A novel andrographolide derivative AL‐1 exerts its cytotoxicity on K562 cells through a ROS‐dependent mechanism

Andrographolide‐lipoic acid conjugate (AL‐1) is a new in‐house synthesized chemical entity, which was derived by covalently linking andrographolide with lipoic acid. However, its anti‐cancer effect and cytotoxic mechanism remains unknown. In this study, we found that AL‐1 could significantly inhibit cell viability of human leukemia K562 cells by inducing G2/M arrest and apoptosis in a dose‐dependent manner. Thirty‐one AL‐1‐regulated protein alterations were identified by proteomics analysis. Gene ontology and ingenuity pathway analysis revealed that a cluster of proteins of oxidative redox state and apoptotic cell death‐related proteins, such as PRDX2, PRDX3, PRDX6, TXNRD1, and GLRX3, were regulated by AL‐1. Functional studies confirmed that AL‐1 induced apoptosis of K562 cells through a ROS‐dependent mechanism, and anti‐oxidant, N‐acetyl‐l ‐cysteine, could completely block AL‐1‐induced cytotoxicity, implicating that ROS generation played a vital role in AL‐1 cytotoxicity. Accumulated ROS resulted in oxidative DNA damage and subsequent G2/M arrest and mitochondrial‐mediated apoptosis. The current work reveals that a novel andrographolide derivative AL‐1 exerts its anticancer cytotoxicity through a ROS‐dependent DNA damage and mitochondrial‐mediated apoptosis mechanism.     

Mitochondrial peroxiredoxin‐5 as potential modulator of mitochondria‐ER crosstalk in MPP+‐induced cell death

Peroxiredoxin‐5 (PRDX5) is an antioxidant enzyme which differs from the other peroxiredoxins with regards to its enzymatic mechanism, its high affinity for organic peroxides and peroxynitrite and its wide subcellular distribution. In particular, the mitochondrial isoform of PRDX5 confers a remarkable cytoprotection toward oxidative stress to mammalian cells. Mitochondrial dysfunction and disruption of Ca²⁺ homeostasis are implicated in neurodegeneration. Growing evidence supports that endoplasmic reticulum (ER) could operate in tandem with mitochondria to regulate intracellular Ca²⁺ fluxes in neurodegenerative processes. Here, we overexpressed mitochondrial PRDX5 in SH‐SY5Y cells to dissect the role of this enzyme in 1‐methyl‐4‐phenylpyridinium (MPP)⁺‐induced cell death. Our data show that mitochondria‐dependent apoptosis triggered by MPP⁺, assessed by the measurement of caspase‐9 activation and mitochondrial DNA damage, is prevented by mitochondrial PRDX5 overexpression. Moreover, PRDX5 overexpression blocks the increase in intracellular Ca²⁺, Ca²⁺‐dependent activation of calpains and Bax cleavage. Finally, using Ca²⁺ channel inhibitors (Nimodipine, Dantrolene and 2‐APB), we show that Ca²⁺ release arises essentially from ER stores through 1,4,5‐inositol‐trisphosphate receptors (IP₃R). Altogether, our results suggest that the MPP⁺ mitochondrial pathway of apoptosis is regulated by mitochondrial PRDX5 in a process that could involve redox modulation of Ca²⁺ transporters via a crosstalk between mitochondria and ER.     

Native LDL‐induced oxidative stress in human proximal tubular cells: multiple players involved

