Angiotensin AT(1) receptor phosphorylation and desensitization in a hepatic cell line. Roles of protein kinase c and phosphoinositide 3-kinase

Endocytosis and signaling of angiotensin II type 1 receptor

A vasoactive octapeptide angiotensin II (Ang II) hormone is the key regulator of the renin-angiotensin system (RAS). It binds with the two different plasma membrane receptors like angiotensin II type 1 (AT1) and type 2 (AT2) and consequence various biological responses occur. Further, AT1 has two subtypes such as AT1A and AT1B. These angiotensin receptors are classified to be G protein-coupled receptors (GPCRs). The main constituent of RAS is the AT1 receptor (AT1R), and its activation, signal transduction, and regulation have been extensively studied. After Ang II stimulation, the ligand-receptor complexes internalized and trafficked through the early endosome, recycling endosome, and some receptors skipped the recycling endosome and trafficked to the lysosome for metabolic degradation. Moreover, some short sequence motifs located in the carboxyl-terminus (CT) of the receptor play a vital role in the internalization, phosphorylation, subcellular trafficking, signaling, and desensitization. Furthermore, in endocytosis, the various proteins interact with the CT region of the receptor. This chapter highlights the basic mechanism of AT1 receptor internalization, trafficking and signaling in both physiological and pathophysiological conditions.

Blocking FcγRIIB in Smooth Muscle Cells Reduces Hypertension

[Figure: see text].

Receptor tyrosine kinase activation induces free fatty acid 4 receptor phosphorylation, β-arrestin interaction, and internalization

FFA4 (Free Fatty Acid receptor 4, previously known as GPR120) is a G protein-coupled receptor that acts as a sensor of long-chain fatty acids, modulates metabolism, and whose dysfunction participates in endocrine disturbances. FFA4 is known to be phosphorylated and internalized in response to agonists and protein kinase C activation. In this paper report the modulation of this fatty acid receptor by activation of receptor tyrosine kinases. Cell-activation with growth factors (insulin, epidermal growth factor, insulin-like growth factor-I, and platelet-derived growth factor) increases FFA4 phosphorylation in a time- and concentration-dependent fashion. This effect was blocked by inhibitors of protein kinase C and phosphoinositide 3-kinase, suggesting the involvement of these kinases in it. FFA4 phosphorylation did not alter agonist-induced FFA4 calcium signaling, but was associated with decreased ERK 1/2 phosphorylation. In addition, insulin, insulin-like growth factor-I, epidermal growth factor, and to a lesser extent, platelet-derived growth factor, induce receptor internalization. This action of insulin, insulin-like growth factor I, and epidermal growth factor was blocked by inhibitors of protein kinase C and phosphoinositide 3-kinase. Additionally, cell treatment with these growth factors induced FFA4-β-arrestin coimmunoprecipitation. Our results evidenced cross-talk between receptor tyrosine kinases and FFA4 and suggest roles of protein kinase C and phosphoinositide 3-kinase in such a functional interaction.

Heterologous phosphorylation-induced formation of a stability lock permits regulation of inactive receptors by β-arrestins

β-Arrestins are key regulators and signal transducers of G protein-coupled receptors (GPCRs). The interaction between receptors and β-arrestins is generally believed to require both receptor activity and phosphorylation by GPCR kinases. In this study, we investigated whether β-arrestins are able to bind second messenger kinase-phosphorylated, but inactive receptors as well. Because heterologous phosphorylation is a common phenomenon among GPCRs, this mode of β-arrestin activation may represent a novel mechanism of signal transduction and receptor cross-talk. Here we demonstrate that activation of protein kinase C (PKC) by phorbol myristate acetate, G_q/11-coupled GPCR, or epidermal growth factor receptor stimulation promotes β-arrestin2 recruitment to unliganded AT₁ angiotensin receptor (AT₁R). We found that this interaction depends on the stability lock, a structure responsible for the sustained binding between GPCRs and β-arrestins, formed by phosphorylated serine-threonine clusters in the receptor's C terminus and two conserved phosphate-binding lysines in the β-arrestin2 N-domain. Using improved FlAsH-based serine-threonine clusters β-arrestin2 conformational biosensors, we also show that the stability lock not only stabilizes the receptor-β-arrestin interaction, but also governs the structural rearrangements within β-arrestins. Furthermore, we found that β-arrestin2 binds to PKC-phosphorylated AT₁R in a distinct active conformation, which triggers MAPK recruitment and receptor internalization. Our results provide new insights into the activation of β-arrestins and reveal their novel role in receptor cross-talk.

AT1-receptor response to non-saturating Ang-II concentrations is amplified by calcium channel blockers

BACKGROUND

Blockers of angiotensin II type 1 receptor (AT₁R) and the voltage gated calcium channel 1.2 (Ca_V1.2) are commonly used for treatment of hypertension. Yet there is little information about the effect of physiological concentrations of angiotensin II (AngII) on AT₁R signaling and whether there is a reciprocal regulation of AT₁R signaling by Ca_V1.2.

