Role of Cytoplasmic Tail of the Type 1A Angiotensin II Receptor in Agonist– and Phorbol Ester–Induced Desensitization

From the Department of Biochemistry (H.T., D.F.G., Y.W., T.I.), Vanderbilt University School of Medicine, Nashville, Tenn, and the Department of Physiology and Biophysics (J.P.P.), University of Louisville (Ky).

Correspondence to Tadashi Inagami, PhD, Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, TN 37232.

	Abstract

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Abstract—To investigate mechanisms underlying the agonist-induced desensitization of the type 1A angiotensin II receptor (AT_1A-R), we have stably expressed in Chinese hamster ovary (CHO) cells the wild-type receptor and truncated mutants lacking varying lengths of the cytoplasmic tail. Assay of inositol 1,4,5-trisphosphate (IP₃) formation in response to agonist demonstrated that the truncated mutants T318, T328, and T348 lacking the last 42, 32, or 12 amino acid residues, respectively, couple with G_q protein with an efficiency similar to that of full-length receptors, whereas coupling of G_q protein was abolished in the T310 truncated mutant devoid of the carboxyl-terminal 50 amino acids. Exposure of CHO/AT_1A-R cells expressing the wild-type AT_1A-R to angiotensin II resulted in rapid and dose-dependent homologous desensitization of receptor-mediated IP₃ formation, which was independent of the receptor internalization. Mastoparan, an activator of G protein–coupled receptor kinase (GRK), induced desensitization of the AT_1A-R. The agonist-induced desensitization of the receptor was largely prevented by heparin, a potent inhibitor of GRK, whereas it was only partially attenuated by a protein kinase C (PKC)-specific inhibitor. The homologous or heterologous desensitization of the receptor was greatly impaired in the truncated mutants T318 and T328, lacking the Ser/Thr-rich (13 or 12 Ser/Thr residues) cytoplasmic tail of the AT_1A-R. Deletion of the last two Ser residues, including one PKC consensus site in the receptor tail, prevented only phorbol 12-myristate 13-acetate–induced desensitization by 30%. Moreover, we found an agonist-induced translocation of a heparin-sensitive kinase activity. The angiotensin II–stimulated heparin-sensitive kinase could phosphorylate a thioredoxin fusion protein containing the entire AT_1A-R cytoplasmic tail (N²⁹⁵ to E³⁵⁹), which lacks consensus phosphorylation sites for GRK1, GRK2, and GRK3. The heparin-sensitive kinase may not be GRK2, GRK3, or GRK6 expressed in CHO/AT_1A-R cells, since angiotensin II did not induce translocation of these receptor kinases. Potential Ser/Thr phosphorylation sites located between S³²⁸ and S³⁴⁷ in the cytoplasmic tail of AT_1A-R seem to play a critical role in the heterologous and homologous desensitization of the receptor. A heparin-sensitive kinase other than GRK2, GRK3, or GRK6 may be involved in the agonist-induced homologous desensitization of the AT_1A-R.

Key Words: angiotensin II • desensitization • AT_1A receptor • phorbol ester

	Introduction

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The type 1 Ang II receptors, AT_1A-R and AT_1B-R, undergo rapid homologous downregulation on exposure to Ang II.^{1 2 3} Multiple mechanisms, such as desensitization and concurrent internalization, seem to be involved in this response to maintain cellular homeostasis. The mechanism of desensitization is complex; however, receptor phosphorylation seems to be an initial and major step. Extensive mutagenesis studies on the ß₂-adrenergic receptor revealed that phosphorylation of two Ser residues located on the third cytoplasmic loop and in the proximal portion of the cytoplasmic tail by protein kinase A is involved in the heterologous desensitization of the receptor, whereas agonist-induced homologous desensitization requires phosphorylation of many Ser/Thr residues in the distal portion of the receptor cytoplasmic tail by ß-ARK. Phosphorylation of the cytoplasmic tail by ß-ARK induces arrestin-like protein binding to the receptor, which has been proposed to block fully the physical coupling of receptor to G_s protein and adenylyl cyclase.⁴ This indicates multiple mechanisms for desensitization by phosphorylation. In the G_q/11-coupled AT_1A-R, the desensitization mechanisms are not well understood. Although the receptor phosphorylation is associated with desensitization, whether they are causally related is not clear.³ Conflicting views exist as to the sites of phosphorylation. Thomas et al⁵ proposed that the second cytoplasmic loop may be the site of phosphorylation responsible for AT_1A-R desensitization, whereas Balmforth et al⁶ reported observations indicating involvement of a long cytoplasmic tail in the receptor desensitization, without identifying specific domains.

The desensitization of the AT₁ receptor was shown to occur independent of the receptor internalization in rat cardiomyocytes.¹ We were able to show that the desensitization of AT_1B-R can occur even if receptor internalization is inhibited and concluded that the two phenomena are distinct receptor functions even though they occur concurrently.² Although AT_1A-R and AT_1B-R share a high degree of structural homology, they have some considerable differences in the cytoplasmic tail region, particularly in positions of Ser/Thr residues. Du et al⁷ have recently reported that AT_1A-R and AT_1B-R are regulated in opposite directions on low salt feeding. Since AT_1A-R is the dominant subtype of the AT₁ receptor in most of tissues, except for the adrenal and pituitary glands, determination of the mechanism of its desensitization has long been overdue.

