The p53-activated Gene, PAG608, Requires a Zinc Finger Domain for Nuclear Localization and Oxidative Stress-induced Apoptosis*

The p53-activated genePAG608, which encodes a nuclear zinc finger protein, is a p53-inducible gene that contributes to p53-mediated apoptosis. However, the mechanisms by which PAG608 is involved in the apoptosis of neuronal cells are still obscure. In this study, we demonstrated that expression of p53 was induced by 100 μm6-hydroxydopamine (6-OHDA), accompanied by increased PAG608 expression in PC12 cells. On the other hand, transient or permanent transfection of antisense PAG608 cDNA into PC12 cells significantly prevented apoptotic cell death induced by 100 μm 6-OHDA or 200 μm hydrogen peroxide but not by 250 μm 1-methyl-4-phenylpyridinium ion. The 6-OHDA-induced activation of caspase-3, DNA fragmentation, loss of mitochondrial membrane potential, and induction of p53 and Bax were also prevented in PC12 cells that stably expressed antisensePAG608 cDNA. These results suggest that PAG608 is associated with the apoptotic pathway induced by these oxidative stress-generating reagents, upstream of the collapse in the mitochondrial membrane potential in PC12 cells. Interestingly, transient transfection with PAG608 cDNA increased p53 expression in both PC12 cells and B65 cells, indicating that PAG608 induced by p53 is able to induce p53 expression in these cells inversely. Furthermore, transient transfection of a truncated mutantPAG608 cDNA, lacking the first zinc finger domain, inhibited 6-OHDA-induced cell death and altered the nuclear and nucleolar localization of wild-type PAG608 in PC12 cells. These results suggest that PAG608 may induce or regulate p53 expression and translocate to the nucleus and nucleolus using its first zinc finger domain during oxidative stress-induced apoptosis of catecholamine-containing cells.

PAG608 expression has been reported to increase in ischemiasensitive brain regions, accompanied by accumulation of p53, caspase-3, and phospho-c-Jun (3)(4)(5), and overexpression of PAG608 resulted in apoptotic cell death of human p53-null tumor cells (1,6). Therefore, PAG608 is thought to be activated by p53 and to be associated with neuronal apoptotic cell death.
The pathogenesis of some neurodegenerative disorders has been linked to apoptosis (7)(8)(9). It is well known that 6-hydroxydopamine (6-OHDA) 1 is a neurotoxin that causes selective degeneration of tyrosine hydroxylase-positive neurons and has been used to generate animal models of Parkinson's disease. Recently, using terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) and hematoxylin staining, apoptotic morphological changes were detected in the substantia nigra of parkinsonian rats lesioned by 6-OHDA (10,11). Moreover, 6-OHDA is known to induce p53 expression and neuronal apoptotic cell death in cultured tyrosine hydroxylaseexpressing neuronal cells (12)(13)(14). Therefore, we investigated whether PAG608 is associated with 6-OHDA-induced apoptotic cell death. We present evidence that PAG608 and p53 expression was increased by 6-OHDA treatment in PC12 cells and that 6-OHDA-induced apoptotic events, such as up-regulation of p53 and Bax, collapse in mitochondrial membrane potential and the appearance of apoptosis-related morphological changes, were inhibited in PC12 cells transfected with antisense PAG608 cDNA. We also found that overexpression of PAG608 increased p53 expression in neuronal cells. Furthermore, the first zinc finger domain-deleted mutant of PAG608 dramatically inhibited 6-OHDA-induced cell death and disturbed the nuclear and nucleolar localization of PAG608, suggesting that this first zinc finger domain is necessary for induction of apoptosis and the nuclear and nucleolar localization of PAG608.

