Inverse agonism of histamine H2 antagonist accounts for upregulation of spontaneously active histamine H2 receptors.

Abstract

Histamine H2 receptors transfected in Chinese hamster ovary (CHO) cells are time-and dosedependently upregulated upon exposure to the H2 antagonists cimetidine and ranitidine. This effect appears to be H2 receptor-mediated as no change in receptor density was observed after H1 or H3 antagonist treatment or after incubation with the structural analogue of cimetidine, VUF 8299, which has no H2 antagonistic effects. By using transfected CHO cells expressing different densities of wild-type H2 receptors or an uncoupled H2Leut24Ala receptor, the histamine H2 receptor was found to display considerable agonist-independent H2 receptor activity. Cimetidine and ranitidine, which both induce H2 receptor upregulation, actually functioned as inverse agonists in those cell lines displaying spontaneous agonistindependent H2 receptor activity. Burimamide, on the other hand, was shown to act as a neutral antagonist and did as expected not induce H2 receptor upregulation after long-term exposure. The displayed inverse agonism of H2 antagonists appears to be a mechanistic basis for the observed H2 antagonist-induced H2 receptor upregulation in transfected CHO cells. These observations shed new light on the pharmacological classification of the H2 antagonists and may offer a plausible explanation for the observed development of tolerance after prolonged clinical use.Following the discovery of burimamide as a selective histamine H2 receptor antagonist (1) various related drugs (cimetidine, ranitidine, famotidine, nizatidine) have proven to be of great importance in the regulation of gastric acid secretion (2). The actual target of these drugs have recently been questioned (3, 4), but is generally considered to be the H2 receptor on the gastric parietal cell (4). As such, the H2 antagonists constitute currently one of the prominent therapies for duodenal and gastric ulcers (5). These drugs have been widely prescribed and are currently also clinically evaluated as immunosuppressants (6) and for the treatment of central nervous system disorders (7-10).The histamine H2 receptor is a member of the large multigene family of G-protein coupled receptors (GPCR) (11). Functionality and expression of members of the GPCR family are generally dynamically regulated after agonist or antagonist exposure (12,13 Studies examining the molecular mechanism underlying H2 receptor function have been greatly facilitated by the cloning of the genes encoding the histamine H2 receptor (25-28). Model systems, expressing a homogeneous population of (mutant) H2 receptors, are currently available for studies regarding H2 receptor function and regulation (18,29,30). In the present study we describe upregulation of H2 receptors after prolonged treatment of transfected Chinese hamster ovary (CHO) cells with some H2 antagonists. In our study, we observed that the histamine H2 receptor shows a spontaneous, histamine-independent activity. For some GPCRs, increased basal agonist-independent receptor activity can be inhibited by certain antagonists referred to as...

Pharmacological chaperones rescue cell-surface expression and function of misfolded V2 vasopressin receptor mutants

“…For instance, treatment of cells expressing the histamine-H2 receptor with the antagonists cimetidine and ranitidine, lead to an increase of the receptor number (29). Such stabilization effects have been more apparent for constitutively active mutant receptors (30)(31)(32), which are known to be less stable than their wild-type counterparts.…”

Expression of a G Protein-coupled Receptor (GPCR) Leads to Attenuation of Signaling by Other GPCRs

“…Compared with other GPCRs, the H2R is unique in that the wildtype receptor possesses a remarkably high degree of constitutive activity. With a receptor density of 300 fmol/mg protein, constitutive H2 receptor activity could be detected in Chinese hamster ovary (CHO-K1) cells (7).…”

Computer modeling of gastric parietal cell: significance of canalicular space, gland lumen, and variable canalicular [K+]

“…The normal stomach maintains a pH of ϳ2 in the absence of food (21,28), requiring that basal acid secretion exceed surface-cell HCO 3 Ϫ secretion (41). In addition, the parietal cell's H2 histamine receptors have been found to have a basal level of activity that can be further lowered by "H2 blockers" that are now known to function as "inverse agonists" (51).…”