Since we have been able to obtain cells expressing AT_1A-R at a high level, which will enable us to quantify the downregulation of the receptor and its functions, we have performed studies directed to the delineation of the receptor domains involved in the desensitization using various mutants of AT_1A-R and a thioredoxin fusion protein containing the entire carboxyl-terminal region of the receptor. To gain insight into the type of kinases involved in this process, we have also applied reagents that stimulate or inhibit PKC and GRKs. We have obtained evidence that the proximal portion (K³¹⁰ to L³¹⁷) of the receptor cytoplasmic tail is involved in the AT_1A-R coupling to G_q and that the potential Ser/Thr phosphorylation sites located between S³²⁸ and S³⁴⁷ in the cytoplasmic tail play a critical role in the agonist- and phorbol ester (PMA)-induced desensitization. Finally, the type and role of protein kinases involved in agonist-induced desensitization of the AT_1A-R were examined.

	Materials and Methods

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Materials
Cell culture media, fetal calf serum, geneticin (G418 sulfate), and protein tyrosine kinase inhibitors were obtained from Life Technologies, Inc; [¹²⁵I]NaI, [-³²P]ATP, and [³H]IP₃ radioreceptor assay kits were purchased from Dupont NEN; CHO cells (CHO-K1) were obtained from American Type Culture Collection; Ang II and [Sar,¹ Ile⁸ ]Ang II were obtained from Peninsula Laboratories; the nonpeptide receptor antagonist losartan and PD124319 were obtained from Research Biochemical Inc; PVDF membranes were purchased from Millipore; GF109203X, W-7, and H-89 were purchased from Calbiochem; the restriction enzymes and calcium phosphate transfection kits were obtained from Promega; expression vector pRc/CMV and the thiofusion expression system were purchased from Invitrogen Corp; PKC (, ß, and ), Src kinase, Src kinase substrate peptide, and PKC assay kits were from Upstate Biotechnology Inc; and the polyclonal antibodies against GRKs and PLCs were from Santa Cruz. All other chemicals were purchased from Sigma Chemical Co.

Stable Expression of WT and Truncated AT_1A-R
The entire coding region of rat kidney AT_1A-R cDNA (an open reading frame encoding 359 amino acid residues) was subcloned into the BstXI site of the pRc/CMV expression vector. Truncated mutants lacking carboxyl-terminal segments of varying lengths were prepared by inserting a stop codon. Sites of truncation are shown in Fig 1. The mutated DNA was confirmed by dideoxynucleotide sequencing using appropriate primers, as described by Sanger et al,⁸ and subcloned into the expression vector pRc/CMV. CHO-K1 cells were grown in Ham's F-12 medium containing 10% fetal calf serum at 37°C in a humidified incubator with 5% CO₂. At 50% confluence, CHO cells were transfected with WT or truncated AT_1A-R plasmid DNA using a calcium phosphate precipitation/glycerol shock procedure previously described.² After selection in G418 (500 µg/mL), resistant colonies were isolated, amplified, and screened for receptor expression by radioligand binding assay. Cells were propagated in flasks with Ham's F-12 medium supplemented with 10% fetal calf serum under a selection pressure of 150 µg/mL G418 in an atmosphere of 95% air and 5% CO₂ at 37°C.

View larger version (26K): [in this window] [in a new window]   Figure 1. Schematic representation of the rat AT1A-R. The sites of truncation for the deletion mutants T310, T318, T328, and T348 AT1A-R are indicated by arrows. Multiple Ser (shaded circle) and Thr (open circle) residues, including three PKC consensus sites (solid circle), are shown. Shown underneath is the amino acid sequence of a portion (F301 to E359) of the AT1A-R cytoplasmic tail. Consensus sites for PKC phosphorylation are underlined.

Radioligand Binding Assay
Monoiodinated ¹²⁵I-[Sar,¹ Ile⁸ ]Ang II was prepared by the lactoperoxidase method⁹ and purified by reversed-phase HPLC as described previously¹⁰ except that a 0% to 80% acetonitrile gradient was applied for 60 minutes with a flow rate of 1.0 mL/min. Binding studies were performed with cells at or near confluence essentially as described previously.² The assay buffer consisted of 50 mmol/L Tris-HCl, pH 7.4, 100 mmol/L NaCl, 5 mmol/L MgCl₂, 0.25% BSA, and 0.5 mg/mL bacitracin. For determination of receptor affinity and density, competition binding studies were performed in the presence of 50 pmol/L ¹²⁵I-[Sar,¹ Ile⁸ ]Ang II and increasing concentrations of unlabeled ligands (10^-12 to 10^-6 mol/L). For measurement of the surface receptor binding capacity, the cells were pretreated as indicated and then incubated in the presence of 100 nmol/L Ang II for the indicated period at 37°C. Cells were then washed with ice-cold 50 mmol/L glycine and 150 mmol/L NaCl, pH 3.0, for 5 minutes at 4°C, followed by two saline washes. The binding studies were performed at near-saturating conditions with 2 nmol/L ¹²⁵I-[Sar,¹ Ile⁸ ]Ang II for 3 hours at 4°C.

Measurement of IP₃ Production
Cells were subcultured into 12-well dishes and were exposed to Ang II at or near confluence; the treatment was stopped by the addition of 1/5 vol of 100% ice-cold trichloroacetic acid to the plates. The cells were then harvested by scraping and transferred to polypropylene tubes. The cell extract was vortexed thoroughly and then centrifuged for 10 minutes at 6000g at 4°C. The supernatant was removed and warmed to room temperature for 15 minutes. Levels of IP₃ in each supernatant were determined by use of a radioimmunoassay kit from Dupont NEN following the manufacturer's instructions.