Dyslipidemia is a well‐established condition proved to accelerate the progression of chronic kidney disease leading to tubulo‐interstitial injury. However, the molecular aspects of the dyslipidemia‐induced renal damage have not been fully clarified and in particular the role played by low‐density lipoproteins (LDLs). This study aimed to examine the effects of native non‐oxidized LDL on cellular oxidative metabolism in cultured human proximal tubular cells. By means of confocal microscopy imaging combined to respirometric and enzymatic assays it is shown that purified native LDL caused a marked increase of cellular reactive oxygen species (ROS) production, which was mediated by activation of NADPH oxidase(s) and by mitochondrial dysfunction by means of a ROS‐induced ROS release mechanism. The LDL‐dependent mitochondrial alterations comprised inhibition of the respiratory chain activity, enhanced ROS production, uncoupling of the oxidative phosphorylation efficiency, collapse of the mtΔΨ, increased Ca²⁺ uptake and loss of cytochrome c. All the above LDL‐induced effects were completely abrogated by chelating extracellular Ca²⁺ as well as by inhibition of the Ca²⁺‐activated cytoplas‐mic phospholipase A2, NADPH oxidase and mitochondrial permeability transition. We propose a mechanicistic model whereby the LDL‐induced intracellular redox unbalance is triggered by a Ca²⁺ inward flux‐dependent commencement of cPLA2 followed by activation of a lipid‐ and ROS‐based cross‐talking signalling pathway. This involves first oxidants production via the plasmamembrane NADPH oxidase and then propagates downstream to mitochondria eliciting redox‐ and Ca²⁺‐dependent dysfunctions leading to cell‐harming conditions. These findings may help to clarify the mechanism of dyslipidemia‐induced renal damage and suggest new potential targets for specific therapeutic strategies to prevent oxidative stress implicated in kidney diseases.     

Differential parameters between cytosolic 2‐Cys peroxiredoxins,PRDX1 and PRDX2

Peroxiredoxins are thiol‐dependent peroxidases that function in peroxide detoxification and H₂O₂ induced signaling. Among the six isoforms expressed in humans, PRDX1 and PRDX2 share 97% sequence similarity, 77% sequence identity including the active site, subcellular localization (cytosolic) but they hold different biological functions albeit associated with their peroxidase activity. Using recombinant human PRDX1 and PRDX2, the kinetics of oxidation and hyperoxidation with H₂O₂ and peroxynitrite were followed by intrinsic fluorescence. At pH 7.4, the peroxidatic cysteine of both isoforms reacts nearly tenfold faster with H₂O₂ than with peroxynitrite, and both reactions are orders of magnitude faster than with most protein thiols. For both isoforms, the sulfenic acids formed are in turn oxidized by H₂O₂ with rate constants of ca 2 × 10³ M^?1 s^?1 and by peroxynitrous acid significantly faster. As previously observed, a crucial difference between PRDX1 and PRDX2 is on the resolution step of the catalytic cycle, the rate of disulfide formation (11 s^?1 for PRDX1, 0.2 s^?1 for PRDX2, independent of the oxidant) which correlates with their different sensitivity to hyperoxidation. This kinetic pause opens different pathways on redox signaling for these isoforms. The longer lifetime of PRDX2 sulfenic acid allows it to react with other protein thiols to translate the signal via an intermediate mixed disulfide (involving its peroxidatic cysteine), whereas PRDX1 continues the cycle forming disulfide involving its resolving cysteine to function as a redox relay. In addition, the presence of C83 on PRDX1 imparts a difference on peroxidase activity upon peroxynitrite exposure that needs further study.     

Caffeine induces matrix metalloproteinase‐2 (MMP‐2) and MMP‐9 down‐regulation in human leukemia U937 cells via Ca2+/ROS‐mediated suppression of ERK/c‐fos pathway and activation of p38 MAPK/c‐jun pathway

Caffeine attenuated invasion of human leukemia U937 cells with characteristic of decreased protein expression and mRNA levels of matrix metalloproteinase‐2 (MMP‐2) and MMP‐9. Down‐regulation of MMP‐2 and MMP‐9 in U937 cells was abrogated by abolishment of caffeine‐elicited increase in intracellular Ca²⁺ concentration and ROS generation. Pretreatment with BAPTA‐AM (Ca²⁺ chelator) and N‐acetylcysteine (ROS scavenger) abolished caffeine‐induced ERK inactivation and p38 MPAK activation. Moreover, caffeine treatment led to MAPK phosphatase‐1 (MKP‐1) down‐regulation and protein phosphatase 2A catalytic subunit (PP2Ac) up‐regulation, which were involved in cross‐talk between p38 MAPK and ERK. Transfection of constitutively active MEK1 or pretreatment with SB202190 (p38 MAPK inhibitor) restored MMP‐2 and MMP‐9 protein expression in caffeine‐treated cells. Caffeine treatment repressed ERK‐mediated c‐Fos phosphorylation but evoked p38 MAPK‐mediated c‐Jun phosphorylation. Knock‐down of c‐Fos and c‐Jun by siRNA reflected that c‐Fos counteracted the effect of c‐Jun on MMP‐2/MMP‐9 down‐regulation. Taken together, our data indicate that MMP‐2/MMP‐9 down‐regulation in caffeine‐treated U937 cells is elicited by Ca²⁺/ROS‐mediated suppression of ERK/c‐Fos pathway and activation of p38 MAPK/c‐Jun pathway. J. Cell. Physiol. 224: 775–785, 2010. © 2010 Wiley‐Liss, Inc.     

