DNA-damage-induced checkpoint pathways in the nematode Caenorhabditis elegans

Exposure to 6-PPD quinone causes ferroptosis activation associated with induction of reproductive toxicity in Caenorhabditis elegans

Exposure to N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ) caused toxicity on Caenorhabditis elegans, including reproductive toxicity. However, the underlying mechanisms for this induced reproductive toxicity by 6-PPDQ remain largely unclear. We examined possible association of ferroptosis activation with reproductive toxicity of 6-PPDQ. In 1-100 μg/L 6-PPDQ exposed nematodes, Fe2+ content was increased, which was accompanied with enhanced lipid peroxidation, increased malonydialdehyde (MDA) content, and decreased L-glutathione (GSH) content. Exposure to 1-100 μg/L 6-PPDQ decreased expressions of ftn-1 encoding ferritin, ads-1 encoding AGPS, and gpx-6 encoding GPX4 and increased expression of bli-3 encoding dual oxidase. After 6-PPDQ exposure, RNAi of ftn-1 decreased ads-1 and gpx-6 expressions and increased bli-3 expression. RNAi of ftn-1, ads-1, and gpx-6 strengthened alterations in ferroptosis related indicators, and RNAi of bli-3 suppressed changes of ferroptosis related indicators in 6-PPDQ exposed nematodes. Meanwhile, RNAi of ftn-1, ads-1, and gpx-6 induced susceptibility, and RNAi of bli-3 caused resistance to 6-PPDQ reproductive toxicity. Moreover, expressions of DNA damage checkpoint genes (clk-2, mrt-2, and hus-1) could be increased by RNAi of ftn-1, ads-1, and gpx-6 in 6-PPDQ exposed nematodes. Therefore, our results demonstrated activation of ferroptosis in nematodes exposed to 6-PPDQ at environmentally relevant concentrations, and this ferroptosis activation was related to reproductive toxicity of 6-PPDQ.

DNA damage responses and p53 in the aging process

The genome is constantly attacked by genotoxic insults. DNA damage has long been established as a cause of cancer development through its mutagenic consequences. Conversely, radiation therapy and chemotherapy induce DNA damage to drive cells into apoptosis or senescence as outcomes of the DNA damage response (DDR). More recently, DNA damage has been recognized as a causal factor for the aging process. The role of DNA damage in aging and age-related diseases is illustrated by numerous congenital progeroid syndromes that are caused by mutations in genome maintenance pathways. During the past 2 decades, understanding how DDR drives cancer development and contributes to the aging process has progressed rapidly. It turns out that the DDR factor p53 takes center stage during tumor development and also plays an important role in the aging process. Studies in metazoan models ranging from Caenorhabditis elegans to mammals have revealed cell-autonomous and systemic DDR mechanisms that orchestrate adaptive responses that augment maintenance of the aging organism amid gradually accumulating DNA damage.

The TP53 signaling network in mammals and worms

The nematode worm Caenorhabditis elegans has been an invaluable model organism for studying the molecular mechanisms that govern cell fate, from fundamental aspects of multicellular development to programmed cell death (apoptosis). The transparency of this organism permits visualization of cells in living animals at high resolution. The powerful genetics and functional genomics tools available in C. elegans allow for detailed analysis of gene function, including genes that are frequently deregulated in human diseases such as cancer. The TP53 protein is a critical suppressor of tumor formation in vertebrates, and the TP53 gene is mutated in over 50% of human cancers. TP53 suppresses malignancy by integrating a variety of cellular stresses that direct it to activate transcription of genes that help to repair the damage or trigger apoptotic death if the damage is beyond repair. The TP53 paralogs, TP63 and TP73, have distinct roles in development as well as overlapping functions with TP53 in apoptosis and repair, which complicates their analysis in vertebrates. C. elegans contains a single TP53 family member, cep-1, that shares properties of all three vertebrate genes and thus offers a simple system in which to study the biological functions of this important gene family. This review summarizes major advances in our understanding of the TP53 family using C. elegans as a model organism.

