Review
α1-Adrenoceptors: function and phosphorylation

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Abstract

This review focuses on α1-adrenoceptor phosphorylation and function. Most of what is currently known is based on studies on the hamster α1B-adrenoceptor. It is known that agonist stimulation leads to homologous desensitization of these receptors and current evidence indicates that such decrease in receptor activity is associated with receptor phosphorylation. Such receptor phosphorylation seems to involve G protein-receptor kinases and the receptor phosphorylation sites have been located in the carboxyl tail (Ser404, Ser408, and Ser410). There is also evidence showing that in addition to desensitization, receptor phosphorylation is associated with internalization and roles of β-arrestins have been observed. Direct activation of protein kinase C leads to receptor desensitization/internalization associated with phosphorylation; the protein-kinase-C-catalyzed receptor phosphorylation sites have been also located in the carboxyl tail (Ser394 and Ser400). Activation of Gq-coupled receptors, such as the endothelin ETA receptor induces α1B-adrenoceptor phosphorylation and desensitization. Such effect involves protein kinase C and a yet unidentified tyrosine kinase. Activation of Gi-coupled receptors, such as the lysophosphatidic acid receptor, also induces α1B-adrenoceptor phosphorylation and desensitization. These effects involve protein kinase C and phosphatidyl inositol 3-kinase. Interestingly, activation of epidermal growth factor receptors also induces α1B-adrenoceptor phosphorylation and desensitization involving protein kinase C and phosphatidyl inositol 3-kinase. A pivotal role of these kinases in heterologous desensitization is evidenced.

Introduction

Adrenoceptors are a heterogeneous group of hormone/neurotransmitter receptors that mediate the central and peripheral actions of the natural adrenergic amines, adrenaline, and noradrenaline. These receptors constitute a subfamily of the seven transmembrane domains/G-protein-coupled receptors, and have been divided into three major types based on their affinities for agonists and antagonists, their coupling to signaling pathways, and their amino acid sequences. The major types are the α1-, the α2- and the β-adrenoceptors (Hieble et al., 1995). Three receptor isoforms have been cloned of each of these three major types (Hieble et al., 1995).

It is well-known that α1-adrenoceptors are mainly coupled to Gq/11 to stimulate phospholipase C activity. This enzyme catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate and the subsequent formation of inositol 1,4,5-trisphosphate and diacylglycerol. These molecules act as second messengers mediating intracellular Ca2+ release and activation of protein kinase C, respectively. Nevertheless, it is now clear that these α1-adrenoceptors can also be coupled to other classes of G proteins and, therefore, are capable of modulating different signaling pathways. Reviews on the structure, subtypes, tissue distribution and signaling of these receptors have been published recently Graham et al., 1996, Garcı́a-Sáinz et al., 1999c, Zhong and Minneman, 1999.

Usually, when cells are exposed to an agent, their subsequent responsiveness is decreased or blunted. This biological phenomenon is called desensitization. Different cellular processes, with different time frames, seem to be involved. These include modulation of receptor function, receptor internalization, recycling to the plasma membrane, degradation and regulation of expression Lefkowitz, 1998, Lefkowitz et al., 1998. We will restrict our review to the initial events, which are associated to receptor phosphorylation.

Phosphorylation of receptors with endogenous protein tyrosine kinase activity is associated to signaling turn-on and dephosphorylation with turning-off Carpenter, 1987, Yarden and Ullrich, 1988. Similarly, dephosphorylation of receptors with guanylyl cyclase activity seems to be involved in desensitization Schulz et al., 1989, Potter and Hunter, 1999. In contrast, phosphorylation of G-protein-coupled receptors is associated to signaling turn-off/desensitization and receptor dephosphorylation with resensitization Premont et al., 1995, Ferguson et al., 1997.

Two major types of desensitization have been distinguished: homologous and heterologous desensitizations. In the homologous type, reduced responsiveness is observed exclusively in the agent (a related agonist) that originally stimulated the cells. In heterologous desensitization a decreased responsiveness is observed in an agent or agents unrelated to the initial stimulus. Certainly, this classification is only operational and both desensitization processes may occur simultaneously in cells. Nevertheless, it has interesting mechanistic implications. In homologous desensitization hormone/neurotransmitter receptors seem to be the molecular targets of the process, whereas in heterologous desensitization, receptors and other distal signaling devices can be affected.