EXPERIMENTAL PROCEDURES
Cell Culture-PC12 cells (Japanese Cancer Research Resources Bank, Osaka, Japan) were cultured at 37°C in 5% CO 2 in RPMI 1640 culture medium (Invitrogen) supplemented with 10% heat-inactivated horse serum, 5% fetal bovine serum, 10 units/ml penicillin, and 10 g/ml streptomycin (Invitrogen). Rat neuroblastoma, B65 cells (European Collection of Animal Cell Culture, Salisbury, UK) were cultured at * This work was supported in part by grants-in-aid for scientific research and for encouragement of young scientists from the Japanese Ministry of Education, Culture, Sports, Science and Technology and by grants-in-aid for comprehensive research on aging and health, research on brain science, research on pharmaceutical and medical safety, and research on specific diseases from the Japanese Ministry of Health, Labor and Welfare. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Construction of Expression Vectors-To obtain rat PAG608 cDNA, reverse transcription-PCR was performed using total RNA isolated from the brain of a male Sprague-Dawley rat (9 weeks old; Charles River, Yokohama, Japan) by the acid guanidinium-phenol-chloroform method (15) using gene-specific primers 5Ј-ggaattccctttcctgccttgatgattct-3Ј and 5Ј-ggggtaccccatcactgaacgtagcccaggttctcc-3Ј harboring an EcoRI and KpnI site, respectively. To construct the antisense PAG608 cDNA expression vector, a fragment of PAG608 cDNA was ligated between the EcoRI and KpnI sites in pcDNA3.1 (ϩ) (Invitrogen). To generate a polyhistidine metal-binding (His)-tagged wild-type PAG608 construct, a cDNA fragment encoding the entire open reading frame of PAG608 was amplified by PCR using the antisense PAG608 expression vector as a template upper primer uHis/PAG, 5Ј-gggaattctgatgattcttttgcagcaggtc-3Ј, and lower primer 5Ј-ccgctcgagcggtcactgaacgtagccca-3Ј and ligated between the EcoRI and XhoI sites in pcDNA3.1/HisA (Invitrogen). His-tagged mutant PAG608 constructs (⌬C582 and ⌬C741) were generated by amplifying cDNA fragments encoding amino acids 1-194 and 1-174 by PCR using the antisense PAG608 expression vector as a template, with upper primer uHis/PAG and lower primers 5Јggatatcctcattgccccgcctctgcagagtc-3Ј and 5Ј-ggatgtcctcagtaaaactggccacttg-3Ј, respectively. The fragments were ligated independently between the EcoRI and EcoRV sites in pcDNA3.1/HisA. His-tagged mutant PAG608 constructs (⌬N207 and ⌬N522) were constructed by amplifying cDNA fragments encoding amino acids 69 -289 and 174 -289 by PCR using the antisense PAG608 expression vector as a template, lower primers lHis/PAG and 5Ј-ggatatcctcactgaacgtagccca-3Ј, respectively, and upper primers 5Ј-ggaattctcttctgcaaactctgc-3Ј, and 5Ј-ggaattcttcgtctggccga-3Ј, respectively. The fragments were ligated independently between the EcoRI and EcoRV sites in pcDNA3.1/HisA. A green fluorescent protein (GFP)-tagged wild-type PAG608 construct was generated by amplifying a cDNA fragment encoding the PAG608 open reading frame by PCR using the antisense PAG608 expression vector as a template, upper primer 5Ј-tctcgaggatgattcttttgcagcaggtc-3Ј and lower primer 5Ј-ccccgggtcactgaacgtagcccaggttct-3Ј, and ligating the product between the XhoI and SmaI sites in pEGFP-C2 (Clontech Laboratories, Palo Alto, CA), respectively.
Transient Transfection Assay-After overnight culture of PC12 cells (1 ϫ 10 5 cells/cm 2 ) in a 60-mm collagen I-coated dish, 10 g of empty or antisense PAG608 cDNA expression vector was cotransfected with 2 g of a pcDNA/Hygro/lacZ plasmid (Invitrogen) encoding the ␤-galactosidase gene into cultured cells using the calcium phosphate method (16). Cells were rinsed with BES (Sigma)-buffered saline 4 h after the transfection and cultured in fresh medium for 36 h. Cells were treated with 100 M 6-OHDA (Sigma) for 8 h and stained with 5-bromo-4-chloro-3indolyl ␤-galactopyranoside (Sigma) solution as reported previously (17).