METHODS

To elucidate these questions, we have studied the Ca²⁺ signaling response to physiological and pharmacological AngII doses in HEK293a cells, vascular smooth muscle cells and cardiomyocytes using a Ca²⁺ sensitive dye as the principal sensor. Intra-cellular calcium recordings were performed in presence and absence of Ca_V1.2 blockers. Semi-quantitative imaging methods were used to assess the plasma membrane expression of AT₁R and G-protein activation.

RESULTS

Repeated exposure to pharmacological (100 nM) concentrations of AngII caused, as expected, a down-regulation of the Ca²⁺ response. In contrast, repeated exposure to physiological (1 nM) AngII concentration resulted in an enhancement of the Ca²⁺ response. The up-regulation of the Ca²⁺ response to repeated 1 nM AngII doses and the down-regulation of the Ca²⁺ response to repeated 100 nM Angll doses were not accompanied by a parallel change of the AT₁R plasma membrane expression. The Ca²⁺ response to 1 nM of AngII was amplified in the presence of therapeutic concentrations of the Ca_V1.2 blockers, nifedipine and verapamil, in vascular smooth muscle cells, cardiomyocytes and HEK293a cells. Amplification of the AT₁R response was also observed following inhibition of the calcium permeable transient receptor potential cation channels, suggesting that the activity of AT₁R is sensitive to calcium influx.

CONCLUSIONS

Our findings have implications for the understanding of hyperactivity of the angiotensin system and for use of Ca²⁺ channel blockers as mono-therapy in hypertension.

International Union of Basic and Clinical Pharmacology. XCIX. Angiotensin Receptors: Interpreters of Pathophysiological Angiotensinergic Stimuli [corrected]

The renin angiotensin system (RAS) produced hormone peptides regulate many vital body functions. Dysfunctional signaling by receptors for RAS peptides leads to pathologic states. Nearly half of humanity today would likely benefit from modern drugs targeting these receptors. The receptors for RAS peptides consist of three G-protein-coupled receptors—the angiotensin II type 1 receptor (AT1 receptor), the angiotensin II type 2 receptor (AT2 receptor), the MAS receptor—and a type II trans-membrane zinc protein—the candidate angiotensin IV receptor (AngIV binding site). The prorenin receptor is a relatively new contender for consideration, but is not included here because the role of prorenin receptor as an independent endocrine mediator is presently unclear. The full spectrum of biologic characteristics of these receptors is still evolving, but there is evidence establishing unique roles of each receptor in cardiovascular, hemodynamic, neurologic, renal, and endothelial functions, as well as in cell proliferation, survival, matrix-cell interaction, and inflammation. Therapeutic agents targeted to these receptors are either in active use in clinical intervention of major common diseases or under evaluation for repurposing in many other disorders. Broad-spectrum influence these receptors produce in complex pathophysiological context in our body highlights their role as precise interpreters of distinctive angiotensinergic peptide cues. This review article summarizes findings published in the last 15 years on the structure, pharmacology, signaling, physiology, and disease states related to angiotensin receptors. We also discuss the challenges the pharmacologist presently faces in formally accepting newer members as established angiotensin receptors and emphasize necessary future developments.

Renal molecular mechanisms underlying altered Na+ handling and genesis of hypertension during adulthood in prenatally undernourished rats

In the present study, we investigated the development of hypertension in prenatally undernourished adult rats, including the mechanisms that culminate in dysfunctions of molecular signalling in the kidney. Dams were fed a low-protein multideficient diet throughout gestation with or without α-tocopherol during lactation. The time course of hypertension development followed in male offspring was correlated with alterations in proximal tubule Na+-ATPase activity, expression of angiotensin II (Ang II) receptors, and activity of protein kinases C and A. After the establishment of hypertension, Ang II levels, cyclo-oxygenase 2 (COX-2) and NADPH oxidase subunit expression, lipid peroxidation and macrophage infiltration were examined in renal tissue. Lipid peroxidation in undernourished rats, which was very intense at 60 d, decreased at 90 d and returned to control values by 150 d. During the prehypertensive phase, prenatally undernourished rats exhibited elevated renal Na+-ATPase activity, type 2 Ang II receptor down-regulation and altered protein kinase A:protein kinase C ratio. Stable late hypertension coexisted with highly elevated levels of Ang II-positive cells in the cortical tubulointerstitium, enhanced increase in the expression of p47phox (NADPH oxidase regulatory subunit), marked down-regulation of COX-2 expression, expanded plasma volume and decreased creatinine clearance. These alterations were reduced when the dams were given α-tocopherol during lactation. The offspring of well-nourished dams treated with α-tocopherol exhibited most of the alterations encountered in the offspring of undernourished dams not treated with α-tocopherol. Thus, alterations in proximal tubule Na+ transport, subcellular signalling pathways and reactive oxygen species handling in renal tissue underpin the development of hypertension.