A potential role of connexin 43 in epidermal growth factor-induced proliferation of mouse embryonic stem cells: involvement of Ca2+/PKC, p44/42 and p38 MAPKs pathways

Abstract. Objectives: The gap junction protein, connexin (Cx), plays an important role in maintaining cellular homeostasis and cell proliferation by allowing communication between adjacent cells. Therefore, this study has examined the effect of epidermal growth factor (EGF) on Cx43 and its relationship to proliferation of mouse embryonic stem cells. Materials and methods: Expressions of Cx43, mitogen‐activated protein kinases (MAPKs) and cell cycle regulatory proteins were assessed by Western blot analysis. Cell proliferation was assayed with [³H]thymidine incorporation. Intercellular communication level was measured by a scrape loading/dye transfer method. Results: The results showed that EGF increased the level of Cx43 phosphorylation in a time‐ (≥5 min) and dose‐ (≥10 ng/mL) dependent manner. Indeed, EGF‐induced increase in phospho‐Cx43 level was significantly blocked by either AG 1478 or herbimycin A (tyrosine kinase inhibitors). EGF increased Ca²⁺ influx and protein kinase C (PKC) translocation from the cytosolic compartment to the membrane compartment. Moreover, pre‐treatment with BAPTA‐AM (an intracellular Ca²⁺ chelator), EGTA (an extracellular Ca²⁺ chelator), bisindolylmaleimide I or staurosporine (PKC inhibitors) inhibited the EGF‐induced phosphorylation of Cx43. EGF induced phosphorylation of p38 and p44/42 MAPKs, and this was blocked by SB 203580 (a p38 MAPK inhibitor) and PD 98059 (a p44/42 MAPK inhibitor), respectively. EGF or 18α‐glycyrrhetinic acid (GA; a gap junction inhibitor) increased expression levels of the protooncogenes (c‐fos, c‐jun and c‐myc), cell cycle regulatory proteins [cyclin D1, cyclin E, cyclin‐dependent kinase 2 (CDK2), CDK4 and p‐Rb], [³H]thymidine incorporation and cell number, but decreased expression levels of the p21^WAF1/Cip1 and p27^Kip1, CDK inhibitory proteins. Transfection of Cx43 siRNA also increased the level of [³H]thymidine incorporation and cell number. EGF, 18α‐GA or transfection of Cx43 siRNA increased 2‐DG uptake and GLUT‐1 protein expression. Conclusions: EGF‐induced phosphorylation of Cx43, which was mediated by the Ca²⁺/PKC, p44/42 and p38 MAPKs pathways, partially contributed to regulation of mouse embryonic stem cell proliferation.     

A key enzyme for flavin synthesis is required for nitric oxide and reactive oxygen species production in disease resistance

