Selective unresponsiveness to the inhibition of p38 MAPK activation by cAMP helps L929 fibroblastoma cells escape TNF-alpha-induced cell death

SARS-CoV-2 nucleocapsid protein enhances the level of mitochondrial reactive oxygen species

Coronavirus disease 2019 (COVID-19) pathogenesis is influenced by reactive oxygen species (ROS). Nevertheless, the precise mechanisms implicated remain poorly understood. The nucleocapsid (N) protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the main driver for this condition, is a structural protein indispensable for viral replication and assembly, and its role in ROS production has not been reported. This study shows that SARS-CoV-2 N protein expression enhances mitochondrial ROS level. Bulk RNA-sequencing suggests of aberrant redox state of the electron transport chain. Accordingly, this protein hinders ATP production but simultaneously augments the activity of complexes I and III, and most mitochondrially encoded complex I and III proteins are upregulated by it. Mechanistically, N protein of SARS-CoV-2 shows significant mitochondrial localization. It interacts with mitochondrial transcription components and stabilizes them. Moreover, it also impairs the activity of antioxidant enzymes with or without detectable interaction.

JNK Cascade-Induced Apoptosis-A Unique Role in GqPCR Signaling

The response of cells to extracellular signals is mediated by a variety of intracellular signaling pathways that determine stimulus-dependent cell fates. One such pathway is the cJun-N-terminal Kinase (JNK) cascade, which is mainly involved in stress-related processes. The cascade transmits its signals via a sequential activation of protein kinases, organized into three to five tiers. Proper regulation is essential for securing a proper cell fate after stimulation, and the mechanisms that regulate this cascade may involve the following: (1) Activatory or inhibitory phosphorylations, which induce or abolish signal transmission. (2) Regulatory dephosphorylation by various phosphatases. (3) Scaffold proteins that bring distinct components of the cascade in close proximity to each other. (4) Dynamic change of subcellular localization of the cascade's components. (5) Degradation of some of the components. In this review, we cover these regulatory mechanisms and emphasize the mechanism by which the JNK cascade transmits apoptotic signals. We also describe the newly discovered PP2A switch, which is an important mechanism for JNK activation that induces apoptosis downstream of the Gq protein coupled receptors. Since the JNK cascade is involved in many cellular processes that determine cell fate, addressing its regulatory mechanisms might reveal new ways to treat JNK-dependent pathologies.

Targeting the TNF-α-TNFR interaction with EGCG to block NF-κB signaling in human synovial fibroblasts

The tumor necrosis factor alpha (TNF-α)-TNF-α receptor (TNFR) interaction plays a central role in the pathogenesis of various autoimmune diseases, particularly rheumatoid arthritis, and is therefore considered a key target for drug discovery. However, natural compounds that can specifically block the TNF-α-TNFR interaction are rarely reported. (-)-Epigallocatechin-3-gallate (EGCG) is the most active, abundant, and thoroughly investigated polyphenolic compound in green tea. However, the molecular mechanism by which EGCG ameliorates autoimmune arthritis remains to be elucidated. In the present study, we found that EGCG can directly bind to TNF-α, TNFR1, and TNFR2 with similar μM affinity and disrupt the interactions between TNF-α and TNFR1 and TNFR2, which inhibits TNF-α-induced L929 cell death, blocks TNF-α-induced NF-κB activation in 293-TNF-α response cell line, and eventually leads to inhibition of TNF-α-induced NF-κB signaling pathway in HFLS and MH7A cells. Thus, regular consumption of EGCG in green tea may represent a potential therapeutic agent for the treatment of TNF-α-associated diseases.

RACK1 Promotes Self-Renewal and Chemoresistance of Cancer Stem Cells in Human Hepatocellular Carcinoma through Stabilizing Nanog

Targeting cancer stem cells (CSCs) has been proposed as a new strategy to eradicate malignancies, including hepatocellular carcinoma (HCC). However, the mechanisms by which CSCs sustain their self-renewal and chemoresistance remain elusive. Nanog is a master transcriptional regulator of stemness, especially in CSCs. Its expression is tightly regulated by the ubiquitin-proteasome system in embryonic stem cells (ESCs). Whether the suppression of Nanog ubiquitination contributes to its over-expression in CSCs has not been explored. In addition, the role of receptor for activated C kinase 1 (RACK1), an adaptor protein implicated in HCC growth, in liver CSC-like traits remains to be determined.

