KLF6 degradation after apoptotic DNA damage

Krüppel-like factor 6 (KLF6) is a cancer gene (www.sanger.ac.uk/genetics/CGP/Census/). Here, we demonstrate that KLF6 protein is rapidly degraded when apoptosis is induced via the intrinsic pathway by cisplatin, adriamycin, or UVB irradiation in multiple cell lines (HCT116, SW40, HepG2, PC3-M, Skov3, NIH-3T3, 293T, GM09706, and MEF, IMR-90). KLF6 degradation occurred in the presence or absence of p53, was associated with ubiquitination, mediated by the proteasome (half-life 16 min, unstimulated), and independent of caspases and calpain. KLF6 was unchanged by apoptosis via the extrinsic/death-receptor pathway. Deregulation of KLF6 stability may alter its tumor suppressor function and/or the response of tumors to chemotherapeutics.     

Cell surface fibronectin and oncogenic transformation

Fibronectin is a large glycoprotein at the cell surface of many different cell types; a related protein is present in plasma. Fibronectin is a dimer of 230,000-dalton subunits and also occurs in larger aggregates; it forms fibrillar networks at the cell surface, between cells and substrata and between adjacent cells, and it is not a typical membrane protein. Cell surface fibronectin is reduced in amount or absent on transformed cells and in many cases its loss correlates with acquisition of tumorigenicity and, in particular, metastatic ability. Exceptions to the correlations with transformation and tumorigenicity exist. Loss of fibronectin and the resulting reduced adhesion appear to be involved in pleiotrpoic alterations in cell behavior and may be responsible for several aspects of the transformed phenotype in vitro. Fibronectin interacts with other macromolecules (collagen/gelatin, fibrin/fibrinogen, proteoglycans) and is apparently connected to microfilaments inside the cell.     

Autophagy-dependent senescence in response to DNA damage and chronic apoptotic stress

Autophagy regulates cell survival and cell death upon various cellular stresses, yet the molecular signaling events involved are not well defined. Here, we established the function of a proteolytic Cyclin E fragment (p18-CycE) in DNA damage-induced autophagy, apoptosis, and senescence. p18-CycE was identified in hematopoietic cells undergoing DNA damage-induced apoptosis. In epithelial cells exposed to DNA damage, chronic but not transient expression of p18-CycE leads to higher turnover of LC3 I/II and increased emergence of autophagosomes and autolysosomes. Levels of p18-CycE, which was generated by proteolytic cleavage of endogenous Cyclin E, were greatly increased by chloroquine and correlated with LC 3II conversion. Preventing p18-CycE genesis blocked conversion of LC3 I to LC3 II. Upon DNA damage, cytoplasmic ataxia-telangiectasia-mutated (ATM) was phosphorylated in p18-CycE-expressing cells resulting in sustained activation of the adenosine-mono-phosphate-dependent kinase (AMPK). These lead to sustained activation of mammalian autophagy-initiating kinase ULK1, which was abrogated upon inhibiting ATM and AMPK phosphorylation. Moreover, p18-CycE was degraded via autophagy followed by induction of senescence. Both autophagy and senescence were prevented by inhibiting autophagy, which leads to increased apoptosis in p18-CycE-expressing cells by stabilizing p18-CycE expression. Senescence was further associated with cytoplasmic co-localization and degradation of p18-CycE and Ku70. In brief, chronic p18-CycE expression-induced autophagy leads to clearance of p18-CycE following DNA damage and induction of senescence. Autophagy inhibition stabilized the cytoplasmic p18-CycE-Ku70 complex leading to apoptosis. Thus, our findings define how chronic apoptotic stress and DNA damage initiate autophagy and regulate cell survival through senescence and/or apoptosis.     

Engineering a DNA damage response without DNA damage

Recent work has achieved the feat of activating the DNA damage checkpoint in the absence of DNA damage, revealing the importance of protein-chromatin associations for the activation, amplification and maintenance of the DNA damage response.     