PC12 cells (1 ϫ 10 5 cells/cm 2 ) were plated on two-well collagen I-coated chamber slides. After an overnight culture, 2 g of empty vector, pcDNA3.1/HisA, or His-tagged mutant PAG608 construct (⌬N522) was cotransfected with 1 g of empty GFP expression vector (pEGFP-C2) into cultured cells using lipofection (LipofectAMINE; Invitrogen) (18). At 24 h after transfection, the cells were treated with 100 M 6-OHDA for a further 24 h. GFP-positive cells were visualized by fluorescence microscopy using a mercury lamp through a 470 -490 nm bandpass filter to excite GFP.
Generation of a Cell Line Stably Expressing Antisense PAG608 -To establish the empty vector-transfected PC12 cell line (PC12/CTL) and antisense PAG608 cDNA-transfected PC12 cell line (PC12/PAG608AS), PC12 cells were seeded at a density of 6 ϫ 10 5 cells/60-mm dish. After overnight culture, 10 g of empty or antisense PAG608 expression vector was transfected into cultured cells using the calcium phosphate method. Cells were rinsed with BES-buffered saline 4 h after transfection. Normal culture medium was added, and the cells were incubated for 72 h in 5% at 37°C. The cells were trypsinized and replated at 4 ϫ 10 5 cells/100-mm dish and then incubated for 24 h. The culture medium was replaced with fresh culture medium containing 400 g/ml Geneti-cin and thereafter replaced every 3-4 days. Individual Geneticin-resistant colonies were isolated 2 weeks later and expanded into cell lines.
Analysis of Cell Viability by a Modification of the Standard MTT Assay-PC12, PC12/PAG608AS and/or PC12/CTL cells (1 ϫ 10 5 cells/ cm 2 ) were plated on each well of 96-well collagen I-coated plates. After a 24-h attachment period, these cells were treated with 50 -100 M 6-OHDA or 50 -200 M hydrogen peroxide (H 2 O 2 ) (Wako). At 24 h after treatment, the cell viability was assessed by quantitative colorimetric assay with WST-1 (DOJINDO, Kumamoto, Japan), a modification of the standard MTT assay (20).

Analysis of Cell Viability by Trypan Blue Exclusion
Assay-PC12 or PC12/PAG608AS cells (5 ϫ 10 5 cells/ml) were plated on each well of six-well collagen I-coated plates as reported previously (21). After a 24-h attachment period, these cells were treated with 250 M 1-methyl-4-phenylpyridinium ion (MPP ϩ ) (Sigma). At 48 and 72 h after treatment, cells were removed from the culture dish by 0.125% trypsin in PBS and were added to 0.5% trypan blue (Wako) in PBS. The cell number of trypan blue exclusion (live) cells was counted using a counting chamber.
Determination of Caspase-3 Activity-To determine caspase 3 activity in PC12 cells and PC12/PAG608AS cells treated with 100 M 6-OHDA, a caspase-3 assay kit (Sigma) was used. PC12 or PC12/ PAG608AS cells (1 ϫ 10 5 cells/cm 2 ) were plated on six-well collagen I-coated plates. After a 24-h attachment period, the cells were treated with 100 M 6-OHDA. At 4 h after treatment, total cell lysates from PC12 cells and PC12/PAG608AS cells were prepared with lysis buffer (50 mM HEPES, pH 7.4, 5 mM CHAPS, and 5 mM dithiothreitol), and then were incubated with 0.2 mM acetyl-Asp-Glu-Val-Asp pnitroanilide (Ac-DEVD-pNA), a colorimetric substrate for caspase-3 at 37°C for 90 min. The activity of caspase-3 was determined by measuring of absorption of paranitroaniline release from Ac-DEVD-pNA.
TUNEL Assay-To detect cells undergoing apoptosis, a colorimetric TUNEL system (Promega, Madison, WI) was used. PC12 or PC12/ PAG608AS cells (1 ϫ 10 5 cells/cm 2 ) were plated on two-well collagen I-coated chamber slides. After a 24-h attachment period, the cells were treated with 100 M 6-OHDA. At 8 h after treatment, the cells were washed with 10 mM PBS and fixed with 4% paraformaldehyde in 0.1 M sodium phosphate buffer. Cells were washed with 10 mM PBS and permeabilized with Triton X-100 for 5 min. After a rinse with 10 mM PBS, each slide was equilibrated and then incubated with terminal dUTP transferase at 37°C for 60 min. The reaction was stopped with 2 ϫ SSC, and the slides were examined under a microscope.
