Chronic treatment with mood stabilizers increases membrane GRK3 in rat frontal cortex.

BACKGROUND: G-protein receptor kinases (GRKs) are a family of serine/threonine kinases involved in the homologous desensitization of agonist activated G-protein coupled receptors (GPCRs). G-protein coupled receptor supersensitivity, possibly as a result of decreased GRK, has been suggested in affective disorders. METHODS: We used immunobloting to determine if chronic, therapeutically relevant doses of lithium (Li+), carbamazepine (CBZ), and valproate (VPA), would increase GRK2/3 protein levels in rat frontal cortex. RESULTS: Chronic Li+ (24%) and CBZ (44%) significantly increased GRK3 in the membrane but not cytosol fractions. Chronic VPA had no effect on GRK3. G-protein receptor kinase 2 protein levels were unchanged by all treatments. The GRK3 membrane to cytosol ratio was increased significantly in Li+ and CBZ treated rats. CONCLUSIONS: These results show that chronically administered Li+ and CBZ, but not VPA, increase the translocation of GRK3 from cytosol to membrane, possibly correcting supersensitivity of GPCRs in bipolar disorder.

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.

GTP-binding-protein-coupled receptor kinases--two mechanistic models.

Six vertebrate protein kinases (G-protein-coupled receptor kinases; GRKs) that regulate the function of G-protein-coupled receptors (GPCRs) were recently cloned; several distinct properties set them apart from conventional second-messenger regulated protein kinases. It appears that GRKs bind GPCR* through two separate sites: a high-affinity site, which involves intracellular loops of the activated receptor, and the lower-affinity site, encompassing the phosphorylation region. The high-affinity interaction may involve complementary structural elements of GRKs and GPCRs* rather than precise amino acid alignment, thus allowing broad and overlapping specificities of these kinases, in spite of differences in the sequences of GPCRs. In addition, GRK structures are modified by several posttranslational modifications, including phosphorylation, autophosphorylation, prenylation, carboxymethylation, and palmitoylation, probably affecting properties of these enzymes. While GRKs phosphorylate and inactivate receptor molecules which are engaged in G-protein activation, controversy surrounds whether GRKs might be activated and phosphorylate unstimulated GPCRs, leading to a desensitization of a larger population of the receptors. In this review, mechanistic aspects of GPCR* phosphorylation related to the distinct properties, regulation and modes of action of GRKs are described.