G protein-coupled receptors: heterologous regulation of homologous desensitization and its implications

Heterologous regulation of muscarinic and beta-adrenergic receptors in rat cardiomyocytes in culture

Previous work indicated that hyperstimulation of muscarinic receptors brings about profound changes not only in the density of the muscarinic receptors, but also of the beta-adrenoceptors in rat heart atria in vivo. We have now investigated whether a similar receptor cross-regulation occurs in cardiomyocytes in vitro. Cardiomyocytes from 3-4 day old rats were exposed to chemical agents on days 5-6 in culture. Densities of muscarinic and beta-adrenergic receptors were measured according to the binding of N-[3H]methylscopolamine and [ H]CGP 12177, respectively, to cell surface membranes and cell homogenates. Exposure of cells to the muscarinic agonist carbachol (1 mmol/l) brought about a profound decrease in the number of muscarinic receptors. The number of beta-adrenoceptors displayed biphasic changes, being augmented after 24 h (by 20-45% on the cell surface and by 29% in the homogenate) and diminished after 48 h and 72 h (after 48 h, decrease by 44-75% on the cell surface and by 36% in the homogenate). These effects of carbachol were not prevented by dimethylaminopropyl-bis-indolylmaleimide, the inhibitor of protein kinase C. Exposure of cells to the beta-adrenoceptor agonist isoprenaline (0.1 mmol/l) strongly diminished the number of beta-adrenoceptors on the cell surface and in the homogenate. The density of muscarinic receptors on the cell surface was diminished by 24-43% after 24 h exposure to isoprenaline and unchanged after 48 h, whereas the concentration of muscarinic receptors in the homogenate was unchanged after 24 h and increased by 20% after 48 h. The isoprenaline-induced decrease in the density of cell surface muscarinic receptors could not be simulated by forskolin and was not abolished by the protein kinase A inhibitors Rp-cAMPS and HA-1004. Dibutyryl cyclic AMP diminished the density of cell surface muscarinic receptors more than that of the beta-adrenergic receptors. Our data reveal a novel phenomenon of a biphasic change (an increase followed by a loss) in the density of beta-adrenoceptors during exposure of cardiocytes to carbachol. Activation of beta-adrenoceptors brings about less conspicuous changes in the density of muscarinic receptors. The observed phenomena of receptor cross-regulation cannot be explained by simple activations of protein kinases A and C.

Enhanced expression of G protein-coupled receptor kinase 2 selectively increases the sensitivity of A2A adenosine receptors to agonist-induced desensitization

1. G protein-coupled receptor kinases (GRKs) are thought to be important in mediating the agonist-induced phosphorylation and consequent desensitization of G protein-coupled receptor (GPCR) responses. We have previously shown that stable expression of a dominant negative mutant G protein-coupled receptor kinase 2 (GRK2) construct in NG108-15 mouse neuroblastoma x rat glioma cells suppresses the agonist-induced desensitization of A2A and A2B adenosine receptor-stimulated adenylyl cyclase activity (Mundell et al., 1997). To further determine the role of GRK2 in agonist-induced desensitization of these adenosine receptors, we stably overexpressed wild type GRK2 in NG108-15 cells. 2. In homogenates prepared from cells overexpressing GRK2, the acute stimulation of adenylyl cyclase by activation of A2A and A2B adenosine receptors was markedly reduced, but could be reversed by pretreating the cells with AD (adenosine deaminase), to remove extracellular adenosine from the medium. On the other hand, acute stimulation of adenylyl cyclase by secretin, iloprost, NaF and forskolin was the same in GRK2 overexpressing cells and plasmid-transfected control cells. 3. Cells overexpressing GRK2 were more sensitive to adenosine receptor agonist-induced desensitization than plasmid-transfected control cells. This effect was selective since the agonist sensitivity of desensitization for secretin and IP-prostanoid receptor-stimulated adenylyl cyclase activity was not affected by GRK2 overexpression. 4. These results further implicate GRK2 as the likely mechanism by which A2 adenosine receptors undergo short-term desensitization in NG108-15 cells, and indicate that even when overexpressed, GRK2 retains its substrate specificity for native receptors in intact cells. Furthermore, the susceptibility of GPCRs to desensitization appears to depend on the level of GRK expression, such that in cells that express high levels of GRK2, low agonist concentrations may be sufficient to trigger GRK-mediated desensitization.