Estimation of Mitochondrial Membrane Potential-PC12 or PC12/ PAG608AS cells (1 ϫ 10 5 cells/cm 2 ) were cultured on two-well collagen I-coated chamber slides for 24 h and then treated with 100 M 6-OHDA. At 4 h after treatment, the cells were washed with 10 mM PBS and stained with 10 g/ml JC-1 (Molecular Probes, Eugene, OR) solution in culture medium for 10 min at 37°C. After staining, mitochondrial membrane potentials in the cells were observed under confocal laser scanning microscopy (Zeiss LSM 510), using the Windows-based Zeiss LSM program (Carl Zeiss, Germany). The 488 nm line of an argon-ion laser attenuated to 5% of the maximal intensity with a neutral density filter and the 543 nm line of a helium-neon laser without attenuation were used to excite the green fluorescent JC-1 monomer. The 543 nm line of a helium-neon laser without attenuation was used to excite red fluorescent J-aggregates formed by JC-1. Light emitted from the green fluorescent JC-1 monomer was collected through a 505-530 nm bandpass filter in multitrack. Light emitted from red fluorescent J aggregates was collected through a 560 nm longpass filter. Filtered beams were detected by photomultiplier tubes. Images were recorded with a 20ϫ objective (Plan NeoFluar, 0.50 NA or 0.30 NA, Zeiss) at 2ϫ zoom (0.9 m/pixel).
Redistribution of GFP-tagged Wild-type PAG608 -PC12 cells (1 ϫ 10 5 cells/cm 2 ) were plated on each well of two-well collagen I-coated chamber slides. After a 24-h attachment period, 2 g of empty or His-tagged wild-type or mutant PAG608 expression vectors were cotransfected with 1 g of GFP-tagged wild-type PAG608 cDNA expression vector into cultured cells by lipofection. At 24 h after transfection, the cells were fixed with 4% paraformaldehyde in 0.1 M sodium phosphate buffer and then stained with 10 g/ml Hoechst 33342. GFPtagged wild-type PAG608 transformant cells and nuclei stained with Hoechst 33342 were visualized by fluorescence microscopy using a mercury lamp through a 470 -490 nm bandpass filter and 360 -370 nm bandpass filter to excite GFP and Hoechst 33342, respectively.
Statistical Analyses-All data were expressed as the mean Ϯ S.E. Differences between groups were examined for statistical significance using one-or two-way ANOVA, followed by a post hoc Fisher's PLSD multiple comparison test. A p value less than 0.05 denoted the presence of a statistically significant difference.

6-OHDA Induces PAG608 in Catecholaminergic Neuronal
Cell Lines-In rat catecholaminergic neuronal PC12 cells and B65 cells, small but significant increases in the proportions of apoptotic cells appeared after exposure to 6-OHDA (PC12 cells, 50 -100 M; B65 cells, 25-50 M) for 8 h (Fig. 1). Therefore, 6-OHDA was used in the following experiments at concentration of 100 M for PC12 cells and 50 M for B65 cells. p53 protein expression was induced in PC12 cells at 3-12 h after treatment with 100 M 6-OHDA (Fig. 2, A and C), coinciding with the 6-OHDA-induced p53 expression described in previous studies (12,22). Changes in PAG608 expression were examined during 6-OHDA-induced cell death of PC12 cells. An increase in PAG608 expression in PC12 cells was detected at 6 -12 h after 6-OHDA treatment (Fig. 2, A and C). In B65 cells, 50 M 6-OHDA treatment also led to induction of PAG608 expression after an increase in p53 (Fig. 2B).
