Báo cáo khoa học: An internal ribosome entry site mediates the initiation of soluble guanylyl cyclase b2 mRNA translation potx

An internal ribosome entry site mediates the initiation of soluble guanylyl cyclase b2 mRNA translation Roberto I. Vazquez-Padron 1 , Si M. Pham 1 , Dania Mateu 1 , Sheik Khan 1 and Abdelouahab Aitouche 2 1 University of Miami Miller School of Medicine, FL, USA 2 National Institutes of Health, Bethesda, MD, USA The soluble guanylyl cyclases (sGCs) are the only receptors for nitric oxide (NO). In the presence of NO, sGCs produce cGMP to initiate the intracellular sig- naling that leads to vascular smooth muscle cell relaxa- tion and synaptic transmission [1,2]. sGCs are heterodimeric enzymes consisting of one a-subunit and one b-subunit. There are two types of a-subunit and b- subunit that may constitute four different isoforms (a1b1, a2b1, a1b2, and a2b2) [3]. sGC a1b1is expressed in most mammalian tissues and has been extensively characterized [4–6]. Nonetheless, informa- tion about other sGCs is very limited. sGC a2b1is specifically produced in the placenta [7,8], and mRNA coding for the b2-subunit has been detected in kidney [9,10] and brain [11]. A search of the Geo Profiles database for sGC b2 gene expression has linked the activation of this gene to neoplastic growth and inflammatory processes. The function of sGC b2 remains elusive. The amino acid sequence of this subunit resembles that of a primi- tive sGC found in insects [12]. This subunit has an isoprenylation consensus site at the C-terminus that, in principle, could target this protein to the membrane. Interestingly, sGC a1b2 in transfected cells is 100-fold less active than the a1b1 heterodimer [11,13]. This finding suggests that sGC b2 could be a dominant neg- ative isoform, desensitizing sGCs and thus inhibiting NO signaling. Recently, a post-transcriptional variant of the sGC b2 that forms active homodimers has been described [14]. Keywords IRES; nitric oxide; soluble guanylyl cyclase; translation; untranslated region Correspondence R. I. Vazquez-Padron, Division of Cardiothoracic Surgery and Vascular Biology Institute, University of Miami Miller School of Medicine, 1600 NW 10th Avenue, RMSB 1063, Miami, FL, 33136, USA Fax: +1 305 243 5636 Tel: +1 305 243 1154 E-mail: rvazquez@med.miami.edu (Received 18 January 2008, revised 8 May 2008, accepted 13 May 2008) doi:10.1111/j.1742-4658.2008.06505.x The soluble guanylyl cyclases (sGC), the receptor for nitric oxide, are hete- rodimers consisting of an a- and b-subunit. This study aimed to investigate the translational mechanism of the sGC b2-subunit. Two mRNA species for sGC b2 were isolated from human kidney. These transcripts had dis- similar 5¢-untranslated regions (5¢-UTRs). The most abundant sGC b2 mRNA showed numerous upstream open reading frames (ORFs) and sta- ble secondary structures that inhibited in vivo and in vitro translation. To evaluate whether these 5¢-UTRs harbored an internal ribosome entry site (IRES) that allows translation by an alternative mechanism, we inserted these regions between the two luciferase genes of a bicistronic vector. Transfection of those genetic constructs into HeLa cells demonstrated that both sGC b2 leaders had IRES activity in a cell-type dependent manner. Finally, the secondary structural model of the sGC b25¢-UTR predicts a Y-type pseudoknot that characterizes the IRES of cellular mRNAs. In con- clusion, our findings suggest that sGC b25¢-UTRs have IRES activity that may permit sGC b2 expression under conditions that are not optimal for scanning-dependent translation. Abbreviations CMV, cytomegalovirus; ECMV, encephalomyocarditis virus; IRES, internal ribosome entry site; NO, nitric oxide; sGC, soluble guanylyl cyclase. 