Caspase-3-mediated cleavage of Cdc6 induces nuclear localization of p49-truncated Cdc6 and apoptosis

Stability of proteins involved in initiation of DNA replication in UV damaged human cells

The protein stability of the initiation factors Orc2, Orc3, Orc4, and Cdc6 was analyzed after UV light exposure in two human cell lines. In the cell line with higher repair capacity, HEK 293, no changes in the cell cycle distribution or in the protein levels of the investigated factors were detected. In HeLa cells that are characterized by lower repair capacity, UV irradiation caused a reduction of the levels of Cdc6, Orc2 and Orc3, but not of Orc4 or triggered apoptosis. The appearance of the truncated 49 kDa form of Cdc6 suggested the involvement of the caspase pathway in the degradation of the proteins. Reduced protein levels of Cdc6 were detected in UV damaged HeLa cells in which the apoptotic process was blocked with the caspase inhibitor Z-VAD-fmk, indicating that the degradation of Cdc6 is mediated by the proteasome pathway instead. In the presence of caffeine, an inhibitor of the cell cycle checkpoint kinases, Cdc6 was stabilized, demonstrating that its degradation is controlled by the DNA damage cell cycle checkpoint. We conclude that in response to DNA damage, the activation of origins of replication can be prevented by the degradation of Cdc6, most likely through the proteasome pathway.

Cell-Cycle Cross Talk with Caspases and Their Substrates

Caspases play central roles in mediating both cell death and inflammation. It has more recently become evident that caspases also drive other biological processes. Most prominently, caspases have been shown to be involved in differentiation. Several stem and progenitor cell types rely on caspases to initiate and execute their differentiation processes. These range from neural and glial cells, to skeletal myoblasts and osteoblasts, and several cell types of the hematopoietic system. Beyond differentiation, caspases have also been shown to play roles in other "noncanonical" processes, including cell proliferation, arrest, and senescence, thereby contributing to the mechanisms that regulate tissue homeostasis at multiple levels. Remarkably, caspases directly influence the course of the cell cycle in both a positive and negative manner. Caspases both cleave elements of the cell-cycle machinery and are themselves substrates of cell-cycle kinases. Here we aim to summarize the breadth of interactions between caspases and cell-cycle regulators. We also highlight recent developments in this area.

Two nuclear export signals of Cdc6 are differentially associated with CDK-mediated phosphorylation residues for cytoplasmic translocation

Cdc6 is cleaved at residues 442 and 290 by caspase-3 during apoptosis producing p49-tCdc6 and p32-tCdc6, respectively. While p32-tCdc6 is unable to translocate into the cytoplasm, p49-tCdc6 retains cytoplasmic translocation activity, but it has a lower efficiency than wild-type Cdc6. We hypothesized that a novel nuclear export signal (NES) sequence exists between amino acids 290 and 442. Cdc6 contains a novel NES in the region of amino acids 300-315 (NES2) that shares sequence similarity with NES1 at residues 462-476. In mutant versions of Cdc6, we replaced leucine with alanine in NES1 and NES2 and co-expressed the mutant constructs with cyclin A. We observed that the cytoplasmic translocation of these mutants was reduced in comparison to wild-type Cdc6. Moreover, the cytoplasmic translocation of a mutant in which all four leucine residues were mutated to alanine was significantly inhibited in comparison to the translocation of wild-type Cdc6. The Crm1 binding activities of Cdc6 NES mutants were consistent with the efficiency of its cytoplasmic translocation. Further studies have revealed that L468 and L470 of NES1 are required for cytoplasmic translocation of Cdc6 phosphorylated at S74, while L311 and L313 of NES2 accelerate the cytoplasmic translocation of Cdc6 phosphorylated at S54. These results suggest that the two NESs of Cdc6 work cooperatively and distinctly for the cytoplasmic translocation of Cdc6 phosphorylated at S74 and S54 by cyclin A/Cdk2.

