|Home | About | Journals | Submit | Contact Us | Français|
Urokinase-type plasminogen activator (uPA) is expressed by lung epithelial cells and regulates fibrin turnover and epithelial cell viability. PMA, LPS, and TNF-alpha, as well as uPA itself, induce uPA expression in lung epithelial cells. PMA, LPS, and TNF-alpha induce uPA expression through increased synthesis as well as stabilization of uPA mRNA, while uPA increases its own expression solely through uPA mRNA stabilization. The mechanism by which lung epithelial cells regulate uPA expression at the level of mRNA stability is unclear. To elucidate this process, we sought to characterize protein-uPA mRNA interactions that regulate uPA expression. Regulation of uPA at the level of mRNA stability involves the interaction of a ~40 kDa cytoplasmic-nuclear shuttling protein with a 66 nt uPA mRNA 3′UTR sequence. We purified the uPA mRNA 3′UTR binding protein and identified it as ribonucleotide reductase M2 (RRM2). We expressed recombinant RRM2 and confirmed its interaction with a specific 66 nt uPA 3′UTR sequence. Immunoprecipitation of cell lysates with anti-RRM2 antibody and RT-PCR for uPA mRNA confirmed that RRM2 binds to uPA mRNA. Treatment of Beas2B cells with uPA or LPS attenuated RRM2-endogenous uPA mRNA interactions, while overexpression of RRM2 inhibited uPA protein and mRNA expression through destabilization of uPA mRNA. LPS exposure of lung epithelial cells translocates RRM2 from the cytoplasm to the nucleus in a time-dependent manner leading to stabilization of uPA mRNA. This newly recognized pathway could influence uPA expression and a broad range of uPA-dependent functions in lung epithelial cells in the context of lung inflammation and repair.
Pulmonary epithelial cells, microvascular endothelial cells and macrophages express urokinasetype plasminogen activator (uPA), a serine protease that catalyzes the conversion of plasminogen to plasmin. By binding to its receptor, uPAR, uPA localizes plasminogen activation at the cell surface. uPA-uPAR complexes serve other functions independent of plasminogen activation, such as cell proliferation, intracellular signaling, differentiation, chemotactic migration and invasion.(1–5) Lung epithelial cells also express PAI-1, which can inhibit uPA activity and promote internalization and recycling of uPA-uPAR-PAI-1 complexes.(6) Previous studies demonstrated rapid increases in circulating and pulmonary concentrations of uPA after endotoxemia or bacterial infection.(7–10) Mice deficient in uPA resist LPS-induced acute lung injury (ALI) and the development of pulmonary edema.(7,11) On the other hand, a defect of uPA-mediated fibrinolysis occurs in Acute Respiratory Distress Syndrome and uPA attenuates the fibrotic response in bleomycin-induced accelerated pulmonary fibrosis. Therefore, uPA appears to play a pivotal role in the pathogenesis of ALI and in pulmonary fibrosis subsequent to ALI.
Recent findings indicated that uPA autoinduction by lung epithelial cells (2,12), endothelial cells and monocytes (13) involve uPA binding to uPAR through its growth factor domain. The potentiation of LPS-induced ALI by uPA (7) and diminished neutrophil recruitment in response to P. aeruginosa pneumonia (14) by uPA- and uPAR-deficient mice underscores the contribution of uPA and uPAR to the development of ALI. Further, increased expression of uPA due to posttranscriptional uPA mRNA stabilization by tumor cells has been implicated in the increased proliferative and invasive potential of cancer cells.(12, 15) We previously reported that uPA expression is upregulated in lung epithelial cells through stabilization of uPA mRNA and that proinflammatory mediators implicated in the pathogenesis of ALI and its repair, stabilizes uPA mRNA.(15) Since elucidation of the underlying mechanism is essential for a better understanding of ALI, we sought to define the regulatory interactions that contribute to the stabilization of uPA mRNA and induce uPA at the posttranscriptional level in lung epithelial cells.
