Phosphorylation of FADD at serine 194 by CKIalpha regulates its nonapoptotic activities

AK2 activates a novel apoptotic pathway through formation of a complex with FADD and caspase-10

Mitochondrial proteins function as essential regulators in apoptosis. Here, we show that mitochondrial adenylate kinase 2 (AK2) mediates mitochondrial apoptosis through the formation of an AK2-FADD-caspase-10 (AFAC10) complex. Downregulation of AK2 attenuates etoposide- or staurosporine-induced apoptosis in human cells, but not that induced by tumour-necrosis-factor-related apoptosis-inducing ligand (TRAIL) or Fas ligand (FasL). During intrinsic apoptosis, AK2 translocates to the cytoplasm, whereas this event is diminished in Apaf-1 knockdown cells and prevented by Bcl-2 or Bcl-X(L). Addition of purified AK2 protein to cell extracts first induces activation of caspase-10 via FADD and subsequently caspase-3 activation, but does not affect caspase-8. AFAC10 complexes are detected in cells undergoing intrinsic cell death and AK2 promotes the association of caspase-10 with FADD. In contrast, AFAC10 complexes are not detected in several etoposide-resistant human tumour cell lines. Taken together, these results suggest that, acting in concert with FADD and caspase-10, AK2 mediates a novel intrinsic apoptotic pathway that may be involved in tumorigenesis.

A novel genotyping strategy based on allele-specific inverse PCR for rapid and reliable identification of conditional FADD knockout mice

The apoptotic adapter protein FADD has been shown to play diverse roles in cell survival and proliferation. FADD knockout embryos died of heart defects, rendering Cre/loxP-mediated conditional FADD knockout mice a unique tool for investigating FADD-dependent nonapoptotic mechanism. Previously, these genetically engineered mice were identified by time-consuming Southern blot or controversial real-time PCR. In this article, we report a novel genotyping strategy based on allele-specific inverse PCR (ASI-PCR) for rapid and reliable identification of conditional FADD knockout mice. In this strategy, the knockout nature of FADD was simply identified by screening the absence of the wild type FADD-specific ASI-PCR product. Using this method, we accurately identified CD4-Cre-mediated T cell specific FADD knockout mice. The whole process can be accomplished in any normal biological laboratory within 12 h using genomic DNA from tail biopsy. The proposed ASI-PCR-based approach is simple, rapid, sensitive, reproducible, and especially suitable for genotyping small amount of spatiotemporally restricted biopsies and large animal population. We believe that the strategy described in this article may be of general utility in genotyping other conditional gene knockout mice.

FasL, Fas, and death-inducing signaling complex (DISC) proteins are recruited to membrane rafts after spinal cord injury

The Fas/CD95 receptor-ligand system plays an essential role in apoptosis that contributes to secondary damage after spinal cord injury (SCI), but the mechanism regulating the efficiency of FasL/Fas signaling in the central nervous system (CNS) is unknown. Here, FasL/Fas signaling complexes in membrane rafts were investigated in the spinal cord of adult female Fischer rats subjected to moderate cervical SCI and sham operation controls. In sham-operated animals, a portion of FasL, but not Fas was present in membrane rafts. SCI resulted in FasL and Fas translocation into membrane raft microdomains where Fas associates with the adaptor proteins Fas-associated death domain (FADD), caspase-8, cellular FLIP long form (cFLIPL ), and caspase-3, forming a death-inducing signaling complex (DISC). Moreover, SCI induced expression of Fas in clusters around the nucleus in both neurons and astrocytes. The formation of the DISC signaling platform leads to rapid activation of initiator caspase-8 and effector caspase-3, and the modification of signaling intermediates such as FADD and cFLIP(L) . Thus, FasL/Fas-mediated signaling after SCI is similar to Fas expressing Type I cell apoptosis.

Regulation of protein kinase CK1alphaLS by dephosphorylation in response to hydrogen peroxide

Low levels of hydrogen peroxide (H(2)O(2)) are mitogenic to mammalian cells and stimulate the hyperphosphorylation of heterogeneous nuclear ribonucleoprotein C (hnRNP-C) by protein kinase CK1alpha. However, the mechanisms by which CK1alpha is regulated have been unclear. Here it is demonstrated that low levels of H(2)O(2) stimulate the rapid dephosphorylation of CK1alphaLS, a nuclear splice form of CK1alpha. Furthermore, it is demonstrated that either treatment of endothelial cells with H(2)O(2), or dephosphorylation of CK1alphaLS in vitro enhances the association of CK1alphaLS with hnRNP-C. In addition, dephosphorylation of CK1alphaLS in vitro enhances the kinase's ability to phosphorylate hnRNP-C. While CK1alpha appears to be present in all metazoans, analysis of CK1alpha genomic sequences from several species reveals that the alternatively spliced nuclear localizing L-insert is unique to vertebrates, as is the case for hnRNP-C. These observations indicate that CK1alphaLS and hnRNP-C represent conserved components of a vertebrate-specific H(2)O(2)-responsive nuclear signaling pathway.

