Protein kinase C and protein kinase A regulate the expression of angiotensin II receptor mRNA in smooth muscle cells

Regulation of the renin-angiotensin system (RAS) in BeWo and HTR-8/SVneo trophoblast cell lines

OBJECTIVES

The renin-angiotensin system (RAS) is implicated in placentation. We determined which RAS pathways are present in two trophoblast cell lines (HTR-8/SVneo and BeWo cells) and the effects of cAMP, which stimulates renal renin.

STUDY DESIGN

The effect of cAMP on RAS gene expression and on prorenin and angiotensin peptides in HTR-8/SVneo and BeWo cells were investigated.

RESULTS

In HTR-8/SVneo cells, prorenin mRNA (REN) and protein, (pro)renin receptor (ATP6AP2) and angiotensin II type 1 receptor (AGTR1) were stimulated by cAMP (P < 0.05, P < 0.05, P < 0.001 and P < 0.05, respectively). HTR-8/SVneo cells also expressed angiotensinogen (AGT) and angiotensin converting enzyme 1 (ACE1), but did not express AGTR2 or ACE2 nor the Ang 1-7 receptor (MAS1). BeWo cells did not express REN, and REN was not inducible by cAMP, but cAMP increased ACE2 and MAS1 (both P < 0.05) and decreased AGT (P < 0.05). BeWo cells expressed AGT, ACE1, ACE2 and MAS1 but not ATP6AP2, AGTR1 nor AGTR2. There was net destruction of Ang II in media from HTR-8/SVneo and BeWo incubations and net production of Ang 1-7 by BeWo and untreated HTR-8/SVneo cells.

CONCLUSION

HTR-8/SVneo cells express REN and produce prorenin as well as expressing other RAS genes likely to regulate Ang II/AT(1)R interactions and respond to cAMP, like renal renin-secreting cells. They are more similar to early gestation placentae and are therefore useful for studying effects of renin/ACE/Ang II/AT₁R on cell function. BeWo cells express the ACE2/Ang 1-7/Mas pathway, which is sensitive to cAMP and therefore are useful for studying the effects of ACE2/Ang 1-7/Mas on trophoblast function.

Role of protein kinase Cβ in phorbol ester-induced c-fos gene expression in neurons of normotensive and spontaneously hypertensive rat brains

We have previously demonstrated that pressure application of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) onto some neurons in the anterior hypothalamic area of rats increases neural activity in vivo and that this PKC activation-induced increase of neural activity is enhanced in spontaneously hypertensive rats (SHR), an animal model for genetic hypertension. Activation of PKC increases expression of the c-fos gene, an important transcription factor and proto-oncogene thought to be a marker of neural activity. To evaluate PKC isoforms responsible for neural activation, we examined which isoforms of PKC are involved in the PKC activation-induced c-fos gene expression in neuronal cultures of Wistar rat and spontaneously hypertensive rat (SHR) brains. PMA increased c-fos gene expression in neuronal cultures of Wistar rat brain and the PMA-induced c-fos gene expression was inhibited by the PKC inhibitor 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7). The PKCalpha,beta,gamma activator thymeleatoxin also increased c-fos gene expression, while the PKCdelta,epsilon activator ingenol did not affect it. In addition, the PMA-induced c-fos gene expression was inhibited by PKCbetaantisense oligonucleotides (AON) but not by PKCalpha and PKCgammaAONs. In SHR brain neuronal cultures, the PMA-induced c-fos gene expression was enhanced as compared with that of Wistar Kyoto rats (WKY), while basal c-fos gene expression was almost the same in both neuronal cultures. The enhancement of PMA-induced c-fos gene expression in SHR brain cultures was abolished by PKCbetaAON. These findings suggest that in rat brain neuronal cultures, PMA increases c-fos gene expression via activation of PKC and that PKCbetaisoforms are partly involved in the PMA-induced c-fos gene expression. In neuronal cultures of SHR brain, it appears that the PMA-induced c-fos gene expression is also enhanced via PKCbeta.