Beas2B and small airway epithelial (SAE) cells were purchased from ATCC (Manassas, VA) and Invitrogen (Carlsbad, CA), respectively. Beas2B cell culture (LHC-9) media and SAE cell culture media (SAGM), penicillin, and streptomycin were purchased from Invitrogen. Tissue culture plastics were from Becton Dickinson Labware (Linclon Park, NJ). Tris-base, aprotinin, dithiothreitol (DTT), phenyl-methylsulfonyl fluoride (PMSF), silver nitrate and ammonium persulfate (APS) were from Sigma Chemical Company (St. Louis, MO). Acrylamide, bisacrylamide, and nitrocellulose were from BioRad Laboratories (Richmond, CA). Anti-uPA antibody was purchased from American Diagnostica (Greenwich, CT), anti-RRM2 and anti-β-actin antibodies were obtained from and Santa Cruz Biotechnologies (Santa Cruz, CA). 32P-labeled UTP and dCTP were purchased from DuPont (Wilmington, DE), and X-ray films were purchased from Eastman Kodak (Rochester, NY).
Human uPA cDNA 3′UTR, and a deletion product containing the previously identified 66 nt uPA mRNA binding protein binding sequence (15) was cloned into pCDNA3.1 vector (Invitrogen) following PCR amplification using full length uPA 3′UTR cDNA as a template. The orientation and sequence of the clones were confirmed by sequencing. The full-length 3′UTR and the deletion product of uPA 3′UTR in pcDNA3.1 vector were linearized with Xba I, purified separately on agarose gels, extracted with phenol-chloroform, and used as a template for in vitro transcription with T7 polymerase. Sense mRNA was transcribed according to the supplier’s (Ambion Inc, Austin TX) protocol, except that 50 μCi (800 Ci/mmol) of [32P] UTP were used to substitute for unlabeled UTP in the reaction mixture. Passage through a NucAway (Ambion) column removed unincorporated radioactivity.
Beas2B cells cultured in 100 mm dishes were treated with LPS (20 μg/ml) for 0–24 h at 37°C. The culture media and the cell lysates were analyzed for changes in uPA and β-actin expression by Western blotting. Total RNA isolated from Beas2B cells treated with LPS for 0–6 h were tested for uPA and β-actin mRNA by RT-PCR using 32P-labeled dCTP in the PCR reaction mixture. The amplified bands were separated on a urea/PAGE using TBE buffer. Afterwards, the gel was dried and autoradiographed. The identity of the amplified PCR product was confirmed by nucleotide sequencing of non-radioactive amplicon. To measure uPA mRNA binding protein activity, the cytosolic and nuclear fractions prepared from the Beas2B cells treated with LPS for 0–24 h were subjected to gel mobility shift assay using the 66 nt uPA mRNA 3′UTR sequence as a probe as we described elsewhere.(15,22) Cytoplasmic and nuclear extracts of LPS exposed Beas2B cells were also immunoblotted for RRM2.
C57B6 mice were kept on a 12:12 h light and dark cycle with free excess to food and water. All experiments were conducted in accordance with institution review board approved protocols. ALI was induced by intratracheal injection of LPS (25 μg/20–25 g mouse) as described previously.(11) Control mice were exposed to saline. BAL fluids and the lung homogenates were prepared 24 h after saline or LPS exposure and analyzed for the changes in uPA expression by Western blotting. Total RNAs isolated from the lung tissues were tested for uPA mRNA expression by RT-PCR as described above. The cytosolic extracts prepared from the lung tissues of mice exposed to PBS or LPS for 24 h were tested for uPA mRNA binding protein activity by gel mobility shift assay using 32P-labled uPA mRNA 3′UTR sequence as probe.
Beas2B cells cultured in 150 mm culture dishes containing LHC-9 media were washed with HBSS. The cells were collected and lysed in an extraction buffer (25 mM Tris-HCl, pH 7.9, 0.5 mM EDTA, and 0.1 mM PMSF) with several freeze-thaw cycles. The lysates were then centrifuged at 12,000 × g for 15 minutes at 4°C, and the supernatants were collected. The protein content was measured with a Pierce BCA protein assay kit using various concentrations of serum albumin as standards.