Norcantharidin inhibits pre-replicative complexes assembly of HepG2 cells

Norcantharidin (NCTD) is currently used for anticancer therapy but the exact mechanism of action remains unknown. Pre-replicative complexes (pre-RCs) are essential for cell DNA replication and highly related to malignant proliferation. Here, we examined the inhibitory effect of NCTD on pre-RC components in HepG2 cells. We showed that NCTD induced degradation of Cdc6 and Mcm2 in a dose-dependent manner. Under 100 μM NCTD concentration, about 70% of Cdc6 and 50% of Mcm2 were degraded. In addition, the nuclear translocation of Mcm6 was inhibited by NCTD. Further studies aiming at G1 synchronous cells showed that, NCTD reduced the chromatin-bound Cdc6, Mcm2 and Mcm6. Moreover, the cells were blocked from entering the S phase and accumulated at the G1 phase when released synchronously into the cell cycle. Consistently, the DNA replication was inhibited by NCTD. Finally, the combination NCTD with Cdc6 depletion lead to more severe cytotoxicity (88%) than NCTD (52%) and Cdc6 depletion (39%) alone. A synergic cytotoxicity was observed between Cdc6 depletion and NCTD. In conclusion, our results demonstrate that NCTD inhibits pre-RC assembly; subsequently blocks the G1 to S transition; and inhibits DNA replication in HepG2 cells. Pre-RCs are an intriguing target for cancer therapy, which merits further investigations for anticancer development.

Fragmented inositol 1,4,5-trisphosphate receptors retain tetrameric architecture and form functional Ca2+ release channels

Inositol 1,4,5-trisphosphate receptor isoforms are a family of ubiquitously expressed ligand-gated channels encoded by three individual genes. The proteins are localized to membranes of intracellular Ca(2+) stores and play pivotal roles in Ca(2+) homeostasis. Previous studies have demonstrated that IP3R1 is cleaved by the intracellular proteases calpain and caspase both in vivo and in vitro. However, the resultant cleavage products are poorly defined, and the functional consequences of these proteolytic events are not fully understood. We demonstrate that IP3R1 is cleaved during staurosporine-induced apoptosis, yielding N-terminal fragments encompassing the ligand-binding domain and the majority of the central modulatory domain together with a C-terminal fragment containing the channel domain and cytosolic tail. Notably, these fragments remain associated with the membrane after initiation of apoptotic cleavage. Furthermore, when recombinant IP3R1 fragments, corresponding to those predicted to be generated by caspase or calpain cleavage, are stably coexpressed in cells, they physically associate and form functional channels. These data provide novel insights regarding the regulation of IP3R1 during proteolysis and provide direct evidence that polypeptide continuity is not required for IP3R activation and Ca(2+) release.

JNK1 phosphorylation of Cdt1 inhibits recruitment of HBO1 histone acetylase and blocks replication licensing in response to stress

In response to environmental stresses, cells activate stress-response genes and inhibit DNA replication. HBO1 histone acetylase is a coactivator both for AP-1 transcription factors responding to stress-activated JNK kinases and also for the Cdt1 licensing factor that ensures that DNA is replicated exactly once per cell cycle. In response to nongenotoxic stress, JNK phosphorylates Jun, an AP-1 transcription factor, leading to increased recruitment of HBO1 and increased transcription of target genes. In addition, JNK phosphorylates Cdt1 on threonine 29, leading to rapid dissociation of HBO1 from replication origins, thereby blocking initiation of DNA replication. Upon relief of stress, HBO1 reassociates with replication origins. Thus, regulated and reciprocal recruitment of the HBO1 coactivator to target genes and replication origins via JNK-mediated phosphorylation of the recruiting transcription and replication licensing factors coordinates the transcriptional and DNA replication response to cellular stress.