Modulation of opioid analgesia, tolerance and dependence by Gs-coupled, GM1 ganglioside-regulated opioid receptor functions

Studies of direct excitatory effects elicited by opioid agonists on various types of neurone have been confirmed and expanded in numerous laboratories following the initial findings reviewed previously by Stanley Crain and Ke-Fei Shen. However, the critical role of the endogenous glycolipid GM1 ganglioside in regulating Gs-coupled, excitatory opioid receptor functions has not been addressed in any of the recent reviews of opioid stimulatory mechanisms. This article by Stanley Crain and Ke-Fei Shen focuses on crucial evidence that the concentration of GM1 in neurones might, indeed, play a significant role in the modulation of opioid receptor-mediated analgesia, tolerance and dependence.

Cyclic AMP signaling and gene regulation

Cyclic adenosine monophosphate (cAMP) is a ubiquitous second messenger produced in cells in response to hormones and nutrients. The production of cAMP is dependent upon the actions of many different proteins that affect its synthesis and degradation. An important function of cAMP is to activate the phosphorylating enzyme, protein kinase A. The key roles of cAMP and protein kinase A in the phosphorylation and regulation of enzyme substrates involved in intermediary metabolism are well known. A newly discovered role for protein kinase A is in the phosphorylation and activation of transcription factors that are critical for the control of the transcription of genes in response to elevated levels of cAMP.

Activation of receptor gene transcription is required to maintain cell sensitization after agonist exposure. Study on neurotensin receptor

Neurotensin (NT) acts through specific G protein-coupled receptors to induce effects in the central nervous system and periphery. In this study we have shown that in the human neuroblastoma cell line CHP 212, an NT agonist, JMV 449, induced high affinity neurotensin receptor (NTR) gene activation. 125I-NT binding of cells challenged with JMV 449 rapidly decreased then reappeared and subsequently stabilized at 50% of the control values after 48 h of agonist exposure. These receptors, which reappeared at the cell surface, are as active as those found in control cells as demonstrated by Ca2+ mobilization. Furthermore, the tyrosine hydroxylase (TH) gene, a known NT target gene, remained activated after prolonged NT agonist exposure in this cell line. In the murine neuroblastoma cell line, N1E-115, NT did not stimulate NTR gene activation but induced NTR mRNA destabilization after long term agonist exposure. In this cell line, NT binding dropped to 15% of control values and remained at this value after agonist treatment. The TH expression, which was originally activated upon NT agonist exposure, decreased to control values after prolonged agonist exposure. These observations combined with the data obtained from a complementary study with HT-29 cells (Souazé, F., Rostène, W., and Forgez, P. (1997) J. Biol. Chem. 272, 10087-10094) revealed the crucial role of agonist-induced receptor gene transcription in the maintenance of cell sensitivity. A model for G protein-coupled receptor regulation induced by prolong and intense agonist stimulation is proposed.

Long-lasting effects of chronic mu-opioid intake on the signal transmission via dopamine D1 receptors in the limbic forebrain of drug deprived rats

Rats orally self-administered the potent and selective mu-opioid receptor agonist etonitazene for 8 weeks (free choice between three opioid solutions and water resulting in low drug intake, or forced intake of a single drug solution resulting in high opioid consumption). The signal transmission in membranes of the limbic forebrain (nucleus accumbens and olfactory tubercle) was studied during acute withdrawal (2 days of abstinence) and after 6 weeks of drug deprivation. Binding experiments with the dopamine (DA) D1 receptor antagonist [3H]SCH23390 revealed in the high consuming rats an increased binding density (Bmax) by 19% during withdrawal and a decreased Bmax by 17% after long-term abstinence compared with drug-naive controls (each P < 0.05). The addition of 500 nM DA reduced the [3H]SCH23390 binding affinity (Kd increased by 60-105%) and density (by 15-23%) in each of the five groups (P < 0.001). During acute withdrawal, the portion of Bmax inhibited by DA increased by 83% in the high consuming rats vs. the controls (P < 0.05). Full concentration-response curves of adenylyl cyclase (AC) stimulation by the DA D1 receptor agonist dihydrexidine and of inhibition of forskolin stimulated AC activity by the GTP analogue guanosine-5'-O-(3-thio)triphosphate (GTPgammaS) were performed: the former revealed a reduced maximum efficacy (Emax decreased by 23-37%), P < 0.001), the latter a reduced effective concentration (EC50 decreased by 60-103%, P < 0.05), in each etonitazene-experienced group vs. the controls.