Ammonium sulfate crystals were added to the lysate prepared above to bring it to 40% saturation and the precipitated proteins were discarded after checking the uPA mRNA binding protein activity. Ammonium sulfate crystals were further added to the supernatant to yield a final saturation of 60%. The precipitated proteins were collected, dissolved, and exhaustively dialyzed against the extraction buffer containing 10% glycerol. uPA 3′UTR mRNA binding activity was assessed using aliquot (10 μg) of dialyzed protein. The 40–60% ammonium sulfate fraction was passed through a blue-sepharose column (90 ml bed volume) in the same buffer containing 100 mM NaCl, and the uPA 3′UTR mRNA binding protein was eluted with a linear gradient (200 ml) of 0.1 –1 M NaCl. Positive fractions were pooled and loaded onto a heparin-Affigel column (90 ml bed volume) in the same buffer. After washing the unbound materials, the uPA mRNA binding protein was eluted with a linear gradient (200 ml) of 0–0.5 M NaCl in the extraction buffer. The fractions containing the uPA mRNA binding protein were pooled and loaded onto a DEAE-sephacel column and eluted with a linear NaCl gradient of 0–1 M NaCl in the extraction buffer. Positive fractions were pooled and passed through a phenyl sepharose column and eluted with a reverse gradient buffer. Positive fractions were pooled, dialyzed and concentrated by ultrafiltration, and loaded onto a mono-Q column fitted to a FPLC system. Unbound proteins were removed with 25 mM Tris-HCl (pH, 7.9) buffer, and bound proteins were eluted with a linear gradient (40 ml) of 0–1 M NaCl in 25 mM Tris-HCl (pH 7.9). Fractions containing uPA mRNA binding protein were analyzed by gel mobility shift assays.
Positive fractions were pooled, dialyzed, and subjected to a final round of affinity purification using an RNA affinity column containing biotin-labeled 66 nt uPA mRNA binding protein binding sequences (15) immobilized to streptavidin agarose, and uPA mRNA binding activity was assessed by gel mobility shift assays.(16–19) The uPA mRNA-protein complex was visualized by autoradiography. This band was excised, pooled from several runs, electroeluted and tested for uPA mRNA 3′UTR binding activity by Northwestern assay using 32P-labeled 66 nt uPA mRNA as a probe. SDS-PAGE analyses yielded a protein with an approximate molecular weight of ~40 kDa. The identity of the eluted protein was analyzed by mass spectroscopy. Database analyses revealed the identity of the binding protein. To determine whether the uPA mRNA binding protein specifically binds to uPA mRNA, we expressed it in a prokaryotic system using BL21 cells and purified the GST fusion protein as described previously (E3–E5). Purified recombinant uPA mRNA binding protein was analyzed for uPA mRNA binding using a gel mobility shift assay. To further confirm the specificity of the binding protein interaction with the 66 nt uPA mRNA 3′UTR sequence, we incubated purified recombinant RRM2 with the 32Plabeled 66 nt uPA mRNA binding sequence in the presence of a molar excess of unlabeled 66 nt uPA mRNA binding protein binding sequence or full length uPA or uPAR mRNA 3′UTR sequences and analyzed the interaction by gel mobility shift assay as described previously. (15, 19) The uPA mRNA binding protein cDNA was then subcloned into a eukaryotic expression vector pcDNA 3.1 and transfected to Beas2B cells. Histidine-tagged binding protein, containing a C-terminal V5 epitope, was expressed in Beas2B cells and was affinity purified using a nickel column. (17–18) Expression of the binding protein was confirmed by Western blotting of eluted fractions using anti-V5 and anti-binding protein antibodies. The fractions containing binding proteins were pooled and tested for uPA mRNA 3′UTR binding by a gel mobility shift assay using the 32P-labeled 66 nt sequence as a probe.
Beas2B cells cultured in LHC-9 media or primary SAE cells maintained in SAGM were transfected with either binding protein cDNA cloned in pcDNA 3.1 or an empty vector using PEI reagent. (20) The cells were lysed and the expression of recombinant protein was confirmed by Western blotting of V5 fusion protein. The conditioned media and cell lysates were tested for uPA expression by Western blotting. Afterwards, the membrane was stripped and sequentially probed with antibody against V5 or β-actin to assess expression of the fusion protein and loading equality. Total RNAs from the Beas2B cells transfected with vector cDNA alone or vector harboring RRM2 cDNA were isolated and analyzed for uPA mRNA by RT-PCR. In order to confirm if inhibition of uPA mRNA by overexpression of RRM2 was due to destabilization of uPA mRNA, Beas2B cells transfected with vector alone or vector harboring RRM2 cDNA were treated with or without LPS for 12 h to induce maximum uPA mRNA. DRB was added to these cells to inhibit ongoing transcription and the total RNA was isolated at different time points. uPA mRNA was analyzed by RT-PCR using 32P-labeled dCTP. Amplified PCR products were separated on a urea/polyacrylamide gel and autoradiographed.
