Fibroblast growth factor 18 stimulates the proliferation of hepatic stellate cells, thereby inducing liver fibrosis.

Liver fibrosis results from chronic liver injury triggered by factors such as viral infection, excess alcohol intake, and lipid accumulation. However, the mechanisms underlying liver fibrosis are not fully understood. Here, we demonstrate that the expression of fibroblast growth factor 18 (Fgf18) is elevated in mouse livers following the induction of chronic liver fibrosis models. Deletion of Fgf18 in hepatocytes attenuates liver fibrosis; conversely, overexpression of Fgf18 promotes liver fibrosis. Single-cell RNA sequencing reveals that overexpression of Fgf18 in hepatocytes results in an increase in the number of Lrat+ hepatic stellate cells (HSCs), thereby inducing fibrosis. Mechanistically, FGF18 stimulates the proliferation of HSCs by inducing the expression of Ccnd1. Moreover, the expression of FGF18 is correlated with the expression of profibrotic genes, such as COL1A1 and ACTA2, in human liver biopsy samples. Thus, FGF18 promotes liver fibrosis and could serve as a therapeutic target to treat liver fibrosis.

A high-sensitivity ELISA for detection of human FGF18 in culture supernatants from tumor cell lines.

Fibroblast growth factor 18 (FGF18) is elevated in several human cancers, such as gastrointestinal and ovarian cancers, and stimulates the proliferation of tumor cells. This suggests that FGF18 may be a promising candidate biomarker in cancer patients. However, the lack of a high-sensitivity enzyme-linked immunosorbent assay (ELISA) does not permit testing of this possibility. In this study, we generated monoclonal antibodies against human FGF18 and developed a high-sensitivity ELISA to measure human FGF18 at concentrations as low as 10 pg/mL. Of the eight tumor cell lines investigated, we detected human FGF18 in culture supernatants from four tumor cell lines, including HeLa, OVCAR-3, BxPC-3, and SW620 cells, albeit the production levels were relatively low in the latter two cell lines. Moreover, the in-house ELISA could detect murine FGF18 in sera from mice overexpressing murine Fgf18 in hepatocytes, although the sensitivity in detecting murine FGF18 was relatively low. This FGF18 ELISA could be a valuable tool to validate FGF18 as a potential biomarker for cancer patients and to test the contribution of FGF18 for various disease models invivo and in vitro.

Generation of transgenic mice expressing a FRET biosensor, SMART, that responds to necroptosis.

Necroptosis is a regulated form of cell death involved in various pathological conditions, including ischemic reperfusion injuries, virus infections, and drug-induced tissue injuries. However, it is not fully understood when and where necroptosis occurs in vivo. We previously generated a Forster resonance energy transfer (FRET) biosensor, termed SMART (the sensor for MLKL activation by RIPK3 based on FRET), which monitors conformational changes of MLKL along with progression of necroptosis in human and murine cell lines in vitro. Here, we generate transgenic (Tg) mice that express the SMART biosensor in various tissues. The FRET ratio is increased in necroptosis, but not apoptosis or pyroptosis, in primary cells. Moreover, the FRET signals are elevated in renal tubular cells of cisplatin-treated SMART Tg mice compared to untreated SMART Tg mice. Together, SMART Tg mice may provide a valuable tool for monitoring necroptosis in different types of cells in vitro and in vivo.

Lewis glycosphingolipids as critical determinants of TRAIL sensitivity in cancer cells.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces cancer cell death and contributes to tumor rejection by cytotoxic lymphocytes in cancer immunosurveillance and immunotherapy. TRAIL and TRAIL receptor agonists have garnered wide popularity as promising agents for cancer therapy. We previously demonstrated that the loss of fucosylation in cancer cells impairs TRAIL sensitivity; however, the precise structures of the fucosylated glycans that regulate TRAIL sensitivity and their carrier molecules remain elusive. Herein, we observed that Lewis glycans among various fucosylated glycans positively regulate TRAIL-induced cell death. Specifically, Lewis glycans on lacto/neolacto glycosphingolipids, but not glycoproteins including TRAIL receptors, enhanced TRAIL-induced formation of the cytosolic caspase 8 complex, without affecting the formation of the membranous receptor complex. Furthermore, type I Lewis glycan expression in colon cancer cell lines and patient-derived cancer organoids was positively correlated with TRAIL sensitivity. These findings provide novel insights into the regulatory mechanism of TRAIL-induced cell death and facilitate the identification of novel predictive biomarkers for TRAIL-related cancer therapies in future.

