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Vol. 19, Issue 5, 2231-2240, May 2008
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*Turku Centre for Biotechnology, Turku University and Åbo Akademi University, FIN-29521 Turku, Finland; Institute of Physiology, University of Luebeck, D-23538 Luebeck, Germany; and Department of Oncology and Radiation Therapy, Turku University Hospital, FIN-20520 Turku, Finland
Submitted November 12, 2007;
Revised February 12, 2008;
Accepted March 3, 2008
Monitoring Editor: Jonathan Weissman
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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; Koumenis and Wouters, 2006; Liu et al., 2006; Thuerauf et al., 2006). Mammalian cells have evolved a molecular machinery to determine whether cells attempt to survive or go into apoptosis in these conditions. The best characterized molecular responses to hypoxia are mediated through a family of dioxygenases that use O2 as a cosubstrate (Bruick and McKnight, 2001; Epstein et al., 2001) and are termed prolyl hydroxylase domain proteins (PHD), also known as HIF prolyl hydroxylases or Egl-9 homologues (EGLN). Three known PHD (PHD1-3) isoforms hydroxylate the 
-subunits of hypoxia-inducible factors 1 and 2 (HIF-1 and -2) under normoxic conditions at two proline residues (Pro402 and 564; Ivan et al., 2001; Jaakkola et al., 2001; Yu et al., 2001; Masson et al., 2004). The hydroxylated HIF-1 is recognized by the von Hippel-Lindau tumor suppressor protein (pVHL) that subsequently leads to ubiquitination and proteosomal destruction of HIF-1 (Kallio et al., 1999; Maxwell et al., 1999; Cockman et al., 2000; Ohh et al., 2000; Tanimoto et al., 2000). Under restricted O2 availability the PHD activity decreases and the degradation of HIF-1 is blocked, activating the transcription of a wide range of genes (Semenza, 2001; Harris, 2002; Pugh and Ratcliffe, 2003; Schofield and Ratcliffe, 2004).
All PHD isoforms have been reported to hydroxylate HIF and to have similar requirements for O2 and cosubstrates FeII and 2-oxoglutarate at least in vitro (Hirsila et al., 2003). However, their function and characteristics also differ in several aspects. Two isoforms, PHD2 and PHD3, are up-regulated transiently by hypoxia, PHD3 showing the most robust induction (Berra et al., 2003; D'Angelo et al., 2003; Appelhoff et al., 2004; Aprelikova et al., 2004; Marxsen et al., 2004). PHD2 demonstrates most abundant mRNA expression across tissues, whereas PHD3 is expressed at mRNA level in nonstressed conditions only in the cardiac and neural tissue (Lieb et al., 2002; Cioffi et al., 2003; Hirsila et al., 2003). Their subcellular localization also differs (Metzen et al., 2003). PHD2 is mainly cytoplasmic, although during tumor progression PHD2 can translocate to the nucleus (Jokilehto et al., 2006). PHD3 in contrast, is expressed both in the nucleus and the cytoplasm. Moreover, a recent report suggests a non-HIF hydroxylation target, ATF-4, for PHD3 (Koditz et al., 2007). Besides hypoxia, PHD3 or the murine homolog SM-20 is up-regulated, e.g., by vascular tissue injury (Wax et al., 1996), ageing of cells (Rohrbach et al., 2005) and nerve growth factor (NGF) removal in neural cells (Lipscomb et al., 1999, 2000; Straub et al., 2003; Lee et al., 2005). Unlike the two other PHD isoforms, PHD3 has been reported to bear apoptotic function. In neural cells PHD3 is required and is sufficient to induce apoptosis after growth factor removal (Lipscomb et al., 1999, 2000; Straub et al., 2003; Lee et al., 2005).