Conventional protein kinase C isoforms mediate phorbol ester-induced lysophosphatidic acid LPA1 receptor phosphorylation

Using C9 cells stably expressing LPA1 receptors fused to the enhanced green fluorescent protein, it was observed that activation of protein kinase C induced a rapid and strong increase in the phosphorylation state of these receptors. Overnight incubation with phorbol esters markedly decreased the amount of conventional (α, βI, βII and γ) and novel (δ) but not atypical (ζ) immunodetected PKC isoforms, this treatment blocks the action of protein kinase on receptor function and phosphorylation. Bis-indolylmaleimide I a general, non-subtype selective protein kinase C inhibitor, and Gö 6976, selective for the isoforms α and β, were also able to block LPA1 receptor desensitization and phosphorylation; hispidin, isoform β-selective blocker partially avoided receptor desensitization. Expression of dominant-negative protein kinase C α or β II mutants and knocking down the expression of these kinase isozymes markedly decreased phorbol ester-induced LPA1 receptor phosphorylation without avoiding receptor desensitization. This effect was blocked by bis-indolyl-maleimide and Gö 6976, suggesting that these genetic interventions were not completely effective. It was also observed that protein kinase C α and β II isozymes co-immunoprecipitate with LPA1 receptors and that such an association was further increased by cell treatments with phorbol esters or lysophosphatidic acid. Our data suggest that conventional protein kinase C α and β isozymes modulate LPA1 receptor phosphorylation state. Receptor desensitization appears to be a more complex process that might involve additional elements.

EGF and angiotensin II modulate lysophosphatidic acid LPA(1) receptor function and phosphorylation state

BACKGROUND

Lysophosphatidic acid (LPA) is a local mediator that exerts its actions through G protein coupled receptors. Knowledge on the regulation of such receptors is scarce to date. Here we show that bidirectional cross-talk exits between LPA(1) and EGF receptors.

METHODS

C9 cells expressing LPA(1) receptor fussed to the enhanced green fluorescent protein were used. We studied intracellular calcium concentration, Akt/PKB phosphorylation, LPA(1) and EGF receptor phosphorylation.

RESULTS

EGF diminished LPA-mediated intracellular calcium response and induced LPA(1) receptor phosphorylation, which was sensitive to protein kinase C inhibitors. Angiotensin II and LPA induced EGF receptor transactivation as evidenced by Akt/PKB phosphorylation through metalloproteinase-catalyzed membrane shedding of heparin-binding EGF and autocrine/paracrine activation of EGF receptors. This process was found to be of major importance in angiotensin II-induced LPA(1) receptor phosphorylation. Attempts to define a role for EGF receptor transactivation in homologous LPA(1) receptor desensitization and phosphorylation suggested that G protein-coupled receptor kinases are the major players in this process, overshadowing other events.

CONCLUSIONS

EGF receptors and LPA(1) receptors are engaged in an intense liaison, in that EGF receptors are capable of modulating LPA(1) receptor function through phosphorylation cascades. EGF transactivation plays a dual role: it mediates some LPA actions, and it modulates LPA(1) receptor function in inhibitory fashion.

GENERAL SIGNIFICANCE

EGF and LPA receptors coexist in many cell types and play key roles in maintaining the delicate equilibrium that we call health and in the pathogenesis of many diseases. The intense cross-talk described here has important physiological and pathophysiological implications.

Mechanisms involved in α1B-adrenoceptor desensitization

α(1B)-Adrenergic receptors mediate many of the actions of the natural catecholamines, adrenaline and noradrenaline. They belong to the seven transmembrane domains G protein-coupled receptor superfamily and exert their actions mainly through activation of Gq proteins and phosphoinositide turnover/calcium signaling. Many hormones and neurotransmitters are capable of inducing α(1B)-adrenergic receptor phosphorylation and desensitization; among them: adrenaline and noradrenaline, phorbol esters, endothelin-I, bradykinin, lysophosphatidic acid, insulin, EGF, PDGF, IGF-I, TGF-β, and estrogens. Key protein kinases for these effects are G protein coupled receptor kinases and protein kinase C. The lipid/protein kinase, phosphoinositide-3 kinase also appears to play a key role, acting upstream of protein kinase C. In addition to the agents employed for cells stimulation, we observed that paracrine/autocrine mediators also participate; these processes include EGF transactivation and sphingosine-1-phosphate production and action. The complex regulation of these receptors unlocks opportunities for therapeutic intervention.

Activation of the epithelial sodium channel by the metalloprotease meprin β subunit

The Epithelial Na(+) Channel (ENaC) is an apical heteromeric channel that mediates Na(+) entry into epithelial cells from the luminal cell surface. ENaC is activated by proteases that interact with the channel during biosynthesis or at the extracellular surface. Meprins are cell surface and secreted metalloproteinases of the kidney and intestine. We discovered by affinity chromatography that meprins bind γ-ENaC, a subunit of the ENaC hetero-oligomer. The physical interaction involves NH(2)-terminal cytoplasmic residues 37-54 of γ-ENaC, containing a critical gating domain immediately before the first transmembrane domain, and the cytoplasmic COOH-terminal tail of meprin β (residues 679-704). This potential association was confirmed by co-expression and co-immunoprecipitation studies. Functional assays revealed that meprins stimulate ENaC expressed exogenously in Xenopus oocytes and endogenously in epithelial cells. Co-expression of ENaC subunits and meprin β or α/β in Xenopus oocytes increased amiloride-sensitive Na(+) currents approximately two-fold. This increase was blocked by preincubation with an inhibitor of meprin activity, actinonin. The meprin-mediated increase in ENaC currents in oocytes and epithelial cell monolayers required meprin β, but not the α subunit. Meprin β promoted cleavage of α and γ-ENaC subunits at sites close to the second transmembrane domain in the extracellular domain of each channel subunit. Thus, meprin β regulates the activity of ENaC in a metalloprotease-dependent fashion.