Methods: In vitro and in vivo assays were performed to investigate the role of RACK1 in liver CSC-like phenotype and murine ESC function. How RACK1 regulates Nanog expression was explored by immunoblotting and immunohistochemistry. The interaction of RACK1 with Nanog and the consequent effects on Nanog ubiquitination and stemness were then analyzed.

Results: RACK1 promotes self-renewal and chemoresistance of human liver CSCs and maintains murine ESC function. Consistently, RACK1 enhances the expression of Nanog in human HCC cells and murine ESCs. The protein levels of RACK1 in clinical HCC tissues positively correlate with those of Nanog. Further exploration indicates that RACK1 directly binds to Nanog, which prevents its recruitment of E3 ubiquitin ligase FBXW8 and ubiquitin-dependent degradation. The interaction with Nanog is essential for RACK1 to promote stemness.

Conclusions: Our data provide novel insights into the regulation of Nanog protein levels, as well the key role of RACK1 to enhance self-renewal and chemoresistance of CSCs in human HCC.

Mitogen-activated protein kinase pathways in sepsis treatment

Sepsis is a characteristic set of systemic reactions to overwhelming infection that remains a major cause of death in critically ill patients. Endotoxins or lipopolysaccharides from gram-negative bacteria play a major role in the pathogenesis by inducing an over-production of inflammatory cytokines, which usually triggers beneficial inflammatory responses but causes tissue injury and lethal multiple organ failure in excessive amounts. The production of inflammatory cytokines depends on the activation of many intracellular signaling pathways, including nuclear factor-κB (NF-κB) pathway and three mitogen-activated protein kinase (MAPK) pathways. This review of important MAPK pathways underscores the essential role of MKP-1 in the negative control of sepsis. Herein is a summary of the roles of MAPK pathways in the production of inflammatory cytokines and the possibility of targeting these pathways for the treatment of sepsis.

RACK1 Promotes Autophagy by Enhancing the Atg14L-Beclin 1-Vps34-Vps15 Complex Formation upon Phosphorylation by AMPK

Autophagy is essential for maintaining tissue homeostasis. Although adaptors have been demonstrated to facilitate the assembly of the Atg14L-Beclin 1-Vps34-Vps15 complex, which functions in autophagosome formation, it remains unknown whether the autophagy machinery actively recruits such adaptors. WD40-repeat proteins are a large, highly conserved family of adaptors implicated in various cellular activities. However, the role of WD40-repeat-only proteins, such as RACK1, in postnatal mammalian physiology remains unknown. Here, we report that hepatocyte-specific RACK1 deficiency leads to lipid accumulation in the liver, accompanied by impaired Atg14L-linked Vps34 activity and autophagy. Further exploration indicates that RACK1 participates in the formation of autophagosome biogenesis complex upon its phosphorylation by AMPK at Thr50. Thr50 phosphorylation of RACK1 enhances its direct binding to Vps15, Atg14L, and Beclin 1, thereby promoting the assembly of the autophagy-initiation complex. These observations provide insight into autophagy induction and establish a pivotal role for RACK1 in postnatal mammalian physiology.

RACK1 antagonizes TNF-α-induced cell death by promoting p38 activation

p38 mitogen-activated protein kinase (MAPK) activity has been reported to either promote or suppress cell death, which depends on cell type and stimulus. Our previous report indicates that p38 exerts a protective role in tumor necrosis factor (TNF)-α-induced cell death in L929 fibroblastoma cells. However, key molecules regulating p38 activation remain unclear. Here, we show that ectopic expression of scaffold protein receptor for activated C kinase 1 (RACK1) suppressed TNF-α-induced cell death in L929 cells, which was associated with enhanced p38 activation. Knockdown of endogenous RACK1 expression exhibited opposite effects. The protective role of RACK1 in TNF-α-induced cell death diminished upon blockade of p38 activation. Therefore, RACK1 antagonizes TNF-α-induced cell death through, at least partially, augmenting p38 activation. Further exploration revealed that RACK1 directly bound to MKK3/6 and enhanced the kinase activity of MKK3/6 without affecting MKK3/6 phosphorylation. Similar effects of RACK1 were also observed in primary murine hepatocytes, another cell type sensitive to TNF-α-induced cell death. Taken together, our data suggest that RACK1 is a key factor involved in p38 activation as well as TNF-α-induced cell death.

