Role Of Receptor-interacting Protein Kinase 1 Research Articles

Role of RIPK1 in SMAC mimetics-induced apoptosis in primary human HIV-infected macrophages

Macrophages serve as viral reservoirs due to their resistance to apoptosis and HIV-cytopathic effects. We have previously shown that inhibitor of apoptosis proteins (IAPs) confer resistance to HIV-Vpr-induced apoptosis in normal macrophages. Herein, we show that second mitochondrial activator of caspases (SMAC) mimetics (SM) induce apoptosis of monocyte-derived macrophages (MDMs) infected in vitro with a R5-tropic laboratory strain expressing heat stable antigen, chronically infected U1 cells, and ex-vivo derived MDMs from HIV-infected individuals. To understand the mechanism governing SM-induced cell death, we show that SM-induced cell death of primary HIV-infected macrophages was independent of the acquisition of M1 phenotype following HIV infection of macrophages. Instead, SM-induced cell death was found to be mediated by IAPs as downregulation of IAPs by siRNAs induced cell death of HIV-infected macrophages. Moreover, HIV infection caused receptor interacting protein kinase-1 (RIPK1) degradation which in concert with IAP1/2 downregulation following SM treatment may result in apoptosis of macrophages. Altogether, our results show that SM selectively induce apoptosis in primary human macrophages infected in vitro with HIV possibly through RIPK1. Moreover, modulation of the IAP pathways may be a potential strategy for selective killing of HIV-infected macrophages in vivo.

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.

RIPK1 contributes to cisplatin-induced apoptosis of esophageal squamous cell carcinoma cells via activation of JNK pathway

AimsPrevious studies have uncovered the function of receptor-interacting protein kinase 1 (RIPK1) to mediate both cell survival and death. Moreover, RIPK1 modulates apoptosis and necroptosis depending on its activity, phosphorylation or ubiquitylation status. Many studies have explained the role or mechanism of RIPK1 in necroptosis. However, the role of RIPK1 has not been elucidated fully in human esophageal squamous cell carcinoma (ESCC) cells. Materials and methodsThe protein and mRNA expression levels of RIPK1 in a panel of ESCC cell lines by Western blot and real-time quantitative reverse transcription PCR (qRT-PCR) were analyzed. MTS assay was used to examine cellular proliferation, flow cytometric analysis to detect apoptosis, mitochondrial membrane potential and reactive oxygen species production. ESCC cells with either inhibitor or overexpressed RIPK1were analyzed to determine cell proliferation, colony formation and apoptosis. Flow cytometry and western blotting assays were used to explore the underlying mechanism. Key findingsIn our study, RIPK1 expression was found to contribute significantly to cisplatin-induced apoptosis in the human ESCC cells. The reduced RIPK1 expression promoted cells proliferation and overexpressed RIPK1 facilitated cell apoptosis. Mechanistic investigations have revealed that the inhibition of proliferation for RIPK1 in ESCC cells was regulated via activation of c-Jun NH2-terminal kinase signaling. Additionally, damages were observed in the mitochondrial membrane, depletion of ATP and increased generation in reactive oxygen species. SignificanceOur findings verified the evidence that RIPK1 can promote cell death in ESCC cells, with potential implications for activating c-Jun NH2-terminal kinase pathway as a novel approach to the disease.

Abstract 2410: RIPK1-mediated induction of mitophagy compromises the viability of extracellular matrix-detached cells

Abstract For cancer cells to survive during ECM-detachment, they must inhibit anoikis and rectify metabolic deficiencies that cause non-apoptotic cell death. Previous studies in ECM-detached cells have linked non-apoptotic cell death to ROS generation, although the mechanistic underpinnings of this link remain poorly defined. Here, we uncover a novel role for receptor-interacting protein kinase-1 (RIPK1) in the modulation of mitochondrial ROS levels and cell viability during ECM-detachment. We find that RIPK1 activation during ECM-detachment results in the induction of PINK1-dependent mitophagy through a mechanism requiring the mitochondrial phosphatase PGAM5. As a consequence of mitophagy induction, ECM-detached cells experience diminished IDH2-mediated NADPH production in the mitochondria and the subsequent elevation in mitochondrial ROS levels leads to non-apoptotic cell death. Furthermore, we find that antagonizing RIPK1 or PGAM5 enhances tumor formation in vivo. Intriguingly, upon mining a meta-analysis of published lung cancer patient microarray data sets, we find that low RIPK1 expression in lung cancer patients correlates with decreased overall and progression-free survival. Thus, RIPK1-mediated induction of mitophagy may be an efficacious target for novel therapeutics aimed at specifically eliminating ECM-detached cancer cells and deterring tumor progression. Citation Format: Mark A. Hawk, Zachary T. Schafer. RIPK1-mediated induction of mitophagy compromises the viability of extracellular matrix-detached cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2410.

