Human T-cell leukemia virus type-I oncoprotein Tax inhibits Fas-mediated apoptosis by inducing cellular FLIP through activation of NF-kappaB

Inhibition of methionine adenosyltransferase II induces FasL expression, Fas-DISC formation and caspase-8-dependent apoptotic death in T leukemic cells

Methionine adenosyltransferase II (MAT II) is a key enzyme in cellular metabolism and catalyzes the formation of S-adenosylmethionine (SAMe) from L-methionine and ATP. Normal resting T lymphocytes have minimal MAT II activity, whereas activated proliferating T lymphocytes and transformed T leukemic cells show significantly enhanced MAT II activity. This work was carried out to examine the role of MAT II activity and SAMe biosynthesis in the survival of leukemic T cells. Inhibition of MAT II and the resultant decrease in SAMe levels enhanced expression of FasL mRNA and protein, and induced DISC (Death Inducing Signaling Complex) formation with FADD (Fas-associated Death Domain) and procaspase-8 recruitment, as well as concomitant increase in caspase-8 activation and decrease in c-FLIP(s) levels. Fas-initiated signaling induced by MAT II inhibition was observed to link to the mitochondrial pathway via Bid cleavage and to ultimately lead to increased caspase-3 activation and DNA fragmentation in these cells. Furthermore, blocking MAT 2A mRNA expression, which encodes the catalytic subunits of MAT II, using a small-interfering RNA approach enhanced FasL expression and cell death, validating the essential nature of MAT II activity in the survival of T leukemic cells.

Involvement of FLIP in 2-methoxyestradiol-induced tumor regression in transgenic adenocarcinoma of mouse prostate model

PURPOSE

The purpose of this study is to investigate whether Fas-associated death domain interleukin-1 converting enzyme like inhibitory protein (FLIP) inhibition is a therapeutic target associated with 2-methoxyestradiol (2-ME2)-mediated tumor regression.

EXPERIMENTAL DESIGN

Expression and levels of FLIP were analyzed using (a) real-time PCR and immunoblot analysis in androgen-independent PC-3 cells treated with the newly formulated 2-ME2 and (b) immunohistochemistry in different Gleason pattern human prostate tumors. Transient transfections and chromatin immunoprecipitation (ChIP) assays were used to identify the transcription factors that regulate FLIP. Involvement of FLIP in 2-ME2-induced tumor regression was evaluated in transgenic adenocarcinoma mouse prostate (TRAMP) mice.

RESULTS

High Gleason pattern (5+5) human prostate tumors exhibit significant increase in FLIP compared with low Gleason pattern 3+3 (P=or<0.04). 2-ME2 reduced the levels and promoter activity of FLIP (P=0.001) in PC-3 cells. Transient expression assays show sequences between -503/+242 being sufficient for 2-ME2-induced inhibition of FLIP promoter activity. Cotransfection experiments show that overexpression of Sp1 activated, whereas Sp3 inhibited, Sp1 transactivation of FLIP promoter activity (P=0.0001). 2-ME2 treatment reduced binding of Sp1 to the FLIP promoter as evidenced by ChIP. Further, levels of FLIP associated with Fas or FADD decreased, whereas cleavage of caspase-8, levels of Bid, and apoptosis increased in response to 2-ME2 treatment in PC-3 cells. Administration of 2-ME2 regressed established prostate tumors in TRAMP mice that were associated with reduced expression of FLIP and Sp1.

CONCLUSION

Targeting Sp1-mediated FLIP signaling pathway may provide a novel approach for prostate cancer management.