The present review focuses on α1-adrenoceptor phosphorylation and function. Reference will be made to what it is known in β2-adrenoceptor phosphorylation, since these receptors have been studied to a much bigger extent. As indicated above, α1-adrenoceptors are heterogeneous and three subtypes have been already cloned, i.e., α1A-, α1B-, and α1D-adrenoceptors (Hieble et al., 1995). Most of what it is known on the phosphorylation of this group of receptors is based on data on the hamster α1B subtype. We will concentrate on this subtype, and at the end of the review, we will address the differences that are likely to exist among the three subtypes, in the regulation of their function by phosphorylation Vázquez-Prado and Garcı́a-Sáinz, 1996, Vázquez-Prado et al., 1997, Vázquez-Prado et al., 2000.

Section snippets

General aspects

It is generally accepted that homologous desensitization involves receptor phosphorylation by G-protein-coupled receptor kinases Ferguson et al., 1997, Krupnick and Benovic, 1998. G protein receptor kinases are a family of at least six serine/threonine protein kinases that phosphorylate G-protein-coupled receptors only in the agonist-bound state. Accordingly, receptors occupied by agonist activate heterotrimeric G proteins, releasing Gβγ complexes. Such membrane-bound Gβγ heterodimers and

General aspects

It has been observed that many G-protein-coupled receptors are desensitized via feedback regulation by second-messenger-stimulated kinases, such as protein kinase A and protein kinase C. This type of desensitization is heterologous, since in principle, any stimulant that can increase cyclic AMP or diacylglycerol has the potential to induce the phosphorylation and desensitization of any G-protein-coupled receptor containing the consensus phosphorylation sites for protein kinase A or protein

Role of protein phosphatases in α1B-adrenoceptor phosphorylation

The phosphorylation state of a phosphoprotein results from the balance between the activities of the protein kinases and protein phosphatases that act on it. However, little is known about the role(s) of protein phosphatases in receptor phosphorylation and function. It has been suggested that endocytosis via clathrin-coated vesicles is crucial for resensitization of some G-protein-coupled receptors (Zhang et al., 1997). Receptors proceed from these vesicles to endosomes where they are

Different α1-adrenoceptor subtypes

Differential regulation within a family of receptors is frequently associated with the susceptibility of members to be modified by phosphorylation. Subtypes of α2- and β-adrenoceptors seem to be subject to desensitization according to their susceptibility as kinase substrates Liggett, 1998, Liggett et al., 1993, Kurose and Lefkowitz, 1994. The information on α1-adrenoceptors phosphorylation/desensitization is far less complete.

When transfected into Rat-1 fibroblasts, these receptors are

Final remarks

Both α1A-adrenoceptors and α1B-adrenoceptors are substrates of protein kinases. However, the α1B subtype seems to be a much better substrate than the α1A subtype, and the former is desensitized to a much greater extent than the latter. It is also clear that not all α1-adrenoceptor phosphorylations result in desensitization at a cellular level. In some cases, such as those observed by us with bradykinin (Medina et al., 1998) for α1B-adrenoceptors and with phorbol esters for α1A-adrenoceptors

Acknowledgements

Research in our laboratory has been partially supported by grants from Dirección General de Asuntos del Personal Académico (IN 200596 and IN 205199), Consejo Nacional de Ciencia y Tecnologı́a (27569N) and Fundación Miguel Alemán.

References (75)

  • D. Diviani et al.

    Effect of different G protein-coupled receptor kinases on phosphorylation and desensitization of the α1B-adrenergic receptor

    J. Biol. Chem.

    (1996)
  • M.I. Fonseca et al.

    Agonist regulation of α1B-adrenergic receptor subcellular distribution and function

    J. Biol. Chem.

    (1995)
  • J.A. Garcı́a-Sáinz et al.

    Protein kinase C-mediated phosphorylation and desensitization of human α1b-adrenoceptors

    Eur. J. Pharmacol.

    (1999)
  • J.R. Hadcock et al.

    Cross-talk between tyrosine kinase and G-protein-linked receptors. Phosphorylation of β2-adrenergic receptors in response to insulin

    J. Biol. Chem.

    (1992)
  • P.L. Hordijk et al.

