Roles of the α1B-Adrenergic Receptor Phosphorylation Domains in Signaling and Internalization

The function of the α1B-adrenergic receptor phosphorylation sites previously detected by mass spectrometry was evaluated by employing mutants, substituting them with non-phosphorylatable amino acids. Substitution of the intracellular loop 3 (IL3) sites did not alter baseline or stimulated receptor phosphorylation, whereas substitution of phosphorylation sites in the carboxyl terminus (Ctail) or both domains (IL3/Ctail) markedly decreased receptor phosphorylation. Cells expressing the IL3 or Ctail receptor mutants exhibited a noradrenaline-induced calcium-maximal response similar to those expressing the wild-type receptor, and a shift to the left in the concentration-response curve to noradrenaline was also noticed. Cells expressing the IL3/Ctail mutant exhibited higher apparent potency and increased maximal response to noradrenaline than those expressing the wild-type receptor. Phorbol ester-induced desensitization of the calcium response to noradrenaline was reduced in cells expressing the IL3 mutant and abolished in cells in which the Ctail or the IL3/Ctail were modified. In contrast, desensitization in response to preincubation with noradrenaline was unaffected in cells expressing the distinct receptor mutants. Noradrenaline-induced ERK phosphorylation was surprisingly increased in cells expressing IL3-modified receptors but not in those expressing receptors with the Ctail or IL3/Ctail substitutions. Our data indicate that phosphorylation sites in the IL3 and Ctail domains mediate and regulate α1B-adrenergic receptor function. Phorbol ester-induced desensitization seems to be closely associated with receptor phosphorylation, whereas noradrenaline-induced desensitization likely involves other elements.

Cell Trafficking and Function of G Protein-coupled Receptors

The G protein-coupled receptors (GPCRs) are plasma membrane proteins that function as sensors of changes in the internal and external milieux and play essential roles in health and disease. They are targets of hormones, neurotransmitters, local hormones (autacoids), and a large proportion of the drugs currently used as therapeutics and for "recreational" purposes. Understanding how these receptors signal and are regulated is fundamental for progress in areas such as physiology and pharmacology. This review will focus on what is currently known about their structure, the molecular events that trigger their signaling, and their trafficking to endosomal compartments. GPCR phosphorylation and its role in desensitization (signaling switching) are also discussed. It should be mentioned that the volume of information available is enormous given the large number and variety of GPCRs. However, knowledge is fragmentary even for the most studied receptors, such as the adrenergic receptors. Therefore, we attempt to present a panoramic view of the field, conscious of the risks and limitations (such as oversimplifications and incorrect generalizations). We hope this will provoke further research in the area. It is currently accepted that GPCR internalization plays a role signaling events. Therefore, the processes that allow them to internalize and recycle back to the plasma membrane are briefly reviewed. The functions of cytoskeletal elements (mainly actin filaments and microtubules), the molecular motors implicated in receptor trafficking (myosin, kinesin, and dynein), and the GTPases involved in GPCR internalization (dynamin) and endosomal sorting (Rab proteins), are discussed. The critical role phosphoinositide metabolism plays in regulating these events is also depicted.

Mutation of putative phosphorylation sites in the free fatty acid receptor 1: Effects on signaling, receptor phosphorylation, and internalization

Free fatty acid receptor 1 phosphorylation sites were studied using mutants, including a) a mutant with T215V in the third intracellular loop (3IL), b) another with changes in the carboxyl terminus (C-term): T287V, T293V, S298A, and c) a mutant with all of these changes (3IL/C-term). Agonist-induced increases in intracellular calcium were similar between cells expressing wild-type or mutant receptors. In contrast, agonist-induced FFA1 receptor phosphorylation was reduced in mutants compared to wild type. Phorbol ester-induced FFA1 receptor phosphorylation was rapid and robust in cells expressing the wild-type receptor and essentially abolished in the mutants. Agonist-induced ERK 1/2 phosphorylation and receptor internalization were decreased in cells expressing the mutant receptors compared to those expressing the wild-type receptor. Our data suggest that the identified sites might participate in receptor phosphorylation, signaling, and internalization.

Roles of receptor phosphorylation and Rab proteins in G protein-coupled receptor function and trafficking

The G Protein-Coupled Receptors form the most abundant family of membrane proteins and are crucial physiological players in the homeostatic equilibrium, which we define as health. They also participate in the pathogenesis of many diseases and are frequent targets of therapeutic intervention. Considering their importance, it is not surprising that different mechanisms regulate their function, including desensitization, resensitization, internalization, recycling to the plasma membrane, and degradation. These processes are modulated in a highly coordinated and specific way by protein kinases and phosphatases, ubiquitin ligases, protein adaptors, interaction with multifunctional complexes, molecular motors, phospholipid metabolism, and membrane distribution. This review describes significant advances in the study of the regulation of these receptors by phosphorylation and endosomal traffic (where signaling can take place); we revisited the bar code hypothesis and include two additional observations: a) that different phosphorylation patterns seem to be associated with internalization and endosome sorting for recycling or degradation, and b) that, surprisingly, phosphorylation of some G protein-coupled receptors appears to be required for proper receptor insertion into the plasma membrane. Significance Statement G protein-coupled receptor phosphorylation is an early event in desensitization/ signaling switching, endosomal traffic, and internalization. These events seem crucial for receptor responsiveness, cellular localization, and fate (recycling/ degradation) with important pharmacological/ therapeutic implications. Phosphorylation sites vary depending on the cells in which they are expressed and on the stimulus that leads to such covalent modification. Surprisingly, evidence suggests that phosphorylation also seems to be required for proper insertion into the plasma membrane for some receptors.

El receptor S1P1 de la esfingosina1-fosfato: avances en el conocimiento de su estructura, función e importancia biomédica

La esfingosina1-fosfato (S1P) es un metabolito intermediario en el catabolismo de la esfingomielina y un “lípido bioactivo”, con la capacidad de funcionar como hormona local al modular las funciones de distintas células y tejidos; en las células regula una gran variedad de respuestas, como la migración, la diferenciación y la sobrevivencia y en los tejidos la angiogénesis, la cardiogénesis, el desarrollo de las extremidades, la formación del sistema linfático y nervioso, entre otros procesos. El receptor S1P1 es un miembro de la familia de receptores acoplados a proteínas G que media muchas de las acciones de la S1P. Este receptor tiene una gran importancia médica por estar involucrado en la patogenia de la esclerosis múltiple. En condiciones fisiológicas el receptor S1P1 se expresa en la superficie de los linfocitos y regula la salida de esas células de los ganglios linfáticos por quimiotaxis. Por otra parte, la esclerosis múltiple es una enfermedad crónica, autoinmune y neurodegenerativa del sistema nervioso central, asociada en muchos casos con una progresión irreversible que conduce a la discapacidad. En la patogenia de esta enfermedad, después de que los linfocitos salen de los ganglios linfáticos, se encuentran en el torrente sanguíneo y atraviesan la barrera hematoencefálica invaden el sistema nervioso central, favorecen la secreción de citocinas inflamatorias que activan a los macrófagos y a otras células, destruyendo las vainas de mielina; además, los linfocitos B producen anticuerpos contra los componentes de la mielina incrementando la desmielinización, el daño axonal y neuronal que caracterizan a los pacientes con esclerosis múltiple.