Permeabilization of CHO Cells
CHO cells bearing AT_1A-R at subconfluence in 12-well plates were washed twice with prewarmed calcium-free PBS and then washed twice with KG buffer (mmol/L: KCl 120, NaCl 30, MgCl₂ 2.5, K₂HPO₄ 1, PIPES 10, EGTA 2, and CaCl₂ 0.5, pH 7.2). Cells were then incubated for 30 seconds with 0.0075% digitonin at room temperature in KG-A buffer (KG buffer supplemented with 5 mmol/L glucose and 2 mmol/L ATP), followed by three washes with KG buffer. Under this condition, >95% of the cells were permeabilized as assessed by staining with trypan blue. After permeabilization, the cells were loaded with mastoparan or low molecular weight heparin (average molecular weight, 3000) by incubation with 100 µmol/L mastoparan or 10 µmol/L heparin for 15 minutes in KG-A buffer at 37°C, respectively.

Desensitization of Receptors
Studies were performed essentially as described previously.² In brief, the mastoparan- or heparin-loaded cells, or cells pretreated with chemicals as indicated, were incubated with or without 100 nmol/L Ang II in serum-free medium for indicated periods at 37°C. The reaction was stopped by washing the cells with 150 mmol/L NaCl/50 mmol/L glycine (pH 3.0) for 5 minutes at 4°C and with prewarmed PBS twice. For measurement of the IP₃ level in response to Ang II, the cells were again stimulated with Ang II (100 nmol/L), and 15 seconds later the IP₃ mass was assayed as described above.

Preparation of the Bacterial Thioredoxin-AT_1A Fusion Protein
A region of the rat AT_1A-R cDNA encoding the entire cytoplasmic tail (N²⁹⁵ to E³⁵⁹) was amplified by polymerase chain reaction and inserted into pBluescript KS(+) TA vector, and DNA was sequenced. Then, the BamHI-SalI fragment was cut and inserted into the same sites of the thiofusion expression plasmid pTrxFus vector in the frame. Induction of pTrxFus-AT_1A–transformed Escherichia coli with tryptophan (100 µg/mL) resulted in the production of a fusion protein of 19 kD (11.6 kD of thioredoxin plus 7.5 kD for N²⁹⁵ to E³⁵⁹). The fusion protein was purified with activated ThioBond resin (Invitrogen Corp) according to manufacturer's protocols.

Preparation of Cell Fractions and Western Blotting
Serum-starved cells were washed twice with PBS and stimulated with 100 nmol/L Ang II for the indicated periods. Then, the cells were washed with ice-cold PBS and scraped into 0.5 mL per dish of lysis buffer (10 mmol/L Tris-HCl, pH 7.4, 5 mmol/L EDTA, 0.1 mmol/L phenylmethylsulfonyl fluoride, 1 mmol/L benzamidine, and 10 µg/mL leupeptin and aprotinin). The cells were allowed to swell for 10 to 15 minutes and were then homogenized three times (30 seconds each) in a Polytron homogenizer (Brinkmann). Unbroken cells and cell nuclei were sedimented at 800g for 3 minutes and discarded. The plasma membranes were collected by centrifugation at 48 000g for 20 minutes and resuspended in lysis buffer. The supernatant, representing the cytosolic fraction, was centrifuged at 100 000g for 60 minutes. Preparations were kept at -80°C and used for Western blot or kinase assay. Samples containing equal amounts of protein (50 to 100 µg) were separated by 7.5% SDS-PAGE gels and transferred to PVDF membranes. Membranes were probed with 0.3 to 0.5 µg/mL GRK polyclonal antibodies, and immunoreactive bands were visualized with the enhanced chemiluminescence detection system (New England Biolabs). For Western blotting of PLCs, CHO/AT_1A-R cells or VSMCs were lysed on ice in lysis buffer containing 1% Triton X-100. The detergent-lysed cell extracts were separated by 7.5% SDS-PAGE gels and subjected to Western blotting with PLC polyclonal antibodies as described above.

In Vitro Phosphorylation of Thioredoxin-AT_1A Fusion Protein
For measuring the PKC-dependent or Src kinase–dependent phosphorylation of thioredoxin-AT_1A fusion protein, purified thioredoxin-AT_1A fusion protein (5 µg) was incubated with PKC (mixture of , ß, and , 50 ng per tube) or Src kinase (19 U per tube) in kinase buffer (mmol/L: Tris-HCl 20, MgCl₂ 10, EGTA 1, and dithiothreitol 2, pH 7.4) containing 1 µmol/L [-³²P]ATP for 30 minutes or 60 minutes at 30°C (final volume, 100 µL). For phosphorylation of the fusion protein by membrane preparations from CHO/AT_1A cells, an aliquot of cell membrane fractions (90 µg) that had been precleared by activated ThioBond resin, was incubated with purified thioredoxin-AT_1A fusion protein (5 µg) in the above kinase buffer containing 50 µmol/L [-³²P]ATP for 30 minutes at 30°C. The reaction was terminated by the addition of 1 mL ice-cold TE buffer (mmol/L: Tris-HCl 10 and EDTA 10, pH 7.4). ThioBond resin was added, collected by centrifugation (13 000g for 10 seconds), and washed four times with 1 mL of TE buffer. Bound fusion protein was dissociated by boiling in 1x SDS-PAGE sample buffer. Proteins were resolved by 15% SDS-PAGE. Gels were then dried and analyzed by autoradiography.

