Abstract
To study phosphorylation of the endogenous type I thyrotropin-releasing hormone receptor in the anterior pituitary, we generated phosphosite-specific polyclonal antibodies. The major phosphorylation site of receptor endogenously expressed in pituitary GH3 cells was Thr(365) in the receptor tail; distal sites were more phosphorylated in some heterologous models. beta-Arrestin 2 reduced thyrotropin-releasing hormone (TRH)-stimulated inositol phosphate production and accelerated internalization of the wild type receptor but not receptor mutants where the critical phosphosites were mutated to Ala. Phosphorylation peaked within seconds and was maximal at 100 nm TRH. Based on dominant negative kinase and small interfering RNA approaches, phosphorylation was mediated primarily by G protein-coupled receptor kinase 2. Phosphorylated receptor, visualized by immunofluorescence microscopy, was initially at the plasma membrane, and over 5-30 min it moved to intracellular vesicles in GH3 cells. Dephosphorylation was rapid (t((1/2)) approximately 1 min) if agonist was removed while receptor was at the surface. Dephosphorylation was slower (t((1/2)) approximately 4 min) if agonist was withdrawn after receptor had internalized. After agonist removal and dephosphorylation, a second pulse of agonist caused extensive rephosphorylation, particularly if most receptor was still on the plasma membrane. Phosphorylated receptor staining was visible in prolactin- and thyrotropin-producing cells in rat pituitary tissue from untreated rats and much stronger in tissue from animals injected with TRH. Our results show that the TRH receptor can rapidly cycle between a phosphorylated and nonphosphorylated state in response to changing agonist concentrations and that phosphorylation can be used as an indicator of receptor activity in vivo.
Highlights
The type I thyrotropin-releasing hormone (TRH)2 receptor is a seven-transmembrane G protein-coupled receptor (GPCR) that is expressed in the anterior pituitary, where it responds to TRH secreted from the hypothalamus, leading to the release of thyroid-stimulating hormone (TSH) from thyrotrophs, which in turn causes release of T3 and T4 from the thyroid
Phosphorylation of Endogenous TRH Receptors in Pituitary Cells—Previous studies of Ala-substituted and truncated TRH receptors suggested that potential phosphosites between residues 350 and 380 of the cytoplasmic tail are important for internalization and desensitization [20, 21]
Phosphorylation was measured by a modified ELISA procedure in which rat pituitary GH3 cells, which endogenously express TRH receptor [22], were incubated with or without TRH, and cells were fixed in culture dishes and incubated with affinity-purified antibody (Ab6959)
Summary
The type I thyrotropin-releasing hormone (TRH)2 receptor is a seven-transmembrane G protein-coupled receptor (GPCR) that is expressed in the anterior pituitary, where it responds to TRH secreted from the hypothalamus, leading to the release of thyroid-stimulating hormone (TSH) from thyrotrophs, which in turn causes release of T3 and T4 from the thyroid. We generated phosphosite-specific polyclonal antibodies against all potential phospho-Ser and -Thr residues in the conserved region of the TRH receptor cytoplasmic tail and used these to study phosphorylation and dephosphorylation of the endogenous TRH receptor in rat pituitary GH3 cells and in rat pituitary tissue.
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