Constitutive phosphorylation mutation in Fas-associated death domain (FADD) results in early cell cycle defects

Fas-associated death domain (FADD) is an adaptor molecule for the death receptor subfamily of the tumor necrosis factor receptor superfamily, but it is also required for cell proliferation. Cell cycle-specific regulation of FADD phosphorylation plays an important role in FADD proliferative function since mice with a mutant form of FADD mimicking constitutive phosphorylation at serine 191 (FADD-D) exhibit defective T cell proliferation. Here we characterized these mice in detail and found that T cell development in 2-4-week-old mice is relatively normal, although mature FADD-D T cells manifest defective G(0) and G(1) to S transition with abnormalities in regulation of p130, p27 degradation, retinoblastoma protein phosphorylation, and CDK2 kinase activity. These downstream defects are further associated with the failure to up-regulate the forkhead box M1 cell cycle transcription factor, FoxM1. FADD-D protein is also mislocalized during cell cycle progression. Thus, regulation of FADD phosphorylation is crucial for proper cell cycle entry.

Death-effector domain-containing protein DEDD is an inhibitor of mitotic Cdk1/cyclin B1

Accumulating evidence has shown that many molecules, including some cyclin-dependent kinases (Cdks) and cyclins, as well as the death-effector domain (DED)-containing FADD, function for both apoptosis and cell cycle. Here we identified that DEDD, which also possesses the DED domain, acts as a novel inhibitor of the mitotic Cdk1/cyclin B1 complex. DEDD associates with mitotic Cdk1/cyclin B1 complexes via direct binding to cyclin B1 and reduces their function. In agreement, kinase activity of nuclear Cdk1/cyclin B1 in DEDD-null (DEDD-/-) embryonic fibroblasts is increased compared with that in DEDD+/+ cells, which results in accelerated mitotic progression, thus exhibiting a shortened G2/M stage. Interestingly, DEDD-/- cells also demonstrated decreased G1 duration, which perhaps enhanced the overall reduction in rRNA amounts and cell volume, primarily caused by the rapid termination of rRNA synthesis before cell division. Likewise, DEDD-/- mice show decreased body and organ weights relative to DEDD+/+ mice. Thus, DEDD is an impeder of cell mitosis, and its absence critically influences cell and body size via modulation of rRNA synthesis.

Expression of nuclear and cytoplasmic phosphorylated FADD in gastric cancers

Fas-associated death domain (FADD) plays a crucial role during death receptor-mediated apoptosis. In addition, FADD possesses apoptosis-independent activities, including cell-cycle regulation and cell proliferation regulated by the phosphorylation of FADD at Ser194. The aim of this study was to explore the possibility whether alteration of phosphorylated FADD (p-FADD) expression might be a characteristic of gastric cancer. We analyzed the expression of p-FADD protein in 60 gastric adenocarcinomas by immunohistochemistry using a tissue microarray approach. In the normal gastric mucosal cells, surface and glandular epithelial cells evenly expressed p-FADD in the nuclei but not in the cytoplasm. In the cancers, p-FADD expression was detected in 38 cases (63%) of the gastric carcinomas, but there was no p-FADD immunostaining in the remaining 22 cancers (37%). Of note, p-FADD immunostaining was observed in cytoplasm/nuclei (20 cancers; 33%) and cytoplasm (18 cancers; 30%). There was no significant association of p-FADD expression with clinocopathological characteristics, including invasion, metastasis, and stage. Our data showed that the expression of p-FADD in gastric cancers was heterogenous in its location compared to the uniform nuclear expression of p-FADD in normal gastric cells. Many of the cancers (67%) were devoid of nuclear p-FADD, suggesting that p-FADD functions in the nucleus may be perturbed in the cancers. Also, p-FADD expression in the cytoplasm in a large fraction of the cancers (63%), not seen in the normal cells, suggested that the cell death functions of p-FADD could be altered in the cancer cells.

Apoptosis in the treatment of cancer: a promise kept?

A common feature of cancer cells is their ability to evade apoptosis as a result of alterations that block cell death signaling pathways. The extensive research efforts that elucidated these signaling pathways over the past decade have set the stage for the development of therapeutic agents that either kill cancer cells selectively or reset their apoptotic threshold. Over the past two years a number of these agents have been evaluated in preclinical and clinical trials. The results of these studies suggest that it might soon be possible to modulate apoptosis in cancer cells for therapeutic benefit.

