Actin-independent exclusion of CD95 by PI3K/AKT signalling: implications for apoptosis

Involvement of CD95 and ligand in CD4+ T-cell and CD8+ T-cell depletion and hepatic cytolysis in patients with chronic viral hepatitis B

Background

Chronic viral hepatitis B (HBV) is characterised by progressive hepatocyte destruction and T-cell depletion. The mechanisms of the CD95-CD95 ligand (CD95L) signalling pathway during this chronic disease and the cirrhotic process remains unclear.

Objective

We evaluated the involvement of the CD95-CD95L receptor-ligand system in T-cell depletion and hepatic cytolysis in patients with chronic HBV.

Methods

This was a cross-sectional study conducted from September to December 2018 at the Yaoundé General Hospital, Cameroon. Four mL of whole blood was collected and analysed. The CD95 and CD95L levels, as well as the CD4+ T-cell and CD8+ T-cell counts, were performed by enzyme-linked immunosorbent assay and flow cytometry.

Results

Of the 130 HBV-positive patients, 36 (27.7%) were cirrhotic and 94 (72.3%) were non-cirrhotic. The cirrhotic patients had significantly elevated CD95 (p < 0.001) and CD95L (p = 0.001) plasma levels, compared with non-cirrhotic patients. The CD4/CD8 ratios were lower in cirrhotic patients, compared to non-cirrhotic patients (p < 0.001). There were statistically significant correlations between CD95 level and CD4⁺ T-cell counts, between CD95 level and CD8⁺ T-cell counts, between CD95 level and the CD4/CD8 ratio, between CD95 level and fibrosis score, and between CD95L level and fibrosis score.

Conclusion

CD95 and CD95L could be involved in T-cell depletion and hepatic cytolysis during the pathogenesis of chronic HBV and could potentially be used as biomarkers for immunological and hepatic monitoring in patients with chronic HBV.

Lipid rafts as signaling hubs in cancer cell survival/death and invasion: implications in tumor progression and therapy: Thematic Review Series: Biology of Lipid Rafts

Cholesterol/sphingolipid-rich membrane domains, known as lipid rafts or membrane rafts, play a critical role in the compartmentalization of signaling pathways. Physical segregation of proteins in lipid rafts may modulate the accessibility of proteins to regulatory or effector molecules. Thus, lipid rafts serve as sorting platforms and hubs for signal transduction proteins. Cancer cells contain higher levels of intracellular cholesterol and lipid rafts than their normal non-tumorigenic counterparts. Many signal transduction processes involved in cancer development (insulin-like growth factor system and phosphatidylinositol 3-kinase-AKT) and metastasis [cluster of differentiation (CD)44] are dependent on or modulated by lipid rafts. Additional proteins playing an important role in several malignant cancers (e.g., transmembrane glycoprotein mucin 1) are also being detected in association with lipid rafts, suggesting a major role of lipid rafts in tumor progression. Conversely, lipid rafts also serve as scaffolds for the recruitment and clustering of Fas/CD95 death receptors and downstream signaling molecules leading to cell death-promoting raft platforms. The partition of death receptors and downstream signaling molecules in aggregated lipid rafts has led to the formation of the so-called cluster of apoptotic signaling molecule-enriched rafts, or CASMER, which leads to apoptosis amplification and can be pharmacologically modulated. These death-promoting rafts can be viewed as a linchpin from which apoptotic signals are launched. In this review, we discuss the involvement of lipid rafts in major signaling processes in cancer cells, including cell survival, cell death, and metastasis, and we consider the potential of lipid raft modulation as a promising target in cancer therapy.

Hepatitis B Spliced Protein (HBSP) Suppresses Fas-Mediated Hepatocyte Apoptosis via Activation of PI3K/Akt Signaling

