THE HUMAN V-3 PITUITARY VASOPRESSIN RECEPTOR - LIGAND-BINDING PROFILEAND DENSITY-DEPENDENT SIGNALING PATHWAYS

The vasopressin (AVP) V-3 pituitary receptor (V3R) is a G protein-coup led corticotropic phenotypic marker that is overexpressed in ACTH-hype rsecreting tumors. Studies of the agonist/antagonist binding profile a nd signal transduction pathways linked to the human V3R have been limi ted because of the scarcity of this protein. To define the signals act ivated by V(3)Rs and the eventual changes triggered by developmental o r pathological receptor regulation, we developed Chinese hamster ovary (CHO)-V-3 cells stably expressing low, medium, or high levels of huma n V(3)Rs (binding capacity, <10, 10-25, and 25-100 pmol/mg, respective ly). The affinity of the V3R for 21 peptide and nonpeptide AVP analogs was clearly distinct from that exhibited by the human V1R and V2R. AV P triggered stimulation of phospholipase C in CHO-V-3 cells (partially sensitive to treatment with pertussis toxin) with a potency directly proportional to receptor density. V3R-mediated arachidonic acid releas e also was also sensitive to pertussis toxin and more efficacious in c ells exhibiting medium than in those with high receptor density. AVP a lso stimulated the pertussis toxin-insensitive uptake of [H-3]thymidin e in CHO-V-3 cells. The concentration-response curves for this effect were monophasic in cells expressing low and medium levels of V(3)Rs; o n the contrary, a biphasic curve was observed in cells with high V3R d ensity. Coupling of V3R to increased production of cAMP was only obser ved in CHOV3 high cells, suggesting a negative relationship between in creased cAMP production and DNA synthesis. Activation of mitogen-activ ated protein kinases by V3R was pertussis toxin insensitive, but was d ependent on activation of phospholipase C and protein kinase C; both t he level and duration of activation were a function of the receptor de nsity. Thus, the human V3R has a pharmacological profile clearly disti nct from that of the human V1R and V2R and activates several signaling pathways via different G proteins, depending on the level of receptor expression. The increased synthesis of DNA and cAMP levels observed i n cells expressing medium and high levels of V(3)Rs, respectively, may represent important events in the tumorigenesis of corticotroph cells .