Nitric oxide (NO) and reactive oxygen species (ROS) play key roles in plant immunity. However, the regulatory mechanisms of the production of these radicals are not fully understood. Hypersensitive response (HR) cell death requires the simultaneous and balanced production of NO and ROS. In this study we indentified NbRibA encoding a bifunctional enzyme, guanosine triphosphate cyclohydrolase II/3,4‐dihydroxy‐2‐butanone‐4‐phosphate synthase, which participates in the biosynthesis of flavin, by screening genes related to mitogen‐activated protein kinase‐mediated cell death, using virus‐induced gene silencing. Levels of endogenous riboflavin and its derivatives, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), which are important prosthetic groups for several enzymes participating in redox reactions, decreased in NbRibA‐silenced Nicotiana benthamiana. Silencing NbRibA compromised not only HR cell death, but also the NO and ROS production induced by INF1 elicitin and a constitutively active form of NbMEK2 (NbMEK2^DD), and also induced high susceptibility to oomycete Phytophthora infestans and ascomycete Colletotrichum orbiculare. Compromised radical production and HR cell death induced by INF1 in NbRibA‐silenced leaves were rescued by adding riboflavin, FMN or FAD. These results indicate that flavin biosynthesis participates in regulating NO and ROS production, and HR cell death.     

Molecular mechanism of diallyl disulfide in cell cycle arrest and apoptosis in HCT‐116 colon cancer cells

Diallyl disulfide (DADS) is the most prevalent oil‐soluble sulfur compound in garlic and inhibits cell proliferation in many cancer cell lines. Here we examined DADS cytotoxicity in a redox‐mediated process, involving reactive oxygen species (ROS) production. In the present study, p53‐independent cell cycle arrest at G2/M phase was observed with DADS treatment, along with time‐dependent increase of cyclin B1. In addition, apoptosis was also observed upon 24‐h DADS treatment accompanied by activation of p53. In HCT‐116 cells, DADS application induced a dose‐dependent increase and time‐dependent changes in ROS production. Scavenging of DADS‐induced ROS by N‐acetyl cysteine or reduced glutathione inhibited cell cycle arrest, apoptosis and p53 activation by DADS. These results suggest that ROS trigger the DADS‐induced cell cycle arrest and apoptosis and that ROS are involved in stress‐induced signaling upstream of p53 activation. Transfection of p53 small interfering RNA prevents the accumulation of cleaved poly(ADP‐ribose) polymerase and sub‐G1 cell population by 65% and 35%, respectively. Moreover, DADS‐induced apoptosis was also prevented by treatment with oligomycin, which is known to prevent p53‐dependent apoptosis by reducing ROS levels in mitochondria. These results suggest that mitochondrial ROS may serve as second messengers in DADS‐induced apoptosis, which requires activation of p53. © 2009 Wiley Periodicals, Inc. J Biochem Mol Toxicol 23:71–79, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/jbt.20266     

Brassinosteroid‐mediated reactive oxygen species are essential for tapetum degradation and pollen fertility in tomato

Phytohormone brassinosteroids (BRs) are essential for plant growth and development, but the mechanisms of BR‐mediated pollen development remain largely unknown. In this study, we show that pollen viability, pollen germination and seed number decreased in the BR‐deficient mutant d^{^im}, which has a lesion in the BR biosynthetic gene DWARF (DWF), and in the bzr1 mutant, which is deficient in BR signaling regulator BRASSINAZOLE RESISTANT 1 (BZR1), compared with those in wild‐type plants, whereas plants overexpressing DWF or BZR1 exhibited the opposite effects. Loss or gain of function in the DWF or BZR1 genes altered the timing of reactive oxygen species (ROS) production and programmed cell death (PCD) in tapetal cells, resulting in delayed or premature tapetal degeneration, respectively. Further analysis revealed that BZR1 could directly bind to the promoter of RESPIRATORY BURST OXIDASE HOMOLOG 1 (RBOH1), and that RBOH1‐mediated ROS promote pollen and seed development by triggering PCD and tapetal cell degradation. In contrast, the suppression of RBOH1 compromised BR signaling‐mediated ROS production and pollen development. These findings provide strong evidence that BZR1‐dependent ROS production plays a critical role in the BR‐mediated regulation of tapetal cell degeneration and pollen development in Solanum lycopersicum (tomato) plants.     

