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J. Biol. Chem., Vol. 282, Issue 21, 15652-15666, May 25, 2007
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1
¶2
From the
Department of Medical Biophysics, the Renal Division and Department of Medicine, St. Michael's Hospital, and the ¶Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Ontario M5S 1A8, Canada
Received for publication, August 31, 2006 , and in revised form, March 28, 2007.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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-dependent. We also demonstrate that sONE transcripts are enriched in the nucleus of normoxic cells and that hypoxia promotes an increase in the level of cytoplasmic and polyribosome-associated, sONE mRNA. The finding that eNOS expression can be regulated by an overlapping cis-antisense transcript in a stimulus-dependent fashion provides evidence that sense/antisense interactions may play a previously unappreciated role in vascular disease pathogenesis. |
INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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(HIF1) has been suggested to negatively regulate HIF1 during prolonged hypoxia (2). Regulation of the sense transcript by antisense RNA can be exerted at the transcriptional, splicing, nuclear export, or translational level. Importantly, antisense transcripts may also regulate the stability of the sense transcript, although the mechanisms implicated have not been defined. Although several orientations between S/AS transcriptional units are possible, tail-to-tail overlaps involving the 3'-UTR predominate (5). Given the prevalence of cis-S/AS interactions, it is vital to understand whether S/AS interactions between 3'-UTR regions are functionally important and to define their mechanisms of regulation. Indeed, trans-S/AS interactions involving microRNAs clearly demonstrate a bias for the 3'-UTR of target mRNAs (6) and have been demonstrated to regulate both mRNA translation and RNA stability, with the former being especially relevant (7). It is possible that functional interactions between S/AS transcripts may result in the generation of short interfering RNA that have the capacity to target either transcript for RNA interference-mediated degradation, although this phenomenon has yet to be directly observed in mammalian cells. A limitation in further understanding these pathways has been the lack of a tractable and biologically relevant model of 3'-UTR cis-S/AS interactions.
We recently identified a natural cis-antisense transcriptional unit to eNOS/NOS3 (8). sONE, also known as NOS3AS/APG9L2, is oriented tail-to-tail to NOS3, and sONE mRNA is complementary to the eNOS mRNA for 662 nucleotides. This overlap includes portions of the 3'-UTR of the eNOS mRNA. sONE and eNOS mRNA were reciprocally expressed in cultured cells. Relative to endothelial cells, levels of sONE RNA were found to be elevated in nonendothelial cell types, such as vascular smooth muscle cells (VSMCs), which express very low basal levels of eNOS RNA (8). Overexpression of sONE in endothelial cells, which normally express high levels of eNOS but not sONE, resulted in reduced expression of eNOS protein, suggesting that sONE can negatively regulate the expression of eNOS. In contrast, RNA interference-mediated knockdown of sONE in vascular smooth muscle cells resulted in increased expression of eNOS RNA (8).
A number of groups, including our own, have observed down-regulation of eNOS expression following exposure of endothelial cells to hypoxia (9–11). eNOS mRNA is constitutively expressed and is highly stable in endothelial cells, with half-life measurements following actinomycin D transcriptional arrest averaging 24–48 h (reviewed in Ref. 12). This is much longer than typical mRNAs (13). Hypoxic stimulation of endothelial cells markedly decreases the stability of eNOS mRNAs (9, 11). The molecular mechanisms responsible for the long half-life of the eNOS transcript in normoxic endothelial cells are not fully understood but are mediated, at least in part, by protein binding to the 3'-UTR of the eNOS mRNA (14–16). The mechanisms responsible for alteration of eNOS RNA stability by hypoxia are not known.