Hepatitis B spliced protein (HBSP) is known to associate with viral persistence and pathogenesis; however, its biological and clinical significance remains poorly defined. Acquired resistance to Fas-mediated apoptosis is thought to be one of the major promotors for hepatitis B virus (HBV) chronicity and malignancy. The purpose of this study was to investigate whether HBSP could protect hepatocytes against Fas-initiated apoptosis. We showed here that HBSP mediated resistance of hepatoma cells or primary human hepatocytes (PHH) to agonistic anti-Fas antibody (CH11)- or FasL-induced apoptosis. Under Fas signaling stimulation, expression of HBSP inhibited Fas aggregation and prevented recruitment of the adaptor molecule Fas-associated death domain (FADD) and procaspase-8 (or FADD-like interleukin-1β-converting enzyme [FLICE]) into the death-inducing signaling complex (DISC) while increasing recruitment of cellular FLICE-inhibitory protein L (FLIP_L) into the DISC. Those effects may be mediated through activation of the phosphoinositide 3-kinase (PI3K)/Akt pathway as evidenced by increased cellular phosphatidylinositol (3,4,5)-trisphosphate (PIP3) content and PI3K activity and enhanced phosphorylation of mTORC2 and PDPK1 as well as Akt itself. Confirmedly, inhibition of PI3K by LY294002 reversed the effect of HBSP on Fas aggregation, FLIP_L expression, and cellular apoptosis. These results indicate that HBSP functions to prevent hepatocytes from Fas-induced apoptosis by enhancing PI3K/Akt activity, which may contribute to the survival and persistence of infected hepatocytes during chronic infection.IMPORTANCE Our study revealed a previously unappreciated role of HBSP in Fas-mediated apoptosis. The antiapoptotic activity of HBSP is important for understanding hepatitis B virus pathogenesis. In particular, HBV variants associated with hepatoma carcinoma may downregulate apoptosis of hepatocytes through enhanced HBSP expression. Our study also found that Akt is centrally involved in Fas-induced hepatocyte apoptosis and revealed that interventions directed at inhibiting the activation or functional activity of Akt may be of therapeutic value in this process.

Hepatitis B Virus Surface Antigen Enhances the Sensitivity of Hepatocytes to Fas-Mediated Apoptosis via Suppression of AKT Phosphorylation

The Fas receptor/ligand system plays a prominent role in hepatic apoptosis and hepatocyte death. Although hepatitis B virus (HBV) surface Ag (HBsAg) is the most abundant HBV protein in the liver and peripheral blood of patients with chronic HBV infection, its role in Fas-mediated hepatocyte apoptosis has not been disclosed. In this study, we report that HBsAg sensitizes HepG2 cells to agonistic anti-Fas Ab CH11-induced apoptosis through increasing the formation of SDS-stable Fas aggregation and procaspase-8 cleavage but decreasing both the expression of cellular FLIP_L/S and the recruitment of FLIP_L/S at the death-inducing signaling complex (DISC). Notably, HBsAg increased endoplasmic reticulum stress and consequently reduced AKT phosphorylation by deactivation of phosphoinositide-dependent kinase-1 (PDPK1) and mechanistic target of rapamycin complex 2 (mTORC2), leading to enhancement of Fas-mediated apoptosis. In a mouse model, expression of HBsAg in mice injected with recombinant adenovirus-associated virus 8 aggravated Jo2-induced acute liver failure, which could be effectively attenuated by the AKT activator SC79. Based on these results, it is concluded that HBsAg predisposes hepatocytes to Fas-mediated apoptosis and mice to acute liver failure via suppression of AKT prosurviving activity, suggesting that interventions directed at enhancing the activation or functional activity of AKT may be of therapeutic value in Fas-mediated progressive liver cell injury and liver diseases.

Nonapoptotic functions of Fas/CD95 in the immune response

CD95 (also known as Fas) is a member of the tumor necrosis factor receptor (TNFR) superfamily. Its cognate ligand, CD95L, is implicated in immune homeostasis and immune surveillance. Mutations in this receptor are associated with a loss of apoptotic signaling and have been detected in an autoimmune disorder called autoimmune lymphoproliferative syndrome (ALPS) type Ia, which shares some clinical features with systemic lupus erythematosus (SLE). In addition, deletions and mutations of CD95 have been described in many cancers, which led researchers to initially classify this receptor as a tumor suppressor. More recent data demonstrate that CD95 engagement evokes nonapoptotic signals that promote inflammation and carcinogenesis. Transmembrane CD95L (m-CD95L) can be cleaved by metalloproteases, releasing a soluble ligand (s-CD95L). Soluble and membrane-bound CD95L show different stoichiometry (homotrimer versus multimer of homotrimers, respectively), which differentially affects CD95-mediated signaling through molecular mechanisms that remain to be elucidated. This review discusses the biological roles of CD95 in light of recent experiments addressing how a death receptor can trigger both apoptotic and nonapoptotic signaling pathways.

A Novel Covalent mTOR Inhibitor, DHM25, Shows in Vivo Antitumor Activity against Triple-Negative Breast Cancer Cells

Constitutive activation of the PI3K/mTOR signaling pathway contributes to carcinogenesis and metastasis in most, if not all, breast cancers. From a chromene backbone reported to inhibit class I PI3K catalytic subunits, several rounds of chemical syntheses led to the generation of a new collection of chromologues that showed enhanced ability to kill PI3K-addicted cancer cells and to inhibit Akt phosphorylation at serine 473, a hallmark of PI3K/mTOR activation. This initial screen uncovered a chromene designated DHM25 that exerted potent antitumor activity against breast tumor cell lines. Strikingly, DHM25 was shown to be a selective and covalent inhibitor of mTOR using biochemical and cellular analyses, modeling, and a large panel of kinase activity assays spanning the human kinome (243 kinases). Finally, in vivo, this novel drug was an efficient inhibitor of growth and metastasis of triple-negative breast cancer cells, paving the way for its clinical application in oncology.

