Changes in cIAP2, survivin and BimEL expression characterize the switch from autophagy to apoptosis in prolonged starvation.

BACKGROUND: Autophagy is a catabolic process involving the engulfment of cytoplasmic content within autophagosomes followed by their delivery to lysosomes. This process is a survival mechanism, enabling cells to cope with nutrient deprivation by degradation and recycling of macromolecules. Yet during continued stress such as prolonged starvation, a switch from autophagy to apoptosis is often detected. OBJECTIVE: In this work, we characterized the temporal dynamics of the transition from autophagy towards apoptosis with the aim of elucidating the molecular mechanism regulating the switch from survival autophagy to apoptotic cell death. RESULTS AND CONCLUSIONS: We defined an inverse relationship between apoptosis and autophagy spanning a period of 72 h, manifested by the sequential reduction in LC3 lipidation and the activation of caspase-3. The transition to apoptosis correlated with a selective decline in the mRNA and protein levels of two anti-apoptotic IAP family proteins, survivin and cIAP2 and a selective increase in the BH3-only protein, BimEL. This 'molecular signature' was common to several cell lines undergoing the switch from autophagy to apoptosis during prolonged starvation. Mechanistically, the increased BimEL protein levels resulted from its reduced binding to its specific E3 ligase, ßTrCP, leading to protein stabilization. Consistent with this, BimEL showed decreased phosphorylation at critical sites previously reported to be essential for binding to the E3 ligase. The decrease in the anti-apoptotic IAPs and the increase in the pro-apoptotic BimEL may thus constitute a molecular switch from autophagy to apoptosis during prolonged starvation.

DAP kinase activates a p19ARF/p53-mediated apoptotic checkpoint to suppress oncogenic transformation.

DAP kinase is a pro-apoptotic calcium-regulated serine/threonine kinase, whose expression is frequently lost in human tumours. Here we show that DAP kinase counteracts oncogene-induced transformation by activating a p19ARF/p53-dependent apoptotic checkpoint. Ectopic expression of DAP kinase suppressed oncogenic transformation of primary embryonic fibroblasts by activating p53 in a p19ARF-dependent manner. Consequently, the fibroblasts underwent apoptosis, characterized by caspase activation and DNA fragmentation. In response to c-Myc or E2F-1, the endogenous DAP kinase protein was upregulated. Furthermore, functional or genetic inactivation of the endogenous DAP kinase reduced the extent of induction of p19ARF/p53 and weakened the subsequent apoptotic responses to c-Myc or E2F-1. These results establish a role for DAP kinase in an early apoptotic checkpoint designed to eliminate pre-malignant cells during cancer development.

DAP-kinase participates in TNF-alpha- and Fas-induced apoptosis and its function requires the death domain.

Death-associated protein (DAP)-kinase is a calcium/calmodulin regulated serine/threonine kinase that carries ankyrin repeats, a death domain, and is localized to the cytoskeleton. Here, we report that this kinase is involved in tumor necrosis factor (TNF)-alpha and Fas-induced apoptosis. Expression of DAP-kinase antisense RNA protected cells from killing by anti-Fas/APO-1 agonistic antibodies. Deletion of the death domain abrogated the apoptotic functions of the kinase, thus, documenting for the first time the importance of this protein domain. Overexpression of a fragment encompassing the death domain of DAP-kinase acted as a specific dominant negative mutant that protected cells from TNF-alpha, Fas, and FADD/MORT1-induced cell death. DAP-kinase apoptotic function was blocked by bcl-2 as well as by crmA and p35 inhibitors of caspases, but not by the dominant negative mutants of FADD/MORT1 or of caspase 8. Thus, it functions downstream to the receptor complex and upstream to other caspases. The multidomain structure of this serine/threonine kinase, combined with its involvement in cell death induced by several different triggers, place DAP-kinase at one of the central molecular pathways leading to apoptosis.

A genetic tool used to identify thioredoxin as a mediator of a growth inhibitory signal.

