Regulation of cytosolic proteins binding cis elements in the 5' leader sequence of the angiotensin AT1 receptor mRNA

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.

Translational regulation of ANG II type 1 receptors in proliferating vascular smooth muscle cells

The current study examined angiotensin receptor (ATR) regulation in proliferating rat aortic vascular smooth muscle cells (VSMCs) in culture. Radioligand competition analysis coupled with RNase protection assays (RPAs) revealed that angiotensin type 1a receptor (AT1aR) densities (Bmax) increased by 30% between 5 and 7 days in culture [Bmax (fmol/mg protein): day 5, 379 ± 8.4 vs. day 7, 481 ± 12, n = 3, P < 0.05] under conditions in which no significant changes in AT1aR mRNA expression occurred [in RPA arbitrary units (AU): day 5, 0.23 ± 0.01 vs. day 7, 0.24 ± 0.04, n = 4] or in mRNA synthesis determined by nuclear run-on assays [AU: day 5, 0.35 ± 0.14 vs. day 7, 0.33 ± 0.11, n = 5]. In contrast, polysome distribution analysis indicated that AT1aR mRNA was more efficiently translated in day 7 cells compared with day 5 [% of AT1aR mRNA in fraction 2 out of total AT1R mRNA recovered from the sucrose gradient: day 5, 20.9 ± 9.9 vs. day 7, 56.8 ± 5.6, n = 3, P < 0.001]. Accompanying the polysome shift was 50% less RNA-protein complex (RPC) formation between VSMC cytosolic RNA binding proteins in day 7 cells compared with 5-day cultures and the 5′ leader sequence (5′LS) of the AT1aR [5′LS RPC (AU): day 5, 0.62 ± 0.15 vs. day 7, 0.23 ± 0.03; n = 4, P < 0.05] and also with exon 2 [Exon 2 RPC (AU): day 5, 35.0 ± 5.7 vs. day 7, 17.2 ± 3.6; n = 4, P < 0.05]. Taken together, these results suggest that AT1aR expression is regulated by translation during VSMC proliferation in part by RNA binding proteins that interact within exon 2 in the 5′LS of the AT1aR mRNA.

Transforming growth factor beta1 regulates angiotensin II type I receptor gene expression in the extravillous trophoblast cell line SGHPL-4

The angiotensin II type 1 (AT1) receptor, transforming growth factor beta1 (TGFbeta1) and Oncostatin M (OSM) control key pathways that may be important during placentation. Although interactions between them exist in other tissues, trophoblast cells have not been investigated. Extravillous trophoblast cells, SGHPL-4, were stimulated with 10 ng/ml TGFbeta1 +/- 100 ng/ml OSM for 24 h. Real-time PCR showed that AT1 expression increased 2.76-fold [95% confidence interval (CI) = 1-6.74, P = 0.05] in response to TGFbeta1 and 4.21-fold (95% CI = 1.33-11.76, P = 0.03) with TGFbeta1 + OSM. Luciferase reporter gene constructs containing three haplotypes of the 59 flanking region of the AT1 receptor gene were transfected into SGHPL-4 and HepG2 cells and stimulated with 0.1, 1 and 10 ng/ml TGFbeta1 and 50 ng/ml OSM. Responses were dose and cell dependent. Luciferase activity increased in HepG2 cells in response to TGFbeta1 alone or together with OSM (P < 0.001); transcriptional activation differed between AT1 receptor gene haplotypes. In SGHPL-4 cells, luciferase activity was reduced on exposure to low concentrations of TGFbeta1 or high concentrations of TGFbeta1 combined with OSM (P = 0.003); the response was unaffected by haplotype. Interaction between AT1 and TGFbeta1 is a novel observation in trophoblast and suggests new avenues for the study of placentation.

Estrogen upregulates renal angiotensin II AT1 and AT2 receptors in the rat

We studied renal AT1 and AT2 receptors in male, female, ovariectomized and ovariectomized-estrogen-treated Wistar-Hanover and Wistar-Kyoto rats. AT1 receptors and AT1A receptor mRNA predominated, with no significant differences between males and females. AT2 receptor expression was restricted in female rats to the capsule, the transition zone between outer and inner medulla, the endothelium lining the papilla, and arcuate arteries and veins. There were no AT2 receptors in male rats, while male mice express substantial numbers of estrogen-dependent AT2 receptors. Arcuate arteries and veins expressed AT1B mRNA in males and females, and AT2 mRNA in females only. AT1 receptor and AT2 receptor expression were estrogen-dependent, with increases in AT1 and AT2 receptor expression after estrogen treatment in ovariectomized rats. Estrogen treatment increased prostaglandin E2 (PGE2) and cGMP concentrations in the renal medulla, and eNOS expression in cortical arteries. In rodents, expression of renal Angiotensin II receptor types is estrogen-dependent, with significant species, strain and area differences. Our results support an important role for AT2 receptors in the regulation of renal function and in the protective effects of estrogen in the kidney.