GnRH signalling pathways and GnRH-induced homologous desensitization in a gonadotrope cell line (alphaT3-1)

Exposure of the gonadotrope cells to gonadotropin-releasing hormone (GnRH) reduces their responsiveness to a new GnRH stimulation (homologous desensitization). The time frame as well as the mechanisms underlying this phenomenon are yet unclear. We studied in a gonadotrope cell line (alphaT3-1) the effects of short as well as long term GnRH pretreatments on the GnRH-induced phospholipases-C (PLC), -A2 (PLA2) and -D (PLD) activities, by measuring the production of IP3, total inositol phosphates (IPs), arachidonic acid (AA) and phosphatidylethanol (PEt) respectively. We demonstrated that although rapid desensitization of GnRH-induced IP3 formation did not occur in these cells, persistent stimulation of cells with GnRH or its analogue resulted in a time-dependent attenuation of GnRH-elicited IPs formation. GnRH-induced IPs desensitization was potentiated after direct activation of PKC by the phorbol ester TPA, suggesting the involvement of distinct mechanisms in the uncoupling exerted by either GnRH or TPA on GnRH-stimulated PI hydrolysis. The levels of individual phosphoinositides remained unchanged under any desensitization condition applied. Interestingly, while the GnRH-induced PLA2 activity was rapidly desensitized (2.5 min) after GnRH pretreatments, the neuropeptide-evoked PLD activation was affected at later times, indicating an important time-dependent contribution of these enzymatic activities in the sequential events underlying the GnRH-induced homologous desensitization processes in the gonadotropes. Under GnRH desensitization conditions, TPA was still able to induce PLD activation and to further potentiate the GnRH-evoked PLD activity. AlphaT3-1 cells possess several PKC isoforms which, except PKCzeta, were differentially down-regulated by TPA (PKCalpha, betaII, delta, epsilon, eta) or GnRH (PKCbetaII, delta, epsilon, eta). In spite of the presence of PKC inhibitors or down-regulation of PKC isoforms by TPA, the desensitizing effect of the neuropeptide on GnRH-induced IPs, AA and PEt formation remained unchanged. In conclusion, in alphaT3-1 cells the GnRH-induced homologous desensitization affects the GnRH coupling with PLC, PLA2 and PLD by mechanism(s) which do not implicate TPA-sensitive PKC isoforms, but likely reflect time-dependent modification(s) on the activation processes of the enzymes.

G protein-coupled receptor kinase 2 (GRK2): mechanisms of regulation and physiological functions

G protein-coupled receptor kinase 2 (GRK2) plays a key role in determining the rate and extent of G protein-coupled receptor (GPCR) desensitization and resensitization. Recent data indicate that GRK2 activity, subcellular distribution and expression are tightly regulated. The important physiological function of GRK2 as a modulator of the efficacy of GPCR signal transduction systems is exemplified by its relevance in cardiovascular physiopathology as well as by its emerging role in the regulation of chemokine receptors.

Pseudomonas pyocyanine alters calcium signaling in human airway epithelial cells

Pseudomonas aeruginosa, an opportunistic human pathogen, causes both acute and chronic lung disease. P. aeruginosa exerts many of its pathophysiological effects by secreting virulence factors, including pyocyanine, a redox-active compound that increases intracellular oxidant stress. Because oxidant stress has been shown to affect cytosolic Ca2+ concentration ([Ca2+]c) in other cell types, we studied the effect of pyocyanine on [Ca2+]c in human airway epithelial cells (A549 and HBE). At lower concentrations, pyocyanine inhibits inositol 1,4,5-trisphosphate formation and [Ca2+]c increases in response to G protein-coupled receptor agonists. Conversely, at higher concentrations, pyocyanine itself increases [Ca2+]c. The pyocyanine-dependent [Ca2+]c increase appears to be oxidant dependent and to result from increased inositol trisphosphate and release of Ca2+ from intracellular stores. Ca2+ plays a central role in epithelial cell function, including regulation of ion transport, mucus secretion, and ciliary beat frequency. By disrupting Ca2+ homeostasis, pyocyanine could interfere with these critical functions and contribute to the pathophysiological effects observed in Pseudomonas-associated lung disease.