20 (S)-Ginsenoside-Rh2 and 20 (R)-Ginsenoside-Rh2 Activate IkappaB Phosphorylation Expression in Human Lung Adenocarcinoma A549 Cells

To study the underlying mechanism of 20 (S)-Ginsenoside-Rh2 and 20 (R)-Ginsenoside-Rh2 inducing apoptosis of human lung adenocarcinoma A549 cells. In this study, cell death rate and cell survival rate were obtained using typan blue staining cell viability assay, and transmission electron microscopy was used to detect cell apoptosis. Meanwhile, IkappaB phosphorylation expression was analysed by western blotting. Results showed that after A549 cells were treated with 30 μg/mL 20(S)-Rh2 and 20(R)-Rh2 for 48h, cell death rate increased significantly compared with the control group (P<0.05), and nuclear condensation, fragmentation, karyopycnosis and apoptotic bodies were found under transmission electron microscope. There were no significant changes of IkappaB expression after treated with 20(S)-Rh2 and 20(R)-Rh2 (P>0.05). After treated with 20(R)-Rh2, p-IkappaB expression increased obviously between 4h-6h (P<0.05). After treated with 20(S)-Rh2, p-IkappaB expression increased obviously between 1h-2h (P<0.05), back to normal over time after 3h, increased significantly again between 4h-6h (P<0.05), which indicated the activation of IkappaB participated in A549 cell apoptosis induced by Rh2. These results demonstrated that 20(S)-Rh2 and 20(R)-Rh2 both have the functions of activating I-kappaB/NF-kappaB signaling pathway, thus promoting A549 cell apoptosis.

Cell division cycle 6, a mitotic substrate of polo-like kinase 1, regulates chromosomal segregation mediated by cyclin-dependent kinase 1 and separase

Defining the links between cell division and DNA replication is essential for understanding normal cell cycle progression and tumorigenesis. In this report we explore the effect of phosphorylation of cell division cycle 6 (Cdc6), a DNA replication initiation factor, by polo-like kinase 1 (Plk1) on the regulation of chromosomal segregation. In mitosis, the phosphorylation of Cdc6 was highly increased, in correlation with the level of Plk1, and conversely, Cdc6 is hypophosphorylated in Plk1-depleted cells, although cyclin A- and cyclin B1-dependent kinases are active. Binding between Cdc6 and Plk1 occurs through the polo-box domain of Plk1, and Cdc6 is phosphorylated by Plk1 on T37. Immunohistochemistry studies reveal that Cdc6 and Plk1 colocalize to the central spindle in anaphase. Expression of T37V mutant of Cdc6 (Cdc6-TV) induces binucleated cells and incompletely separated nuclei. Wild-type Cdc6 but not Cdc6-TV binds cyclin-dependent kinase 1 (Cdk1). Expression of wild-type Plk1 but not kinase-defective mutant promotes the binding of Cdc6 to Cdk1. Cells expressing wild-type Cdc6 display lower Cdk1 activity and higher separase activity than cells expressing Cdc6-TV. These results suggest that Plk1-mediated phosphorylation of Cdc6 promotes the interaction of Cdc6 and Cdk1, leading to the attenuation of Cdk1 activity, release of separase, and subsequent anaphase progression.

Betulin induces mitochondrial cytochrome c release associated apoptosis in human cancer cells

We examined whether betulin, a naturally abundant compound, has anticancer functions in human cancer cells. The results showed that betulin significantly inhibited cell viability in cervix carcinoma HeLa cells, hepatoma HepG2 cells, lung adenocarcinoma A549 cells, and breast cancer MCF-7 cells with IC(50) values ranging from 10 to 15 microg/mL. While betulin exhibited only moderate anticancer activity in other human cancer cells such as hepatoma SK-HEP-1 cells, prostate carcinoma PC-3, and lung carcinoma NCI-H460, with IC(50) values ranging from 20 to 60 microg/mL, it showed minor growth inhibition in human erythroleukemia K562 cells (IC(50) > 100 microg/mL). We further investigated the mechanism of anticancer activity by betulin, using HeLa cells as an experimental model. Betulin (10 microg/mL) induces apoptotic cell death, as evidenced by morphological characteristics such as membrane phosphatidylserine translocation, nuclear condensation/fragmentation, and apoptotic body formation. A kinetics analysis showed that the depolarization of mitochondrial membrane potential and the release of mitochondrial cytochrome c occurred as early as 30 min after treatment with betulin. Betulin, unlike its chemical derivative betulinic acid, did not directly trigger mitochondrial cytochrome c release in isolated mitochondria. Importantly, Bax and Bak were rapidly translocated to the mitochondria 30 min after betulin treatment. The sequential activation of caspase-9 and caspase-3/-7 and the cleavage of poly(ADP-ribose) polymerase (PARP) were observed behind those mitochondrial events. Furthermore, specific downregulation of either caspase-9, Bax, or Bak by siRNA effectively reduced PARP cleavage and caspase-3 activation. Taken together, the lines of evidence demonstrate that betulin triggers apoptosis of human cancer cells through the intrinsic apoptotic pathway.