Proscillaridin A Sensitizes Human Colon Cancer Cells to TRAIL-Induced Cell Death.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a cytotoxic cytokine that induces cancer cell death by binding to TRAIL receptors. Because of its selective cytotoxicity toward cancer cells, TRAIL therapeutics, such as recombinant TRAIL and agonistic antibodies targeting TRAIL receptors, have garnered attention as promising cancer treatment agents. However, many cancer cells acquire resistance to TRAIL-induced cell death. To overcome this issue, we searched for agents to sensitize cancer cells to TRAIL-induced cell death by screening a small-molecule chemical library consisting of diverse compounds. We identified a cardiac glycoside, proscillaridin A, as the most effective TRAIL sensitizer in colon cancer cells. Proscillaridin A synergistically enhanced TRAIL-induced cell death in TRAIL-sensitive and -resistant colon cancer cells. Additionally, proscillaridin A enhanced cell death in cells treated with TRAIL and TRAIL sensitizer, the second mitochondria-derived activator of caspase mimetic. Proscillaridin A upregulated TRAIL receptor expression, while downregulating the levels of the anti-cell death molecules, cellular FADD-like IL-1ß converting enzyme-like inhibitor protein and Mcl1, in a cell type-dependent manner. Furthermore, proscillaridin A enhanced TRAIL-induced cell death partly via O-glycosylation. Taken together, our findings suggest that proscillaridin A is a promising agent that enhances the anti-cancer efficacy of TRAIL therapeutics.

The scaffold-dependent function of RIPK1 in dendritic cells promotes injury-induced colitis.

Receptor interacting protein kinase 1 (RIPK1) is a cytosolic multidomain protein that controls cell life and death. While RIPK1 promotes cell death through its kinase activity, it also functions as a scaffold protein to promote cell survival by inhibiting FADD-caspase 8-dependent apoptosis and RIPK3-MLKL-dependent necroptosis. This pro-survival function is highlighted by excess cell death and perinatal lethality in Ripk1-/- mice. Recently, loss of function mutation of RIPK1 was found in patients with immunodeficiency and inflammatory bowel diseases. Hematopoietic stem cell transplantation restored not only immunodeficiency but also intestinal inflammatory pathology, indicating that RIPK1 in hematopoietic cells is critical to maintain intestinal immune homeostasis. Here, we generated dendritic cell (DC)-specific Ripk1-/- mice in a genetic background with loss of RIPK1 kinase activity and found that the mice developed spontaneous colonic inflammation characterized by increased neutrophil and Ly6C+ monocytes. In addition, these mice were highly resistant to injury-induced colitis. The increased colonic inflammation and the resistance to colitis were restored by dual inactivation of RIPK3 and FADD, but not by inhibition of RIPK3, MLKL, or ZBP1 alone. Altogether, these results reveal a scaffold activity-dependent role of RIPK1 in DC-mediated maintenance of colonic immune homeostasis.

MIND bomb 2 prevents RIPK1 kinase activity-dependent and -independent apoptosis through ubiquitylation of cFLIPL.

Mind bomb 2 (MIB2) is an E3 ligase involved in Notch signalling and attenuates TNF-induced apoptosis through ubiquitylation of receptor-interacting protein kinase 1 (RIPK1) and cylindromatosis. Here we show that MIB2 bound and conjugated K48- and K63-linked polyubiquitin chains to a long-form of cellular FLICE-inhibitory protein (cFLIPL), a catalytically inactive homologue of caspase 8. Deletion of MIB2 did not impair the TNF-induced complex I formation that mediates NF-κB activation but significantly enhanced formation of cytosolic death-inducing signalling complex II. TNF-induced RIPK1 Ser166 phosphorylation, a hallmark of RIPK1 death-inducing activity, was enhanced in MIB2 knockout cells, as was RIPK1 kinase activity-dependent and -independent apoptosis. Moreover, RIPK1 kinase activity-independent apoptosis was induced in cells expressing cFLIPL mutants lacking MIB2-dependent ubiquitylation. Together, these results suggest that MIB2 suppresses both RIPK1 kinase activity-dependent and -independent apoptosis, through suppression of RIPK1 kinase activity and ubiquitylation of cFLIPL, respectively.