Monoamine oxidase‐A is a novel driver of stress‐induced premature senescence through inhibition of parkin‐mediated mitophagy

Cellular senescence, the irreversible cell cycle arrest observed in somatic cells, is an important driver of age‐associated diseases. Mitochondria have been implicated in the process of senescence, primarily because they are both sources and targets of reactive oxygen species (ROS). In the heart, oxidative stress contributes to pathological cardiac ageing, but the mechanisms underlying ROS production are still not completely understood. The mitochondrial enzyme monoamine oxidase‐A (MAO‐A) is a relevant source of ROS in the heart through the formation of H₂O₂ derived from the degradation of its main substrates, norepinephrine (NE) and serotonin. However, the potential link between MAO‐A and senescence has not been previously investigated. Using cardiomyoblasts and primary cardiomyocytes, we demonstrate that chronic MAO‐A activation mediated by synthetic (tyramine) and physiological (NE) substrates induces ROS‐dependent DNA damage response, activation of cyclin‐dependent kinase inhibitors p21^cip, p16^ink4a, and p15^ink4b and typical features of senescence such as cell flattening and SA‐β‐gal activity. Moreover, we observe that ROS produced by MAO‐A lead to the accumulation of p53 in the cytosol where it inhibits parkin, an important regulator of mitophagy, resulting in mitochondrial dysfunction. Additionally, we show that the mTOR kinase contributes to mitophagy dysfunction by enhancing p53 cytoplasmic accumulation. Importantly, restoration of mitophagy, either by overexpression of parkin or inhibition of mTOR, prevents mitochondrial dysfunction and induction of senescence. Altogether, our data demonstrate a novel link between MAO‐A and senescence in cardiomyocytes and provides mechanistic insights into the potential role of MAO‐dependent oxidative stress in age‐related pathologies.     

Non‐apoptotic function of caspases in a cellular model of hydrogen peroxide‐associated colitis

Oxidative stress, caused by reactive oxygen species (ROS), is a major contributor to inflammatory bowel disease (IBD)‐associated neoplasia. We mimicked ROS exposure of the epithelium in IBD using non‐tumour human colonic epithelial cells (HCEC) and hydrogen peroxide (H₂O₂). A population of HCEC survived H₂O₂‐induced oxidative stress via JNK‐dependent cell cycle arrests. Caspases, p21^WAF1 and γ‐H2AX were identified as JNK‐regulated proteins. Up‐regulation of caspases was linked to cell survival and not, as expected, to apoptosis. Inhibition using the pan‐caspase inhibitor Z‐VAD‐FMK caused up‐regulation of γ‐H2AX, a DNA‐damage sensor, indicating its negative regulation via caspases. Cell cycle analysis revealed an accumulation of HCEC in the G₁‐phase as first response to oxidative stress and increased S‐phase population and then apoptosis as second response following caspase inhibition. Thus, caspases execute a non‐apoptotic function by promoting cells through G₁‐ and S‐phase by overriding the G₁/S‐ and intra‐S checkpoints despite DNA‐damage. This led to the accumulation of cells in the G₂/M‐phase and decreased apoptosis. Caspases mediate survival of oxidatively damaged HCEC via γ‐H2AX suppression, although its direct proteolytic inactivation was excluded. Conversely, we found that oxidative stress led to caspase‐dependent proteolytic degradation of the DNA‐damage checkpoint protein ATM that is upstream of γ‐H2AX. As a consequence, undetected DNA‐damage and increased proliferation were found in repeatedly H₂O₂‐exposed HCEC. Such features have been associated with neoplastic transformation and appear here to be mediated by a non‐apoptotic function of caspases. Overexpression of upstream p‐JNK in active ulcerative colitis also suggests a potential importance of this pathway in vivo.     