Angiotensin-induced EGF receptor transactivation inhibits insulin signaling in C9 hepatic cells

To investigate the potential interactions between the angiotensin II (Ang II) and insulin signaling systems, regulation of IRS-1 phosphorylation and insulin-induced Akt activation by Ang II were examined in clone 9 (C9) hepatocytes. In these cells, Ang II specifically inhibited activation of insulin-induced Akt Thr(308) and its immediate downstream substrate GSK-3alpha/beta in a time-dependent fashion, with approximately 70% reduction at 15 min. These inhibitory actions were associated with increased IRS-1 phosphorylation of Ser(636)/Ser(639) that was prevented by selective blockade of EGFR tyrosine kinase activity with AG1478. Previous studies have shown that insulin-induced phosphorylation of IRS-1 on Ser(636)/Ser(639) is mediated mainly by the PI3K/mTOR/S6K-1 sequence. Studies with specific inhibitors of PI3K (wortmannin) and mTOR (rapamycin) revealed that Ang II stimulates IRS-1 phosphorylation of Ser(636)/Ser(639) via the PI3K/mTOR/S6K-1 pathway. Both inhibitors blocked the effect of Ang II on insulin-induced activation of Akt. Studies using the specific MEK inhibitor, PD98059, revealed that ERK1/2 activation also mediates Ang II-induced S6K-1 and IRS-1 phosphorylation, and the impairment of Akt Thr(308) and GSK-3alpha/beta phosphorylation. Further studies with selective inhibitors showed that PI3K activation was upstream of ERK, suggesting a new mechanism for Ang II-induced impairment of insulin signaling. These findings indicate that Ang II has a significant role in the development of insulin resistance by a mechanism that involves EGFR transactivation and the PI3K/ERK1/2/mTOR-S6K-1 pathway.

Validation of an in vitro model for assessment of androstenedione hepatotoxicity using the rat liver cell line clone-9

Androstenedione, a naturally occurring steroid hormone, has been used to enhance athletic performance. Little is known, however, about its hepatotoxicity. Clone-9 cells, a non-transformed epithelial cell line that was originally isolated from normal liver of a 4-week old Sprague-Dawley rat, were used as an in vitro model to assess the hepatotoxic potential of androstenedione. The cultures were treated with androstenedione for 24 h at 37 degrees C in 5% CO(2) at concentrations of 0-100 microg ml(-1). After the treatment period, the cells and the culture supernatants were assayed for markers of cytotoxicity which included: release of liver enzymes, cell viability, cellular double-stranded DNA content, oxidative stress, steatosis, cellular ATP content, caspase-3 activity, the mitochondrial permeability transition and induction of cytochrome P450 activity. Significant concentration-dependent differences from control were observed in some endpoints at medium concentrations of 10 microg ml(-1) and above. These in vitro findings were compared with comparable endpoints obtained from an in vivo study of androstenedione toxicity in female Sprague-Dawley rats. Of the eight endpoints that could be compared between the two studies, only three (lipid accumulation, ATP depletion and P450 activity) appeared to be concordant. This suggests that, under the experimental conditions used, the clone-9 cells were not a good model for androstenedione hepatotoxicity.

Differential signaling pathways in angiotensin II- and epidermal growth factor-stimulated hepatic C9 cells

Caveolin1 (Cav1) is an important component of the plasmamembrane microdomains, such as caveolae/lipid rafts, that are associated with angiotensin II type 1 (AT(1)) and epidermal growth factor (EGF) receptors in certain cell types. The interactions of Cav1 with other signaling molecules that mediate AT(1) receptor function were analyzed in angiotensin II (Ang II)- and EGF-stimulated hepatic C9 cells. This study demonstrated that cholesterol-rich domains mediate the actions of early upstream signaling molecules such as Src and intracellular Ca(2+) in cells stimulated by Ang II, but not by EGF, and that Cav1 has a scaffolding role in the process of mitogen-activated protein kinase activation. Furthermore, Cav1 phosphorylation by Ang II and EGF was regulated by intracellular Ca(2+) and Src, further indicating reciprocal interactions among Cav1, Src, and intracellular Ca(2+) through the AT(1) receptor. Phosphorylation of Cav1 and the EGF receptor by Ang II, but not of extracellular signal-regulated kinase 1/2, was dependent on intracellular Ca(2+). The phosphatidylinositol 3-kinase inhibitors, 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride (LY294002) and wortmannin, differentially modulated both Cav1 and EGF receptor activation by Ang II through intracellular Ca(2+). These findings further demonstrate the importance of Cav1 in conjunction with the receptor-mediated signaling pathways involved in cell proliferation and survival. It is clear that differential signaling pathways are operative in Ang II- and EGF-stimulated C9 cells and that cholesterol-enriched microdomains are essential components in cellular signaling processes that are dependent on specific agonists and/or cell types.