6-OHDA-induced Apoptotic Cell Death Is Inhibited by Transient Transfection of Antisense PAG608 cDNA- Fig. 3 shows the effects of inhibition of PAG608 expression (by transient transfection of an antisense PAG608 cDNA) on 6-OHDA-induced apoptotic cell death in PC12 cells. There were no morphological differences between ␤-galactosidase-positive PC12 cells transfected with the antisense PAG608 cDNA expression vector and empty vector (Fig. 3, A and B). A recent study reported that apoptosis-related morphological changes in PC12 cells were detectable 8 h after treatment with 6-OHDA (23). Therefore, we exposed transfected PC12 cells to 100 M 6-OHDA for 8 h and examined cell morphology; ϳ11% of ␤-galactosidase-positive cells transfected with an empty vector exhibited shrunken cell size and condensed chromatin after treatment with 6-OHDA (Fig. 3, C and E). In contrast, cells transfected with antisense PAG608 cDNA remained predominantly normal and healthy after the addition of 6-OHDA, although they were ␤-galactosidase-positive (Fig. 3,  D and E).
Attenuation of 6-OHDA and H 2 O 2 -induced Apoptotic Cell Death in Cells Stably Expressing Antisense PAG608 -To confirm the association of PAG608 with apoptotic cell death, we generated a clone of stable transformant PC12 cells, which were transfected with an empty vector (PC12/CTL) or an antisense PAG608 cDNA expression vector (PC12/PAG608AS) and then selected with Geneticin. Western blot analyses using anti-PAG608 antibody revealed that the expression of PAG608 decreased in PC12/PAG608AS cells (Fig. 4A). At 24 h after treatment with 100 M 6-OHDA, the numbers of surviving naive and empty vector-transfected PC12 cells diminished to 52.6 Ϯ 5.0% and 31.8 Ϯ 2.7%, respectively, of the control (Fig. 4B). In contrast to these control cells, cell death induced by 6-OHDA was prevented in PC12/PAG608AS cells (Fig. 4B), consistent with the results after transient transfection of antisense PAG608 cDNA shown in Fig. 3 (Fig. 5, A and B).
Because it has been reported that MPP ϩ causes inhibition of cell proliferation and cell death of PC12 cells, we examined the viability of PC12/PAG608AS cells treated with 250 M MPP ϩ using trypan blue exclusion as reported previously (21). Treatment with MPP ϩ inhibited cell proliferation and led to cell death of PC12/PAG608AS cells at 48 and 72 h as well as naive PC12 cells (Fig. 6).
Antisense PAG608 Rescued PC12 Cells from 6-OHDA-induced Apoptotic Events-It has been reported that caspase-3 mediates 6-OHDA-induced apoptosis in PC12 cells (22). Fig. 7 shows the inhibitory effects of PAG608 expression on caspase-3 activity and DNA fragmentation in PC12 cells after treatment with 6-OHDA. At 4 h after 6-OHDA treatment, no significant increase in the activity of caspase-3 was observed in PC12/ PAG608AS cells in contrast to the increase in naive PC12 cells (Fig. 7A). Furthermore, no appearance of DNA fragmentation in PC12/PAG608 cells treated with 6-OHDA for 8 h was detected by TUNEL assay, although the TUNEL-positive signals were increased in 6-OHDA-treated naive PC12 cells (Fig. 7B).
PAG608 could potentially mediate 6-OHDA-induced apoptotic cell death via a collapse in mitochondrial membrane potential because the mitochondrial membrane potential is reportedly decreased during p53-induced apoptosis, and 6-OHDA is known to alter mitochondrial membrane potential and membrane permeability in PC12 cells (24 -26). Therefore, using JC-1, which is an indicator of mitochondrial membrane were analyzed by Western blot with goat anti-PAG608 antibody. Ponceau S staining was performed to control for protein loading. Cell viability of antisense PAG608 cDNA transformants 24 h after 100 M 6-OHDA treatment was measured by a modification of the standard MTT assay (B). Data are the mean Ϯ S.E. (n ϭ 6) and represent the percentage of surviving cells relative to the number of cells in each untreated group. *p Ͻ 0.001 between the two indicated groups (one-way ANOVA followed by Fisher's PLSD post hoc test). potential, we examined changes in the mitochondrial membrane potential in PC12/PAG608AS cells after treatment with 6-OHDA (Fig. 8). JC-1 exhibits potential-dependent accumulation in mitochondria, as indicated by a fluorescence emission shift from green (488 nm) for the reduced form of JC-1 to red (543 nm) for the oxidized form. Consequently, loss of mitochondrial membrane potential is indicated by green fluorescence. The intensity of green fluorescence was maintained in PC12 cells treated with 100 M 6-OHDA for 4 h (Fig. 8B). Untreated PC12/PAG608AS cells exhibited red fluorescence, which was similar to naive PC12 cells (Fig. 8, A and C). In contrast to 6-OHDA-treated PC12 cells, the intensity of red fluorescence in PC12/PAG608AS cells treated with 100 M 6-OHDA was similar to that in untreated PC12 cells (Fig. 8D).