In light of the potential for sONE to regulate eNOS post-transcriptionally, and considering that eNOS RNA is destabilized by chronic hypoxia, it was of interest to determine if sONE plays a functional role in the hypoxic down-regulation of eNOS. Here we describe robust hypoxic induction of sONE mRNA expression in various cell types in vitro and vascular tissues in vivo. sONE was not induced at the transcriptional level but was up-regulated by post-transcriptional stabilization of the sONE transcript. Additionally, sONE expression was increased in the absence of functional VHL activity, but HIF2 was not involved in sONE induction. Importantly, sONE functionally regulates eNOS expression during hypoxia, since RNA interference-mediated ablation of sONE transcripts resulted in increased levels of eNOS mRNA and protein in the setting of hypoxia. To our knowledge, this is the first report of an antisense RNA playing a functional role in regulating gene expression in response to cellular stimulation.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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(19), were cultured in Dulbecco's modified Eagle's medium containing puromycin and 10% fetal bovine serum. Cell Treatment and RNA Extraction—Cells were grown in atmospheric O2 (21%) or in a temperature- and humidity-controlled hypoxia chamber (ThermoForma, Marietta, OH). Less than 1% O2 was maintained by the use of Ultra High Purity gas in a sealed chamber (5% CO2, 10% H2, 85% N2; Praxair, Mississauga, ON). To mimic hypoxia, cells were treated with a 140 µM concentration of the iron-chelator desferrioxamine (DFO) (Sigma). After treatment, RNA was extracted as described previously (17).
Extraction of Nuclear and Cytoplasmic RNA—Nuclear and cytoplasmic extracts were generated from normoxic and hypoxic HUVEC. Cells were washed with phosphate-buffered saline, pelleted, and resuspended in lysis buffer (50 mM Tris-HCl, pH 8.0, 140 mM NaCl, 1.5 mM MgCl2, 0.5% (v/v) Nonidet P-40, 1 mM dithiothreitol, and 0.2 units/µl RNaseOUT (Invitrogen)) for 5 min on ice. Nuclei were pelleted at 300 x g for 2 min. RNA was extracted from the supernatant (cytoplasmic fraction) and the nuclei pellet using guanidinium-based extraction protocols. Validation of the efficiency of nuclear/cytoplasmic partitioning was performed by quantifying levels of unspliced, pre-mRNAs (see below for primer sequences), which are known to be highly enriched in the nucleus due to their short half-life (20).
In Vivo Expression of sONE in Hypoxic Rats—Male Sprague-Dawley rats (200–250 g) were exposed to 10% O2 for 16 h or 7 days, and RNA was extracted from the descending thoracic aorta, as previously described (21). All experiments were performed in accordance with the standards of the Canadian Council on Animal Care, and ethics approval was granted by the institutional animal care committee of the University of Toronto Faculty of Medicine and St. Michael's Hospital.
Determination of RNA Half-life—HUVEC were grown in normoxic or hypoxic conditions for 16 h, after which the RNA polymerase inhibitor DRB (60 µM; Sigma) was added prior to RNA extraction at various time points following transcriptional arrest. 1 h post-DRB addition was used as time 0, and RNA half-life was assessed by quantifying eNOS and sONE RNA levels following transcriptional arrest using real-time RT-PCR (see below). Half-life was estimated by calculating the exponential decay rate of sONE and eNOS RNA using a line-of-best-fit algorithm in Microsoft Excel.
Quantitative Real Time RT-PCR—2 µg of total cellular RNA was treated with heparinase (1 unit; Sigma-Aldrich) and DNase I (5 units; Fermenatas; Burlington, ON) and used in first-strand cDNA synthesis using random primers and SuperScript II reverse transcriptase (Invitrogen) according to the manufacturer's recommendations. cDNA was diluted to a final volume of 100 µl. All quantitative RT-PCR analyses were performed in triplicate on 2 µl of cDNA using the ABI PRISM 7900 HT sequence detection system (Applied Biosystems, Foster City, CA) using Taqman® or SYBR® green technology (Applied Biosystems), as previously described (8) (See Table 1 for primer sequences). Normalization to the reference genes cyclophilin A, TATA-binding protein (TBP), or 18S yielded similar results. Serial 10-fold dilutions of plasmids corresponding to target amplicons were used to develop standard curves so that copy numbers could be deduced by absolute quantification. For the determination of eNOS (exon 11/12) and sONE (exon 7/8) copy numbers, the cycle at which the exponential portion of the amplification curve for 1000 copies of target amplicon passed through the cycle threshold (CT1000) (threshold = 0.2) was 35.3 ± 0.4 and 31.4 ± 0.3 cycles, respectively. Additionally, relative quantification was determined using the 
CT method (22) with similar results.
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For quantification of unspliced pre-mRNA, total cellular RNA was reverse transcribed as above. Reactions were also performed in the absence of reverse transcriptase to ensure the absence of genomic DNA contamination. Genomic contamination was routinely <5% of the total signal. Primer sequences are located in Table 1.