Angiotensin II type 2 receptor decreases ischemia reperfusion induced fluid leak

BACKGROUND

Intravascular volume loss from ischemia-reperfusion (IR) injury is a major clinical concern. We hypothesize that angiotensin II decreases IR-mediated microvascular fluid leak via the angiotensin II type 2 receptor in a cAMP dependent manner. We therefore sought to determine hydraulic permeability after IR of venules treated with 1) angiotensin II, 2) angiotensin II and cAMP synthesis inhibitor, and 3) angiotensin II and an angiotensin II type 2 receptor antagonist.

METHODS

Rat mesenteric post-capillary venules were micro-cannulated to measure hydraulic permeability (L(p)). IR was achieved by placing animals in a 5% oxygen environment and preventing venular flow, after which blood flow was allowed to resume. L(p) was measured after IR and treatment with 1) angiotensin II (20 nm), 2) angiotensin II (20 nm) + cAMP synthesis inhibitor (DDA,10uM), and 3) angiotensin II (20 nm) + type 2 receptor antagonist (PD-123319, 300 mum) (n=6 in each group).

RESULTS

Compared with the seven-fold increase in L(p) because of IR alone: 1) angiotensin II attenuated the seven-fold increase by 50% (P<0.005), 2) cAMP inhibition did not change the effect of angiotensin II on leak, and the type 2 receptor antagonist completely blocked the effects of angiotensin II on IR mediated leak.

CONCLUSION

Treatment with angiotensin II attenuated increases in hydraulic permeability because of IR by 50%. Inhibition of cAMP synthesis did not change the effect of angiotensin II on IR mediated leak, and angiotensin II type 2 receptor inhibition completely blocked the effects of angiotensin II. These results indicate that angiotensin II decreases IR induced leak via the angiotensin II type 2 receptor in a cAMP independent manner. A better understanding of mediators that reduce intravascular fluid loss from IR-induced microvascular dysfunction may help clinicians treat uncontrolled fluid extravasation that occurs during shock and sepsis.

AT1 receptor blocker-insensitive mutant AT1A angiotensin receptors reveal the presence of G protein-independent signaling in C9 cells

Although mutant receptors are highly useful to dissect the signal transduction pathways of receptors, they are difficult to study in physiological target tissues, due to the presence of endogenous receptors. To study AT(1) angiotensin receptors in their physiological environment, we constructed a mutant receptor, which differs only from the AT(1A) receptor in its reduced affinity for candesartan, a biphenylimidazole antagonist. We have determined that the conserved S109Y substitution of the rat AT(1A) receptor eliminates its candesartan binding, without exerting any major effect on its angiotensin II and peptide angiotensin receptor antagonist binding, internalization kinetics, beta-arrestin binding, and potency or efficacy of the inositol phosphate response. To demonstrate the usefulness of this mutant receptor in signal transduction studies, we combined it with substitution of the highly conserved DRY sequence with AAY, which abolishes G protein activation. In rat C9 hepatocytes the S109Y receptor caused ERK activation with the same mechanism as the endogenous AT(1) receptor. After combination with the DRY/AAY mutation G protein-independent ERK activation was detected demonstrating that this approach can be used to study the angiotensin II-stimulated signaling pathways in cells endogenously expressing AT(1) receptors.

Prooxidant activity and toxicity of nordihydroguaiaretic acid in clone-9 rat hepatocyte cultures

Nordihydroguaiaretic acid (NDGA) is a polyphenol. It is present at high concentrations in the leaves of the evergreen desert shrub, Larrea tridentate (Creosote bush), which has a long history of medicinal use traditionally by the native Americans and Mexicans. It is generally believed that the antioxidant properties of NDGA are responsible for the medicinal value of this desert shrub. The clone-9 rat hepatocyte cultures were used as an in vitro model to assess the hepatotoxic potential of NDGA and to determine whether it exhibits any prooxidant activity. The hepatocyte cultures were treated with NDGA for 2 h at 37 degrees C at concentrations of 0-100 microM. After the treatment period the cells, the culture supernatants and cell lysates were assayed for evaluation of prooxidant activity and toxicity of NDGA. Oxidative stress level and oxidative cell injury as measured by the peroxidation of membrane lipids and DNA double-strand breaks were used to index prooxidant activity. Cytotoxicity as measured by the leakage of the liver enzyme lactate dehydrogenase (LDH) into the culture medium, mitochondrial function and extent of cell proliferation were used as the endpoints of toxicity. Significant concentration-dependent differences were observed in these biomarkers over the concentration range examined demonstrating the prooxidant activity and toxicity of NDGA in clone-9 rat hepatocyte cultures.