Protein aggregation is induced in response to various cellular stress and when a cell's capacity to degrade misfolded proteins is exceeded as well as during degenerative processes (Wojcik et al., 1996; Johnston et al., 1998; Kopito, 2000; Garcia-Mata et al., 2002). The aggregates generally contain components of the 26S proteasome, diverse ubiquitylated proteins and chaperones such as HSP70 and TRiC. Both cytotoxic and cytoprotective functions have been reported for these structures. In some situations the accumulation of protein aggregates impairs the function of the ubiquitin-proteasomal system (Bence et al., 2001) and may lead to caspase-dependent apoptosis (Kristiansen et al., 2005). Protein aggregation has been implicated in the pathogenesis of several neurodegenerative diseases, such as Parkinson's, Huntington's, and Alzheimer's diseases and amyotrophic lateral sclerosis (ALS) as well as alcoholic liver disease (Kopito and Sitia, 2000; Garcia-Mata et al., 2002; Ardley and Robinson, 2004; Olanow et al., 2004).
The protein aggregation is a tightly regulated and dynamic process. The number, size, subcellular localization, and protein content of the aggregates vary depending on the aggregating protein and cell type. Multiple aggregates varying in size and scattered around the cytoplasm are in many instances referred to as aggresome-like structures. The small cytoplasmic protein aggregates can be transported along the microtubules (MT) toward the perinuclear region where they converge and form large structures at the microtubule organizing center (MTOC) and are termed aggresomes. Aggresomes may disrupt the organization of cytoskeleton, leading to formation of vimentin cage around the aggresome. Aggresomes are induced by various proteins such as -synuclein (Lee and Lee, 2002) and HDAC6 deacetylase in Parkinson's disease (Kawaguchi et al., 2003) and prions in prion-associated disease (Kristiansen et al., 2005). Smaller scattered protein aggregates or aggresome-like structures are detected, for example, with PLIC-1 (Heir et al., 2006) and p62/SQSTM1 (Zatloukal et al., 2002; Paine et al., 2005; Pankiv et al., 2007).
PHD3 is undetectable or expressed at low level in normoxic cancer cells. The expression is strongly induced during hypoxia (Appelhoff et al., 2004; Aprelikova et al., 2004; Marxsen et al., 2004). However, in hypoxia PHD3 remains mainly inactive and the full activity is restored upon reoxygenation. Here we have studied the expression and function of PHD3 in oxygenated conditions. We show that PHD3 forms subcellular aggregates. The aggregation is dependent on both oxygen and PHD3 catalytic activity. Forced PHD3 expression induced the formation of protein aggregates in an oxygen- and hydroxylase activity–dependent manner. Furthermore, we show that the PHD3-induced aggregates bear an apoptosis-inducing potential in HeLa cells.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). Cells were cultured in DMEM, supplemented with 10% FCS, penicillin-streptomycin, and L-glutamine. Cells were cultured in humidified air containing 5% CO2 at 37°C. For hypoxia treatments air was replaced by nitrogen to reach 1% oxygen in a hypoxia workstation (In vivo2, Ruskinn Technology, Bridgend, United Kingdom). Cells were transfected at optimal confluence using either FuGene HD (Roche Applied Science, Lewes, East Sussex, United Kingdom) or Effectene transfection reagent (Qiagen, Chatsworth, CA) according to manufacturer's protocols.
PHD3-EGFP construct, where enhanced green fluorescent protein (EGFP) is fused to the C-terminus of PHD3, has been described previously (Metzen et al., 2003). PHD3R206K-EGFP was made by QuikChange Site-directed Mutagenesis Kit (Stratagene, La Jolla, CA) using 5'-CCCTCTTACGCAACCAAATATGCTATGACTGTCTGG-3' oligonucleotide (the mutated residue is underlined). For red light-emitting EGFP variant PHD3-dsRed, complete PHD3 ORF was acquired by PCR and cloned into pDsRed-Monomer-N1 vector (Clontech, Heidelberg, Germany) using HindIII and SmaI sites. PCR primer oligonucleotides used were 5'-GAGCAAGCTTTTATGCCCCTGGGACAC-3' and 5'-CCTCCCGGGTCAGTCTTCAGTGAGG-3'.
Hydroxylase inhibitor dimethyloxaloylglycine (DMOG) was obtained from Cayman Chemical (Ann Arbor, MI) and used at 40 µM or as indicated. Cobalt chloride (Sigma-Aldrich, Poole, Dorset, United Kingdom) was used as 100 µM final concentration. Nocodazole (Sigma-Aldrich) was used at 5 µM. The proteosomal inhibitor MG132 (Z-leu-leu-Al; Sigma-Aldrich) was used at 10 µM.