Loss of sensitivity to growth inhibitory polypeptides is likely to be one of the events that participates in the formation of some tumors and might be caused by inactivation or loss of the genetic elements that transduce these extracellular signals. The isolation of such a gene was achieved by randomly inactivating genes by an anti-sense complementary DNA expression library followed by direct selection for growth in the presence of an inhibitory polypeptide. Thus, a gene whose inactivation conveyed growth resistance to interferon-gamma (IFN-gamma) was isolated. Sequence analysis showed complete identity with human thioredoxin, a dithiol reducing agent, implicated here in the IFN-gamma-mediated growth arrest of HeLa cells.

Tumor necrosis factor reduces c-myc expression and cooperates with interferon-gamma in HeLa cells.

The suppression of the c-myc nuclear oncogene is associated with growth arrest and may therefore be directly controlled by naturally occurring growth inhibitors. The effect of tumor necrosis factor (TNF) and of interferon-gamma (IFN-gamma) on c-myc expression was investigated in HeLa cells, which respond to these cytokines by a specific arrest in the G0/G1 phase of the cell cycle. Northern blot and nuclear transcription analyses indicated that each cytokine reduced within 1 to 3 hours the c-myc messenger RNA levels as a result of transcriptional inhibition. Adding the two cytokines together at saturating levels resulted in enhanced inhibition of c-myc transcription and of the c-myc messenger RNA steady-state levels. While the reduction of c-myc messenger RNA by IFN-gamma was dependent on new protein synthesis, the inhibitory effect of TNF on c-myc messenger RNA was direct and was not abrogated by cycloheximide. The differential effect of the protein synthesis inhibitor and the cooperative inhibitory effects of the two cytokines when added together suggest that IFN-gamma and TNF reduce c-myc transcription through different molecular mechanisms.

Absence of TGF-beta receptors and growth inhibitory responses in retinoblastoma cells.

The responses of retinoblastoma tumor cells and normal retinal cells to various growth inhibitory factors were examined. Whereas fetal retinal cells were highly sensitive to the antimitogenic effects of transforming growth factor beta 1 (TGF-beta 1), retinoblastoma tumor cell lines were all resistant to this factor. Binding assays and affinity labeling of these cells with radioiodinated TGF-beta 1 revealed that the cells did not have TGF-beta receptors. The retinoblastoma cells lacked the three affinity-labeled proteins of 65, 95, and 300 kilodaltons typically seen in human cell lines and thus differed from normal retinal cells and from other types of neuroectodermal tumors that display the normal pattern of receptors. Loss of TGF-beta receptors, which is a rare event among tumor cells, may represent one mechanism through which these cells escape from negative control and form retinoblastomas.

The autophagic inducer smARF interacts with and is stabilized by the mitochondrial p32 protein.

The alternative reading frame (ARF) mRNA encodes two pro-death proteins, the nucleolar p19ARF and a shorter mitochondrial isoform, named smARF (hsmARF in human). While p19ARF can inhibit cell growth by causing cell cycle arrest or type I apoptotic cell death, smARF is able to induce type II autophagic cell death. Inappropriate proliferative signals generated by proto-oncogenes, such as c-Myc and E2F1, can elevate both p19ARF and smARF proteins. Here, we reveal a novel means of regulation of smARF protein steady state levels through its interactions with the mitochondrial p32. The p32 protein physically interacts with both human and murine smARF, and colocalizes with these short isoforms to the mitochondria. Remarkably, knocking down p32 protein levels significantly reduced the steady state levels of smARF by increasing its turn over. As a consequence, the ability of ectopically expressed smARF to induce autophagy and to cause mitochondrial membrane dissipation was significantly reduced. In contrast, the protein levels of full-length p19ARF, which mainly resides in the nucleolus, were not influenced by p32 depletion, suggesting that p32 exclusively stabilizes the mitochondrial smARF protein. Thus the interaction with p32 provides a means of specifically regulating the expression of the recently identified autophagic inducer, smARF, and adds yet another layer of complexity to the multifaceted regulation of the ARF gene.

DAP kinase regulates JNK signaling by binding and activating protein kinase D under oxidative stress.

