p53 deficiency fails to prevent increased programmed cell death in the Bcl-X(L)-deficient nervous system

Botulinum Toxin Type A: Assessing The Effects on The Brain Stem

BACKGROUND

In this study, our aim is to investigate the possible effects of Botulinum toxin type A administrations in the early and late periods on the brain stem.

METHODS

Eighteen white New Zealand rabbits were used in this study with the subjects being divided into three groups. Group I received 0.05 mL sterile saline to the left anterior auricular muscles. Group II and III were injected with Botulinum toxin type A (Botox, Allergan) to the left anterior auricular muscles. Group II was sacrificed 5 days after application and Group III was sacrificed 12 weeks after application; brain stem tissues were then taken. The samples were examined with Caspase 3, 8, and 9 immunohistochemical stainings.

RESULTS

In the control group with Caspase-3 immune staining, moderate-to-strong immune reactivity was seen in a small number of neurons. In the Caspase-8 and 9 immune stainings, the immune reactive neurons were seen in greater numbers when compared with the Caspase-3 immune reactive neurons. In the early and late period, groups with Caspase-8 and 9 immune stainings, the immune reactive neurons were seen in greater numbers and in the wider area when compared with the Caspase-3 immune reactive neurons. No significant differences were recognized in the Caspase immune stainings between the early and late period groups. The results were statistically supported.

CONCLUSION

It was concluded that Botulinum toxin type A application did not trigger apoptosis in stem cell tissues.

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Characterization of the mechanism of inhibin α-subunit gene in mouse anterior pituitary cells by RNA interference

Inhibin, a member of the transforming growth factor-β [TGF-β] superfamily, is a suppressor of follicle-stimulating hormone [FSH] release through pituitary–gonadal negative feedback loop to regulate follicular development. In this study, Inhibin α-subunit [Inha] gene was knocked down successfully in mice primary anterior pituitary cells at both transcriptional and translational levels by RNAi-Ready pSIREN-RetroQ-ZsGreen Vector mediated recombinant pshRNA vectors. The results indicated that inhibin silencing significantly promoted apoptosis by up-regulating Caspase-3, Bax and Bcl-2 genes without affecting p53 both at transcriptional and translational levels. Furthermore, it markedly impaired the progression of G1 phase of cell cycle and decreased the amount of cells in S phase [as detected by flow cytometry]. Inhibin silencing resulted in significant up-regulation of mRNA and protein expressions of Gondotropin releasing hormone receptors [GnRHR] and down-regulated mRNA levels of β-glycans with parellel change in the amount of its protein expression. Silencing of inhibin-a significantly increased [P<0.05] activin-β concentration without affecting FSH and LH levels in anterior pituitary cells. These findings revealed that up regulation of GnRH receptors by silencing inhibin a-subunit gene might increase the concentration of activin-β in the culture medium. Inhibin a silencing resulted in increased mRNA and protein expressions of inhibinβ which may demonstrate that both inhibin subunits co-participate in the regulation of reproductive events in anterior pituitary cells. This study concludes that inhibin is a broad regulatory marker in anterior pituitary cells by regulating apoptosis, cellular progression and simultaneously by vital fluctuations in the hormonal signaling.

Migfilin sensitizes cisplatin-induced apoptosis in human glioma cells in vitro

AIM

Filamin binding LIM protein 1, also known as migfilin, is a skeleton organization protein that binds to mitogen-inducible gene 2 at cell-extracellular matrix adhesions. The aim of this study was to investigate the role of migfilin in cisplatin-induced apoptosis in human glioma cells, to determine the functional domains of migfilin, and to elucidate the molecular mechanisms underlying the regulation of cisplatin-related chemosensitivity.

METHODS

The human glioma cell lines Hs683, H4, and U-87 MG were transfected with pEGFP-C2-migfilin to elevate the expression level of migfilin. RNA interference was used to reduce the expression of migfilin. To determine the functional domains of migfilin, U-87 MG cells were transfected with plasmids of migfilin deletion mutants. After treatment with cisplatin (40 μmol/L) for 24 h, the cell viability was assessed using the MTS assay, and the cell apoptotic was examined using the DAPI staining assay and TUNEL analysis. Expression levels of apoptosis-related proteins were detected by Western blot analysis.