Mechanisms of vascular angiotensin II surface receptor regulation by epidermal growth factor

We investigated mechanisms by which epidermal growth factor (EGF) reduces angiotensin II (AngII) surface receptor density and stimulated actions in vascular smooth muscle cells (VSMC). EGF downregulated specific AngII radioligand binding in intact cultured rat aortic smooth muscle cells but not in cell membranes and also inhibited AngII-stimulated contractions of aortic segments. Inhibitors of cAMP-dependent kinases, PI-3 kinase, MAP kinase, cyclooxygenase, and calmodulin did not prevent EGF-mediated downregulation of AngII receptor binding, whereas the EGF receptor kinase inhibitor AG1478 did. Total cell AngII AT1a receptor protein content of EGF-treated and untreated cells, measured by immunoblotting, did not differ. Actinomycin D or cytochalasin D, which interacts with the cytoskeleton, but not the protein synthesis inhibitor cycloheximide, prevented EGF from downregulating AngII receptor binding. Consistently, EGF inhibited AngII-stimulated formation of inositol phosphates in the presence of cycloheximide but not in the presence of actinomycin D or cytochalasin D. In conclusion, EGF needs an intact signal transduction pathway to downregulate AngII surface receptor binding, possibly by altering cellular location of the receptors.

Angiotensin II and cAMP regulate AT(1)-mRNA expression in rat cardiomyocytes by transcriptional mechanism

Mechanisms of angiotensin II and cAMP regulating the expression of angiotensin II type 1 (AT(1)) receptor mRNA were studied in neonatal rat cardiomyocytes. Angiotensin II induced a transient decrease of AT(1)-mRNA expression in time- and dose-dependent manner. Maximal decrease (49.2 +/- 9.5% of control) occurred at 6 h of angiotensin II (10 nmol/l) treatment. AT(1) receptor antagonists 4-ethyl-2-n-propyl-1-[2'-(1H-tetrazol-5-yl)biphenyl-4-yl)methyl]imidazole-5-carboxylic acid (DMP811) and losartan as well as 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7) reversed the down-regulation of AT(1)-mRNA expression. 6 h of phorbol 12-myristate 13-acetate (PMA) stimulation caused a decrease of AT(1)-mRNA level. Treatment by angiotensin II plus actinomycin D for 6 h produced the same effect as actinomycin D alone. These results suggest that angiotensin II down-regulates AT(1)-mRNA level of rat cardiomyocytes by inhibiting the transcription of AT(1) gene, which is mediated by AT(1) receptor and related to the activation of protein kinase C. Stimulation by forskolin plus 3-isobutyl-1-methyl-xanthine (IBMX) decreased the expression of AT(1)-mRNA to 68.1 +/- 21.5% of control at 6 h treatment; while increased to 207.9 +/- 27.1% of control at 48 h treatment. A series of 5'-upstream deletion mutants of AT(1A) promoter were produced and then were recombined with pGL(3) basic vector utilizing luciferase as reporter gene. Among all the constructors, p(-201/+ 74)Luc was of the highest luciferase activity (5.9 times higher than control) after stimulation by forskolin for 48 h. Further deletion from -201 to -61 resulted in a large decrease of activity. These results indicate that cAMP induces a time-dependent bi-directional regulation of AT(1)-mRNA expression. The cAMP responsible element (CRE) cis-element located in the region -201/-61 of rat AT(1A) promoter is forskolin inducible, which may mediate the up-regulation of AT(1)-mRNA expression induced by cAMP long-lasting stimulation.