Phosducin expression in NG 108-15 hybrid cells enhances prostaglandin E1 stimulated adenylate cyclase activity

Phosducin (Phd), a cytosolic protein, has been proposed to compete with certain receptor kinases for Gbetagamma of heterotrimeric G proteins, and may inhibit GTPase activity of G alpha s. These suggestions together with the enhancing effect of Phd on odorant-induced cAMP accumulation let us assume a stimulatory action of the protein on intracellular signaling. Therefore, this investigation was designed to examine the excitatory effect of PGE1 on signal transmission in neuroblastoma x glioma hybrid cells (NG 108-15) overexpressing Phd. The neuronal cells stably expressing Phd were found to display a 3 to 4-fold increased sensitivity to PGE1 as compared to wild type cells, using cAMP accumulation as measure. Examination of membranes prepared from Phd-overexpressing cells revealed an elevated GTPase activity as indicated by the formation of 32Pi upon PGE1 challenge. This activity was inhibited by exogenous Phd. In addition, receptor independent stimulation of adenylate cyclase by forskolin reveals an increased formation of cAMP in Phd expressing cells, which is accompanied by an increased binding of [3H]forskolin. The findings let us propose that Phd elevates intracellular levels of functional G alpha s which accounts for the increased response to PGE1.

A serine cluster prevents recycling of the V2 vasopressin receptor

Receptor recycling plays a critical role in the regulation of cellular responsiveness to environmental stimuli. Agonist-promoted phosphorylation of G protein-coupled receptors has been related to their desensitization, internalization, and sequestration. Dephosphorylation of internalized G protein-coupled receptors by cytoplasmic phosphatases has been shown to be pH-dependent, and it has been postulated to be necessary for receptors to recycle to the cell surface. The internalized V2 vasopressin receptor (V2R) expressed in HEK 293 cells is an exception to this hypothesis because it does not recycle to the plasma membrane for hours after removal of the ligand. Because this receptor is phosphorylated only by G protein-coupled receptor kinases (GRKs), the relationship between recycling and GRK-mediated phosphorylation was examined. A nonphosphorylated V2R, truncated upstream of the GRK phosphorylation sites, rapidly returned to the cell surface after removal of vasopressin. Less-drastic truncations of V2R revealed the presence of multiple phosphorylation sites and suggested a key role for a serine cluster present at the C terminus. Replacement of any one of Ser-362, Ser-363, or Ser-364 with Ala allowed quantitative recycling of full-length V2R without affecting the extent of internalization. Examination of the stability of phosphate groups incorporated into the recycling S363A mutant V2Rs revealed that the recycling receptor was dephosphorylated after hormone withdrawal, whereas the wild-type V2R was not, providing molecular evidence for the hypothesis that GRK sites must be dephosphorylated prior to receptor recycling. These experiments uncovered a role for GRK phosphorylation in intracellular sorting and revealed a GRK-dependent anchoring domain that blocks V2R recycling.

Inhibition of constitutive signaling of Kaposi's sarcoma-associated herpesvirus G protein-coupled receptor by protein kinases in mammalian cells in culture

Kaposi's sarcoma-associated herpesvirus (KSHV)/human herpesvirus 8, which is consistently present in tissues of patients with Kaposi's sarcoma and primary effusion lymphomas, contains a gene that encodes a G protein-coupled receptor (KSHV-GPCR). We recently showed that KSHV-GPCR exhibits constitutive signaling via activation of phosphoinositide-specific phospholipase C and stimulates cell proliferation and transformation. In this study, we determined whether normal cellular mechanisms could inhibit constitutive signaling by KSHV-GPCR and thereby KSHV-GPCR-stimulated proliferation. We show that coexpression of GPCR-specific kinases (GRKs) and activation of protein kinase C inhibit constitutive signaling by KSHV-GPCR in COS-1 monkey kidney cells and in mouse NIH 3T3 cells. Moreover, GRK-5 but not GRK-2 inhibits KSHV-GPCR-stimulated proliferation of rodent fibroblasts. These data provide evidence that cell regulatory pathways of receptor desensitization may be therapeutic targets in human diseases involving constitutively active receptors.

Regulation of H1-receptor coupling and H1-receptor mRNA by histamine in bovine tracheal smooth muscle

1. Pretreatment of bovine tracheal smooth muscle (BTSM) with histamine (1-100 microM, 1 h) induced a concentration-dependent desensitization of the contractile response to subsequently administered histamine, with a reduction of the maximum response of 72 +/- 8% (n = 5) following pre-exposure to 100 microM histamine. In contrast, concentration-response curves to the muscarinic agonist, methacholine were not affected following histamine pretreatment, indicating a homologous desensitization. Furthermore, concentration-response curves to NaF, a G-protein activator, were not altered following histamine pre-incubation. 2. The histamine H1-receptor (H1R) desensitization could be antagonized by mepyramine (an H1-receptor antagonist, 1 microM) but not by cimetidine (an H2-receptor antagonist, 10 microM), indicating that the desensitization occurred via stimulation of histamine H1-receptors, without evidence for the involvement of histamine H2-receptors. 3. Indomethacin (10 microM) did not block the H1R desensitization, suggesting no involvement of prostaglandins. Furthermore, histamine pre-incubation in calcium free medium still induced a functional uncoupling of H1R. 4. GF 109203X, a protein kinase C (PKC) inhibitor, and H-7, a non-selective kinase inhibitor, did not antagonize the homologous H1R desensitization. 5. The steady-state level of H1R mRNA, assessed by Northern blot analysis, was not affected by prolonged histamine exposure (100 microM, 0.5, 1, 2, 4, 16 and 24 h). 6. These results suggest that histamine induces desensitization of the H1R at the level of the receptor protein, which involves a mechanism independent of PKC, PKA, PKG and calcium influx, suggesting the involvement of a receptor-specific kinase.