Association analysis between the Cdc6 G1321A polymorphism and the risk for non-Hodgkin lymphoma and hepatocellular carcinoma

Cdc6 play crucial roles in DNA replication and carcinogenesis. The biological significance of the Cdc6 G1321A polymorphism (V441I, rs13706) is still not elucidated. Here we examined the influence of this polymorphism on the function of Cdc6 and the individual's susceptibility to non-Hodgkin lymphoma (NHL) and hepatocellular carcinoma (HCC). Unconditional logistic regression analysis revealed that the risk for NHL was significantly reduced in both AG heterozygotes [odds ratio (OR)=0.67, P=0.019] and AA homozygotes (OR=0.54, P=0.026), compared with GG homozygotes. Further stratification by subtypes of NHL showed that the AG as well as combined AA/AG genotypes were associated with decreased risk for B-cell-NHL (OR=0.62, P=0.011 and OR=0.61, P=0.006, respectively), especially for diffuse large B-cell lymphoma (DLBCL, OR=0.63, P=0.025 and OR=0.62, P=0.012, respectively). In addition, male individuals with the AA genotype displayed borderline significantly reduced risk for HCC (OR=0.48, P=0.054). Interestingly, the G1321A polymorphism did not affect caspase-mediated cleavage of Cdc6 during etoposide-induced apoptosis, but it was predicted to alter the secondary structure of Cdc6 mRNA. Our data provide the first evidence that the Cdc6 G1321A polymorphism is associated with decreased risk of cancer. Further studies are necessary to confirm the general validity of our findings.

A caspase-dependent cleavage of CDC25A generates an active fragment activating cyclin-dependent kinase 2 during apoptosis

The cellular level of the CDC25A phosphatase is tightly regulated during both the normal and genotoxic-perturbed cell cycle. Here, we describe a caspase-dependent cleavage of this protein at residue D223 in non-genotoxic apoptotic conditions. This specific proteolysis generates a catalytically active C-terminal fragment that localizes to the nuclear compartment. Accumulation of this active CDC25A fragment leads to reduced inhibitory phosphorylation of the CDC25A substrate cyclin-dependent kinase 2 (CDK2) on Tyr15. Moreover, CDK2 was found stably associated with this fragment, as well as with an ectopically expressed CDC25A224-525 truncation mutant that mimicks the cleavage product. Ectopic expression of this mutant induced CDK2 Tyr15 dephosphorylation, whereas its catalytically inactive version did not. Finally, this 224-525 mutant initiated apoptosis when transfected into HeLa cells, whereas its catalytic inactive form did not. Altogether, this study demonstrates for the first time that caspase-dependent cleavage of CDC25A is a central step linking CDK2 activation with non-genotoxic apoptotic induction.

Generation of an indestructible cyclin B1 by caspase-6-dependent cleavage during mitotic catastrophe

Overriding the G(2) DNA damage checkpoint permits precocious entry into mitosis that ultimately leads to mitotic catastrophe. Mitotic catastrophe is manifested by an unscheduled activation of CDK1, caspase activation and apoptotic cell death. We found that although cyclin B1 was required for mitotic catastrophe, it was cleaved into a approximately 35 kDa protein by a caspase-dependent mechanism during the process. Cyclin B1 cleavage occurred after Asp123 in the motif ILVD(123) downward arrow, and mutation of this motif attenuated the cleavage. Cleavage was abolished by a pan-caspase inhibitor as well as by specific inhibitors for the effector caspase-6 and the initiator caspase-8. Cleavage created a truncated cyclin B1 lacking part of the NH(2)-terminal regulatory domain that included the destruction box sequence. Although cleavage of cyclin B1 itself was not absolutely required for mitotic catastrophe, expression of the truncated product enhanced cell death. In support of this, ectopic expression of this truncated cyclin B1 was not only sufficient to induce mitotic block and apoptosis but also enhanced mitotic catastrophe induced by ionizing radiation and caffeine. These data underscore a possible linkage between mitotic and apoptotic functions by caspase-dependent processing of mitotic activators.