Inhibition of 6-OHDA-induced p53 and Bax Expression in Antisense PAG608-expressing Cells-Because the mitochondrial membrane potential in PC12/PAG608AS cells was not disrupted by 6-OHDA (Fig. 8D), we investigated the effects of inhibition of PAG608 expression on several apoptosis-related molecules that are particularly associated with a collapse in mitochondrial membrane potential. Western blot analyses showed that expression of both p53 and Bax was increased by 100 M 6-OHDA in naive PC12 cells. In contrast, neither p53 nor Bax expression was induced by 100 M 6-OHDA in PC12/ PAG608AS cells (Fig. 9, A and B). Treatment of both naive PC12 and PC12/PAG608AS cells with 100 M 6-OHDA induced no changes in the expression of Bcl-2 ( Fig. 9), which prevents oxidative stress-induced loss of mitochondrial membrane potential and consequent apoptosis (27,28).
Effect of PAG608 Overexpression on p53 Expression in Catecholaminergic Neuronal Cells-Because 6-OHDA-induced p53 expression was completely inhibited in PC12/PAG608AS cells (Fig. 9A), the effects of overexpression of PAG608 on p53 expression in PC12 cells were examined using transfection of expression vectors encoding His-tagged PAG608 or His alone (control). At 24 -48 h after transient transfection with the His-tagged PAG608 expression vector, Western blot analyses confirmed expression of His-tagged PAG608 in PC12 cells. Moreover, p53 protein levels increased in His-tagged PAG608overexpressing PC12 cells at the same time points that Histagged PAG608 expression was observed, but this was not seen in PC12 cells overexpressing His alone (Fig. 10A). To provide additional evidence that PAG608 regulates p53 expression in other catecholaminergic neuronal cells, the His-tagged PAG608 expression vector was transfected into B65 cells. p53 expression was increased in B65 cells transfected with Histagged PAG608, in temporal association with His-tagged PAG608 expression (Fig. 10B). In contrast, transient transfection of His-tagged PAG608 expression vector failed to cause an increase in p53 expression in mouse glioma C6 cells (data not shown).
Increased expression of p53 in response to several stresses is the result of inhibition of MDM2 activity via direct interaction of partner proteins (29). To investigate whether PAG608 and MDM2 can engage in a protein-protein interaction, PC12 cells were cultured for 48 h after the transfection with His-tagged PAG608 expression vector. Then, cell extracts were subjected to immunoprecipitation using anti-His or anti-PAG608 antibody followed by Western blot analysis using anti-MDM2 antibody. Unexpectedly, MDM2 was not brought down with anti-His and anti-PAG608 antibodies (data not shown).
N-terminal Truncation of PAG608 Prevents 6-OHDA-induced Cell Death-PAG608 contains three C 2 H 2 -type zinc finger domains and a putative nuclear localization signal (2), and these domains appear to be essential for 6-OHDA-induced cell death. To clarify the roles of the zinc finger domains of PAG608 in 6-OHDA-induced cell death, we generated a His-tagged mutant PAG608 expression vector, His-⌬N522, which encoded the first zinc finger domain-truncated PAG608 (Fig. 11). An expression vector encoding GFP was cotransfected with His-⌬N522 or the control vector into PC12 cells. There was no difference between the number of GFP-positive PC12 cells transfected with ⌬N522 and empty control vector (Fig. 12, A, B, and E). After treatment with 100 M 6-OHDA, the number of GFPpositive cells transfected with control vector was reduced significantly to 25.8% of control (Fig. 12, C and E). In contrast, ⌬N522-transfected PC12 cells were resistant to 6-OHDAinduced cell death (Fig. 12, D and E). Furthermore, three other His-tagged mutant PAG608 expression vectors, ⌬N207, ⌬C582, and ⌬C741, were generated (Fig. 11) to evaluate the roles of other domains of PAG608 in 6-OHDA-induced cell death. After treatment with 6-OHDA, the number of GFP-positive cells transfected with these three mutant constructs decreased to levels similar to those noted after transfection with the control vector (data not shown).
