Involvement of upstream open reading frames in regulation of rat V(1b) vasopressin receptor expression

Responsiveness to DDAVP in Cushing's disease is associated with USP8 mutations through enhancing AVPR1B promoter activity

PURPOSE

To clarify the characteristics of Cushing's disease (CD) patients who respond to the desmopressin (DDAVP) test and its underlying mechanisms.

METHODS

Forty-seven patients with CD who underwent DDAVP testing were included. Patients were divided into two groups: DDAVP test (+) (adrenocorticotropic hormone [ACTH] levels increased by ≥ 1.5-fold during the DDAVP test) and DDAVP test (-) (ACTH levels increased by < 1.5-fold). AVP receptor expression levels in these tumors were quantified using quantitative RT-PCR and immunohistochemistry. AVP receptor promoter activity was analyzed using a dual-luciferase reporter assay system.

RESULTS

Females (96.9%) and USP8 mutants (85.7%) were more prevalent in the DDAVP test (+) than in the DDAVP test (-). Indeed, the ACTH and cortisol responsiveness to DDAVP was greater in USP8 mutation positive tumors than that in USP8 wild type tumors (3.0-fold vs. 1.3-fold, 1.6-fold vs. 1.1-fold, respectively). Responsiveness to DDAVP was correlated with the expression levels of AVPR1B, but not with those of AVPR2. Comparably, Avpr1b promoter activity was enhanced by the overexpression of mutant USP8 compared to the wild type.

CONCLUSIONS

We found that the responsiveness of ACTH to DDAVP in CD was greater in tumors with USP8 mutations. The present data suggest that USP8 mutations upregulate the AVPR1B promoter activity. Additionally, we showed that the DDAVP test can predict the presence of USP8 mutations.

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.

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.

uAUG and uORFs in human and rodent 5′untranslated mRNAs

The control of translation is a fundamental mechanism in the regulation of gene expression. Among the cis-acting elements that play a role in translation regulation are upstream open reading frames (uORFs) and upstream AUG (uAUGs) located in the 5'UTR of mRNAs. We present here a genome-wide analysis of uAUGs and uORFs in a curated set of human and rodent mRNAs. Our study shows that the occurrence of uAUGs is suppressed more strongly than that of uORFs and that in-frame uAUGs are more strongly suppressed than out-of-frame uAUGs. A very similar pattern of uAUG/uORF frequency was also observed in mouse mRNAs. The analysis of orthologous 5'UTR sequences revealed a remarkable degree of evolutionary conservation only of those uORFs which acquired some functional activity. Our data suggest that besides leaky scanning and reinitiation, which likely occur with variable and gene-specific efficiency, the ribosome-shunt mechanism, eventually coupled to reinitiation after uORF translation, may be a widespread mode of translation regulation in eukaryotes.

Regulation of vasopressin V1b receptors and stress adaptation

Vasopressin (VP) regulates pituitary corticotroph function by acting upon plasma membrane G-protein receptors of the V1b subtype (V1bR), coupled to calcium-phospholipid signaling. The number of these receptors in the anterior pituitary varies during stress in direct correlation with corticotroph responsiveness, suggesting that the V1bR plays an important role during adaptation of the hypothalamic-pituitary-adrenal (HPA) axis to stress. The molecular regulation of pituitary V1bR involves transcriptional and translational mechanisms. V1bR gene transcription, which is necessary to maintain V1bR mRNA levels, depends on a number of responsive elements in the promoter region, of which the stretch of GA repeats near the transcription start point (GAGA box) is essential. Although transcriptional activation is necessary to maintain V1bR mRNA levels, the lack of correlation between VP binding and V1bR mRNA suggests that V1bR content is mainly regulated at the translational level. Two potential mechanisms by which the 5' untranslated region (5'UTR) of the V1bR mediates negative and positive regulation of V1bR translation were identified. This includes the repressor effect of small open reading frames (ORF) present upstream of the main V1bR ORF, and an internal ribosome entry site (IRES), which activates V1bR translation. The existence of multiple loci of regulation for the V1bR at transcriptional and translational levels provides a mechanism to facilitate plasticity of regulation of the number of pituitary vasopressin receptors according to physiological demand.