Protein Detection, Cell Imaging, and Fluorescence-activated Cell Sorting Analysis
For Western blotting, cells were harvested in 1x SDS sample buffer. Proteins were detected by enhanced chemiluminescence (Pierce, Bonn, Germany). For immunocytochemistry, cells were fixed on cover slips with PTEMF (100 mmol/L PIPES, pH 6.8, 10 mmol/L EGTA, 1 mmol/L MgCl, 0.2% Triton X-100, and 4% formaldehyde). Fixed cells were then treated with primary and secondary antibodies and mounted with Mowiol-DABCO (Carpinteria, CA) on cover glass. Histograms of colocalization were generated with the LSM META software.
Endogenous PHD3 was detected by immunocytochemistry using two independent specific primary antibodies at 1:2500 dilution: Rabbit polyclonal PHD3 (cat. no. BET-A300-327A, Bethyl Laboratories, Montgomery, TX) was used in Supplementary Figure 1 and Figure 6B and rabbit polyclonal PHD3 (cat. no. NB100-139, Novus Biologicals, Littleton, CO) in all other experiments. Other primary antibodies used with indicated dilutions were as follows: rabbit polyclonal PHD2 at 1:2500 dilution (cat. no. NB100-137, Novus Biologicals), rabbit polyclonal activated caspase-3 antibody at 1:400 dilution (cat. no. G748, Promega, Madison, WI), mouse monoclonal HSP70 at 1:1000 (clone SPA-810, Stressgen, San Diego, CA), rabbit polyclonal anti-20S proteasome core subunits at 1:2000 (cat. no. PW8155, Biomol, Hamburg, Germany) and beta-Actin (AC-40, Sigma). Hemagglutinin (HA)-ubiquitin was detected using antibody against HA at 1:1000 dilution (clone 3F10, Roche Diagnostics, Mannheim, Germany). Cy3- or Cy5-conjugated secondary antibodies were used as at 1:1000 dilution (Jackson Immunochemicals, West Grove, PA). Cell nuclei were stained with Hoechst 33342 (Sigma).
Quantification of the EGFP amount (Figure 1D) was done using ImageJ image analysis software (NIH, http://rsb.info.nih.gov/ij/) after imaging the cells with LSM510 Meta confocal microscope. Cell perimeter was defined manually and the mean intensity of the region of interest (the entire cell) was measured using ImageJ analysis option. Flow cytometric analysis (FACS) was done using FACSCalibur flow cytometer (Becton Dickinson, San Jose, CA). To study the cell cycle and in particular the amount of sub-G1 population (apoptotic cells), the cells were fixed with methanol and stained with propidium iodide solution. Fluorochromes (GFP) were excited with 488 nm laser and GFP was measured using 530/30 bandpass filter. Propidium iodide was measured using 670 longpass filter. For FACS analysis CellQuest software (Becton Dickinson) was used. The sub-G1 population was studied from the GFP-positive and -negative populations separately.