The stress-activated kinase JNK mediates key cellular responses to oxidative stress. Here we show that DAP kinase (DAPk), a cell death promoting Ser/Thr protein kinase, plays a main role in oxidative stress-induced JNK signaling. We identify protein kinase D (PKD) as a novel substrate of DAPk and demonstrate that DAPk physically interacts with PKD in response to oxidative stress. We further show that DAPk activates PKD in cells and that induction of JNK phosphorylation by ectopically expressed DAPk can be attenuated by knocking down PKD expression or by inhibiting its catalytic activity. Moreover, knockdown of DAPk expression caused a marked reduction in JNK activation under oxidative stress, indicating that DAPk is indispensable for the activation of JNK signaling under these conditions. Finally, DAPk is shown to be required for cell death under oxidative stress in a process that displays the characteristics of caspase-independent necrotic cell death. Taken together, these findings establish a major role for DAPk and its specific interaction with PKD in regulating the JNK signaling network under oxidative stress.

Initial characterization of a spontaneous interferon secreted during growth and differentiation of Friend erythroleukemia cells.

A gradual increase in the level of 2',5'-oligoadenylate synthetase takes place in Friend erythroleukemia cells after a shiftdown in the rate of cell growth. The increase is about 5-fold after entry of cells into the stationary phase of growth, but much higher (25-fold) when reduction in growth accompanies cell differentiation. In the latter case, the enzyme increase is similar to that which can be induced in these cells by exogenous interferon (IFN). The increase in 2',5'-oligoadenylate synthetase was shown to be due to a spontaneous secretion of IFN by the cells themselves: it is completely abolished if antiserum to murine type I IFN is added to the culture medium. In attempts to isolate some of this spontaneously secreted IFN, we show that it is stable at pH 2, not neutralized by antiserum to type II IFN, and that it also differs from the known IFN species induced by Sendai virus in Friend cells. The major component of this spontaneously secreted IFN is 20,000 M(r) and differs from the corresponding virus-induced 20,000-M(r) IFN by its lower affinity for antiserum to type I IFN and its antigenic characterization as beta-murine IFN. The major component of the spontaneous IFN also exhibits a higher ratio of antigrowth to antiviral activity than the Sendai-induced IFNs. We suggest that Friend cells produce this specific type of IFN for the regulation of their growth and differentiation.

Recessive genetic deregulation abrogates c-myc suppression by interferon and is implicated in oncogenesis.

In a previous study we demonstrated that many hematopoietic tumor cells are resistant to the inhibitory effects that interferon exerts on c-myc mRNA expression without losing other receptor-mediated intracellular responses (M. Einat, D. Resnitzky, and A. Kimchi, Nature [London] 313:597-600). We report here that this partial resistance was overridden in two independent stable somatic cell hybrids prepared by fusion between sensitive and resistant cells. The c-myc mRNA transcribed from the active allele of the resistant parent cell was reduced by interferon within the context of the cell hybrid. It was therefore concluded that changes in the cis-acting sequences of c-myc were not involved in this type of relaxed regulation and that resistance resulted rather from inactivation or loss of postreceptor elements which operate in trans. The growth-stimulating effect that this genetic deregulation might have on cells was tested in experimental systems of cell differentiation in which an autocrine interferon is produced. For that purpose we isolated variant clones of M1 myeloid cells which were partially resistant to alpha and beta interferons and tested their growth behavior during in vitro-induced differentiation. The resistant clones displayed higher proliferative activity on days 2 and 3 of differentiation than did the sensitive clones, which stopped proliferating. The loss of c-myc responses to the self-produced interferon disrupted the normal cessation of growth during differentiation and therefore might lead cells along the pathway of neoplasia.

Interferons and interleukin-6 suppress the DNA-binding activity of E2F in growth-sensitive hematopoietic cells.

