DAPK2 regulates oxidative stress in cancer cells by preserving mitochondrial function.

Death-associated protein kinase (DAPK) 2 is a serine/threonine kinase that belongs to the DAPK family. Although it shows significant structural differences from DAPK1, the founding member of this protein family, DAPK2 is also thought to be a putative tumour suppressor. Like DAPK1, it has been implicated in programmed cell death, the regulation of autophagy and diverse developmental processes. In contrast to DAPK1, however, few mechanistic studies have been carried out on DAPK2 and the majority of these have made use of tagged DAPK2, which almost invariably leads to overexpression of the protein. As a consequence, physiological roles of this kinase are still poorly understood. Using two genetically distinct cancer cell lines as models, we have identified a new role for DAPK2 in the regulation of mitochondrial integrity. RNA interference-mediated depletion of DAPK2 leads to fundamental metabolic changes, including significantly decreased rate of oxidative phosphorylation in combination with overall destabilised mitochondrial membrane potential. This phenotype is further corroborated by an increase in the production of mitochondrial superoxide anions and increased oxidative stress. This then leads to the activation of classical stress-activated kinases such as ERK, JNK and p38, which is observed on DAPK2 genetic ablation. Interestingly, the generation of oxidative stress is further enhanced on overexpression of a kinase-dead DAPK2 mutant indicating that it is the kinase domain of DAPK2 that is important to maintain mitochondrial integrity and, by inference, for cellular metabolism.

DAPK2 is a novel modulator of TRAIL-induced apoptosis.

Targeting molecules involved in TRAIL-mediated signalling has been hailed by many as a potential magic bullet to kill cancer cells efficiently, with little side effects on normal cells. Indeed, initial clinical trials showed that antibodies against TRAIL receptors, death receptor (DR)4 and DR5, are well tolerated by cancer patients. Despite efficacy issues in the clinical setting, novel approaches to trigger TRAIL-mediated apoptosis are being developed and its clinical potential is being reappraised. Unfortunately, as observed with other cancer therapies, many patients develop resistance to TRAIL-induced apoptosis and there is thus impetuous for identifying additional resistance mechanisms that may be targetable and usable in combination therapies. Here, we show that the death-associated protein kinase 2 (DAPK2) is a modulator of TRAIL signalling. Genetic ablation of DAPK2 using RNA interference causes phosphorylation of NF-κB and its transcriptional activity in several cancer cell lines. This then leads to the induction of a variety of NF-κB target genes, which include proapoptotic DR4 and DR5. DR4 and DR5 protein expression is correspondingly increased on the cell surface and this leads to the sensitisation of resistant cells to TRAIL-induced killing, in a p53-independent manner. As DAPK2 is a kinase, it is imminently druggable, and our data thus offer a novel avenue to overcome TRAIL resistance in the clinic.

Control of gp130 expression by the mitogen-activated protein kinase ERK2.

Interleukin (IL)-6-type cytokines such as IL-6, oncostatin M (OSM) and leukaemia inhibitory factor (LIF) signal through receptor complexes that are critically dependent on gp130. The latter is the common signal-transducing molecule that couples these cytokines to their downstream effectors, Janus kinases (JAKs) and signal transducers and activators of transcription (STATs). IL-6-type cytokine signalling additionally involves the recruitment and activation of extracellular signal-regulated kinase (ERK) 1 and ERK2. Both STATs and ERKs regulate responses mediated by members of the IL-6 family. Here, we show that ERK2, but not ERK1, also controls the expression and function of gp130 per se, as silencing ERK2 in human osteosarcoma U2OS cells inhibits the expression of gp130. This does not simply reflect quantitative differences between ERK1 and ERK2, and the effects are not restricted to osteosarcoma cells, as they can be extended to several other cancer cell types analysed to date (such as breast, prostate, lung and cervical cancer cells). Importantly, ERK2 binds to the GP130 promoter, where it perhaps interacts with the transcriptional machinery. Indeed, its role in the transcriptional regulation of the GP130 gene was corroborated using luciferase reporter assays and messenger RNA stability experiments. Considering the pivotal role that gp130 has in cancer and inflammation these data thus identify novel non-overlapping functions for ERK1 and ERK2 that are biologically relevant.

Multiple kinases in the interferon-gamma response.