A novel functional polymorphism in the Cdc6 promoter is associated with the risk for hepatocellular carcinoma

Cdc6 is essential for DNA replication and its deregulation is involved in carcinogenesis. To date, the biological significance of the polymorphism in Cdc6 promoter is still unknown. In this study, we aimed to evaluate the influence of the Cdc6 -515A>G polymorphism (rs4134994) on the individual's susceptibility to cancer and on the function of Cdc6. The Cdc6 -515A>G polymorphism was genotyped in 387 hepatocellular carcinoma (HCC) and 389 age- and sex-matched healthy subjects. The association between the genotypes and the risk for HCC was then estimated by unconditional logistic regression analysis with adjustment for age, sex and HBV status. Compared with the AA homozygotes, the homozygous GG genotype (adjusted OR=0.36, 95% confidence interval (CI)=0.18-0.72, P=0.004) or the combined AG/GG genotypes (adjusted OR=0.56, 95% CI=0.36-0.86, P=0.008) were statistically significantly associated with the reduced risk for HCC. Moreover, the analysis using luciferase reporter system showed that the G-allelic Cdc6 promoter displayed a decreased transcriptional activity compared with the A-allelic one. These results indicate that the individuals with G allele may have reduced Cdc6 expression and are therefore in reduced risk for HCC. Further investigation using electrophoretic mobility shift assay (EMSA) revealed that the G allele had a stronger binding strength to nuclear protein(s) which might function as negative regulator(s) for Cdc6 transcription. Our findings suggest that the -515A>G polymorphism may affect the Cdc6 promoter binding affinity with nuclear protein(s) and in turn the Cdc6 expression, which consequently modulates the individual's susceptibility to HCC.

Subcellular localization determines the delicate balance between the anti- and pro-apoptotic activity of Livin

Livin is a member of the Inhibitor of Apoptosis Protein family which inhibits apoptosis induced by a variety of stimuli. We previously identified Livin and demonstrated that following apoptotic stimuli, Livin is cleaved by effector caspases to produce a truncated form with paradoxical pro-apoptotic activity. In the present study, we reveal that while full-length Livin shows diffuse cytoplasmic localization, truncated Livin (tLivin) is found in a peri-nuclear distribution with marked localization to the Golgi apparatus. Using mutation analysis, we identified two domains that are crucial for the pro-apoptotic activity of tLivin: the N-terminal region of tLivin which is exposed by cleavage, and the RING domain. We demonstrate that, of the N-terminal sequence, only the first N-terminal glycine residue dictates the peri-nuclear distribution of tLivin. However, while the perinuclear localization of tLivin is essential, it is not sufficient for tLivin to exert its pro-apoptotic function. Once tLivin is properly localized, an intact RING domain enables its pro-apoptotic function.

Cdc45 degradation during differentiation and apoptosis

Cell division cycle protein 45 (Cdc45) is crucial for the initiation as well as the elongation process of eukaryotic DNA replication. Our findings suggested that the Cdc45 protein is ubiquitylated and degraded by the proteasome pathway in human cells. Firstly, the fate of Cdc45 protein after induction of terminal differentiation of cultured human cells was significantly decelerated by application of proteasomal inhibitors. Secondly, we identified various putative destruction boxes and one KEN-box in the amino acid sequence of vertebrate Cdc45, which indicate that Cdc45 seems to be a novel substrate of the anaphase promoting complex/cyclosome. Thus, the evidences for ubiquitylation of Cdc45 refer the first posttranslational modification of this essential replication factor. We also found, that the human Cdc45 protein was not cleaved during apoptosis of cultured cells. This is in accordance with reports demonstrating the absence of indiscriminative cleavage of replication proteins during the programmed cell death.