Transcription factor E2F binds to cellular promoters of certain growth- and cell cycle-controlling genes and forms distinct heteromeric complexes with other nuclear proteins. We show here that alpha and beta interferons (alpha, beta) and interleukin-6 abolished the E2F-containing DNA-binding complexes in Daudi Burkitt lymphoma cells and in M1 myeloblastic cells, which responded to the cytokines by suppression of c-myc transcription. Time kinetics studies showed that the abolishment of E2F complexes coincided with reduction of c-myc expression and that both molecular events preceded the cell cycle block in G0/G1 phase. In contrast, the pattern of E2F complexes remained unchanged in an interferon-treated growth-resistant Daudi cell mutant that displayed relaxed regulation of c-myc. All of the DNA-binding E2F complexes, including those containing the retinoblastoma protein (pRB), cyclin A-p33cdk2, and the free forms of E2F, were reduced by interferons or interleukin-6. Their abolishment was unperturbed by pharmacological treatments that alleviated the cyclin A and pRB responses to interferon. Thus, changes in cyclin A expression and pRB phosphorylation are not primary events that influence the pattern of E2F responses to cytokines. Addition of EDTA to cell extracts of interferon-treated Daudi cells restored the DNA-binding activity of E2F, resulting in the appearance of a single E2F complex that exclusively contained pRB. It is suggested that the regulation of E2F by growth-inhibitory cytokines that induce cell cycle exit takes place at the level of the DNA-binding activity, and by that mean it differs basically from the phase-specific regulation of E2F in cycling cells.

Complementation by wild-type p53 of interleukin-6 effects on M1 cells: induction of cell cycle exit and cooperativity with c-myc suppression.

Stable transfection of M1 myeloid leukemia cells with a temperature-sensitive mutant of p53 results in two phenomena that are manifested exclusively at the permissive temperature. On one hand, activation of wild-type p53 by the temperature shift induced an apoptotic type of cell death which could be inhibited by interleukin-6 (IL-6) (E. Yonish-Rouach, D. Resnitzky, J. Lotem, L. Sachs, A. Kimchi, and M. Oren, Nature 352:345-347, 1991). On the other hand, as reported in this work, activated p53 complemented the antiproliferative effects of IL-6 in M1 cells. A shift to the permissive temperature concomitant with or early after IL-6 treatment imposed a novel pattern of cell cycle arrest in which about 95% of the cells were retained within a G0-like quiescent state. This phase was characterized by 2N DNA content and low RNA and protein content. On the molecular level, activation of wild-type p53 transrepressed the c-myc gene but not the cyclin A, D1, or D2 gene, which are all independently suppressed by IL-6 in M1 cells. To further analyze whether c-myc inhibition mediates or complements p53 effects, the p53-transfected M1 cells were infected with a retroviral vector expressing deregulated c-myc, refractory to p53 or IL-6 action. It was found that the process of cell death was not interrupted at all in these M1 c-myc-p53 double transfectants, suggesting that the transrepression of c-myc is not a major obligatory event mediating p53-induced cell death. In addition, some of the antiproliferative effects of activated p53, manifested in the presence of IL-6, could still be transmitted in the background of constitutive c-myc. Yet the context of deregulated c-myc interfered with the final accumulation of cells within a G0-like phase, suggesting complementary interactions between the outcome of p53 activation and of c-myc suppression in the control of cell cycle arrest.

DAP-5, a novel homolog of eukaryotic translation initiation factor 4G isolated as a putative modulator of gamma interferon-induced programmed cell death.

A functional approach to gene cloning was applied to HeLa cells in an attempt to isolate cDNA fragments which convey resistance to gamma interferon (IFN-gamma)-induced programmed cell death. One of the rescued cDNAs, described in this work, was a fragment of a novel gene, named DAP-5. Analysis of a DAP-5 full-length cDNA clone revealed that it codes for a 97-kDa protein that is highly homologous to eukaryotic translation initiation factor 4G (eIF4G, also known as p220). According to its deduced amino acid sequence, this novel protein lacks the N-terminal region of eIF4G responsible for association with the cap binding protein eIF4E. The N-terminal part of DAP-5 has 39% identity and 63% similarity to the central region of mammalian p220. Its C-terminal part is less homologous to the corresponding region of p220, suggesting that it may possess unique functional properties. The rescued DAP-5 cDNA fragment which conveyed resistance to IFN-gamma-induced cell death was expressed from the vector in the sense orientation. Intriguingly, it comprised part of the coding region which corresponds to the less conserved C-terminal part of DAP-5 and directed the synthesis of a 28-kDa miniprotein. The miniprotein exerted a dual effect on HeLa cells. Low levels of expression protected the cells from IFN-gamma-induced programmed cell death, while high levels of expression were not compatible with continuous cell growth. The relevance of DAP-5 protein to possible changes in a cell's translational machinery during programmed cell death and growth arrest is discussed.