The LPA3 Receptor: Regulation and Activation of Signaling Pathways

The lysophosphatidic acid 3 receptor (LPA₃) participates in different physiological actions and in the pathogenesis of many diseases through the activation of different signal pathways. Knowledge of the regulation of the function of the LPA₃ receptor is a crucial element for defining its roles in health and disease. This review describes what is known about the signaling pathways activated in terms of its various actions. Next, we review knowledge on the structure of the LPA₃ receptor, the domains found, and the roles that the latter might play in ligand recognition, signaling, and cellular localization. Currently, there is some information on the action of LPA₃ in different cells and whole organisms, but very little is known about the regulation of its function. Areas in which there is a gap in our knowledge are indicated in order to further stimulate experimental work on this receptor and on other members of the LPA receptor family. We are convinced that knowledge on how this receptor is activated, the signaling pathways employed and how the receptor internalization and desensitization are controlled will help design new therapeutic interventions for treating diseases in which the LPA₃ receptor is implicated.

Effect of docosahexaenoic acid, phorbol myristate acetate, and insulin on the interaction of the FFA4 (short isoform) receptor with Rab proteins

Human embryonic kidney (HEK) 293 cells were co-transfected with plasmids for the expression of mCherry fluorescent protein-tagged FFA4 receptors and the enhanced green fluorescent protein-tagged Rab proteins involved in retrograde transport and recycling, to study their possible interaction through Förster Resonance Energy Transfer (FRET), under the action of agents that induce FFA4 receptor phosphorylation and internalization through different processes, i.e., the agonist, docosahexaenoic acid, the protein kinase C activator phorbol myristate acetate, and insulin. Data indicate that FFA4 receptor internalization varied depending on the agent that induced the process. Agonist activation (docosahexaenoic acid) induced an association with early endosomes (as suggested by interaction with Rab5) and rapid recycling to the plasma membrane (as indicated by receptor interaction with Rab4). More prolonged agonist stimulation also appears to allow the FFA4 receptors to interact with late endosomes (interaction with Rab9), slow recycling (interaction with Rab 11), and target to degradation (Rab7). Phorbol myristate acetate, triggered a rapid association with early endosomes (Rab5), slow recycling to the plasma membrane (Rab11), and some receptor degradation (Rab7). Insulin-induced FFA4 receptor internalization appears to be associated with interaction with early endosomes (Rab5) and late endosomes (Rab9) and fast and slow recycling to the plasma membrane (Rab4, Rab11). Additionally, we observed that agonist- and PMA-induced FFA4 internalization was markedly reduced by paroxetine, which suggests a possible role of G protein-coupled receptor kinase 2.

Effects of agonists and phorbol esters on α1A-adrenergic receptor-Rab protein interactions

In a cell line, stably expressing α1A-adrenoceptors fused to the mCherry red fluorescent protein, noradrenaline, methoxamine, and oxymetazoline induced concentration-dependent increases in intracellular calcium. All of these agents increase α1A-adrenoceptor phosphorylation and internalization. Transient co-expression of these receptors with Rab proteins tagged with the enhanced Green Fluorescent Protein was employed to estimate α1A-adrenoceptor-Rab interaction using Förster Resonance Energy Transfer. Noradrenaline and methoxamine increased α1A-adrenoceptor interaction with Rab5 and Rab7 but did not modify it with Rab9. Oxymetazoline induced adrenoceptor interaction with Rab5 and Rab9 and only an insignificant increase in Rab7 signal. Phorbol myristate acetate increased α1A-adrenoceptor interaction with Rab5 and Rab9 but did not modify it with Rab7. The agonists and the active phorbol ester, all of which induce receptor phosphorylation and internalization, favor receptor interaction with Rab5, i.e., association with early endosomes. Cell stimulation with phorbol myristate acetate induced the α1A-adrenoceptors to interact with the late endosomal marker, Rab9, suggesting that the receptors are directed to slow recycling endosomes once they have transited to the Trans-Golgi network to be retrieved to the plasma membrane. The agonists noradrenaline and methoxamine likely induce a faster recycling and might direct some of the adrenoceptors toward degradation and/or very slow recycling to the plasma membrane. Oxymetazoline produced a mixed pattern of interaction with the Rab proteins. These data indicate that α1A-adrenoceptor agonists can trigger different vesicular traffic and receptor fates within the cells.

Canonical and non-canonical Wnt signaling are simultaneously activated by Wnts in colon cancer cells

The Wnt signaling pathway is a crucial regulator of the intestinal epithelium homeostasis and is altered in most colon cancers. While the role of aberrant canonical, β-catenin-dependent Wnt signaling has been well established in colon cancer promotion, much less is known about the role played by noncanonical, β-catenin-independent Wnt signaling in this type of cancer. This work aimed to characterize the noncanonical signal transduction pathway in colon cancer cells. To this end, we used the prototype noncanonical ligand, Wnt5a, in comparison with Wnt3a, the prototype of a canonical β-catenin activating ligand. The analysis of the expression profile of Wnt receptors in colon cancer cell lines showed a clear increase in both level expression and variety of Frizzled receptor types expressed in colon cancer cells compared with non-malignant cells. We found that Wnt5a activates a typical Wnt/Ca++ - noncanonical signaling pathway in colon malignant cells, inducing the hyperphosphorylation of Dvl1, Dvl2 and Dvl3, promoting Ca++ mobilization as a result of phospholipase C (PLC) activation via pertussis toxin-sensitive G-protein, and inducing PLC-dependent cell migration. We also found that while the co-receptor Ror2 tyrosine kinase activity is not required for Ca++ mobilization-induced by Wnt5a, it is required for the inhibitory effects of Wnt5a on the β-catenin-dependent transcriptional activity. Unexpectedly, we found that although the prototype canonical Wnt3a ligand was unique in stimulating the β-catenin-dependent transcriptional activity, it also simultaneously activated PLC, promoted Ca++ mobilization, and induced Rho kinase and PLC-dependent cell migration. Our data indicate, therefore, that a Wnt ligand can activate at the same time the so-called Wnt canonical and noncanonical pathways inducing the formation of complex signaling networks to integrate both pathways in colon cancer cells.

Glycogen Synthase Kinase-3 modulates α1A-adrenergic receptor action and regulation

The possibility that glycogen synthase kinase 3 (GSK3) could modulate α_1A-adrenergic receptor (α_1A-AR) function and regulation was tested employing LNCaP and HEK293 cells transfected to express the enhanced green fluorescent protein-tagged human α_1A-AR. Receptor phosphorylation and internalization, intracellular free calcium, α_1A-AR-GSK3 colocalization, and coimmunoprecipitation were studied. The effects of the pharmacological GSK3 inhibitor, SB-216763, and the coexpression of a dominant-negative mutant of this kinase, as well as the signaling, desensitization, and internalization of receptors with S229, S258, S352, and S381 substitutions for alanine or aspartate, were also determined. SB-216763 inhibited agonist- and phorbol myristate acetate (PMA)-mediated α_1A-AR phosphorylation, reduced oxymetazoline-induced desensitization, and magnified that induced by PMA. Agonists and PMA increased receptor-GSK3 colocalization and coimmunoprecipitation. Expression of a dominant-negative GSK3 mutant reduced agonist- but not PMA-induced receptor internalization. α_1A-AR with the GSK3 putative target sites mutated to alanine exhibited reduced phosphorylation and internalization in response to agonists and increased PMA-induced desensitization. Agonist-induced, but not PMA-induced, receptor-β arrestin intracellular colocalization was diminished in cells expressing the GSK3 putative target sites mutated to alanine. Our data indicated that GSK3 exerts a dual action on α_1A-AR participating in agonist-mediated desensitization and internalization and avoiding PMA-induced desensitization.

