Alpha 1b-adrenoceptor-mediated calcium oscillation is specific for the S phase in cell cycle and dependent on the extracellular calcium

Pharmacological lineage analysis revealed the binding affinity of broad-spectrum substance P antagonists to receptors for gonadotropin-releasing peptide

A group of synthetic substance P (SP) antagonists, such as [Arg(6),D-Trp(7,9),N(Me)Phe(8)]-substance P(6-11) and [D-Arg(1),D-Phe(5),D-Trp(7,9),Leu(11)]-substance P, bind to a range of distinct G-protein-coupled receptor (GPCR) family members, including V1a vasopressin receptors, and they competitively inhibit agonist binding. This extended accessibility enabled us to identify a GPCR subset with a partially conserved binding site structure. By combining pharmacological data and amino acid sequence homology matrices, a pharmacological lineage of GPCRs that are sensitive to these two SP antagonists was constructed. We found that sensitivity to the SP antagonists was not limited to the Gq-protein-coupled V1a and V1b receptors; Gs-coupled V2 receptors and oxytocin receptors, which couple with both Gq and Gi, also demonstrated sensitivity. Unexpectedly, a dendrogram based on the amino acid sequences of 222 known GPCRs showed that a group of receptors sensitive to the SP antagonists are located in close proximity to vasopressin/oxytocin receptors. Gonadotropin-releasing peptide receptors, located near the vasopressin receptors in the dendrogram, were also sensitive to the SP analogs, whereas α1B adrenergic receptors, located more distantly from the vasopressin receptors, were not sensitive. Our finding suggests that pharmacological lineage analysis is useful in selecting subsets of candidate receptors that contain a conserved binding site for a ligand with broad-spectrum binding abilities. The knowledge that the binding site of the two broad-spectrum SP analogs partially overlaps with that of distinct peptide agonists is valuable for understanding the specificity/broadness of peptide ligands.

Differences in the subcellular localization of alpha1-adrenoceptor subtypes can affect the subtype selectivity of drugs in a study with the fluorescent ligand BODIPY FL-prazosin

G protein-coupled receptor (GPCR) subtypes are differentially distributed in the cell; however, it remains unclear how this affects the subtype selectivity of particular drugs. In the present study, we used flow cytometry analysis with the fluorescent ligand, BODIPY FL-prazosin, to study the relationship between the subcellular distribution of subtype receptors and the subtype-selective character of ligands using alpha1a and alpha1b-adrenoceptors (ARs). Alpha1a-ARs predominantly localize inside the cell, while alpha1b-ARs on the cell surface. Flow cytometry analysis and confocal laser-scanning micrographs of living cells showed that BODIPY FL-prazosin can label not only alpha1-ARs on the cell surface, but also those localized inside the cell. Furthermore, flow cytometry analysis of alpha1A-AR-selective drug, KMD-3213, and alpha1B-AR-selective drug, CEC, revealed that the major determinant of the subtype selectivity of each drug is different. The alpha1A-AR selectivity of KMD-3213 can be explained by its much higher affinity for alpha1a-AR than alpha1b-AR (affinity-dependent selectivity), while the alpha1B-AR selectivity of the hydrophilic alkylating agent CEC is due to preferential inactivation of alpha1-ARs on the cell surface (receptor localization-dependent selectivity). This study illustrates that factors in addition to the affinity of the drug for the receptor, such as subcellular localization of the receptor, should be taken into account in assessing the subtype selectivity of a drug.

Differential mechanism for the cell surface sorting and agonist-promoted internalization of the alpha1B-adrenoceptor

1. Alpha1B-adrenoceptors are localized at a steady state in the plasma membrane in untreated cells, and internalize to intracellular vesicles when exposed to agonist. Flow cytometry analysis with an anti-N-terminus-antibody (1B-N1-C, (Hirasawa et al., 1996)) facilitated the quantification of cell surface alpha1B-adrenoceptor. Also, the cellular distribution of alpha1B-adrenoceptors was visually monitored by immunocytochemical confocal microscopy. 2. Utilizing this combined approach, we have examined the molecular mechanism for cellular trafficking of alpha1B-adrenoceptors, including the process of sorting of the synthesized receptor protein to the cell surface, and the agonist-induced internalization. The two processes were separately examined by using alpha1B-adrenoceptor inducible DDT1MF-2 cells for the sorting process and CHO cells stably expressing alpha1B-adrenoceptors for the agonist-promoted internalization. 3. We examined the effects of cytochalasin D and mycalolide B (actin depolymerization agents), demecolcine (a microtubule disrupting agent), brefeldin A (an inhibitor of vesicular transport and Golgi function), bafilomycin A1 (a specific inhibitor of the vacuolar proton pump) or hyperosmotic sucrose treatment (that may inhibit clathrin-mediated endocytosis) on these processes. 4. We found that the agonist-promoted internalization was blocked by cytochalasin D and mycalolide B, while the cell surface sorting process was specifically blocked by brefeldin A, indicating that the two processes involve different components of the cellular endocytic machinery. 5. The experimental approach as exemplified in this study would provide a valuable system to study further the molecular mechanism(s) of cellular trafficking of G protein-coupled receptors.

Subtype-specific differences in subcellular localization of alpha1-adrenoceptors: chlorethylclonidine preferentially alkylates the accessible cell surface alpha1-adrenoceptors irrespective of the subtype

Selective inactivation of alpha1B-adrenoceptor (AR) by the site-directed alkylating agent chlorethylclonidine (CEC) has been used as one of major pharmacological criteria to subclassify alpha1-AR; however, the mechanism for the differential CEC sensitivity of the two subtypes is uncertain, and the extent of CEC inactivation varies depending on the treatment employed. In this study, we examined the correlation between the subcellular localization of alpha1-AR subtypes (alpha1A and alpha1B) and CEC sensitivity. Constructing alpha1-AR tagged with the FLAG epitope at the amino terminus and/or green fluorescent protein (GFP) at the carboxyl terminus, we examined the subcellular distribution of alpha1-ARs expressed in COS-7 cells. Flow cytometry analysis showed that most populations of GFP-expressing alpha1B-AR cells, but very few GFP-expressing alpha1A-AR cells, were detected by the anti-amino terminus antibodies. The immunocytochemical and GFP-fluorescence confocal micrographs showed that alpha1A-ARs predominantly localize intracellularly, whereas alpha1B-ARs localize on the cell surface. Furthermore, CEC (10 microM) treatment of intact cells resulted in an inactivation of approximately 42% of alpha1A-ARs and 93% of alpha1B-ARs, whereas treatment of the membrane preparations resulted in an inactivation of approximately 83% of alpha1A-ARs and 88% of alpha1B-ARs, respectively. Together, the results showed that a hydrophilic alkylating agent CEC preferentially inactivates alpha1-AR on the cell surface irrespective of its subtype, and that the subtype-specific subcellular localization rather than the receptor structure is a major determinant for CEC inactivation of alpha1-AR. Subtype-specific subcellular localization suggests an additional class of functional properties that provide new insight into drug action.