For quantification of rat sONE expression in hypoxic rat tissues, 2 µg of RNA was used for first-strand synthesis using random hexamers, and cDNA was diluted to 100 µl. Primers for real time RT-PCR analysis were designed to a conserved region of rat sONE (Table 1). In addition, rat VEGF-A, 18S, and TBP levels were determined (Table 1). The CT method (22) was used to determine the change in sONE and VEGF expression, relative to 18S and TBP.
RNA Polymerase II (Pol II) Chromatin Immunoprecipitation (ChIP) Assays—ChIP was performed as described previously (23). 5 µg of anti-Pol II antibody (N-20; Santa Cruz Biotechnology, Inc., Santa Cruz, CA) was added per immunoprecipitation, and control immunoprecipitations containing no antibody were performed in parallel. Pol II and control immunoprecipitated DNA was suspended in 40 µl of water. An input sample (18 µl of 1800 µl total) was removed prior to immunoprecipitation. The cross-links were reversed, and the input sample was diluted 1:10. Primers were directed to sONE exon 3 to detect transcription of both the long and short sONE transcripts (Table 1, sONE pre-mRNA primers). Primers used to amplify the hypoxia response element (HRE) of the VEGF-A promoter (-934 to -785) were 5'-CTA ACC CCG GAA CCA CAC AGC-3' and 5'-CTG GCC TGC AGA CAT CAA AGT G-3'. The number of copies of the sONE exon 3 amplicon or the VEGF HRE amplicon were determined in 2 µl of bound chromatin, 2 µl of a no antibody control, and 2 µl of a diluted input sample using serial dilutions of genomic DNA. Immunoprecipitated (IP) DNA was calculated by subtracting the number of copies in the no antibody control from the bound chromatin and dividing by input.
sONE RNA Interference—Three StealthTM small interfering RNAs (siRNA) duplexes were designed to recognize a region in common with the long and short sONE transcripts (Invitrogen). sONE 414 (sense, GGG ACA UCU GUU CCU UUG CCC UUA U; antisense, AUA AGG GCA AAG GAA CAG AUG UCC C) targets exon 3 of the sONE transcript, whereas sONE 513 (sense, AGG ACG GCA AGA CUG AGC UUU CUU U; antisense, AAA GAA AGC UCA GUC UUG CCG UCC U) and sONE 579 (sense, GGC ACA GCU CUA AGU UUC UUG GGC A; antisense, UGC CCA AGA AAC UUA GAG CUG UGC C) target exon 4 of sONE. 40 nM sONE siRNA was transfected into 90% confluent HUVEC grown on 60-mm gelatin-coated tissue culture plates using 33 µl of OligofectamineTM transfection reagent (Invitrogen) in a total volume of 2000 µl. A 40 nM concentration of a GC-matched control (StealthTM RNA interference negative control medium GC; Invitrogen) was transfected into parallel plates. Following 3 h of transfection at 37 °C in Opti-MEM medium (Invitrogen), M199 medium (Invitrogen) containing fetal bovine serum (Hyclone), heparin, and endothelial cell growth supplement (Biomedical Technologies) was added. Cells were cultured for 18 h and were then transferred to the hypoxia chamber and grown for an additional 24 h, after which RNA and protein were extracted.
Western Blots—Western blotting was performed using 20 µg of total cellular protein samples as in Ref. 24. Lamin A/C (1:100 dilution; mouse monoclonal IgM, anti-lamin A/C (sc-346; Santa Cruz Biotechnology), 
-actin (1:200 dilution; horseradish peroxidase-conjugated goat polyclonal IgG, anti-actin (sc-1615; Santa Cruz Biotechnology)), or -tubulin (1:5000 dilution; mouse monoclonal, anti--tubulin (T-5168); Sigma)) Western blots were used as loading controls. eNOS was detected using an anti-eNOS antibody (C-20) (1:200 dilution; rabbit polyclonal IgG; Santa Cruz Biotechnology). HIF2 was detected using an anti-HIF2 antibody (ab13704) (1:1000 dilution; rabbit polyclonal IgG; Novus Biologicals, Littleton, CO). HA-tagged VHL was detected using an anti-HA antibody (12CA5) (1:250 dilution; mouse monoclonal; Roche Applied Science). Goat anti-rabbit IgG horseradish peroxidase-linked secondary antibody (1:3000 dilution; catalog number 12-348; Upstate%20Biotechnology">Upstate Biotechnology) was used as a secondary antibody for eNOS and HIF2 Western blots, and goat anti-mouse IgG-horseradish peroxidase secondary antibody (1:3000; catalog number 172-1011; Bio-Rad) was used as a secondary for lamin A/C, tubulin, and HA Western blots. SuperSignal West Pico chemiluminescent substrate (Pierce) and the Fluor-S Max Multi-Imager (model 170-772; Bio-Rad) were used for signal analysis.