The α1-adrenoceptor-mediated human hyperplastic prostate cells proliferation is impaired by EGF receptor inhibition

Benign prostatic hyperplasia (BPH) is an aging-related and progressive disease linked to an up-regulation of α₁-adrenoceptors. The participation of EGF receptors (EGFR) in the GPCRs' signalosome has been described but so far data about the contribution of these receptors to prostatic stromal hyperplasia are scanty. We isolated and cultured vimentin-positive prostate stromal cells obtained from BPH patients. According to intracellular Ca²⁺ measurements, cell proliferation and Western blotting assays, these cultured hyperplastic stromal cells express functional α_1-adrenoceptors and EGFR, and proliferate in response to the α_1-adrenoceptor agonist phenylephrine. Interestingly, in these cells the inhibition of EGFR signaling with GM6001, CRM197, AG1478 or PD98059 was associated with full blockage of α₁-adrenoceptor-mediated cell proliferation, while cell treatment with each inhibitor alone did not alter basal cell growth. Moreover, the co-incubation of AG1478 (EGFR inhibitor) with α_1A/α_1D-adrenoceptor antagonists showed no additive inhibitory effect. These findings highlight a putative role of EGFR signaling to α₁-adrenoceptor-mediated human prostate hyperplasia, suggesting that the inhibition of this transactivation cascade could be useful to reduce BPH progression.

Sites phosphorylated in human α1B-adrenoceptors in response to noradrenaline and phorbol myristate acetate

Phosphorylation of the human α_1B-adrenergic receptor (fused with the green fluorescent protein) was studied employing the inducible Flp-ln HEK293 T-Rex system for expression. Serine/alanine substitutions were performed in five sites corresponding to those previously identified as phosphorylation targets in the hamster ortholog. Desensitization was decreased in these mutants but receptor phosphorylation was still clearly detected. The protein phosphorylation of the wild-type receptor (fused to the green fluorescent protein) was studied, using mass spectrometry, under baseline and stimulated conditions (noradrenaline or phorbol myristate acetate). Basal phosphorylation was detected at sites located at the intracellular loop 3 and carboxyl terminus, and the number of sites detected increased under agonist activation and stimulation of protein kinase C. The phosphorylation patterns differed under the distinct conditions. Three of the phosphorylation sites detected in this work corresponded to those observed in the hamster receptor. The phosphorylation sites detected included the following: a) at the intracellular loop 3: serines 246, 248, 257, 267, and 277; and threonines 252, 264, and 268, and b) at the carboxyl terminus: serines 396, 400, 402, 406, 423, 425, 427, 455, and 470, and threonines 387, 392, 420, and 475. Our data indicate that complex phosphorylation patterns exist and suggest the possibility that such differences could be relevant in receptor function and subcellular localization.

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.

Updates in the function and regulation of α1 -adrenoceptors

α₁ -Adrenoceptors are seven transmembrane domain GPCRs involved in numerous physiological functions controlled by the endogenous catecholamines, noradrenaline and adrenaline, and targeted by drugs useful in therapeutics. Three separate genes, whose products are named α_1A -, α_1B -, and α_1D - adrenoceptors, encode these receptors. Although the existence of multiple α₁ -adrenoceptors has been acknowledged for almost 25 years, the specific functions regulated by each subtype are still largely unknown. Despite the limited comprehension, the identification of a single class of subtype-selective ligands for the α_1A - adrenoceptors, the so-called α-blockers for prostate dysfunction, has led to major improvement in therapeutics, demonstrating the need for continued efforts in the field. This review article surveys the tissue distribution of the three α₁ -adrenoceptor subtypes in the cardiovascular system, genitourinary system, and CNS, highlighting the functions already identified as mediated by the predominant activation of specific subtypes. In addition, this review covers the recent advances in the understanding of the molecular mechanisms involved in the regulation of each of the α₁ -adrenoceptor subtypes by phosphorylation and interaction with proteins involved in their desensitization and internalization. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Different phosphorylation patterns regulate α1D-adrenoceptor signaling and desensitization

Human α_1D-adrenoceptors (α_1D-ARs) are a group of the seven transmembrane-spanning proteins that mediate many of the physiological and pathophysiological actions of adrenaline and noradrenaline. Although it is known that α_1D-ARs are phosphoproteins, their specific phosphorylation sites and the kinases involved in their phosphorylation remain largely unknown. Using a combination of in silico analysis, mass spectrometry and site directed mutagenesis, we identified distinct α_1D-AR phosphorylation patterns during noradrenaline- or phorbol ester-mediated desensitizations. We found that the G protein coupled receptor kinase, GRK2, and conventional protein kinases C isoforms α/β, phosphorylate α_1D-AR during these processes. Furthermore, we showed that the phosphorylated residues are located in the receptor's third intracellular loop (S300, S323, T328, S331, S332, S334) and carboxyl region (S441, T442, T477, S486, S492, T507, S515, S516, S518, S543) and are conserved among orthologues but are not conserved among the other human α₁-adrenoceptor subtypes. Additionally, we found that phosphorylation in either the third intracellular loop or carboxyl tail was sufficient to regulate calcium signaling desensitization. By contrast, mutations in either of these two domains significantly altered mitogen activated protein kinase (ERK) pathway and receptor internalization, suggesting that they have differential regulatory mechanisms. Our data provide new insights into the functional repercussions of these posttranslational modifications in signaling outcomes and desensitization.

Effects of arachidonic acid on FFA4 receptor: Signaling, phosphorylation and internalization

Arachidonic acid increased intracellular calcium, in cells expressing green fluorescent protein-tagged human FFA4 receptors, with an EC₅₀ of ~40µM. This action was not blocked by cyclooxygenase or lipoxigenase inhibitors but it was inhibited by AH7614, a FFA4 antagonist. Arachidonic acid induced ERK activation accompanied by EGF receptor transactivation. However, EGF transactivation was not the major mechanism through which the fatty acid induced ERK phosphorylation, as evidenced by the inability of AG1478 to block it. Arachidonic acid increased FFA4 receptor phosphorylation that reached its maximum within 15min with an EC₅₀ of ~30µM; inhibitors of protein kinase C partially diminish this effect and AH7614 blocked it. Arachidonic acid induced rapid and sustained Akt/PKB phosphorylation and FFA4 - β-arrestin interaction. Confocal microscopy evidenced that FFA4 receptor activation and phosphorylation were associated to internalization. In conclusion, arachidonic acid is a bona fide FFA4 receptor agonist.

Protein Kinase C Activation Promotes α1B-Adrenoceptor Internalization and Late Endosome Trafficking through Rab9 Interaction. Role in Heterologous Desensitization

Upon agonist stimulation, α_1B-adrenergic receptors couple to G_q proteins, calcium signaling and protein kinase C activation; subsequently, the receptors are phosphorylated, desensitized, and internalized. Internalization seems to involve scaffolding proteins, such as β-arrestin and clathrin. However, the fine mechanisms that participate remain unsolved. The roles of protein kinase C and the small GTPase, Rab9, in α_1B-AR vesicular traffic were investigated by studying α_1B-adrenergic receptor-Rab protein interactions, using Förster resonance energy transfer (FRET), confocal microscopy, and intracellular calcium quantitation. In human embryonic kidney 293 cells overexpressing Discosoma spp. red fluorescent protein (DsRed)-tagged α_1B-ARs and enhanced green fluorescent protein--tagged Rab proteins, pharmacological protein kinase C activation mimicked α_1B-AR traffic elicited by nonrelated agents, such as sphingosine 1-phosphate (i.e., transient α_1B-AR-Rab5 FRET signal followed by a sustained α_1B-AR-Rab9 interaction), suggesting brief receptor localization in early endosomes and transfer to late endosomes. This latter interaction was abrogated by blocking protein kinase C activity, resulting in receptor retention at the plasma membrane. Similar effects were observed when a dominant-negative Rab9 mutant (Rab9-GDP) was employed. When α_1B-adrenergic receptors that had been mutated at protein kinase C phosphorylation sites (S396A, S402A) were used, phorbol ester-induced desensitization of the calcium response was markedly decreased; however, interaction with Rab9 was only partially decreased and internalization was observed in response to phorbol esters and sphingosine 1-phosphate. Finally, Rab9-GDP expression did not affect adrenergic-mediated calcium response but abolished receptor traffic and altered desensitization. Data suggest that protein kinase C modulates α_1B-adrenergic receptor transfer to late endosomes and that Rab9 regulates this process and participates in G protein-mediated signaling turn-off.