Death-associated protein kinase-related protein 1, a novel serine/threonine kinase involved in apoptosis.

In this study we describe the identification and structure-function analysis of a novel death-associated protein (DAP) kinase-related protein, DRP-1. DRP-1 is a 42-kDa Ca(2+)/calmodulin (CaM)-regulated serine threonine kinase which shows high degree of homology to DAP kinase. The region of homology spans the catalytic domain and the CaM-regulatory region, whereas the remaining C-terminal part of the protein differs completely from DAP kinase and displays no homology to any known protein. The catalytic domain is also homologous to the recently identified ZIP kinase and to a lesser extent to the catalytic domains of DRAK1 and -2. Thus, DAP kinase DRP-1, ZIP kinase, and DRAK1/2 together form a novel subfamily of serine/threonine kinases. DRP-1 is localized to the cytoplasm, as shown by immunostaining and cellular fractionation assays. It binds to CaM, undergoes autophosphorylation, and phosphorylates an exogenous substrate, the myosin light chain, in a Ca(2+)/CaM-dependent manner. The truncated protein, deleted of the CaM-regulatory domain, was converted into a constitutively active kinase. Ectopically expressed DRP-1 induced apoptosis in various types of cells. Cell killing by DRP-1 was dependent on two features: the status of the catalytic activity, and the presence of the C-terminal 40 amino acids shown to be required for self-dimerization of the kinase. Interestingly, further deletion of the CaM-regulatory region could override the indispensable role of the C-terminal tail in apoptosis and generated a "superkiller" mutant. A dominant negative fragment of DAP kinase encompassing the death domain was found to block apoptosis induced by DRP-1. Conversely, a catalytically inactive mutant of DRP-1, which functioned in a dominant negative manner, was significantly less effective in blocking cell death induced by DAP kinase. Possible functional connections between DAP kinase and DRP-1 are discussed.

A novel form of DAP5 protein accumulates in apoptotic cells as a result of caspase cleavage and internal ribosome entry site-mediated translation.

Death-associated protein 5 (DAP5) (also named p97 and NAT1) is a member of the translation initiation factor 4G (eIF4G) family that lacks the eIF4E binding site. It was previously implicated in apoptosis, based on the finding that a dominant negative fragment of the protein protected against cell death. Here we address its function and two distinct levels of regulation during apoptosis that affect the protein both at translational and posttranslational levels. DAP5 protein was found to be cleaved at a single caspase cleavage site at position 790, in response to activated Fas or p53, yielding a C-terminal truncated protein of 86 kDa that is capable of generating complexes with eIF4A and eIF3. Interestingly, while the overall translation rate in apoptotic cells was reduced by 60 to 70%, in accordance with the simultaneous degradation of the two major mediators of cap-dependent translation, eIF4GI and eIF4GII, the translation rate of DAP5 protein was selectively maintained. An internal ribosome entry site (IRES) element capable of directing the translation of a reporter gene when subcloned into a bicistronic vector was identified in the 5' untranslated region of DAP5 mRNA. While cap-dependent translation from this transfected vector was reduced during Fas-induced apoptosis, the translation via the DAP5 IRES was selectively maintained. Addition of recombinant DAP5/p97 or DAP5/p86 to cell-free systems enhanced preferentially the translation through the DAP5 IRES, whereas neutralization of the endogenous DAP5 in reticulocyte lysates by adding a dominant negative DAP5 fragment interfered with this translation. The DAP5/p86 apoptotic form was more potent than DAP5/p97 in these functional assays. Altogether, the data suggest that DAP5 is a caspase-activated translation factor which mediates cap-independent translation at least from its own IRES, thus generating a positive feedback loop responsible for the continuous translation of DAP5 during apoptosis.