Janus kinases (JAKs) and signal transducers and activators of transcription (STATs) are essential for responses to interferons (IFNs), most cytokines, and some growth factors. JAK/STAT signaling is not, however, sufficient for a full IFN-gamma response. Here, a convenient, robust, and quantitative flow cytometry-based kinome-wide siRNA screen has identified nine additional kinases as required for the IFN-gamma class II HLA response, seven for an antiviral response, and two for the cytopathic response to encephalomyocarditis virus (EMCV). As one example, inhibition of the IFN-gamma response by siRNA to ataxia telangiectasia-mutated (ATM) differentially affects a spectrum of IFN-gamma-stimulated mRNAs, with inhibitions being seen as early as 1 h after IFN-gamma stimulation. The implication of ATM, with its previously recognized function in chromatin decondensation, in the control of transcription early in the IFN-gamma response highlights both a role for ATM in cytokine responses and a possible correlation with the chromatin decondensation recently observed in response to IFN-gamma in mammalian cells. This work has, therefore, revealed the simplicity, power, and convenience of quantitative flow cytometry-based siRNA screens, a requirement for ATM and multiple additional kinases in the IFN-gamma response and a possible requirement for two of these kinases in the cytopathic response to EMCV.

A completely foreign receptor can mediate an interferon-gamma-like response.

A tripartite receptor comprising the external region of the erythropoietin (Epo) receptor, the transmembrane and JAK-binding domains of the gp130 subunit of the interleukin-6 (IL-6) receptor, and a seven amino acid STAT1 recruitment motif (Y440) from the interferon (IFN)-gamma receptor, efficiently mediates an IFN-gamma-like response. An analogous completely foreign chimeric receptor in which the Y440 motif is replaced with the Y905 motif from gp130 also mediates an IFN-gamma-like response, but less efficiently. The IFNGR1 signal-transducing subunit of the IFN-gamma receptor is tyrosine phosphorylated through the chimeric receptors and the endogenous IL-6 and OSM receptors. Cross phosphorylation of IFNGR1 is not, however, required for the IFN-gamma-like response through the chimeric receptors, nor does it mediate an IFN-gamma-like response to IL-6 or OSM. The data argue strongly for modular JAK/STAT signalling and against any rigid structural organization for the "pathways" involved. They emphasize the likely high degree of overlap between the signals generated from disparate JAK-receptor complexes and show that relatively minor changes in such complexes can profoundly affect the response.

Analysis of gene expression using high-density and IFN-gamma-specific low-density cDNA arrays.

The combination of high and low density cDNA filter array technology potentially permits both the identification of subsets of induced genes and convenient and rapid multisample expression profiling of such subsets under a variety of conditions. The JAK/STAT1 pathway for IFN-gamma signaling in human cells has been well characterized, but the extent and importance of additional pathways remain to be established. Here, using high-density filter arrays of the RZPD UniGene set, we identified 18 novel IFN-gamma-inducible genes. Expression profiling was carried out using low-density arrays representing both novel and known IFN-gamma-inducible genes. Initial experiments failed to detect evidence for any novel non-JAK-dependent pathways in cells expressing a kinase-dead JAK2. The data, however, validated the potential of the combined methods in establishing rapid and convenient expression profiling of several hundred genes in response to any ligand of choice.

Activation of SAPK/JNK by camptothecin sensitizes androgen-independent prostate cancer cells to Fas-induced apoptosis.

We have previously shown that the androgen-independent prostate cancer cells DU145, despite expressing Fas and FasL, were resistant to anti-Fas-induced apoptosis, and that this resistance could be overcome by pretreating the cells with sublethal doses of camptothecin. Here, we provide evidence that SAPK/JNK activity is required for camptothecin sensitization to anti-Fas-induced apoptosis. Camptothecin, but not Fas ligation, was shown to activate SAPK/JNK in a time-dependent manner, and to induce c-Jun expression. The effects were more prominent in cells treated with both camptothecin and anti-Fas. The expression levels of MKP-1, a phosphatase which regulates SAPK/JNK and which has been implicated in prostate cancer resistance to apoptosis, remained unchanged. Inhibition of caspases had no effect on the SAPK/JNK activation, suggesting that this activation is an upstream event in the Fas-signalling pathway, and is independent of caspase activity. Antisense oligonucleotides targeted to JNK1 and JNK2 reversed the effect of camptothecin. These results suggest that stress kinase activation can significantly influence the fate of androgen-independent prostate cancer cells following Fas receptor ligation.

