Translational regulation of angiotensin II type 1A receptor. Role of upstream AUG triplets

Angiotensin-(3-4) normalizes blood pressure, decreases Na+ and energy intake, but preserves urinary Na+ excretion in overweight hypertensive rats

Hypertension, one of the most common and severe comorbidities of obesity and overweight, is a worldwide epidemic affecting over 30% of the population. We induced overweight in young male rats (aged 58 days) by exposure to a hypercaloric high lipid (HL) diet in which 70% of the calories originated from fat. The HL diet also contained 33 or 57% higher Na⁺ than the control (CTR) diet. Over the following weeks the HL rats gradually became overweight (490 ± 12 g vs 427 ± 7 g in the CTR group after 15 weeks) with high visceral fat. They developed elevated systolic blood pressure (SBP) (141 ± 1.9 mmHg), which was fully restored to CTR values (128 ± 1.1 mmHg) by oral administration of Ang-(3-4) (Val-Tyr), the shortest renin-angiotensin-derived peptide. The overweight rats had lower plasma Na⁺ concentration that augmented to CTR values by Ang-(3-4) treatment. Na⁺ ingestion was depressed by 40% as result of the Ang-(3-4) treatment, whereas the urinary excretion of Na⁺ (U_NaV) remained unmodified. The preservation of U_NaV after Ang-(3-4) treatment - despite the sharp decrease in the dietary Na⁺ intake - can be ascribed to the normalization of renal type 1 angiotensin II receptors and Na⁺-transporting ATPases, both up-regulated in overweight rats. These renal effects complete a counterregulatory action on elevated renin-angiotensin activity that allows the high SBP to be normalized and body Na⁺ homeostasis to be restored concomitantly in overweight rats.

Early kidney damage induced by subchronic exposure to PM2.5 in rats

BACKGROUND

Particulate matter exposure is associated with respiratory and cardiovascular system dysfunction. Recently, we demonstrated that fine particles, also named PM_2.5, modify the expression of some components of the angiotensin and bradykinin systems, which are involved in lung, cardiac and renal regulation. The endocrine kidney function is associated with the regulation of angiotensin and bradykinin, and it can suffer damage even as a consequence of minor alterations of these systems. We hypothesized that exposure to PM_2.5 can contribute to early kidney damage as a consequence of an angiotensin/bradykinin system imbalance, oxidative stress and/or inflammation.

RESULTS

After acute and subchronic exposure to PM_2.5, lung damage was confirmed by increased bronchoalveolar lavage fluid (BALF) differential cell counts and a decrease of surfactant protein-A levels. We observed a statistically significant increment in median blood pressure, urine volume and water consumption after PM_2.5 exposure. Moreover, increases in the levels of early kidney damage markers were observed after subchronic PM_2.5 exposure: the most sensitive markers, β-2-microglobulin and cystatin-C, increased during the first, second, sixth and eighth weeks of exposure. In addition, a reduction in the levels of specific cytokines (IL-1β, IL-6, TNF-α, IL-4, IL-10, INF-γ, IL-17a, MIP-2 and RANTES), and up-regulated angiotensin and bradykinin system markers and indicators of a depleted antioxidant response, were also observed. All of these effects are in concurrence with the presence of renal histological lesions and an early pro-fibrotic state.

CONCLUSION

Subchronic exposure to PM_2.5 induced an early kidney damage response that involved the angiotensin/bradykinin systems as well as antioxidant and immune imbalance. Our study demonstrates that PM_2.5 can induce a systemic imbalance that not only affects the cardiovascular system, but also affects the kidney, which may also overall contribute to PM-related diseases.

A 5'-upstream short open reading frame encoded peptide regulates angiotensin type 1a receptor production and signalling via the β-arrestin pathway

AUG sequences and short open reading frames are commonly present in the 5'-leader sequence of G protein-coupled receptor mRNAs. The presence of these upstream AUG sequences has been demonstrated to inhibit downstream receptor translation efficiency and, most recently, receptor signal transduction. A seven amino acid peptide encoded by a short open reading frame in exon 2 of the angiotensin type 1a receptor has been shown to inhibit non-G protein-coupled signalling of angiotensin II, without altering the classical G protein-coupled pathway activated by the ligand. This finding may lead to the development of a new class of angiotensin receptor antagonists with activities biased for one, but not all, of the signalling cascades activated by angiotensin II, which could have therapeutic implications for the myriad hormones and neurotransmitters that signal through G protein-coupled receptors.