Regenerating islet-derived protein (Reg)3ß plays a crucial role in attenuation of ileitis and colitis in mice.

Regenerating islet-derived protein (Reg)3ß belongs to a member of the Reg family of proteins and has pleiotropic functions, including antimicrobial activity and tissue repair. However, whether Reg3ß plays a protective role in the development of colitis and ileitis has not been fully investigated. We generated transgenic mice expressing a short form of cellular FLICE-inhibitory protein (cFLIPs) that promotes necroptosis, a regulated form of cell death. cFLIPs transgenic (CFLARs Tg) mice develop severe ileitis in utero. Although Reg3ß is undetectable in the small intestine of wild-type embryos, its expression is aberrantly elevated in the small intestine of CFLARs Tg embryos. To test whether elevated Reg3ß attenuates or exacerbates ileitis in CFLARs Tg mice, we generated a Reg3b -/- strain. Reg3b -/- mice grew to adulthood without apparent abnormalities. Deletion of Reg3b in CFLARs Tg mice exacerbated the embryonic lethality of CFLARs Tg mice. Dextran sulfate sodium-induced colitis, characterized by body weight loss and infiltration of neutrophils, was exacerbated in Reg3b -/- compared to wild-type mice. Moreover, the expression of Interleukin 6, an inflammatory cytokine and Chitinase-like 3, a marker for tissue repair macrophages was elevated in the colon of Reg3b -/- mice compared to wild-type mice after DSS treatment. Together, these results suggest that attenuation of colitis and ileitis is a result of Reg3ß's real function.

Necroptosis of Intestinal Epithelial Cells Induces Type 3 Innate Lymphoid Cell-Dependent Lethal Ileitis.

A short form of cellular FLICE-inhibitory protein encoded by CFLARs promotes necroptosis. Although necroptosis is involved in various pathological conditions, the detailed mechanisms are not fully understood. Here we generated transgenic mice wherein CFLARs was integrated onto the X chromosome. All male CFLARs Tg mice died perinatally due to severe ileitis. Although necroptosis was observed in various tissues of CFLARs Tg mice, large numbers of intestinal epithelial cells (IECs) died by apoptosis. Deletion of Ripk3 or Mlkl, essential genes of necroptosis, prevented both necroptosis and apoptosis, and rescued lethality of CFLARs Tg mice. Type 3 innate lymphoid cells (ILC3s) were activated and recruited to the small intestine along with upregulation of interleukin-22 (Il22) in CFLARs Tg mice. Deletion of ILC3s or Il22 rescued lethality of CFLARs Tg mice by preventing apoptosis, but not necroptosis of IECs. Together, necroptosis-dependent activation of ILC3s induces lethal ileitis in an IL-22-dependent manner.

Blockade of TNF receptor superfamily 1 (TNFR1)-dependent and TNFR1-independent cell death is crucial for normal epidermal differentiation.

BACKGROUND: A delicate balance between cell death and keratinocyte proliferation is crucial for normal skin development. Previous studies have reported that cellular FLICE (FADD-like ICE)-inhibitory protein plays a crucial role in prevention of keratinocytes from TNF-α-dependent apoptosis and blocking of dermatitis. However, a role for cellular FLICE-inhibitory protein in TNF-α-independent cell death remains unclear. OBJECTIVE: We investigated contribution of TNF-α-dependent and TNF-α-independent signals to the development of dermatitis in epidermis-specific Cflar-deficient (CflarE-KO) mice. METHODS: We examined the histology and expression of epidermal differentiation markers and inflammatory cytokines in the skin of CflarE-KO;Tnfrsf1a+/- and CflarE-KO;Tnfrsf1a-/- mice. Mice were treated with neutralizing antibodies against Fas ligand and TNF-related apoptosis-inducing ligand to block TNF-α-independent cell death of CflarE-KO;Tnfrsf1a-/- mice. RESULTS: CflarE-KO;Tnfrsf1a-/- mice were born but experienced severe dermatitis and succumbed soon after birth. CflarE-KO;Tnfrsf1a+/- mice exhibited embryonic lethality caused by massive keratinocyte apoptosis. Although keratinocytes from CflarE-KO;Tnfrsf1a-/- mice still died of apoptosis, neutralizing antibodies against Fas ligand and TNF-related apoptosis-inducing ligand substantially prolonged survival of CflarE-KO;Tnfrsf1a-/- mice. Expression of inflammatory cytokines, such as Il6 and Il17a was increased; conversely, expression of epidermal differentiation markers was severely downregulated in the skin of CflarE-KO;Tnfrsf1a-/- mice. Treatment of primary keratinocytes with IL-6 and, to a lesser extent, IL-17A suppressed expression of epidermal differentiation markers. CONCLUSION: TNF receptor superfamily 1 (TNFR1)-dependent or TNFR1-independent apoptosis of keratinocytes promotes inflammatory cytokine production, which subsequently blocks epidermal differentiation. Thus blockade of both TNFR1-dependent and TNFR1-independent cell death might be an alternative strategy to treat skin diseases when treatment with anti-TNF-α antibody alone is not sufficient.