Beas2B cells were treated with PBS or uPA for 24 h, or with LPS for 0–24 h, in LHC-9 media. Afterwards, the media were aspirated, cells washed with cold PBS, and cell lysates prepared were cleared with mouse IgG. The precleared lysates were subsequently subjected to immunoprecipation using anti-RRM2 antibody. The immune complexes were washed with the binding buffer, and the RNA isolated and the associated uPA mRNA was amplified by RT-PCR in the presence of 32P-labeled dCTP. The PCR products were separated by urea-PAGE. After electrophoresis, the gel was dried and subjected to autoradiography.
The statistical differences between various experimental conditions were analyzed by Student’s t test.
To determine how LPS alters uPA expression in human lung epithelial cells, we treated Beas2B cells with LPS for 0–24 h and tested both conditioned media and cell lysates for uPA antigen level by Western blotting. The results showed that LPS induced uPA expression in a time-dependent manner with maximum effect between 3–24 h after LPS exposure (Fig 1A). LPS treatment likewise induced uPA mRNA within 3–6 h (Fig 1B). We previously reported that treatment of Beas2B and primary SAE cells with LPS, uPA or TNF-α, induced uPA protein and mRNA expression in a similar manner.(21) We further reported that uPA induction by TNF-α involved, at least in part, stabilization of uPA mRNA (15,21), whereas uPA induced its own expression solely through uPA mRNA stabilization at the posttranscriptional level.(2) Additionally, we found that posttranscriptional control of uPA expression involves specific binding of a ~40 kDa uPA mRNABp to a 66 nt uPA mRNA 3′UTR destabilization determinant.(15,22) We hypothesized that altered interaction of the uPA mRNABp with the uPA mRNA 3′UTR in the cytoplasm may contribute to the induction of uPA expression by LPS since the latter induces both TNF-α and uPA expression during sepsis.(23–25) To test this postulate, we prepared cytosolic and nuclear extracts from Beas2B cells exposed to LPS for 0–24 h, and analyzed them for uPA mRNABp binding activity. Cytosolic fractions of LPS treated Beas2B cells showed decreased uPA mRNABp interaction with the 66 nt uPA mRNA 3′UTR binding sequence in a temporal fashion (Fig 1C). In contrast, uPA mRNA binding activity in the nuclear fractions gradually increased and reached a plateau between 6–24 h after LPS exposure, indicating nuclear translocation of uPA mRNABp. To determine if LPS induces uPA expression in vivo, we analyzed the BAL fluids and lung homogenates of mice exposed to intratracheal LPS for uPA protein and mRNA expression. Our results clearly showed the induction of both uPA protein and uPA mRNA after LPS exposure (Fig 2). Further analysis of cytoplasmic preparations of LPS-exposed mouse lung tissues for uPA mRNABp and uPA mRNA interaction by gel mobility shift assay revealed significant inhibition of the uPA mRNABp interaction with the 66 nt uPA 3′UTR mRNA sequence after LPS exposure (Fig 2C).
To further characterize the mechanism by which uPA expression is controlled at the posttranscriptional level in lung epithelial cells, we purified the uPA mRNABp from Beas2B cell lysates using conventional as well as uPA mRNA affinity column chromatography. Purification yielded a preparation enriched with a ~40 kDa protein having uPA mRNA binding activity, as confirmed by gel mobility shift (Fig 3A) and UV cross-linking (Fig 3B) assays, SDS-PAGE and silver staining (Fig 3C) or Northwestern assay (Fig 3D). Mass spectrophotometric analysis and database search indicated a high homology (69%) of this polypeptide to RRM2. To further evaluate the identity of the purified uPA mRNABp as RRM2, we subjected various protein fractions eluted from the Mono-Q column to Northwestern blotting using 32P-labeled 66 nt uPA mRNA binding sequence as a probe, and found that one specific fraction showed uPA mRNA binding activity at the approximate molecular weight position of ~40 kDa. To confirm that the enriched fraction co-migrates with RRM2, we stripped the same membrane and subjected it to Western blotting using anti-RRM2 antibody as a probe. As shown in Fig 3E, RRM2 antigens were indeed detected in the same fraction that exhibited uPA 3′UTR mRNA binding activity.