	Results

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Desensitization of the AT_1A-R–Mediated IP₃ Formation
To investigate mechanisms underlying the agonist-induced desensitization of AT_1A-R, we expressed cloned rat WT AT_1A-R in CHO cells that lack endogenous AT₁ receptors. The binding of ¹²⁵I-[Sar,¹ Ile⁸ ]Ang II, a peptidic Ang II receptor antagonist, to the cells bearing WT AT_1A-R was dose-dependently inhibited by Ang II, [Sar,¹ Ile⁸ ]Ang II, and an AT_1A-R–selective antagonist, losartan. In contrast, PD124319, a selective AT₂ receptor antagonist, had no effect on the binding of ¹²⁵I-[Sar,¹ Ile⁸ ]Ang II (data not shown). The AT₁ receptor activates PLC through G_q/11.¹¹ Previous studies of the AT_1A-R have shown that Ang II–induced formation of IP₃ was inhibited 75% by the tyrosine kinase inhibitor genistein and was associated with the activation of PLC₁ in VSMCs lacking PLC_ß1.¹² Western blot analysis demonstrated that PLC_ß1(150 kD), PLC_ß3 (150 kD), and PLC₁(145 kD) were clearly detected in CHO cells expressing high levels of WT AT_1A-R (CHO/AT_1A-R cells), whereas only PLC_ß3 and PLC₁ were detected in VSMCs (Fig 2). To determine whether Ang II–induced formation of IP₃ was sensitive to the tyrosine kinase inhibitors or calmodulin inhibitor W-7, we measured IP₃ response to Ang II in CHO/AT_1A-R cells with three different tyrosine inhibitors (genistein, herbimycin-A, and lavendustin A) or W-7. The results (not presented) showed that neither compound affected the IP₃ response to Ang II in intact cells. This observation suggests that the IP₃ response to Ang II in CHO/AT_1A-R cells is probably mediated by PLC_ß1 through G_q/11.¹¹

View larger version (32K): [in this window] [in a new window]   Figure 2. Immunoblots showing the expression of PLCs. Equal amounts of cell lysates (100 µg) were separated by 7.5% SDS-PAGE and transferred to PVDF membranes. The blocked membranes then were probed with polyclonal antibodies (Ab) to PLCß1 through PLCß4, PLC1, and PLC2. The position of PLC is indicated by the arrows.

To ascertain whether agonist-induced desensitization of AT_1A-R expressed in CHO cells occurs, we determined the effect of a pretreatment with Ang II on the subsequent maximal Ang II–stimulated IP₃ formation. The pretreatment with Ang II resulted in a subsequent attenuation of IP₃ response to Ang II. By varying the time of preexposure to the agonist from 1 minute to 30 minutes, we found that Ang II rapidly induced receptor desensitization as early as 1 minute (P<.05), with complete desensitization observed by pretreatment for 15 minutes (Fig 3A). Attenuation of IP₃ production was maximal when cells were pretreated with 10 to 100 nmol/L Ang II, and half-maximal effects were observed after pretreatment with 3 nmol/L Ang II (Fig 3B).

View larger version (12K): [in this window] [in a new window]   Figure 3. Time course and dose response of the Ang II–induced desensitization of AT1A-R. CHO/AT1A-R cells were incubated with 100 nmol/L Ang II for the indicated periods (A) or with different concentrations of the agonist for 15 minutes (B). After acid washing, cells were restimulated with 100 nmol/L Ang II, and the IP3 mass generated in 15 seconds was measured by radioreceptor binding assay as described in "Experimental Procedures." Results are expressed as percentage of levels in cells not restimulated with Ang II. Data are mean±SE of three separate experiments. A 100% response represents the complete desensitization of the receptor.

Involvement of GRKs in Agonist-Induced Homologous Desensitization of AT_1A-R
We and others^{2 13 14} have shown that PKC is involved in the heterologous desensitization of the AT₁ receptor. Treatment of CHO/AT_1A-R cells with PMA (100 nmol/L) for 10 minutes completely abolished receptor-mediated production of IP₃ with no effect on the basal level of IP₃ (Fig 4). This effect of PMA was completely antagonized by pretreating the cells with 10 µmol/L of a newly tested in vitro and in vivo nontoxic PKC-specific inhibitor, GF109203X¹⁵ (Fig 4). Consistent with the effects of the PKC inhibitor, downregulation of PKC by overnight incubation with PMA (1 µmol/L)¹⁶ completely blocked the desensitization of the AT_1A-R by PMA (data not shown). Like AT_1B-R, desensitization of AT_1A-R by a low Ang II concentration (1 nmol/L) was abolished by GF109203X, whereas the desensitizing effect at a high agonist concentration (100 nmol/L) was only partially prevented by GF109203X (Fig 4). The partial attenuation was seen even at a high dose of this compound (20 µmol/L) or with PKC depletion by overnight treatment with 1 µmol/L PMA (data not shown). These results suggests that a PKC-independent pathway, probably involving GRKs, contributes to the homologous desensitization of the AT_1A-R in transfected CHO cells.

View larger version (18K): [in this window] [in a new window]   Figure 4. PKC-dependent desensitization of AT1A-R. CHO/AT1A-R cells were pretreated with 10 µmol/L GF109203X (GF) for 30 minutes to inhibit PKC and then incubated for an additional 10 minutes with 100 nmol/L PMA or 15 minutes with 1 or 100 nmol/L Ang II. Cells were acid-washed and restimulated with 100 nmol/L Ang II for 15 seconds, and IP3 mass was determined by radioreceptor binding assay. Results are expressed as percentage of levels in cells not restimulated with Ang II. Data are mean±SE of four experiments. A 100% response represents the complete desensitization of the receptor.

