ASK1 signaling regulates phase-specific glial interactions during neuroinflammation.

Neuroinflammation is well known to be associated with neurodegenerative diseases. Apoptosis signal-regulating kinase 1 (ASK1) is a mitogen-activated protein kinase kinase kinase that has been implicated in neuroinflammation, but its precise cellular and molecular mechanisms remain unknown. In this study, we generated conditional knockout (CKO) mice that lack ASK1 in T cells, dendritic cells, microglia/macrophages, microglia, or astrocytes, to assess the roles of ASK1 during experimental autoimmune encephalomyelitis (EAE). We found that neuroinflammation was reduced in both the early and later stages of EAE in microglia/macrophage-specific ASK1 knockout mice, whereas only the later-stage neuroinflammation was ameliorated in astrocyte-specific ASK1 knockout mice. ASK1 deficiency in T cells and dendritic cells had no significant effects on EAE severity. Further, we found that ASK1 in microglia/macrophages induces a proinflammatory environment, which subsequently activates astrocytes to exacerbate neuroinflammation. Microglia-specific ASK1 deletion was achieved using a CX3CR1CreER system, and we found that ASK1 signaling in microglia played a major role in generating and maintaining disease. Activated astrocytes produce key inflammatory mediators, including CCL2, that further activated and recruited microglia/macrophages, in an astrocytic ASK1-dependent manner. Astrocyte-specific analysis revealed CCL2 expression was higher in the later stage compared with the early stage, suggesting a greater proinflammatory role of astrocytes in the later stage. Our findings demonstrate cell-type-specific roles of ASK1 and suggest phase-specific ASK1-dependent glial cell interactions in EAE pathophysiology. We propose glial ASK1 as a promising therapeutic target for reducing neuroinflammation.

Killing of Latently HIV-Infected CD4 T Cells by Autologous CD8 T Cells Is Modulated by Nef.

The role of HIV-specific CD8 T cell activity in the course of HIV infection and the way it affects the virus that resides in the latent reservoir resting memory cells is debated. The PBMC of HIV-infected patients contain HIV-specific CD8 T cells and their potential targets, CD4 T cells latently infected by HIV. CD4 T cells and CD8 T cells procured from PBMC of HIV-infected patients were co-incubated and analyzed: Formation of CD8 T cells and HIV-infected CD4 T cell conjugates and apoptosis of these CD4 T cells were observed by fluorescence microscopy with in situ PCR of HIV LTR DNA. Furthermore, conjugation of CD8 T cells with CD4 T cells and apoptosis of CD4 T cells was observed and quantified by imaging flow cytometry using anti-human activated caspase 3 antibody and TUNEL assay. The conjugation activity and apoptosis were found to be much higher in patients with acute HIV infection or AIDS compared to patients in chronic infection on antiretroviral therapy (ART) or not. Patients on ART had low grade conjugation and apoptosis of isolated CD69, CD25, and HLA-DR-negative CD4 T cells (latent reservoir cells) by CD8 T cells. Using in situ PCR The latent reservoir CD4 T cells were shown to contain most of the HIV DNA. We demonstrate in HIV-infected patients, that CD8 T cells conjugate with and kill HIV-infected CD4 T cells, including HIV-infected resting memory CD4 T cells, throughout the course of HIV infection. We propose that in HIV-infected patients CD4 T cell annihilation is caused in part by ongoing activity of HIV-specific CD8 T cells. HIV Nef protein interacts with ASK 1 and inhibits its pro-apoptotic death signaling by Fas/FasL, thus protecting HIV-infected cells from CD8 T cells killing. A peptide that interrupts Nef-ASK1 interaction that had been delivered into CD4 T cells procured from patients on ART resulted in the increase of their apoptosis inflicted by autologous CD8 T cells. We suggest that elimination of the HIV-infected latent reservoir CD4 T cells can be achieved by Nef inhibition.

ASK1 inhibitor treatment suppresses p38/JNK signalling with reduced kidney inflammation and fibrosis in rat crescentic glomerulonephritis.