Detection of molecular events during apoptosis by flow cytometry.

Apoptosis describes an intrinsic cell suicide program that may be activated by both endogenous and exogenous stimuli. This method of cell death is characterized by specific morphological features including chromatin condensation, nuclear fragmentation, cell shrinkage, membrane blebbing, and the formation of membrane-bound vesicles termed apoptotic bodies (1). Apoptosis has come to be referred to as the physiological mode of cell death, as it allows cellular destruction in the absence of an associated inflammatory response. In contrast, necrosis is a pathological mode of cell death that occurs under circumstances of severe cellular injury/trauma. Necrotic cell death involves cell swelling and organelle disruption, followed by lysis and release of cellular debris. This form of cell death may cause damage to surrounding tissue due to the inappropriate triggering of an inflammatory response (2).

Camptothecin sensitizes androgen-independent prostate cancer cells to anti-Fas-induced apoptosis.

Despite expressing both Fas and Fas ligand, DU145 and LNCaP prostate cancer cells were resistant to anti-Fas-induced cell death. Resistance to Fas-mediated cytotoxicity could be overcome in DU145, but not in LNCaP, cells by pretreating cells with sublethal doses of cytotoxic drugs, such as camptothecin. Activated caspases were shown to be required for this cytotoxicity. Indeed, poly(ADP-Ribose) polymerase was shown to be proteolytically cleaved in cells treated with camptothecin plus anti-Fas, but not in cells treated with anti-Fas only. Moreover, pretreatment of cells with ZVAD completely blocked camptothecin-mediated Fas-induced apoptosis. Sensitization of cells to Fas-induced cell death did not involve up-regulation of Fas or FasL, and it was independent of alterations in the cell cycle. Reactive oxygen intermediates (ROI) have been shown to be important mediators of drug-induced apoptosis. Here, we demonstrate that treatment of DU145 cells with camptothecin, anti-Fas, or both, did not alter the intracellular levels of peroxide or superoxide anion.

Molecular and cellular biology of prostate cancer-the role of apoptosis as a target for therapy.

Despite the high incidence and mortality rate of prostate cancer, the molecular mechanisms underlying the progression of the disease remain to be fully elucidated. This paper reviews the current state of knowledge on prostatic carcinogenesis, focusing not only on the molecular genetics of prostate cancer, but also on the role of oncogenes, tumour suppressor genes, and growth factors. The remarkable ability of prostate cancer cells to survive androgen withdrawal and apoptosis are discussed, with particular emphasis on how apoptosis may be manipulated from a therapeutic viewpoint.

Chemotherapeutic drug-induced apoptosis in human leukaemic cells is independent of the Fas (APO-1/CD95) receptor/ligand system.

The potential role of the Fas (CD95/APO-1) receptor/ligand system in chemotherapeutic drug-induced apoptosis was examined in a number of human leukaemic cell lines. Flow cytometric profiles of doxorubicin-treated HL-60, K562, U937 and Jurkat cells failed to show any significant increase in Fas or Fas ligand expression over 24 h, despite the induction of significant levels of apoptosis in these cells. Although preincubation of human leukaemic cells with a neutralizing anti-Fas IgG antibody blocked anti-Fas IgM-induced apoptosis, this strategy failed to inhibit chemotherapeutic drug-induced apoptosis. To determine whether recruitment of the Fas/Fas ligand complex during drug-induced apoptosis was a cell-specific event we utilized the CEM cell line. Doxorubicin treatment of CEM cells over 24 h failed to show any up-regulation in Fas or Fas ligand protein levels as detected by flow cytometry. Furthermore, neutralizing anti-Fas IgG Ab failed to inhibit chemotherapeutic drug-induced apoptosis in CEM cells. The present studies do, however, demonstrate a role for anti-Fas IgM Ab in producing a cytotoxic synergistic effect when used in combination with chemotherapeutic drugs. Low-dose anti-Fas IgM treatment in combination with doxorubicin, methotrexate, camptothecin and etoposide produced an augmented cytoxicity in CEM cells. Taken together these observations demonstrate that although recruitment of the Fas/APO-1/CD95 receptor/ligand system is not a necessary requirement for chemotherapeutic drug-induced apoptosis, combination of anti-Fas IgM and drug treatment produces a synergistic cytotoxic effect which may prove useful in the treatment of human leukaemias.