Novel Structural Approaches to Study GPCR Regulation

BACKGROUND

Upon natural agonist or pharmacological stimulation, G protein-coupled receptors (GPCRs) are subjected to posttranslational modifications, such as phosphorylation and ubiquitination. These posttranslational modifications allow protein-protein interactions that turn off and/or switch receptor signaling as well as trigger receptor internalization, recycling or degradation, among other responses. Characterization of these processes is essential to unravel the function and regulation of GPCR.

METHODS

In silico analysis and methods such as mass spectrometry have emerged as novel powerful tools. Both approaches have allowed proteomic studies to detect not only GPCR posttranslational modifications and receptor association with other signaling macromolecules but also to assess receptor conformational dynamics after ligand (agonist/antagonist) association.

RESULTS

this review aims to provide insights into some of these methodologies and to highlight how their use is enhancing our comprehension of GPCR function. We present an overview using data from different laboratories (including our own), particularly focusing on free fatty acid receptor 4 (FFA4) (previously known as GPR120) and α1A- and α1D-adrenergic receptors. From our perspective, these studies contribute to the understanding of GPCR regulation and will help to design better therapeutic agents.

A Latin American Perspective on G Protein-Coupled Receptors

G protein-coupled receptors are sensors that interact with a large variety of elements, including photons, ions, and large proteins. Not surprisingly, these receptors participate in the numerous normal physiologic processes that we refer to as health and in its perturbations that constitute disease. It has been estimated that a large percentage of drugs currently used in therapeutics target these proteins, and this percentage is larger when illegal drugs are included. The state of the art in this field can be defined with the oxymoron "constant change," and enormous progress has been made in recent years. A group of scientists working in Latin America were invited to contribute minireviews for this special section to present some of the work performed in this geographical region and foster further international collaboration.

New Multi-target Antagonists of α1A-, α1D-Adrenoceptors and 5-HT1A Receptors Reduce Human Hyperplastic Prostate Cell Growth and the Increase of Intraurethral Pressure

Benign prostatic hyperplasia (BPH) is characterized by stromal cell proliferation and contraction of the periurethral smooth muscle, causing lower urinary tract symptoms. Current BPH treatment, based on monotherapy with α1A-adrenoceptor antagonists, is helpful for many patients, but insufficient for others, and recent reports suggest that stimulation of α1D-adrenoceptors and 5-hydroxytryptamine (serotonin) (5-HT)1A receptors contributes to cell proliferation. In this study, we investigated the potential of three N-phenylpiperazine derivatives (LDT3, LDT5, and LDT8) as multi-target antagonists of BPH-associated receptors. The affinity and efficacy of LDTs were estimated in isometric contraction and competition-binding assays using tissues (prostate and aorta) and brain membrane samples enriched in specific on- or off-target receptors. LDTs' potency was estimated in intracellular Ca(2+) elevation assays using cells overexpressing human α1-adrenoceptor subtypes. The antiproliferative effect of LDTs on prostate cells from BPH patients was evaluated by viable cell counting and 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyl tetrazolium bromide assays. We also determined LDTs' effects on rat intraurethral and arterial pressure. LDT3 and LDT5 are potent antagonists of α1A-, α1D-adrenoceptors, and 5-HT1A receptors (Ki values in the nanomolar range), and fully inhibited phenylephrine- and 5-HT-induced proliferation of BPH cells. In vivo, LDT3 and LDT5 fully blocked the increase of intraurethral pressure (IUP) induced by phenylephrine at doses (ED50 of 0.15 and 0.09 μg.kg(-1), respectively) without effect on basal mean blood pressure. LDT3 and LDT5 are multi-target antagonists of key receptors in BPH, and are capable of triggering both prostate muscle relaxation and human hyperplastic prostate cell growth inhibition in vitro. Thus, LDT3 and LDT5 represent potential new lead compounds for BPH treatment.

α1B-adrenergic receptors differentially associate with Rab proteins during homologous and heterologous desensitization

Internalization of G protein-coupled receptors can be triggered by agonists or by other stimuli. The process begins within seconds of cell activation and contributes to receptor desensitization. The Rab GTPase family controls endocytosis, vesicular trafficking, and endosomal fusion. Among their remarkable properties is the differential distribution of its members on the surface of various organelles. In the endocytic pathway, Rab 5 controls traffic from the plasma membrane to early endosomes, whereas Rab 4 and Rab 11 regulate rapid and slow recycling from early endosomes to the plasma membrane, respectively. Moreover, Rab 7 and Rab 9 regulate the traffic from late endosomes to lysosomes and recycling to the trans-Golgi. We explore the possibility that α_1B-adrenergic receptor internalization induced by agonists (homologous) and by unrelated stimuli (heterologous) could involve different Rab proteins. This possibility was explored by Fluorescence Resonance Energy Transfer (FRET) using cells coexpressing α_1B-adrenergic receptors tagged with the red fluorescent protein, DsRed, and different Rab proteins tagged with the green fluorescent protein. It was observed that when α_1B-adrenergic receptors were stimulated with noradrenaline, the receptors interacted with proteins present in early endosomes, such as the early endosomes antigen 1, Rab 5, Rab 4, and Rab 11 but not with late endosome markers, such as Rab 9 and Rab 7. In contrast, sphingosine 1-phosphate stimulation induced rapid and transient α_1B-adrenergic receptor interaction of relatively small magnitude with Rab 5 and a more pronounced and sustained one with Rab 9; interaction was also observed with Rab 7. Moreover, the GTPase activity of the Rab proteins appears to be required because no FRET was observed when dominant-negative Rab mutants were employed. These data indicate that α_1B-adrenergic receptors are directed to different endocytic vesicles depending on the desensitization type (homologous vs. heterologous).