1Alpha,25-dihydroxyvitamin D3 modulation of adipocyte reactive oxygen species production

Objective: We have previously shown 1α,25‐dihydroxyvitamin D₃ [1α,25‐(OH)₂D₃] to inhibit mitochondrial uncoupling protein 2 (UCP2) expression in adipocytes and that in vivo suppression of calcitriol levels with calcium‐rich diets increases UCP2 expression. Because UCP2 plays a significant role in the clearance of reactive oxygen species (ROS), we studied the effect of calcitriol on ROS production and ROS‐induced adipocyte proliferation. Research Methods and Procedures: ROS production in human and murine adipocytes was stimulated by high glucose (30 mM) or H₂O₂ (100 nM). Results: Both approaches resulted in increased ROS production by 27% to 100% (p < 0.05) and increased cell proliferation by 15% to 39% (p < 0.03). These effects were augmented by the addition of mitochondrial uncoupling inhibitor guanosine 5′‐diphosphate (GDP; 100 μM) or 1α,25‐(OH)₂D₃ (10 nM) and attenuated by UCP2 overexpression, suggesting that inhibition of mitochondrial uncoupling suppresses clearance of ROS and increases adipocyte proliferation. The addition of α ± tocopherol (1 μM) inhibited cell proliferation in adipocytes treated with either H₂O₂ or high glucose, indicating that ROS plays a major role in the regulation of cell proliferation in adipocytes. Moreover, stimulation of ROS with high glucose and H₂O₂ resulted in a 2‐ to 5‐fold increase in adipocyte intracellular calcium ([Ca²⁺]i; p < 0.001), and calcium channel antagonism (nifedipine, 10 μM) suppressed ROS induced calcium influx and cell proliferation, indicating that [Ca²⁺]i may also regulate ROS production and exert a mitogenic effect in adipocytes. Discussion: These data support a role of 1α,25‐(OH)₂D₃, UCP2, and [Ca²⁺]i in the regulation of adipocyte ROS production.     

Role of LOX‐1 in monocyte adhesion‐triggered redox,Akt/eNOS and Ca2+ signaling pathways in endothelial cells

This study was conducted to examine the role of lectin‐like oxidized low‐density lipoprotein receptor‐1 (LOX‐1) in monocyte adhesion‐induced redox‐sensitive, Akt/eNOS and Ca²⁺ signaling pathways in endothelial cells (ECs). LOX‐1 was blocked by an antibody‐neutralizing LOX‐1 TS92 or small interfering RNA. In cultured human aortic ECs, monocyte adhesion activated Rac1 and p47^phox, and increased NADPH oxidase activity and reactive oxygen species (ROS) generation within 30 min and NF‐κB phosphorylation within 1 h, resulting in redox‐sensitive gene expression. Akt and eNOS phosphorylation was induced 15 min after adding monocytes and returned to control level after 30 min, whereas NO production was not altered by monocyte adhesion. Blockade of LOX‐1 blunted the monocyte adhesion‐triggered redox‐sensitive signaling pathway and Akt/eNOS phosphorylation in ECs. Both endothelial intracellular Ca²⁺ mobilization and Ca²⁺ influx caused by monocyte attachment were markedly attenuated by pretreatment of ECs with TS92. This suggests that LOX‐1 is involved in redox‐sensitive, Akt/eNOS and Ca²⁺ signaling pathways in monocyte adhesion to ECs independent of oxidized low‐density lipoprotein (ox‐LDL). Furthermore, blockade of Ca²⁺ inhibited monocyte adhesion‐triggered Rac1 and p47^phox activation and ROS generation in ECs, whereas Ca²⁺ signaling was suppressed by blockade of NADPH oxidase and ROS generation. Finally, TS92 blocked the monocyte adhesion to ECs stimulated with or without tumor necrosis factor‐α or ox‐LDL. We provide evidence that LOX‐1 plays a role in redox‐sensitive, Akt/eNOS and Ca²⁺ signaling pathways in monocyte adhesion to ECs independent of the ox‐LDL–LOX‐1 axis. J. Cell. Physiol. 220: 706–715, 2009. © 2009 Wiley‐Liss, Inc.     

