Marc Thibonnier

CLONING AND CHARACTERIZATION OF THE HUMAN V3 PITUITARY VASOPRESSIN RECEPTOR

Arginine‐vasopressin (AVP) plays a determinant role in the normal ACTH response to stress in mammals. We cloned a human cDNA coding a 424 amino acid G‐protein coupled receptor structurally related to the vasopressin/oxytocin receptor family. When expressed in COS cells, this receptor binds AVP with a high affinity (K d = 0.55 ± 0.13 nM) and is functionally coupled to phospholipase C. Competition studies with peptidic or non peptidic AVP analogues reveal that it is pharmacologically distinct from V1a and V2 AVP receptors and therefore it is designated V3. RT‐PCR analysis shows that the human V3 receptor is expressed in normal pituitary and also in kidney, but is undetectable in liver, myometrium and adrenal gland. Northern blot analysis reveals a ∼4.8 kb messenger in human corticotropic pituitary adenomas.

Molecular Pharmacology of Human Vasopressin Receptors

Vasopressin (AVP) and oxytocin (OT) are cyclic nonapeptides whose actions are mediated by activation of specific G protein-coupled receptors (GPCRs) currently classified into V1-vascular (V1R), V2-renal (V2R) and V3-pituitary (V3R) AVP receptors and OT receptors (OTR). The cloning of the different members of the AVP/OT family of receptors now allows the extensive molecular pharmacological characterization of a single AVP/OT receptor subtype in stably transfected mammalian cell lines. The human V1-vascular (CHO-V1), V2-renal (CHO-V2), V3-pituitary (CHO-V3) and oxytocin (CHO-OT) receptors stably expressed in CHO cells display distinct binding profiles for 18 peptide and 5 nonpeptide AVP/OT analogs. Several peptide and nonpeptide compounds have a greater affinity for the V1R than AVP itself. V2R peptide agonists and antagonists tend to be non-selective ligands whereas nonpeptide V2R antagonists are potent and subtype-selective. None of the 22 AVP/OT analogs tested has a better affinity for the human V3R than AVP itself. Several peptide antagonists do not select well between V1R and OTR. These results underscore the need for developing specific and potent analogs interacting specifically with a given human AVP/OT receptor subtype.

Signal transduction of V1-vascular vasopressin receptors

This review covers the recent developments gained in the exploration of V1-vascular vasopressin (AVP) receptors. We examine the different radioligands available for the pharmacological characterization of these receptors. The immediate transmembrane signaling of V1-vascular AVP receptors involves ligand-receptor complex formation, receptor lateral mobility and internalization, coupling to a Gq protein, activation of phospholipases A2, C and D, translocation and activation of protein kinase C, production of inositol 1,4,5-triphosphate and 1,2-diacylglycerol, mobilization of intracellular calcium, alteration of intracellular pH with activation of the Na+/H+ exchanger, calmodulin activation and myosin light chain phosphorylation. The secondary nuclear signal mechanisms triggered by activation of V1-vascular AVP receptors includes tyrosine phosphorylation, induction of gene expression and protein synthesis.

Signal transduction pathways of the human V1-vascular, V2-renal, V3-pituitary vasopressin and oxytocin receptors.

Vasopressin (VP) and oxytocin (OT) are cyclic nonapeptides whose actions are mediated by stimulation of specific G protein-coupled receptors (GPCRs) currently classified into V1-vascular (V1R), V2-renal (V2R) and V3-pituitary (V3R) VP receptors and OT receptors (OTR). The recent cloning of the different members of the VP/OT family of receptors now allows the extensive characterization of the molecular determinants involved in ligand binding and signal transduction pathways coupled to a given VP/OT receptor subtype in stably transfected mammalian cell lines. In this article, we review the present knowledge of the signal transduction pathways coupled to the different VP/OT receptor subtypes and we present new observations derived from the study of each human VP or OT receptor subtype stably expressed in CHO cells.

Molecular pharmacology and modeling of vasopressin receptors.

