Mechanism of Inverse Agonist Action of Sarpogrelate at the Constitutively Active Mutant of Human 5-HT_2A Receptor Revealed by Molecular Modeling

Abstract

We previously reported that sarpogrelate, a selective 5-HT 2A antagonist, showed a potent inverse agonist activity to constitutively active mutant (C322K) of human 5-HT 2A receptor (5-HT 2A R). However, it remains to be unknown about the actual mechanism of this mutant for its constitutive activation as well as inverse agonist activity of sarpogrelate. Our model shows that mutation (C322K) of 5-HT 2A R causes electronic repulsion between positively charged Arg173(3.50) and Lys322(6.34) residues resulting outward movement of the C-terminus of transmembrane helix (TMH) III. This motion of TMH III leads to a partially active structure of the receptor, which may be a key step in receptor activation. The structural model of the partially active receptor also indicates that the binding of sarpogrelate to the constitutively active receptor causes an inward swing of TMH III to an inactive receptor structure. Therefore, the present study may suggest that the electronic repulsion causing outward movement of the C-terminus of TMH III may be the key step for constitutive activation of mutant C322K of 5-HT 2A R and the inward movement of TMH III causes the inverse agonist activity of sarpogrelate.Key words 5-HT 2A receptor; mutation; constitutive activity; molecular modeling; electronic repulsion It has been established that many G-protein coupled receptors (GPCRs) can exist in a constitutively active form in the absence of agonist.1-3) Certain molecules could act as inverse agonists and reduce the levels of constitutive activity and functional cellular responses. [3][4][5][6] Inverse agonism is very common among GPCR antagonists.7) It has become apparent over the past few years that a number of compounds that had been thought to be neutral antagonists at GPCRs possess negative efficacy and have been termed inverse agonists. This was first shown for μ-opiate and α 2 -adrenergic receptors 1,8) but has subsequently been extended to many GPCRs. 9,10) Considering the therapeutic implication, it is suggested that all new antagonists should be routinely tested for their potential inverse agonistic activity in future drug development programs. 11,12) This concept is of particular importance for the 5-HT 2A receptor (5-HT 2A R) where the drugs used to treat cardiovascular diseases had been assumed to be antagonists at this receptor. It has, however, been shown using potentiation of basal inositol phosphate activity 13) that all of the drugs tested were inverse agonists and there was a good correlation between inverse agonist potency and ligand binding affinity. In the previous study, we demonstrated that sarpogrelate, a selective 5-HT 2A antagonist, had been shown to be a potent inverse agonist at the constitutive active human 5-HT 2A R. 14) Patients are treated chronically with this drug and the effects of an inverse agonist on the activity of 5-HT 2A R may be quite different to those of an antagonist. Owing to the important therapeutic activity (i.e. to treat ischemic diseases) of sarpogrelate, it is very important to understand t...

New therapeutic opportunities for 5-HT2 receptor ligands

“…Such a mechanism was also suggested in an ischemic hind limb model in diabetic mice where sarpogrelate restored perfusion through the stimulation of the eNOS/Akt pathway involving the 5-HT 1B receptor (Iwabayashi, et al, 2012). Nevertheless, an effect in the absence of 5-HT could also happen due to the inverse agonist action of the drug (Hossain, et al, 2012;Muntasir, et al, 2006). In humans, sarpogrelate given as a single 200 mg oral administration also increased basal and maximal (adenosine triphosphate) average peak velocity of the coronary blood flow in patients with coronary artery disease.…”

Serotonin and Cardiovascular Diseases

“…Such a mechanism was also suggested in an ischemic hindlimb model in diabetic mice where sarpogrelate restored perfusion through stimulation of endothelial NOS/Akt pathway involving 5-HT 1B receptors (Iwabayashi, et al, 2012). Nevertheless, an effect in the absence of serotonin could also happen due to an inverse agonist action of the drug (Hossain, et al, 2012;Muntasir, et al, 2006). In humans, sarpogrelate given as a single 200 mg oral administration also increased basal and maximal (adenosine triphosphate) average peak velocity of coronary blood flow in patients with coronary artery disease.…”