The phosphoinositide-dependent protein kinase 1 inhibitor, UCN-01, induces fragmentation: possible role of metalloproteinases

Phosphoinositide-dependent protein kinase 1 (PDK1) is a key enzyme, master regulator of cellular proliferation and metabolism; it is considered a key target for pharmacological intervention. Using membranes obtained from DDT1 MF-2 cells, phospho-PDK1 was identified by Western blotting, as two major protein bands of Mr 58-68 kDa. Cell incubation with the PDK1 inhibitor, UCN-01, induced a time- and concentration-dependent decrease in the amount of phospho-PDK1 with a concomitant appearance of a ≈42 kDa phosphorylated fragment. Knocking down PDK1 diminished the amount of phospho-PDK1 detected in membranes, accompanied by similarly decreased fragment generation. UCN-01-induced fragment generation was also observed in membranes from cells stably expressing a myc-tagged PDK1 construct. Other PDK1 inhibitors were also tested: OSU-03012 induced a clear decrease in phospho-PDK1 and increased the presence of the phosphorylated fragment in membrane preparations; in contrast, GSK2334470 and staurosporine induced only marginal increases in the amount of PDK1 fragment. Galardin and batimastat, two metalloproteinase inhibitors, markedly attenuated inhibitor-induced PDK1 fragment generation. Metalloproteinases 2, 3, and 9 co-immunoprecipitated with myc-PDK1 under baseline conditions and this interaction was stimulated by UCN-01; batimastat also markedly diminished this effect of the PDK1 inhibitor. Our results indicate that a series of protein kinase inhibitors, namely UCN-01 and OSU-03012 and to a lesser extent GSK2334470 and staurosporine induce PDK1 fragmentation and suggest that metalloproteinases could participate in this effect.

Visualizing G protein-coupled receptors in action through confocal microscopy techniques

G protein-coupled receptors constitute one of the most abundant entities in cellular communication. Elucidation of their structure and function as well as of their regulation began 30-40 years ago and the advance has markedly increased during the last 15 years. They participate in a plethora of cell functions such as regulation of metabolic fluxes, contraction, secretion, differentiation, or proliferation, and in essentially all activities of our organism; these receptors are targets of a large proportion of prescribed and illegal drugs. Fluorescence techniques have been used to study receptors for many years. The experimental result was usually a two-dimensional (2D) micrograph. Today, the result can be a spatiotemporal (four-dimensional, 4D) movie. Advances in microscopy, fluorescent protein design, and computer-assisted analysis have been of great importance to increase our knowledge on receptor regulation and function and create opportunities for future research. In this review we briefly depict the state of the art of the G protein-coupled receptor field and the methodologies used to study G protein-coupled receptor location, trafficking, dimerization, and other types of receptor-protein interaction. Fluorescence techniques now permit the capture of receptor images with high resolution and, together with a variety of fluorescent dyes that color organelles (such as the plasma membrane or the nucleus) or the cytoskeleton, allow researchers to obtain a much clearer idea of what is taking place at the cellular level. These developments are changing the way we explore cell communication and signal transduction, permitting deeper understanding of the physiological and pathophysiological processes.

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.

Differential phosphorylation, desensitization, and internalization of α1A-adrenoceptors activated by norepinephrine and oxymetazoline

Loss of response on repetitive drug exposure (i.e., tachyphylaxis) is a particular problem for the vasoconstrictor effects of medications containing oxymetazoline (OXY), an α1-adrenoceptor (AR) agonist of the imidazoline class. One cause of tachyphylaxis is receptor desensitization, usually accompanied by phosphorylation and internalization. It is well established that α1A-ARs are less phosphorylated, desensitized, and internalized on exposure to the phenethylamines norepinephrine (NE), epinephrine, or phenylephrine (PE) than are the α1B and α1D subtypes. However, here we show in human embryonic kidney-293 cells that the low-efficacy agonist OXY induces G protein-coupled receptor kinase 2-dependent α1A-AR phosphorylation, followed by rapid desensitization and internalization (∼40% internalization after 5 minutes of stimulation), whereas phosphorylation of α1A-ARs exposed to NE depends to a large extent on protein kinase C activity and is not followed by desensitization, and the receptors undergo delayed internalization (∼35% after 60 minutes of stimulation). Native α1A-ARs from rat tail artery and vas deferens are also desensitized by OXY, but not by NE or PE, indicating that this property of OXY is not limited to recombinant receptors expressed in cell systems. The results of the present study are clearly indicative of agonist-directed α1A-AR regulation. OXY shows functional selectivity relative to NE and PE at α1A-ARs, leading to significant receptor desensitization and internalization, which is important in view of the therapeutic vasoconstrictor effects of this drug and the varied biologic process regulated by α1A-ARs.

Sphingosine 1-phosphate-mediated α1B-adrenoceptor desensitization and phosphorylation. Direct and paracrine/autocrine actions

Sphingosine-1-phosphate-induced α1B-adrenergic receptor desensitization and phosphorylation were studied in rat-1 fibroblasts stably expressing enhanced green fluorescent protein-tagged adrenoceptors. Sphingosine-1-phosphate induced adrenoceptor desensitization and phosphorylation through a signaling cascade that involved phosphoinositide 3-kinase and protein kinase C activities. The autocrine/paracrine role of sphingosine-1-phosphate was also studied. It was observed that activation of receptor tyrosine kinases, such as insulin growth factor-1 (IGF-I) and epidermal growth factor (EGF) receptors increased sphingosine kinase activity. Such activation and consequent production of sphingosine-1-phosphate appear to be functionally relevant in IGF-I- and EGF-induced α1B-adrenoceptor phosphorylation and desensitization as evidenced by the following facts: a) expression of a catalytically inactive (dominant-negative) mutant of sphingosine kinase 1 or b) S1P1 receptor knockdown markedly reduced this growth factor action. This action of sphingosine-1-phosphate involves EGF receptor transactivation. In addition, taking advantage of the presence of the eGFP tag in the receptor construction, we showed that S1P was capable of inducing α1B-adrenergic receptor internalization and that its autocrine/paracrine generation was relevant for internalization induced by IGF-I. Four distinct hormone receptors and two autocrine/paracrine mediators participate in IGF-I receptor-α1B-adrenergic receptor crosstalk.

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.

Roles of the α1A-adrenergic receptor carboxyl tail in protein kinase C-induced phosphorylation and desensitization

Noradrenaline- and tetradecanoyl phorbol acetate (TPA)-induced phosphorylation and functional desensitization of the following receptors were studied: (1) wild-type bovine α(1A)- and hamster α(1B)-adrenergic receptors (ARs), (2) chimeric ARs in which the carboxyl terminus tails were exchanged (α(1AB)- and α(1BA)-ARs), and (3) carboxyl terminus-truncated α(1A)-ARs fussed to enhanced green fluorescent protein. Noradrenaline and TPA pronouncedly increased α(1B)-AR phosphorylation while TPA markedly desensitized these receptors. In contrast, TPA-induced desensitization and TPA- and noradrenaline-induced phosphorylation of α(1A)-ARs were clearly of lesser magnitude. Chimeric ARs with exchanged carboxyl terminus tails showed that the extent of phosphorylation reflected the carboxyl domain rather than the receptor core. Surprisingly, there was no correlation between phosphorylation and functional desensitization, i.e., activation of protein kinase C clearly desensitized both chimeric receptors to a similar extent. Interestingly, TPA and noradrenaline increased carboxyl terminus-truncated α(1A)-AR phosphorylation and TPA also induced receptor desensitization. We were unable to detect carboxyl terminus-truncated α(1A)-AR internalization after 5-min stimulations with noradrenaline or TPA. Our results suggest the following: (a) the α(1A)-AR carboxyl terminus tail was not essential for signaling or desensitization; (b) carboxyl terminus tail exchange "transplanted" the phosphorylation pattern of the receptors, but the functional consequences of such a transplant were very limited; (c) α(1A)-AR desensitization was not associated to receptor internalization.