Cellular FLICE-Inhibitory Protein Regulates Tissue Homeostasis.

Cellular FLICE-inhibitory protein (cFLIP) is structurally related to caspase-8, but lacks its protease activity. Cflip gene encodes several splicing variants including short form (cFLIPs) and long form (cFLIPL). cFLIPL is composed of two death effector domains at the N terminus and a C-terminal caspase-like domain, and cFLIPs lacks the caspase-like domain. Our studies reveal that cFLIP plays a central role in NF-κB-dependent survival signals that control apoptosis and programmed necrosis. Germline deletion of Cflip results in embryonic lethality due to enhanced apoptosis and programmed necrosis; however, the combined deletion of the death-signaling regulators, Fadd and Ripk3, prevents embryonic lethality in Cflip-deficient mice. Moreover, tissue-specific deletion of Cflip reveals cFLIP as a crucial regulator that maintains tissue homeostasis of immune cells, hepatocytes, intestinal epithelial cells, and epidermal cells by preventing apoptosis and programmed necrosis.

Depletion of myeloid cells exacerbates hepatitis and induces an aberrant increase in histone H3 in mouse serum.

Tissue-resident macrophages and bone marrow (BM)-derived monocytes play a crucial role in the maintenance of tissue homeostasis; however, their contribution to recovery from acute tissue injury is not fully understood. To address this issue, we generated an acute murine liver injury model using hepatocyte-specific Cflar-deficient (CflarHep-low ) mice. Cellular FLICE-inhibitory protein expression was down-regulated in Cflar-deficient hepatocytes, which thereby increased susceptibility of hepatocytes to death receptor-induced apoptosis. CflarHep-low mice developed acute hepatitis and recovered with clearance of apoptotic hepatocytes at 24 hours after injection of low doses of tumor necrosis factor α (TNFα), which could not induce hepatitis in wild-type (WT) mice. Depletion of Kupffer cells (KCs) by clodronate liposomes did not impair clearance of dying hepatocytes or exacerbate hepatitis in CflarHep-low mice. To elucidate the roles of BM-derived monocytes and neutrophils in clearance of apoptotic hepatocytes, we examined the effect of depletion of these cells on TNFα-induced hepatitis in CflarHep-low mice. We reconstituted CflarHep-low mice with BM cells from transgenic mice in which human diphtheria toxin receptor (DTR) was expressed under control of the lysozyme M (LysM) promoter. TNFα-induced infiltration of myeloid cells, including monocytes and neutrophils, was completely ablated in LysM-DTR BM-reconstituted CflarHep-low mice pretreated with diphtheria toxin, whereas KCs remained present in the livers. Under these experimental conditions, LysM-DTR BM-reconstituted CflarHep-low mice rapidly developed severe hepatitis and succumbed within several hours of TNFα injection. We found that serum interleukin-6 (IL-6), TNFα, and histone H3 were aberrantly increased in LysM-DTR BM-reconstituted, but not in WT BM-reconstituted, CflarHep-low mice following TNFα injection. CONCLUSION: These findings indicate an unexpected role of myeloid cells in decreasing serum IL-6, TNFα, and histone H3 levels via the suppression of TNFα-induced hepatocyte apoptosis. (Hepatology 2017;65:237-252).

Novel method to rescue a lethal phenotype through integration of target gene onto the X-chromosome.