Regulation of PRDX1 peroxidase activity by Pin1

Pin1 isomerizes the phosphorylated Ser/Thr-Pro peptide bonds and regulates the functions of its binding proteins by inducing conformational changes. Involvement of Pin1 in the aging process has been suggested based on the phenotype of Pin1-knockout mice and its interaction with lifespan regulator protein, p66^Shc. In this study, we utilize a proteomic approach and identify peroxiredoxin 1 (PRDX1), another regulator of aging, as a novel Pin1 binding protein. Pin1 binds to PRDX1 through interacting with the phospho-Thr⁹⁰-Pro⁹¹ motif of PRDX1, and this interaction is abolished when the Thr⁹⁰ of PRDX1 is mutated. The Pin1 binding motif, Thr-Pro, is conserved in the 2-Cys PRDXs, PRDX1–4 and the interactions between Pin1 and PRDX2–4 are also demonstrated. An increase in hydrogen peroxide buildup and a decrease in the peroxidase activity of 2-Cys PRDXs were observed in Pin1^?/? mouse embryonic fibroblasts (MEFs), with the activity of PRDXs restored when Pin1 was re-introduced into the cells. Phosphorylation of PRDX1 at Thr⁹⁰ has been shown to inhibit its peroxidase activity; however, how exactly the activity of PRDX1 is regulated by phosphorylation still remains unknown. Here, we demonstrate that Pin1 facilitates the protein phosphatase 2A-mediated dephosphorylation of PRDX1, which helps to explain the accumulation of the inactive phosphorylated form of PRDX1 in Pin1^?/? MEFs. Collectively, we identify Pin1 as a novel PRDX1 binding protein and propose a mechanism for Pin1 in regulating the metabolism of reactive oxygen species in cells.     

p53 signalling controls cell cycle arrest and caspase‐independent apoptosis in macrophages infected with pathogenic Leptospira species

Pathogenic Leptospira species, the causative agents of leptospirosis, have been shown to induce macrophage apoptosis through caspase‐independent, mitochondrion‐related apoptosis inducing factor (AIF) and endonuclease G (EndoG), but the signalling pathway leading to AIF/EndoG‐based macrophage apoptosis remains unknown. Here we show that infection of Leptospira interrogans caused a rapid increase in reactive oxygen species (ROS), DNA damage, and intranuclear foci of 53BP1 and phosphorylation of H2AX (two DNAdamage indicators) in wild‐type p53‐containing mouse macrophages and p53‐deficient human macrophages. Most leptospire‐infected cells stayed at the G₁ phase, whereas depletion or inhibition of p53 caused a decrease of the G₁‐phase cells and the early apoptotic ratios. Infection with spirochaetes stimulated a persistent activation of p53 and an early activation of Akt through phosphorylation. The intranuclear translocation of p53, increased expression of p53‐dependent p21^Cip1/WAF1 and pro‐apoptotic Bcl‐2 family proteins (Bax, Noxa and Puma), release of AIF and EndoG from mitochondria, and membrane translocation of Fas occurred during leptospire‐induced macrophage apoptosis. Thus, our study demonstrated that ROS production and DNA damage‐dependent p53‐Bax/Noxa/Puma‐AIF/EndoG signalling mediates the leptospire‐induced cell cycle arrest and caspase‐independent apoptosis of macrophages.     

Dysfunctional telomeres induce p53‐dependent and independent apoptosis to compromise cellular proliferation and inhibit tumor formation

Aging is associated with progressive telomere shortening, resulting in the formation of dysfunctional telomeres that compromise tissue proliferation. However, dysfunctional telomeres can limit tumorigenesis by activating p53‐dependent cellular senescence and apoptosis. While activation of both senescence and apoptosis is required for repress tumor formation, it is not clear which pathway is the major tumor suppressive pathway in vivo. In this study, we generated Eμ‐myc; Pot1b ^?/? mouse to directly compare tumor formation under conditions in which either p53‐dependent apoptosis or senescence is activated by telomeres devoid of the shelterin component Pot1b. We found that activation of p53‐dependent apoptosis plays a more critical role in suppressing lymphoma formation than p53‐dependent senescence. In addition, we found that telomeres in Pot1b^?/?; p53^?/? mice activate an ATR‐Chk1‐dependent DNA damage response to initiate a robust p53‐independent, p73‐dependent apoptotic pathway that limited stem cell proliferation but suppressed B‐cell lymphomagenesis. Our results demonstrate that in mouse models, both p53‐dependent and p53‐independent apoptosis are important to suppressing tumor formation.