Profiling capacity for coordination and systems change: the relative contribution of stakeholder relationships in interorganizational collaboratives

In response to increasing demands for greater coordination and collaboration among community institutions, interorganizational collaboratives (i.e., coalitions, partnerships, coordinating councils) have emerged as a popular mechanism for strengthening the capacity of a community system to respond to public and social issues. This study adopts a network approach to explore the relative importance of dense networks of cooperative relationships among members of interorganizational collaboratives for two outcomes of effectiveness: improving interorganizational coordination and fostering systems change. Based on survey and social network data collected from 48 different collaboratives, findings indicate that, relative to other key characteristics of collaboratives identified in previous literature, cooperative stakeholder relationships were the strongest predictor of systems change outcomes. However, for coordination outcomes, stakeholder relationships were overshadowed in importance by the leadership and decision making capacity of the collaborative. Collectively, findings suggest key differences in the requisite capacity profiles for coordination and systems change outcomes.

Involvement of c-Jun N-terminal kinase in G2/M arrest and FasL-mediated apoptosis induced by a novel indoloquinoline derivative, IQDMA, in K562 cells

N'-(11H-Indolo[3,2-c]quinolin-6-yl)-N,N-dimethylethane-1,2-diamine (IQDMA), an indoloquinoline derivative, synthesized in our laboratory, has been demonstrated to be an effective anti-tumor agent in human leukemia cells. Treatment of K562 cells with IQDMA resulted in G2/M phase cell cycle arrest, presumably involving the concomitant up-regulation of p21 and apoptosis through up-regulation of FasL and sequential activation of caspase-8 and caspase-3. In contrast to the lack of appreciable effect on the phosphorylation of ERK and p38 MAPK, activation of JNK was noted when K562 cells were exposed to IQDMA. Moreover, IQDMA-mediated G2/M phase arrest and apoptosis were reversed after treatment with the JNK-specific inhibitors, SP600125 and JNK inhibitor 1. Further investigation showed that SP600125 reduced the activation of FasL, caspase-3, caspase-8, and led to a marked decline of p21. Taken together, our data show that JNK plays an important role in IQDMA-mediated G2/M arrest and apoptosis of K562 cancer cells.

The nucleotide excision repair pathway is required for UV-C-induced apoptosis in Caenorhabditis elegans

Ultraviolet (UV) radiation is a mutagen of major clinical importance in humans. UV-induced damage activates multiple signaling pathways, which initiate DNA repair, cell cycle arrest and apoptosis. To better understand these pathways, we studied the responses to UV-C light (254 nm) of germ cells in Caenorhabditis elegans. We found that UV activates the same cellular responses in worms as in mammalian cells. Both UV-induced apoptosis and cell cycle arrest were completely dependent on the p53 homolog CEP-1, the checkpoint proteins HUS-1 and CLK-2, and the checkpoint kinases CHK-2 and ATL-1 (the C. elegans homolog of ataxia telangiectasia and Rad3-related); ATM-1 (ataxia telangiectasia mutated-1) was also required, but only at low irradiation doses. Importantly, mutation of genes encoding nucleotide excision repair pathway components severely disrupted both apoptosis and cell cycle arrest, suggesting that these genes not only participate in repair, but also signal the presence of damage to downstream components of the UV response pathway that we delineate here. Our study suggests that whereas DNA damage response pathways are conserved in metazoans in their general outline, there is significant evolution in the relative importance of individual checkpoint genes in the response to specific types of DNA damage.

Ionizing radiations in Caenorhabditis elegans induce poly(ADP-ribosyl)ation, a conserved DNA-damage response essential for survival

Poly(ADP-ribosyl)ation is one of the first responses to DNA damage in mammals. Although it is involved in base excision repair, its exact role has not been ascertained yet. Poly(ADP-ribose) polymerase-1 (PARP-1) and PARP-2 mediate most of the poly(ADP-ribosyl)ation response in mammals and are well conserved in evolution. Their respective homologues PME-1 and PME-2 are found in the nematode Caenorhabditis elegans, a well-known genetically tractable model currently used in DNA damage response research. Here we report the functional analysis of PME-1 and PME-2 in presence of DNA damage. Worms irradiated with high doses of ionizing radiations displayed a sharp drop in their NAD(+) content immediately after treatment, and a biphasic increase in poly(ADP-ribose). The physiological importance of the poly(ADP-ribosyl)ation response was highlighted when worms were preincubated with mammalian PARP inhibitors (3AB, DHQ, PJ34) and irradiated. The embryonic survival rate of the progeny was significantly decreased in a dose-dependent manner. The inhibitor 3AB had a weak effect on embryonic survival, followed closely by DHQ. However, PJ34, a member of the phenantridinone family, was very effective even when used at low concentration (100nM). In vitro PARP assay using recombinant PME-1 and PME-2 showed a similar pattern of inhibition where 3AB and DHQ were weak inhibitors, and PJ34 a stronger one. Inhibitors affect mostly the poly(ADP-ribose) polymers elongation at high concentrations. These results suggest that poly(ADP-ribosyl)ation in response to DNA damage is an ancient and very important biochemical process protecting DNA from deleterious modification.