Angiotensin receptors and actions in guinea pig enteric nervous system

Actions of ANG II on electrical and synaptic behavior of enteric neurons in the guinea pig small intestine were studied. Exposure to ANG II depolarized the membrane potential and elevated neuronal excitability. The number of responding neurons was small, with responses to ANG II in 32% of submucosal neurons and 25% of myenteric neurons. Hyperpolarizing responses were evoked by ANG II in 45% of the neurons. The hyperpolarizing responses were suppressed by alpha2-noradrenergic receptor antagonists, which suggested that the hyperpolarizing responses reflected stimulation of norepinephrine release from sympathetic neurons. Exposure to ANG II enhanced the amplitude and prolonged the duration of noradrenergic inhibitory postsynaptic potentials and suppressed the amplitude of both fast and slow excitatory postsynaptic potentials. The selective ANG II(1) receptor (AT1R) antagonists, ZD-7115 and losartan, but not a selective AT2R antagonist (PD-123319), suppressed the actions of ANG II. Western blot analysis and RT-PCR confirmed expression of AT1R protein and the mRNA transcript for the AT1R in the enteric nervous system. No expression of AT2R protein or mRNA was found. Immunoreactivity for AT1R was expressed by the majority of neurons in the gastric antrum and small and large intestine. AT1R immunoreactivity was coexpressed with calbindin, choline acetyltransferase, calretinin, neuropeptide Y, and nitric oxide synthase in subpopulations of neurons. The results suggest that formation of ANG II might have paracrine-like actions in the enteric nervous system, which include alterations in neuronal excitability and facilitated release of norepinephrine from sympathetic postganglionic axons. The enhanced presence of norepinephrine is expected to suppress fast and slow excitatory neurotransmission in the enteric microcircuits and to suppress neurogenic mucosal secretion.

Phosphorylation and desensitization of the lysophosphatidic acid receptor LPA1

In C9 cells, LPA (lysophosphatidic acid) induced inositol phosphate production, increased intracellular calcium concentration and inhibited adenylate cyclase activity. These responses were abolished in cells challenged with active phorbol esters. Action of phorbol esters was blocked by inhibitors of PKC (protein kinase C) and by its down-regulation. LPA1 receptor phosphorylation was observed in response to phorbol esters. The effect was rapid (t1/2 approximately 1 min), intense (2-fold) and sustained (at least 60 min). PKC inhibitors markedly decreased the LPA1 receptor phosphorylation induced by phorbol esters. LPA1 receptor tagged with the green fluorescent protein internalized in response to PKC activation. In addition, LPA and angiotensin II were also capable of inducing LPA1 receptor phosphorylation, showing that LPA1 receptor can be subjected to homologous and heterologous desensitization.

Agonist-Induced Interactions between Angiotensin AT1and Epidermal Growth Factor Receptors

In rat hepatic C9 cells, angiotensin II (Ang II)-induced activation of angiotensin type 1 (AT(1)) receptors (AT(1)-Rs) stimulates extracellular signal-regulated kinase (ERK) 1/2 phosphorylation via transactivation of the endogenous epidermal growth factor (EGF) receptor (EGF-R) by a protein kinase C (PKC) delta/Src/Pyk2-dependent pathway. This leads to phosphorylation of the EGF-R as well as its subsequent internalization. On the other hand, EGF-induced activation of the EGF-R in C9 cells was found to cause phosphorylation of the AT(1)-R. This was prevented by selective inhibition of the intrinsic tyrosine kinase activity of the EGF-R by AG1478 [4-(3'-chloroanilino)-6,7-dimethoxy-quinazoline] and was reduced by inhibition of PKC and phosphoinositide 3-kinase. EGF-induced AT(1)-R phosphorylation was associated with a decrease in membrane-associated AT(1)-Rs and a reduced inositol phosphate response to Ang II. Agonist activation of endogenous AT(1)-Rs and EGF-Rs induced the formation of a multireceptor complex containing both the AT(1)-R and the transactivated EGF-R. The dependence of these responses on caveolin was indicated by the finding that cholesterol depletion of C9 cells abolished Ang II-induced inositol phosphate production, activation of Akt/PKB and ERK1/2, and AT(1)-R internalization. Confocal microscopy demonstrated that caveolin-1 was endogenously phosphorylated and was distributed on the plasma membrane in patches that undergo redistribution during Ang II stimulation. Agonist-induced phosphorylation and association of caveolin 1 with the AT(1)-R was observed, consistent with a scaffolding role of caveolin during transactivation of the EGF-R by Ang II. The EGF-induced AT(1)-R/caveolin association was abolished by AG1478, suggesting that activation of the EGF-R promotes the association of caveolin and the AT(1)-R.

Effects of adrenomedullin on cell proliferation in rat adventitia induced by lysophosphatidic acid

Lysophosphatidic acid (LPA) is a bioactive phospholipid having growth factor-like activity on fibroblasts and is involved in cardiovascular diseases such as hypertension and heart failure by inducing vascular remodeling, characterized by fibroblast proliferation and migration in adventitia. Among various bioactive factors that LPA works with, adrenomedullin (ADM) is a multiple functional peptide with an important cytoprotective effect against cardiovascular damage. We studied rat aortic adventitia to explore the possible paracrine/autocrine interaction between endogenous ADM and LPA. LPA stimulation of the adventitia to secrete ADM and express its mRNA was concentration dependent. ADM inhibited LPA-induced proliferation in adventitial cells and attenuated the activity of mitogen-activated protein kinase (MAPK) stimulated by LPA. In contrast, treatment with specific antagonists of the ADM receptor potentiated the LPA-induced proliferation in adventitial cells. We concluded that LPA stimulates the adventitia to produce and secrete ADM, which in turn regulates the vascular biological effects of LPA.