DNA damage after chemotherapy correlates with tumor response and survival in small cell lung cancer patients

To explore the induction of chemotherapy (CT) DNA damage and its correlation with tumor response and patient survival, we undertook the present study in 20 small cell lung cancer (SCLC) patients. All patients underwent the same treatment based on CT courses of carboplatin and etoposide. Blood samples were taken before and immediately after CT and every 12 weeks during follow-up. Nuclear DNA damage was determined through the variations in three mitochondrial pseudogene mutations in DNA of peripheral blood mononuclear cells. They were detected by mutation-specific PCR and assessed by a semiquantitative method. The relative level of mutation rose after chemotherapy in all cases. Among the 11 patients (55%) with higher relative levels of mutations, 9 (82%) of them achieved a complete response. In contrast, of the 9 patients (45%) with lower relative levels of mutations, only 2 (18%) achieved a complete response, displaying a statistically significant difference (P=0.02). The overall survival for patients with marked genomic damage was 18 months (range 10-24), and for patients with low degree of DNA damage, it was 12 months (range 5-15) (P=0.002). Genomic damage detected after chemotherapy treatment correlates positively with tumor response and patient survival.     

TOR signaling is a determinant of cell survival in response to DNA damage

The conserved TOR (target of rapamycin) kinase is part of a TORC1 complex that regulates cellular responses to environmental stress, such as amino acid starvation and hypoxia. Dysregulation of Akt-TOR signaling has also been linked to the genesis of cancer, and thus, this pathway presents potential targets for cancer chemotherapeutics. Here we report that rapamycin-sensitive TORC1 signaling is required for the S-phase progression and viability of yeast cells in response to genotoxic stress. In the presence of the DNA-damaging agent methyl methanesulfonate (MMS), TOR-dependent cell survival required a functional S-phase checkpoint. Rapamycin inhibition of TORC1 signaling suppressed the Rad53 checkpoint-mediated induction of ribonucleotide reductase subunits Rnr1 and Rnr3, thereby abrogating MMS-induced mutagenesis and enhancing cell lethality. Moreover, cells deleted for RNR3 were hypersensitive to rapamycin plus MMS, providing the first demonstration that Rnr3 contributes to the survival of cells exposed to DNA damage. Our findings support a model whereby TORC1 acts as a survival pathway in response to genotoxic stress by maintaining the deoxynucleoside triphosphate pools necessary for error-prone translesion DNA polymerases. Thus, TOR-dependent cell survival in response to DNA-damaging agents coincides with increased mutation rates, which may contribute to the acquisition of chemotherapeutic drug resistance.     

Chk1 suppresses a caspase-2 apoptotic response to DNA damage that bypasses p53, Bcl-2, and caspase-3

Evasion of DNA damage-induced cell death, via mutation of the p53 tumor suppressor or overexpression of prosurvival Bcl-2 family proteins, is a key step toward malignant transformation and therapeutic resistance. We report that depletion or acute inhibition of checkpoint kinase 1 (Chk1) is sufficient to restore gamma-radiation-induced apoptosis in p53 mutant zebrafish embryos. Surprisingly, caspase-3 is not activated prior to DNA fragmentation, in contrast to classical intrinsic or extrinsic apoptosis. Rather, an alternative apoptotic program is engaged that cell autonomously requires atm (ataxia telangiectasia mutated), atr (ATM and Rad3-related) and caspase-2, and is not affected by p53 loss or overexpression of bcl-2/xl. Similarly, Chk1 inhibitor-treated human tumor cells hyperactivate ATM, ATR, and caspase-2 after gamma-radiation and trigger a caspase-2-dependent apoptotic program that bypasses p53 deficiency and excess Bcl-2. The evolutionarily conserved "Chk1-suppressed" pathway defines a novel apoptotic process, whose responsiveness to Chk1 inhibitors and insensitivity to p53 and BCL2 alterations have important implications for cancer therapy.     

The DNA damage response: ten years after

The DNA damage response (DDR), through the action of sensors, transducers, and effectors, orchestrates the appropriate repair of DNA damage and resolution of DNA replication problems, coordinating these processes with ongoing cellular physiology. In the past decade, we have witnessed an explosion in understanding of DNA damage sensing, signaling, and the complex interplay between protein phosphorylation and the ubiquitin pathway employed by the DDR network to execute the response to DNA damage. These findings have important implications for aging and cancer.     