Acute and subchronic exposure to air particulate matter induces expression of angiotensin and bradykinin-related genes in the lungs and heart: Angiotensin-II type-I receptor as a molecular target of particulate matter exposure

Background

Particulate matter (PM) adverse effects on health include lung and heart damage. The renin-angiotensin-aldosterone (RAAS) and kallikrein-kinin (KKS) endocrine systems are involved in the pathophysiology of cardiovascular diseases and have been found to impact lung diseases. The aim of the present study was to evaluate whether PM exposure regulates elements of RAAS and KKS.

Methods

Sprague–Dawley rats were acutely (3 days) and subchronically (8 weeks) exposed to coarse (CP), fine (FP) or ultrafine (UFP) particulates using a particulate concentrator, and a control group exposed to filtered air (FA). We evaluated the mRNA of the RAAS components At1, At2r and Ace, and of the KKS components B1r, B2r and Klk-1 by RT-PCR in the lungs and heart. The ACE and AT₁R protein were evaluated by Western blot, as were HO-1 and γGCSc as indicators of the antioxidant response and IL-6 levels as an inflammation marker.

We performed a binding assay to determinate AT₁R density in the lung, also the subcellular AT₁R distribution in the lungs was evaluated. Finally, we performed a histological analysis of intramyocardial coronary arteries and the expression of markers of heart gene reprogramming (Acta1 and Col3a1).

Results

The PM fractions induced the expression of RAAS and KKS elements in the lungs and heart in a time-dependent manner. CP exposure induced Ace mRNA expression and regulated its protein in the lungs. Acute and subchronic exposure to FP and UFP induced the expression of At1r in the lungs and heart. All PM fractions increased the AT₁R protein in a size-dependent manner in the lungs and heart after subchronic exposure. The AT₁R lung protein showed a time-dependent change in subcellular distribution. In addition, the presence of AT₁R in the heart was accompanied by a decrease in HO-1, which was concomitant with the induction of Acta1 and Col3a1 and the increment of IL-6. Moreover, exposure to all PM fractions increased coronary artery wall thickness.

Conclusion

We demonstrate that exposure to PM induces the expression of RAAS and KKS elements, including AT₁R, which was the main target in the lungs and the heart.

A novel CARD containing splice-isoform of CIITA regulates nitric oxide synthesis in dendritic cells

MHC class II expression is controlled mainly at transcriptional level by class II transactivator (CIITA), which is a non-DNA binding coactivator and serves as a master control factor for MHC class II genes expression. Here, we describe the function of a novel splice-isoform of CIITA, DC-expressed caspase inhibitory isoform of CIITA (or DC-CASPIC), and we show that the expression of DCCASPIC in DC is upregulated upon lipopolysaccharides (LPS) induction. DC-CASPIC localizes to mitochondria, and protein-protein interaction study demonstrates that DC-CASPIC interacts with caspases and inhibits its activity in DC. Consistently, DC-CASPIC suppresses caspases-induced degradation of nitric oxide synthase-2 (NOS2) and subsequently promotes the synthesis of nitric oxide (NO). NO is an essential regulatory molecule that modulates the capability of DC in stimulating T cell proliferation/activation in vitro; hence, overexpression of DC-CASPIC in DC enhances this stimulation. Collectively, our findings reveal that DC-CASPIC is a key molecule that regulates caspases activity and NO synthesis in DC.

Inhibition of vasopressin V1b receptor translation by upstream open reading frames in the 5'-untranslated region

The 5'-UTR of the vasopressin V1b receptor (V1bR) mRNA contains small open reading frames (ORF) located upstream (u) of the main ORF encoding the V1bR. The ability of the three proximal uORFs to be translated into peptides and their influence on V1bR translation was examined using fusion constructs of uORFs and V5 epitope, or ATG/ATA uORF mutations in the V1bR cDNA. In vitro translation and western blot analysis after transfection of uORF1-V5 or uORF2-V5 into cells revealed that uORF1 can be translated. As predicted by computer analysis, in vitro translation using a rabbit reticulocyte/canine microsome system, immunohistochemistry and western blot in membranes of transfected cells with uORF1-V5 revealed translocation of the uORF1 peptide into membrane fractions. In vitro translation of V1bR cDNA with mutations of the two uORFs proximal to the initiating methionine, uORFs 1 and 2 (Mut 1-2), or uORF2 (Mut 2) showed significantly increased translation of a 46 kDa band corresponding to the V1bR, compared with wild-type (WT) V1bR, an effect that was attenuated by cotranslation of uORF1-V5. Consistently, VP-induced inositol phosphate formation was higher in Chinese hamster ovay cells transfected with Mut 1-2 than with WT V1bR. Immunohistochemical and western blot analysis, using an antibody against uORF1, revealed peptide immunoreactivity in rat pituitary but not in liver. Pituitary uORF immunoreactivity increased following glucocorticoid administration. The present study shows that uORFs in the 5'-UTR of the V1bR mRNA inhibit V1bR translation, and suggests that translation of a 38-amino acid membrane peptide encoded by uORF1 exerts tonic inhibition of V1bR translation.