Bortezomib-induced apoptosis in mature T-cell lymphoma cells partially depends on upregulation of Noxa and functional repression of Mcl-1

Bortezomib, a proteasome inhibitor that was originally developed as an inhibitor of nuclear factor-κB pathways, is currently used for the treatment of multiple myeloma (MM) and mantle cell lymphoma (MCL). The mechanisms of action of this antitumor agent have been studied by several investigators. Here, we explore the underlying mechanisms of bortezomib-induced apoptosis in cutaneous T-cell lymphoma (CTCL) and adult T-cell leukemia/lymphoma (ATLL) at the level of mitochondrial membrane injury. In all cell lines including (KMS-12-PE [MM], HUT78 [CTCL], ATN1 [ATLL], and MT4 [ATLL]), antiapoptotic factors such as c-Flip and XIAP were downregulated after exposure to bortezomib, probably via inhibition of nuclear factor-κB signaling. In addition, among the members of the BH3-only family, upregulation of Noxa was consistently seen at both the transcriptional and protein levels in a p53-independent manner after exposure to bortezomib. Repression of Noxa by small interfering RNA partially rescued CTCL and ATLL cells from bortezomib-induced apoptosis. Immunoprecipitation assays indicated time-dependent binding of Noxa and Mcl-1 in all cell types, suggesting that functional repression of Mcl-1 led to the loss of mitochondrial outer membrane potential. Similar results were also obtained in primary tumor cells from patients with ATLL. Taken together, we conclude that bortezomib-induced apoptosis in ATLL and CTCL cells at least partly depends on the upregulation of Noxa and functional repression of Mcl-1, as is also the case in MM and malignant melanoma.

Stimulation of interleukin-13 expression by human T-cell leukemia virus type 1 oncoprotein Tax via a dually active promoter element responsive to NF-kappaB and NFAT

The human T-cell leukemia virus type 1 (HTLV-1) Tax oncoprotein transforms human lymphocytes and is critical for the pathogenesis of HTLV-1-induced adult T-cell leukaemia. In HTLV-transformed cells, Tax upregulates interleukin (IL)-13, a cytokine with proliferative and anti-apoptotic functions that is linked to leukaemogenesis. Tax-stimulated IL-13 is thought to result in autocrine stimulation of HTLV-infected cells and thus may be relevant to their growth. The causal transactivation of the IL-13 promoter by Tax is predominantly dependent on a nuclear factor of activated T cells (NFAT)-binding P element. Here, it was shown that the isolated IL-13 Tax-responsive element (IL13TaxRE) was sufficient to mediate IL-13 transactivation by Tax and NFAT1. However, cyclosporin A, a specific NFAT inhibitor, revealed that Tax transactivation of IL13TaxRE or wild-type IL-13 promoter was independent of NFAT and that NFAT did not contribute to IL-13 upregulation in HTLV-transformed cells. By contrast, Tax stimulation was repressible by an efficient nuclear factor (NF)-kappaB inhibitor (IkBaDN), indicating the requirement for NF-kappaB. The capacity of NF-kappaB to stimulate IL13TaxRE was demonstrated by a strong response to NF-kappaB in reporter assays and by direct binding of NF-kappaB to IL13TaxRE. Thus, IL13TaxRE in the IL-13 promoter represents a dually active promoter element responsive to NF-kappaB and NFAT. Together, these results indicate that Tax causes IL-13 upregulation in HTLV-1-infected cells via NF-kappaB.

Novel cell death by downregulation of eEF1A1 expression in tetraploids

When duplicated sister chromatids are not properly compacted in mitosis, chromosomes are mis-segregated, inducing genetically unstable tetraploidy known to facilitate aneuploid malignancies. Here, we show that tetraploid cells produced by impaired chromosomal condensation are eliminated by a novel type of cell death different from caspase-dependent apoptosis. The cell death was associated with downregulation of eukaryotic translation elongation factor-1 alpha 1 (eEF1A1/EF-1alpha) expression in conjunction with accumulation of its mRNA in processing bodies (P bodies). Importantly, expression of exogenous eEF1A1 was shown to inhibit the caspase-independent cell death, and a similar cell death was observed after inducing the expression of short hairpin RNA specific for eEF1A1. Furthermore, the number of spontaneously arising binucleated cells was indicated to increase several fold during 1- to 2-week cultivation after initiation of exogenous eEF1A expression. Taken together, the novel cell death machinery should help to eliminate abnormal tetraploid cells and inhibit tumorigenesis.