We then immunoprecipitated RRM2 proteins from the cytoplasmic fractions of Beas2B cells treated with LPS for 0–12 h. Northwestern blotting of immunoprecipitated RRM2 using 32Plabeled 66 nt uPA mRNA 3′UTR binding sequence as a probe demonstrated that LPS treatment inhibits RRM2 binding with the uPA 3′UTR mRNA sequence in Beas2B cells in a timedependent manner (Fig 3F). However, when LPS treated Beas2B cell lysates were tested for RRM2 expression, no significant inhibition of RRM2 levels by LPS was observed (Fig 3G). We next isolated RRM2 proteins using cytoplasmic extracts prepared from the lungs of mice exposed to PBS or LPS, and tested them for uPA 3′UTR mRNA binding activity by Northwestern assay (Fig 3H). Consistent with the responses found in Beas2B cells, RRM2 proteins isolated from mice exposed to intratracheal LPS showed reduced uPA mRNA binding activity compared to control mice exposed to PBS.
To independently determine that RRM2 binds to the uPA mRNA 3′UTR, we next expressed RRM2 in E. coli and affinity-purified rRRM2 using a GST Sepharose column (Fig 4A). Purified rRRM2 protein was tested for uPA mRNA 3′UTR binding activity. Gel mobility shift assay demonstrated specific binding of rRRM2 with a 66 nt 3′UTR determinant of uPA mRNA (Fig 4B). We expressed RRM2 in Beas2B cells and likewise confirmed its ability to specifically interact with the 66 nt uPA mRNA 3′UTR sequence. To confirm the specificity of the interaction of RRM2 with the uPA mRNA, we incubated rRRM2 protein with 32P-labeled 66 nt uPA mRNA 3′UTR sequence in the presence of a 200-fold molar excess of unlabeled 66 nt uPA 3′UTR binding sequence, full length uPAR or uPA 3′UTR sequence. The results showed that rRRM2 binds the uPA mRNA 3′UTR through the 66 nt sequence, that the interaction was specific and that it could be inhibited by self competition (Fig 4C).
We then sought to confirm whether RRM2 binds to endogenous uPA mRNA in Beas2B cells and if the increased uPA expression due to posttranscriptional uPA mRNA stabilization involves altered RRM2-uPA mRNA interaction. RRM2 protein was immunoprecipitated from the cytoplasmic extracts of Beas2B cells treated with PBS, uPA for 24 h, or LPS for 0–24 h using non-specific mouse IgG or an anti-RRM2 monoclonal antibody. Total RNAs isolated from the mouse IgG and anti-RRM2-immune complexes were analyzed for associated uPA mRNA by RT-PCR. As shown in Fig 4D, uPA mRNA was found to co-precipitate with the RRM2-anti- RRM2-immune complex in PBS treated Beas2B cells. Interestingly, treatment of cells with either uPA or LPS suppressed uPA mRNA co-precipitation with the RRM2 protein. No uPA mRNA was detected when immunoprecipitation reactions were performed with non-specific mouse IgG, indicating that RRM2 specifically interacts with endogenous uPA mRNA. Since LPS inhibits RRM2 interaction with uPA mRNA without inhibiting RRM2 expression, we next analyzed the cytoplasmic and nuclear extracts of Beas2B cell exposed to LPS for varying time for RRM2 by Western blotting. Our results showed translocation of RRM2 from the cytoplasm to the nucleus in a temporal fashion after LPS treatment. These results also suggest that increased uPA expression by lung epithelial cells following exogenous uPA or LPS treatment may be associated with reduced RRM2 interaction with cytoplasmic uPA mRNA due to relocation of RRM2 to the nucleus.
To determine whether RRM2 inhibits uPA expression in lung epithelial cells, we overexpressed RRM2 in Beas2B cells and tested for changes in the expression of uPA protein. We found that the expression of RRM2 in Beas2B cells led to the attenuation of uPA expression, compared to control Beas2B cells transfected with empty vector DNA (Fig 5A). RRM2 overexpression similarly inhibited uPA expression in primary SAE cells. We next tested if RRM2 overexpression by lung epithelial cells inhibits uPA mRNA. As shown in Fig 5B, transfection of Beas2B cells with RRM2 cDNA suppressed uPA mRNA expression, indicating that the effect was controlled at the mRNA level.