Effect of CD95 on inflammatory response in rheumatoid arthritis fibroblast-like synoviocytes

OBJECTIVE

Many CD95-expressing cells don't always undergo apoptosis after stimulation with CD95 ligation. The purpose of this paper is to investigate the role of expression of CD95 (Fas/Apo1) on inflammatory response in fibroblast-like synoviocytes (FLS) obtained from rheumatoid arthritis (RA) and to evaluate the role of phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB or Akt) pathways within this process.

METHODS

The expression levels of CD95 were monitored by immunohistochemistry and reverse transcription polymerase chain reaction (RT-PCR). Apoptotic cells were detected by in situ apoptosis detection (TUNEL) assay. The RA-FLS were treated with agonistic anti-CD95 antibody or CD95 siRNA. Then the proliferation was detected by CCK-8, and mRNA level of inflammatory cytokines was detected by RT-PCR. After the RA-FLS were treated with agonistic anti-CD95 antibody, the total Akt and pAkt protein expression was analyzed by Western blot, and the changes mentioned above were observed while pre-incubated with the PI3K inhibitor LY294002.

RESULTS

A significant increase of CD95 antigen was found in RA compared with osteoarthritis (OA) samples, while apoptosis in RA synovial tissue was not obvious. Low concentrations of agonistic anti-CD95 antibody could promote RA-FLS growth and interleukin-6 (IL-6) mRNA expression, while high concentrations could induce apoptosis. And both of these phenomena could be inhibited by CD95 siRNA. Agonistic anti-CD95 antibody could stimulate the expression of pAkt, and PI3K specific inhibitor LY294002 could induce opposite changes.

CONCLUSION

Stimulation of CD95 could promote RA-FLS proliferation and inflammation, and activation of the PI3K/Akt signaling pathway might be the possible mechanism.

Nrf2 expression and activity in human T lymphocytes: stimulation by T cell receptor activation and priming by inorganic arsenic and tert-butylhydroquinone

The transcription factor nuclear factor-erythroid 2-related-2 (Nrf2) controls cellular redox homeostasis and displays immunomodulatory properties. Nrf2 alters cytokine expression in murine T cells, but its effects in human T lymphocytes are unknown. This study investigated the expression and activity of Nrf2 in human activated CD4(+) T helper lymphocytes (Th cells) that mediate the adaptive immune response. Th cells were isolated from peripheral blood mononuclear cells and activated with antibodies against CD3 and CD28, mimicking physiologic Th cell stimulation by dendritic cells. Nrf2 is hardly detectable in unstimulated Th cells. Activation of Th cells rapidly and strongly increases the levels of Nrf2 protein by increasing NRF2 gene transcription. Th cell activation also enhances mRNA and protein levels of Nrf2 target genes encoding antioxidant enzymes. Blocking Nrf2 expression using chemical inhibitors or siRNAs prevents these gene inductions. Pretreatment with inorganic arsenic, a Nrf2 inducer that does not alter NRF2 gene expression, increases protein level and transcriptional activity of Nrf2 induced by Th cell stimulation. Inorganic arsenic enhances nuclear translocation of Nrf2, its interaction with the coactivator protein p300, and its DNA binding activity. Inhibition of Nrf2 expression abrogates the effects of inorganic arsenic on mRNA levels of antioxidant genes, but does not alter the expression of IL-2, TNF-α, interferon-γ, or IL-17 in Th cells activated in the absence or presence of the metalloid. In conclusion, this study demonstrates for the first time that stimulation of human Th cells increases transcription of the NRF2 gene and activity of the Nrf2 protein. However, modulation of Nrf2 levels does not modify the secretion of inflammatory cytokines from these T lymphocytes.

The CD95/CD95L signaling pathway: a role in carcinogenesis

Apoptosis is a fundamental process that contributes to tissue homeostasis, immune responses, and development. The receptor CD95, also called Fas, is a member of the tumor necrosis factor receptor (TNF-R) superfamily. Its cognate ligand, CD95L, is implicated in immune homeostasis and immune surveillance, and various lineages of malignant cells exhibit loss-of-function mutations in this pathway; therefore, CD95 was initially classified as a tumor suppressor gene. However, more recent data indicate that in different pathophysiological contexts, this receptor can transmit non-apoptotic signals, promote inflammation, and contribute to carcinogenesis. A comparison with the initial molecular events of the TNF-R signaling pathway leading to non-apoptotic, apoptotic, and necrotic pathways reveals that CD95 is probably using different molecular mechanisms to transmit its non-apoptotic signals (NF-κB, MAPK, and PI3K). As discussed in this review, the molecular process by which the receptor switches from an apoptotic function to an inflammatory role is unknown. More importantly, the biological functions of these signals remain elusive.

