Functional selectivity of adenosine receptor ligands

Abstract

Adenosine receptors are plasma membrane proteins that transduce an extracellular signal into the interior of the cell. Basically every mammalian cell expresses at least one of the four adenosine receptor subtypes. Recent insight in signal transduction cascades teaches us that the current classification of receptor ligands into agonists, antagonists, and inverse agonists relies very much on the experimental setup that was used. Upon activation of the receptors by the ubiquitous endogenous ligand adenosine they engage classical G protein-mediated pathways, resulting in production of second messengers and activation of kinases. Besides this well-described G protein-mediated signaling pathway, adenosine receptors activate scaffold proteins such as β-arrestins. Using innovative and sensitive experimental tools, it has been possible to detect ligands that preferentially stimulate the β-arrestin pathway over the G protein-mediated signal transduction route, or vice versa. This phenomenon is referred to as functional selectivity or biased signaling and implies that an antagonist for one pathway may be a full agonist for the other signaling route. Functional selectivity makes it necessary to redefine the functional properties of currently used adenosine receptor ligands and opens possibilities for new and more selective ligands. This review focuses on the current knowledge of functionally selective adenosine receptor ligands and on G protein-independent signaling of adenosine receptors through scaffold proteins.

Fig. 1

Adenosine receptor ligands. The structures of ligands that are mentioned in the text are shown. The ligands are indicated in the text with a bold number when they are mentioned for the first time. At moments when structural information can contribute to the discussion, the ligands are indicated in the text with a bold number as well. Abbreviations: 2CdA 2-chloro-2′-deoxyadenosine, 3′dA 3′-deoxyadenosine, CADO 2-chloro-adenosine, CCPA 2-chloro-N⁶-cyclopentyladenosine, CHA N⁶-cyclohexyladenosine, Cl-IB-MECA 2-chloro-N⁶-(3-iodobenzyl)-5′-N-methylcarboxamidoadenosine, CPA N⁶-cyclopentyladenosine, CPeCA 5′-N-cyclopentyl-carboxamidoadenosine, DBXRM 1,3-dibutylxanthine-7-riboside-5′-N-methylcarboxamide, DPCPX 8-cyclopentyl-1,3-dipropylxanthine, DPMA N⁶-[2-(3,5-Dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine, IB-MECA N⁶-(3-iodobenzyl)-5′-N-methylcarboxamidoadenosine, MECA 5′-N-methylcarboxamidoadenosine, NECA 5′-N-ethylcarboxamidoadenosine, R-PIA N⁶-(1-methyl-2-phenylethyl)adenosine

Fig. 1

Adenosine receptor ligands. The structures of ligands that are mentioned in the text are shown. The ligands are indicated in the text with a bold number when they are mentioned for the first time. At moments when structural information can contribute to the discussion, the ligands are indicated in the text with a bold number as well. Abbreviations: 2CdA 2-chloro-2′-deoxyadenosine, 3′dA 3′-deoxyadenosine, CADO 2-chloro-adenosine, CCPA 2-chloro-N⁶-cyclopentyladenosine, CHA N⁶-cyclohexyladenosine, Cl-IB-MECA 2-chloro-N⁶-(3-iodobenzyl)-5′-N-methylcarboxamidoadenosine, CPA N⁶-cyclopentyladenosine, CPeCA 5′-N-cyclopentyl-carboxamidoadenosine, DBXRM 1,3-dibutylxanthine-7-riboside-5′-N-methylcarboxamide, DPCPX 8-cyclopentyl-1,3-dipropylxanthine, DPMA N⁶-[2-(3,5-Dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine, IB-MECA N⁶-(3-iodobenzyl)-5′-N-methylcarboxamidoadenosine, MECA 5′-N-methylcarboxamidoadenosine, NECA 5′-N-ethylcarboxamidoadenosine, R-PIA N⁶-(1-methyl-2-phenylethyl)adenosine

