Regulation of tyrosine kinase cascades by G-protein-coupled receptors
Introduction
Although the role of G-protein-coupled receptors (GPCRs) in the regulation of intermediary metabolism is well established, they have only recently been recognized as important mediators of cellular growth and differentiation [1]. It is now known that many GPCRs regulate mitogen-activated protein kinase (MAPK) cascades, leading to activation of the extracellular-signal-regulated kinases (ERKs), Jun amino-terminal kinase/stress-activated protein kinase, and p38 MAPK, which function as transcriptional regulators 2, 3.
Several lines of evidence indicate that current models of GPCR signaling are insufficient to account for these mitogenic signals. In most cases, stimulation of the ERK cascade follows GPCR-mediated activation of the small GTP-binding protein (G protein), p21Ras 4, 5. These Ras-dependent signals are frequently dissociated from the regulation of classical G-protein effectors, such as phospholipase C (PLC) and adenylyl cyclase 6, 7, 8. Rather, the mechanisms by which GPCRs mediate Ras activation bear striking similarity to those employed by RTKs, involving tyrosine protein phosphorylation and recruitment of Ras guanine-nucleotide-exchange factors (GEFs) to membrane-associated Ras-activation complexes assembled on tyrosine phosphoprotein scaffolds [9].
This review focuses on new insights into the mechanisms whereby GPCRs regulate tyrosine protein kinase activity and induce the formation of mitogenic signaling complexes. Recent data have implicated three distinct classes of tyrosine kinase as proximal mediators of GPCR-induced Ras activation; RTKs, focal adhesion kinases (FAKs), and Src family kinases. Depending upon the receptor and cell type studied, each has been shown to contribute to the assembly of signaling complexes that lead to Ras activation. In addition, GPCRs have been found in complex with Jak and Src nonreceptor tyrosine kinases, indicating that the GPCR itself can function in a scaffolding role.
Section snippets
‘Transactivation’ of receptor tyrosine kinases
Several groups have found that the mechanisms of GPCR-mediated activation of the ERK cascade closely parallel those employed by RTKs. In fibroblasts, stimulation of the lysophosphatidic acid (LPA) receptor, α-thrombin receptor, M2 muscarinic receptor, or α2A adrenergic receptor rapidly induces activation of Ras and ERK. GPCR activation induces a rapid increase in tyrosine phosphorylation of adaptor proteins such as Shc and Gab1, and the association of Shc and Grb2 9, 10, 11, 12, 13, 14••. Shc
G-protein-coupled receptor signaling via focal adhesion complexes
The focal adhesion represents an alternative locus for the GPCR-induced assembly of Ras-activation complexes (Figure 2). Focal adhesions are points of cell attachment to the extracellular matrix and of cytoskeletal anchoring to the plasma membrane. Single-transmembrane-domain receptors that lack intrinsic enzymatic activity, called integrins, form the backbone of focal adhesions. Integrin binding to extracellular matrix proteins results in assembly of αβ integrin heterodimers, which nucleate
GPCRs as signaling platforms
The receptor interaction with G proteins is catalytic — one receptor activates multiple G-proteins before coupling is terminated by the regulated processes of receptor desensitization and sequestration 43, 44. The termination of G-protein coupling involves the formation of stable complexes between receptors and β-arrestins. Agonist exposure leads to GPCR phosphorylation mediated by second-messenger-regulated protein kinases and GPCR kinases (GRKs). The GRK phosphorylation of GPCRs, followed by
Conclusions
Depending upon receptor and cell type, GPCRs employ multiple strategies for the activation of mitogenic signaling pathways. Most of these involve crosstalk between GPCRs and tyrosine phosphoprotein scaffolds, including transactivated RTKs and focal adhesions. In some cases, the GPCR itself forms part of a signaling complex, exemplified by the formation of complexes between the AT1 receptor and Jak2, and also between the β2 adrenergic receptor, β-arrestin and c-Src.
Recruitment and activation of
Acknowledgements
Robert J Lefkowitz is an investigator with the Howard Hughes Medical Institute. The authors thank M Holben and D Addison for excellent secretarial assistance.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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