A Role for Receptor-Interacting Protein Kinase-1 in Neutrophil Extracellular Trap Formation in Patients with Systemic Lupus Erythematosus: a Preliminary Study

Background/Aims: Neutrophil extracellular traps (NETs) are known to play an important role in systemic lupus erythematosus (SLE) by triggering innate and adaptive immune responses. The molecular mechanisms responsible for their formation in SLE are still unclear. In this study, we aim to characterize the role of the receptor-interacting protein kinase-1 (RIPK1), a homologous serine/threonine kinase previously implicated in the regulation of necroptosis and tissue injury, in decreasing neutrophil death and formation of NETs, and to investigate the clinical implications of RIPK1 in SLE. Methods: Patients with SLE (n = 50) and healthy donors (n = 35) were enrolled in in vitro studies. Management of SLE patients was evaluated using the SLE disease activity index 2000 (SLEDAI-2K) score and laboratory variables. The mRNA level of RIPKs was measured by quantitative polymerase chain reaction (qPCR). Intracellular RIPK1 and RIPK3 production by peripheral blood leukocytes was detected by four-color flow cytometry and confirmed by automatic western blotting. TNF-α, IFN-γ, IL-1β, IL-2, IL-8, IL-18, and RIPK1 were measured by enzyme-linked immunosorbent assay. Cell death was assayed by Sytox green dye from peripheral neutrophils stimulated by RIPK-1-stabilizer necrostatin-1 (nec-1) and phorbol 12-myristate 13-acetate (PMA). Immunofluorescence staining and confocal microscopy were used to detect NET formation ex vivo. Quantification of NETs was determined by fluorescence spectrometry. Results: IFN-γ, IL-1β, IL-8, and IL-18 levels in serum were increased in SLE patients compared to controls. However, the expression of TNF-α, IL-2, and RIPK1 were decreased. In addition, we observed significant differences in the expression of RIPK1 in peripheral blood leukocytes. Of all the leukocytes, RIPK1 expression was significantly lower in neutrophils. Furthermore, we studied NETs formation in neutrophils of SLE with decreased RIPK1 expression, and these show increased susceptibility to NETosis, when stimulated with PMA and/or nec-1. Importantly, RIPK1 expression in neutrophils negatively correlated with ESR, CRP, 24-hour urine total protein, and the disease activity index in SLE. Conclusion: These data represent the first report of decreased RIPK1 expression in neutrophils of SLE patients and imply that RIPK1 may be involved in neutrophil death and NET formation. We suggest that RIPK1 is a potential biomarker to predict disease activity.

RIPK1 protects hepatocytes from Kupffer cells-mediated TNF-induced apoptosis in mouse models of PAMP-induced hepatitis

The severity of liver diseases is exacerbated by the death of hepatocytes, which can be induced by the sensing of pathogen associated molecular patterns (PAMPs) derived from the gut microbiota. The molecular mechanisms regulating these cell death pathways are poorly documented. In this study, we investigated the role of the receptor interacting protein kinase 1 (RIPK1), a protein known to regulate cell fate decisions, in the death of hepatocytes using two in vivo models of PAMP-induced hepatitis. Hepatitis was induced in mice by independent injections of two different bacterial PAMPs: lipopolysaccharide (LPS) and unmethylated CpG oligodeoxynucleotide (CpG-DNA) motifs. The role of RIPK1 was evaluated by using mice specifically lacking RIPK1 in liver parenchymal cells (Ripk1LPC-KO). Administration of liposome-encapsulated clodronate served to investigate the role of Kupffer cells in the establishment of the disease. Etanercept, a tumor necrosis factor (TNF)-decoy receptor, was used to study the contribution of TNF-α during LPS-mediated liver injury. Whereas RIPK1 deficiency in liver parenchymal cells did not trigger basal hepatolysis, it greatly sensitized hepatocytes to apoptosis and liver damage following a single injection of LPS or CpG-DNA. Importantly, hepatocyte death was prevented by previous macrophage depletion or by TNF inhibition. Our data highlight the pivotal function of RIPK1 in maintaining liver homeostasis in conditions of macrophage-induced TNF burst in response to PAMPs sensing. Excessive death of hepatocytes is a characteristic of liver injury. A new programmed cell death pathway has been described involving upstream death ligands such as TNF and downstream kinases such as RIPK1. Here, we show that in the presence of LPS liver induced hepatic injury was due to secretion of TNF by liver macrophages, and that RIPK1 acts as a powerful protector of hepatocyte death. This newly identified pathway in the liver may be helpful in the management of patients to predict their risk of developing acute liver failure.