Alpha interferon suppresses the cyclin D3 and cdc25A genes, leading to a reversible G0-like arrest.

Alpha interferon is a potent growth inhibitor of Daudi Burkitt's lymphoma cells. We show here that alpha-interferon signaling interacted simultaneously with several components of the basic cell cycle machinery, causing cells to enter into a state that had many features characteristic of the G0 state. Within a few hours after alpha-interferon treatment, cyclin D3 mRNA and protein levels dropped to undetectable levels and, in parallel, the activities of cyclin A- and cyclin E-associated kinases were significantly reduced. The latter resulted from the rapid alpha-interferon-mediated elimination of cdc25A, a phosphatase that is required for antagonism of negative tyrosine phosphorylation of cdk2 in cyclin-cdk complexes. This regulation represents a novel mechanism through which an external inhibitory cytokine interacts with the cell cycle machinery. At later time points after alpha-interferon treatment, the levels of the 55-kDa slowly migrating hyperphosphorylated form of cyclin E and of cyclin A were also reduced. The antiproliferative effects were reversible, and cultures from which alpha interferon was removed reentered S phase after a lag that typically corresponded to approximately two doubling times. During this lag period, the expression of cyclin D3 and cyclin A, as well as of the cdc25A phosphatase, continued to be switched off, in spite of the removal of alpha interferon from the cell surface. In contrast, c-myc, which represents another downstream target gene that is subjected to negative regulation by alpha interferon, was relieved from suppression much earlier, concomitant with the decay in early signaling of the cytokine. The delayed pattern of cyclin reexpression provides evidence that alpha-interferon signaling imposes a G0-like state on this system.

p53-mediated cell death: relationship to cell cycle control.

M1 clone S6 myeloid leukemic cells do not express detectable p53 protein. When stably transfected with a temperature-sensitive mutant of p53, these cells undergo rapid cell death upon induction of wild-type (wt) p53 activity at the permissive temperature. This process has features of apoptosis. In a number of other cell systems, wt p53 activation has been shown to induce a growth arrest. Yet, wt 53 fails to induce a measurable growth arrest in M1 cells, and cell cycle progression proceeds while viability is being lost. There exists, however, a relationship between the cell cycle and p53-mediated death, and cells in G1 appear to be preferentially susceptible to the death-inducing activity of wt p53. In addition, p53-mediated M1 cell death can be inhibited by interleukin-6. The effect of the cytokine is specific to p53-mediated death, since apoptosis elicited by serum deprivation is refractory to interleukin-6. Our data imply that p53-mediated cell death is not dependent on the induction of a growth arrest but rather may result from mutually incompatible growth-regulatory signals.

Transfection of fibroblasts with activated c-myc confers resistance to antigrowth effects of interferon.

The Go/G1 to S transition in growth factor-stimulated Balb/c 3T3 fibroblasts is efficiently blocked by interferons (IFNs). In the present communication we studied whether deregulated expression of exogenously introduced c-myc changes the growth sensitivity to type I IFNs (alpha + beta). Constructs that link the two coding exons of c-myc to the long terminal repeat (LTR) of Ha-MS virus were introduced into the 3T3 fibroblasts. The steady state levels of exogenous c-myc mRNA in the individual stable clones were 3-10 fold higher than the endogenous mRNA levels in non transfected Balb/c 3T3 cells. The expression directed from the c-myc-construct was found to be completely resistant to inhibition by IFN while it was still partially responsive to addition or depletion of growth factors. To test the possible phenotypic changes after this genetic manipulation, the cell cycle distribution of individual c-myc-transfected clones was analyzed during the growth factor controlled transition from resting phase to proliferating state. We find that entry into S phase of the c-myc-transfected clones became completely resistant to inhibition by low IFN concentrations which blocked this process in the parental cell line and the control clones. It is concluded that deregulated expression of c-myc resulting from substitution of the authentic promoters and the first c-myc exon with a viral promoter reduces the sensitivity of synchronized fibroblasts to the antimitogenic effects of IFN.

The involvement of protein kinase C in mediating growth suppressive signals of interferons in hematopoietic cells.