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The experimental conditions for FRAP have been described previously elsewhere (Phair and Misteli, 2000). Briefly, from cells transiently expressing PHD3-EGFP a region of interest (ROI) was selected and imaged (prebleach) followed by photobleaching of the ROI with argon laser 488 nm at maximum power for 100 iterations. The recovery of the bleached region was imaged with 2% laser power with 2-s intervals. The mobile fraction and the half-life of the recovery was calculated using FRApCalc program (Dr. Rolf Sara, Turku Centre for Biotechnology).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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; Appelhoff et al., 2004; Aprelikova et al., 2004; Marxsen et al., 2004). In hypoxia, however, PHD3 remains mainly inactive (Hirsila et al., 2003) and is reactivated after reoxygenation. Using squamous cancer cells (SCC2) as a model, we studied the expression pattern of PHD3 in normoxia and hypoxia and after short reoxygenation (Figure 1). To validate the specificity of the PHD3 staining, we evaluated two commercial antibodies raised against PHD3 and performed small interfering RNA (siRNA) experiments. The two anti-PHD3 antibodies showed an indistinguishable staining pattern (Supplementary Figure 1A). Moreover, transfection of a previously validated PHD3 siRNA (Berra et al., 2003) before PHD3 immunocytochemistry caused complete loss of the PHD3 signal compared with control siRNA (Supplementary Figure 1B). Normoxic SCC2 cells showed negligible PHD3 expression. After 48-h hypoxia PHD3 demonstrated expression within the nucleus and cytoplasm as reported previously (Metzen et al., 2003). Interestingly, the SCC2 cells also showed a punctuate or aggregating PHD3 expression pattern in hypoxia. After the subsequent 1-h reoxygenation, a subset of the cells demonstrated increased aggregation of PHD3 seen as an increase in the amount and size of the aggregates (Figure 1A). Besides SCC2 cells, subcellular PHD3 aggregates were also detected in PC12 cells and these were increased in size and amount in a subset of cells in normoxic conditions as compared with hypoxia (Figure 1B). To study the nature of the aggregates we used HeLa cells, which have a low or undetectable normoxic PHD3 level, with ectopically expressed PHD3-EGFP. In addition to cells that showed a dispersed PHD3 expression pattern, a large subset of the cells demonstrated subcellular aggregates (Figure 1C). Costaining with laminA/C (not shown) demonstrated that the aggregates were mainly located in the cytoplasmic or perinuclear compartments. The aggregation of PHD3 did not depend on the tag used because other PHD3 fusion proteins with different, both N- and C-terminal, tags displayed similar aggregates (Figure 1D). Quantification of the PHD3-EGFP expression level from aggregate-forming and nonforming cells showed that the aggregation did not directly correlate to the amount of the expressed PHD3 within a given cell (Figure 1, E and F). Similarly to HeLa cells, the aggregation of PHD3-EGFP was detected in PC12 (Figure 1G) and SCC2 cells (Figure 1H). These aggregates were termed PHD3 bodies.
Next we asked whether the formation of the PHD3 bodies depends on oxygen availability (Figure 2). HeLa cells expressing ectopical PHD3-EGFP were exposed to hypoxia (1% O2) or normoxia (21% O2) for 8 h. The normoxic aggregation was strongly reduced in hypoxic conditions. Hypoxia reduced the size of the aggregates and the amount of aggregates within the cells as well as the amount of cells displaying aggregates (Figure 2, B, E, and F).
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The PHD3 bodies strongly resembled some proteins that form aggresome-like structures, such as p62/SQSTM1. We asked whether the oxygen dependency is a more general feature of protein aggregation by using EGFP-p62. Cells transfected with EGFP-p62 were exposed to normoxia or 8-h hypoxia (Figure 2, G and H). p62 formed cytoplasmic aggregates in normoxia as described previously (Zatloukal et al., 2002; Pankiv et al., 2007). The formation of p62 aggregates were not altered by hypoxic exposure, suggesting that oxygen is not in general required for protein aggregation.
PHD3 Bodies Colocalize with Proteosomal Components
The aggresome-like structures contain components of the 26S proteasome. To study whether PHD3 localizes with the proteosomal components, cells were cotransfected with PHD3 and a structural component of the 20S proteasome, LMP2 (Reits et al., 1997). A red variant of EGFP fused to PHD3 (PHD3-dsRed) was used to detect the colocalization of PHD3 with LMP2-EGFP. Cells expressing both constructs showed nearly complete colocalization 1 d after transfection (Figure 3, A and B). We further studied the localization of the endogenous proteasomal components using an antibody against the 20S proteasome together with ectopic PHD3-EGFP. Similarly to LMP2, the endogenous 20S demonstrated colocalization with PHD3 (Figure 3, C and D, Supplementary Video 1). Besides proteasomal components, the protein aggregates may contain various chaperones and ubiquitin. In line with this, we detected colocalization of PHD3 with endogenous HSP70 chaperone (Figure 3E) as well as with ectopically expressed HA-ubiquitin (Figure 3F). However, we did not detect vimentin reorganization upon PHD3 expression and the PHD3 bodies were not surrounded by a vimentin cage (Supplementary Figure 2).