RESULTS

Overexpression of migfilin significantly enhanced cisplatin-induced apoptosis in Hs683, H4, and U-87 MG cells, whereas downregulation of migfilin expression inhibited the chemosensitivity of these cell lines. The N-terminal region of migfilin alone was able to enhance the cisplatin-induced apoptosis. However, despite the existence of the N-terminal region, mutants of migfilin with any one of three LIM domains deleted led to a function loss. Furthermore, apoptotic proteins (PARP and caspase-3) and the anti-apoptotic protein Bcl-xL were modulated by the expression level of migfilin in combination with cisplatin.

CONCLUSION

The LIM1-3 domains of migfilin play a key role in sensitizing glioma cells to cisplatin-induced apoptosis through regulation of apoptosis-related proteins.

Mechanisms of neurotoxicity induced in the developing brain of mice and rats by DNA-damaging chemicals

It is not widely known how the developing brain responds to extrinsic damage, although the developing brain is considered to be sensitive to diverse environmental factors including DNA-damaging agents. This paper reviews the mechanisms of neurotoxicity induced in the developing brain of mice and rats by six chemicals (ethylnitrosourea, hydroxyurea, 5-azacytidine, cytosine arabinoside, 6-mercaptopurine and etoposide), which cause DNA damage in different ways, especially from the viewpoints of apoptosis and cell cycle arrest in neural progenitor cells. In addition, this paper also reviews the repair process following damage in the developing brain.

Deficiency of pro-apoptotic Hrk attenuates programmed cell death in the developing murine nervous system but does not affect Bcl-x deficiency-induced neuron apoptosis

The BCL-2 family includes both pro- and anti-apoptotic proteins, which regulate programmed cell death during development and in response to various apoptotic stimuli. The BH3-only subgroup of pro-apoptotic BCL-2 family members is critical for the induction of apoptotic signaling, by binding to and neutralizing anti-apoptotic BCL-2 family members. During embryonic development, the anti-apoptotic protein BCL-X(L) plays a critical role in the survival of neuronal populations by regulating the multi-BH domain protein BAX. In this study, the authors investigated the role of Harakiri (HRK), a relatively recently characterized BH3-only molecule in disrupting the BAX-BCL-X(L) interaction during nervous system development. Results indicate that HRK deficiency significantly reduces programmed cell death in the nervous system. However, HRK deficiency does not significantly attenuate the widespread apoptosis seen in the Bcl-x (-/-) embryonic nervous system, indicating that other BH3-only molecules, alone or in combination, may regulate BAX activation in immature neurons.

Regulation of Bax activation and apoptotic response to UV irradiation by p53 transcription-dependent and -independent pathways

The Trp53 tumor suppressor gene product (p53) functions in the nucleus to regulate proapoptotic genes, whereas cytoplasmic p53 directly activates proapoptotic Bcl-2 proteins to permeabilize mitochondria and initiate apoptosis. Here, we demonstrate that both p53 transcription-dependent and -independent pathways contribute to UV-induced apoptosis. First we show that Pifithrin-alpha, a small molecule inhibitor of p53 transcriptional activity, delays Bax translocation and cell death by UV irradiation. Then using CHX (cycloheximide) to prevent new protein expression in response to p53, we also find that Bax translocation and cell death by UV irradiation are delayed. Furthermore we find that overexpression of Bcl-x(L), an inhibitor of cytoplasmic p53 after UV irradiation, prevents cell death. Finally, we observe that Pifithrin-alpha and CHX effectively inhibit PUMA expression by UV irradiation. Taken together, these data indicate that the nuclear p53 promotes PUMA expression, which then displaces cytoplasmic p53 from Bcl-x(L), allowing p53 to induce mitochondrial permeabilization, thereby triggering apoptosis.

Loss of BH3-only protein Bim inhibits apoptosis of hemopoietic cells in the fetal liver and male germ cells but not neuronal cells in bcl-x-deficient mice

Members of the Bcl-2 family include pro- and antiapoptotic proteins that regulate programmed cell death of developing tissues and death in response to cellular damage. In developing mice, the antiapoptotic Bcl-x(L) is necessary for survival of neural and hematopoietic cells, and consequently, bcl-x-deficient mice die around Day 13.5 of embryogenesis. Furthermore, adult bcl-x(+/-) heterozygous male mice have reduced fertility because of testicular degeneration. Bax, a multi-BH (Bcl-2 homology) domain proapoptotic member of the Bcl-2 family, is regulated by Bcl-x(L) and is required for the neuropathological abnormalities seen in bcl-x-deficient embryos. The BH3 domain only subgroup of the Bcl-2 family includes proapoptotic members that are essential for the initiation of apoptotic signaling. In this study, we investigated the role for Bim, a BH3 domain only protein, in the embryonic lethality and increased developmental cell death in bcl-x-deficient animals and the perturbed testicular function in bcl-x(+/-) adults. Our studies show that bim deficiency attenuates hematopoietic cell death in the fetal liver of bcl-x-deficient animals, indicating that Bim contributes to programmed cell death in this cell population. In addition, we found that testicular degeneration of adult bcl-x(+/-) males was rescued by concomitant Bim deficiency. However, concomitant Bim deficiency had no effect on the embryonic lethality and widespread nervous system abnormalities caused by bcl-x deficiency. Our work identifies Bim as an important regulator of bcl-x deficiency-induced cell death during hematopoiesis and testicular development.