3598 FEBS Journal 275 (2008) 3598–3607 ª 2008 The Authors Journal compilation ª 2008 FEBS The human sGC b2 gene lies on chromosome 13q14.3 and expands for more than 73 kb. It consists of 17 exons, of which four correspond to the 5¢-UTR. This 5¢-leader has numerous upstream initiation co- dons (uAUG) and stable, predictable secondary struc- tures that difficult gene translation by the conventional scanning mechanism [15–17]. The translation of genes bearing complex 5¢-UTRs occur through alternative mechanisms such as IRES and ribosome jumping. The IRES direct ribosome binding straight to the initiation codon, avoiding the ribosome migration along the mRNA leader sequence [18,19]. This mechanism was originally discovered in picornavirus RNAs. Recently, a number of IRES have been identified in eukaryotic mRNAs coding for proteins associated with stress, cell growth, and cell death [20–25]. This study aimed to investigate the translational mechanism of the sGC b2-subunit. We found that sGC b2 translation is mediated by an IRES. This is the first example of an IRES being identified in one of the sGC genes. Results Cloning of the sGC b25¢-UTRs The 5¢-UTR of sGC b2 was isolated from human kid- ney RNA by RT-PCR. Two cDNA fragments of 298 bp and 195 bp were amplified using specific prim- ers (Fig. 1A). The 5¢-UTR of 298 bp was the most abundant in kidney, and several tumors and their nor- mal adjacent tissues (Fig. S1 Supplementary materials). The 298 bp fragment corresponded to the full-length 5¢-UTR with a perfect match to sGC b2 GeneBank sequences NP004120 and AF038499 (Fig. 1A). The less abundant cDNA fragment of 195 bp showed a new and rare sGC b25¢-UTR that lacked exon III. This alternative sGC b2 mRNA has a new start codon that would generate a polypeptide with an N-terminus longer than the one encoded by the full-length 5¢-UTR (Fig. 1C). The 5¢-UTR of sGC b2 mRNA contains numerous uAUGs and stable predictable secondary structures The full-length sGC b25¢-UTR was characterized of having nine uAUGs that could potentially initiate translation (Table 1) [26]. Only the one at +116 bp was in frame with the sGC b2 ORF though it did not overlap the sGC b2 primary sequence. This uORF could potentially block access of ribosomes to the sGC b2 start codon to inhibit the scanning translation of this gene. The sGC b25¢-UTR had a calculated fold- ing energy values up to )72.2 kcalÆmol )1 and predicted Y-type stem–loop structures (Fig. 2). This kind of structural motif is often detected in IRES of cellular mRNAs [27–30]. The sGC b25¢-UTR inhibits in vitro translation of a downstream reporter gene The sGC b25¢-UTR was inserted upstream to the FLuc gene under the control of the T7 promoter and the SV40 polyadenylation signal (Fig. 3A). Equimolar amounts of genetic constructs with and without the A B C Fig. 1. Cloning and sequencing of sGC b2 5¢-UTRs. (A) Isolation of sGC b25¢-UTRs by RT-PCR. The arrows indicate amplified DNA fragments. (B) Nucleotide sequence of the 298 bp 5¢-UTR. (C) Nucleotide sequence of the 195 bp 5¢-UTR. Upstream AUGs (uAUG) are show in bold and underlined. The authentic initiation codon is identified by the amino acid sequence. The exons are limited by arrows. R. I. Vazquez-Padron et al. The sGC b25¢-UTR contains an IRES FEBS Journal 275 (2008) 3598–3607 ª 2008 The Authors Journal compilation ª 2008 FEBS 3599 sGC b2 leader were transcribed ⁄ translated in the pres- ence of 7-methyl-G(5¢)ppp(5¢) guanosine cap analog in reactions supplemented with [ 35 S]Met. In vitro transla- tion produced luciferase and background proteins probably generated by proteolysis and protein aggrega- tion. The presence of the sGC b2 leader decreased luciferase protein three-fold as compared to the reac- tion performed with the control plasmid pGEM-FLuc (Fig. 3B,C). On the other hand, the absence of cap analog slightly decreased the in vitro translation of sGC b2-FLuc. Similar results were obtained by