Effect of transforming viruses on molecular mechanisms associated with cancer

Viruses have been linked to approximately 20% of all human tumors worldwide. These transforming viruses encode viral oncoproteins that interact with cellular proteins to enhance viral replication. The transcriptional and post-transcriptional effects of these viral oncoproteins ultimately result in cellular transformation. Historically, viral research has been vital to the discovery of oncogenes and tumor suppressors with more current research aiding in unraveling some mechanisms of carcinogenesis. Interestingly, since transforming viruses affect some of the same pathways that are dysregulated in human cancers, their study enhances our understanding of the multistep process of tumorigenesis. This review will examine the cellular mechanisms targeted by oncogenic human viruses and the processes by which these effects contribute to transformation. In particular, we will focus on three transforming viruses, human T-cell leukemia virus type-I, hepatitis B virus and human papillomavirus. These viruses all encode specific oncogenes that promote cell cycle progression, inhibit DNA damage checkpoint responses and prevent programmed cell death in an effort to promote viral propagation. While the transforming properties of these viruses are probably unintended consequences of replication strategies, they provide excellent systems in which to study cancer development.

Cellular FLICE-like inhibitory protein (C-FLIP): a novel target for cancer therapy

Cellular FLICE-like inhibitory protein (c-FLIP) has been identified as a protease-dead, procaspase-8-like regulator of death ligand-induced apoptosis, based on observations that c-FLIP impedes tumor necrosis factor-alpha (TNF-alpha), Fas-L, and TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis by binding to FADD and/or caspase-8 or -10 in a ligand-dependent fashion, which in turn prevents death-inducing signaling complex (DISC) formation and subsequent activation of the caspase cascade. c-FLIP is a family of alternatively spliced variants, and primarily exists as long (c-FLIP(L)) and short (c-FLIP(S)) splice variants in human cells. Although c-FLIP has apoptogenic activity in some cell contexts, which is currently attributed to heterodimerization with caspase-8 at the DISC, accumulating evidence indicates an anti-apoptotic role for c-FLIP in various types of human cancers. For example, small interfering RNAs (siRNAs) that specifically knocked down expression of c-FLIP(L) in diverse human cancer cell lines, e.g., lung and cervical cancer cells, augmented TRAIL-induced DISC recruitment, and thereby enhanced effector caspase stimulation and apoptosis. Therefore, the outlook for the therapeutic index of c-FLIP-targeted drugs appears excellent, not only from the efficacy observed in experimental models of cancer therapy, but also because the current understanding of dual c-FLIP action in normal tissues supports the notion that c-FLIP-targeted cancer therapy will be well tolerated. Interestingly, Taxol, TRAIL, as well as several classes of small molecules induce c-FLIP downregulation in neoplastic cells. Efforts are underway to develop small-molecule drugs that induce c-FLIP downregulation and other c-FLIP-targeted cancer therapies. In this review, we assess the outlook for improving cancer therapy through c-FLIP-targeted therapeutics.

MG-132 Sensitizes TRAIL-Resistant Prostate Cancer Cells by Activating c-Fos/c-Jun Heterodimers and Repressing c-FLIP(L)