Life in the Fas lane: differential outcomes of Fas signaling

Fas, also known as CD95 or APO-1, is a member of the tumor necrosis factor/nerve growth factor superfamily. Although best characterized in terms of its apoptotic function, recent studies have identified several other cellular responses emanating from Fas. These responses include migration, invasion, inflammation, and proliferation. In this review, we focus on the diverse cellular outcomes of Fas signaling and the molecular switches identified to date that regulate its pro- and anti-apoptotic functions. Such switches occur at different levels of signal transduction, ranging from the receptor through to cross-talk with other signaling pathways. Factors identified to date including other extracellular signals, proteins recruited to the death-inducing signaling complex, and the availability of different intracellular components of signal transduction pathways. The success of therapeutically targeting Fas will require a better understanding of these pathways, as well as the regulatory mechanisms that determine cellular outcome following receptor activation.

The effect of sea anemone (H. magnifica) venom on two human breast cancer lines: death by apoptosis

Venom from the sea anemone, Heteractis magnifica, has multiple biological effects including, cytotoxic, cytolytic and hemolytic activities. In this study, cytotoxicity induced by H. magnifica venom was investigated using the crystal violet assay on human breast cancer T47D and MCF7 cell lines and normal human breast 184B5 cell line. Apoptosis was also assayed via Annexin V-flourescein isothiocyanate and propidium iodide (PI) staining followed by flow cytometric analysis. Cell cycle progression and mitochondria membrane potential were studied via flow cytometry following PI and JC-1 staining respectively. H. magnifica venom induced significant reductions in viable cell numbers and increases in apoptosis in T47D and MCF7 in dose-dependent manners. A significant apoptosis-related increase in the sub G1 peak of the cell cycle in both breast cancer cell lines was also observed. Moreover, treatment by venom cleaved caspase-8, caspase-9, and activated caspase-3. Overall, H. magnifica venom was highly cytotoxic to T47D and MCF7 human breast cancer cells, and the phenomenon could be the killing phenomenon via the death receptor-mediated and the mitochondria-mediated apoptotic pathways. Consequently, H. magnifica venom has potential for the development of a breast cancer therapeutic.

CD95-mediated cell signaling in cancer: mutations and post-translational modulations

Apoptosis has emerged as a fundamental process important in tissue homeostasis, immune response, and during development. CD95 (also known as Fas), a member of the tumor necrosis factor receptor (TNF-R) superfamily, has been initially cloned as a death receptor. Its cognate ligand, CD95L, is mainly found at the plasma membrane of activated T-lymphocytes and natural killer cells where it contributes to the elimination of transformed and infected cells. According to its implication in the immune homeostasis and immune surveillance, and since several malignant cells of various histological origins exhibit loss-of-function mutations, which cause resistance towards the CD95-mediated apoptotic signal, CD95 has been classified as a tumor suppressor gene. Nevertheless, this assumption has been recently challenged, as in certain pathophysiological contexts, CD95 engagement transmits non-apoptotic signals that promote inflammation, carcinogenesis or liver/peripheral nerve regeneration. The focus of this review is to discuss these apparent contradictions of the known function(s) of CD95.

New advances in the diagnosis and treatment of autoimmune lymphoproliferative syndrome

PURPOSE OF REVIEW

Autoimmune lymphoproliferative syndrome (ALPS) is a disorder of disrupted lymphocyte homeostasis, resulting from mutations in the Fas apoptotic pathway. Clinical manifestations include lymphadenopathy, splenomegaly, and autoimmune cytopenias. A number of new insights have improved the understanding of the genetics and biology of ALPS. These will be discussed in this review.

RECENT FINDINGS

A number of key observations have been made recently that better define the pathophysiology of ALPS, including the characterization of somatic FAS variant ALPS, the identification of haploinsufficiency as a mechanism of decreased Fas expression, and the description of multiple genetic hits in FAS in some families that may explain the variable penetrance of the disease. In addition, ALPS has been shown to be a more common condition, as patients diagnosed with other disorders, including Evans syndrome and common variable immune deficiency, have been found to have ALPS. Finally, the treatment of the disease has changed as splenectomy and rituximab have been shown to have unexpected ALPS-specific toxicities, and mycophenolate mofetil and sirolimus have been demonstrated to have marked activity against the disease.

SUMMARY

On the basis of novel advances, the diagnostic algorithm and recommended treatment for ALPS have changed significantly, improving quality of life for many patients.