POT1 protects telomeres from a transient DNA damage response and determines how human chromosomes end

The hallmarks of telomere dysfunction in mammals are reduced telomeric 3' overhangs, telomere fusions, and cell cycle arrest due to a DNA damage response. Here, we report on the phenotypes of RNAi-mediated inhibition of POT1, the single-stranded telomeric DNA-binding protein. A 10-fold reduction in POT1 protein in tumor cells induced neither telomere fusions nor cell cycle arrest. However, the 3' overhang DNA was reduced and all telomeres elicited a transient DNA damage response in G1, indicating that extensive telomere damage can occur without cell cycle arrest or telomere fusions. RNAi to POT1 also revealed its role in generating the correct sequence at chromosome ends. The recessed 5' end of the telomere, which normally ends on the sequence ATC-5', was changed to a random position within the AATCCC repeat. Thus, POT1 determines the structure of the 3' and 5' ends of human chromosomes, and its inhibition generates a novel combination of telomere dysfunction phenotypes in which chromosome ends behave transiently as sites of DNA damage, yet remain protected from nonhomologous end-joining.     

DNA‐PK suppresses a p53‐independent apoptotic response to DNA damage

p53 is required for DNA damage‐induced apoptosis, which is central to its function as a tumour suppressor. Here, we show that the apoptotic defect of p53‐deficient cells is nearly completely rescued by inactivation of any of the three subunits of the DNA repair holoenzyme DNA‐dependent protein kinase (DNA‐PK). Intestinal crypt cells from p53 nullizygous mice were resistant to radiation‐induced apoptosis, whereas apoptosis in DNA‐PK_cs/p53, Ku80/p53 and Ku70/p53 double‐null mice was quantitatively equivalent to that seen in wild‐type mice. This p53‐independent apoptotic response was specific to the loss of DNA‐PK, as it was not seen in ligase IV (Lig4)/p53 or ataxia telangiectasia mutated (Atm)/p53 double‐null mice. Furthermore, it was associated with an increase in phospho‐checkpoint kinase 2 (CHK2), and cleaved caspases 3 and 9, the latter indicating engagement of the intrinsic apoptotic pathway. This shows that there are two separate, but equally effective, apoptotic responses to DNA damage: one is p53 dependent and the other, engaged in the absence of DNA‐PK, does not require p53.     

The DNA damage response pathway regulates the alternative splicing of the apoptotic mediator Bcl-x

Alternative splicing often produces effectors with opposite functions in apoptosis. Splicing decisions must therefore be tightly connected to stresses, stimuli, and pathways that control cell survival and cell growth. We have shown previously that PKC signaling prevents the production of proapoptotic Bcl-x(S) to favor the accumulation of the larger antiapoptotic Bcl-x(L) splice variant in 293 cells. Here we show that the genotoxic stress induced by oxaliplatin elicits an ATM-, CHK2-, and p53-dependent splicing switch that favors the production of the proapoptotic Bcl-x(S) variant. This DNA damage-induced splicing shift requires the activity of protein-tyrosine phosphatases. Interestingly, the ATM/CHK2/p53/tyrosine phosphatases pathway activated by oxaliplatin regulates Bcl-x splicing through the same regulatory sequence element (SB1) that receives signals from the PKC pathway. Convergence of the PKC and DNA damage signaling routes may control the abundance of a key splicing repressor because SB1-mediated repression is lost when protein synthesis is impaired but is rescued by blocking proteasome-mediated protein degradation. The SB1 splicing regulatory module therefore receives antagonistic signals from the PKC and the p53-dependent DNA damage response pathways to control the balance of pro- and antiapoptotic Bcl-x splice variants.     

c-Abl tyrosine kinase regulates caspase-9 autocleavage in the apoptotic response to DNA damage

Activation of the initiator caspase-9 is essential for induction of apoptosis by developmental signals, oncogenic transformation, and genotoxic stress. The c-Abl tyrosine kinase is also involved in the apoptotic response to DNA damage. The present results demonstrate that c-Abl binds directly to caspase-9. We show that c-Abl phosphorylates caspase-9 on Tyr-153 in vitro and in cells treated with DNA damaging agents. Moreover, inhibition of c-Abl with STI571 blocked DNA damage-induced autoprocessing of caspase-9 to the p35 subunit and activation of caspase-3. Caspase-9(Y153F) also attenuated DNA damage-induced processing of caspase-9 to p35, activation of caspase-3, and apoptosis. These findings indicate that caspase-9 autoprocessing is regulated by c-Abl in the apoptotic response to genotoxic stress.