Polyribosome Profiling—Polyribosome profiling was performed as in Ref. 25. Briefly, normoxic and hypoxic HUVEC were washed in ice-cold phosphate-buffered saline containing 100 µg/ml cycloheximide (Sigma) and harvested in lysis buffer (100 mM KCl, 5 mM MgCl2, 10 mM HEPES, pH 7.4, 100 µg/ml cycloheximide, and 1000 units/ml RNaseOUT (Invitrogen)) at room temperature. Cellular extracts were passed through a 27-gauge needle, and the nuclei were pelleted by centrifugation (12,000 x g, 4 °C, 5 min). The supernatant was centrifuged again (12,000 x g, 4 °C, 5 min) to remove residual cellular debris, and the resulting supernatant was layered onto 10-ml sucrose gradients (15–45%). Following ultracentrifugation (SW Ti 41 rotor, 35,000 rpm, 4 °C, 2 h) (Beckman), 12–15 fractions (750 µl) were collected using a programmable density gradient fraction collector (Isco; PerkinElmer Life Sciences) with continuous spectrophotometric measurements at A254 nm. RNA was extracted from each of the fractions using guanidinium-based protocols. All of the RNA from each fraction was DNase-treated and reverse transcribed using random hexamers and diluted to a final volume of 50 µl.
Statistical Analysis—Unless otherwise indicated, all results are the mean ± S.E. of at least three independent experiments. One-way analysis of variance with a Newmann-Keuls post hoc test or a paired t test was used, as appropriate. A p value of <0.05 was considered statistically significant.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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30 and 20% of the mRNA levels of control HUVEC following 24- and 48-h treatment with DFO, respectively. The observed kinetics of eNOS expression was consistent with previously reported findings (28). Importantly, we observed that sONE gene expression was reciprocal in nature to the expression of eNOS. sONE gene expression began to increase after 2 h of DFO treatment and reached a maximum following 24 h of treatment, when mRNA levels increased more than 15-fold with respect to levels in control HUVEC. We have shown previously that treatment of endothelial cells with the histone deacetylase inhibitor, trichostatin A, up-regulates sONE RNA and that this is followed by a decrease in the levels of eNOS RNA (8). Similar to our previous observation that sONE RNA levels were increased by trichostatin A prior to a decrease in eNOS RNA (8), DFO also resulted in an increase in sONE prior to the fall in eNOS RNA levels. The expression of VEGF confirmed that DFO was successful at mimicking hypoxia (Fig. 1A).
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50-fold by 24 h (Fig. 1B). Although eNOS levels were relatively unchanged during short durations of hypoxia (1–4 h), eNOS levels significantly decreased following 6 and 24 h. eNOS RNA levels decreased by more than 65% in cells exposed to hypoxia for 24 h. VEGF was induced by hypoxic conditions similarly to sONE, confirming that HUVEC were indeed hypoxic (Fig. 1B). In addition to decreasing the levels of eNOS RNA, hypoxia also resulted in decreased levels of eNOS protein following a 24-h exposure to hypoxic conditions (Fig. 1C). Together, these studies indicate that sONE is hypoxia-inducible and that the expression of sONE negatively correlates with the expression of eNOS in endothelial cells.
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sONE Is Hypoxia-inducible in Vivo—To study the regulation of sONE in vivo, rats were exposed to hypoxic conditions (10% oxygen), and sONE expression was analyzed in the aorta. Previous studies have demonstrated that hypoxia causes a decrease in eNOS expression in rat aortas (30). Real time RT-PCR analysis of sONE expression revealed that sONE was induced 2-fold by 16 h of hypoxia and more than 3-fold after 7 days of hypoxia treatment (Fig. 3). In these same samples, VEGF RNA expression increased 3-fold following 7 days of hypoxia (Fig. 3). The findings of these experiments suggest that sONE is hypoxia-inducible both in vitro and in vivo. The physiological relevance of sONE induction in vivo will need to be addressed in future studies.