Nuclear Localization of GFP-tagged Wild-type PAG608 Is Disrupted by a Truncated Mutant, ⌬N522-PAG608 is reportedly localized in the nucleus and nucleolus of human tumor cells (1). We first confirmed PAG608 accumulation in the nucleus and nucleolus of PC12 cells, using transfection of a GFPtagged wild-type PAG608 expression vector (data not shown). We then examined the effects of expression of a truncated PAG608 on the nuclear and nucleolar localization of PAG608 in PC12 cells. Fig. 13 shows changes in the subcellular localization of GFP-tagged wild-type PAG608 in PC12 cells after cotransfection with either a construct encoding His-tagged wildtype PAG608 or truncated mutant PAG608 (design described in Fig. 11) and nuclear staining with Hoechst 33342. The nuclear and nucleolar localization of GFP-tagged PAG608 in PC12 cells was not affected by cotransfection with the control vector or His-tagged wild-type PAG608 expression vector (Fig. 13 A, B,  G, and H). In contrast to these vectors, the signal of GFPtagged PAG608 in PC12 cells was distributed diffusely throughout the whole cell when His-⌬N522 was cotransfected (Fig. 13, C and I). Cotransfection of the other three His-tagged mutant PAG608 expression vectors (except His-⌬N522) had no effect on nuclear and nucleolar localization of GFP-tagged PAG608 (Fig. 13, D-F and J-L). DISCUSSION PAG608 expression is dependent on p53 (1, 5), which is increased and activated in response to various stimuli, such as reactive oxygen species and DNA-damaging reagents, to induce apoptosis and/or cell cycle arrest (30 -32). Catecholaminergic neurotoxin 6-OHDA has been reported to induce an increase in p53 expression and caspase-mediated apoptosis in the PC12 cell line (12,22). In the present study, the addition of 6-OHDA to PC12 cells resulted in elevation of PAG608 expression (6 -24 h) after an increase in p53 expression (3-12 h) (Fig. 2). Furthermore, transient or permanent inhibition of PAG608 expression prevented cell death induced by 6-OHDA and H 2 O 2 in PC12 cells (Figs. 3 and 4). Recently, it has been reported that addition and overexpression of catalase, which metabolizes H 2 O 2 , can rescue cells from 6-OHDA-induced toxicity (31, 34), implying that the toxic effect of 6-OHDA is caused by the generation of H 2 O 2 through autoxidation of the toxin.
It has been reported that generation of reactive oxygen species and inhibition of oxidative phosphorylation are considered important mediators of neuronal cell death in Parkinson's disease (35) and that treatment with MPP ϩ produces the same biochemical defect as detected in the substantia nigra of Parkinson's disease because MPP ϩ inhibits complex I of the mitochondrial electron transport chain (36). However, the effect of MPP ϩ on mitochondrial membrane potential, release of cytochrome c, and caspase-3 activity is less than that of 6-OHDA (37). Furthermore, MPP ϩ induces cell death and inhibits cell proliferation without induction of p53 expression (21). Taken together with these reports, it is considered that MPP ϩ induces cell death through action mechanism different from 6-OHDA. Indeed, the present study shows that inhibition of PAG608 expression had no effects on cell death and prevention of cell proliferation in PC12 cells by induced by MPP ϩ (Fig. 6). Therefore, these results suggest that PAG608 is associated with the p53-mediated apoptotic pathway induced by 6-OHDA and H 2 O 2 , which generates reactive oxygen species in PC12 cells.