insert- ing the alternative sGC b2 between T7 promoter and the reporter gene. This leader also decreased luciferase activity three-fold with respect to the control vector. These findings suggested that a region of stable sec- ondary structure was obstructing gene translation and ⁄ or that factors that were not present in the rabbit reticulocyte lysates were required for in vitro protein production. The sGC b25¢-UTR inhibits in vivo transcription and translation of a downstream reporter gene Next, the sGC b25¢-UTR was inserted between the cytomegalovirus (CMV) promoter and the FLuc gene to study its effects on downstream gene expression (Fig. 4A). Genetic constructs were cotransfected with the transfection control plasmid (pRL-TK) in HeLa cells. The levels of firefly and renilla mRNAs were measured by TaqMan RT-PCR. Total RNAs were treated with DNase to avoid contamination. HeLa cells transfected with the control plasmid, the pFLuc, produced two times (2.1 ± 1.32, n = 4) more FLuc mRNA than those transfected with the pFLuc b2. The ratio of renilla mRNA between the two groups was 0.97 ± 0.13, which indicated no differences in trans- fection. This result suggested that the sGC b25¢-UTR may have a negative effect on the gene transcription or mRNA stability. The latter seemed less likely, as northern blot analysis showed no signals of RNA deg- radation (Fig. 4B). However, after adjusting the lucif- erase activity on the basis of mRNA levels, the luciferase activity in cells transfected with pFLuc-b2 was 1.8 times lower than in those transfected with the control plasmid (Fig. 4C). This agrees with the in vitro results and suggests the presence of stable secondary structure in the sGC b25¢-UTR that may inhibit both transcription and scanning translation. It also suggests that the initiation of sGC b2 translation is through an alternative mechanism that is less efficient than scan- ning translation. The 5¢-UTR of sCG b2 mRNA harbors an IRES The sGC b25¢-UTR was then inserted downstream to the hairpin in the pSL3 bicistronic vector to determine whether this sequence contained an IRES (Fig. 5A). This plasmid incorporates the renilla and firefly lucife- rases as first and second cistrons, respectively. The Fig. 2. Predicted secondary structure of the sGC b25¢-UTR. Predicted RNA secondary structures of the sGC b 25¢-UTR calculated by the MFOLD algorithm. Both secondary structures have extendable and stable stem loops with a semiconserved Y-structure characteristic of the cellular IRES. The dots denote the position of the uAUG in the sec- ondary structures. The folding energy (DG) appears below each structure. Table 1. Characteristics of the initiation codons of the sGC b25¢- UTR. AUG Position Context sequence Consensus a uORF length b In frame c uAUG 54 AACaugU + 75 No uAUG 58 UGUaugG + 11 No uAUG 116 AACaugG ++ 20 Yes uAUG 151 AAGaugC ) 2No uAUG 159 UUCaugA ) 39 No uAUG 171 ACAaugU + 35 No uAUG 175 UGUaugA ) 24 No uAUG 178 AUGaugA ) 23 No uAUG 234 UCCaugG + 15 No iAUG 282 AAGaugU + NA Yes a Degree of agreement with the Kozac consensus sequence for the initiation of protein translation. b Number of amino acids encoding for the upstream ORF (uORF) of the sGC b25¢-UTR. c Whether this ATG is in frame with the initiation codon. The sGC b25¢-UTR contains an IRES R. I. Vazquez-Padron et al. 3600 FEBS Journal 275 (2008) 3598–3607 ª 2008 The Authors Journal compilation ª 2008 FEBS hairpin between cistrons inhibits scanning translation and eliminates the possibility of ribosomal read- through. The relative IRES activity (ratio of firefly to renilla luciferase) was determined in transiently trans- fected HeLa cells. The insertion of the sense-oriented sGC b25¢-UTR in the intercistronic position increased the translation of the downstream cistron 6.3-fold over the control (pSL3) and two-fold over the vector con- taining