CKI and CKII mediate the FREQUENCY-dependent phosphorylation of the WHITE COLLAR complex to close the Neurospora circadian negative feedback loop

The eukaryotic circadian oscillators consist of circadian negative feedback loops. In Neurospora, it was proposed that the FREQUENCY (FRQ) protein promotes the phosphorylation of the WHITE COLLAR (WC) complex, thus inhibiting its activity. The kinase(s) involved in this process is not known. In this study, we show that the disruption of the interaction between FRQ and CK-1a (a casein kinase I homolog) results in the hypophosphorylation of FRQ, WC-1, and WC-2. In the ck-1a(L) strain, a knock-in mutant that carries a mutation equivalent to that of the Drosophila dbt(L) mutation, FRQ, WC-1, and WC-2 are hypophosphorylated. The mutant also exhibits ~32 h circadian rhythms due to the increase of FRQ stability and the significant delay of FRQ progressive phosphorylation. In addition, the levels of WC-1 and WC-2 are low in the ck-1a(L) strain, indicating that CK-1a is also important for the circadian positive feedback loops. In spite of its low accumulation in the ck-1a(L) strain, the hypophosphorylated WCC efficiently binds to the C-box within the frq promoter, presumably because it cannot be inactivated through FRQ-mediated phosphorylation. Furthermore, WC-1 and WC-2 are also hypophosphorylated in the cka(RIP) strain, which carries the disruption of the catalytic subunit of casein kinase II. In the cka(RIP) strain, WCC binding to the C-box is constantly high and cannot be inhibited by FRQ despite high FRQ levels, resulting in high levels of frq RNA. Together, these results suggest that CKI and CKII, in addition to being the FRQ kinases, mediate the FRQ-dependent phosphorylation of WCs, which inhibit their activity and close the circadian negative feedback loop.

Caspases at the crossroads of immune-cell life and death

Caspases are responsible for crucial aspects of inflammation and immune-cell death that are disrupted in a number of genetic autoimmune and autoinflammatory diseases. The caspase family of proteases can be divided into pro-apoptotic and pro-inflammatory members based on their substrate specificity and participation in separate signalling cascades. However, as discussed here, evidence has emerged over the past few years that a number of the caspases thought to be involved solely in apoptosis also contribute to specific aspects of immune-cell development, activation and differentiation, and can even protect cells from some forms of cell death.

cFLIP regulation of lymphocyte activation and development

Cellular caspase-8 (FLICE)-like inhibitory protein (cFLIP) was originally identified as an inhibitor of death-receptor signalling through competition with caspase-8 for recruitment to FAS-associated via death domain (FADD). More recently, it has been determined that both cFLIP and caspase-8 are required for the survival and proliferation of T cells following T-cell-receptor stimulation. This paradoxical finding launched new investigations of how these molecules might connect with signalling pathways that link to cell survival and growth following antigen-receptor activation. As discussed in this Review, insight gained from these studies indicates that cFLIP and caspase-8 form a heterodimer that ultimately links T-cell-receptor signalling to activation of nuclear factor-kappaB through a complex that includes B-cell lymphoma 10 (BCL-10), mucosa-associated-lymphoid-tissue lymphoma-translocation gene 1 (MALT1) and receptor-interacting protein 1 (RIP1).

FADD phosphorylation is critical for cell cycle regulation in breast cancer cells

Anti-oestrogen therapy is effective for control of hormone receptor-positive breast cancers, although the detailed molecular mechanisms, including signal transduction, remain unclear. We demonstrated here that long-term tamoxifen treatment causes G2/M cell cycle arrest through c-jun N-terminal kinase (JNK) activation, which is dependent on phosphorylation of Fas-associated death domain-containing protein (FADD) at 194 serine in an oestrogen (ER) receptor-positive breast cancer cell line, MCF-7. Expression of a dominant negative mutant form of MKK7, a kinase upstream of JNK, or mutant FADD (S194A) in MCF-7 cells suppressed the cytotoxicity of long-term tamoxifen treatment. Of great interest, similar signallings could be evoked by paclitaxel, even in an ER-negative cell line, MDA-MB-231. In addition, immunohistochemical analysis using human breast cancer specimens showed a close correlation between phosphorylated JNK and FADD expression, both being significantly reduced in cases with metastatic potential. We conclude that JNK-mediated phosphorylation of FADD plays an important role in the negative regulation of cell growth and metastasis, independent of the ER status of a breast cancer, so that JNK/FADD signals might be promising targets for cancer therapy.

Nonapoptotic functions of FADD-binding death receptors and their signaling molecules

Death receptors (DRs) are surface receptors that when triggered have the capacity to induce apoptosis in cells by forming the death-inducing signaling complex (DISC). The first protein recruited to form the DISC is the adaptor protein FADD/Mort1. Some members of the DR family, CD95 and the TRAIL receptors DR4 and DR5, directly bind FADD, whereas others, such as TNF receptor I and DR3, initially bind another adaptor protein, TRADD, which then recruits FADD. While all DRs can activate both apoptotic and non-apoptotic pathways, it has been widely assumed that the main physiological role of FADD-binding death receptors is to trigger apoptosis. However, recent work has ascribed multiple non-apoptotic activities to these receptors and/or the signaling components of the DISC.