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising anticancer agent because it induces apoptosis in cancer cells but not in normal cells. Unfortunately, some cancer cells develop resistance to TRAIL-induced apoptosis. Therefore, it is clinically relevant to determine the molecular mechanisms that differentiate between TRAIL-sensitive and TRAIL-resistant tumors. Previously, we have shown that the antiapoptotic molecule cellular-FLICE-inhibitory protein long isoform [c-FLIP(L)] is necessary and sufficient to maintain resistance to TRAIL-induced apoptosis. We have found that c-FLIP(L) is transcriptionally regulated by the activator protein-1 (AP-1) family member protein c-Fos. Here, we report that MG-132, a small-molecule inhibitor of the proteasome, sensitizes TRAIL-resistant prostate cancer cells by inducing c-Fos and repressing c-FLIP(L). c-Fos, which is activated by MG-132, negatively regulates c-FLIP(L) by direct binding to the putative promoter region of the c-FLIP(L) gene. In addition to activating c-Fos, MG-132 activates another AP-1 family member, c-Jun. We show that c-Fos heterodimerizes with c-Jun to repress transcription of c-FLIP(L). Therefore, MG-132 sensitizes TRAIL-resistant prostate cancer cells by activating the AP-1 family members c-Fos and c-Jun, which, in turn, repress the antiapoptotic molecule c-FLIP(L).

Rapid up-regulation of c-FLIP expression by BCR signaling through the PI3K/Akt pathway inhibits simultaneously induced Fas-mediated apoptosis in murine B lymphocytes

Cross-linking of BCR rapidly induces protection of B cells from Fas-mediated apoptosis, which has been assumed one of the important survival mechanisms of B cells during antigen stimulation. In the mouse B cell line A20, which is sensitive to Fas-mediated apoptosis, stimulation of BCR inhibited apoptosis induced via Fas upstream of caspase-8 activation with an associated rapid increase in the expression of both short and long forms of cellular caspase-8/FLICE-inhibitory protein (c-FLIP). The c-FLIP competitively inhibited the recruitment of caspase-8 to the death-inducing signaling complex (DISC), which took as long as 3h to form after the stimulation of Fas in A20 cells. Knockdown of c-FLIP by a short hairpin RNA-expressing method rendered BCR-stimulated A20 cells sensitive to Fas-mediated apoptosis. The BCR-induced rapid expression of c-FLIP was not affected by inactivation of NF-kappaB, but was inhibited by either treatment with a PI3K inhibitor, LY294002, or expression of a dominant negative PI3K p85 subunit, both of which suppressed phosphorylation of Akt and sensitized BCR-stimulated A20 cells to Fas-mediated apoptosis. Overexpression of constitutively active Akt was shown not only to up-regulate c-FLIP expression but also to render A20 cells resistant to Fas-mediated apoptosis. Moreover, treatment with LY294002 also suppressed BCR-induced up-regulation of c-FLIP expression in spleen B cells. Taken together, BCR-stimulation was shown to rapidly trigger a survival signal against simultaneously or ongoingly stimulated Fas-mediated apoptosis by promoting a PI3K/Akt signaling pathway-mediated up-regulation of c-FLIP expression.

Manipulation of the nuclear factor-kappaB pathway and the innate immune response by viruses

Viral and microbial constituents contain specific motifs or pathogen-associated molecular patterns (PAMPs) that are recognized by cell surface- and endosome-associated Toll-like receptors (TLRs). In addition, intracellular viral double-stranded RNA is detected by two recently characterized DExD/H box RNA helicases, RIG-I and Mda-5. Both TLR-dependent and -independent pathways engage the IkappaB kinase (IKK) complex and related kinases TBK-1 and IKKvarepsilon. Activation of the nuclear factor kappaB (NF-kappaB) and interferon regulatory factor (IRF) transcription factor pathways are essential immediate early steps of immune activation; as a result, both pathways represent prime candidates for viral interference. Many viruses have developed strategies to manipulate NF-kappaB signaling through the use of multifunctional viral proteins that target the host innate immune response pathways. This review discusses three rapidly evolving areas of research on viral pathogenesis: the recognition and signaling in response to virus infection through TLR-dependent and -independent mechanisms, the involvement of NF-kappaB in the host innate immune response and the multitude of strategies used by different viruses to short circuit the NF-kappaB pathway.