The toxins 6-OHDA and H 2 O 2 are known to reduce the mitochondrial membrane potential, increase caspase-3 activity, and produce DNA fragmentation, which are indicators of apoptotic cell damage (22,27,38). In this study, an increase in caspase-3 activity and DNA fragmentation in antisense PAG608-expressing PC12 cells (PC12/PAG608AS) after 6-OHDA treatment was significantly less than that in naive PC12 cells (Fig. 7). Furthermore, the mitochondrial membrane potential in PC12/PAG608AS cells was not disturbed by treatment with 6-OHDA, whereas that in naive PC12 cells was reduced (Fig. 8), indicating that PAG608 is associated with 6-OHDA-induced apoptosis upstream of the collapse in mitochondrial membrane potential. Translocation of Bax from the cytosol to mitochondrial membranes has been reported in some apoptotic events (39,40). Recently it was demonstrated that Bax interacts with a voltage-dependent anion channel and adenine nucleotide translocator, components of the permeability transition pore complex, which precedes loss of mitochondrial membrane potential (41,42). Moreover, Pastorino et al. (43) suggested that cell death resulting from overexpression of Bax is mediated by mitochondrial permeability transition. Conversely, a Bax-independent p53-related pathway was also reported, whereby overexpression of p53 (by infection with p53containing adenovirus) induced apoptotic cell death through a Bax-independent decrease in mitochondrial membrane potential (24), and the accumulation of p53 in the mitochondria preceded the change in mitochondrial membrane potential (25). These reports, taken together, suggested that alternation of the mitochondria membrane potential in the p53-induced apoptosis cascade may or may not require Bax induction. In the present study, 6-OHDA failed to increase the expression of either p53 or Bax in PC12/PAG608AS cells (Fig. 9, A and B). Although it is not clear whether p53 can induce Bax expression in PC12 cells, less induction of p53 and Bax after 6-OHDA treatment may have prevented reduction of mitochondrial membrane potential in antisense PAG608-expressing PC12 cells. In other words, PAG608 may mediate 6-OHDA-induced p53 and Bax induction in catecholaminergic cells.
Although PAG608 expression has been reported to be induced by p53 in vitro and in vivo (1), to our knowledge there are no studies showing that PAG608 can regulate p53 expression. The present results showed that overexpression of PAG608 by transient transfection with His-tagged PAG608 cDNA caused a marked increase in p53 expression in both PC12 cells and B65 cells (Fig. 10) but not in C6 cells. Thus, the present study is the first to show that PAG608-induced p53 expression is neuronal cell-specific. p53 expression and function are controlled by several mechanisms. One well known mechanism is regulation of protein stability. Recently, several reports showed that p53 is regulated by various molecules such as MDM2, ARF, and c-Abl. MDM2 plays a role in exporting p53 from the nucleus to the cytoplasm and promotes degeneration of p53 in the proteasome (44). In contrast, both ARF and c-Abl directly bind to MDM2 and inhibit MDM2 activity, which lead to an increase in p53 protein levels (29,45,46). In the present study, however, a direct interplay between PAG608 and MDM2 was not unexpectedly observed using immunoprecipitation. Although further studies are needed to clarify the mechanism underlying PAG608-induced p53 expression, the present results suggest that PAG608 induced by p53 could positively regulate p53 expression during reactive oxygen species-induced apoptosis of neuronal cells.
To our knowledge, only two zinc finger proteins, PAG608 and JAZ, are reported to be located in the nucleus and nucleolus and to be involved in apoptosis. Interestingly, the amino acid sequences of the first and second zinc finger domains of PAG608 are homologous to those of JAZ (47). The zinc finger domains of JAZ are required for nuclear and nucleolar localization as well as for apoptosis (47). In this study, 6-OHDAinduced cell death was prevented by transfection of a mutant PAG608 expression vector, ⌬N522, which lacks the first zinc finger domain (Fig. 12, D and E). Moreover, overexpression of this truncated PAG608 mutant led to altered localization of GFP-tagged wild-type PAG608 (Fig. 13C). These results suggest that PAG608 interacts with itself or other molecules via the first zinc finger domain to facilitate translocation into the nucleus and nucleolus and that this event is an important process for reactive oxygen species-induced apoptosis.