Involvement of G protein-coupled receptor kinase 5 in homologous desensitization of the thyrotropin receptor.

Homologous desensitization of G protein-coupled receptors involves agonist-dependent phosphorylation of receptors by G protein-coupled receptor kinases (GRKs). To identify GRK(s) that play a role in homologous desensitization of the thyrotropin (TSH) receptor, thyroid cDNA was amplified by polymerase chain reaction using degenerate oligonucleotide primers from highly conserved regions in GRK family. GRK5 is found in the predominant isoform expressed in the thyroid. Rat GRK5 cDNA was then isolated, which encodes a 590-amino acid protein with 95% homology to human and bovine homologs. Northern blot identified GRK5 mRNA of approximately 3, 8, and 10 kilobases with highest expression levels in lung > heart, kidney, colon > thyroid. In functional studies using a normal rat thyroid FRTL5 cells, overexpression of GRK5 by transfecting the plasmid capable of expressing the sense GRK5 RNA suppressed basal cAMP levels and augmented the extent of TSH receptor desensitization, whereas suppression of endogenous GRK5 expression by transfecting the antisense GRK5 construct increased basal cAMP levels and attenuated the extent of receptor desensitization. Although exogenously overexpressed GRK6 also enhanced TSH receptor desensitization, we conclude that GRK5, the predominant GRK isoform in the thyroid, appears to be mainly involved in homologous desensitization of the TSH receptor.