TGF-β1 regulation of human AT1 receptor mRNA splice variants harboring exon 2

At least four alternatively spliced mRNAs can be synthesized from the human AT(1)R (hAT(1)R) gene that differ only in the inclusion or exclusion of exon 2 and/or 3. RT-PCR experiments demonstrate that splice variants harboring exon 2 accounts for at least 30% of all the hAT(1)R mRNA transcripts expressed in the human tissues investigated. Since exon 2 contains two upstream AUGs or open reading frames (uORFs), we hypothesized that these AUGs would inhibit the translation of the downstream hAT(1)R protein ORF harbored in exon 4. This study demonstrates that the inclusion of exon 2 in hAT(1)R mRNA transcripts dramatically reduces hAT(1)R protein levels (nine-fold) and significantly attenuates Ang II responsiveness ( approximately four-fold). Interestingly, only when both AUGs were mutated in combination were the hAT(1)R density and Ang II signaling levels comparable with those values obtained using mRNA splice variants that did not include exon 2. This observation is consistent with a model where the majority of the ribosomes likely translate uORF#1 and are then unable to reinitiate at the downstream hAT(1)R ORF, in part due to the presence of AUG#2 and to the short intercistronic spacing. Importantly, TGF-beta(1) treatment (4ng/ml for 4h) of fibroblasts up-regulated hAT(1)R mRNA splice variants, which harbored exon 2, six-fold. Since AT(1)R activation is closely associated with cardiovascular disease, the inclusion of exon 2 by alternative splicing represents a novel mechanism to reduce the overall production of the hAT(1)R protein and possibly limit the potential pathological effects of AT(1)R activation.

A splice variant acquiring an extra transcript leader region decreases the translation of glutamine synthetase gene

The expression of glutamine synthetase (GS), catalysing the ATP-dependent conversion of glutamate and ammonia into glutamine, is transcriptionally and post-transcriptionally regulated. The genomic structure of dog GS shown in the present study is basically similar to that of other mammals in that it is composed of seven exons and six introns. Using 5'-cRACE (where cRACE stands for circular rapid amplification of cDNA ends) and reverse transcriptase-PCR, we identified an additional exon (120 bp) in the first intron, designated in the present study as exon 1'. By means of alternative splicing, the GS gene produces an altered form of GS transcript with 5'-untranslated region (UTR) containing the exon 1'. This alternative transcript is abundantly expressed in brain, whereas it is found at lower levels in other tissues. In the human and mouse GS genes, extra exons are also found at the corresponding site of the intron 1 but with different sizes. An exon-trapping experiment for the GS gene in COS-7, Madin-Darby canine kidney and SK-N-SH cells revealed that the pattern of alternative splicing is variable in different cell types. The propensity of forming a secondary structure is predicted to be considerably higher in the presence of extra 5'-UTR, suggesting the possibility of a translational effect. To test this, we performed a reporter assay for fusions with different 5'-UTRs, demonstrating that the long form with extra 5'-UTR was translated 20- and 10-fold less than the short one in SK-N-SH and Neuro-2A cells respectively. Similarly, translations of human and mouse transcripts with extra 5'-UTRs were less efficient, showing 6-8-fold reductions in SK-N-SH cells. Furthermore, when we mutated an ATG sequence contained in the exon 1', the suppression of translation was partially relieved, suggesting that the negative regulation by an extra 5'-UTR is, to some extent, due to an abortive translation from the upstream ATG.