Identification and characterization of functional nongenomic progesterone receptors on human sperm membrane

The presence of functional nongenomic progesterone (P) receptors in human spermatozoa has been investigated by equilibrium binding studies in intact spermatozoa, ligand blot and Western blot analysis of sperm lysates, as well as determination of the effects of the steroid on sperm intracellular Ca2+ concentrations. Binding experiments were performed using progesterone-11alpha-glucuronide-[125I]iodotyramine as tracer. Computer analysis of competition curves using different steroids as competitors indicated the presence of two distinct binding sites for P. The high affinity site (Kd in the nanomolar range) appears to be specific for P, whereas the low affinity one (Kd in the micromolar range) binds with equal affinity 11beta-hydroxyprogesterone (11betaOHP) and 17alpha-hydroxyprogesterone (17alphaOHP). A significant correlation exists among affinity constants (as determined by binding studies) and EC50 values for the effects of P, 11betaOHP, and 17alphaOHP on intracellular Ca2+ in fura-2-loaded spermatozoa, strongly indicating the involvement of P-binding sites in the biological effect of the steroid. In particular, dose-response curves for P were biphasic, with an EC50 in the nanomolar range and another in the micromolar range. Conversely, curves for 11betaOHP and 17alphaOHP were monophasic, with an EC50 just in the micromolar range. Ligand blot analysis of sperm total lysates performed with peroxidase-conjugated P revealed the presence of two binding proteins of 54 and 57 kDa that were specific for P. Indeed, peroxidase-conjugated P binding was blocked by the simultaneous presence of the unconjugated steroid. Using alpha c262 antibody, which is directed against the P-binding domain of genomic receptor, we detected two proteins of similar molecular mass (54 and 57 kDa), whereas using antibodies directed against the DNA-binding and N-terminal domains of the genomic P receptors, the two proteins were not detected. In addition, p54 and p57 appear to be mostly localized in sperm membranes and virtually absent in the cytoplasm. The involvement of these proteins in the biological effects of P is indicated by the strong inhibitory effect of alpha c262 on P-induced acrosome reaction of capacitated human spermatozoa.

Peptidergic activation of transcription and secretion in chromaffin cells. Cis and trans signaling determinants of pituitary adenylyl cyclase-activating polypeptide (PACAP)