AVP receptors represent a logical target for drug development. As a new class of therapeutic agents, orally active AVP analogs could be used to treat several human pathophysiological conditions including neurogenic diabetes insipidus, the syndrome of inappropriate secretion of AVP (SIADH), congestive heart failure, arterial hypertension, liver cirrhosis, nephrotic syndrome, dysmenorrhea, and ocular hypertension. By immunoprecipitation and immunoblotting, we elucidated the phosphorylation pattern of green fluorescent protein-tagged AVP receptors and showed interactions with the specific kinases PKC and GRK5 that are agonist-, time- and receptor subtype-dependent. The tyrosine residue of the NPWIY motif present in the 7th helix of AVP receptors is rapidly and transiently phosphorylated after agonist stimulation. This phosphorylation is instrumental in the genesis of the mitogenic cascade linked to the activation of this receptor, presumably by establishing key intramolecular contacts and by participating in the creation of a scaffold of proteins that produce the activation of downstream kinases. The random screening of chemical entities and optimization of lead compounds recently resulted in the development of orally active non-peptide AVP receptor agonists and antagonists. Furthermore, the identification of the molecular determinants of receptor-ligand interactions should facilitate the development of more potent and very selective orally active compounds via the approach of structure-based drug design. We developed three-dimensional molecular docking models of peptide and non-peptide ligands to the human V1 vascular, V2 renal and V3 pituitary AVP receptors. Docking of the peptide hormone AVP to the receptor ligand binding pockets reflects its dual polar and non-polar structure, but is receptor subtype-specific. The characteristics of non-peptide AVP analogs docking to the receptors are clearly distinct from those of peptide analogs docking. Molecular modeling of the results of site-directed mutagenesis experiments performed in CHO cells stably transfected with the human AVP receptor subtypes revealed that non-peptide antagonists establish key contacts with a few amino acid residues of the receptor subtypes that are different from those involved in agonist binding. Moreover, these interactions are species-specific. These findings provide further understanding of the signal transduction pathways of AVP receptors and new leads for elucidation of drug-receptor interactions and optimization of drug design. NOTE TO THE READER: The recent cloning and molecular characterization of AVP/OT receptor subtypes call for the revision of their nomenclature. For the sake of clarity and reference to their main site of expression, we call the V1a receptor the V1 vascular receptor, the V2 receptor the V2 renal receptor and the V1b or V3 receptor the V3 pituitary receptor in the present review.

Effects of the Nonpeptide V1 Vasopressin Receptor Antagonist SR49059 in Hypertensive Patients

We assessed the clinical and pharmacological profile of the orally active V(1) vascular vasopressin (AVP) receptor nonpeptide antagonist SR49059 (SR) during the osmotic stimulation of AVP release in hypertensive patients. In a double-blind crossover-versus-placebo study, 24 untreated stage I or II essential hypertensive patients (12 whites and 12 blacks) received a single 300 mg oral dose of SR 2 hours before the stimulation of AVP secretion with a 5% hypertonic saline infusion. Hemodynamic, humoral, and hormonal parameters were monitored for up to 28 hours after drug administration. SR did not alter blood pressure or heart rate before the saline infusion and did not reduce the blood pressure increment induced by the hypertonic saline infusion. However, the blood pressure peak at the end of the hypertonic saline infusion was slightly lower in the presence of SR (P=0.04). Heart rate was significantly faster between 4 and 6 hours after SR administration (P=0.02). The rise in plasma sodium and osmolality triggered by the saline infusion was not modified by SR, but AVP release was slightly greater in the presence of SR (P<0.0003). AVP-induced aggregation of blood platelets in vitro was significantly reduced by SR, with a peak effect 2 hours after drug administration that coincided with the SR peak plasma concentration. Plasma renin activity and aldosterone before and after the saline infusion were not modified by SR. Urine volume and osmolality were not altered by SR administration. SR effects were similar in the 2 ethnic groups as well as in salt-sensitive versus salt-resistant patients. In a situation of AVP osmotic release and volume expansion in hypertensive patients, a single oral dose of the V(1) vascular AVP receptor nonpeptide antagonist SR49059, which is able to block AVP-induced platelet aggregation, exerts a transient vasodilation effect that is not associated with a sustained blood pressure reduction. SR49059 is a pure V(1) vascular receptor antagonist that is devoid of V(2) renal receptor actions.

Comparison of the hormonal and renal effects of captopril in severe essential and renovascular hypertension

Abstract Captopril was administered for 6 days to 26 severely hypertensive hospitalized patients, 17 with essential hypertension and 9 with renovascular hypertension. All the patients were on a low sodium diet and were treated after the 3rd day in the hospital with progressively increasing doses of captopril (75 to 450 mg/day). Mean blood pressure decreased from 142 ± 20 to 117 ± 18 mm Hg in essential hypertension and from 136 ± 11 to 106 ± 10 mm Hg in renovascular hypertension. Plasma renin activity and aldosterone as well as urinary aldosterone and vasopressin were higher in renovascular hypertension than in essential hypertension. In both groups, captopril decreased similarly plasma aldosterone and urinary aldosterone and vasopressin, whereas plasma renin activity increased. These identical changes in blood pressure and hormonal variables were accompanied by different modifications in urinary sodium excretion, plasma volume and plasma creatinine. Urinary sodium excretion was higher (3.06 ± 1.38 mmol/mmol of urinary creatinine) in treated essential hypertension than in treated renovascular hypertension (1.77 ± 1.01, p These data demonstrate that captopril is able to decrease similarly the activity of the renin-angiotensin-aldosterone-vasopressin system in essential and renovascular hypertension, whereas its renal effects are different. Marked increases in plasma creatinine were observed among patients with renal artery stenosis. These observations suggest caution in the use of converting enzyme inhibitors in patients with renovascular hypertension, especially those on a low sodium diet.