Reconstitution of angiotensin receptor mRNA down-regulation in vascular smooth muscle. Post-transcriptional control by protein kinase a but not mitogenic signaling directed by the 5'-untranslated region

Cell surface receptor activation generally leads to changes in mRNA abundance, which may involve regulatory targets in processes working at the post-transcriptional level. Many types of agonists down-regulate vascular smooth muscle angiotensin receptor (AT(1)-R) gene expression, but it is unclear which of these activate post-transcriptional mechanisms. To reconstitute faithfully the normal AT(1)-R mRNA regulatory environment, tetracycline-suppressible promoters drive highly accurate recombinant AT(1)-R mRNA mimics in vascular smooth muscle cells that co-express an endogenous AT(1)-R mRNA. Down-regulation of the latter occurs shortly after stimulating mitogenic receptors or by using forskolin, but only cAMP signaling reduces expression of the recombinant AT(1)-R mRNA. Transcription of the recombinant mRNA is unaffected by cAMP signaling. Deletions of the AT(1)-R mRNA 3'-untranslated region do not impair cAMP-mediated down-regulation. Both loss of function and gain of function mutants show the response is mediated by the 5'-untranslated region. These observations provide the first direct functional evidence for modulation of vascular AT(1)-R gene expression by a mechanism involving a protein kinase A-regulated post-transcriptional process.

B2 kinin receptor upregulation by cAMP is associated with BK-induced PGE2 production in rat mesangial cells

In the rat mesangial cell (MC), activation of the bradykinin B2 receptor (B2R) by bradykinin (BK) is associated with both phospholipase C (PLC) and A2 (PLA2) activities and with inhibition of adenosine 3',5'-cyclic monophosphate (cAMP) formation leading to cell contraction. Because cAMP plays an important role in the regulation of gene expression in general, we investigated the effect of increasing the intracellular cAMP concentration ([cAMP]i) in mesangial cells on the B2 mRNA expression, on the density of B2 receptor binding sites, on the BK-induced increase in both the free cytosolic Ca2+ concentration ([Ca2+]i), and in the prostaglandin E2 (PGE2) production. Forskolin, PGE2, and cAMP analog, 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP), were used to increase [cAMP]i. Twenty-four-hour treatment with forskolin, PGE2, and 8-BrcAMP resulted in significant increases in B2 receptor binding sites, which were inhibited by cycloheximide. The maximum B2 receptor mRNA expression (160% above control) was observed in cells treated during 24 h with forskolin and was prevented by actinomycin D. In contrast, the D-myo-inositol 1,4,5-trisphosphate (IP3) formation and the BK-induced increase in [Ca2+]i, reflecting activation of PLC, were not affected by increased levels of [cAMP]i. However the BK-induced PGE2 release, reflecting PLA2 activity, was significantly enhanced. These data bring new information regarding the dual signaling pathways of B2 receptors that can be differentially regulated by cAMP.

The vascular smooth muscle type I angiotensin II receptor mRNA is destabilized by cyclic AMP-elevating agents

Although processes involved in mRNA degradation play a significant role in dictating steady state mRNA levels, the influence of cell surface signaling on mRNA stability control is understood incompletely. In this study, the effects of cAMP-elevating agents on type I angiotensin II receptor (AT1-R) mRNA levels were assessed in cultured rat aortic vascular smooth muscle cells (VSMCs). AT1-R mRNA levels are rapidly reduced by forskolin treatment, in which the maximal effect yields an 80% reduction in AT1-R mRNA levels after 6 hr of treatment. The rate of AT1-R mRNA decay in response to forskolin is greater than its apparent intrinsic decay, as assessed in the presence of the transcriptional inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, suggesting forskolin treatment destabilizes the AT1-R mRNA. Nuclear run-on analysis indicates forskolin treatment does not affect transcription of the AT1-R gene in VSMCs, implying induced AT1-R mRNA destabilization accounts for the entire effect of forskolin in decreasing AT1-R mRNA levels. Dose-effect studies that assessed AT1-R mRNA levels and cAMP production were conducted using forskolin and the beta-adrenergic receptor agonist isoproterenol as agonists. Isoproterenol is almost 3 orders of magnitude more potent at eliciting the reduction in AT1-receptor mRNA levels than it is at stimulating cAMP production. Similarly, forskolin elicits reductions in AT1-R mRNA, which occur at concentrations that fail to elicit a detectable production of cAMP. However, protein kinase A activity is stimulated maximally by isoproterenol and forskolin concentrations that do not stimulate detectable cAMP production. These data provide evidence that the mechanism for down-regulation of AT1-R mRNA levels by cAMP-elevating agents in VSMCs occurs via a PKA-regulated mRNA destabilization pathway.

