Roles of mitogen-activated protein kinase pathways during Escherichia coli-induced apoptosis in U937 cells

Francisella tularensis disrupts TLR2-MYD88-p38 signaling early during infection to delay apoptosis of macrophages and promote virulence in the host

Francisella tularensis is a zoonotic pathogen and the causative agent of tularemia. F. tularensis replicates to high levels within the cytosol of macrophages and other host cells while subverting the host response to infection. Critical to the success of F. tularensis is its ability to delay macrophage apoptosis to maintain its intracellular replicative niche. However, the host-signaling pathway(s) modulated by F. tularensis to delay apoptosis are poorly characterized. The outer membrane channel protein TolC is required for F. tularensis virulence and its ability to suppress apoptosis and cytokine expression during infection of macrophages. We took advantage of the F. tularensis ∆tolC mutant phenotype to identify host pathways that are important for activating macrophage apoptosis and that are disrupted by the bacteria. Comparison of macrophages infected with wild-type or ∆tolC F. tularensis revealed that the bacteria interfere with TLR2-MYD88-p38 signaling at early times post infection to delay apoptosis, dampen innate host responses, and preserve the intracellular replicative niche. Experiments using the mouse pneumonic tularemia model confirmed the in vivo relevance of these findings, revealing contributions of TLR2 and MYD88 signaling to the protective host response to F. tularensis, which is modulated by the bacteria to promote virulence. IMPORTANCE Francisella tularensis is a Gram-negative intracellular bacterial pathogen and the causative agent of the zoonotic disease tularemia. F. tularensis, like other intracellular pathogens, modulates host-programmed cell death pathways to ensure its replication and survival. We previously identified the outer membrane channel protein TolC as required for the ability of F. tularensis to delay host cell death. However, the mechanism by which F. tularensis delays cell death pathways during intracellular replication is unclear despite being critical to pathogenesis. In the present study, we address this gap in knowledge by taking advantage of ∆tolC mutants of F. tularensis to uncover signaling pathways governing host apoptotic responses to F. tularensis and which are modulated by the bacteria during infection to promote virulence. These findings reveal mechanisms by which intracellular pathogens subvert host responses and enhance our understanding of the pathogenesis of tularemia.

6H2L, a novel synthetic derivative of bifendate, induces apoptosis in hepatoma cells via mitochondrial and MAPK pathway

Hepatocellular carcinoma (HCC) is a major cause of cancer-related death worldwide. Our previous study indicated that 6H2L, a novel synthetic bifendate derivative, shows multidrug resistance reversal activity, while its antitumor effect has not been revealed. Here, the potent antitumor effects of 6H2L on hepatoma cells both in vitro and in vivo were investigated. 6H2L inhibited cell viability of HepG2 and SMMC-7721 cells with less sensitivity to normal human liver L-02 cells. 6H2L induced apoptosis in hepatoma cells. It upregulated Bax expression, while simultaneously decreasing Bcl-2 expression. Further elucidation of the mechanism revealed that 6H2L induced mitochondrial dysfunction, with transmitochondrial membrane potential collapse and cytochrome c release, which activated caspase-9 and caspase-3 and subsequently cleaved PARP, suggesting that 6H2L induced apoptosis via triggering mitochondrial pathway. Moreover, 6H2L decreased the phosphorylation of ERK1/2, whereas it increased the expression of p-JNK and p-p38. Then, specific inhibitors of the mitogen-activated protein kinase (MAPK) pathway were employed to confirm the roles of the MAPK pathway in the apoptosis-inducing effects of 6H2L. Additionally, 6H2L obviously inhibited the tumor growth in H22-bearing ICR mice. Meanwhile, 6H2L remarkably up-regulated Bax while suppressing Bcl-2 in tumors. Importantly, neither significant weight loss, white blood cell (WBC) count, nor histopathological abnormalities of major organs were observed in the mice receiving 6H2L treatment, indicating that 6H2L exerted strong anticancer activities with low toxicity in vivo. In contrast, fluorouracil inhibited tumor growth with significant decreased body weight and WBC count. Taken together, these results suggested 6H2L is a potential therapeutic candidate for HCC.