The mRNA structure has potent regulatory effects on type 2 iodothyronine deiodinase expression

Type 2 deiodinase (D2) is a selenoenzyme catalyzing the activation of T(4) to T(3). D2 activity/mRNA ratios are often low, suggesting that there is significant posttranscriptional regulation. The D2 mRNA in higher vertebrates is more than 6 kb, containing long 5' and 3' untranslated regions (UTRs). The D2 5'UTRs are greater than 600 nucleotides and contain 3-5 short open reading frames. These full-length 5'UTRs reduce the D2 translation efficiency approximately 5-fold. The inhibition by human D2 5'UTR is localized to a region containing the first short open reading frame encoding a tripeptide-MKG. This inhibition was abolished by mutating the AUG start codon and weakened by modification of the essential purine of the Kozak consensus. Deletion of the 3.7-kb 3'UTR of the chicken D2 mRNA increased D2 activity approximately 3.8-fold due to an increase in D2 mRNA half-life. In addition, alternatively spliced D2 mRNA transcripts similar in size to the major 6- to 7-kb D2 mRNAs but not encoding an active enzyme are present in both human and chicken tissues. Our results indicate that a number of factors reduce the D2 protein levels. These mechanisms, together with the short half-life of the protein, ensure limited expression of this key regulator of T(4) activation.

Evidence for translational repression of the SOCS-1 major open reading frame by an upstream open reading frame

The suppressor of cytokine signalling 1 protein (SOCS-1) belongs to a novel family of cytokine inducible factors which function as inhibitors of the JAK/STAT pathway. While SOCS-1 previously has been described as a single-exon gene, here we present evidence for an additional 5' exon, separated by a 509 bp intron from exon 2. Exon 1 and part of exon 2 contain an open reading frame of 115 nt, ending one nucleotide upstream of the major reading frame. Using SOCS-1-promoter/luciferase constructs, we investigated which sequences are involved in the regulation of SOCS-1 expression. Serial promoter deletion clones indicate the localization and functionality of SP1, interferon-stimulated responsive elements (ISRE), and interferon-gamma-activated sites (GAS) promoter elements in the SOCS-1 5' flanking region. We present evidence that the upstream open reading frame (uORF) represses the translation of the downstream major open reading frame (mORF). Mutating the start codon of the uORF relieves this repression. Our data indicate that expression of the SOCS-1 protein is repressed on translational level by a mechanism, which bears similarities to that postulated for genes like retinoic acid receptor beta2 (RARbeta2), S-adenosylmethionine-decarboxylase (AdoMetDC), Bcl-2, and others.

Identification of an alternatively spliced seprase mRNA that encodes a novel intracellular isoform

Seprase is a homodimeric 170-kDa integral membrane gelatinase that is related to the ectoenzyme dipeptidyl peptidase IV. We have identified an alternatively spliced seprase messenger from the human melanoma cell line LOX that encodes a novel truncated isoform, seprase-s. The splice variant mRNA is generated by an out-of-frame deletion of a 1223-base pair exonic region that encodes part of the cytoplasmic tail, transmembrane, and the membrane proximal-central regions of the extracellular domain (Val(5) through Ser(412)) of the seprase 97-kDa subunit (seprase-l). The seprase-s mRNA has an elongated 5' leader (548 nucleotides) that harbors at least two upstream open reading frames that inhibit seprase-s expression from a downstream major open reading frame. Deletion mutagenesis of the wild type splice variant cDNA confirms that initiation of the seprase-s coding sequence begins with an ATG codon that corresponds to Met(522) of seprase-l. The seprase-s open reading frame encodes a 239-amino acid polypeptide with an M(r) approximately 27,000 that precisely overlaps the carboxyl-terminal catalytic region of seprase-l.

Metazoan nuclear genes for mitoribosomal protein S12

We have characterized nuclear genes for mitoribosomal protein S12 (mt-rps12) a major component of the ribosomal accuracy centre, in human, mouse and Drosophila melanogaster. In human and Drosophila, and probably also in mouse, there is a single intron within the coding region, located in the mitochondrial targeting pre-sequence. In humans, the mRNA structure is highly suggestive of translational regulation. In all three species, there is an amino-acid substitution with respect to eubacterial homologues in a residue implicated in aminoglycoside resistance. The only viable mutant allele of the Drosophila gene, associated with a bang-sensitive phenotype (paralysis upon mechanical vibration, arising from a mechanoreceptor cell defect) also has a novel substitution in a conserved region implicated in translational fidelity. Given the involvement of the mitoribosomal accuracy centre in human sensorineural deafness by virtue of rRNA mutations, our results indicate that this fly mutant may be a useful animal model of this disorder, and earmark the gene for mt-rps12 as a candidate in human hearing impairment.