Dissecting how receptor tyrosine kinases modulate G protein-coupled receptor function

Receptor tyrosine kinases and G protein-coupled receptors modulate physiological processes and are also involved in the pathogenesis of some diseases. These receptors have intense bidirectional crosstalks leading to interactions in their signaling pathways and also modulation of the receptors themselves. In some cases, the receptor tyrosine kinases phosphorylate G protein-coupled receptors whereas in others phosphoinositide 3-kinase, protein kinase B and protein kinase C are key elements in these crosstalks. Two paracrine/ autocrine processes also participate, i.e., epidermal growth factor transactivation and sphingosine 1-phosphate generation and signaling. G proteins seem to mediate actions of receptor tyrosine kinases, but how this takes place is far from completely understood; some models are presented. Recent data indicate that the mitogen activated protein kinase cascade also mediate crosstalks. In the present perspective these processes are outlined using information from receptors that have been intensively studied, and important gaps in our knowledge are indicated.

Effect of inhibitors of mitogen-activated protein kinase kinase on alpha(1B)-adrenoceptor phosphorylation

1 Mitogen-activated protein kinases mediate hormone/neurotransmitter action on proliferation and differentiation and participate in receptor regulation. The effect of inhibitors of mitogen-activated kinase kinase (MEK) on alpha(1B)-adrenoceptor phosphorylation state and function was studied using different cell lines. It was observed that at nanomolar concentrations the MEK inhibitors, PD98059 (2'-amino-3'-methoxyflavone) and UO126 [1,4-(diamino-2,3-dicyano/1,4-bis-(2-aminophenylthio)-butadiene], increased alpha(1B)-adrenoceptor phosphorylation and diminished the functional response of this receptor to noradrenaline. These agents did not alter the action of lysophosphatidic acid. 2 Staurosporine (IC(50) approximately 0.8 nm) (a general protein kinase inhibitor) and bis-indolyl-maleimide I (IC(50) approximately 200 nm) (a selective protein kinase C inhibitor) inhibited PD98059-induced alpha(1B)-adrenoceptor phosphorylation. In contrast, neither wortmannin (phosphoinositide 3-kinase inhibitor) nor genistein (protein tyrosine kinase inhibitor) had any effect. The data suggest the possibility that MEK might exert control on the activity of the enzymes that regulate receptor phosphorylation, such as G-protein-coupled receptor kinases, protein kinase C or serine/threonine protein phosphatases. 3 Coimmunoprecipitation studies showed a constant association of total extracellular signal-regulated kinase 2 (ERK2) with alpha(1B)-adrenoceptors. Association of phospho-ERK 1/2 to alpha(1B)-adrenoceptors increased not only in response to agonist but also in response to agents that increase alpha(1B)-adrenoceptor and ERK1/2 phosphorylation [such as endothelin-1, phorbol 12-myristate-13-acetate (PMA) and epidermal growth factor (EGF)]; not surprisingly, PD98059 decreased this effect. 4 Our data show that blockade of MEK activity results in increased alpha(1B)-adrenoceptor phosphorylation, diminished adrenoceptor function and perturbation of receptor-ERK1/2 interaction.

Receptor tyrosine kinases regulate alpha1D-adrenoceptor signaling properties: phosphorylation and desensitization

Human alpha(1D)-adrenoceptors (truncated at the amino terminus (Delta1-79) to increase their membrane expression) were stably expressed in Rat-1 fibroblasts (1-1.5 pmol/mg protein). The receptors were functional as evidenced by a robust increase in intracellular calcium in response to noradrenaline. Using this cell line, the possibility that activation of receptor tyrosine kinases could modulate this adrenoceptor subtype was studied. It was observed that cell preincubation with insulin, IGF-I, EGF or PDGF markedly reduced the intracellular calcium increase observed in response to noradrenaline. Inhibitors of PI3K and PKC essentially blocked insulin-, IGF-I- and EGF-induced desensitizations. Interestingly, PDGF-induced alpha(1D)-adrenergic desensitization was only partially ameliorated by PI3K inhibitors and was not affected by those of PKC. Insulin, IGF-I, EGF and PDGF induced concentration-dependent increases in the phosphorylation state of alpha(1D)-adrenoceptors; phosphorylation took place on serine residues. Inhibitors of PI3K and PKC markedly reduced the effects of insulin, IGF-I and EGF on this parameter. These inhibitors only marginally reduced PDGF-induced alpha(1D)-adrenoceptors phosphorylation. The ability of IGF-I to induce alpha(1D)-adrenergic desensitization and phosphorylation was confirmed in cells expressing non-truncated rat alpha(1D)-adrenoceptors. Our data indicate that the function and phosphorylation state of alpha(1D)-adrenoceptors is modulated by activation of receptor tyrosine kinases. Insulin, IGF-I and EGF actions take place through the action of PI3K and PKC; additional pathway(s) seem to participate in PDGF-induced alpha(1D)-adrenoceptor desensitization and phosphorylation.

Roles of c-Src in alpha1B-adrenoceptor phosphorylation and desensitization

1 The role of the protein tyrosine kinase, c-Src, on the function and phosphorylation of alpha1B-adrenoceptors (alpha1B-AR) and their association with G-protein-coupled receptor kinase (GRK) isozymes was studied. 2 Inhibitors of this kinase (PP2 and Src Inhibitor II) decreased ( approximately 50-75%) noradrenaline- (NA) and phorbol myristate acetate-mediated receptor phosphorylation. Expression of a dominant-negative mutant of c-Src similarly reduced receptor phosphorylation induced by the natural agonists, active phorbol esters and endothelin-1 (ET-1). 3 c-Src, GRK2, GRK3 and GRK5 coimmunoprecipitate with alpha1B-ARs in the basal state. In cells treated with NA or phorbol myristate acetate the amount of coimmunoprecipitated GRK2 and GRK3 increased ( approximately 2- to 3-fold), while treatment with ET-1 only augmented the amount of coimmunoprecipitated GRK2 ( approximately 2-fold). The Src inhibitor, PP2, markedly attenuated all these increases. 4 Cell pretreatment with PP2 amplified the increase in intracellular-free calcium observed with NA, in the basal state and after the stimulation (desensitization) induced by ET-1. 5 The data suggest a role of c-Src in alpha1B-AR desensitization/phosphorylation and in the interaction of these ARs with GRKs.

Regulation of LPA receptor function by estrogens

17beta-Estradiol induced LPA(1) receptor desensitization in C9 cells stably expressing LPA(1) receptors and transiently expressing estrogen receptor alpha. Such desensitization was evidenced by a reduction in lysophosphatidic acid-mediated Ca(2+)mobilization and it was associated to receptor phosphorylation and internalization. These effects of 17beta-estradiol were rapid (taking place over 5 min) and were blocked by the estrogen receptor antagonist ICI 182780. Similarly, inhibitors of phosphoinositide 3-kinase (wortmannin and LY294002) and of protein kinase C (staurosporine and Gö 6976) blocked 17beta-estradiol-induced LPA(1) receptor desensitization and phosphorylation. Confocal microscopy evidenced LPA(1) receptor internalization in response to 17beta-estradiol treatment. Association between LPA(1) receptors and protein kinase C alpha was suggested by co-immunoprecipitation assays. Protein kinase C alpha was associated with LPA(1) receptors in the absence of stimulus and such association further increased in a dynamic fashion in response to 17beta-estradiol. The results demonstrated that in C9 cells estrogens modulate LPA(1) action through estrogen receptor alpha with the participation of protein kinase C alpha and phosphoinositide 3-kinase.