We previously reported that uPA mRNABp (15,22) regulates uPA mRNA expression at the posttranscriptional level which is now identified as RRM2. We therefore sought to clarify whether inhibition of uPA mRNA and protein expression following RRM2 overexpression in lung epithelial cells involves destabilization of uPA mRNA at the posttranscriptional level. Beas2B cells, with or without LPS treatment, were treated with DRB to inhibit uPA mRNA synthesis, and the posttranscriptional uPA mRNA decay was analyzed. Our results showed that overexpression of RRM2 destabilized basal uPA mRNA compared to the lung epithelial cells stably transfected with empty vector DNA (Fig 5C). RRM2 overexpression also prevented LPS-induced stabilization of uPA mRNA in Beas2B cells compared to control cells transfected with vector DNA only.
We recently reported that p53 inhibits uPA protein and mRNA expression in lung epithelial cells. (26) The process involves binding of p53 with a 35 nt 3′UTR sequences of uPA mRNA and destabilization of the uPA mRNA transcript. Our earlier findings (15) indicate that RRM2 binding with a 66 nt 3′UTR determinant resides upstream of the 35 nt p53 binding 3′UTR sequence. In addition, earlier reports (27–28) demonstrated that p53 controls RRM2 through a protein-protein interaction. We therefore hypothesized that the inhibitory effect of RRM2 on uPA expression at the posttranscriptional level is controlled by p53. To test this possibility, we initially analyzed basal RRM2 expression in p53-deficient (H1299) cells transfected with pcDNA3.1 harboring p53 cDNA to determine if reintroduction of p53 in p53-deficient cells affects RRM2 expression. These responses were compared with naïve p53-deficient cells and H1299 cells transfected with pcDNA3.1 only. As shown in Fig 6A, transfection of p53-deficient cells with p53 cDNA failed to induce RRM2 expression. We then immunoprecipitated RRM2 and analyzed for associated p53 by Western blotting, confirming the interaction of p53 with RRM2 protein in p53-deficient cells transfected with p53 cDNA (Fig 6B). To clarify if LPS treatment alters p53-induced inhibition of uPA expression, we exposed naïve p53-deficient cells or p53-deficient cells transfected with vector DNA or p53 cDNA to PBS, LPS or the amino-terminal fragment (ATF) of uPA for 24 h. The conditioned media were analyzed for uPA by Western blotting. Consistent with our earlier report, (26) reintroduction of p53 in p53-deficient cells inhibited uPA expression. However, treatment with either LPS or ATF, which mimics full length uPA in terms of uPA expression, partially reversed the inhibitory effects of p53 (Fig 6C). We next immunoprecipitated p53 from the cell lysates of naïve p53-deficient H1299 cells, or the same cells expressing vector DNA (pcDNA 3.1) alone or p53 cDNA in pcDNA 3.1 and tested for RRM2 by Western blotting. We also exposed p53 cDNA transfected H1299 cells to PBS, LPS, or ATF of uPA and cytosolic extracts were immunoprecipitated for p53 and immunoblotted for RRM2 to assess changes in p53-RRM2 interaction. We found p53 and RRM2 interactions in the cytosolic fractions of H1299 cells transfected with p53 cDNA. However, treatment of these with either LPS or ATF abolished p53 and RRM2 interaction (Fig 6D). These results demonstrate that the changes in the interaction between p53 and RRM2 following LPS injury are associated with the induction of uPA expression at the posttranscriptional level in lung epithelial cells.
Proinflammatory cytokines such as TNF-α induce uPA expression in multiple cell types, including lung epithelial cells. uPA likewise induces its own expression in lung epithelial cells (2), endothelial and myeloid cells. (13) LPS induces both TNF-α and uPA expression in multiple cell types including lung epithelial cells.(15,23–25) Stabilization of uPA mRNA at the posttranscriptional level, at least in part in the case of TNF-α (15, 21) and exclusively in the case of uPA (2, 26), contributes to increased uPA mRNA and protein expression. Posttranscriptional regulation of uPA involves the interaction of a ~40 kDa cytoplasmic-nuclear shuttling protein with a 66 nt destabilization determinant present in the uPA mRNA 3′UTR. (15) Here, we extended our earlier study, purified the uPA mRNABp from the lysates of lung epithelial cells, identified it as RRM2 and characterized the role of the RRM2-uPA mRNA 3′UTR in the regulation of uPA mRNA stability and uPA expression by lung epithelial cells.