We then studied the role of GRKs in agonist-induced homologous desensitization of AT_1A-R. Mastoparan, a wasp venom peptide, has been known to mimic the agonist-bound (active form) GPCRs and activate GRKs.^{17 18} Incubation of permeabilized CHO/AT_1A-R cells with 100 µmol/L mastoparan for 10 minutes completely abolished the receptor-mediated IP₃ response to Ang II (Fig 5). On the other hand, pretreatment of the permeabilized cells with 10 µmol/L heparin, a potent GRK inhibitor,^{19 20} for 20 minutes largely prevented the Ang II–induced desensitization of the AT_1A-R–mediated IP₃ response. The pretreatment with heparin decreased the basal level of IP₃ only slightly (Fig 5).

View larger version (19K): [in this window] [in a new window]   Figure 5. GRK-dependent desensitization of AT1A-R. CHO/AT1A-R cells were permeabilized by 0.0075% digitonin for 30 seconds and then treated with 100 µmol/L mastoparan or with 10 µmol/L heparin for 15 minutes in KG-A buffer before incubation with 100 nmol/L Ang II for 15 minutes. Cells were acid-washed and then restimulated with 100 nmol/L Ang II for 15 seconds, and IP3 mass was determined by radioreceptor binding assay. Changes of IP3 mass are expressed as percentage of that seen in control cells not restimulated with Ang II. Data are mean±SE of four independent experiments.

These results indicate that PKC plays a major role in the heterologous desensitization of AT_1A-R and that a heparin-sensitive GRK(s) may be involved in the agonist-induced homologous desensitization of the receptor in transfected CHO cells.

Impaired Heterologous or Homologous Desensitization of Truncated Mutants of AT_1A-R
To address the possible role of the cytoplasmic tail in the regulation of AT_1A-R, we constructed truncated mutants of AT_1A-R and stably expressed them in CHO cells. As shown in Fig 1, T310 and T318 truncation removes all Ser/Thr residues, including three potential PKC phosphorylation sites in the cytoplasmic tail of the AT_1A-R; T328 truncation removes all but the Ser³²⁶; and T348 truncation removes a potentially farnesylated cysteine and two Ser residues in the tail, including one PKC phosphorylation site, S³⁴⁸AK. Clonal, stably transfected cells expressing T310, T318, or T348 AT_1A-R were prepared and matched with CHO/AT_1A-R cells expressing WT AT_1A-R at a high level. Cells expressing the T328 receptor were compared with cells bearing low levels of WT AT_1A-R (Table). As shown in the Table, Scatchard analysis revealed that the dissociation constants (K_ds) for ¹²⁵I-[Sar,¹ Ile⁸ ]Ang II among the WT AT_1A-R and its mutants were similar, indicating that all the mutants possessed similar ligand binding affinity. Assay of IP₃ formation in response to Ang II demonstrated that truncated mutants (T318, T328, and T348) of AT_1A-R couple with effector PLC with an efficiency similar to that of full-length receptors. By contrast, the IP₃ formation on Ang II stimulation was barely detected in cells expressing high levels of truncated mutant T310, indicating the region between K³¹⁰ and L³¹⁷ in the cytoplasmic tail of the AT_1A-R that is involved in the receptor coupling to G_q protein (Table).

As shown in Fig 6, pretreatment of cells expressing WT AT_1A-R or mutant T348 with 100 nmol/L Ang II abolished the IP₃ response of the cells to a subsequent maximal stimulation with Ang II. By contrast, the Ang II–induced attenuation of the IP₃ response was markedly impaired in both cell clones expressing T318 or T328 AT_1A-R, indicating that potential Ser/Thr phosphorylation sites located between S³²⁸ and S³⁴⁷ in the cytoplasmic tail are important for the agonist-induced desensitization. In addition to the impaired agonist-induced desensitization, the PMA-dependent heterologous desensitization of the receptor was also dramatically impaired in the truncated mutants T318 and T328 (Fig 6). Furthermore, deletion of the last two Ser residues (mutant T348), including one PKC phosphorylation site in the receptor tail, prevented PMA-induced desensitization of the AT_1A-R by 30% (Fig 6).

View larger version (25K): [in this window] [in a new window]   Figure 6. Impaired homologous and heterologous desensitization of truncated mutants of AT1A-R. Cells expressing WT or truncated mutants of AT1A-R were pretreated with 100 nmol/L Ang II for 15 minutes or 100 nmol/L PMA for 10 minutes. Cells were acid-washed and restimulated with 100 nmol/L Ang II for 15 seconds. IP3 mass was determined by radioreceptor binding assay. Results are expressed as percent of levels in cells not restimulated with Ang II. Data are mean±SE of three independent experiments. A 100% response represents the complete desensitization of the receptor.

Phosphorylation of Thioredoxin-AT_1A Fusion Protein by PKC
To determine whether the cytoplasmic tail of AT_1A-R could be phosphorylated by PKC, we generated a thioredoxin fusion protein containing the entire carboxyl tail of the AT_1A-R (Fig 7A). The purified thioredoxin-AT_1A fusion protein was then used in an in vitro PKC phosphorylation assay. It was not a surprise that the thioredoxin-AT_1A fusion protein was highly phosphorylated by PKC (mixture of , ß, and ) (Fig 7B), since there are three potential PKC phosphorylation sites in the carboxyl tail of the AT_1A-R. However, the thioredoxin-AT_1A fusion protein was not phosphorylated by Src kinase, even though a Src kinase substrate peptide (KVEKIGEGTYGVVKK)²¹ was highly phosphorylated (205 728±17 235 cpm, 8-fold incorporation over background). There was no phosphorylation of the thioredoxin protein by PKC or Src kinase (Fig 7B).