Phosphoinositide 3-kinase cascade facilitates mu-opioid desensitization in sensory neurons by altering G-protein-effector interactions

Signaling via G-protein-coupled receptors undergoes desensitization after prolonged agonist exposure. Here we investigated the role of phosphoinositide 3-kinase (PI3K) and its downstream pathways in desensitization of micro-opioid inhibition of neuronal Ca2+ channels. In cultured mouse dorsal root ganglion neurons, two mechanistically different forms of desensitization were observed after acute or chronic treatment with the micro agonist [D-Ala2, N-MePhe4, Gly-ol5]-enkephalin (DAMGO). Chronic DAMGO desensitization was heterologous in nature and significantly attenuated by blocking the activity of PI3K or mitogen-activated protein kinase (MAPK). A combined application of PI3K and MAPK inhibitors showed no additive effect, suggesting that these two kinases act in a common pathway to facilitate chronic desensitization. Acute DAMGO desensitization, however, was not affected by the inhibitors. Furthermore, upregulation of the PI3K-Akt pathway in mutant mice lacking phosphatase and tensin homolog, a lipid phosphatase counteracting PI3K, selectively enhanced chronic desensitization in a PI3K- and MAPK-dependent manner. Using the prepulse facilitation (PPF) test, we further examined changes in the voltage-dependent component of DAMGO action that requires direct interactions between betagamma subunits of G-proteins and Ca2+ channels. DAMGO-induced PPF was diminished after chronic treatment, suggesting disruption of G-protein-channel interactions. Such disruption could occur at the postreceptor level, because chronic DAMGO also reduced GTPgammaS-induced PPF that was independent of receptor activation. Again, inhibition of PI3K or MAPK reduced desensitization of PPF. Our data suggest that the PI3Kcascade involving MAPK and Akt enhances micro-opioid desensitization via postreceptor modifications that interfere with G-protein-effector interactions.

Akt is a major downstream target of PI3-kinase involved in angiotensin II-induced proliferation

Different signal transduction cascades have been implicated in angiotensin II (Ang II)-mediated cell growth, such as the extracellular signal-regulated kinase 1/2 (ERK1/2) and the phosphatidylinositol 3-kinase (PI3K) pathways. To identify the downstream targets of PI3K involved in Ang II-induced proliferation, we used both rat aortic smooth muscle (RASM) cells and a CHO cell line stably expressing the rat AT1A receptor. The ERK1/2 and PI3K pathways are independently activated and implicated in Ang II-mediated DNA synthesis and cell number increase in these 2 cell lines. In addition, a specific inhibitor of Akt inhibited Ang II-induced Akt phosphorylation, DNA synthesis and proliferation in CHO-AT1A or RASM cells. A dominant-negative mutant of Akt was also found to selectively block Ang II-induced proliferation of CHO-AT1A cells. To further elucidate the signaling events leading to Akt activation, we used an AT1 receptor mutant (AT1AD74E), deficient for Gq protein coupling, and the intracellular calcium chelator BAPTA-AM. Although altered Akt and ERK1/2 activation was observed in the CHO-AT1AD74E cell line, blockade of intracellular calcium elevation did not affect phosphorylation of these kinases. These results provide the first evidence of a specific and necessary role of Akt in Ang II-induced proliferation through a Gq protein-dependent calcium-independent pathway.

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.

Independence of angiotensin II-induced MAP kinase activation from angiotensin type 1 receptor internalization in clone 9 hepatocytes

The agonist-induced internalization of several G protein-coupled receptors is an obligatory requirement for their activation of MAPKs. Studies on the relationship between endocytosis of the angiotensin II (Ang II) type 1 receptor (AT1-R) and Ang II-induced ERK1/2 activation were performed in clone 9 (C9) rat hepatic cells treated with inhibitors of endocytosis [sucrose, phenylarsine oxide (PAO), and concanavalin A]. Although Ang II-induced endocytosis of the AT1-R was prevented by sucrose and PAO, and was partially inhibited by concanavalin A, there was no impairment of Ang II-induced ERK activation. However, the specific epidermal growth factor receptor (EGF-R) kinase inhibitor, AG1478, abolished Ang II-induced activation of ERK1/2. Sucrose and PAO also inhibited EGFinduced internalization of the EGF-R in C9 cells, and the inability of these agents to impair EGF-induced ERK activation suggested that the latter is also independent of receptor endocytosis. In COS-7 cells transiently expressing the rat AT1A-R, Ang II also caused ERK activation through EGF-R transactivation. Furthermore, a mutant AT1A-R with truncated carboxyl terminus and impaired internalization retained full ability to activate ERK1/2 in response to Ang II stimulation. These findings demonstrate that Ang II-induced ERK1/2 activation in C9 hepatocytes is independent of both AT1-R and EGF-R endocytosis and is mediated by transactivation of the EGF-R.

p32 (gC1qBP) is a general protein kinase C (PKC)-binding protein; interaction and cellular localization of P32-PKC complexes in ray hepatocytes