Vasopressinergic regulation of the hypothalamic pituitary adrenal axis and stress adaptation

Vasopressin (VP) stimulates pituitary ACTH secretion through interaction with receptors of the V1b subtype (V1bR, V3R), located in the plasma membrane of the pituitary corticotroph, mainly by potentiating the stimulatory effects of corticotropin releasing hormone (CRH). Chronic stress paradigms associated with corticotroph hyperresponsiveness lead to preferential expression of hypothalamic VP over CRH and upregulation of pituitary V1bR, suggesting an important role for VP during adaptation of the hypothalamic-pituitary-adrenal (HPA) axis to stress. Vasopressinergic regulation of ACTH secretion depends on the number of V1bRs as well as coupling of the receptor to phospholipase C (PLC) in the pituitary. Regulation of V1bR gene transcription may involve a number of regulatory elements in the promoter region, of which a GAGA box was shown to be essential. Although V1bR gene transcription is necessary to maintain V1bR mRNA levels, the lack of correlation between VP binding and V1bR mRNA suggests that regulation of mRNA translation is a major regulatory step of the number of V1bRs. V1bR translation appears to be under tonic inhibition by upstream minicistrons and positive regulation through protein kinase C (PKC) activation of an internal ribosome entry site (IRES) in the 5' untranslated region (5'UTR) of the mRNA. The data provide mechanisms by which regulation of hypothalamic VP and pituitary V1bR content contribute to controlling HPA axis activity during chronic stress.

Alternative splicing: a novel mechanism to fine-tune the expression and function of the human AT1 receptor

Activation of the angiotensin II type 1 (AT(1)) receptor is closely involved in the pathogenesis of cardiovascular diseases; therefore, aberrant regulation of the production of this receptor might play a role in these disorders. Currently, there is strong evidence to suggest that the predominant mechanism regulating the number of AT(1) receptors is the modulation of mRNA stability. Here, we discuss the importance of alternative splicing as an additional post-transcriptional mechanism regulating human AT(1) receptor number and function.

Recognition of a novel stage of betaherpesvirus latency in human herpesvirus 6

Latency-associated transcripts of human herpesvirus 6 (H6LTs) (K. Kondo et al. J. Virol. 76:4145-4151, 2002) were maximally expressed at a fairly stable intermediate stage between latency and reactivation both in vivo and in vitro. H6LTs functioned as sources of immediate-early protein 1 at this stage, which up-regulated the viral reactivation.

Untranslated regions of mRNAs

Gene expression is finely regulated at the post-transcriptional level. Features of the untranslated regions of mRNAs that control their translation, degradation and localization include stem-loop structures, upstream initiation codons and open reading frames, internal ribosome entry sites and various cis-acting elements that are bound by RNA-binding proteins.

Constraints on reinitiation of translation in mammals

The efficiency of reinitiation in mammalian translation systems depends in part on the size and arrangement of upstream open reading frames (upORFs). The gradual decrease in reinitiation as an upORF is lengthened, confirmed here using a variety of sequences, might reflect time-dependent loss of protein factors required for reinitiation. Consistent with the idea that the duration of elongation is what matters, reinitiation was nearly abolished when a pseudoknot that causes a pause in elongation was inserted into a short upORF. Control experiments showed that this transient pause in elongation had little effect on the final protein yield when the pseudoknot was moved from the upORF into the main ORF. Thus, the deleterious effect of slowing elongation is limited to the reinitiation mode. Another aspect of reinitiation investigated here is whether post-termination ribosomes can scan backwards to initiate at AUG codons positioned upstream from the terminator codon. Earlier studies that raised this possibility may have been complicated by the occurrence of leaky scanning along with reinitiation. Re-examination of the question, using constructs that preclude leaky scanning, shows barely detectable reinitiation from an AUG codon positioned 4 nt upstream from the terminator codon and no detectable reinitiation from an AUG codon positioned farther upstream. These experiments carried out with synthetic transcripts help to define the circumstances under which reinitiation may be expected to occur in the growing number of natural mRNAs that deviate from the simple first AUG rule.