the EMCV IRES (Fig. 5B). The IRES activity of the bicistronic vector with the antisense leader was two times less than that of the one with the sense sGC A B C Fig. 3. The sGC b 25¢-UTR inhibits the in vitro translation of the downstream reporter gene. (A) Genetic constructs for in vitro trans- lation. Both 5¢-UTRs were cloned downstream of the T7 promoter of the pGEM-11ZP(+) vector. (B) The in vitro translation products were resolved via SDS ⁄ PAGE, and [ 35 S]Met-labeled polypeptides were detected by autoradiography. The pGEM-11ZP(+) vector was used as control. The relative position of the luciferase is indicated. (C) Luciferase activity from in vitro transcription ⁄ translation reac- tions in the presence or absence of 7-methyl-G(5¢)ppp(5¢) guanosine cap (Cap). The error bars represent the mean ± standard deviation of five independent experiments. *P < 0.01 with respect to the Cap-FLuc group as calculated by one-way ANOVA followed by the Duncan test (n = 5). pCMV A C B FLuc pFLuc pFLuc pA sGC β2 5′ UTR pFLuc-β2 pFLuc-β2 β2 -FLuc mRNA FLuc mRNA 100 80 90 20 40 60 * pFLuc β2 Relative luciferase activity/mRNA Fig. 4. The sGC b25¢-UTR reduces in vivo transcription and transla- tion of a downstream reporter gene. (A) Monocistronic reporter vec- tors. The sGC b25¢-UTR was inserted between pCMV and the firefly luciferase gene. (B) Northern blot analysis of monocistronic mRNAs. Total mRNAs from transfected cells were blotted to nylon membranes and probed with a radiolabeled DNA specific for lucifer- ase. (C) Relative luciferase activity of transfected HeLa cells. The luciferase activity was normalized to the transfection control activity of pRL-TK and to the mRNA concentration determined by TaqMan RT-PCR. The error bars represent the mean ± standard deviation. *P < 0.01 as calculated by a t-test of unequal variances (n = 4). R. I. Vazquez-Padron et al. The sGC b25¢-UTR contains an IRES FEBS Journal 275 (2008) 3598–3607 ª 2008 The Authors Journal compilation ª 2008 FEBS 3601 b25¢-UTR. However, the mRNA harboring the anti- sense leader was not totally bicistronic as detected by RT-PCR (data not shown). These data suggested that the 5¢-UTR of sGC b 2 harbors an IRES only in the sense orientation. IRES activity in the 5¢-UTR of sGC b2 mRNA is neither due to the presence of a cryptic promoter nor to splicing sites in the bicistronic mRNA To demonstrate that the IRES activity observed in the sGC b25¢-UTR was not due to the production of monocistronic mRNA by splicing [31], total RNAs from transfected HeLa cells with the bicistronic vectors pSL3 (control) and pSL3-b2 were analyzed by northern blot. The full-length mRNA and a faint upper band were detected (Fig. 5C). The band above the bicistronic mRNA was likely pro- duced by an incomplete transcription termination of those genetic constructs. The absence of firefly lucif- erase monocistronic mRNAs ruled out the possibility of splicing sites within the sGC b25¢-UTR. This was further confirmed by RT-PCR. However, three times more firefly luciferase mRNA was observed in the control cells than in those transfected with pSL3- b2. The L-myc and Apaf-1 IRESs also decreased the abundance of luciferase mRNA in HeLa cells [20,25]. To exclude the possibility that the sGC b2 5¢-UTR contains cryptic promoters, it was inserted upstream of the FLuc gene in the promoterless plas- mid pGL3-Basic. The pFLuc with the CMV pro- moter was used as positive control (Fig. 6A). The absence of promoter abolished the luciferase activity in transfected HeLa cells regardless of the presence of the sGC b25¢-UTR (Fig. 6B). This experiment demonstrated that no cryptic promoter resides in this 5¢-UTR. The alternative sGC b25¢-UTR promotes the IRES-mediated translation The ability of alternative sGC b25¢-UTRs to promote internal ribosome entry on a bicistronic mRNA was also assessed (Fig. 7A). This alternative leader lacks exon III along with five uAUGs that may have inhibitory effects on translation (Fig. 1C). The alternative leader increased the IRES activity 53-fold over pSL3 (control) and six-fold over the full-length 5¢-UTR (Fig. 7B). The mRNA containing this 5¢-UTR b2 was completely bicistronic, as shown by RT-PCR with two independent sets of primers (Fig. 7C). No cryptic promoters were found in this leader either (Fig. 6). Exon III alone slightly, but not significantly, increased IRES activity over the control (one-way anova analysis, n = 6, Fig. 7B). The ele- vated IRES activity in the alternative 5¢-UTR may be explained by the appearance of more efficient IRES structures and ⁄ or the absence of inhibitory uORFs. The alternative leader also possess an in- frame AUG at +53 that may allow a more efficient translation of the downstream cistron (Fig. 1C). Taken together, these data suggest that both sGC b2 5¢-UTR isoforms are able to promote IRES-mediated translation. EMCV pCMV RLuc FLuc h pSL3 pA 1.0 1.6 3.2 Kb sGC β 2 5′UTR pSL3- β 2 pSL3-EMCV 1.0 1.6 0.3 3.5 Kb Kb β 2 -FLuc mRNA FLuc mRNA 3 4 5 8 6 2 2 6 8 * Relative IRES activity folds control (pSL3) 2 4 pSL3 EMCV β 2 0 C B A Fig. 5. The sGC b25¢-UTR has IRES activity. (A) Genetic constructs utilised to demonstrate the presence of IRES in the sGC b2 5¢-UTR. The EMCV IRES was used as a positive control. (B) Rela- tive IRES activity of HeLa cells transfected with bicistronic genetic constructs. Relative IRES activity was calculated by the ratio of fire- fly and renilla luciferase activity. The error bars represent the mean ± standard deviation of five independent experiments. *P < 0.01 as calculated by one-way ANOVA analysis followed by the Duncan test. (C) Northern blot analysis that shows the integrity of the bicistronic mRNA containing the sGC b25¢-UTR. The sGC b25¢-UTR contains an IRES R. I. Vazquez-Padron et al. 3602 FEBS Journal 275 (2008) 3598–3607 ª 2008 The Authors Journal compilation ª 2008 FEBS The sGC b2 IRES is functional in a wide range of cell lines To investigate how widely the sGC b2 IRES is utilized in mammalian cells, a panel of cell lines from different tissues was used. These cell lines were transfected using either pSL3 (control) or pSL3-b2. The sGC b25¢-UTR was most active in human cervix epithelial adenocarci- noma (HeLa) cells and moderately active in trans- formed monkey kidney (Cos7) cells, in human lung carcinoma (CCL-185) cells, and in human hepatocellu- lar carcinoma (Hep G2) cells (Fig. 8). Discussion The mechanism by which sGC b2 mRNA is translated has remained elusive thus far. Herein, we present evidences that the human sGC b2 mRNA is translated via a nonconventional mechanism and that its 5¢-UTR has a strong IRES element comparable with that of picornaviruses [32,33]. Our demonstration of IRES activity in the 5¢-UTR of sGC b2 accounts for the translation of this gene despite the presence of abortive uORFs and stable secondary structures in its leader sequence that may inhibit scanning-dependent transla- tion [18]. We initially characterized the sGC b25¢-UTR in vitro and in vivo. The use of monocistronic genetic construct demonstrated that the insertion of the sGC b25¢-UTR upstream of the FLuc gene inhibited its 60 50 30 40 Relative luciferase activity Folds of control (pGL3-Basic) 1 pGL3-basic β2 pFLuc 0 Aβ2 Fig. 6. The sGC b25¢-UTRs have no cryptic promoters. (A) Promo- terless genetic constructs. The pFLuc was used as a positive con- trol. (B) Genetic constructs were transfected into HeLa cells, and FLuc activity was measured and normalized to transfection effi- ciency and protein content. The data are expressed as fold increase over pGL3-Basic. The error bars represent the mean ± standard deviation of four independent experiments. FLucpCMV RLuc pSL3-A- β 2 β 2 5′UTR I I II III