Mutually exclusive subsets of BH3-only proteins are activated by the p53 and c-Jun N-terminal kinase/c-Jun signaling pathways during cortical neuron apoptosis induced by arsenite

The c-Jun N-terminal protein kinase (JNK)/c-Jun and p53 pathways form distinct death-signaling modules in neurons that culminate in Bax-dependent apoptosis. To investigate whether this signaling autonomy is due to recruitment of particular BH3-only proteins, we searched for a toxic signal that would activate both pathways in the same set of neurons. We show that arsenite activates both the JNK/c-Jun and p53 pathways in cortical neurons, which together account for >95% of apoptosis, as determined by using the mixed-lineage kinase (JNK/c-Jun) pathway inhibitor CEP11004 and p53-null mice. Despite the coexistence of both pathways in at least 30% of the population, Bim mRNA and protein expression was increased only by the JNK/c-Jun signaling pathway, whereas Noxa and Puma mRNA and Puma protein expression was entirely JNK/c-Jun independent. About 50% of Puma/Noxa expression was p53 dependent, with the remaining signal being independent of both pathways and possibly facilitated by arsenite-induced reduction in P-Akt. However, functionally, Puma was predominant in mediating Bax-dependent apoptosis, as evidenced by the fact that more than 90% of apoptosis was prevented in Puma-null neurons, although Bim was still upregulated, while Bim- and Noxa-null neurons died similarly to wild-type neurons. Thus, the p53 and JNK/c-Jun pathways can activate mutually exclusive subclasses of BH3-only proteins in the same set of neurons. However, other factors besides expression may determine which BH3-only proteins mediate apoptosis.

PUMA couples the nuclear and cytoplasmic proapoptotic function of p53

The Trp53 tumor suppressor gene product (p53) functions in the nucleus to regulate proapoptotic genes, whereas cytoplasmic p53 directly activates proapoptotic Bcl-2 proteins to permeabilize mitochondria and initiate apoptosis. Here, we demonstrate that a tripartite nexus between Bcl-xL, cytoplasmic p53, and PUMA coordinates these distinct p53 functions. After genotoxic stress, Bcl-xL sequestered cytoplasmic p53. Nuclear p53 caused expression of PUMA, which then displaced p53 from Bcl-xL, allowing p53 to induce mitochondrial permeabilization. Mutant Bcl-xL that bound p53, but not PUMA, rendered cells resistant to p53-induced apoptosis irrespective of PUMA expression. Thus, PUMA couples the nuclear and cytoplasmic proapoptotic functions of p53.

Molecular Regulation of Acute Ethanol-Induced Neuron Apoptosis

Ethanol is a potent neurotoxin particularly for the developing nervous system. Intrauterine exposure to ethanol during the last trimester of human gestation can produce a broad spectrum of neuropathologic consequences. This period of human brain development is roughly equivalent to the first week of rodent postnatal life and acute exposure of neonatal mice to ethanol produces massive neuronal apoptosis throughout the brain. We have previously demonstrated that ethanol-induced neuron apoptosis is critically dependent on expression of Bax, a proapoptotic member of the Bcl-2 family. To further define the molecular pathway regulating ethanol-induced neuron apoptosis, we analyzed the effects of acute ethanol exposure on cerebellar internal granule cell neurons both in vivo and in vitro. Ethanol produced extensive Bax-dependent caspase-3 activation and neuron apoptosis in the cerebellar internal granule cell layer, which was maximal at approximately 6 hours postadministration. This effect was recapitulated in vitro and required new gene transcription, protein translation, Bax expression, and caspase activation. Ethanol-induced neuron death was independent of p53 expression and was unaffected by deficiency in the proapoptotic Bcl-2 family members Bid or Bad. These studies indicate that ethanol activates an intrinsic apoptotic death program in neurons that is likely to contribute to the neuropathologic effects of human fetal alcohol exposure.