Pituitary adenylyl cyclase-activating polypeptide (PACAP) is a potent endogenous secretagogue for chromaffin cells. Chromogranin A is the major soluble core component in secretory vesicles. Since chromogranin A is secreted along with catecholamines, we asked whether PACAP regulates expression of the chromogranin A gene in PC12 rat chromaffin cells, so as to resynthesize the just-secreted protein, and whether such biosynthetic regulation is coupled mechanistically to catecholamine secretion. PACAP activated the endogenous chromogranin A gene by four- to fivefold. Proportional results (seven- to eightfold activation) were obtained with a transfected 1,200-bp mouse chromogranin A promoter/luciferase reporter construct. A series of chromogranin A promoter 5' deletion mutant/luciferase reporter constructs narrowed down the PACAP response element to a proximal region containing the cAMP response element (CRE box), at (-71 bp)5'-TGACGTAA-3'(-64 bp). Site-directed point mutations of the CRE site suppressed PACAP-induced trans-activation of the promoter. Thus, the proximal CRE box is entirely necessary for the chromogranin A promoter response to PACAP. Transfer of the CRE box to a neutral, heterologous promoter also conferred activation by PACAP, suggesting that the CRE domain is also sufficient to mediate the transcriptional response to PACAP. Expression of a dominant-negative mutant (KCREB) of the CRE-binding factor CREB markedly diminished trans-activation of the chromogranin A promoter by PACAP. Cotransfection of expression plasmids encoding the protein kinase A inhibitor, or an inactive protein kinase A (PKA) catalytic beta subunit, inhibited both forskolin and PACAP activation of chromogranin A transcription, revealing that PACAP-induced trans-activation is highly dependent on PKA. By contrast, inhibition of protein kinase C (by chronic exposure to phorbol ester) had no effect on transcriptional activation by PACAP. The potent PACAP/vasoactive intestinal peptide (VIP) type I receptor antagonist PACAP6-38 impaired both chromogranin A transcription or catecholamine secretion triggered by PACAP38, while the PACAP/VIP type II receptor antagonist (p-Chloro-D-Phe6, Leu17)-VIP had little or no ability to antagonize the PACAP38 effect. The agonist VIP was approximately 100- to 1,000-fold less potent than PACAP in stimulating either secretion or transcription. Thus, PACAP-evoked chromogranin A transcription and catecholamine secretion are likely mediated by the PACAP/VIP type I receptor isoform. Although the calcium channel antagonists Zn2+ (100 microM), nifedipine (10 microM), or ruthenium red (10 microM), or the cytosolic calcium chelator BAPTA-AM (50 microM) each strongly impaired PACAP-induced secretion, transcriptional activation of chromogranin A remained unaltered. Therefore, we propose that PACAP signals to chromogranin A transcription through the CRE in cis, and through PKA and CREB in trans. By contrast, a pathway involving cytosolic calcium entry through L-type voltage-dependent channels is required for PACAP to evoke catecholamine secretion.

Rat beta-adrenergic receptor kinases 1 and 2 in mouse neuroblastoma X rat glioma NG 108-15 hybrid cells

Beta-adrenergic receptor kinase (betaARK, EC 2.7.1.-) has been implicated in the phosphorylation of G protein-coupled receptors, including opioid receptors. Since delta-opioid receptors of mouse neuroblastoma x rat glioma hybrid cells (NG 108-15) desensitize upon activation, this investigation was designed to find out whether NG 108-15 cells contain betaARK activity. Using the reverse transcription polymerase chain reaction technique, we identified two mRNAs, one coding for rat betaARK1 and the other for rat betaARK2. No hint was found for the presence of mouse betaARK. Examining the cytosolic betaARK activity in these hybrid cells using rhodopsin as substrate, we found a strict functional dependence on the presence of exogenous G protein subunit Gbetagamma. This relationship reflects a characteristic for betaARK1 and 2 out of the known G protein-coupled receptor kinases. Finally, highly purified recombinant betaARK1 proved active to phosphorylate enriched delta-opioid receptor preparations in an opioid agonist-dependent manner. The results reported here provide the basis to study more closely the molecular function of G protein-coupled receptor kinases in a cell line (NG 108-15) most frequently used to investigate acute and chronic opioid actions.

Activation of endothelin ETA receptors induces phosphorylation of alpha1b-adrenoreceptors in Rat-1 fibroblasts

The effect of endothelin-1 on the phosphorylation of alpha1b-adrenoreceptors, transfected into rat-1 fibroblasts, was studied. Basal alpha1b-adrenoreceptor phosphorylation was markedly increased by endothelin-1, norepinephrine, and phorbol esters. The effect of endothelin-1 was dose dependent (EC50 approximately 1 nM), reached its maximum 5 min after stimulation, and was inhibited by BQ-123, an antagonist selective for ETA receptors. Endothelin-1-induced alpha1b-adrenoreceptor phosphorylation was attenuated by staurosporine or genistein and essentially abolished when both inhibitors were used together. The effect of norepinephrine was not modified by either staurosporine or genistein alone, and it was only partially inhibited when both were used together. These data suggest the participation of protein kinase C and tyrosine kinase(s) in endothelin-1-induced receptor phosphorylation. However, phosphoaminoacid analysis revealed the presence of phosphoserine and traces of phosphothreonine, but not of phosphotyrosine, suggesting that the putative tyrosine kinase(s), activated by endothelin, could act in a step previous to receptor phosphorylation. The effect of endothelin-1 on alpha1b-adrenoreceptor phosphorylation was not mediated through pertussis toxin-sensitive G proteins. Calcium mobilization induced by norepinephrine was diminished by endothelin-1. Norepinephrine and endothelin-1 increased [35S]GTPgammaS binding to control membranes. The effect of norepinephrine was abolished in membranes obtained from cells pretreated with endothelin-1. Interestingly, genistein plus staurosporine inhibited this effect of the endothelial peptide. Endothelin-1 did not induce alpha1b-adrenoreceptor internalization. Our data indicate that activation of ETA receptors by endothelin-1 induces alpha1b-adrenoreceptor phosphorylation and alters G protein coupling.