Posttranscriptional mechanisms contribute to osmotic regulation of ANG type 1 receptors in cultured rat renomedullary interstitial cells

Previously, we showed that ANG II receptors in cultured rat renomedullary interstitial cells (RMICs) are osmotically regulated (19). The current study examined the mechanisms underlying this osmotic regulation in RMICs cultured in isoosmotic (300 mosmol/kgH2O) and hyperosmotic (600 mosmol/kgH2O) conditions. Radioligand competition analysis coupled with RNase protection assays (RPA) and ligand-mediated receptor internalization studies revealed that RMICs primarily express the type 1a angiotensin receptor (AT(1a)R). When cultured under hyperosmotic conditions, the density (B(max)) of AT1R in RMIC membranes decreased by 31% [B(max) (pmol/mg protein): 300 mosmol/kgH2O, 6.44 +/- 0.46 vs. 600 mosmol/kgH2O, 4.42 +/- 0.37, n = 8, P < 0.01], under conditions in which no detectable changes in AT(1a)R mRNA expression or in the kinetics of ligand-mediated AT1R internalization were observed. RNA electromobility shift assays showed that RNA protein complex (RPC) formation between RMIC cytosolic RNA binding proteins and the 5' leader sequence (5'LS) of the AT(1a)R was increased 1.5-fold under hyperosmotic conditions [5'LS RPC (arbitrary units): 300 mosmol/kgH2O, 0.79 +/- 0.08 vs. 600 mosmol/kgH2O, 1.17 +/- 0.07, n = 4, P < 0.01]. These results suggest that the downregulation of AT(1a)R expression in RMICs cultured under hyperosmotic conditions is regulated at the posttranscriptional level by RNA binding proteins that interact within the 5'LS of the AT(1a)R mRNA. The downregulation of AT(1a)R expression under hyperosmotic conditions may be an important mechanism by which the activity of ANG II is regulated in the hyperosmotic renal medulla.

Corticotropin releasing hormone receptors: two decades later

Hypothalamic corticotropin releasing hormone (CRH) regulates pituitary ACTH secretion and mediates behavioral and autonomic responses to stress, through interaction with type 1 plasma membrane receptors (CRHR1) located in pituitary corticotrophs and the brain. Although the CHRI are essential for ACTH responses to stress, their number in the pituitary gland does not correlate with corticotroph responsiveness, suggesting that activation of a small number of receptors is sufficient for maximum ACTH production. CRH binding and hybridization studies in adrenalectomized, glucocorticoid-treated or stressed rats revealed divergent changes in CRH receptors and CRH1 mRNA in the pituitary, with a reduction in receptor binding but normal or elevated expression of CHR1 mRNA levels. Western blot analysis of CRHR1 protein in pituitary membranes from adrenalectomized rats showed unchanged receptor mRNA levels and increased CRHR1 protein, despite binding down-regulation, suggesting that decreased binding is due to homologous desensitization, rather than reduced receptor synthesis. In contrast, decreased CRH binding following glucocorticoid administration is associated with a reduction in CRHR1 protein, suggesting inhibition of CRH1 mRNA translation. The regulation of CRHR1 translation may involve binding of cytosolic proteins, and a minicistron in the 5'-UTR of the CRHR1 mRNA. It is likely that post-transcriptional regulatory mechanisms that permit rapid changes in CRH receptor activity are important for adaptation of corticotroph responsiveness to continuous changes in physiological demands.

Growth hormone regulation of glomerular AT1 angiotensin receptors in adult uninephrectomized male rats

Sex differences exist in the mechanisms initiating early compensatory renal growth after unilateral nephrectomy (UNX); remnant kidney growth is growth hormone (GH) independent in adult female rats and GH dependent in adult male rats. The present study determined whether sex differences also exist in angiotensin type 1 receptor (AT1R) regulation during early remnant kidney (REM) growth after UNX, and if so, whether GH modulates AT1R expression after UNX in the male rat. Scatchard analysis of radioligand binding in glomeruli demonstrated that 48 h post-UNX, AT1R density (Bmax) was significantly decreased by 20% in female REM compared with control kidneys. In contrast, male REM glomerular Bmax was significantly increased by 28% compared with control kidneys. Furthermore, GH-suppressed male rats displayed attenuated REM growth, which was associated with a 35% decrease in AT1R Bmax. Losartan treatment also decreased REM AT1R Bmax by 55%. The activity of mRNA binding proteins that bind to the 5' leader sequence of the AT1R was regulated by UNX and GH treatment in an inverse manner to AT1R expression. These findings suggest that in rats 1) there are sex differences in the regulation of glomerular AT1R expression after UNX; 2) the increase in AT1R binding sites in the male REM is regulated by GH and mediates early remnant kidney growth; and 3) AT1R 5' leader sequence mRNA binding proteins play a role in UNX and GH regulation of glomerular AT1Rs in both males and females.