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Protein aggregates may be transported along the microtubules, and in the case of aggresome formation they are transported toward the perinuclear region. Inhibition of microtubule polymerization by nocodazole abrogates the transportation. Supporting the hypothesis of PHD3 transport along the microtubuli, 5 µM nocodazole nearly completely inhibited the aggregation of PHD3 (Figure 4A). Moreover, suggesting the requirement of proteasomal function for the localization of the PHD3 bodies, the inhibition of proteosomal function by MG132 caused PHD3 to cluster along the microtubuli (Figure 4A).
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PHD3 Expression Activates Aggregation of Proteasomal Components in a Hydroxylase Activity–dependent Manner
The experiments with forced PHD3 expression suggested that PHD3 could activate the aggregation of proteasomes. To study this more closely, we compared the aggregation induced by proteosomal inhibition to that induced by PHD3 expression. Forced PHD3 expression activated aggregation of the 20S proteasomal subunit, and visible 20S aggregates were detected only within cells with aggregating PHD3 (Figure 5, C and D). The 20S aggregation was comparable to that induced by MG132 (Figure 5B). Importantly, the inhibition of the PHD3 hydroxylase activity either by DMOG (Figure 5, E and F) or by the hydroxylase activity inhibiting mutation (Figure 5, G and H) inhibited the aggregation of 20S by PHD3. We further compared PHD1, -2, and -3 in their ability to induce proteasomal aggregation. In contrast to EGFP-PHD3, neither EGFP-PHD1 nor EGFP-PHD2 expression elicited the formation of visible 20S aggregates in normoxic conditions (Supplementary Figure 3).
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PHD3-induced Aggregates Activate Apoptosis in an Oxygen- and Hydroxylase Activity–dependent Manner
PHD3 induces caspase-dependent apoptosis under oxygenated conditions in neural cells when accumulated after NGF withdrawal (Lipscomb et al., 2000; Straub et al., 2003). Moreover, proteins that induce proteasomal aggregation have been reported to activate caspase-3–dependent apoptosis, and aggresomes can impair the proteosomal function (Bennett et al., 2005; Kristiansen et al., 2005). Using HeLa cells as a model, we asked whether PHD3 has apoptosis-inducing capacity when forced to be expressed upon reoxygenation. Apoptotic cell death, as measured by nuclear fragmentation, was detected with forced PHD3 expression during normoxia and after long reoxygenation (24 h), but not after short reoxygenation (4 h; Figure 6A) after overnight hypoxia. In contrast, PHD2 expression did not lead to cell death under any of these conditions. In line with the fact that HeLa cells are resistant to reoxygenation-induced cell death, the endogenous hypoxia-induced PHD3 was rapidly degraded during reoxygenation, reaching normoxic background levels in less than 2 h (Figure 6B).
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). HeLa cells were transfected with PHD3-EGFP and exposed for 8 h to DMOG followed by the detection of apoptosis by nuclear fragmentation and caspase-3 activation. Both the nuclear fragmentation and the caspase-3 activation induced by PHD3 were strongly inhibited by DMOG (Figure 6, C–E), indicating that intact cellular hydroxylase activity is required for PHD3 to activate apoptosis. Similarly to DMOG, the genetic inactivation of PHD3 (PHD3R206K) led to the loss of the apoptotic activity as measured by nuclear fragmentation (Figure 6C) or caspase-3 activation (Figure 6, D and E), indicating that the apoptosis-activating capacity requires PHD3's own enzymatic activity. FACS studies further showed that by increasing the amount of the transfected PHD3, the number of cells undergoing apoptosis was augmented (Figure 6F and Supplementary Figure 4). The data demonstrated that elevated normoxic PHD3 levels activate apoptosis in HeLa cells in an oxygen- and PHD3 hydroxylase activity–dependent manner.