sONE Induction and eNOS Repression Are Reversible upon Reoxygenation of Hypoxic Endothelial Cells—The reversibility of the hypoxic induction of sONE was determined by returning cells grown in hypoxic conditions (24 h), to a normoxic environment for 24 h (Fig. 4). Following reoxygenation of hypoxic cells, levels of sONE RNA returned to the levels found in normoxic cells (Fig. 4). Similarly, VEGF RNA levels decreased upon reoxygenation of hypoxic cells. Importantly, eNOS RNA levels also returned to baseline levels (Fig. 4), suggesting that repression of eNOS by hypoxia is reversible and that eNOS re-expression is correlated with sONE down-regulation.
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55% of total sONE mRNA at base line, suggesting that the long and short variants are similarly abundant in HUVEC (Fig. 5B). Comparison of the hypoxic induction of the short and long sONE transcripts revealed that both transcripts were hypoxia-inducible. However, the shorter variant increased more than 70-fold with longer durations (24 h) of hypoxia, whereas the longer variant increased 18-fold (Fig. 5B). This suggests that hypoxia regulates both the short and long sONE transcripts but that long term hypoxia has a more potent effect on the shorter transcript. sONE RNA Is Stabilized by Hypoxia, whereas Transcription of sONE Is Not Induced by Hypoxia—Several hypoxia-inducible genes are up-regulated by both transcriptional and post-transcriptional mechanisms (reviewed in Ref. 31). To determine whether sONE was transcriptionally induced by hypoxia in HUVEC, the loading of RNA Pol II within the sONE transcriptional unit was monitored by a ChIP assay. Exon 3 of sONE, a genomic region that is common to both the short and the long transcriptional units, was assessed. In normoxic cells, signals generated for Pol II ChIPs were considerably higher than background (Fig. S2), although very little steady-state sONE mRNA is detectable in this cell type. Hypoxia did not significantly change the transcription of the gene throughout the duration of hypoxia (Fig. 6A). In contrast, RNA Pol II was recruited to the HRE of the VEGF gene during exposure of HUVEC to hypoxia (Fig. 6A). Levels of Pol II at the VEGF HRE were significantly above background in normoxic cells (Fig. S2) but were much lower than levels found at exon 3 of sONE. This is consistent with the low transcriptional activity of the VEGF gene in normoxic cells.
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2.3 h (2.27 ± 0.97 h) in normoxic HUVEC, and the half-life increased nearly 2-fold (1.81 ± 0.16, p < 0.05) following hypoxia, to a half-life of 4.1 h (4.12 ± 1.94 h). A representative experiment is shown in Fig. 6C. As a control, the stability of eNOS RNA was also assessed. As we have previously published (9), hypoxia resulted in a decrease in the half-life of eNOS RNA (half-life of eNOS in hypoxia/normoxia = 0.31 ± 0.19; p < 0.05). These results suggest that sONE transcript stabilization, rather than transcriptional induction, occurs during hypoxia.
The Induction of sONE by Hypoxia is VHL-dependent—The VHL protein regulates gene transcription by ubiquitylating the subunit of HIFs and targeting them for proteasomal degradation in an oxygen-dependent manner (33, 34). Consequently, cells that are deficient in VHL functional activity constitutively express hypoxia-inducible genes (35). VHL also regulates genes (36) and cellular function (37) in a HIF-independent manner. To elucidate the role of VHL in regulating sONE RNA levels, we utilized renal epithelial cells (786-O) that are deficient in functional VHL protein (18). These cells were then stably transfected with an expression cassette that constitutively expresses either wild-type or mutant HA-tagged VHL. The mutant VHL protein (C162F) expressed in these cells cannot bind to HIF proteins, and HIF is therefore constitutively present in these cells under normoxic conditions (38). Western blotting revealed similar expression of wild-type and mutant HA-VHL (Fig. 7A). sONE was expressed at 3.5-fold higher levels in those cell lines lacking functional VHL activity (Fig. 7B). As expected, levels of VEGF were also expressed at higher levels in cells lacking wild-type VHL (Fig. 7B). 