The loss-of-function mutations of serine protease inhibitor, Kazal type 1 (SPINK1) gene are associated with human chronic pancreatitis, but the underlying mechanisms remain unknown. We previously reported that mice lacking Spink3, the murine homologue of human SPINK1, die perinatally due to massive pancreatic acinar cell death, precluding investigation of the effects of SPINK1 deficiency. To circumvent perinatal lethality, we have developed a novel method to integrate human SPINK1 gene on the X chromosome using Cre-loxP technology and thus generated transgenic mice termed "X-SPINK1". Consistent with the fact that one of the two X chromosomes is randomly inactivated, X-SPINK1 mice exhibit mosaic pattern of SPINK1 expression. Crossing of X-SPINK1 mice with Spink3+/- mice rescued perinatal lethality, but the resulting Spink3-/-;XXSPINK1 mice developed spontaneous pancreatitis characterized by chronic inflammation and fibrosis. The results show that mice lacking a gene essential for cell survival can be rescued by expressing this gene on the X chromosome. The Spink3-/-;XXSPINK1 mice, in which this method has been applied to partially restore SPINK1 function, present a novel genetic model of chronic pancreatitis.

c-FLIP maintains tissue homeostasis by preventing apoptosis and programmed necrosis.

As a catalytically inactive homolog of caspase-8, a proapoptotic initiator caspase, c-FLIP blocks apoptosis by binding to and inhibiting caspase-8. The transcription factor nuclear factor κB (NF-κB) plays a pivotal role in maintaining the homeostasis of the intestine and the liver by preventing death receptor-induced apoptosis, and c-FLIP plays a role in the NF-κB-dependent protection of cells from death receptor signaling. Because c-Flip-deficient mice die in utero, we generated conditional c-Flip-deficient mice to investigate the contribution of c-FLIP to homeostasis of the intestine and the liver at developmental and postnatal stages. Intestinal epithelial cell (IEC)- or hepatocyte-specific deletion of c-Flip resulted in perinatal lethality as a result of the enhanced apoptosis and programmed necrosis of the IECs and the hepatocytes. Deficiency in the gene encoding tumor necrosis factor-α (TNF-α) receptor 1 (Tnfr1) partially rescued perinatal lethality and the development of colitis in IEC-specific c-Flip-deficient mice but did not rescue perinatal lethality in hepatocyte-specific c-Flip-deficient mice. Moreover, adult mice with interferon (IFN)-inducible deficiency in c-Flip died from hepatitis soon after depletion of c-FLIP. Pretreatment of IFN-inducible c-Flip-deficient mice with a mixture of neutralizing antibodies against TNF-α, Fas ligand (FasL), and TNF-related apoptosis-inducing ligand (TRAIL) prevented hepatitis. Together, these results suggest that c-FLIP controls the homeostasis of IECs and hepatocytes by preventing cell death induced by TNF-α, FasL, and TRAIL.

Interleukin-11 links oxidative stress and compensatory proliferation.

Apoptotic cells can stimulate the compensatory proliferation of surrounding cells to maintain tissue homeostasis. Although oxidative stress is associated with apoptosis and necrosis, whether it contributes to compensatory proliferation is unknown. Here, we showed that interleukin-11 (IL-11), a member of the IL-6 family of proinflammatory cytokines, was produced by cells in an oxidative stress-dependent manner. IL-11 production depended on the activation in dying cells of extracellular signal-regulated kinase 2, which in turn caused the phosphorylation and accumulation of the transcription factor Fra-1 by preventing its proteasome-dependent degradation. Fra-1 was subsequently recruited to the Il11 promoter and activated gene transcription. Upon acute liver injury in mice, IL-11 was mainly produced by hepatocytes in response to reactive oxygen species that were presumably released from dying hepatocytes. IL-11 that was secreted by the dying cells then induced the phosphorylation of the transcription factor STAT3 in adjacent healthy hepatocytes, which resulted in their compensatory proliferation. Furthermore, an IL-11 receptor (IL-11R) agonist enhanced the proliferation of hepatocytes and ameliorated oxidative stress upon acetaminophen-induced liver injury. Conversely, the effects of acetaminophen were exacerbated in mice deficient in the IL-11R α subunit. Together, these results suggest that IL-11 provides a functional link between oxidative stress and compensatory proliferation.

Crucial role for autophagy in degranulation of mast cells.

