Reviewα1-Adrenoceptors: function and phosphorylation
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.
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