786-O cells reconstituted with wild-type HA-VHL were also exposed to hypoxic conditions. Hypoxia resulted in up-regulation of both sONE and VEGF transcripts in these cells (Fig. 7B). It has been previously demonstrated that 786-O cells (VHL-negative) express high levels of HIF2 protein but negligible levels of HIF1 protein (39). To determine whether elevated levels of HIF2 were responsible for sONE induction in the absence of functional VHL, 786-O cells stably expressing an shRNA cassette targeting the HIF2 transcript (19) were monitored for sONE expression. These cells express greatly reduced levels of HIF2 protein compared with cells transfected with empty vector (Fig. 7C). However, sONE expression was not altered in HIF2 knockdown cells (Fig. 7D). As a control, the expression of VEGF and CXCR4, two well characterized HIF-dependent transcripts (40, 41), were also monitored in these cells (Fig. 7D). VEGF and CXCR4 transcript levels were greatly diminished in two independent clones expressing HIF2 shRNAs. This suggests that HIF2 is not involved in sONE induction in the absence of functional VHL activity. To determine whether transcription of the sONE gene was altered by a lack of VHL activity, RNA Pol II ChIPs were performed in 786-O cells expressing wild-type or mutant VHL. A lack of functional VHL did not result in an increase in sONE transcription (Fig. 7E), suggesting that transcriptional induction was not responsible for the increased sONE levels observed in these cells. In contrast, we observed an increase in the amount of Pol II at the VEGF HRE in cells lacking wild-type VHL (Fig. 7E). These results suggest that sONE induction requires inhibition of VHL activity and that HIF2 is not involved in sONE induction, and they are consistent with our earlier conclusions in hypoxic endothelial cells, namely that transcriptional activation of the sONE gene is not responsible for the observed increase in sONE RNA.
sONE mRNA Is Enriched in the Nucleus Basally, but Cytoplasmic Accumulation Is Induced by Hypoxia and sONE Becomes Associated with Polyribosomes—It is currently not known whether sONE mRNAs are translated in endothelial cells or whether sONE plays a primarily noncoding regulatory role. Interestingly, the sONE transcript is a potential candidate for nonsense-mediated decay due to the presence of a translation stop codon in exon 8 of a 12-exon gene. Nonsense-mediated decay-regulated transcripts are often degraded before entering the cytoplasm (42) or remain associated with their transcriptional unit (43). Additionally, sense/antisense pairs have a propensity to be enriched in the nucleus (44), and RNA editing of double-stranded RNA also results in nuclear accumulation (45). It was therefore of interest to define the subcellular distribution of sONE mRNAs. RNA was isolated from nuclear and cytoplasmic subcellular fractions, and sONE and eNOS pre-mRNA and mRNA were quantified using real time RT-PCR (Fig. 8A). Although pre-mRNA levels for both sONE and eNOS were highly enriched in the nucleus, as expected, levels of sONE mRNA were also enriched in the nuclear fraction. In contrast, eNOS mRNA did not demonstrate nuclear enrichment. As a further control, pre-mRNA and mRNA levels of CD31 and CD34 were also monitored in these fractions, confirming that pre-mRNAs are generally enriched in the nucleus, whereas mRNAs are not. These results suggest that sONE mRNA is atypical in that it is enriched in the nucleus under basal conditions. To determine whether changes in whole cell sONE mRNA levels were attributed to an increase in nuclear or cytoplasmic levels of sONE, sONE mRNA levels were quantified in both of these fractions in normoxic and hypoxic cells (Fig. 8B). sONE mRNA levels increased only modestly in the nucleus, whereas cytoplasmic levels increased more than 30-fold (Fig. 8B). In contrast, the cellular distribution of eNOS mRNA was not altered, since eNOS mRNA levels decreased in both the cytoplasm and nucleus (Fig. 8B).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Cellular hypoxia provokes the induction of several genes through both transcriptional and post-transcriptional mechanisms. Upon dissection of the mechanisms that regulate increased sONE expression during hypoxia, it was discovered that transcription of sONE/NOS3AS was not significantly modulated in cells exposed to hypoxia or in cells containing mutated VHL protein. Additionally, and in contrast to the regulation of VEGF or CXCR4, we found that HIF proteins were not involved in the regulation of sONE in cells containing mutant VHL. These findings are consistent with a lack of transcriptional regulation of the sONE gene by hypoxic stimulation.
