Structure of the promoter for the rat Fas antigen gene

NFkappaB in neurons? The uncertainty principle in neurobiology

Nuclear factor kappaB (NFkappaB) is a dynamically modulated transcription factor with an extensive literature pertaining to widespread actions across species, cell types and developmental stages. Analysis of NFkappaB in a complex environment such as neural tissue suffers from a difficulty in simultaneously establishing both activity and location. Much of the available data indicate a profound recalcitrance of NFkappaB activation in neurons, as compared with most other cell types. Few studies to date have sought to distinguish between the various combinatorial dimers of NFkappaB family members. Recent research has illuminated the importance of these problems, as well as opportunities to move past them to the nuances manifest through variable activation pathways, subunit complexity and target sequence preferences.

Mechanism of Fas-mediated cell death and its enhancement by TNF-alpha in human salivary gland adenocarcinoma cell line HSG

Fas-mediated cell death in a human salivary gland adenocarcinoma cell line (HSG) was induced by treatment of the cells with agonistic anti-Fas antibody (CH-11), and this cell death was enhanced by pretreatment with tumor necrosis factor alpha (TNF-alpha). The mode of cell death was apoptosis, because it was accompanied by caspase activation and the cleavage of poly(ADP-ribose) polymerase. The TNF-alpha treatment of the cells increased the expression of Fas, which was accompanied by the activation of nuclear factor kappaB (NFkappaB). These results suggest that the enhancement of the apoptosis caused by TNF-alpha resulted from increased sensitivity of the HSG cells to CH-11-mediated apoptosis due to induction of Fas protein by TNF-alpha via the activation of NFkappaB. In order to elucidate the apoptosis signaling pathway, we examined the effect of various caspase inhibitors on the apoptosis induced by CH-11. Fas-mediated apoptosis of HSG cells was slightly inhibited by the caspase-9 inhibitor although it was mainly inhibited by that for caspase-8. Based on this finding, we consider CH-11-induced apoptosis in HSG cells to be mainly mediated by the type I death signaling pathway that is caused by a caspase cascade initiated by the activation of caspase-8 at the death-inducing signaling complex (DISC).

Okadaic acid stimulates expression of Fas receptor and Fas ligand by activation of nuclear factor kappa-B in human oral squamous carcinoma cells

In the present study, we used western blot and RT-PCR analysis to examine the expression of proteins and mRNAs of Fas receptor and Fas ligand in human oral squamous carcinoma SCC-25 cells treated with okadaic acid. Treatment with okadaic acid enhanced the expression of proteins and mRNAs of both Fas receptor and Fas ligand in SCC-25 cells. The amount of IkappaB-alpha in whole cell lysates decreased, while the level of NF-kappaB in nucleus increased, in the okadaic acid-treated cells. Okadaic acid-treatment also alters the cellular localization of NF-kappaB, from cytoplasm to nuclei. To investigate the activation of NF-kappaB in okadaic acid-treated SCC-25 cells, we performed electrophoretic mobility gel shift assay using nuclear extracts and the consensus oligonucleotide for NF-kappaB DNA binding site. The binding of nuclear proteins to the oligonucleotide of NF-kappaB increased when the cells had been treated with 20 nM okadaic acid for 4 h. We transfected the cells with pFLF1, which has the promoter region of Fas receptor gene containing NF-kappaB binding site. A luciferase reporter gene assay demonstrated that the activity in the cells transfected with pFLF1 and treated with 20 nM okadaic acid increased in a time-dependent manner and that the activity was more than three-fold over that in the control cells. Our results suggest that NF-kappaB activated at early stages in the okadaic acid-treated SCC-25 cells stimulated the promoter activity of Fas receptor in the cells leading to the apoptotic death of these cells.

H(2)O(2) induces upregulation of Fas and Fas ligand expression in NGF-differentiated PC12 cells: modulation by cAMP

Fas, (APO-1/CD95), a transmembrane glycoprotein belonging to the tumor necrosis (TNF) receptor superfamily, transduces apoptotic death upon crosslinking by its cognate ligand (FasL). As upregulation of Fas/FasL expression occurs in neuropathological conditions (e.g., stroke, central nervous system [CNS] trauma and seizures) associated with oxidative damage, we questioned whether reactive oxygen species (ROS) can directly affect Fas and FasL expression in neuronal cells. Utilizing rat PC12 cells neuronally differentiated with nerve growth factor (NGF), we observed that concentrations of H(2)O(2) inducing apoptotic cell death rapidly trigger the expression of Fas mRNA and protein as well as FasL mRNA. Although NGF-addition to naive PC12 downregulated constitutive Fas and FasL transcription, the H(2)O(2)-induced Fas and FasL mRNA upregulation invariably occurred either in the presence or in the absence of NGF. Similarly, phorbol 1,2-myristate 1, 3-acetate (PMA), a potent protein kinase C (PKC) activator, did not modify Fas and FasL mRNA upregulation subsequent to H(2)O(2) exposure. On the contrary, forskolin and dibutyryl cAMP, which elevate intracellular cAMP by independent mechanisms, both counteracted H(2)O(2)-induced Fas, but not FasL, mRNA upregulation and increased constitutive expression of FasL mRNA. Altogether, our data show that oxidative stress is a major stimulus in eliciting Fas and FasL expression in NGF-differentiated PC12 cells. Moreover, we describe here for the first time the existence of cAMP-dependent mechanism(s) modulating Fas and FasL expression.

