Chapter Seven - The Inflammatory Signal Adaptor RIPK3: Functions Beyond Necroptosis
Introduction
In pathology, the term necrosis is used to describe gross histological damage caused by cell death. It is defined at the cellular and morphological level by cell and organelle swelling and membrane rupture. On the other hand, the term apoptosis describes cell death marked by cellular shrinkage, chromatin condensation, and cellular fragmentation (Kerr et al., 1972). Although the different cell death modes were originally defined by morphology, we now know that they also exhibit distinct biological functions and are regulated by unique mechanisms. Genetic studies in Caenorhabditis elegans laid the foundation for discovery of numerous apopotosis genes and the signaling network that regulates it. These discoveries unveiled the important physiological roles of apoptosis in embryonic development (Ellis and Horvitz, 1986), immune homeostasis (Burger et al., 2014), and cancer (Hockenbery et al., 1990, Tsujimoto et al., 1985). In contrast, since necrotic cell death is often observed when cells are exposed to excessive physical or chemical stresses, it was considered to be an unprogrammed and accidental cell death. However, accumulating evidence shows that necrosis can in fact be induced by dedicated regulatory signaling pathways and thus the long-standing dogma that necrosis represents unregulated cell death is being challenged.
Necroptosis is a type of regulated necrosis which is controlled by receptor interacting protein kinase 3 (RIPK3) and its downstream effector mixed lineage kinase domain-like (MLKL) (Chan et al., 2014). Upon ligand binding, a variety of cell surface receptors, such as tumor necrosis factor (TNF) superfamily death receptors (Vercammen et al., 1998a, Vercammen et al., 1998b), toll-like receptors (TLRs) (He et al., 2011), interferon receptors (IFNRs) (Thapa et al., 2011, Thapa et al., 2013), and T-cell receptor (Ch’en et al., 2011, Lu et al., 2011, Osborn et al., 2010, Zhang et al., 2011), induce necroptosis through phosphorylation-driven activation of the RIPK3-MLKL signaling pathway. Germline Ripk3-deficient (Ripk3−/−) mice are widely used to examine the physiological functions of necroptosis. Many infectious and noninfectious inflammatory disease models were attenuated in Ripk3−/− mice (Chan et al., 2014). These observations bolster the premise that RIPK3 promotes the release of intracellular immunogenic contents through necroptosis to elicit inflammatory responses (Kaczmarek et al., 2013). However, recent evidence indicates that RIPK3 also exhibits necroptosis-independent functions and that the amelioration of inflammatory phenotypes in Ripk3−/− mice could at least in part be attributed to these nonnecroptotic signaling functions (Moriwaki and Chan, 2014). In this review, we summarize our current knowledge of how RIPK3 executes necrotic and nonnecrotic functions. We will also discuss how these distinct functions of RIPK3 cooperate to promote inflammation in physiology.
Section snippets
Phosphorylation-Driven Activation of RIPK3
RIPK3 is a cytosolic serine/threonine kinase that consists of an active kinase domain at the amino terminus (Moriwaki and Chan, 2013). Essential amino acids for enzymatic activity of typical protein kinases are conserved in RIPK3, including the catalytic triad (Lys50, Glu60, and Asp160 in human RIPK3) and the DFG motif (Asp160, Phe161, and Gly162 in human RIPK3). RIPK3 also carries a unique homotypic protein–protein interaction domain, called RIP homotypic interaction motif (RHIM), at the
Endogenous Necroptosis Inhibitory Proteins
Necrotic cell death is a highly inflammatory type of cell death due to the release of intracellular immunogenic content that stimulates innate immune cells and subsequently inflammation. As such, there are cellular mechanisms put in place to keep in check the potential deleterious effects of necroptosis. For instance, the initiator caspase, caspase 8, not only induces apoptosis, but also has a critical role in necroptosis inhibition by cleavage of the crucial necroptosis regulators RIPK1,
How Does RIPK3 Promote Inflammation?
The germline Ripk3−/− mice generated by Newton and Dixit have been instrumental in the study of physiological functions of RIPK3 and necroptosis over the years (Newton et al., 2004). Ripk3−/− mice exhibited reduced inflammatory phenotypes in viral infection models as well as sterile inflammatory diseases in the kidney (Linkermann et al., 2013), heart (Luedde et al., 2014, Zhang et al., 2016), blood vessel (Lin et al., 2013, Meng et al., 2015), pancreas (He et al., 2009), brain (Vitner et al.,
RIPK3 in NF-κB Activation
RIPK3 was originally identified as an RIPK1 binding protein with homology to RIPK1 and RIPK2, both of which were known NF-κB inducers (Sun et al., 1999, Yu et al., 1999). Similar to these other RIP kinase family members, early studies showed that overexpression of RIPK3 also alters NF-κB activation (Kasof et al., 2000, Meylan et al., 2004, Sun et al., 1999, Yu et al., 1999). However, the results were confusing since different studies have shown activating as well as inhibitory effects on NF-κB
Concluding Remarks
Genetic evidence has provided strong rationale that RIPK3 is a therapeutic target in treating various inflammatory diseases. Indeed, several types of RIPK3 inhibitors targeting its kinase activity have been reported in recent years (Fauster et al., 2015, Mandal et al., 2014, Najjar et al., 2015, Rodriguez et al., 2016). However, as we have discussed here, RIPK3 can promote inflammation independent of its kinase activity and necroptosis. Therefore, it will be important to determine which RIPK3
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2022, GastroenterologyCitation Excerpt :Collectively, our work supports the notion that hepatocytes from both mice and humans transcriptionally repress RIPK3 and prevent necroptosis. In addition to its essential role in necroptosis, RIPK3 has more recently been implicated in inflammation signaling.46,47 This has made us revisit many studies that used RIPK3-deficient mice to implicate necroptosis in disease processes.
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2022, Journal of Biological ChemistryCitation Excerpt :It is important to consider nocodazole-challenged cells with enhanced p-MLKL is likely not necroptotic death but rather an accumulation of intracellular p-MLKL that fails to translocate from the nucleus to other cellular compartments over time due to nocodazole-altered microtubule dynamic instability (38, 47). Notably, staurosporine inhibited p-MLKL signal at 4 and 24 h with signal recovery by 48 h (Fig. 3D), which may be the result of cross talk between apoptotic and necroptotic signaling pathways (48, 49). Jurkat and MV-4-11 were more sensitive to induction of apoptotic effector cCAS3 than necroptotic effector p-MLKL (Fig. S5) and reflect cell type–specific responses with respect to leukemia subtypes.