Arg32) and light blue (Ser86) are essential for binding to TNFR2, whereas the residues shown in yellow (Asp143) and green (Ala145) are essential for binding to TNFR1. The upper image shows the top view of the molecule (facing towards the N- and C-termini) and the lower image is flipped by 90° along the horizontal axis.

Figure 2. Biological activities of tumour necrosis factor (TNF) on different cells and tissues. Almost all nucleated cell types in the body express TNF receptors and are thus potentially reactive to TNF. Among the countless effects described for TNF, the most important is its role as a central regulator of inflammation and immunity. Monocytes and macrophages are the main producers of TNF, although numerous cell types can express TNF in response to diverse stimuli. Abbreviations: IL, interleukin; IFN, interferon; GM-CSF, granulocyte–macrophage colony-stimulating factor; PGE, prostaglandin E; MHC, major histocompatibility complex.

Figure 3. Schematic representation of the major signalling pathways induced by TNFR1. The cellular mechanisms of TNFR1 signalling are an area of intense research and the presented scheme, therefore, covers only the best-described signalling pathways. Initiated by binding of TNF (shown in dark green) to TNFR1 (extracellular four CRDs blue colour and intracellular DD orange colour) an initial signalling complex (complex I) is formed at the plasma membrane, leading to the activation of diverse MAPKKKs (ASK1, MEKK1 and MEKK3; for details see text). Further downstream kinases of the MAPK cascade become activated, finally resulting in the activation of transcription factors (c-Jun, ATF2 and CHOB). cIAP-mediated ubiquitylation of RIP1 leads to recruitment of the TAK1–TAB2–TAB3 complex resulting in activation of the IKK complex possibly in cooperation with MEKK3. Activated IKK complex leads to degradation of the inhibitor of the transcription factor NF-B that translocates into the nucleus to induce gene transcription of several antiapoptotic, proliferative and inflammatory genes. Signalling pathways requiring TRAF2 are indicated by red arrows and those dependent on RIP1 are shown in blue arrows. Formation of the secondary signalling complex (complex II) results from clustering, deubiquitylation events, internalisation and change of interaction partners, like the recruitment of FADD and caspases 8 and/or 10 and also RIP3. On sufficient initiator caspase activity, effector caspases can be activated by two pathways either directly by active caspase 8/10 or indirectly by the mitochondrion, that is, the apoptotic programme is executed. The mitochondrial pathway can be initiated/enhanced by proteolytically processed forms of the proapoptotic Bcl-2 family member Bid (tBid and jBid). The subsequent release of the mitochondrial proapoptotic factor second mitochondria-derived activator of caspase (Smac, also known as DIABLO) counteracts caspase inhibitors from the IAP family. Likewise, released cytochrome c leads to the formation of a multiprotein complex called the ‘apoptosome’. Finally, effector caspases accomplish the apoptotic destruction of the cell. On effective blockade of initiator caspases, for example, by FLIP_L, the alternative necroptotic programme is started by RIP1/RIP3 kinase activities within complex II. This leads to the mitochondrion via MLKL and PGAM5 and finally results in programmed necrosis using sparsely defined pathways likely including ROS (reactive oxygen species) formation and JNK activation. The exact composition of both complex I and II is not fully defined or may vary. For more details see text. Apoptotic pathways are highlighted in green and necroptotic pathways in blue.

Figure 4. Schematic representation of signalling pathways induced by TNFR2. On binding of membrane-bound TNF (mTNF), TRAF2 and cIAPs are recruited to the cytoplasmic domain of TNFR2. This can result in the activation of the IKK complex consisting of IKK, and and induction of the canonical NF-B pathway resulting in the nuclear translocation of the p50/relA dimer. Alternatively, only IKK is activated resulting in activation of the noncanonical NF-B pathway, which leads to the nuclear translocation of the p52/relB dimer. Although not formally shown for TNFR2, it is likely that the NF-B-inducing kinase (NIK) is involved in activation of the noncanonical NF-B pathway as it has been shown that other members of the TNFSF, such as BAFF and CD40L, use this pathway. In addition, TNFR2 can activate the JNK pathway. In contrast to TNFR1, little is known about the molecular mechanisms of how TNFR2 stimulation activates JNK or IKKs. Using a so far undefined mechanism, TNFR2 stimulation can further activate phosphatidylinositol 3 kinase (PI3 K) and thus initiate the conversion of phosphatidylinositol 2-phosphate (PIP2) to PIP3 in the plasma membrane. This results in the recruitment of PKB/Akt to the membrane followed by its phosphorylation by PDK1 and mTOR. Phosphorylated PKB/Akt can phosphorylate and thus activate IKK, providing a possible link to the induction of the canonical and/or noncanonical NF-B signalling pathway by TNFR2. Because signalling via both TNFR1 (complex I) and TNFR2 requires recruitment of TRAF2 and cIAPs into the receptor complex (red arrows), activation of either receptor can interfere with the signalling of the other receptor. Moreover, because activation of TNFR2 is followed by the degradation of TRAF2, this limits the availability of this signal transducer for TNFR1 signalling via complex I and therefore may favour the formation of TNFR1 complex II and thus the induction of apoptosis or necroptosis.