Cytokine induction of Fas gene expression in insulin-producing cells requires the transcription factors NF-kappaB and C/EBP

Fas-mediated cell death may play a role in the autoimmune destruction of pancreatic beta-cells in type 1 diabetes. beta-Cells do not express Fas under physiological conditions, but Fas mRNA and protein are induced in cytokine-exposed mouse and human islets, rendering the beta-cells susceptible to Fas ligand-induced apoptosis. The aim of the present study was to investigate the molecular regulation of Fas by cytokines in rat beta-cells and in insulin-producing RINm5F cells. Fas mRNA expression was increased 15-fold in fluorescence-activated cell sorting-purified rat beta-cells exposed to interleukin (IL)-1beta, whereas gamma-interferon had no effect. Transfection experiments of rat Fas promoter-luciferase reporter constructs into purified rat beta-cells and RINm5F insulinoma cells identified an IL-1beta-responsive region between nucleotides -223 and -54. Inactivation of two adjacent NF-kappaB and C/EBP sites in this region abolished IL-1beta-induced Fas promoter activity in RINm5F cells. Binding of NF-kappaB and C/EBP factors to their respective sites was confirmed by gel shift assays. In cotransfection experiments, NF-kappaB p65 transactivated the Fas promoter. NF-kappaB p50 and C/EBPbeta overexpression had no effect by themselves on the Fas promoter activity, but when cotransfected with p65, each factor inhibited transactivation by p65. These results suggest a critical role for NF-kappaB and C/EBP factors in cytokine-regulation of Fas expression in insulin-producing cells.

Malignant transformation of B lymphoma cell line BJAB by Epstein-Barr virus-encoded small RNAs

EBV-encoded small RNAs (EBERs) are non-polyadenylated and abundantly transcribed RNAs, whose functions have not yet been fully elucidated. Here, we report that the EBV-negative B lymphoma cell line, BJAB, was rendered more malignant and resistant to apoptosis by EBERs. EBER-transfected cells exhibited enhanced growth potential in SCID mice as well as in soft agar, and showed resistance to apoptotic stimuli in comparison with the vector control. EBERs inhibited the activity of the double-stranded RNA-dependent protein kinase, PKR, which is reputed to act as a tumor-suppressor. These results suggest that EBERs play an important role in the pathogenesis of EBV-associated malignancies through the inhibition of PKR.

NFkappaB mediates apoptosis through transcriptional activation of Fas (CD95) in adenoviral hepatitis

NFkappaB is an essential survival factor in several physiological conditions such as embryonal liver development and liver regeneration. However, NFkappaB is also a main mediator of the cellular response to a variety of extracellular stress stimuli, and it has been shown that some viral-induced host cell apoptosis appears to be dependent on NFkappaB activation. The activation of NFkappaB upon viral infection may be a rapid way of initiating an innate immune response against the viral particles. We have assessed the role of NFkB during the early phase of adenoviral hepatitis in a nude mouse model using an adenoviral vector expressing a mutant form of IkappaBalpha. Administration of a LacZ-expressing adenoviral vector induces NFkB DNA and correlates with the up-regulation of Fas (CD95) mRNA, but not FasL (CD95L) mRNA, during the early phase of adenoviral hepatitis. The rapid increase in NFkappaB DNA binding after adenoviral infection of the liver could be very effectively inhibited by IkappaBalpha. Compared with the LacZ control virus, the IkappaBalpha-expressing adenoviral vector inhibits the increase of Fas (CD95) mRNA expression, in particular in the very early phase of the hepatitis. Reporter gene experiments in hepatoma cell lines with a Fas promoter-luciferase construct indicated that the repression of Fas (CD95) mRNA by IkappaBalpha was transcriptionally mediated. The functional relevance of the NFkappaB-dependent increase in Fas (CD95) transcription was assessed by caspase 3 assays and terminal dUTP nick-end labeling tests. Compared with the control, IkappaBalpha adenoviral infection resulted in reduced caspase 3 activity during the early phase of viral hepatitis and in a prevention of liver cell apoptosis 24 h after adenoviral administration. Therefore our study demonstrates a new pro-apoptotic function of NFkappaB in Fas (CD95)-mediated apoptosis of hepatocytes. Interestingly, NFkappaB mediates liver cell apoptosis upon viral infection even in a phase where tumor necrosis factor-alpha is already induced, as shown by the time curves of tumor necrosis factor-alpha serum levels. Therefore, the pro- or anti-apoptotic role of NFkappaB appears to be more determined by the nature of the death stimulus than by the origin of the tissue.