Cdc6 stability is regulated by the Huwe1 ubiquitin ligase after DNA damage

The Cdc6 protein is an essential component of pre-replication complexes (preRCs), which assemble at origins of DNA replication during the G1 phase of the cell cycle. Previous studies have demonstrated that, in response to ionizing radiation, Cdc6 is ubiquitinated by the anaphase promoting complex (APC(Cdh1)) in a p53-dependent manner. We find, however, that DNA damage caused by UV irradiation or DNA alkylation by methyl methane sulfonate (MMS) induces Cdc6 degradation independently of p53. We further demonstrate that Cdc6 degradation after these forms of DNA damage is also independent of cell cycle phase, Cdc6 phosphorylation of the known Cdk target residues, or the Cul4/DDB1 and APC(Cdh1) ubiquitin E3 ligases. Instead Cdc6 directly binds a HECT-family ubiquitin E3 ligase, Huwe1 (also known as Mule, UreB1, ARF-BP1, Lasu1, and HectH9), and Huwe1 polyubiquitinates Cdc6 in vitro. Degradation of Cdc6 in UV-irradiated cells or in cells treated with MMS requires Huwe1 and is associated with release of Cdc6 from chromatin. Furthermore, yeast cells lacking the Huwe1 ortholog, Tom1, have a similar defect in Cdc6 degradation. Together, these findings demonstrate an important and conserved role for Huwe1 in regulating Cdc6 abundance after DNA damage.

Geminin cleavage during apoptosis by caspase-3 alters its binding ability to the SWI/SNF subunit Brahma

Geminin has been proposed to coordinate cell cycle and differentiation events through balanced interactions with the cell cycle regulator Cdt1 and with homeobox transcription factors and chromatin remodeling activities implicated in cell fate decisions. Here we show that Geminin is cleaved in primary cells and cancer cell lines induced to undergo apoptosis by a variety of stimuli. Geminin targeting is mediated by caspase-3 both in vivo and in vitro. Two sites at the carboxyl terminus of Geminin (named C1 and C2) are cleaved by the caspase, producing truncated forms of Geminin. We provide evidence that Geminin cleavage is regulated by phosphorylation. Casein kinase II alters Geminin cleavage at site C1 in vitro, whereas mutating phosphorylation competent Ser/Thr residues proximal to site C1 affects Geminin cleavage in vivo. We show that truncated Geminin produced by cleavage at C1 can promote apoptosis. In contrast, Geminin cleaved at site C2 has lost the ability to interact with Brahma (Brm), a catalytic subunit of the SWI/SNF chromatin remodeling complex, while binding efficiently to Cdt1, indicating that targeting of Geminin during apoptosis differentially affects interactions with its binding partners.

Production of reactive oxygen species in response to replication stress and inappropriate mitosis in fission yeast

Previous studies have indicated that replication stress can trigger apoptosis-like cell death, accompanied (where tested) by production of reactive oxygen species (ROS), in mammalian cells and budding yeast (Saccharomyces cerevisiae). In mammalian cells, inappropriate entry into mitosis also leads to cell death. Here, we report similar responses in fission yeast (Schizosaccharomyces pombe). We used ROS- and death-specific fluorescent stains to measure the effects of mutations in replication initiation and checkpoint genes in fission yeast on the frequencies of ROS production and cell death. We found that certain mutant alleles of each of the four tested replication initiation genes caused elevated ROS and cell death. Where tested, these effects were not enhanced by checkpoint-gene mutations. Instead, when cells competent for replication but defective in both the replication and damage checkpoints were treated with hydroxyurea, which slows replication fork movement, the frequencies of ROS production and cell death were greatly increased. This was a consequence of elevated CDK activity, which permitted inappropriate entry into mitosis. Thus, studies in fission yeast are likely to prove helpful in understanding the pathways that lead from replication stress and inappropriate mitosis to cell death in mammalian cells.