Activation of p38 mitogen-activated protein kinase (MAPK) and c-Jun amino terminal kinase (JNK) is prominent in human crescentic glomerulonephritis. p38 and JNK inhibitors suppress crescentic disease in animal models; however, the upstream mechanisms inducing activation of these kinases in crescentic glomerulonephritis are unknown. We investigated the hypothesis that apoptosis signal-regulating kinase 1 (ASK1/MAP3K5) promote p38/JNK activation and renal injury in models of nephrotoxic serum nephritis (NTN); acute glomerular injury in SD rats, and crescentic disease in WKY rats. Treatment with the selective ASK1 inhibitor, GS-444217 or vehicle began 1 hour before nephrotoxic serum injection and continued until animals were killed on day 1 (SD rats) or 14 (WKY rats). NTN resulted in phosphorylation (activation) of p38 and c-Jun in both models which was substantially reduced by ASK1 inhibitor treatment. In SD rats, GS-444217 prevented proteinuria and glomerular thrombosis with suppression of macrophage activation on day 1 NTN. In WKY rats, GS-444217 reduced crescent formation, prevented renal impairment and reduced proteinuria on day 14 NTN. Macrophage activation, T-cell infiltration and renal fibrosis were also reduced by GS-444217. In conclusion, GS-444217 treatment inhibited p38/JNK activation and development of renal injury in rat NTN. ASK1 inhibitors may have therapeutic potential in rapidly progressive glomerulonephritis.

ASK1/2 signaling promotes inflammation in a mouse model of neutrophilic dermatosis.

Mice homozygous for the Tyr208Asn amino acid substitution in the carboxy terminus of Src homology region 2 (SH2) domain-containing phosphatase 1 (SHP-1) (referred to as Ptpn6spin mice) spontaneously develop a severe inflammatory disease resembling neutrophilic dermatosis in humans. Disease in Ptpn6spin mice is characterized by persistent footpad swelling and suppurative inflammation. Recently, in addition to IL-1α and IL-1R signaling, we demonstrated a pivotal role for several kinases such as SYK, RIPK1, and TAK1 in promoting inflammatory disease in Ptpn6spin mice. In order to identify new kinases involved in SHP-1-mediated inflammation, we took a genetic approach and discovered apoptosis signal-regulating kinases 1 and 2 (ASK1 and ASK2) as novel kinases regulating Ptpn6-mediated footpad inflammation. Double deletion of ASK1 and ASK2 abrogated cutaneous inflammatory disease in Ptpn6spin mice. This double deletion further rescued the splenomegaly and lymphomegaly caused by excessive neutrophil infiltration in Ptpn6spin mice. Mechanistically, ASK regulates Ptpn6spin-mediated disease by controlling proinflammatory signaling in the neutrophils. Collectively, the present study identifies SHP-1 and ASK signaling crosstalk as a critical regulator of IL-1α-driven inflammation and opens future avenues for finding novel drug targets to treat neutrophilic dermatosis in humans.

Immune profiling of human prostate epithelial cells determined by expression of p38/TRAF-6/ERK MAP kinases pathways.

The aim of the present work was to study the immune profiling of prostate epithelial cells by the expression of ASK-1/p38 and Raf-1/ERK MAP Kinases signaling pathways mediated by TRAF-6. Immunohistochemical and Western blot analyses for TRAF-6, ASK-1, MEK-6, p38, Raf-1, MEK-1, ERK-1, ERK-2 and PSA were carried out in 5 samples of normal prostate gland, 24 samples of BPH and 19 samples of PC. Immunoreaction to TRAF-6 was found in the cytoplasm of epithelial cells of BPH and tumor cells of PC samples. For patients with the profile (TRAF-6+), optical densities revealed a weak immunoexpression of ASK-1 in PC compared to BPH patients. Whereas, immunoexpression to Raf-1 was higher in PC than in BPH. According to the expression of ASK-1 and Raf-1, two main profiles were identified: (TRAF-6+, ASK-1+, Raf-1+) and (TRAF-6+, ASK-1+, RAF-1-) in both BPH and PC. In addition, ASK-1/p38 axis expression was increased in BPH. Raf-1/ERK signaling pathway was increased in PC samples. On the other hand, representing of individual signaling protein expression enclosing each of p38 and ERK MAP Kinases according to TRAF-6+ showed a qualitative behavior of ASK61/p38 and Raf-1/ERK signaling pathways and a dynamic expression of PSA associated with immune and inflammatory process. These findings suggest that prostate epithelial cell could able an immune and inflammatory setting.