Dynamic distribution of chemoattractant receptors in living cells during chemotaxis and persistent stimulation

While the localization of chemoattractant receptors on randomly oriented cells has been previously studied by immunohistochemistry, the instantaneous distribution of receptors on living cells undergoing directed migration has not been determined. To do this, we replaced cAR1, the primary cAMP receptor of Dictyostelium, with a cAR1-green fluorescence protein fusion construct. We found that this chimeric protein is functionally indistinguishable from wild-type cAR1. By time-lapse imaging of single cells, we observed that the receptors remained evenly distributed on the cell surface and all of its projections during chemotaxis involving turns and reversals of polarity directed by repositioning of a chemoattractant-filled micropipet. Thus, cell polarization cannot result from a gradient-induced asymmetric distribution of chemoattractant receptors. Some newly extended pseudopods at migration fronts showed a transient drop in fluorescence signals, suggesting that the flow of receptors into these zones may slightly lag behind the protrusion process. Challenge with a uniform increase in chemoattractant, sufficient to cause a dramatic decrease in the affinity of surface binding sites and cell desensitization, also did not significantly alter the distribution profile. Hence, the induced reduction in binding activity and cellular sensitivity cannot be due to receptor relocalization. The chimeric receptors were able to "cap" rapidly during treatment with Con A, suggesting that they are mobile in the plane of the cell membrane. This capping was not influenced by pretreatment with chemoattractant.

Molecular cloning of rat G-protein-coupled receptor kinase 6 (GRK6) from brain tissue, and its mRNA expression in different brain regions and peripheral tissues

The rat G-protein-coupled receptor kinase 6 (GRK6) cDNA was cloned from rat brain tissue by a combination of reverse-transcription polymerase chain reactions (RT-PCR), based on homology to the cloned human GRK6, and rapid amplification of cDNA ends (RACE-PCR). We obtained a clone of 2817 bp with an open reading frame of 1731 bp encoding for a protein of 576 amino acids that is 96.7% identical and 97.9% similar to its human counterpart. mRNA was detectable in all brain areas examined. In addition, GRK6 was expressed in skeletal muscle, small intestine, aorta, liver, heart, lung, thymus, stomach, uterus and kidney.

Structure-function analysis of inhibitory adenosine receptor regulation

Pharmacological and molecular cloning studies have revealed the presence of four adenosine receptor (AR) subtypes, termed A1, A2A, A2B and A3. Given that the A1 and A3ARs can both bind adenosine and couple productively to inhibitory G-proteins, the significance of the existence of multiple inhibitory AR subtypes remains obscure, although one possibility is that these receptors are regulated in a subtype-specific manner. In this review, we summarize our investigations into the mechanisms underlying the agonist-induced desensitization of inhibitory AR function. The results of this work demonstrate that while the A1AR desensitizes slowly over a time course of several hours, the A3AR desensitizes within minutes of agonist exposure. Molecular biological studies have begun to delineate the structural requirements responsible for these differences, and will provide a basis for future experiments designed to determine whether the ability of an inhibitory AR receptor subtype to 'turn-off' at a specific rate has implications for the physiological role of that receptor.

GTP-binding-protein-coupled receptor kinases--two mechanistic models

Six vertebrate protein kinases (G-protein-coupled receptor kinases; GRKs) that regulate the function of G-protein-coupled receptors (GPCRs) were recently cloned; several distinct properties set them apart from conventional second-messenger regulated protein kinases. It appears that GRKs bind GPCR* through two separate sites: a high-affinity site, which involves intracellular loops of the activated receptor, and the lower-affinity site, encompassing the phosphorylation region. The high-affinity interaction may involve complementary structural elements of GRKs and GPCRs* rather than precise amino acid alignment, thus allowing broad and overlapping specificities of these kinases, in spite of differences in the sequences of GPCRs. In addition, GRK structures are modified by several posttranslational modifications, including phosphorylation, autophosphorylation, prenylation, carboxymethylation, and palmitoylation, probably affecting properties of these enzymes. While GRKs phosphorylate and inactivate receptor molecules which are engaged in G-protein activation, controversy surrounds whether GRKs might be activated and phosphorylate unstimulated GPCRs, leading to a desensitization of a larger population of the receptors. In this review, mechanistic aspects of GPCR* phosphorylation related to the distinct properties, regulation and modes of action of GRKs are described.