    Protein tyrosine phosphorylation induced by lysophosphatidic acid in rat-1 fibroblasts. Evidence that phosphorylation of MAP kinase is mediated by the Gi-p21ras pathway

    J. Biol. Chem.

    (1994)
  • V. Karoor et al.

    Insulin-like growth factor receptor-1 stimulates phosphorylation of the β2-adrenergic receptor in vivo on sites distinct from those phosphorylated in response to insulin

    J. Biol. Chem.

    (1996)
  • H. Kurose et al.

    Differential desensitization and phosphorylation of three cloned and transfected α2-adrenergic receptor subtypes

    J. Biol. Chem.

    (1994)
  • A.-L. Lattion et al.

    Truncation of the receptor carboxyl terminus impairs agonist-dependent phosphorylation and desensitization of the α1B-adrenergic receptor

    J. Biol. Chem.

    (1994)
  • L.M.F. Leeb-Lundberg et al.

    Regulation of adrenergic receptor function by phosphorylation: I. Agonist-promoted desensitization of α1-adrenoceptors coupled to inositol phospholipid metabolism in DDT1 MF-2 smooth muscle cells

    J. Biol. Chem.

    (1987)
  • R.J. Lefkowitz

    G protein-coupled receptors. III. New roles for receptor kinases and β-arrestins in receptor signaling and desensitization

    J. Biol. Chem.

    (1998)
  • C.J. Lynch et al.

    Inhibition of hepatic α1-adrenergic effects and binding by phorbol myristate acetate

    J. Biol. Chem.

    (1985)
  • L.C. Medina et al.

    Crosstalk: phosphorylation of α1b-adrenoceptors induced through activation of bradykinin B2 receptors

    FEBS Lett.

    (1998)
  • L.E. Rameh et al.

    The role of phosphoinositide 3-kinase lipid products in cell function

    J. Biol. Chem.

    (1999)
  • L.A. Selbie et al.

    G protein-coupled receptor crosstalk: the fine-tuning of multiple receptor-signalling pathways

    Trends Pharmacol. Sci.

    (1998)
  • M. Shih et al.

    Dynamic complexes of β2-adrenergic receptors with protein kinases and phosphatases and the role of gravin

    J. Biol. Chem.

    (1999)
  • A. Ullrich et al.

    Signal transduction by receptors with tyrosine kinase activity

    Cell

    (1990)
  • J. Vázquez-Prado et al.

    Activation of endothelin ETA receptors induce phosphorylation of α1b-adrenoceptors in rat-1 fibroblasts

    J. Biol. Chem.

    (1997)
  • M.P. Wymann et al.

    Structure and function of phosphoinositide 3-kinase

    Biochim. Biophys. Acta

    (1998)
  • J. Zhang et al.

    A central role for β-arrestins and clathrin-coated vesicle-mediated endocytosis in β2-adrenergic receptor resensitization. Differential regulation of receptor resensitization in two distinct cell types

    J. Biol. Chem.

    (1997)
  • J. Zhang et al.

    Dynamin and β-arrestin reveal distinct mechanisms for G protein-coupled receptor internalization

    J. Biol. Chem.

    (1996)
  • H. Zhong et al.

    α1-Adrenoceptor subtypes

    Eur. J. Pharmacol.

    (1999)
  • E. Zwick et al.

    Critical role of calcium-dependent epidermal growth factor receptor transactivation in PC12 cell membrane depolarization and bradykinin signaling

    J. Biol. Chem.

    (1997)
  • R. Alcántara-Hernández et al.

    Protein phosphatase–protein kinase interplay modulates α1b-adrenoceptor phosphorylation: effects of okadaic acid

    Br. J. Pharmacol.

    (2000)
  • D.R. Allesi et al.

    Mechanism of activation and function of protein kinase B

    Curr. Opin. Genet. Dev.

    (1998)
  • T. Awaji et al.

    Real-time optical monitoring of ligand-mediated internalization of α1b-adrenoceptor with green fluorescent protein

    Mol. Endocrinol.

    (1998)
  • S.W. Bahouth et al.

    Enhanced desensitization and phosphorylation of the β1-adrenergic receptor in rat adipocyte by peroxovanadate

    Mol. Pharmacol.

    (1996)
  • J.F. Beeler et al.

    Regulation of hepatocyte plasma membrane α1-adrenergic receptors by 4β-phorbol 12-myristate 13-acetate

    Biochem. J.

    (1995)
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