Lys29-linkage of ASK1 by Skp1-Cullin 1-Fbxo21 ubiquitin ligase complex is required for antiviral innate response.

Protein ubiquitination regulated by ubiquitin ligases plays important roles in innate immunity. However, key regulators of ubiquitination during innate response and roles of new types of ubiquitination (apart from Lys48- and Lys63-linkage) in control of innate signaling have not been clearly understood. Here we report that F-box only protein Fbxo21, a functionally unknown component of SCF (Skp1-Cul1-F-box protein) complex, facilitates Lys29-linkage and activation of ASK1 (apoptosis signal-regulating kinase 1), and promotes type I interferon production upon viral infection. Fbxo21 deficiency in mice cells impairs virus-induced Lys29-linkage and activation of ASK1, attenuates c-Jun N-terminal kinase (JNK) and p38 signaling pathway, and decreases the production of proinflammatory cytokines and type I interferon, resulting in reduced antiviral innate response and enhanced virus replication. Therefore Fbxo21 is required for ASK1 activation via Lys29-linkage of ASK1 during antiviral innate response, providing mechanistic insights into non-proteolytic roles of SCF complex in innate immune response.

Thymoquinone inhibits TNF-α-induced inflammation and cell adhesion in rheumatoid arthritis synovial fibroblasts by ASK1 regulation.

Tumor necrosis factor-α (TNF-α) is a pro-inflammatory cytokine produced by monocytes/macrophage that plays a pathological role in rheumatoid arthritis (RA). In this study, we investigate the effect of thymoquinone (TQ), a phytochemical found in Nigella sativa, in regulating TNF-α-induced RA synovial fibroblast (RA-FLS) activation. Treatment with TQ (1-5µM) had no marked effect on the viability of human RA-FLS. Pre-treatment of TQ inhibited TNF-α-induced interleukin-6 (IL-6) and IL-8 production and ICAM-1, VCAM-1, and cadherin-11 (Cad-11) expression in RA-FLS (p<0.01). Evaluation of the signaling events showed that TQ inhibited TNF-α-induced phospho-p38 and phospho-JNK expression, but had no inhibitory effect on NF-κB pathway, in RA-FLS (p<0.05; n=4). Interestingly, we observed that selective down-regulation of TNF-α-induced phospho-p38 and phospho-JNK activation by TQ is elicited through inhibition of apoptosis-regulated signaling kinase 1 (ASK1). Furthermore, TNF-α selectively induced phosphorylation of ASK1 at Thr845 residue in RA-FLS, which was inhibited by TQ pretreatment in a dose dependent manner (p<0.01). Pre-treatment of RA-FLS with ASK1 inhibitor (TC ASK10), blocked TNF-α induced expression of ICAM-1, VCAM-1, and Cad-11. Our results suggest that TNF-α-induced ASK1-p38/JNK pathway is an important mediator of cytokine synthesis and enhanced expression of adhesion molecule in RA-FLS and TQ, by selectively inhibiting this pathway, may have a potential therapeutic value in regulating tissue destruction observed in RA.

8ß-hydroxy-3-oxopimar-15-ene exerts anti-inflammatory effects by inhibiting ROS-mediated activation of the TRAF6-ASK1-p38 signaling pathway.

The flying squirrel's droppings (Pteropus pselaphon) have been used for improving the blood circulation, arresting bleeding to treat hematological disorders, and reducing pain. Here, 8ß-hydroxy-3-oxopimar-15-ene (OXO), one of main constituents of P. pselaphon, was examined for its anti-inflammatory activity in murine macrophages. We found that OXO significantly suppressed LPS-induced nitric oxide (NO) without exerting cytotoxic effects on RAW 264.7 cells. OXO inhibited the expression of LPS-induced iNOS and COX-2 protein and their mRNA in a dose-dependent manner. Also, TNF-α, IL-6, and PGE2 secretion was decreased by OXO in LPS-stimulated macrophages. These inflammatory biomarkers were attributed to the suppression of LPS-induced activation of p38 MAPK and subsequent activation of two components of AP-1 (c-Jun and c-Fos), but not of ERK, JNK, NF-κB. Moreover, OXO inhibited LPS-induced intracellular reactive oxygen species (ROS) production and co-incubation of OXO and hydrogen peroxide (H2O2) suppressed the phosphorylation of p38 in a concentration-dependent manner. In addition, OXO completely disrupted the formation of TRAF6-ASK complex in the cells. Therefore, we demonstrate here that OXO can potentially inhibit several biomarkers related to inflammation through inhibition of ROS-mediated activation of TRAF6-ASK1-p38 pathway.