The aim of this study was to identify cellular proteins that bind protein kinase C (PKC) and may influence its activity and its localization. A 32-kDa PKC-binding protein was purified to homogeneity from the Triton X-100-insoluble fraction obtained from hepatocytes homogenates. The protein was identified by NH(2)-terminal amino acid sequencing as the previously described mature form of p32 (gC1qR). Recombinant p32 was expressed as a glutathione S-transferase fusion protein, affinity-purified, and tested for an in vitro interaction with PKC using an overlay assay approach. All PKC isoforms expressed in rat hepatocytes interacted in vitro with p32, but the binding dependence on PKC activators was different for each one. Whereas PKCdelta only binds to p32 in the presence of PKC activators, PKCzeta and PKCalpha increase their binding when they are in the activated form. Other PKC isoforms such as beta, epsilon, and theta bind equally well to p32 regardless of the presence of PKC activators, and PKCmu binds even better in their absence. It was also found that p32 is not a substrate for any of the PKC isoforms tested, but interestingly, its presence had a stimulatory effect (2-fold for PKCdelta) on PKC activity. We also observed in vivo interaction between PKC and p32 by immunofluorescence and confocal microscopy. A time course of phorbol ester treatment of cultured rat hepatocytes (C9 cells) showed that PKCtheta and p32 are constitutively associated in vivo, whereas PKCdelta activation is required for its association with p32. Our data also showed that phorbol ester treatment induces a transient translocation of p32 from the cytoplasm to the cell nucleus. Together, these findings suggest that p32 may be a regulator of PKC location and function.

Calcium-independent activation of extracellularly regulated kinases 1 and 2 by angiotensin II in hepatic C9 cells: roles of protein kinase Cdelta, Src/proline-rich tyrosine kinase 2, and epidermal growth receptor trans-activation

Agonist activation of endogenous angiotensin II (Ang II) AT(1) receptors expressed in hepatic C9 cells markedly stimulated inositol phosphate production, phosphorylation of the proline-rich tyrosine kinase PyK-2, and ERK activation. Ang II caused activation of protein kinase C delta (PKCdelta) in C9 cells, and its stimulatory actions on Pyk2 and extracellularly regulated kinase (ERK) phosphorylation were abolished by PKC depletion and selective inhibition of PKCdelta by rottlerin, but not by Ca(2+)-chelators. These effects, and the similar actions of the Src kinase inhibitor PP2 indicate the involvement of PKCdelta and Src kinase in ERK activation. In C9 cells, phorbol-12-myristate-13-acetate (PMA) caused much greater phosphorylation of Pyk2 and ERK than the Ca(2+) ionophore ionomycin, and the effects of PMA and Ang II were abolished in PKC-depleted cells. Ang II increased the association of Pyk2 with Src and with the epidermal growth factor receptor (EGF-R). EGF caused much greater tyrosine phosphorylation of the EGF-R than Ang II and PMA. Ang II-induced activation of ERK, but not Pyk2, was prevented by inhibition of EGF receptor phosphorylation by AG 1478 and of Src kinase by PP1. Ang II also increased the association of the adaptor protein Grb2 with the EGF-R. These findings indicate that Src and Pyk2 act upstream of the EGF-R and that the majority of Ang II-induced ERK phosphorylation is dependent on trans-activation of the EGF-R. Ang II-induced ERK activation in C9 cells is initiated by a PKCdelta-dependent but Ca(2+)-independent mechanism and is mediated by the Src/Pyk2 complex through trans-activation of the EGF-R.

Agonist-induced signaling, desensitization, and internalization of a phosphorylation-deficient AT1A angiotensin receptor

An analysis of the functional role of a diacidic motif (Asp236-Asp237) in the third intracellular loop of the AT1A angiotensin II (Ang II) receptor (AT1-R) revealed that substitution of both amino acids with alanine (DD-AA) or asparagine (DD-NN) residues diminished Ang II-induced receptor phosphorylation in COS-7 cells. However, Ang II-stimulated inositol phosphate production, mitogen-activated protein kinase, and AT1 receptor desensitization and internalization were not significantly impaired. Overexpression of dominant negative G protein-coupled receptor kinase 2 (GRK2)K220M decreased agonist-induced receptor phosphorylation by approximately 40%, but did not further reduce the impaired phosphorylation of DD-AA and DD-NN receptors. Inhibition of protein kinase C by bisindolylmaleimide reduced the phosphorylation of both the wild-type and the DD mutant receptors by approximately 30%. The inhibitory effects of GRK2K220M expression and protein kinase C inhibition by bisindolylmaleimide on agonist-induced phosphorylation were additive for the wild-type AT1-R, but not for the DD mutant receptor. Agonist-induced internalization of the wild-type and DD mutant receptors was similar and was unaltered by coexpression of GRK2K220M. These findings demonstrate that an acidic motif at position 236/237 in the third intracellular loop of the AT1-R is required for optimal Ang II-induced phosphorylation of its carboxyl-terminal tail by GRKs. Furthermore, the properties of the DD mutant receptor suggest that not only Ang II-induced signaling, but also receptor desensitization and internalization, are independent of agonist-induced GRK-mediated phosphorylation of the AT1 receptor.