BACKGROUND: Autophagy plays a crucial role in controlling various biological responses including starvation, homeostatic turnover of long-lived proteins, and invasion of bacteria. However, a role for autophagy in development and/or function of mast cells is unknown. OBJECTIVE: To investigate a role for autophagy in mast cells, we generated bone marrow-derived mast cells (BMMCs) from mice lacking autophagy related gene (Atg) 7, an essential enzyme for autophagy induction. METHODS: Bone marrow-derived mast cells were generated from bone marrow cells of control and IFN-inducible Atg7-deficient mice, and morphologic and functional analyses were performed. RESULTS: We found that conversion of type I to type II light chain (LC3)-II, a hallmark of autophagy, was constitutively induced in mast cells under full nutrient conditions, and LC3-II localized in secretory granules of mast cells. Although deletion of Atg7 did not impair the development of BMMCs, Atg7(-/-) BMMCs showed severe impairment of degranulation, but not cytokine production on FcεRI cross-linking. Intriguingly, LC3-II but not LC3-I was co-localized with CD63, a secretory lysosomal marker, and was released extracellularly along with degranulation in Atg7(+/+) but not Atg7(-/-) BMMCs. Moreover, passive cutaneous anaphylaxis reactions were severely impaired in mast cell-deficient WBB6F1-W/W(V) mice reconstituted with Atg7(-/-) BMMCs compared with Atg7(+/+) BMMCs. CONCLUSION: These results suggest that autophagy is not essential for the development but plays a crucial role in degranulation of mast cells. Thus, autophagy might be a potential target to treat allergic diseases in which mast cells are critically involved.

Fusion of OTT to BSAC results in aberrant up-regulation of transcriptional activity.

OTT/RBM15-BSAC/MAL/MKL1/MRTF-A was identified as a fusion transcript generated by t(1;22)(p13;q13) in acute megakaryoblastic leukemia. Previous studies have shown that BSAC (basic, SAP, and coiled-coil domain) activates the promoters containing CArG boxes via interaction with serum response factor, and OTT (one twenty-two) negatively regulates the development of megakaryocytes and myeloid cells. However, the mechanism by which OTT-BSAC promotes leukemia is largely unknown. Here we show that OTT-BSAC, but not BSAC or OTT strongly activates several promoters containing a transcription factor Yin Yang 1-binding sequence. In addition, although BSAC predominantly localizes in the cytoplasm and its nuclear translocation is considered to be regulated by the Rho-actin signaling pathway, OTT-BSAC exclusively localizes in the nucleus. Moreover, OTT interacts with histone deacetylase 3, but this interaction is abolished in OTT-BSAC. Collectively, these functional and spatial changes of OTT and BSAC caused by the fusion might perturb their functions, culminating in the development of acute megakaryoblastic leukemia.

An antiapoptotic protein, c-FLIPL, directly binds to MKK7 and inhibits the JNK pathway.

Inhibition of NF-kappaB activation increases susceptibility to tumor necrosis factor (TNF)alpha-induced cell death, concurrent with caspases and prolonged c-Jun N-terminal kinase (JNK) activation, and reactive oxygen species (ROS) accumulation. However, the detailed mechanisms are unclear. Here we show that cellular FLICE-inhibitory protein (c-FLIP) is rapidly lost in NF-kappaB activation-deficient, but not wild-type fibroblasts upon TNFalpha stimulation, indicating that NF-kappaB normally maintains the cellular levels of c-FLIP. The ectopic expression of the long form of c-FLIP (c-FLIPL) inhibits TNFalpha-induced prolonged JNK activation and ROS accumulation in NF-kappaB activation-deficient fibroblasts. Conversely, TNFalpha induces prolonged JNK activation and ROS accumulation in c-Flip-/- fibroblasts. Moreover, c-FLIPL directly interacts with a JNK activator, MAP kinase kinase (MKK)7, in a TNFalpha-dependent manner and inhibits the interactions of MKK7 with MAP/ERK kinase kinase 1, apoptosis-signal-regulating kinase 1, and TGFbeta-activated kinase 1. This stimuli-dependent interaction of c-FLIPL with MKK7 might selectively suppress the prolonged phase of JNK activation. Taken that ROS promote JNK activation and activation of the JNK pathway may promote ROS accumulation, c-FLIPL might block this positive feedback loop, thereby suppressing ROS accumulation.