Binding of HIV-1 gp120 to DC-SIGN promotes ASK-1-dependent activation-induced apoptosis of human dendritic cells.

During disease progression to AIDS, HIV-1 infected individuals become increasingly immunosuppressed and susceptible to opportunistic infections. It has also been demonstrated that multiple subsets of dendritic cells (DC), including DC-SIGN⁺ cells, become significantly depleted in the blood and lymphoid tissues of AIDS patients, which may contribute to the failure in initiating effective host immune responses. The mechanism for DC depletion, however, is unclear. It is also known that vast quantities of viral envelope protein gp120 are shed from maturing HIV-1 virions and form circulating immune complexes in the serum of HIV-1-infected individuals, but the pathological role of gp120 in HIV-1 pathogenesis remains elusive. Here we describe a previously unrecognized mechanism of DC death in chronic HIV-1 infection, in which ligation of DC-SIGN by gp120 sensitizes DC to undergo accelerated apoptosis in response to a variety of activation stimuli. The cultured monocyte-derived DC and also freshly-isolated DC-SIGN⁺ blood DC that were exposed to either cross-linked recombinant gp120 or immune-complex gp120 in HIV⁺ serum underwent considerable apoptosis after CD40 ligation or exposure to bacterial lipopolysaccharide (LPS) or pro-inflammatory cytokines such as TNFα and IL-1ß. Furthermore, circulating DC-SIGN⁺ DC that were isolated directly from HIV-1⁺ individuals had actually been pre-sensitized by serum gp120 for activation-induced exorbitant apoptosis. In all cases the DC apoptosis was substantially inhibited by DC-SIGN blockade. Finally, we showed that accelerated DC apoptosis was a direct consequence of excessive activation of the pro-apoptotic molecule ASK-1 and transfection of siRNA against ASK-1 significantly prevented the activation-induced excessive DC death. Our study discloses a previously unknown mechanism of immune modulation by envelope protein gp120, provides new insights into HIV immunopathogenesis, and suggests potential therapeutic approaches to prevent DC depletion in chronic HIV infection.

Ectopic expression of human MutS homologue 2 on renal carcinoma cells is induced by oxidative stress with interleukin-18 promotion via p38 mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase (JNK) signaling pathways.

Human MutS homologue 2 (hMSH2), a crucial element of the highly conserved DNA mismatch repair system, maintains genetic stability in the nucleus of normal cells. Our previous studies indicate that hMSH2 is ectopically expressed on the surface of epithelial tumor cells and recognized by both T cell receptor γδ (TCRγδ) and natural killer group 2 member D (NKG2D) on Vδ2 T cells. Ectopically expressed hMSH2 could trigger a γδ T cell-mediated cytolysis. In this study, we showed that oxidative stress induced ectopic expression of hMSH2 on human renal carcinoma cells. Under oxidative stress, both p38 mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase (JNK) pathways have been confirmed to mediate the ectopic expression of hMSH2 through the apoptosis-signaling kinase 1 (ASK1) upstream and activating transcription factor 3 (ATF3) downstream of both pathways. Moreover, renal carcinoma cell-derived interleukin (IL)-18 in oxidative stress was a prominent stimulator for ectopically induced expression of hMSH2, which was promoted by interferon (IFN)-γ as well. Finally, oxidative stress or pretreatment with IL-18 and IFN-γ enhanced γδ T cell-mediated cytolysis of renal carcinoma cells. Our results not only establish a mechanism of ectopic hMSH2 expression in tumor cells but also find a biological linkage between ectopic expression of hMSH2 and activation of γδ T cells in stressful conditions. Because γδ T cells play an important role in the early stage of innate anti-tumor response, γδ T cell activation triggered by ectopically expressed hMSH2 may be an important event in immunosurveillance for carcinogenesis.

Myeloid cell death associated with Toll-like receptor 7/8-mediated inflammatory response. Implication of ASK1, HIF-1 alpha, IL-1 beta and TNF-alpha.