Aggregation of PHD3 Coincides with Apoptosis
Both the emergence of the PHD3 bodies (Figure 2) and the PHD3-induced apoptosis (Figure 6) were strictly dependent on oxygen availability and PHD3 hydroxylase activity. We asked whether the aggregation of PHD3 coincides with PHD3-induced apoptosis. First, FACS analysis showed that the PHD3-induced apoptosis is time-dependent. Twenty-four hours after transfection a clearly increased apoptosis was detected, but maximal apoptosis was seen 48 h after transfection (Figure 7A). Next, apoptosis was determined by nuclear fragmentation in PHD3-EGFP expressing HeLa cells. Twenty-four hours after transfection 60% of the transfected cells that showed nuclear fragmentation displayed aggregated PHD3. Forty-eight hours after transfection nearly all apoptotic cells displayed aggregated PHD3. In contrast, within the cells with no visible signs of nuclear fragmentation, the PHD3 aggregation was negligible (Figure 7B). Similarly to the nuclear fragmentation, 48 h after transfection a clear correlation between caspase activation and PHD3 aggregation was seen. Cells with visible PHD3 aggregates showed activated caspase-3 in contrast to the cells with dispersed PHD3 (Figure 7C). The two cell populations (cells with aggregated and dispersed PHD3) showed a mirror image 48 h after transfection; more than 90% of the cells with PHD3 aggregates displayed activated caspase-3. Vice versa, <5% of cells with nonaggregating PHD3 demonstrated activated caspase-3 (Figure 7D).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Several lines of evidence imply that the ability of PHD3 to activate apoptosis and to form aggresome-like structures are crucial biological mechanisms of the oxygen-sensing pathway to regulate cell fate. First, the activation of protein aggregation has been recognized in the past years as an important and tightly controlled response to regulate cell survival upon stressed conditions. In keeping with this, only few proteins have been reported to bear a proteasomal aggregation–inducing capacity. Second, the wild type, but not the mutated PHD3 induces aggregation. If this were a cytotoxic response to misfolded overexpressed protein, either both proteins or the mutant form only, would be expected to activate aggregation and cell death. Third, both the PHD3 body formation and its apoptotic function are prevented by hypoxia or by inhibiting the hydroxylase activity. In the case of a plain nonspecific cytotoxicity one would expect hypoxia or the hydroxylase inhibition to have an additive effect, rather than reversal, on the PHD3-induced cell death. Finally, the endogenous PHD3 when aggregated upon reoxygenation partially colocalizes with proteasomal aggregates. However, unlike in stressed neural cells, the endogenous PHD3 does not activate apoptosis after reoxygenation in cancer cells. This is in line with the fact that cancer cells are resistant to reoxygenation-induced cell death and with the rapid oxygen-induced degradation of PHD3 in these cells. It needs to be emphasized that even with forced expression PHD3 requires 24–48 h to activate cell death. One may argue that PHD3 could block the proteosomal system when artificially overexpressed as it is degraded by the proteasomal system (Nakayama et al., 2004). Several lines of evidence argue against this. The vast majority of proteins degraded by the proteasome, such as HIF-1, HIF-2, and PHD2 in the oxygen-sensing pathway, are not aggregated even when forced to express (Kallio et al., 1998; Metzen et al., 2003; Acker et al., 2005; Figure 2). Moreover, if the overexpressed PHD3 sterically blocked the proteasome, the PHD3R206K should have a similar effect, which was not the case.
Both protective and survival-impairing functions have been described for aggresome-like structures. In the HeLa cell strain used in this study, the aggregation induced by PHD3 clearly functioned as an apoptosis-inducing factor. However, we also detected HeLa cell strains that did not undergo apoptosis with elevated PHD3 (not shown). Interestingly, although in these strains PHD3 did aggregate, it neither activated the aggregation of the proteasomal system nor was PHD3 localized to aggresome-like structures. Furthermore, these aggregates did not respond to hypoxia nor to hydroxylase inhibition (not shown), which, along the fact that PHD3 aggregation was seen only in a subpopulation of HeLa cells (Figure 2E), indicated that PHD3 aggregation is a cell type–specific response and that other regulatory components are required for the aggregation. Moreover, it is also clear that PHD3 requires the hydroxylase activity to activate aggregation. This suggests the existence of PHD3 hydroxylation targets that are expressed in normoxic conditions. Previous findings have demonstrated an interaction between PHD3 and the TRiC chaperone (Masson et al., 2004), which is a known component of protein aggregates. However, rather than being a hydroxylation target of PHD3, TRiC may regulate the function of PHD3. Whether TRiC takes part in targeting PHD3 into the aggregates is an open question. One non-HIF target for PHD3, ATF-4, has been recently characterized (Koditz et al., 2007). Interestingly, ATF-4 is a transcription factor that mediates the unfolded protein response. However, although ATF-4 is an intriguing target for PHD3 in the aggregates, it is a nuclear protein and PHD3 bodies are mainly found in the cytoplasm.