Differential regulation and function of Fas expression on glial cells

Fas/Apo-1 is a member of the TNF receptor superfamily that signals apoptotic cell death in susceptible target cells. Fas or Fas ligand (FasL)-deficient mice are relatively resistant to the induction of experimental allergic encephalomyelitis, implying the involvement of Fas/FasL in this disease process. We have examined the regulation and function of Fas expression in glial cells (astrocytes and microglia). Fas is constitutively expressed by primary murine microglia at a low level and significantly up-regulated by TNF-alpha or IFN-gamma stimulation. Primary astrocytes express high constitutive levels of Fas, which are not further affected by cytokine treatment. In microglia, Fas expression is regulated at the level of mRNA expression; TNF-alpha and IFN-gamma induced Fas mRNA by approximately 20-fold. STAT-1alpha and NF-kappaB activation are involved in IFN-gamma- or TNF-alpha-mediated Fas up-regulation in microglia, respectively. The cytokine TGF-beta inhibits basal expression of Fas as well as cytokine-mediated Fas expression by microglia. Upon incubation of microglial cells with FasL-expressing cells, approximately 20% of cells underwent Fas-mediated cell death, which increased to approximately 60% when cells were pretreated with either TNF-alpha or IFN-gamma. TGF-beta treatment inhibited Fas-mediated cell death of TNF-alpha- or IFN-gamma-stimulated microglial cells. In contrast, astrocytes are resistant to Fas-mediated cell death, however, ligation of Fas induces expression of the chemokines macrophage inflammatory protein-1beta (MIP-1beta), MIP-1alpha, and MIP-2. These data demonstrate that Fas transmits different signals in the two glial cell populations: a cytotoxic signal in microglia and an inflammatory signal in the astrocyte.

IFN-gamma-dependent delay of in vivo tumor progression by Fas overexpression on murine renal cancer cells

The role of Fas in the regulation of solid tumor growth was investigated. Murine renal carcinoma (Renca) cells were constitutively resistant to Fas-mediated killing in vitro, but exhibited increased expression of Fas and sensitivity to Fas-mediated killing after exposure to IFN-gamma and TNF. Transfected Renca cells overexpressing Fas were efficiently killed in vitro upon exposure to anti-Fas Ab (Jo2). When Fas-overexpressing Renca cells were injected into syngenic BALB/c mice, there was a consistent and significant delay in tumor progression, reduced metastasis, and prolonged survival that was not observed for Renca cells that overexpressed a truncated nonfunctional Fas receptor. The delay of in vivo tumor growth induced by Fas overexpression was not observed in IFN-gamma-/- mice, indicating that IFN-gamma is required for the delay of in vivo tumor growth. However, there was a significant increase of infiltrated T cells and in vivo apoptosis in Fas-overexpressing Renca tumors, and Fas-overexpressing Renca cells were also efficiently killed in vitro by T cells. In addition, a strong therapeutic effect was observed on Fas-overexpressing tumor cells by in vivo administration of anti-Fas Ab, confirming that overexpressed Fas provides a functional target in vivo for Fas-specific ligands. Therefore, our findings demonstrate that Fas overexpression on solid tumor cells can delay tumor growth and provides a rationale for therapeutic manipulation of Fas expression as a means of inducing tumor regression in vivo.

Regulatable expression of the interferon-induced double-stranded RNA dependent protein kinase PKR induces apoptosis and fas receptor expression

PKR is an interferon-induced dsRNA-dependent protein kinase involved in the antiviral response as well as in cell growth and differentiation. Studies using a transdominant negative mutant of PKR also have implicated the kinase in tumor suppression and apoptosis. However, functional studies of PKR have been hampered by the lack of a suitable expression system. In this study, we used a tetracycline-regulated inducible system in NIH3T3 cells to investigate the involvement of PKR in programmed cell death (apoptosis). We show that expression of wild-type PKR causes apoptosis and correlates with increased mRNA levels for the Fas receptor, a member of the tumor necrosis family of proteins. Expression of an inactive form of PKR (K296R) or the vector alone did not induce apoptosis or elevate Fas mRNA levels. Our results clearly demonstrate that expression of an active form of PKR triggers apoptosis, possibly through upregulation of the Fas receptor.