Fig. 1

Adenosine receptor ligands. The structures of ligands that are mentioned in the text are shown. The ligands are indicated in the text with a bold number when they are mentioned for the first time. At moments when structural information can contribute to the discussion, the ligands are indicated in the text with a bold number as well. Abbreviations: 2CdA 2-chloro-2′-deoxyadenosine, 3′dA 3′-deoxyadenosine, CADO 2-chloro-adenosine, CCPA 2-chloro-N⁶-cyclopentyladenosine, CHA N⁶-cyclohexyladenosine, Cl-IB-MECA 2-chloro-N⁶-(3-iodobenzyl)-5′-N-methylcarboxamidoadenosine, CPA N⁶-cyclopentyladenosine, CPeCA 5′-N-cyclopentyl-carboxamidoadenosine, DBXRM 1,3-dibutylxanthine-7-riboside-5′-N-methylcarboxamide, DPCPX 8-cyclopentyl-1,3-dipropylxanthine, DPMA N⁶-[2-(3,5-Dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine, IB-MECA N⁶-(3-iodobenzyl)-5′-N-methylcarboxamidoadenosine, MECA 5′-N-methylcarboxamidoadenosine, NECA 5′-N-ethylcarboxamidoadenosine, R-PIA N⁶-(1-methyl-2-phenylethyl)adenosine

Fig. 2

Functional selectivity of the adenosine A ₁ receptor. The A₁R can activate Gα _s and Gα _q/11 proteins besides the classical Gα _i pathway. The non-selective agonist NECA has higher intrinsic activity for these alternative pathways than A₁R-selective agonists. The intrinsic activities of A₁R-selective agonists such as CHA, CPA, and CPeCA for the alternative G proteins differ amongst each other as well. Allosteric modulators such as 2A3BT-class PAMs can bias signaling of orthosteric ligands. The endogenous enzyme ADA appears to function as a natural extracellular allosteric modulator of A₁Rs, possibly in close cooperation with the intracellular scaffold protein Hsc73. Signaling through G proteins and β-arrestins converges at the level of ERK1/2 activation. Interaction with scaffold proteins like 4.1G protein may favor functional selectivity by stabilizing distinct receptor conformations. See references in the text for more detailed information

Fig. 3

Potential for functional selectivity of the adenosine A _2A receptor. Although no functionally selective ligands for the A_2AR have been identified to date, this adenosine receptor in particular bears potential for biased signaling. Its exceptionally long carboxyl terminus interacts with a variety of scaffold proteins such as β-arrestins, α-actinin, ARNO, calmodulin, NECAB2, USP4, and TRAX. Besides the classical activation of Gα _s proteins by A_2ARs, interaction with Ga _15/16 proteins of the Gα_q/11 family has been reported. Both G protein-dependent and -independent pathways can lead to phosphorylation of ERK1/2 by A_2AR. Several lines of research indicate that interaction with scaffold proteins tethers A_2ARs to microdomains in the plasma membrane, where they may engage selective signal transduction pathways. See references in the text for more detailed information

Fig. 4

Potential for functional selectivity of the adenosine A _2B receptor. Similar to the A_2AR no functionally selective ligands have been identified for the A_2BR. Besides coupling of the A _2B R to Gα _s, coupling to Gα _q/11 and possibly Gα _i proteins has been described. ERK1/2 is activated with a remarkably higher potency than observed for Gα_s-mediated cAMP accumulation, indicating divergent signal transduction pathways that potentially can be regulated with biased ligands. Interaction with the extracellular proteins ADA and netrin or with the membrane-bound netrin-receptor DCC may influence A_2BR signaling in an allosteric manner. In addition, DCC and A_2BR both bind the intracellular proteins ezrin and NHERF-2 that may be involved in anchoring A_2BR to the cytoskeleton and/or forming a signaling complex. See references in the text for more detailed information

Fig. 5

Functional selectivity of the adenosine A ₃ receptor. Ligands with a bias towards β-arrestin-mediated signaling versus a Gα _i-dependent pathway have been identified. Ligands that are antagonists for Gα _i-mediated cAMP inhibition such as DMPA, CCPA, MRS1760, and MRS542 act as partial agonists for β-arrestin translocation. Interestingly, amongst the compounds that act as full agonists for both pathways, i.e., NECA, MRS3558, IB-MECA, Cl-IB-MECA, CGS21680, and CPA, both the nonspecific agonist NECA and the A₃R-specific agonist MRS3558 show faster β-arestin translocation rates. DBXRM, a full agonist for the Gα _i pathway, was a partial agonist for β-arrestin signaling. Besides these evident examples of functional selectivity, there are conflicting reports of the effects A₃R ligands have on apoptosis and proliferation. Since in many of these studies very high ligand concentrations were used, care must be taken when drawing conclusions about biased effects on cell growth. See references in the text for more detailed information