We found that sONE mRNA transcripts were labile in normoxic vascular endothelial cells but became stabilized in hypoxic endothelium. Several hypoxia-inducible transcripts are stabilized during hypoxia in a 3'-UTR-dependent fashion (41, 52). For example, VHL has been shown to interact with HuR in normoxic cells and to inhibit HuR-dependent stabilization of VEGF transcripts (53). Upon hypoxic stimulation, this interaction is compromised, and HuR binds to the 3'-UTR of VEGF, resulting in transcript stabilization. HuR is also involved in the nuclear/cytoplasmic trafficking of RNAs (54). Nuclear HuR may play a role in the stabilization of nuclear VEGF transcripts and may facilitate the transport of VEGF mRNAs to the cytoplasm for translation. Whether the increase in sONE transcripts in cells containing mutant VHL is the result of a loss of HuR regulation and determination of the relative role of HuR in the hypoxic regulation of sONE expression will need to be addressed in future studies. Importantly, we demonstrated that sONE mRNAs were predominantly nuclear in normoxic cells and that hypoxia resulted in transcript stabilization and an increase in cytoplasmic levels of sONE mRNA. HuR binds to the elongin-binding site of VHL (amino acids 157–184 (53)), which is a hot spot for mutation (55). Indeed, the mutant VHL protein that we used in our studies (C162F) would be expected to interfere with HuR/VHL interactions, allowing HuR to mediate the stabilization of hypoxia-inducible transcripts, such as VEGF. Hypoxia increased the expression of both the short and the long variants of sONE. These transcripts share a common 3'-UTR but originate from two separate transcriptional start sites. This suggests that shared regions of the transcripts, possibly the 3'-UTR, may be involved in the stabilization of sONE during hypoxia.
What is the normal biological role of sONE? The open reading frame of the longer sONE transcript is homologous to a known yeast autophagy gene, yAtg9p (8, 56). Autophagy is the degradation pathway for intracellular components triggered by nutrient deprivation (57) and exogenous stimuli that limit nutrient and energy availability, including hypoxia (58). Overexpression of the ORF of the longer sONE variant has been demonstrated to result in autophagosome formation (56). However, since sONE RNA is expressed at very low levels in normoxic endothelial cells, and considering that sONE mRNA is enriched in the nucleus, there is currently no evidence that the native sONE ORF produces a functional protein in normoxic endothelial cells. In contrast to the longer sONE variant, the shorter variant shares only minimal homology with the ORF of yAtg9p (8). We have demonstrated that the shorter variant is the predominant mRNA induced by hypoxia. Moreover, it was the major mRNA associated with polyribosomes. It is important to note that polyribosome association does not necessitate sONE translation in hypoxic endothelial cells. For example, microRNAs localize to polyribosomes (59, 60) to inhibit the translation of target mRNAs that are already engaged on the ribosome (61). We cannot exclude a model in which sONE localizes to polysomes for the purpose of inhibiting the translation of eNOS mRNAs. The amount of eNOS mRNA actively engaged with polyribosomes was decreased during hypoxia, and total eNOS protein levels were decreased in hypoxic cells. Additionally, sONE knockdown in hypoxic cells resulted in an increase in eNOS protein levels. In regard to the biological role of sONE in human endothelial cell types, we favor a model in which sONE plays an important noncoding biological role, specifically that sONE mRNA regulates eNOS mRNA post-transcriptionally during hypoxia. We posit that interaction between eNOS and sONE mRNAs results in the destabilization of eNOS mRNA and may also interfere with its translation.
We anticipate that the significant overlap (662 nucleotides) between the human eNOS and sONE mRNAs is key to the mechanism by which sONE regulates eNOS expression in endothelial cells. The overlap between eNOS and sONE mRNA (662 nt) is atypical compared with the median of known cis-S/AS pairs (139 nt) (62). Several models could be put forward for the regulation of eNOS RNA levels by sONE. eNOS mRNA is highly stable in endothelial cells, and changes in RNA stability and adenylation of the eNOS transcript have been implicated in the response of endothelial cells to several physiological and pathological stimuli, including shear stress (63), exposure to inflammatory cytokines (15, 64), and hypoxia (9). A multiprotein complex binds to the eNOS 3'-UTR in endothelial cells to actively stabilize eNOS RNA (14, 15). eNOS and sONE may compete for RNA-stabilizing proteins, or sONE/eNOS RNA interactions may prevent the formation of this stability complex on the eNOS 3'-UTR. Competition for stabilizing complexes has previously been observed in the regulation of CHOP RNA by its overlapping transcript, MetRS (65). Since eNOS and sONE mRNA demonstrate reciprocal expression patterns in cells, either eNOS or sONE may be preferentially stabilized in a given cell type. However, hypoxia results in increased sONE expression even in cell types with relatively high levels of sONE RNA, suggesting that hypoxia exerts an additional stabilizing effect. Since sONE mRNA is enriched in the nucleus of normoxic cells, but cytoplasmic levels increase following hypoxia, sONE mRNA could regulate eNOS in either the nucleus or the cytoplasm during hypoxia. We considered whether interactions between eNOS and sONE mRNA might prevent the nuclear export and cytoplasmic stabilization of eNOS mRNA. However, we did not observe a change in the nuclear/cytoplasmic distribution of eNOS mRNAs in hypoxic cells, suggesting that nuclear export of eNOS was not affected.