Phosphorylation, desensitization and internalization of human alpha1B-adrenoceptors induced by insulin-like growth factor-I

The effect of insulin-like growth factor-I (IGF-I) on human alpha(1B)-adrenoceptor function, phosphorylation state and cellular location was studied. Rat-1 fibroblasts were transfected with a plasmid construction containing enhanced green fluorescent protein joined to the carboxyl terminus of the human alpha(1B)-adrenoceptor. Receptors were identified by radioligand binding and photoaffinity labeling, and were immunoprecipitated with an antiserum generated against the enhanced green fluorescent protein. The receptor was functional, as evidenced by noradrenaline action on intracellular calcium and inositol phosphate production. IGF-I had no significant effect by itself on these parameters but markedly reduced the effects of noradrenaline. IGF-I induced alpha(1B)-adrenoceptor phosphorylation, which was markedly reduced by the following agents: pertussis toxin, a metalloproteinase inhibitor, diphtheria toxin mutant CRM 197, an epidermal growth factor (EGF) receptor intrinsic kinase activity inhibitor, and by phosphoinositide 3-kinase and protein kinase C inhibitors. IGF-I action appears to involve activation of a pertussis toxin-sensitive G protein, shedding of heparin-binding EGF and autocrine activation of EGF receptors. G protein subunits and phosphotyrosine residues stimulate phosphoinositide 3-kinase activity leading to activation of protein kinase C, which in turn phosphorylates alpha(1B)-adrenoceptors. Confocal fluorescent microscopy showed that alpha(1B)-adrenoceptors fussed to the green fluorescent protein were located in plasma membrane and intracellular vesicles in the basal state. IGF-I induced receptor redistribution favoring the intracellular location; this effect was blocked by hypertonic sucrose and concanavalin A. Our data show that IGF-I induces alpha(1B)-adrenoceptor desensitization associated to receptor phosphorylation and internalization.

Insulin-like growth factor-I induces alpha(1B)-adrenergic receptor phosphorylation through G beta gamma and epidermal growth factor receptor transactivation

IGF-I induces alpha(1B)-adrenoceptor (alpha(1B)-AR) phosphorylation. The effect of IGF-I was rapid and transient, reaching near-maximal values at 10 min and decreasing after 30 min; it was observed at low IGF-I concentrations (EC(50) approximately 10 ng/ml) and was associated to receptor desensitization as evidenced by a decreased alpha(1B)-adrenergic effect on intracellular calcium and production of inositol phosphates. The effect of IGF-I was markedly decreased in cells treated with pertussis toxin suggesting involvement of pertussis toxin-sensitive G proteins. Transfection of the carboxyl terminus of the beta-adrenergic receptor kinase or the Deltap85 mutant of phosphoinositide 3-kinase (PI3K) markedly decreased the alpha(1B)-AR phosphorylation induced by IGF-I without decreasing the receptor phosphorylation induced by noradrenaline. Inhibitors of PI3K and protein kinase C blocked IGF-I-induced alpha(1B)-AR phosphorylation. In addition, it was observed that AG1478, an inhibitor of the epidermal growth factor (EGF) receptor kinase, and BB-94, a metalloproteinase inhibitor, also diminished IGF-I-induced adrenoceptor phosphorylation. The data clearly show that IGF-I triggers a complex signaling pathway, which leads to the phosphorylation and desensitization of a serpentine G protein-coupled receptor, suggesting the following hypothetical model: 1) stimulation of IGF-I receptors activate pertussis toxin-sensitive G proteins; 2) the growth factor action activates metalloproteinases, which catalyze heparin binding-EGF shedding, and transactivation of EGF receptors, and 3) dissociated Gbetagamma subunits and phosphotyrosine residues seem to trigger PI3K activity, which leads to activation of protein kinase C, resulting in alpha(1B)-AR phosphorylation and desensitization.

Role of epidermal growth factor receptor transactivation in alpha1B-adrenoceptor phosphorylation

Phosphorylation of G protein-coupled receptors is one of the earliest events that regulate their function. Current evidence indicates that homologous desensitization of these receptors mainly involves G protein-coupled receptor kinases whereas in heterologous desensitization second messenger-activated kinases play key roles. Recent data show that transactivation of EGF (epidermal growth factor) receptors may also play a role in receptor phosphorylation. The role of this process was studied for the alpha1B-adrenoceptor phosphorylation induced by agents acting through different processes using inhibitors to block the EGF receptor transactivation process at different levels. Experiments were performed using transfected rat-1 fibroblasts that express alpha1B-adrenoceptors in a stably fashion. A metalloproteinase inhibitor, an anti-heparin-binding-EGF-selective antibody, and a selective EGF-receptor kinase inhibitor blocked the alpha1B-adrenoceptor phosphorylation induced by noradrenaline or endothelin-1. Our results indicate that shedding of heparin-binding-EGF, transactivation of EGF receptors plays a more general role in alpha1B-adrenoceptor phosphorylation than previously anticipated. It is possible that other receptors/channels could be modulated through a similar pathway.

Estrogens Cross-Talk to α1b-Adrenergic Receptors

beta-Estradiol induced alpha1b-adrenergic receptor desensitization in U373 MG cells stably expressing alpha1b-adrenoceptors, as evidenced by a reduction in the adrenergic-mediated Ca2+ mobilization; desensitization was associated with receptor phosphorylation and internalization. These effects of beta-estradiol were rapid (taking place during 15 min) and were blocked by the estrogen receptor antagonist ICI 182,780 (faslodex). Likewise, inhibitors of phosphoinositide 3-kinase [wortmannin and 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002)] and of protein kinase C [staurosporine, 3-[1-[3-(amidinothio)propyl-1H-indol-3-yl]-3-(1-methyl-1H-indol-3-yl) maleimide (Ro31-8220), and rottlerin] blocked the desensitization and phosphorylation of alpha1b-adrenoceptors induced by estradiol. The formation of a complex was suggested by coimmunoprecipitation assays. The regulatory and catalytic subunits of phosphoinositide 3-kinase (p85 and p110) and protein kinase C delta were associated with alpha1b-adrenoceptors in the absence of stimulus, and such association further increased in a dynamic fashion in response to beta-estradiol. In cells cotransfected with the estrogen receptor alpha and alpha1b-adrenoceptors, beta-estradiol induced phosphorylation, desensitization and internalization of the adrenergic receptors; pretreatment with ICI 182,780 inhibited these effects. Our data support the idea that estrogens modulate alpha1b-adrenergic action through estrogen receptor alpha.

Okadaic acid increases the phosphorylation state of alpha1A-adrenoceptors and induces receptor desensitization

Okadaic acid, a protein phosphatase inhibitor, and phorbol myristate acetate, an activator of protein kinase C, increased the phosphorylation state of alpha1A-adrenergic receptors. The effects of these agents were of similar magnitude but that of okadaic acid developed more slowly. Wortmannin (inhibitor of phosphoinositide 3-kinase), but not staurosporine (inhibitor of protein kinase C), abolished the effect of okadaic acid on the alpha1A-adrenoceptor phosphorylation state. The effect of phorbol myristate acetate on this parameter was blocked by staurosporine and only partially inhibited by wortmannin. Okadaic acid markedly increased the co-immunoprecipitation of both the catalytic and regulatory subunits of phosphatidylinositol 3-kinase and of Akt/protein kinase B with the adrenoceptor and only marginally increases receptor association with protein kinase C epsilon. Okadaic acid induced desensitization of alpha1A-adrenoceptors as evidenced by a decreased ability of noradrenaline to increase intracellular calcium. Such desensitization was fully reverted by wortmannin. Our data indicate that inhibition of serine/threonine protein phosphatases increases the phosphorylation state of alpha1A-adrenergic receptor and alters the adrenoceptor function.