Role of renal nerves in afferent arteriolar reactivity in angiotensin-induced hypertension

The objective of this study was to determine the contribution of renal nerves to the enhanced afferent arteriolar reactivity observed in angiotensin II (Ang II)-induced hypertension. Uninephrectomized Sprague-Dawley rats were divided into four groups: sham rats, renal-denervated rats, Ang II-infused (at 40 ng/min for 13 days) rats, and Ang II-infused+renal-denervated rats. With the use of an implanted arterial catheter, mean arterial pressure (MAP) was monitored in conscious rats. Ang II infusion resulted in a progressive increase in MAP from 98 +/- 1 (day 0) to 166 +/- 7 mm Hg (day 13). This increase in MAP was attenuated in denervated rats and averaged 136 +/- 3 mm Hg on day 13. Kidneys were harvested on day 13 for microcirculatory experiments or measurement of intrarenal Ang II levels. Basal afferent arteriolar diameter was similar in all groups, and group averages ranged from 19.6 to 20.7 microns. Chronic Ang II infusion increased intrarenal Ang II levels. Renal denervation did not alter this effect. Increasing perfusion pressure from 100 to 160 mm Hg reduced afferent arteriolar diameter significantly by 11.2 +/- 0.6% in the sham group and by a similar degree in the remaining three groups. Superfusion with Ang II (10 nmol/L) reduced afferent arteriolar diameter by 34.3 +/- 2.0% in the sham group. This response was enhanced in Ang II-infused (62.3 +/- 3.4%) but not in renal-denervated or Ang II-infused+renal-denervated rats. Additionally, the enhanced afferent arteriolar reactivity to Ang II was not influenced by adrenergic receptor blockade. The afferent arteriolar response to norepinephrine was enhanced in renal-denervated, Ang II-infused, and Ang II-infused+renal-denervated rats compared with sham controls. Administration of the calcium ionophore A23187 decreased afferent arteriolar diameter similarly in all four groups. These results indicate that renal nerves contribute to the development of hypertension and to the enhanced afferent arteriolar responsiveness to Ang II elicited by chronic Ang II infusion.

Down-regulation of beta3 adrenoreceptor gene expression in brown fat cells is transient and recovery is dependent upon a short-lived protein factor

The regulation of the expression of the beta3 adrenoreceptor gene was examined in the brown adipose tissue of intact mice and in murine brown fat primary cell cultures. Both in vivo and in vitro, high levels of beta3 receptor mRNA were observed. Acute cold exposure of mice resulted in a marked and rapid down-regulation of beta3 gene expression; this down-regulation was, however, transient. Similarly, in brown fat cell cultures, norepinephrine addition led to down-regulation of beta3 gene expression, with a lag phase of 30 min and with an apparent half-life of beta3 mRNA of approximately 30 min. This down-regulation was stimulated via the beta3 receptors themselves and mediated via cAMP; the apparent affinity of norepinephrine was extremely high (<1 nM). The degradation rate after actinomycin was identical to that after norepinephrine and was not affected by the presence of norepinephrine; thus, the down-regulation was due to cessation of transcription but not to an increased rate of degradation. Notably, inhibition of protein synthesis by cycloheximide also led to down-regulation. The norepinephrine-induced down-regulation was transient; spontaneous recovery occurred after approximately 18 h and was not due to depletion of adrenergic agent. Recovery did not occur in the presence of cycloheximide. After recovery, the cells showed a functional desensitization of the down-regulation process itself (EC50 now approximately 10 nM). It is concluded that a down-regulated state cannot explain the functional desensitization of beta3 adrenergic responsiveness observed in brown fat cells isolated from cold-acclimated animals (i.e. physiologically chronically adrenergically stimulated brown fat cells); since the beta3 receptor is not subject to desensitization via phosphorylation processes, no satisfactory explanation for the functional desensitization exists as yet. A model is presented for the down-regulation/recovery process, involving the participation of a phosphorylatable short-lived transcription factor.