Translational repression of C. elegans p53 by GLD-1 regulates DNA damage-induced apoptosis

p53 is a tumor suppressor gene whose regulation is crucial to maintaining genome stability and for the apoptotic elimination of abnormal, potentially cancer-predisposing cells. C. elegans contains a primordial p53 gene, cep-1, that acts as a transcription factor necessary for DNA damage-induced apoptosis. In a genetic screen for negative regulators of CEP-1, we identified a mutation in GLD-1, a translational repressor implicated in multiple C. elegans germ cell fate decisions and related to mammalian Quaking proteins. CEP-1-dependent transcription of proapoptotic genes is upregulated in the gld-1(op236) mutant and an elevation of p53-mediated germ cell apoptosis in response to DNA damage is observed. Further, we demonstrate that GLD-1 mediates its repressive effect by directly binding to the 3'UTR of cep-1/p53 mRNA and repressing its translation. This study reveals that the regulation of cep-1/p53 translation influences DNA damage-induced apoptosis and demonstrates the physiological importance of this mechanism.

Genome-wide RNAi identifies p53-dependent and -independent regulators of germ cell apoptosis in C. elegans

We used genome-wide RNA interference (RNAi) to identify genes that affect apoptosis in the C. elegans germ line. RNAi-mediated knockdown of 21 genes caused a moderate to strong increase in germ cell death. Genetic epistasis studies with these RNAi candidates showed that a large subset (16/21) requires p53 to activate germ cell apoptosis. Apoptosis following knockdown of the genes in the p53-dependent class also depended on a functional DNA damage response pathway, suggesting that these genes might function in DNA repair or to maintain genome integrity. As apoptotic pathways are conserved, orthologues of the worm germline apoptosis genes presented here could be involved in the maintenance of genomic stability, p53 activation, and fertility in mammals.

Tales of cannibalism, suicide, and murder: Programmed cell death in C. elegans

"Life is pleasant. Death is peaceful. It's the transition that's troublesome," said Isaac Asimov. Indeed, much scientific work over the last hundred years centered around attempts either to stave off or to induce the onset of death, at both the organismal and the cellular levels. In this quest, the nematode C. elegans has proven an invaluable tool, first, in the articulation of the genetic pathway by which programmed cell death proceeds, and also as a continuing source of inspiration. It is our purpose in this Chapter to familiarize the reader with the topic of programmed cell death in C. elegans and its relevance to current research in the fields of apoptosis and cell corpse clearance.

Caenorhabditis elegans HUS-1 is a DNA damage checkpoint protein required for genome stability and EGL-1-mediated apoptosis

BACKGROUND

The inability to efficiently repair DNA damage or remove cells with severely damaged genomes has been linked to several human cancers. Studies in yeasts and mammals have identified several genes that are required for proper activation of cell cycle checkpoints following various types of DNA damage. However, in metazoans, DNA damage can induce apoptosis as well. How DNA damage activates the apoptotic machinery is not fully understood.

RESULTS

We demonstrate here that the Caenorhabditis elegans gene hus-1 is required for DNA damage-induced cell cycle arrest and apoptosis. Following DNA damage, HUS-1 relocalizes and forms distinct foci that overlap with chromatin. Relocalization does not require the novel checkpoint protein RAD-5; rather, relocalization appears more frequently in rad-5 mutants, suggesting that RAD-5 plays a role in repair. HUS-1 is required for genome stability, as demonstrated by increased frequency of spontaneous mutations, chromosome nondisjunction, and telomere shortening. Finally, we show that DNA damage increases expression of the proapoptotic gene egl-1, a response that requires hus-1 and the p53 homolog cep-1.

CONCLUSIONS

Our findings suggest that the RAD-5 checkpoint protein is not required for HUS-1 to relocalize following DNA damage. Furthermore, our studies reveal a new function of HUS-1 in the prevention of telomere shortening and mortalization of germ cells. DNA damage-induced germ cell death is abrogated in hus-1 mutants, in part, due to the inability of these mutants to activate egl-1 transcription in a cep-1/p53-dependent manner. Thus, HUS-1 is required for p53-dependent activation of a BH3 domain protein in C. elegans.