Programmed cell death or apoptosis is an important part of the host innate immune defence, especially against ssRNA viruses (influenza virus, HIV-1, ebola virus, hepatitis C virus and many others). Viral ssRNA is recognised by endosomal Toll-like receptors 7 and 8 (TLR7/8) which induce further stages of immune defence against these pathogens. Some of the immune cells die because of inflammatory stress allowing for the selection of those cells which are resistant to stress-induced apoptosis and which are used in further stages of the host immune response. On the other hand, apoptosis could be used as an instrument to suppress the function of activated inflammatory cells. However, the mechanisms underlying death of the inflammatory cells associated with stress induced by ligands of TLR7/8 remain unclear. In this study we have found that programmed death of human myeloid cells from different cell lines associated with ligand-induced TLR7/8-mediated inflammatory stress depends on activation of apoptosis signal-regulating kinase 1 (ASK1). This enzyme is, however, not required for the production of pro-inflammatory cytokines - TNF-α and IL-1ß. We have found that released IL-1ß and TNF-α are involved in apoptosis of myeloid cells associated with TLR7/8-mediated inflammatory stress. The pro-apoptotic effect of released TNF-α in this case is much lower compared to that of IL-1ß.

Role of apoptosis-regulating signal kinase 1 in innate immune responses by Mycobacterium bovis bacillus Calmette-Guérin.

Mycobacterium bovis bacillus Calmette-Guérin (BCG) induces innate immune responses through Toll-like receptor (TLR) 2 and TLR4. We investigated the role of apoptosis-regulating signal kinase (ASK) 1 in reactive oxygen species (ROS)-mediated innate immune responses induced by BCG mycobacterial infection. In macrophages, M. bovis BCG stimulation resulted in rapid activation of mitogen-activated protein kinases (MAPKs), secretion of inflammatory cytokines, such as tumor necrosis factor (TNF)-alpha and interleukin (IL)-6, and ROS generation in a TLR2- and TLR4-dependent manner. M. bovis BCG-induced ROS production led to robust activation of ASK1 upstream of the c-jun-N-terminal kinase and p38 MAPK, but not extracellular-regulated kinase 1/2. Blocking ASK1 activity markedly attenuated M. bovis BCG-induced TNF-alpha and IL-6 production by macrophages. Both TLR2 and TLR4 were required for optimal activation of ASK1 in response to M. bovis BCG. Furthermore, we present evidence that TNF receptor-associated factor (TRAF) 6 activities were essential for ROS-mediated ASK1 activation by M. bovis BCG. Finally, ASK1 activities were required for effective control of intracellular mycobacterial survival. Thus, the results of this study suggest a novel role of the TLR-ROS-TRAF6-ASK1 axis in the innate immune response to mycobacteria as a signaling intermediate.

ASK1-p38 MAPK-p47phox activation is essential for inflammatory responses during tuberculosis via TLR2-ROS signalling.

The roles of intracellular reactive oxygen species (ROS) and related signalling pathways in mycobacterial infection are largely unknown. Here we show that tuberculin purified protein derivative (PPD)/Toll-like receptor (TLR) 2/ROS signalling through activation of apoptosis-regulating signal kinase (ASK) 1 and p47phox pathways is responsible for the induction of proinflammatory responses during tuberculosis (TB) infection. Tuberculin PPD stimulation resulted in rapid activation of mitogen-activated protein kinases (MAPKs) and an early burst of ROS in monocytes/macrophages in a TLR2-dependent manner. PPD-induced ROS production led to robust activation of ASK1 upstream of p38 MAPK, via TLR2. Interestingly, phosphorylation of the cytosolic NADPH oxidase subunit p47phox and ASK1 activation are mutually dependent on PPD/TLR2-mediated signalling. Furthermore, active pulmonary TB patients showed upregulated ROS generation, as well as enhanced activation of ASK1/p38/p47phox pathways in their primary monocytes compared with healthy controls, which suggests a systemic primed status during TB. Taken together, these results indicate that activation of the ASK1/p38 MAPK/p47phox cascade plays a central role in PPD/TLR2-induced ROS generation and suggests the existence of a 'ROS/ASK1' inflammatory amplification feedback loop in monocytes/macrophages. The altered regulation of this axis with an increasing free-radical burden may contribute to the immunopathogenesis of human TB.