The possible involvement of protein kinase C in transducing the growth suppressive signals of interferons was studies in this work in two different hematopoietic cell lines. Chronic exposure of human Burkitt lymphoma and mouse M1 myeloblastic cell lines to phorbol myristate acetate (PMA), reduced by more than 90% the PKC protein levels and enzymatic activity in cell extracts. The depletion of PKC from cells abrogated the ability of IFN (alpha + beta) to arrest cell growth at the G0/G1 resting phase of the cell cycle. In contrast, other responses to IFN such as the induction of (2'-5') oligoadenylate synthetase gene, continued to take place at the same dose response pattern thus excluding the possibility that early targets in the pathway, such as the number or affinity of IFN cell surface receptors might be affected by PMA. The same prolonged treatment of M1 cells with PMA did not interfere with the ability of another cytokine, transforming growth factor beta (TGF-beta), to induce the normal type of G0/G1 arrest further supporting the specificity of the effect towards IFN responses. Unexpectedly, depletion of PKC from cells did not interfere with the negative effects of IFN on c-myc mRNA and protein expression in spite of the direct involvement of this molecular event in growth responses to IFN. The putative PKC dependent molecular event could therefore function either downstream to or in combination with the reduction in c-myc protein levels, providing a necessary but not a sufficient step to arrest cell cycle progression at the G0/G1 phase.

Death associated proteins (DAPs): from gene identification to the analysis of their apoptotic and tumor suppressive functions.

The process of apoptosis (programmed cell death) has become the subject of intensive and extensive research over the past few years. Various approaches are being used to identify and study genes which function as positive mediators of apoptosis. Here, we address a novel approach of gene cloning aimed at isolating intracellular death promoting genes by utilizing a functional screen. This method, called TKO, was based on transfection of cells with an anti-sense cDNA library, followed by the selection of transfectants which survived in the continuous presence of a killing cytokine-interferon-gamma. It led to the identification of five novel apoptotic genes and to the finding that a known protease-cathepsin D, is actively recruited to the death process. The five novel apoptotic genes (named DAP genes for: Death Associated Proteins) code for proteins which display a diverse spectrum of biochemical activities. The list comprises a novel type of calcium/calmodulin-regulated kinase which carries ankyrin repeats and a death domain (DAP-kinase), a nucleotide-binding protein (DAP-3), a small proline-rich cytoplasmic protein (DAP-1), and a novel homolog of the eIF4G translation initiation factor (DAP-5). Extensive studies proved that these genes are critical for mediating cell death initiated by interferon-gamma, and in some of the tested cases also cell death induced by Fas/APO-1, TNF-alpha, and a detachment from extracellular matrix. Moreover, one of these genes, DAP-kinase, was recently found to display strong tumor suppressive activities, coupling the control of apoptosis to metastasis. The advantage of functional approaches of gene cloning is that they select the relevant rate limiting genes along the death pathways in a complete unbiased manner. As a consequence, novel targets and unpredicted mechanisms emerged. A few examples illustrating this important point will be discussed. One relates to the calcium/calmodulin-dependent DAP-kinase, which is localized to the actin microfilaments. It was found that the correct localization of DAP-kinase to the microfilament network was critical for the execution of the apoptotic process, and more specifically for the disruption of the stress fibers--a typical hallmark of apoptosis. Another important breakthrough step in our understanding of apoptotic processes relates to the identification and analysis of the DAP-5 gene. The structure/ function features of this novel translation regulator resemble the proteolytically cleaved eIF4G which appears in cells upon infection with some RNA viruses and which directs cap-independent translation. Thus, the rescue of DAP-5 highlighted the importance of regulation of protein translation in certain apoptotic systems. Finally, the isolation of cathespin D by our method suggests that lysosomal proteases are recruited during apoptosis, in addition to the well known caspase family of proteases, and that a unique pattern of regulation affecting the processing of this protease takes place. The major challenge now is to analyse how these diverse DAP gene activities constitute biochemical pathway(s) leading to programmed cell death, and what is their functional position with respect to other known positive mediators and suppressors of apoptosis such as the Bcl2 and caspase family members.