View larger version (37K): [in this window] [in a new window]   Figure 7. Phosphorylation of thioredoxin-AT1A fusion protein by PKC and a heparin-sensitive kinase from the membrane of Ang II–treated CHO/AT1A-R cells. A, Expression and purification of the thioredoxin-AT1A fusion protein. Shown is a Coomassie blue stain of the purified fusion protein indicating the relative position on a 12% SDS-PAGE gel. Indicated on the left are the positions of molecular weight standards. ß-ME indicates ß-mercaptoethanol. B, Phosphorylation of the fusion protein by PKC. Thioredoxin control protein (Thio.) or thioredoxin-AT1A fusion protein (Thio.-AT1A) was incubated with PKC (mixture of , ß, and ) or Src kinase for indicated times at 30°C. C, CHO/AT1A-R cells pretreated with 1 µmol/L PMA overnight to deplete PKC and then exposed to 100 nmol/L Ang II for 15 minutes as indicated. The membranes were prepared and incubated with thioredoxin (Thio.) or thioredoxin-AT1A fusion protein (Thio.-AT1A) in the absence or presence of 100 nmol/L heparin for 30 minutes at 30°C. The reaction was stopped, and ThioBond resin (Invitrogen Corp) was added and collected. Bound fusion protein was dissociated and resolved by 15% SDS-PAGE. The position of the thioredoxin-AT1A fusion protein is indicated by the arrow.

Ang II Did Not Induce Translocation of GRK2, GRK3, or GRK6
Agonist-induced translocation of GRK1, GRK2, or GRK3 is considered the first step involved in the homologous desensitization of rhodopsin or ß-adrenergic receptors, respectively.²² To determine whether Ang II could induce translocation of GRK from cytosol to the plasma membranes, CHO/AT_1A-R cells were pretreated with 1 µmol/L PMA overnight to deplete PKC and then exposed to 100 nmol/L Ang II for 15 minutes. Plasma membranes were prepared, and the kinase activity associated with the membranes was determined using thioredoxin-AT_1A fusion protein as a substrate. As shown in Fig 7C, the phosphorylation of the thioredoxin-AT_1A fusion protein was increased dramatically by membranes derived from Ang II–treated CHO/AT_1A-R cells depleted of PKC. This indicated that the cytoplasmic tail of AT_1A-R could be phosphorylated by a PKC-independent mechanism. Moreover, the Ang II–induced kinase activity in the membrane fraction was completely inhibited by the GRK inhibitor heparin (100 nmol/L) (Fig 7C) but not by the PKC-specific inhibitor GF109203X or the PKA inhibitor H-89 (data not shown). To determine which individual GRK is selectively activated (by translocation) on Ang II stimulation, CHO/AT_1A-R cells or VSMCs were stimulated with Ang II for different time periods, and the cytosol and plasma membranes were prepared. Western blot experiments showed that GRK2 was almost equally distributed in cytosol or plasma membranes of the CHO/AT_1A-R cells, whereas GRK3 was mainly present in cytosol (Fig 8A). Ang II treatment did not induce translocation of GRK2 or GRK3 in CHO/AT_1A-R cells, since the protein level of GRK2 or GRK3 in the cytosolic and membrane fractions was not changed by Ang II stimulation (Fig 8A). Similar results with GRK2 and GRK3 were obtained before and after Ang II treatment in VSMCs (data not shown). In addition, acute PMA treatment had no effect on the translocation of GRK2 or GRK3 in either CHO/AT_1A-R cells (Fig 8A) or VSMCs (data not shown). However, translocation of GRK2 was observed by agonist UK14,304 treatment in MDCK cells stably expressing _2A-adrenergic receptors²³ (Fig 8B). GRK5 was undetectable in CHO/AT_1A-R cells but was detected in VSMCs (Fig 8C). GRK6 was detected only in the plasma membranes of both CHO/AT_1A-R cells (Fig 8D) and VSMCs (not shown). The localization of GRK6 was not changed on Ang II stimulation of CHO/AT_1A-R cells (Fig 8D) or VSMCs (not shown). On the other hand, PKC was rapidly activated by Ang II in CHO/AT_1A-R cells as well as in VSMCs (not shown), as reported previously.^{24 25}

View larger version (54K): [in this window] [in a new window]   Figure 8. No translocation of GRK2, GRK3, and GRK6 on Ang II stimulation. A, Serum-starved CHO/AT1A-R cells were stimulated with 100 nmol/L Ang II for different time periods or with 100 nmol/L PMA for 10 minutes as indicated. The cytosol (C) and membrane (M) were prepared and subjected to Western blotting with polyclonal antibodies to GRK2 or GRK3. B, MDCK cells expressing 2A-adrenergic receptor were stimulated with 1 µmol/L agonist UK14,304 (UK) for 10 minutes. Membranes were prepared and subjected to Western blotting with GRK2 antibody. C, Total detergent-lysed cell extracts from VSMCs and CHO/AT1A-R cells were subjected to Western blotting with GRK5 polyclonal antibody. D, Membrane fractions from CHO/AT1A-R cells treated with 100 nmol/L Ang II or 100 nmol/L PMA as indicated were subjected to Western blotting with GRK6 polyclonal antibody.