Melatonin Induces Apoptosis and Inhibits the Proliferation of Cancer Cells via Reactive Oxygen Species-mediated MAPK and mTOR Pathways

Background:

Recent human and animal studies have demonstrated the oncostatic properties of N-acetyl-5-methoxytryptamine (melatonin) in different types of cancer. However, in few cancer cell lines including colorectal cancer cell line (HT-29), acute T cell leukemia cell line (JURKAT) and cervical cancer cell line (HeLa), precise oncostatic mechanism induced by melatonin is yet to be described.

Objectives:

The aim of this study is to investigate the effects of melatonin in HT-29, JURKAT and HeLa cells and to determine the underlying molecular mechanism.

Methods:

Cell viability was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay while cell cycle, apoptosis and membrane potential were analysed by flow cytometry. Reactive oxygen species (ROS) was detected by 2',7'.dichlorofluorescein diacetate(DCFH-DA) staining. Protein expressions were determined by Western blot.

Results:

Our results showed that melatonin suppressed cell proliferation, increased the number of sub G1 hypodiploid cells and cell cycle arrest in HT-29, JURKAT and HeLa cells. Besides, melatonin also induced early and late apoptosis, although there were marked variations in responses between different cell lines (sensitivity; HeLa > HT-29 >JURKAT). Apart from that, staining with DCHF-DA demonstrated ROS production that was induced in a dose-dependent manner in HeLa, HT-29 and JURKAT cells. Moreover, the apoptotic process and oncostatic effect of melatonin were seen to be associated with extracellular-signal-regulated kinase (ERK) and stress-activated protein kinase/c-Jun NH (2)-terminal kinase (SAPK-JNK) signalling cascades in HeLa cells. In HT-29 and JURKAT cells, melatonin induced apoptosis via activation of p38 mitogen-activated protein kinases (p38), ERK and SAPK-JNK signalling pathways. In all three cell lines, the apoptotic event was triggered by the mammalian target of rapamycin (mTOR)-mediated activation of the downstream target rapamycininsensitive companion of mTOR (RICTOR) and/or regulatory-associated protein of mTOR (RAPTOR) proteins.

Conclusions:

Our findings confirm that melatonin induces apoptosis through reactive oxygen speciesmediated dysregulated mitogen-activated protein kinase (MAPK) and mTOR signalling pathways in these cancer cell lines.

Apoptosis induced by para-phenylenediamine involves formation of ROS and activation of p38 and JNK in chang liver cells

para-Phenylenediamine (p-PD) is a suspected carcinogen, but it has been widely used as a component in permanent hair dyes. In this study, the mechanism of p-PD-induced cell death in normal Chang liver cells was investigated. The results demonstrated that p-PD decreased cell viability in a dose-dependent manner. Cell death via apoptosis was confirmed by enhanced DNA damage and increased cell number in the sub-G1 phase of the cell cycle, using Hoechst 33258 dye staining and flow cytometry analysis. Apoptosis via reactive oxygen species generation was detected by the dichlorofluorescin diacetate staining method. Mitogen-activated protein kinase (MAPK) activation was assessed by western blot analysis and revealed that p-PD activated not only stress-activated protein kinase (SAPK)/c-Jun N-terminal kinases (JNK) and p38 MAPK but also extracellular signal-regulated kinase (ERK). Cytotoxicity and apoptosis induced by p-PD were markedly enhanced by ERK activation and selectively inhibited by ERK inhibitor PD98059, thus indicating a negative role of ERK. In contrast, inhibition of p38 MAPK activity with the p38-specific inhibitor SB203580 moderately inhibited cytotoxicity and apoptosis induction by p-PD. Similarly, SP600125, an inhibitor of SAPK/JNK, moderately inhibited cytotoxicity and apoptosis induced by p-PD, thus implying that p38 MAPK and SAPK/JNK had a partial role in p-PD-induced apoptosis. Western blot analysis revealed that p-PD significantly increased phosphorylation of p38 and SAPK/JNK and decreased phosphorylation of ERK. In conclusion, the results demonstrated that SAPK/JNK and p38 cooperatively participate in apoptosis induced by p-PD and that a decreased ERK signal contributes to growth inhibition or apoptosis.

Signalling profiles of circulating leucocytes in patients recovered from reactive arthritis

OBJECTIVES

Reactive arthritis (ReA) is a sterile joint inflammation triggered by a remote infection and associated with human leucocyte antigen (HLA)-B27. Its pathogenesis is unknown, but abnormal response to microbial structures or endogenous inflammatory mediators may be involved. We studied responses in leucocyte signalling profiles in patients with previous ReA after a full recovery.