Multiple sites of control of type-1 corticotropin releasing hormone receptor levels in the pituitary

Hypothalamic corticotropin releasing hormone (CRH) stimulates pituitary ACTH secretion through interaction with type 1 CRH receptors (CRH-R1), the number of which varies during alterations of the hypothalamic-pituitary-adrenal (HPA) axis. CRH-R1 are essential for ACTH responses to stress but CRH receptor content in the pituitary does not correlate with corticotroph responsiveness. This indicates that a small number of receptors is sufficient for full ACTH responses probably through post-receptor interaction with vasopressin (VP) signaling. CRH binding and hybridization studies in adrenalectomized, glucocorticoid-treated or stressed rats revealed divergent levels of CRH receptors and CRH-R1 mRNA in the pituitary, with binding reductions but normal or elevated CRH-R1 mRNA levels during alterations of the HPA axis. Western blot analysis of CRH-R1 protein in pituitary membranes from adrenalectomized rats show unchanged CRH-R1 mRNA levels, but reduced CRH binding associated with significant increases in CRH-R1 protein, suggesting that the decrease in binding is due to homologous desensitization and not to reduced receptor synthesis. In contrast, decreased CRH binding following glucocorticoid administration is associated with reduction in CRH-R1 protein suggesting inhibition of CRH-R1 mRNA translation. Regulation of CRH-R1 translation may involve binding of cytosolic proteins, and a minicistron in the 5'UTR of the CRH-R1 mRNA. Post-transcriptional regulatory mechanisms allowing rapid changes in CRH receptor activity are important for adaptation of corticotroph responsiveness to continuous change in physiological demand.

Use of positron emission tomography to study AT1 receptor regulation in vivo

Increased sodium intake and enhanced sodium sensitivity are implicated in the pathogenesis of hypertension and in the control of a major regulator of BP, the type 1 angiotensin receptor (AT(1) receptor). An in vivo technique to study changes of renal AT(1) receptors by dietary sodium was developed that uses positron emission tomography (PET). PET revealed that renal cortical AT(1) receptor binding was increased in sodium-loaded compared with sodium-deprived dogs, which correlated with ex vivo estimations of AT(1) receptor numbers. Plasma renin activity, angiotensin II, and aldosterone were inversely related to changes in AT(1) receptor binding. These results demonstrate, for the first time in vivo, that the renal AT(1) receptor is inversely related to the activity of the renin angiotensin system, which may provide a compensatory mechanism to prevent inappropriate fluctuations in arterial BP. The ability to measure AT(1) receptor binding in vivo has potential significance for clinical studies of AT(1) receptors, because PET is a noninvasive imaging technique that is readily applicable in humans.

Angiotensin II binding to renomedullary interstitial cells is regulated by osmolality

Angiotensin II (Ang II) AT(1A) receptors are localized to renomedullary interstitial cells (RMIC) in the inner stripe of the outer medulla but not in the inner medulla. Thus, there seems to be a correlation between decreases in AT(1A) receptor binding to RMIC and increases in interstitial osmolality, suggesting that osmolality is important in determining Ang II binding to RMIC. Cultured RMIC were incubated in media of differing osmolalities (330, 630, 930, and 1230 mOsm/kg H(2)O). (125)I-[Sar(1), Ile(8)] Ang II binding to AT(1A) receptors on RMIC grown in hyperosmolal media (930 mOsm/kg H(2)O) was reduced compared with isoosmolal (330 mOsm/kg H(2)O) media and was progressively reduced with further increases of osmolality. Similar studies were performed using bradykinin (BK) as a control peptide. Binding of the BK receptor ligand (125)I-[HPP-Hoe 140] to B(2) receptors was not affected by varying osmolality of the media. Reverse transcriptase-PCR demonstrated the presence of the mRNA expression for both AT(1A) and B(2) receptors at each osmolality. The conclusion is that osmolality modulates Ang II binding to RMIC; in these cells, this phenomenon is restricted to Ang II as BK binding is not affected. Osmolality-induced changes in Ang II binding may modulate the actions of this peptide on RMIC and provide an important mechanism by which these cells modulate renal medullary function.