Although sONE might affect the formation of stabilizing complexes on the eNOS 3'-UTR, an alternative model for eNOS destabilization involves the RNA interference pathway. If long double-stranded mRNAs were to form between these two transcripts, they could be targeted for processing by Dicer to form siRNAs (66). Generation of siRNAs from naturally occurring sense-antisense partners has been observed in plants (67), but no evidence has been found for their existence in mammalian cells (68, 69). However, siRNAs generated from bidirectional transcription of the 5'-UTR of L1 retrotransposons have been identified in human cells that play a role in suppressing retrotransposition (70). Others have argued that the RNA interference pathway is not importantly involved in the regulation of natural S/AS genes (71). This recent work demonstrated that sense and antisense RNAs are often present in different subcellular compartments and do not physically interact in the cytoplasm. Long double-stranded RNA species are known to activate the protein kinase R pathway and elicit global translational inhibition (reviewed in Ref. 72). Compartmentalizing sense and antisense transcripts may avoid the activation of this pathway under physiological conditions. Although we demonstrate that sONE mRNA is basally enriched in the nucleus, sONE levels were dramatically increased in the cytoplasm following hypoxia. This important nuance could allow for the de novo physical interaction between the mRNAs in the cytoplasm. Whether such interaction might activate the protein kinase R pathway is currently not known, but global translational inhibition is known to occur in hypoxic cells (73). Additionally, the functional importance of interactions that occur at or near the site of transcription should not be discounted, especially considering the fact that RNA interference can occur in the nucleus (74, 75). Finally, even following hypoxic stimulation of HUVEC, there remains a 100–1000-fold difference in the copy numbers between eNOS and sONE transcripts. It is especially noteworthy that the RNA-induced silencing complex (RISC) is able to amplify the effect of guide strand-mediated RNA degradation in the RNA interference pathway (76). Determination of whether the RNA interference pathway is involved in eNOS/sONE regulation will require future validation.
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FOOTNOTES |
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The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. 1 and 2.
1 Recipient of a Canada Graduate Scholarship from the Natural Sciences and Engineering Research Council of Canada.
2 To whom correspondence should be addressed: University of Toronto, Medical Sciences Bldg., Rm. 7358, 1 King's College Circle, Toronto, Ontario M5S 1A8, Canada. Tel.: 416-978-2441; Fax: 416-978-8765; E-mail: p.marsden{at}utoronto.ca.
3 The abbreviations used are: S/AS, sense/antisense; DFO, desferrioxamine; eNOS, endothelial nitric-oxide synthase; HA, hemagglutinin; HIF, hypoxia-inducible factor; HRE, hypoxia response element; HUVEC, human umbilical vein endothelial cell(s); MVEC, dermal microvascular endothelial cell(s); ORF, open reading frame; RT, reverse transcription; VEGF, vascular endothelial growth factor; VHL, von Hippel-Lindau; VSMC, vascular smooth muscle cell(s); UTR, untranslated region; shRNA, short hairpin RNA; siRNA, small interfering RNA; Pol, polymerase; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; ChIP, chromatin immunoprecipitation; TBP, TATA-binding protein.
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ACKNOWLEDGMENTS |
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shRNA reagents, Mindy Maynard of Dr. Michael Ohh's laboratory for technical assistance on the exposure of cells to hypoxic conditions, and Jeff He and Dr. Michael Ward (St. Micheal's Hospital, Univerisity of Toronto) for RNA samples from hypoxic rat aortas. |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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