RIP3, an Energy Metabolism Regulator

That Switches TNF-Induced Cell Death from Apoptosis to Necrosis describes steps that led to the discovery of protein kinase receptor-interacting protein 3 (RIP3), a molecular switch that changes TNF-induced cell death from Apoptosis to Necrosis. Necrosis is a caspase independent cell death induced in the presence of caspase inhibitor zVAD and simulation of TNF. Tumor necrosis factor (TNF) is an inflammatory cytokine that induces necrotic cell death as well as transcription factor activation and inflammatory and immunological responses.

It brings up necrotic cell death by increase Reactive Oxygen Species (ROS) accumulation.

Receptor-interacting protein 1 (RIP1) is a kinase required in both Apoptosis and Necrosis and is downstream of TNF. RIP3 is a kinase essential for Necrosis, that responds to TNF? death-inducing cytokines. RIP3 is recruited by RIP1 and the two form a necrosis-inducing complex. RIP3 stimulates the production of ROS, which is required for TNF-induced cell death. zVAD is a known caspase inhibitor. Modulation of energy metabolism also has an important role in choice of the pathways of apoptosis and necrosis. Discovery of RIP3 was the last piece to understanding the precise mechanism of necrosis. The study was conducted because the precise mechanism and key players of necrotic cell death were unknown at the time.

The regulator that enables the switch from TNF-induced apoptosis to TNF-induced necrosis was unknown. Knowing the exact mechanism and each component is necessary for understanding the process and development of potential new drug, that can be utilized in the treatment of diseases. The dependence of caspase was studied in NIH 3T3 cells that came from different sources but were derived from the same population – called A cells and N cells. zVAD inhibited the TNF-induced cell death in A cells but enhanced it in N cells. The difference could be explained by different gene expression. The expression of RIP3 (amongst other genes) was increased only in N cells. They increased expression of decreased genes and decreased expression of increased genes in N cells. They were able to inhibit cell death by zVAD in N cells with RIP3 knockdown. This discovery made them focus their research on RIP3.

They confirmed the RIP3 expression in both A and N cells by Western blots. They used shRNA or cDNA delivery by lentiviral vectors. The control was by targeting lacZ, which exhibited no effect on the type of cell death. The RIP3 expression in A cells switched the death type to necrosis. zVAD enhanced the cell death in this case. Not functional RIP3 mutant had no effect. zVAD is needed for the induction of cell death only in some cell lines. RIP3 always required for necrosis mediated by zVAD. zVAD cannot induce necroptosis in RIP3 deficient cells. RIP3 is therefore essential for some forms of necrosis. RIP1 is always required in the process of activation of necrosis. The RIP1 expression in A and N cells didn`t show any difference.

Knockdown of RIP1 in N cells inhibited necrosis and blocked the TNF-induced necrosis in L929 cells. Both RIP1 and RIP3 were found to be essential for necrosis. zVAD inhibited cell death in RIP3 knockdown cells only. They found out that MEF cell line is missing both RIP1 and RIP3 expression. RIP3 overexpression induced cell death in RIP1 deficient cells. Cell death in RIP1/RIP3 deficient cells was similar to RIP3 only deficient cells. RIP1 therefore induces apoptosis in the absence of RIP3. RIP1 overexpression induces apoptosis in cells with RIP3 overexpression. zVAD only induces cell dead in cells when RIP3 is present. They found targets of RIP3 by flagging RIP3 and treating cells with TNF and zVAD combination. They identified 7 metabolic enzymes in the RIP3 complex. Those enzymes are PYGL, GLUL, FBP2, FH, GLT25D1 and IDH1. PYGL, GLUL and GLUD1 were confirmed to interact with RIP3 in 293T cells.

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