pSL3- β 2 IV 729 bp A- β 2 5′UTR PCR #2 PCR #1 525 bp pSL3-EIII- β 2 III 576 bp 525 bp pSL-3 422 bp 525 bp Ladder Plus (bp) pSL-3 pSL3- β 2 pSL3-A β 2 +RT pSL3-A β 2 –RT 850 1000 650 500 400 300 PCR #1 850 1000 650 500 PCR #2 65 * 35 45 55 Relative IRES activity folds control (pSL3) 5 15 25 pSL3 β2A-β2 EIII-β2 C B A II IV Fig. 7. The alternative sGC b25¢-UTR promotes IRES-mediated translation (A) Bicistronic genetic constructs utilized in this experi- ment. Exons within the 5¢-UTR are indicated (see Fig. 1B and C). (B) Relative IRES activity recovered from HeLa cells transfected with above genetic constructs. The error bars represent the mean ± standard deviation of six independent experiments. *P < 0.01 as calculated by one-way ANOVA analysis followed by the Duncan test. (C) RT-PCR analysis to demonstrate bicistronic RNA integrity using two sets of primers, PCR#1 and PCR#2 [as indicated in (A)]. The first lane is the 1 kb ladder Plus DNA molecular marker. Lane 5 shows the reverse transcriptase-negative control for each set of primers. R. I. Vazquez-Padron et al. The sGC b25¢-UTR contains an IRES FEBS Journal 275 (2008) 3598–3607 ª 2008 The Authors Journal compilation ª 2008 FEBS 3603 translation. This is consistent with the observations that only low levels of the sGC b2 protein are detected in tissues and with the low levels of b2-based sGC obtained in transfected cells [34, 9]. Transfection of cells with a bicistronic construct containing the sGC b25¢-UTR between cistrons dem- onstrated the presence of an IRES. The IRES activity was found, although differing in degree, in several human cell lines. The differences in the sGC b2 IRES- mediated translation among cell lines may be due to the levels of translation factors require for a proper IRES function. Recently, the idea of IRES-mediated translation in eukaryotes has been challenged on the basis of the meth- ods typically used for IRES identification [31]. IRES activity in the cells transiently transfected with bicistron- ic vectors is often confused with aberrant RNA cleav- age, splicing, and ⁄ or the presence of the cryptic promoter within the construct itself that lead to the formation of a low amount of monocistronic messengers translated via the conventional scanning mechanism. No evidence of either RNA cleavage or cryptic pro- moter was found in cells transfected with the bicistronic constructs containing the sGC b25¢-UTR. These data confirmed that the sGC b2 gene could be produced by an IRES-mediated translational mechanism. We also investigated whether the alternative sGC b2 5¢-UTR had IRES activity. The IRES activity in this sequence was six-fold stronger than that in the full- length and most abundant 5¢-UTR. This suggests that the ability of the sGC b25¢-UTR to initiate IRES translation resides in exons I, II and IV. It also sug- gests that the production of sGC b2 could be regulated at the post-transcriptional level through differential splicing, and at the translational level through an IRES. We can predict that under conditions where high levels of protein are required, more alternative sGC b2 mRNA is produced along with the factors needed for an efficient IRES-mediated translation. It is important to note that cellular IRES are often found in mRNA coding for regulatory proteins which expression is timely coordinated within the cell [18]. Furthermore, the IRES allows translation of tran- scripts in situations where cap-dependent translation is attenuated, such as stress, apoptosis, mitotic-phase transition, and development. There are several lines of evidence indicating a possible role of sGC b2 as nega- tive modulator of NO signaling. For example, Gupta et al. demonstrated that cotransfection of sGC b2 along with a1 and b1 into Cos7 cells blunted sGC activity in response to NO [13]. In addition, they found that the expression of