METHOD

The study comprised 10 HLA-B27-positive healthy subjects with a history of Yersinia enterocolitica-triggered ReA (B27+ReA+) and 20 healthy reference subjects, of whom 10 carried HLA-B27 (B27+ReA-) and 10 did not (B27-ReA-). Phosphospecific fluorescent monoclonal antibodies and flow cytometry were used to determine activation of nuclear factor kappa B (NF-κB), signal transducers and activators of transcription (STATs) 1, 3, 5, and 6, and two mitogen-activated protein (MAP) kinases, p38 and extracellular signal-regulated kinase (ERK)1/2, in monocytes, lymphocytes, lymphocyte subsets, and neutrophils. B27+ReA+ and B27-ReA- whole-blood samples were incubated with Yersinia with or without infliximab to study the role of tumour necrosis factor (TNF) in lymphocyte subset activation. Samples of the three subject groups were studied using soluble bacterial or endogenous stimuli. Fluorescence levels were determined as relative fluorescence units (RFU) and the proportion of positively fluorescing cells.

RESULTS

The intracellular activation of circulating leucocytes in response to soluble stimuli was consistently comparable in B27+ReA+, B27+ReA-, and B27-ReA- subjects. Infliximab inhibited Yersinia-induced lymphocyte NF-κB phosphorylation similarly in B27+ReA+ and B27-ReA- groups.

CONCLUSIONS

ReA susceptibility is not reflected in leucocyte signalling profiles elicited by phlogistic stimuli. However, the possibility remains that aberrations occur in response to combinations of stimuli, such as those associated with leucocyte adhesion.

Escherichia coli induces apoptosis in human monocytic U937 cells through the Fas/FasL signaling pathway

Apoptosis is a genetically regulated cellular suicide mechanism that plays an essential role in development and in defense of multicellular organism. Escherichia coli (E. coli) can induce monocyte apoptosis; however, the mechanism is not clear. This study determines if Fas/FasL regulates E. coli-induced human monocyte line U937 cell apoptosis. We found that infection of U937 cells with E. coli induced rapid cell death in a dose- and time-dependent manner displaying the characteristic features of apoptosis. Moreover, opsonized E. coli induced U937 apoptosis with a higher apoptotic rate (53.29 ± 5.83%) than non-opsonized E. coli (19.37 ± 2.56%). Studying the underlying mechanisms we found that the E. coli-induced apoptosis was associated with a more prominent induction expression of Fas/FasL in a time- and dose-dependent manner. Furthermore, E. coli treatment resulted in a significant increase in the levels of DR5, TRAIL, and FADD, but exerted no statistically significant effects on the levels of DR4. The activity of caspase-8 enzyme increased in infection groups, positively correlated with apoptosis rate. Taken together, these results clearly indicate that receptor-mediated phagocytosis of E. coli induces apoptosis. Moreover, our findings suggest a possible regulatory role of Fas/FasL in the pathway of E. coli infection.

Steroid receptor coactivator 3 is required for clearing bacteria and repressing inflammatory response in Escherichia coli-induced septic peritonitis

Steroid receptor coactivator 3 (SRC-3) is a multifunctional protein that plays an important role in regulation of bacterial LPS-induced inflammation. However, its involvement in host defense against bacterial infection remains unclear. In this study, we used SRC-3 knockout mice to assess the role of SRC-3 in antibacterial defense in Escherichia coli-induced septic peritonitis. After E. coli bacteria were injected i.p., SRC-3-deficient mice exhibited excessive local and systemic inflammatory responses and more severe bacterial burdens, leading to a significantly higher mortality compared with wild-type mice. Peritoneal macrophages of SRC-3-deficient mice showed a decrease in bacterial phagocytosis in culture and an increase in apoptosis, which was consistent with the defective bacterial clearance observed in SRC-3-deficient mice. Accordingly, SRC-3 null macrophages expressed much lower levels of scavenger receptor A, the antioxidant enzyme catalase, and antiapoptotic gene Bcl-2. Collectively, our data demonstrate that SRC-3 is important not only in modulating the local and systemic inflammation but also in intensifying bacterial clearance, which highlights a pivotal role of SRC-3 in the host defense system against bacterial infection.