The elusive alpha(1D)-adrenoceptor: molecular and cellular characteristics and integrative roles

alpha(1)-Adrenoceptors seem to play key roles in cardiovascular, genitourinary, and central nervous system functions. This review will be focused on alpha(1D)-adrenoceptors. These receptors have intrinsic activity, and many of the more commonly used antagonists are in reality inverse agonists. alpha(1D)-Adrenoceptors are phosphorylated in the basal state, and the natural agonists, adrenaline and noradrenaline, increase their phosphorylation; similar effects are induced by direct activation of protein kinase C and through activation of nonadrenergic receptors. Interestingly, a large proportion of alpha(1D)-adrenoceptors are located in intracellular vesicles. Such intracellular location can be changed to surface expression through the use of inverse agonists and coexpression of alpha(1B)-adrenoceptors, which seem to act as pharmacological chaperons for proper plasma membrane insertion. The alpha(1D)-adrenoceptor amino terminus seems to contain a signal that keeps the receptor intracellularly, but interaction with other proteins may also contribute. The precise relationship between the intrinsic activity, phosphorylation, and intracellular location is currently unknown. alpha(1D)-Adrenoceptor activation induces contraction in a variety of vessels, and a role in the control of blood pressure has been suggested. Studies using young prehypertensive and adult spontaneously hypertensive rats as well as knockout mice suggest that vascular alpha(1D)-adrenoceptors are involved in the genesis/maintenance of hypertension.

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.

Insulin induces α1B-adrenergic receptor phosphorylation and desensitization

The ability of insulin to induce alpha1B-adrenoceptor phosphorylation and desensitization was tested in two model systems: rat-1 cells that stably express alpha1B-adrenoceptors, through transfection, and endogenously express insulin receptors and DDT1 MF2 cells that endogenously express both receptors. Insulin induced concentration-dependent increases in the phosphorylation state of the adrenergic receptors in the two models with similar EC50 values (0.5-2 nM). The effect was rapid in the two systems but it was sustained in rat-1 cells and transient in DDT1 MF2 cells. In both cell lines, the insulin-mediated phosphorylation of alpha1B-adrenoceptors was blocked by wortmannin and LY 294002, and by staurosporine and bisindolylmaleimide I, indicating that the effect involved phosphoinositide 3-kinase and protein kinase C activities. The adrenoceptor phosphorylation induced by insulin was associated to desensitization as evidences by a diminished elevation of intracellular calcium in response to noradrenaline. Inhibitors of phosphoinositide 3-kinase and protein kinase C blocked the functional desensitization induced by insulin.

Human alpha1D-adrenoceptor phosphorylation and desensitization

Rat-1 fibroblast were transfected with a plasmid containing the cDNA of the human alpha(1D)-adrenoceptor. A cell line was isolated that stably expressed the receptor as evidenced by BMY 7378-sensitive noradrenaline-induced increases in intracellular calcium concentration. The effect of noradrenaline was blocked by active phorbol esters; such blockade was mediated by protein kinase C (PKC) as evidenced by its inhibition by staurosporine or the downregulation of this protein kinase. Radioligand binding experiments showed expression of receptors with high affinity for [3H]tamsulosin (K(D) 0.30 +/- 0.05 nM) but low density (B(max) 35 +/- 4 fmol/mg protein). The receptors had the expected orders of potency for agonists (adrenaline = noradrenaline > oxymetazoline) and antagonists (BMY 7378 > 5-methyl-urapidil = phentolamine). Photoaffinity labeling identified the receptor as a band of M(r) 70-80kDa, which could be immunoprecipitated with a selective anti-alpha(1D)-adrenoceptor antiserum. In cells metabolically labeled with radioactive phosphate the adrenoceptor was identified as a phosphoprotein whose phosphorylation state was increased by the agonist, noradrenaline, and by phorbol myristate acetate. The data indicate that the human alpha(1D)-adrenoceptor function was regulated through phosphorylation by PKC.

Peroxovanadate induces $alpha;1B-adrenoceptor phosphorylation and association with protein kinase C

Peroxovanadate induced a marked increase in the phosphorylation state of alpha(1B)-adrenoceptors. The effect was dose-dependent (EC(50) approximately 2 microM) and rapid, reaching its maximum in 5 min and remaining at this level for 30 min. Hydrogen peroxide also increased alpha(1B)-adrenoceptor phosphorylation but to a lesser extent, in an ephemeral fashion, and only at high (millimolar) concentrations. The effect of peroxovanadate was blocked by inhibitors of protein kinase C such as staurosporine and rottlerin and only partially reduced by genistein and inhibitors of phosphoinositide 3-kinase. Protein kinase C alpha, delta and epsilon are associated with the alpha(1B)-adrenoceptor under basal conditions, as reflected by coimmunoprecipitation. Such association was increased by peroxovanadate for all isoforms. In contrast, hydrogen peroxide increased only the association of the epsilon isoform to the adrenoceptor. Peroxovanadate decreased the ability of noradrenaline to increase intracellular calcium, indicating that the receptor phosphorylation induced has functional consequences.

Lysophosphatidic acid induces alpha1B-adrenergic receptor phosphorylation through G beta gamma, phosphoinositide 3-kinase, protein kinase C and epidermal growth factor receptor transactivation

Lysophosphatidic acid (LPA) induces alpha(1B)-adrenoceptor phosphorylation through pertussis toxin-sensitive G proteins, phosphoinositide 3-kinase (PI3K) and protein kinase C (PKC). Here we showed that transfection of the carboxyl terminus of the beta-adrenergic receptor kinase (betaARK) or the Deltap85 mutant of PI3K markedly decreased the alpha(1B)-adrenoceptor phosphorylation induced by LPA without decreasing the receptor phosphorylations induced by active phorbol esters or noradrenaline. In addition, it was observed that inhibitors of epidermal growth factor (EGF) receptor kinase and of metalloproteinases and an anti-heparin binding-EGF antibody also diminish LPA-induced phosphorylation; such partial inhibitions were not additive, indicating that they occur through a common process. Our data indicate that stimulation of LPA receptors activates pertussis-toxin-sensitive G proteins. Dissociated Gbetagamma subunits initiate two processes: one of them involving activation of metalloproteinases, heparin binding-EGF shedding and transactivation of EGF receptors and another independent of these events. Both processes triggered PI3K activity, which lead to activation of PKC and this to alpha(1B)-adrenoceptor phosphorylation. This is the first demonstration of a role of EGF receptor transactivation in the phosphorylation of a G protein-coupled receptor.

G protein-coupled receptor cross-talk: pivotal roles of protein phosphorylation and protein-protein interactions

G protein-coupled receptors are dynamically regulated. Such regulation is frequently associated with covalent posttranslational modifications, such as phosphorylation, and with regulatory elements. G protein-coupled receptor kinases and casein kinase 1alpha play key roles in agonist-dependent receptor phosphorylations. Cross-talk between different receptors frequently involves second messenger-activated proteins, such as protein kinase C and protein kinase A. There is some evidence indicating that such kinases may not only turn off receptors but also switch their coupling to different G proteins. Receptor tyrosine kinases may phosphorylate and regulate G protein-coupled receptors and recent evidence indicates that other kinases, such as Akt/protein kinase B and phosphoinositide 3-kinase, may participate in such regulations as integrators of signalling. Recent approaches have shed new light on G protein-coupled receptor interactions that provide novel mechanisms of action and regulation. G protein-coupled receptor activities go beyond G proteins and receptors can be partners of exquisitely assembled signalling complexes through molecular bridges composed of multidomain proteins. The possibilities of interaction increase enormously through the diversity of structural and functional domains present in complex proteins, many of them just known as predicted sequences.