the b2-subunit was increased in the kidney of hypertensive rats where sGC activity was diminished [13]. Collectively, these findings support the hypothesis that sGC b2 could be a modu- lator of NO signaling under stress or pathological con- ditions where IRES translation is favored. Future studies will be needed to determine the specific role of IRES-mediated translation of sGC b2 in the control of NO signaling. Experimental procedures Cloning of 5¢-UTRs of the human sGC b2 gene The sGC b25¢-UTRs were isolated from human kidney total RNA (Stratagene, La jolla, CA, USA) by RT-PCR with the following primers: 5¢-GCTTGGTGCTGCATCT CAATCCC-3¢ (forward) and 5¢-CTTCAGAATTGAAAG TATTCTCC-3¢ (reverse). Exon III was isolated in a similar manner but using 5¢-AAAGGTACCAACTTCTGC AGAAGTAC-3¢ (forward) and 5¢-CCATGGCTCGAGC CAGAATGTTGCAGG-3¢ (reverse) as primers. The PCR conditions per cycle were set as 30 s at 95 °C, 30 s at 50 °C, and 30 s at 72 °C. PCR products were resolved on a 2% agarose gel, and the intensity of each band was mea- sured using NIH imagej software (NIH, Bethesda, MA, USA). PCR products were further cloned into a pMos- Blue-TA vector (Amersham Biosciences, Piscataway, NJ, USA), and their sequences were determined at Sequetech (Mountain View, CA, USA). Genetic constructs Genetic constructs are depicted at the tops of the relevant figures. pSL3 was generated from pSL-EMCV by deleting Fig. 8. The sGC b25¢-UTR is active in a range of cell lines. Cell lines were transfected with the pSL3-b2 bicistronic vector using Lipofectamine 2000. Luciferase activity was measured and normal- ized on the basis of transfection control and protein contents. The error bars represent the mean ± standard deviation of five indepen- dent experiments. The sGC b25¢-UTR contains an IRES R. I. Vazquez-Padron et al. 3604 FEBS Journal 275 (2008) 3598–3607 ª 2008 The Authors Journal compilation ª 2008 FEBS the IRES [35]. pSL3-b2, pSL3-Ab2 and pSL3-EIII-b2 were generated by inserting the corresponding sGC b25¢-UTR into the pSL-3. PGEM-FLuc and pGEM-b2 were generated by either cloning the firefly luciferase (FLuc) alone or fused to the sGC b25¢-UTR downstream of the T7 promoter in the pGEM-11ZP(+) (Promega, Madison, WI , USA). The pSL-FLuc was constructed by NcoI removal of the renilla luciferase gene (RLuc) from the bicistronic vector pSL 3. The sGC b25¢-UTR was KpnI-XhoI inserted in pSL-FLuc to generate pSL-b2. pGL3-b2 was generated by inserting the sGC b25¢-UTR upstream of the FLuc gene in the pro- moterless plasmid pGL-3 Basic (Promega). Cell culture and transient transfection Transformed monkey kidney (Cos7), human lung carci- noma (CCL-185), human hepatocellular carcinoma (HepG2) and human cervix epithelial adenocarcinoma (HeLa) cells were obtained from American Tissue Culture Collection (Manassas, VA, USA). Cells were cultured under conventional conditions in DMEM supplemented with penicillin ⁄ streptomycin and 10% fetal bovine serum. Transfections were performed with Lipofectamine 2000 (Invitrogen, Carlsbad, CA, USA). Cells were cultured with Opti-MEMÒ I Reduced Serum Medium for 1 h before transfection. The DNA–Lipofectamine suspension (100 lL) containing 1.6 lg of plasmid and 4 lL of Lipo- fectamine 2000 was added drop by drop to the cells. After 4 h of incubation, 1 mL of growth medium was added and cells were cultured for 48 additional hours. Luciferase activity The firefly luciferase activity of cells transfected with mono- cistronic reporter constructs was measured with the Lucifer- ase Assay System (Promega). In these experiments, luciferase activity was normalized on the basis of the renilla luciferase activity of the transfection control vector pRL- TK (Promega) and of the mRNA levels determined by TaqMan Real Time PCR. The luciferase expression of the bicistronic vector was determined using the dual luciferase