Failed clearance of aneuploid embryonic neural progenitor cells leads to excess aneuploidy in the Atm-deficient but not the Trp53-deficient adult cerebral cortex

Aneuploid neurons populate the normal adult brain, but the cause and the consequence of chromosome abnormalities in the CNS are poorly defined. In the adult cerebral cortex of three genetic mutants, one of which is a mouse model of the human neurodegenerative disease ataxia-telangiectasia (A-T), we observed divergent levels of sex chromosome (XY) aneuploidy. Although both A-T mutated (Atm)- and transformation related protein 53 (Trp53)-dependent mechanisms are thought to clear newly postmitotic neurons with chromosome abnormalities, we found a 38% increase in the prevalence of XY aneuploidy in the adult Atm-/- cerebral cortex and a dramatic 78% decrease in Trp53-/- mutant mice. A similar 43% decrease in adult XY aneuploidy was observed in DNA repair-deficient Xrcc5-/- mutants. Additional investigation found an elevated incidence of aneuploid embryonic neural progenitor cells (NPCs) in all three mutants, but elevated apoptosis, a likely fate of embryonic NPCs with severe chromosome abnormalities, was observed only in Xrcc5-/- mutants. These data lend increasing support to the hypothesis that hereditary mutations such as ATM-deficiency, which render abnormal cells resistant to developmental clearance, can lead to late-manifesting human neurological disorders.

Metastasis-associated protein 1 deregulation causes inappropriate mammary gland development and tumorigenesis

Emerging data suggest that metastasis-associated protein 1 (MTA1) represses ligand-dependent transactivation functions of estrogen receptor-alpha in cultured breast cancer cells and that MTA1 is upregulated in human breast tumors. However, the role of MTA1 in tumorigenesis in a physiologically relevant animal system remains unknown. To reveal the role of MTA1 in mammary gland development, transgenic mice expressing MTA1 under the control of the mouse mammary tumor virus promoter long terminal repeat were generated. Unexpectedly, we found that mammary glands of these virgin transgenic mice exhibited extensive side branching and precocious differentiation because of increased proliferation of ductal and alveolar epithelial cells. Mammary glands of virgin transgenic mice resemble those from wild-type mice in mid-pregnancy and inappropriately express beta-casein, cyclin D1 and beta-catenin protein. Increased ductal growth was also observed in the glands of ovariectomized female mice, as well as of transgenic male mice. MTA1 dysregulation in mammary epithelium and cancer cells triggered downregulation of the progesterone receptor-B isoform and upregulation of the progesterone receptor-A isoform, resulting in an imbalance in the native ratio of progesterone receptor A and B isoforms. MTA1 transgene also increased the expression of progesterone receptor-A target genes Bcl-XL (Bcl2l1) and cyclin D1 in mammary gland of virgin mice, and, subsequently, produced a delayed involution. Remarkably, 30% of MTA1 transgenic females developed focal hyperplastic nodules, and about 7% exhibited mammary tumors within 18 months. These studies establish, for the first time, a potential role of MTA1 in mammary gland development and tumorigenesis. The underlying mechanism involves the upregulation of progesterone receptor A and its targets, Bcl-XL and cyclin D1.

HIV associated neurodegeneration requires p53 in neurons and microglia

HIV infection of the central nervous system leads to HIV-associated dementia (HAD) in a substantial subset of infected individuals. The pathogenesis of neuronal dysfunction in HAD is not well understood, but previous studies have demonstrated evidence for activation of apoptotic pathways. The tumor suppressor transcription factor p53 is an apical mediator of neuronal apoptosis following a variety of injurious stimuli. To determine whether p53 participates in HAD, we exposed cerebrocortical cultures from wild-type and p53 deficient mice to the neurotoxic HIV envelope protein gp120. Using neuron/microglia co-culture of mixed p53 genotype, we observed that both neurons and microglia require p53 for gp120 induced neuronal apoptosis. Additionally, accumulation of p53 protein in neurons was recently reported in post-mortem cortical tissue from a small group of HAD patients. Using a much larger cohort of HAD cases, we extend this finding and report that p53 protein also increases in non-neuronal cells, including microglia. Taken together these findings demonstrate a novel role for p53 in the microglial response to gp120. Additionally, these findings, in conjunction with a recent report that monocytes expressing HIV-Tat also secrete neurotoxins that promote p53 activation, suggest that distinct HIV proteins may converge on the p53 pathway to promote neurotoxicity.