Norcantharidin inhibits DNA replication and induces apoptosis with the cleavage of initiation protein Cdc6 in HL-60 cells

Norcantharidin (NCTD), a demethylated form of cantharidin, is currently used as an anti-cancer drug in China. However, the exact anti-cancer mechanism of NCTD on human cancer cells remains poorly understood. In the present study, NCTD inhibited proliferation and DNA replication effectively in HL-60 cells. DNA replication-initiation protein Cdc6 was cleaved after 12 h treatment with NCTD. This cleavage generated a truncated Cdc6 fragment with a relative molecular weight of 49 kDa and elongated treatment with NCTD resulted in a complete loss of Cdc6. In addition, we found that Cdc6 was present in both non-chromatin- and chromatin-bound fractions in the untreated HL-60 cells, and NCTD treatment led to the cleavage of Cdc6 in both fractions. NCTD-induced cleavage of Cdc6 was prevented by pre-treatment with caspase-3 inhibitor, suggesting the involvement of caspase-3 activity in the process. Furthermore, NCTD treatment resulted in apoptotic changes including granular nuclear morphology, DNA laddering and sub-G1 arrest in HL-60 cells. In conclusion, our study reveals that NCTD can inhibit DNA replication, and induce apoptosis and caspase-3-dependent cleavage of Cdc6. The anti-cancer effect of NCTD may be closely associated with the dysfunction of Cdc6 and our report is the first to put forward this point of view.

Cleavage of Cdc6 by caspase-3 promotes ATM/ATR kinase-mediated apoptosis of HeLa cells

We show that caspase-3 cleaves Cdc6 at D²⁹⁰/S and D⁴⁴²/G sites, producing p32-tCdc6 (truncated Cdc6) and p49-tCdc6, respectively, during etoposide- or tumor necrosis factor (TNF)-α–induced apoptosis. The expression of these tCdc6 proteins, p32- and p49-tCdc6, promotes etoposide-induced apoptosis. The expression of tCdc6 perturbs the loading of Mcm2 but not Orc2 onto chromatin and activates ataxia telangiectasia mutated (ATM) and ATM and Rad-3 related (ATR) kinase activities with kinetics similar to that of the phosphorylation of Chk1/2. The activation kinetics are consistent with elevated cellular levels of p53 and mitochondrial levels of Bax. The tCdc6-induced effects are all suppressed to control levels by expressing a Cdc6 mutant that cannot be cleaved by caspase-3 (Cdc6-UM). Cdc6-UM expression attenuates the TNF-α–induced activation of ATM and caspase-3 activities. When ATM or ATR is down-expressed by using the small interfering RNA technique, the TNF-α– or tCdc6-induced activation of caspase-3 activities is suppressed in the cells. These results suggest that tCdc6 proteins act as dominant-negative inhibitors of replication initiation and that they disrupt chromatin structure and/or induce DNA damage, leading to the activation of ATM/ATR kinase activation and p53–Bax-mediated apoptosis.

Ubiquitylation, phosphorylation and Orc2 modulate the subcellular location of Orc1 and prevent it from inducing apoptosis

Previous studies have suggested that the activity of the mammalian origin recognition complex (ORC) is regulated by cell-cycle-dependent changes in its Orc1 subunit. Here, we show that Orc1 modifications such as mono-ubiquitylation and hyperphosphorylation that occur normally during S and G2-M phases, respectively, can cause Orc1 to accumulate in the cytoplasm. This would suppress reassembly of pre-replication complexes until mitosis is complete. In the absence of these modifications, transient expression of Orc1 rapidly induced p53-independent apoptosis, and Orc1 accumulated perinuclearly rather than uniformly throughout the nucleus. This behavior mimicked the increased concentration and perinuclear accumulation of endogenous Orc1 in apoptotic cells that arise spontaneously in proliferating cell cultures. Remarkably, expression of Orc1 in the presence of an equivalent amount of Orc2, the only ORC subunit that did not induce apoptosis, prevented induction of apoptosis and restored uniform nuclear localization of Orc1. This would promote assembly of ORC-chromatin sites, such as occurs during the transition from M to G1 phase. These results provide direct evidence in support of the regulatory role proposed for Orc1, and suggest that aberrant DNA replication during mammalian development could result in apoptosis through the appearance of 'unmodified' Orc1.