assay system (Pro- mega) in a Tuner Biosystems Lumminometer Model TD 20 ⁄ 20 (Mountain View, CA, USA). All assays were performed in triplicate on three to six different occasions. Final values were expressed as fold of the experimental control, indicated on each figure. Northern blotting, RT-PCR, and TaqMan real time RT-PCR Total RNA was prepared with the RNeasy Midi kit (Qia- gen, Valencia, CA, USA). Four micrograms of RNA were denatured in formaldehyde and separated on 1% formalde- hyde ⁄ agarose gels. RNA was capillary blotted onto a nylon membrane. Filters were UV cross-linked, and hybridiza- tions were performed in Ultrahigh hybridization solution (Ambion, Austin, TX, USA). The hybridization probe specific for luciferase were prepared with the T7 Ribrop- robe system (Promega) in the presence of [ 32 P]CTP[aP]. Total RNAs were treated with DNase before retrotran- scription. RT-PCRs were carried out with no retrotranscript- ed RNAs (RT-Minus control) to rule out any possibility of DNA contamination. The integrity of bicistronic mRNA was also assessed by RT-PCR with two set of primers. PCR#1 (F-5¢-AACTTTCGAAGTCATGGTGG-3¢ and R-5¢-GAC TTTCCAAAATGTCGTAATAACC-3¢) amplified the inter- cistronic region. PCR#2 (F-5¢-TTCCATCTTCCAGCGGA TAG-3¢ and R-5¢-CTACGTGCAAGTGATGATTTAC-3¢) amplified a 525 bp internal fragment of the first cistron. The PCR conditions per cycle were set as 30 s at 95 °C, 30 s at 52 °C, and 1 min at 72 °C. Firefly and renilla luciferase mRNAs were quantified using custom TaqMan Gene Expression Assays, respec- tively, according to the manufacturer’s instructions (Applied Biosystems, Foster City, CA, USA). Probes were FAM ⁄ MGB and primers were not limited. Real-time PCR was performed on an ABI Prism 7500 Fast Real-Time PCR System (96-well plate). In vitro transcription and translation In vitro coupled transcription and translation were performed with the TnT Quick Coupled Transcrip- tion ⁄ Translation System (Promega). Reactions were per- formed with 10 fm of circular plasmids in the presence of [ 35 S]Met at 37 °C for 90 min. The translated proteins were resolved via 12.5% SDS ⁄ PAGE and labeled polypeptides were detected by autoradiography. The band intensity was measured using NIH imagej software. The luciferase activity of in vitro translated proteins was measured as described above. Secondary structure modeling Minimal free energy calculations and secondary structure predictions were generated using the web-implemented ver- sion of the mfold algorithm incorporating version 3.0 of the Turner rules. Acknowledgements We thank Mireya Hernandez for her technical assis- tance and Deborah Georges for her assistance with the preparation of the manuscript. This work was supported by NIH grant R01 HL63426 granted to Dr Si M. Pham. R. I. Vazquez-Padron et al. The sGC b25¢-UTR contains an IRES FEBS Journal 275 (2008) 3598–3607 ª 2008 The Authors Journal compilation ª 2008 FEBS 3605 References 1 Andreopoulos S & Papapetropoulos A (2000) Molecu- lar aspects of soluble guanylyl cyclase regulation. Gen Pharmacol 34, 147–157. 2 Lucas KA, Pitari GM, Kazerounian S, Ruiz-Stewart I, Park J, Schulz S, Chepenik KP & Waldman SA (2000) Guanylyl cyclases and signaling by cyclic GMP. 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Any queries (other than missing material) should be directed to the corre- sponding author for the article. R. I. Vazquez-Padron et al. The sGC b25¢-UTR contains an IRES FEBS Journal 275 (2008) 3598–3607 ª 2008 The Authors Journal compilation ª 2008 FEBS 3607 . An internal ribosome entry site mediates the initiation of soluble guanylyl cyclase b2 mRNA translation Roberto I. Vazquez-Padron 1 , Si M. Pham 1 , Dania. investigate the translational mechanism of the sGC b2- subunit. We found that sGC b2 translation is mediated by an IRES. This is the first example of an IRES