Summary of Pathway: 

 It is essential for a cell to regulate the way that it responds to external stress and stimuli. Damaged cells may respond to their environment in a multitude of ways, and when their vital functions have been substantially affected, one of these possible responses is the initiation of self-mediated cell death. This process is the result of a signaling cascade that begins when a small ligand called TNF (tumor necrosis factor) binds to its transmembrane receptor on a damaged cell’s plasma membrane. The activated receptor complex then recruits additional molecules within the cell to its intracellular domain, including RIPK1 and TRADD. RIPK1 is an essential protein in the pathway because it begins the process of determining whether the cell will undergo necroptosis (a form of cell death that does not involve caspases) or apoptosis (conventional programmed cell death). There are 3 other primary checkpoints in the pathway that are dependent on phosphorylation of downstream proteins, the ubiquitination of complex II, (which is activated by RIPK1), and a transcriptional checkpoint (where the level of transcription of “prosurvival genes” determines whether the cell death process will be inhibited or not). If the pathway continues, the levels of RIPK3 (which promotes necroptosis) and caspase 8 (necessary for apoptosis) control which death process will take place. Overall, the TNF pathway is capable of inducing cell death via these two methods, and creates an inflammatory response in the cell by upregulating a variety of other cytokines (proinflammatory molecules).

Key Functions of TNF Pathway

1. Stimulation of Proinflammatory Response:

Although TNF signaling plays an essential role in initiating programmed cell death, one of its primary functions is to produce an inflammatory response in damaged cells. This occurs because the pathway causes the activation of the transcription factor NF-kb, which upregulates transcription of genes promoting cell survival and inflammatory responses to cell stress. Therefore, in cells where high levels of cell survival factors prevent programmed cell death from occurring, a severe inflammatory reaction may occur. 

  Induction of Necroptosis or Apoptosis: 

The other significant outcome of the TNF pathway is cell death. The pathway can initiate either apoptosis (using the protein caspase 8) or necroptosis (a programmed form of necrosis, inflammatory cell death). The signaling molecule RIPK1 (Receptor-interacting serine/threonine-protein kinase 1) is essential in determining whether any type of cell death will occur, as activated RIPK1 is necessary for the activation of caspase 8 (for apoptosis) and RIPK3 (for necroptosis). The collection of proteins responsible for triggering either apoptosis or necroptosis is called “complex II”, and becomes active when activated RIPK1 (which is recruited after TNF binds to its transmembrane receptor), and it is present in the cytosol of all cells. This entire complex is regulated by the transcriptional checkpoint of the TNF pathway because signaling cascade can increase the transcription of survival factors (such as cFLIP). These molecules can inhibit both apoptosis and necroptosis, effectively stopping the pathway from causing cell death.  

Key Molecules Involved in TNF Pathway

1. TNF and TNF-R1 -  TNF (tumor necrosis factor) is extremely important because it is the signal that initiates the entire pathway. It binds to TNF receptor 1 (TNF-R1) and causes the intracellular portion of the receptor to trimerize. This provides binding sites for downstream molecules in the pathway to bind. Once these additional molecules have bound, the entire structure is known as complex I.
2. RIPK1 - This molecule is recruited to the intracellular portion of the activated TNF receptor (TNF-R1). It plays several key roles in the pathway, including activating complex II molecules in order for apoptosis and/or necroptosis to occur. However, through ubiquitination and phosphorylation RIPK1 is also responsible for continuing the signaling cascade that involves MAPK and eventually activates the transcription factor NF-kb. As mentioned previously, this factor increases transcription of survival factors that actually inhibit apoptosis and necroptosis. As a result, RIPK1 can be involved in both triggering cell death and preventing it in the TNF pathway. 
3. NF-kb - This is a transcription factor that promotes the transcription of anti-apoptotic (and anti-necroptic) survival factors (like cFLIP). 
4. Caspase 8 and RIPK3 - these proteins promote the onset of apoptosis (Caspase 8) and necroptosis (RIPK3). Their relative levels after the activation of RIPK1 determines which process will occur. 
5. Cellular FLICE (FADD-like IL-1β-converting enzyme)-inhibitory protein (cFLIP) - This is a survival factor that prevents the TNF pathway from proceeding with cell death. It inhibits apoptosis by binding and inactivating caspase 8 and eventually halts necroptosis in the cell as well. 

 The TNF Pathway and Human Diseases

 Tumor necrosis factor was discovered because the signaling pathway it initiated was capable of inducing cell death in tumors. However, more recent research has shown that the pathway  is highly active in conditions that resulted in severe inflammatory reactions in patients. Examples are inflammatory bowel disease (IBS), psoriasis, ankylosing spondylitis, and rheumatoid arthritis. The involvement of the pathway was confirmed when inflammation symptoms were reduced after patients received an NF-kb (a downstream transcription factor in the pathway) inhibiting agent. Therefore, although the pathway is capable of causing cell death through apoptosis and necroptosis, it is commonly seen as an activator of inflammatory molecules (such as cytokines) in human diseases. However, there have also been reports that downregulating TNF activity also increases the risk of infection in these patients, suggesting that the cell death pathway may be related to antibacterial activities as well.

 Latest Progress in TNF Pathway Research:

 The importance of the TNF pathway in maintaining tissue homeostasis has recently been discovered. This implies that both the initiation of inflammatory responses and the induction of cell death do not occur in isolation - the processes that occur in one cell affect the surround tissue as well. There are several questions that remain unanswered with regards to these cell-cell interactions. One concept to be investigated is the threshold between cell survival and death. What conditions within the cell increase or decrease the levels of survival factors that are transcribed under the control of NF-kb How does this affect the extent to which apoptosis and necroptosis are inhibited? Answering these questions will help us understand how the tissues in a cell work collectively to effectively minimize the damage from external/environmental stress.

References: 

1. Annibaldi, Alessandro, and Pascal Meier. “Checkpoints in TNF-Induced Cell Death: Implications in Inflammation and Cancer.” Trends in Molecular Medicine, vol. 24, no. 1, 2018, pp. 49–65., doi:10.1016/j.molmed.2017.11.002.
2. Bradley, Jr. “TNF-Mediated Inflammatory Disease.” The Journal of Pathology, vol. 214, no. 2, 2008, pp. 149–160., doi:10.1002/path.2287.
3. Linkermann, Andreas, and Douglas R. Green. “Necroptosis.” The New England journal of medicine 370.5 (2014): 455–465. PMC. Web. 14 Sept. 2018.
4. Zhang, Jianke et al. “RIP1-Mediated Regulation of Lymphocyte Survival and Death Responses.” Immunologic research 51.2-3 (2011): 227–236. PMC. Web. 14 Sept. 2018.