Expression of RRM2 inhibited uPA protein expression by lung epithelial cells and this response involves the suppression of uPA mRNA expression. LPS and uPA inhibited RRM2 binding to uPA mRNA 3′UTR, suggesting that RRM2 attenuated uPA expression. This is consistent with our earlier observation that TNF-α-mediated induction of uPA in lung epithelial cells is associated with parallel inhibition of the cytoplasmic uPA mRNABp interaction with the 66 nt uPA mRNA 3′UTR sequence due to translocation of uPA mRNABp to the nucleus.(15) The uPA mRNA destabilizing effect of the RRM2-uPA mRNA interaction was confirmed by the time dependent translocation of uPA mRNA binding activity from cytoplasm to the nucleus with accumulation of RRM2 proteins in the latter compartment after LPS treatment. This is further supported by the destabilization of uPA mRNA in resting as well as LPS-treated lung epithelial cells that overexpressed RRM2.
Ribonucleotide reductase is an important enzyme involved in the synthesis of DNA and responsible for the reduction of ribonucleotides to their corresponding deoxyribonucleotides. Three subunits of ribonucleotide reductase, RRM1, RRM2 and p53R2, provide a balanced supply of nucleotide precursors for DNA synthesis. RRM2 interacts with RRM1 to form a heterotertramer complex which is catalytically active. We recently reported that tumor suppressor protein, p53 inhibits uPA expression through destabilization of uPA mRNA.(26) The process involves sequence specific interaction of p53 with a 35 nucleotide destabilization determinant present in the uPA mRNA 3′UTR. Previous studies (27–28) suggested that p53 and RRM2 directly interact to control ribonucleotide reductase activity during DNA damage. Interestingly, the RRM2 binding sequence on uPA mRNA 3′UTR resides adjacent but upstream to the p53 binding sequence, supporting their potential for coordinate interaction. Increased translocation of RRM2 from the cytoplasm to the nucleus after UV irradiation (29–30) is consistent with our earlier report that uPA mRNABp shuttles to the nucleus following stimulation of lung epithelial cells with TNF-α, leading to stabilization of uPA mRNA and induction of uPA expression. RRM2 and its mode of regulation of uPA expression through cytoplasmic-nuclear shuttling are illustrated in a schematic diagram (Fig 7).
We reported earlier that uPA induces its own expression as well as that of uPAR through obliteration of p53. We therefore inferred that p53 must have contributed to RRM2-mediated inhibition of uPA expression by lung epithelial cells. Cells lacking p53 express a significant amount of uPA and reintroduction of p53 inhibits uPA expression through destabilization of uPA mRNA.(26) p53 and RRM2 independently interact with unique 3′UTR sequences that contain information for mRNA degradation (12,26). p53 interacts with RRM2 without significant changes in RRM2 expression. However, treatment of the cells with LPS inhibits cytosolic RRM2 and p53 interaction due to translocation of RRM2 to the nucleus. These changes are associated with a parallel induction of uPA expression through mRNA stabilization. Based on our present findings and prior reports (15,21,26) it is unlikely that p53 directly interferes with RRM2 binding to uPA mRNA since they independently bind to two destabilization determinants present in the 3′UTR (15,26). However, the possibility of p53 being involved in the translocation of RRM2 from the cytoplasm to the nucleus after LPS treatment is not yet ruled out and could be a subject of future study. This observation demonstrates that RRM2 maintains a complex network of cellular functions depending upon the external stimuli, metabolic state of the cell and the localization of RRM2.
This newly recognized paradigm is to our knowledge the first description of RRM2 regulating uPA expression in any cell type. If operative in vivo, this pathway could contribute to the regulation of uPA expression under normal conditions and in pathophysiologic states including various forms of ALI and its most severe form; acute respiratory distress syndrome.
This work was supported in part by grants from Flight Attendant Medical Research Institute Clinical Innovator Award (FAMRI-ID-082380) and NHLBI R21-HL093547.
Author disclosure: S.S. has received from the sponsored grants from FAMRI and National Institutes of Health for more than $100,001.