G protein-coupled receptor kinase 2 involvement in desensitization of corticotropin-releasing factor (CRF) receptor type 1 by CRF in murine corticotrophs

Hypothalamic CRF stimulates synthesis and secretion of ACTH via CRF receptor type 1 (CRFR1) in the anterior pituitary gland. After agonist- activated stimulation of receptor signaling, CRFR1 is down-regulated and desensitized. Generally, it is thought that G protein-coupled receptors may be desensitized by G protein-coupled receptor kinases (GRKs). However, the role of GRKs in corticotropic cells has not been determined. In this study we focused on involvement of GRKs in desensitization of CRFR1 by CRF in corticotropic cells. We found that GRK2 (but not GRK3) mRNA and protein were expressed in rat anterior pituitary cells and AtT-20 cells (a line of mouse corticotroph tumor cells). To determine the role of GRK2 in CRF-induced desensitization of CRFR1 in mouse corticotrophs, AtT-20 cells were transfected with a dominant-negative mutant GRK2 construct. CRF desensitized the cAMP-dependent response by CRFR1. Desensitization of CRFR1 by CRF was significantly less in AtT-20 cells transfected with the dominant-negative mutant GRK2 construct compared with desensitization in control (an empty vector-transfected) AtT-20 cells. Furthermore, pretreatment with a protein kinase A inhibitor also partially blocked desensitization of CRFR1 by CRF. These results suggest that GRK2 is involved in CRF- induced desensitization of CRFR1 in AtT-20 cells, and the protein kinase A pathway may also have an important role in desensitization of CRFR1 by CRF seen in corticotropic cells.