Cytoplasmic and nuclear signaling pathways of V1-vascular vasopressin receptors

We studied the cytoplasmic and nuclear signaling pathways of V1-vascular AVP receptors of human platelets, primary cultures of renal glomerular mesangial cells, and established cultures of the A7r5 aortic smooth muscle cell line. The immediate transmembrane signals are triggered by the formation of ligand-receptor complexes as illustrated by binding experiments with [3H]AVP (Kd = 2.50 nM), d(CH2)5Tyr(Me)AVP (Kd = 0.62 nM), the linear V1 antagonist phenylacetyl-D-Tyr(Et)-Phe-Val-Asn-Lys-Pro-[125I]Tyr-NH2 (Kd = 1.42 nM) or by fluorescence experiments with linear antagonists like phenylacetyl-D-Tyr(Et)-Phe-Gln-Asn-Lys-Pro-Arg-NH2 coupled to biotin and made fluorescent by labeling with tetramethylrhodamine-avidin. We used several approaches (radioreceptor binding, radioactive labeling, autoradiographic, enzymatic, photoaffinity labeling, and immunoblotting procedures) to identify the guanine nucleotide regulatory protein coupled to V1-vascular vasopressin receptors. AVP-stimulated GTPase activity of human platelet membranes was blocked by pretreatment with antibodies specific for the C-terminal of the newly described Gq alpha protein. In the presence of MgCl2, AVP increased labeling by the photoreactive GTP analog [alpha-32P]azidoanilido GTP of a platelet membrane protein of apparent molecular mass of 42 kDa. AVP effect was reversed by the specific V1-vascular antagonist d(CH2)5Tyr(Me)AVP and labeling was completely abolished by GTP gamma s.(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormal regulation of antidiuretic hormone in idiopathic edema

Idiopathic edema is characterized by impaired water excretion, particularly in the upright posture. Indirect evidence has shown that antidiuretic hormone is involved in this disease. For this reason, we measured urinary arginine vasopressin by radioimmunoassay before and during water loading (15 ml/kg) in 10 normal women and in 10 subjects with idiopathic edema in both the supine and upright postures. Daily sodium intake was 100 meq. Renin and aldosterone were concomitantly investigated, and abnormally high values were observed both in the recumbent and upright postures. Basal values for urinary arginine vasopressin were identical in control subjects and in patients with idiopathic edema. The water load significantly reduced urinary arginine vasopressin in normal women in both positions, but in those with idiopathic edema only in the supine position. In those with idiopathic edema, assumption of the upright posture was accompanied by a transient decrease in glomerular filtration, a major decrease in osmolar clearance and no decrease in urinary arginine vasopressin after water loading. Significant correlations were established between urinary arginine vasopressin and osmolar or volemic parameters in normal women, but these correlations were not found in those with idiopathic edema in either position. Arginine vasopressin regulation was abnormal in idiopathic edema, and this hormone was believed to play a part in the pathogenesis of this disease.

Acute antihypertensive and hormonal effects of a calcium antagonist in essential hypertension.

In a double-blind versus placebo preliminary study conducted in seven mildly hypertensive patients, the 10-mg capsule form of nifedipine was able to reduce significantly systolic and diastolic blood pressure (-14%) for 3 h. In a single-dose, cross-over study, captopril (1 mg/kg) and nifedipine (20 mg) significantly reduced blood pressure in 12 patients with moderate essential hypertension, but the mean maximum arterial pressure reduction was faster and greater with nifedipine than with captopril (-23 +/- 2% at 37 +/- 15 min and -17 +/- 1% at 86 +/- 25 min, respectively). Nifedipine did not significantly alter the renin angiotensin aldosterone system in supine and upright positions, and the blood pressure drop it induced was not related to the initial level of activation of that system. Associated with the stimulation of the sympathetic nervous system, an increased release of vasopressin was noted during nifedipine administration. Finally, nifedipine, a calcium antagonist, was a potent antihypertensive drug through its vasodilating properties. It provoked specific hormonal alterations, i.e., stimulation of catecholamines and vasopressin release, whereas the renin angiotensin aldosterone system was not significantly altered.