The PHD3 bodies differ from classical aggresomes such as the cystic fibrosis transmembrane regulator, presenilin-1 (Johnston et al., 1998) or HDAC6 (Kawaguchi et al., 2003). First, we did not detect complete localization of PHD3 to the MTOC, which is seen with aggresomal proteins. Even at the late stages of PHD3-induced apoptosis, several aggregates, which were localized mainly at the perinuclear region, were detected. Second, aggresomes are surrounded by a vimentin cage and this was not detected with the PHD3 bodies. The PHD3 bodies more resembled some aggresome-like structures, such as p62/SQSTM1, that show multiple scattered cytoplasmic proteasome- and chaperone-containing structures (Paine et al., 2005). It is unclear whether the PHD3 bodies represent functionally similar class to, for example, the p62/SQSTM1 bodies but respond differentially to the oxygen levels.
Nearly complete colocalization of PHD3 bodies with proteasomal components was detected with ectopical PHD3 and only partial with the endogenous PHD3. This implies that strongly elevated normoxic PHD3 is required for global PHD3-induced proteasomal aggregation. Moreover, it suggests that only a subset of the endogenous PHD3 aggregates are associated with proteasomes and may represent the larger endogenous PHD3 aggregates that are more pronounced in oxygenated conditions. LMP-2, which partially colocalized with the endogenous PHD3, is a component of immunoproteasomes (Puttaparthi et al., 2007). Therefore it is an intriguing possibility that a subset of the PHD3 bodies could in fact represent immunoproteasomes.
To our knowledge the only physiological event in which the endogenous PHD3 has been reported to induce apoptosis, is in stressed neural cells. Noticeably, in these conditions PHD3 is expressed under normoxia for long time periods (72 h), and the PHD3-activated apoptosis requires both oxygen and intact hydroxylase activity (Lee et al., 2005). Whether the activation of protein aggregation by PHD3 plays a role either in stressed neural cells or during hypoxia, reoxygenation injury in some other cell types remains to be investigated. Perhaps an even more tempting function for PHD3 exists in the neurodegenerative processes. Aggresomes or aggresome-like structures are components of a number of neurodegenerative disorders including Parkinson's, Alzheimer's, and Huntington's diseases as well as motoneuronal diseases (Garcia-Mata et al., 2002; Ardley and Robinson, 2004; Olanow et al., 2004). It is not completely clear whether the aggregates represent a protective response or whether they are the actual cause of the diseases potentially by impairing the proteasomal system. However, it is tempting to speculate that the aggregation activating function of PHD3 is a feature of neurodegenerative disorders.
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ACKNOWLEDGMENTS |
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Footnotes |
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Address correspondence to: Panu M. Jaakkola (panjaa{at}utu.fi)
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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300 cells. Means ± SD of three experiments are shown. (B) Western blotting analysis of endogenous PHD3 in normoxia (N), 48-h hypoxia (H), or cobalt chloride treatment and reoxygenation after 48-h hypoxia at the indicated time points. (C) HeLa cells expressing control plasmid, PHD3-EGFP, mutant PHD3 (PHD3R206K-EGFP) or PHD3-EGFP exposed to 8-h hydroxylase inhibition (DMOG) were analyzed for apoptosis by nuclear fragmentation (NF). (D) The same conditions were used to study the activated caspase-3 by immunocytochemistry (red). Caspase-3 activation induced by PHD3 was suppressed by the inhibition of PHD3 hydroxylase activity (DMOG and PHD3R206K). Hoechst was used to counterstain the nuclei (blue). Magnification, x400. (E) Quantification of caspase-3 activation from three independent experiments. For each experiment 