The basal subcellular distribution of beta-adrenergic receptor kinase is independent of G-protein beta gamma subunits

beta-Adrenergic receptor kinase (beta ARK-1 or GRK2) is a key regulatory protein involved in the regulation of G-protein-coupled receptors which associates with microsomal and plasma membranes. beta gamma Subunits of G-proteins have been suggested to mediate agonist-dependent membrane translocation of beta ARK, but their possible role in maintaining the complex subcellular distribution of the kinase is not known. In this study we show that lovastatin-mediated inhibition of G gamma subunits isoprenylation in HEK-293 cells stably transfected with beta ARK1 leads to a significant release of G beta subunits to the cytosol without causing changes in total particulate beta ARK or in the association of this kinase to plasma or microsomal membrane fractions. In addition, transient overexpression of mutant forms of G gamma unable to become isoprenylated resulted in a marked sequestration of G beta to the soluble compartment, but caused no rearrangement in the distribution of cotransfected beta ARK. These results indicate that anchoring of beta ARK to cellular membranes under basal conditions is independent of the availability of heterotrimeric G-protein subunits.

Identification of a short sequence highly divergent between beta-adrenergic-receptor kinases 1 and 2 that determines the affinity of binding to betagamma subunits of heterotrimeric guanine-nucleotide-binding regulatory proteins

A 28-residue peptide (peptide G), derived from the C-terminal (W643-S670) of the beta-adrenergic receptor kinase (betaARK), was previously identified as the critical domain for binding to the betagamma subunits of the heterotrimeric guanine-nucleotide-binding regulatory protein (G betagamma). We observed that the 18-amino-acid core of this domain is poorly conserved between betaARK1 and betaARK2 and so may provide the basis for differences in G betagamma-binding properties. Specific antibodies raised against 18-residue peptides derived from the divergent sequences (peptides P1 and P2 for betaARK1 and betaARK2, respectively) competitively inhibited G betagamma-activation of the related betaARK subtype, confirming the involvement of this region in binding to G betagamma. Peptides P1 and P2 inhibited G betagamma-stimulated activity of both betaARK1 and betaARK2, with P2 being significantly more potent than P1 (IC50 of 179+/-5 microM for P2 and >500 microM for P1). The 28-residue peptides G showed the same relative inhibitory activities (IC50 = 48+/-5 microM for G2 and 146+/-8 microM for G1). This relative order of potency G2 > G1 approximately P2 > P1 was confirmed in a direct G betagamma-binding assay. No binding selectivity for the beta1, beta2, beta3 and beta4 G beta subtypes was observed. The EC50 value for G betagamma-activation of betaARK1 was about double of that for betaARK2, indicating a higher affinity between G betagamma and betaARK2, which is the expected result based on the findings with the peptides. These findings show that the 18-residue peptides P represent the shortest sequence of betaARK that can bind to G betagamma and provide a demonstration of a functional difference between the G betagamma binding domains of betaARK1 and betaARK2.

G protein-coupled receptor kinase GRK4. Molecular analysis of the four isoforms and ultrastructural localization in spermatozoa and germinal cells

G protein-coupled receptor kinase 4 (GRK4) presents some peculiar characteristics that make it a unique member within the GRK multigene family. For example, this is the only GRK for which four splice variants (GRK4alpha, -beta, -gamma, -delta) have been identified. We developed a simple assay to study kinase activity, and we found that GRK4alpha, but not GRK4beta, -gamma, and -delta, was able to phosphorylate rhodopsin in an agonist-dependent manner. GRK4alpha kinase activity was inhibited by Ca2+/calmodulin (CaM) (IC50 = 80 nM), and a direct interaction between GRK4alpha and Ca2+/CaM was revealed using CaM-conjugated Sepharose 4B. The other three GRK4 isoforms did not interact with CaM in parallel experiments. The present investigation also aimed to define cellular and ultrastructural localization of GRK4. A substantial expression of GRK4 mRNA was only found in testis and in the spermatogonia cell line GC-1 spg. Specific GRK4 immunoreactivity was only found on sperm membranes, and immunochemical and ultrastructural analyses showed that it is associated to the acrosomal membranes and to the outer mitochondrial membranes. GRK4gamma was the only detectable isoform in human sperm. We concluded that: i) only GRK4alpha can phosphorylate rhodopsin and that this activity is inhibited by CaM; ii) the other three isoforms do not phosphorylate rhodopsin and do not interact with CaM; and iii) the association of GRK4 with highly specialized sperm organelles, which are essential for fertilization, strongly indicates that this kinase is involved in this process.