Constitutive activity of G protein coupled receptors and drug action
Introduction
An interesting and new concept in G protein coupled receptor (GPCR)-mediated drug action is the occurrence of constitutive activity of GPCRs in the absence of agonist stimulation. Originally reported for the δ-opioid receptor in NG108-15 cells (Costa and Herz, 1989), this GPCR activity has now been described for a variety of GPCRs (Milligan et al., 1995). Concomitant with the notion of agonist-independent signalling it has been realised that the constitutive GPCR activity can be inhibited by some antagonists (inverse agonists), but not by all antagonists (neutral antagonists) introducing the need for a pharmacological reclassification of GPCR antagonists (Milligan et al., 1995). Moreover, as previously described for agonists also for inverse agonist a full spectrum of (negative) intrinsic activity from −1 to 0 can be observed (Milligan et al., 1995; Leurs et al., 1998). Neutral antagonists, i.e., compounds with actually no intrinsic activity, are quite rare. Nevertheless, for some GPCRs, ligands with virtually no (negative) intrinsic activity are found. For example, the H2 receptor (the first described H2 antagonists burimamide) has been identified as a ligand with very low (Alewijnse et al., 1998) or no intrinsic activity (Smit et al., 1996). Burimamide is able to block the effects of both an agonist (histamine) and an inverse agonist (cimetidine) on the cAMP levels in CHO cells expressing the H2 receptor (Fig. 1), with apparent pKB values that match its pKi value (Alewijnse et al., 1998).
Section snippets
Constitutive activity and the two-state model
To explain the above described findings the GPCR protein is thought to isomerise between (at least) two different states, an inactive (R) and active (R*) conformation. In this oversimplified scheme the R* state is considered to be responsible for effective G protein activation and the equilibrium between R and R* determines the level of basal, constitutive GPCR signalling (Milligan et al., 1995; Kenakin, 1996; Leurs et al., 1998). Agonist binding is thought to shift the equilibrium towards R*,
Constitutive activity in a cellular context
It is accepted for some time that agonists can have different intrinsic activities in different cell systems. Similarly, the degree of constitutive GPCR activity and the concomitant inverse agonistic behaviour of GPCR ligands are also highly dependent upon the cellular context. As for GPCR agonism, one of the main determinants for constitutive GPCR signalling is the expression level of the respective GPCR. As can be seen in Fig. 2A, the transient expression of the histamine H1 receptor in COS-7
CAM GPCRs and human disease
The structural features of the GPCR that determine the level of constitutive activity are poorly understood. Important information has been obtained from studies, with so-called constitutively active mutant (CAM) receptors. Studies with adrenergic CAM receptors have indicated that the intracellular loop close to TM6 is of main importance (Samama et al., 1993). However, recent data suggest that mutations throughout the GPCR protein can result in constitutive activation (Pauwels and Wurch, 1998).
Agonist-like auto-antibodies and inverse agonists
Agonist-independent GPCR activation can in vivo also occur by other means. The presence of auto-antibodies against GPCRs with agonist-like properties has been associated with human disorders (Borda and Sterinborda, 1996; Hoebeke, 1996), such as Graves' disease (antibodies with TSH-like activity), Chagas disease (antibodies against β1-, β2 adrenergic and muscarinic m2 receptors), idiopathic dilated cardiomyopathy (anti-muscarinic m2 and anti-β1 receptor) and forms of malignant hypertension
Long-term exposure to inverse agonists
Long-term agonist exposure is known to result in the downregulation of GPCRs. Extending the concept of inverse agonism to GPCR regulation, long-term exposure of cells expressing constitutively active GPCRs to inverse agonists should result in an upregulation of GPCRs. Inverse agonist treatment of cells expressing β2-CAM receptors indeed results in sensitisation of adenylyl cyclase and is accompanied by an increase in binding sites (Pei et al., 1994). Also constitutively active wild-type
Conclusions
The new concept of constitutive GPCR activity has received considerable attention in recent years. For a proper understanding of drug action the molecular aspects underlying the negative intrinsic activity of inverse agonists should be understood. It is clear that constitutive activity of GPCRs highly depends on the `cellular context' and, thus, will be tissue specific and differ among species and individuals. Disorders, originating from constitutively signalling of GPCRs (e.g., due to
Acknowledgements
The research of the authors is supported by the EU BIOMED 2 programme `Inverse Agonism. Implications for drug research'.
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