	Discussion

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In the present study, we have used four truncated mutants of AT_1A-R (T310, T318, T328, and T348) lacking the potential carboxyl-terminal phosphorylation sites but retaining the normal binding of Ang II intact. We showed that the proximal portion (K³¹⁰ to L³¹⁷) of the receptor cytoplasmic tail is involved in the AT_1A-R coupling to G_q, whereas the middle portion (S³²⁸ to S³⁴⁷) containing the potential Ser/Thr (eight Ser and two Thr residues) phosphorylation sites plays an important role in the agonist- and PMA-induced desensitization of the AT_1A-R. Furthermore, the thioredoxin-AT_1A fusion protein containing the entire cytoplasmic tail of the AT_1A-R can be heavily phosphorylated by PKC and by a membrane-associated heparin-sensitive kinase from Ang II–treated CHO/AT_1A-R cells depleted of PKC.

It has been noted that Ang II action is accompanied by a rapid and profound desensitization known as tachyphylaxis. However, the mechanisms by which Ang II receptors desensitize are still not clear. It seems that receptor sequestration or internalization does not play a major role in causing the homologous desensitization of the AT_1A-R expressed in CHO cells, since the complete desensitization induced by 15 minutes of incubation with 100 nmol/L Ang II still occurred even when the receptor internalization was almost completely blocked by concanavalin A² or phenylarsine oxide treatment (loss of membrane receptors: 12±2% with concanavalin A+Ang II or 8±0.6% with phenylarsine oxide+Ang II versus 50±4% with Ang II alone). Similar findings have been reported for the AT₁ receptors in cardiomyocytes,¹ AT_1B-R expressed in CHO cells,² AT_1A-R expressed in 293 cells,³ and other PLC-linked GPCRs.^{26 27} Thus, rapid desensitization of the AT_1A-R–mediated IP₃ formation may well be a consequence of the agonist-induced biochemical modification of the receptor, such as receptor phosphorylation.

The cytoplasmic tail of the AT_1A-R contains multiple Ser/Thr residues (13 of the last 59 amino acids), and there are three consensus PKC phosphorylation sites (S³³¹TK, S³³⁸YR, and S³⁴⁸AK).^{28 29} Removal of all Ser/Thr residues in the tail (truncated mutant T318) or removal of all Ser/Thr residues but the S³²⁶ (mutant T328) dramatically impaired agonist- and PMA-induced desensitization of the receptor. Deletion of the last two Ser residues, including one PKC consensus site, prevented only the PMA-induced heterologous desensitization by 30%, suggesting that two other PKC consensus sites (S³³¹TK and S³³⁸YR) may chiefly contribute to the PKC-dependent desensitization of the AT_1A-R. Since the immunoprecipitable antibody against the AT₁ receptor is not available, we cannot measure the receptor phosphorylation on agonist or PMA stimulation. We made and prepared a purified thioredoxin-AT_1A fusion protein that contains the entire cytoplasmic tail of AT_1A-R. The thioredoxin-AT_1A fusion protein was highly phosphorylated by PKC and by a membrane-associated heparin-sensitive kinase from Ang II–treated CHO/AT_1A-R cells depleted of PKC, but not by Src kinase in vitro despite the presence of four tyrosine residues in the receptor cytoplasmic tail. Taken together, these results indicate that the phosphorylation of the receptor cytoplasmic tail is involved in the homologous and heterologous desensitization of the AT_1A-R and that the potential Ser/Thr phosphorylation sites important for the AT_1A-R desensitization are located between S³²⁸ and S³⁴⁷ (containing eight Ser and two Thr residues) of the receptor cytoplasmic tail. Studies are now in progress to identify the major phosphorylation site(s) by the agonist or PMA treatment.

The PKC-specific inhibitor GF109203X completely suppressed the desensitization of AT_1A-R by Ang II at a low concentration (1 nmol/L). This suggests that PKC plays a major role in heterologous desensitization at a near-physiological agonist concentration. The desensitizing effect at a higher agonist concentration (100 nmol/L) is only partially prevented by the inhibition of PKC. Previous studies have shown that the PKC inhibitor staurosporine suppressed agonist-induced Ang II receptor desensitization in Xenopus oocytes.¹⁴ Pfeilschifter and coworkers^{30 31 32} have obtained evidence implicating PKC in the Ang II–induced desensitization in glomerular mesangial cells. We have demonstrated that PKC is only partially involved in agonist-induced desensitization of AT_1B-R in transfected CHO cells.² Similar findings have also been reported for several other PLC-linked GPCRs.^{33 34} In contrast, PKC depletion or treatment with the selective PKC inhibitor RO31–7519 did not affect the rapid agonist-induced desensitization of the Ang II receptor in neonatal cardiac myocytes.¹ Recently, Oppermann et al³ reported that PKC inhibition with staurosporine reduced agonist-induced phosphorylation of AT_1A-R by 40% to 50% in transfected 293 cells but did not affect the agonist-induced desensitization of the receptor. In aggregate, these studies and ours suggest that the agonist-induced desensitization of AT_1A-R may only be partially regulated by PKC in a cell type–specific manner.