Nerve growth factor withdrawal-mediated apoptosis in naive and differentiated PC12 cells through p53/caspase-3-dependent and -independent pathways

Programmed cell death is regulated in response to a variety of stimuli, including the tumor suppressor protein p53, that can mediate cell cycle arrest through p21/Waf1 and apoptosis through the Bcl-2/Bax equilibrium and caspases. Neuronal cell apoptosis has been reported to require p53, whereas other data suggest that neuronal cell death may be independent of p53. Comparison of wild type PC12 to a temperature-sensitive PC12 cell line that depresses the normal function of p53 has permitted investigation of the importance of p53 in a variety of cell functions. This study examined the role of p53 in trophic factor withdrawal-mediated apoptosis in both naïve and differentiated PC12 cells. Our data show that as PC12 cells differentiate they are more poised to undergo apoptosis than their undifferentiated counterparts. Survival assays with XTT (sodium 3'-1-(phenylaminocarbonyl)-3,4-tetrazolium-bis(4-methoxy-6-nitro)benzene sulfonic acid) and TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling) demonstrated that lack of p53 is initially protective against apoptosis. The window of protection is about 20 h for naïve and 36 h for differentiated cells. Apoptosis involved caspases 3, 6, and 9. However, caspase 3 activation was absent in cells lacking p53, concomitant with the delayed apoptosis. When the expression of caspase 3 was silenced with interference RNA, wild type PC12 cells revealed a morphology and biochemistry similar to PC12[p53ts] cells, indicating that caspase 3 accounts for the observed delay in apoptosis in p53 dysfunction. These results suggest that p53 is important, but not essential, in factor withdrawal-mediated apoptosis. Parallel pathways of caspase-mediated apoptosis are activated later in the absence of functional p53.

Caspase regulation of genotoxin-induced neural precursor cell death

Neural precursor cells (NPCs) critically regulate brain morphogenesis and recent studies have revealed an unexpectedly high frequency of NPC chromosomal abnormalities and apoptosis in the developing brain. We have shown previously that the apoptotic response of NPCs to genotoxic agents is dependent on p53 and caspase-9, but not Bax or caspase-3 expression. In this study, we found that NPCs deficient in Apaf-1, or both the pro-apoptotic multidomain Bcl-2 family members Bax and Bak, were resistant to cytosine arabinoside and gamma-irradiation-induced apoptosis. Inhibitors of gene transcription, protein translation, and caspase activity also blocked genotoxin-induced NPC apoptosis. Although caspase-3 and caspase-6 were both cleaved in response to DNA damage, neither of these effector caspases was critical for apoptosis. Genotoxin-induced NPC death was accompanied by the generation of reactive oxygen species and could be inhibited by several known antioxidants. Conversely, DNA damage-induced reactive oxygen species generation was inhibited significantly by gene disruption of p53, Apaf-1, or caspase-9, and combined deficiency of Bax and Bak, but not by caspase-3 or caspase-6 deficiency. These studies suggest that caspase-9 activation is both necessary and sufficient for genotoxin-induced neural precursor cell reactive oxygen species generation and death.

c-Myc augments gamma irradiation-induced apoptosis by suppressing Bcl-XL

Alterations in MYC and p53 are hallmarks of cancer. p53 coordinates the response to gamma irradiation (gamma-IR) by either triggering apoptosis or cell cycle arrest. c-Myc activates the p53 apoptotic checkpoint, and thus tumors overexpressing MYC often harbor p53 mutations. Nonetheless, many of these cancers are responsive to therapy, suggesting that Myc may sensitize cells to gamma-IR independent of p53. In mouse embryo fibroblasts (MEFs) and in E micro -myc transgenic B cells in vivo, c-Myc acts in synergy with gamma-IR to trigger apoptosis, but alone, when cultured in growth medium, it does not induce a DNA damage response. Surprisingly, c-Myc also sensitizes p53-deficient MEFs to gamma-IR-induced apoptosis. In normal cells, and in precancerous B cells of E micro -myc transgenic mice, this apoptotic response is associated with the suppression of the antiapoptotic regulators Bcl-2 and Bcl-X(L) and with the concomitant induction of Puma, a proapoptotic BH3-only protein. However, in p53-null MEFs only Bcl-X(L) expression was suppressed, suggesting levels of Bcl-X(L) regulate the response to gamma-IR. Indeed, Bcl-X(L) overexpression blocked this apoptotic response, whereas bcl-X-deficient MEFs were inherently and selectively sensitive to gamma-IR-induced apoptosis. Therefore, MYC may sensitize tumor cells to DNA damage by suppressing Bcl-X.