RIP1-dependent Bid cleavage mediates TNFα-induced but Caspase-3-independent cell death in L929 fibroblastoma cells

L929 fibroblastoma cells (L929-A) and L929 fibrosarcoma cells (L929-N) are different cell lines that are commonly used to study the cytotoxicity of tumor necrosis factor alpha (TNFα). TNFα has been reported to induce necrosis in both of these cell lines. However, comparing the TNFα-induced cell death in these two cell lines, we found that, unlike the L929-N cells that show typical RIP3-dependent necrosis, TNFα-induced cell death in L929-A cells is pan-caspase inhibitor Z-VAD-FMK (Z-VAD)-sensitive, which does not depend on RIP3. We also confirmed that the cell death signal in the L929-A cells was initiated through cytosol-preassembled ripoptosome and that the knockdown of either Caspase-8 or RIP1 protein blocked cell death. Compared with the L929-N cells, the L929-A cell line had lower levels of constitutive and inducible TNFα autocrine production, and the pan-caspase inhibitors Z-VAD or Q-VD did not kill the L929-A cells as they affect the L929-N cells. Moreover, the L929-A cells expressed less RIP3 protein than the L929-N cells; therefore, TNFα failed to induce RIP3-dependent necroptosis. In addition, the ripoptosome-mediated cell death signal was transduced to the mitochondria through Caspase-8-mediated and RIP1 kinase activity-dependent Bid cleavage. The RIP1 kinase inhibitor Necrostatin-1 (Nec-1) or Caspase-8 knockdown completely blocked Bid cleavage, and the knockdown of Bid or Bax/Bak prevented TNFα-induced cell death in the L929-A cells. Although the activation of Bax/Bak decreased the mitochondrial membrane potential, the levels of mitochondrial intermembrane space proteins, including cytochrome-c (cyt-C) and Smac, declined, and western blotting and immunofluorescence staining analysis did not determine whether these proteins were redistributed to the cytosol. In addition, the mitochondrial outer membrane protein Tom20 was also reduced, indicating that the reduced mitochondria proteins may be induced by the reduced mitochondria numbers. No efficient cyt-C release was observed; therefore, the limited activation and cleavage of downstream caspases, including Caspase-9, Caspase-7, Caspase-6 and Caspase-3, was insufficient to kill the cells. The Caspase-9, Caspase-6 and Caspase-3/7 inhibitors or Caspase-9 and -3 knockdown also failed to block cell death, and the overexpression of Bcl-2 also did not abrogate cell death. Moreover, the dead cells showed necrotic-like but not apoptotic characteristics under transmission electronmicroscopy, and these features were significantly different from mitochondrial apoptosis, indicating that the effector caspases were not the executioners of cell death. These new discoveries show that TNFα-induced cell death in L929-A cells is different than typical RIP3-dependent necrosis and Caspase-8/Caspase-3-mediated apoptosis. These results highlight that caution is necessary when using different L929 cells as a model to investigate TNFα-induced cell death.

Prolonged pretreatment of mice with cholera toxin, but not isoproterenol, alleviates acute lethal systemic inflammatory response

Isoproterenol, a synthetic non-selective β-adrenergic agonist, is often used during the immediate postoperative period after open heart surgery for its chronotropic and vasodilatory effects. It has been demonstrated that isoproterenol pretreatment followed by immediate LPS administration leads to reduced tumor necrosis factor-α (TNF-α) response in vivo. However, sepsis never happens immediately after the surgery, but rather severe immune dysfunction occurs at least 24h later. It remains elusive what effects isoproterenol might exert to innate immunity during the period. In this scenario, we investigated the effects of 24-h isoproterenol pretreatment on septic shock induced by experimental endotoxemia and bacterial peritonitis, with cholera toxin as another cAMP elevator. Unexpectedly, we found that isoproterenol and cholera toxin exhibited distinct effects in acute lethal systemic inflammatory response. Isoproterenol worsened liver injury without enhancing NK/NKT activity. Meanwhile, cholera toxin but not isoproterenol showed dramatically reduced TNF-α response in LPS induced septic shock. Our data provide a caution for the clinical use of isoproterenol and suggest that isoproterenol has cAMP-independent functions.