A wasp venom peptide, mastoparan, has been shown to mimic the agonist-bound (active form) GPCRs and activate GRKs.^{17 18} Challenging the permeabilized cells expressing WT AT_1A-R with mastoparan completely abolished the receptor-mediated IP₃ response to Ang II. Pretreatment of the permeabilized cells with the GRK inhibitor heparin largely prevented the agonist-induced desensitization of AT_1A-R. Furthermore, treatment of the cells depleted of PKC with Ang II induced the translocation of a heparin-sensitive kinase activity, which could phosphorylate the thioredoxin-AT_1A fusion protein containing the entire cytoplasmic tail of AT_1A-R. These data indicate that the heparin-sensitive kinase may be mainly involved in the agonist-induced homologous desensitization of AT_1A-R. Six members of the GRK family have been cloned to date³⁵ and, on the basis of structural similarities, have been divided into three subfamilies: (1) rhodopsin kinase (GRK1), (2) the ß-adrenergic receptor kinase subfamily (GRK2 and GRK3), and (3) the GRK4 subfamily (GRK4, GRK5, and GRK6). GRKs also seem to have different patterns of expression in tissues. GRK1 and GRK4, for instance, are primarily expressed in retinal cells and testis, respectively, suggesting very specific functions. Whereas, other GRKs are expressed in many tissues, suggesting their broad roles in regulating the functions of GPCRs. Overexpression (>20-fold) of either GRK2, GRK3, or GRK5 was shown to enhance agonist-induced AT_1A-R phosphorylation equally well,³ suggesting a role for GRKs (GRK2, GRK3, and GRK5) in the agonist-induced phosphorylation and desensitization of the AT_1A-R. Since active GPCRs (agonist-bound form), in the presence of charged phospholipids, can directly associate with and activate GRKs,^{36 37} it is possible that overexpression (20-fold) of either GRK2, GRK3, or GRK5 could result in nonspecific direct interaction between Ang II–bound AT_1A-R with the overexpressed GRK, leading to the enhanced agonist-induced phosphorylation of AT_1A-R, since experiments did not show a significant difference between individual GRKs in their ability to enhance receptor phosphorylation in response to Ang II.³ As described above, we have demonstrated that the cytoplasmic tail of AT_1A-R plays a critical role in the agonist-induced homologous desensitization of the receptor. The cytoplasmic tail of AT_1A-R does not contain the consensus phosphorylation sites for GRK1 (the presence of acidic residue on the C-terminal side of a Ser/Thr), GRK2, and GRK3 (the presence of acidic residue localized to the N-terminal side of a Ser/Thr).^{37 38} Stimulation of CHO/AT_1A-R cells or VSMCs with Ang II did not induce translocation of GRK2 and GRK3. GRK5 was not detected in CHO/AT_1A-R cells but was detected only in the membrane fraction of quiescent VSMCs. GRK6 was detected only in the plasma membranes of both CHO/AT_1A-R cells and VSMCs. The localization of GRK6 was not affected by Ang II stimulation of either CHO/AT_1A-R cells or VSMCs. Moreover, we immunoprecipitated GRK6 and measured its catalytic activity using a soluble GRK substrate, casein, according to method of Pitcher et al.³⁹ The activity of GRK6 was not affected by Ang II treatment in CHO/AT_1A-R cells (not shown). Therefore, it seems that GRK2, GRK3, or GRK6 might not be involved in the agonist-induced homologous desensitization of the AT_1A-R, at least in CHO/AT_1A-R cells and VSMCs. Recently, a novel heparin-sensitive 40-kD receptor kinase, designated as MEK, was purified from porcine cerebellum. It was identified in the CHO cells expressing the m3-muscarinic receptor, and the activity of MEK in the membrane fraction was increased after the agonist stimulation.⁴⁰

In summary, the data presented have demonstrated that potential Ser/Thr phosphorylation sites located between S³²⁸ and S³⁴⁷ in the cytoplasmic tail of AT_1A-R play a critical role in the heterologous and homologous desensitization of the receptor. Although a recent study⁵ reported that the cytoplasmic tail of AT_1A-R is not involved in desensitization, the preliminary observation needs to be reconsidered, since no acid-washing step was taken to remove the receptor-bound Ang II before a second challenge of the cells with agonist during the measurement of Ca²⁺ mobilization. However, the precise role played by the cytoplasmic tail of the AT_1A-R in the complex series of biochemical mechanisms underlying receptor regulation needs to be explored further. The present results with the T310 truncated mutant indicate that the proximal portion (K³¹⁰ to L³¹⁷) of the AT_1A-R cytoplasmic tail is directly involved in receptor coupling to G_q. Recently, T. Sano (unpublished data, 1997) detailed experiments showing that a specific sequence, Y³¹²-F³¹³-L³¹⁴, in the proximal portion of the rat AT_1A-R cytoplasmic tail is essential for coupling and activation of G_q. Clearly, more work is needed to identify the Ser/Thr residues between S³²⁸ and S³⁴⁷ involved in the homologous and heterologous desensitization of AT_1A-R.

	Selected Abbreviations and Acronyms

 

Ang II = angiotensin II ß-ARK = ß2-adrenergic receptor kinase AT1A-R, AT1B-R = AT1A and AT1B receptor CHO = Chinese hamster ovary GPCR = G protein–coupled receptor GRK = G protein–coupled receptor kinase IP3 = inositol 1,4,5-trisphosphate MDCK = Madin-Darby canine kidney MEK = muscarinic receptor kinase PKC = protein kinase C PLC = phospholipase C PMA = phorbol 12-myristate 13-acetate PVDF = polyvinylidene difluoride VSMC = vascular smooth muscle cell WT = wild-type

	Acknowledgments

 
This study was supported in part by National Institutes of Health grants HL-58205, HL-35323, and DK-20593. The authors thank J.R. Keefer for the MDCK cells expressing _2A-adrenergic receptor and Dr E.J. Landon for reading this manuscript.

Received May 28, 1997; accepted January 5, 1998.

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