Caspases target only two architectural components within the core structure of the nuclear pore complex

Caspases were recently implicated in the functional impairment of the nuclear pore complex during apoptosis, affecting its dual activity as nucleocytoplasmic transport channel and permeability barrier. Concurrently, electron microscopic data indicated that nuclear pore morphology is not overtly altered in apoptotic cells, raising the question of how caspases may deactivate nuclear pore function while leaving its overall structure largely intact. To clarify this issue we have analyzed the fate of all known nuclear pore proteins during apoptotic cell death. Our results show that only two of more than 20 nuclear pore core structure components, namely Nup93 and Nup96, are caspase targets. Both proteins are cleaved near their N terminus, disrupting the domains required for interaction with other nucleoporins actively involved in transport and providing the permeability barrier but dispensable for maintaining the nuclear pore scaffold. Caspase-mediated proteolysis of only few nuclear pore complex components may exemplify a general strategy of apoptotic cells to efficiently disable huge macromolecular machines.

Apoptosis in budding yeast caused by defects in initiation of DNA replication

Apoptosis in metazoans is often accompanied by the destruction of DNA replication initiation proteins, inactivation of checkpoints and activation of cyclin-dependent kinases, which are inhibited by checkpoints that directly or indirectly require initiation proteins. Here we show that, in the budding yeast Saccharomyces cerevisiae, mutations in initiation proteins that attenuate both the initiation of DNA replication and checkpoints also induce features of apoptosis similar to those observed in metazoans. The apoptosis-like phenotype of initiation mutants includes the production of reactive oxygen species (ROS) and activation of the budding-yeast metacaspase Yca1p. In contrast to a recent report that activation of Yca1p only occurs in lysed cells and does not contribute to cell death, we found that, in at least one initiation mutant, Yca1p activation occurs at an early stage of cell death (before cell lysis) and contributes to the lethal effects of the mutation harbored by this strain. Apoptosis in initiation mutants is probably caused by DNA damage associated with the combined effects of insufficient DNA replication forks to completely replicate the genome and defective checkpoints that depend on initiation proteins and/or replication forks to restrain subsequent cell-cycle events until DNA replication is complete. A similar mechanism might underlie the proapoptotic effects associated with the destruction of initiation and checkpoint proteins during apoptosis in mammals, as well as genome instability in initiation mutants of budding yeast.

Activation of budding yeast replication origins and suppression of lethal DNA damage effects on origin function by ectopic expression of the co-chaperone protein Mge1

Initiation of DNA replication in eukaryotes requires the origin recognition complex (ORC) and other proteins that interact with DNA at origins of replication. In budding yeast, the temperature-sensitive orc2-1 mutation alters these interactions in parallel with defects in initiation of DNA replication and in checkpoints that depend on DNA replication forks. Here we show that DNA-damaging drugs modify protein-DNA interactions at budding yeast replication origins in association with lethal effects that are enhanced by the orc2-1 mutation or suppressed by a different mutation in ORC. A dosage suppressor screen identified the budding yeast co-chaperone protein Mge1p as a high copy suppressor of the orc2-1-specific lethal effects of adozelesin, a DNA-alkylating drug. Ectopic expression of Mge1p also suppressed the temperature sensitivity and initiation defect conferred by the orc2-1 mutation. In wild type cells, ectopic expression of Mge1p also suppressed the lethal effects of adozelesin in parallel with the suppression of adozelesin-induced alterations in protein-DNA interactions at origins, stimulation of initiation of DNA replication, and binding of the precursor form of Mge1p to nuclear chromatin. Mge1p is the budding yeast homologue of the Escherichia coli co-chaperone protein GrpE, which stimulates initiation at bacterial origins of replication by promoting interactions of initiator proteins with origin sequences. Our results reveal a novel, proliferation-dependent cytotoxic mechanism for DNA-damaging drugs that involves alterations in the function of initiation proteins and their interactions with DNA.