The Gβγ-Src signaling pathway regulates TNF-induced necroptosis via control of necrosome translocation

Formation of multi-component signaling complex necrosomes is essential for tumor necrosis factor α (TNF)-induced programmed necrosis (also called necroptosis). However, the mechanisms of necroptosis are still largely unknown. We isolated a TNF-resistant L929 mutant cell line generated by retrovirus insertion and identified that disruption of the guanine nucleotide-binding protein γ 10 (Gγ10) gene is responsible for this phenotype. We further show that Gγ10 is involved in TNF-induced necroptosis and Gβ2 is the partner of Gγ10. Src is the downstream effector of Gβ2γ10 in TNF-induced necroptosis because TNF-induced Src activation was impaired upon Gγ10 knockdown. Gγ10 does not affect TNF-induced activation of NF-κB and MAPKs and the formation of necrosomes, but is required for trafficking of necrosomes to their potential functioning site, an unidentified subcellular organelle that can be fractionated into heterotypic membrane fractions. The TNF-induced Gβγ-Src signaling pathway is independent of RIP1/RIP3 kinase activity and necrosome formation, but is required for the necrosome to function.

Chronic psychological stress induces the accumulation of myeloid-derived suppressor cells in mice

Chronic psychological stress has been shown to adversely impact immune system functions and compromise host defenses against various infections. However, the underlying mechanisms remain elusive. Recent studies have demonstrated that myeloid-derived suppressor cells (MDSCs) play an important role in regulating immunity. It is of interest to explore whether or not chronic psychological stress plays immunosuppressive functions partially by inducing MDSCs accumulation. In this work, we report that chronic psychological stress led to the accumulation of CD11b+Gr1+ cells in the bone marrow of BALB/c mice. Repeated β-agonist infusion showed no such effect. However, β-adrenergic blockade, but not glucocorticoids blockade, partially reversed the accumulation of CD11b+Gr1+ cells under the condition of chronic psychological stress, suggesting catecholamines collaborate with other factors to induce the accumulation. Further exploration indicates that cyclooxygenase 2 (COX-2)-prostaglandin E2 (PGE2) loop might act downstream to induce the accumulation. A majority of the accumulated CD11b+Gr1+ cells were Ly6G+Ly6C(low) immature neutrophils, which inhibited cytokine release of macrophages as well as T cell responsiveness. Moreover, the accumulated CD11b+Gr1+ cells under the condition of chronic psychological stress expressed multiple inhibitory molecules. Taken together, our data demonstrate for the first time that chronic psychological stress induces MDSCs accumulation in mice, which can contribute to immunosuppression.

Cyclic AMP suppresses TGF-β-mediated adaptive Tregs differentiation through inhibiting the activation of ERK and JNK

The second messenger cAMP is involved in the regulation of many cellular activities partially through modulating the MAPK pathways. The role of cAMP in TGF-β-mediated adaptive Tregs differentiation remains elusive. In this work, we show that cAMP inhibits antigen-nonspecific proliferation of murine CD4+ T cells without significant promotion of apoptosis. Moreover, cAMP suppresses TGF-β-induced expression of forkhead transcription factor Foxp3. 6-MB-cAMP, a site-selective activator of PKA, mimics the role of cAMP in TGF-β-induced Foxp3 expression. Further exploration reveals that TGF-β activates ERK and JNK, but not p38. cAMP and 6-MB-cAMP block TGF-β-induced activation of ERK and JNK through transcription-independent manner and transcription-dependent manner, respectively. Since direct inhibition of ERK or JNK activity mimics the effects of cAMP during this process, our work suggests that cAMP suppresses TGF-β-mediated adaptive Tregs differentiation through, at least partially, inhibiting the activation of ERK and JNK.

RACK1 promotes the proliferation of THP1 acute myeloid leukemia cells

The receptor for activated C kinase 1 (RACK1), an adaptor protein implicated in the regulation of multiple signaling pathways, has been reported to contribute to the survival of leukemic progenitor cells by enhancing the activity of glycogen synthase kinase 3β (GSK3β). However, it remains unknown whether RACK1 also contributes to the oncogenic growth of acute myeloid leukemia (AML) cells. Here, we report that transient or stable silencing of endogenous RACK1 expression by RACK1 short hairpin RNAs (shRNAs) led to impaired proliferation of THP1 AML cells without inducing terminal differentiation. Further exploration revealed that RACK1 loss-of-function resulted in reduced GSK3β activity. GSK3β shRNA treatment showed similar effects to RACK1 loss-of-function. Our data collectively suggest that RACK1 contributes to THP1 cell proliferation through, at least partially, enhancing GSK3β activity. Thus, targeting RACK1 may have some important therapeutic implications in the treatment of AML.