A 64 kDa protein is a candidate for a thyrotropin-releasing hormone receptor in prolactin-producing rat pituitary tumor cells (GH4C1 cells)

Synthesis and characterization of a high-affinity photoactivatable analogue of thyrotropin-releasing hormone

An analogue of thyrotropin-releasing hormone (TRH, pGlu-His-ProNH2), i.e. pGlu-His-ProNH-(CH2)6-(4-azidosalicylamide) (TRH-ASA), has been synthesized and, in a radioiodinated form (TRH-IASA), characterized and used as a photoaffinity reagent to label the TRH receptor on rat pituitary GH4C1 cells. TRH-IASA bound to GH4C1 cells with high affinity (Kd = 8 nM), comparable with that of TRH binding. The binding of TRH-IASA was competitive with binding of TRH, two TRH analogues and a TRH receptor antagonist, chlordiazepoxide. TRH-IASA did not bind to or label GH12C1 cells, which lack functional TRH receptors. Labelling of GH4C1 cells with TRH-IASA followed by SDS/PAGE and autoradiography of membrane proteins demonstrated labelling of a single polypeptide which ran as a diffuse band between 71 and 91 kDa, centred at 76 kDa. No change in this labelling pattern was observed as a function of the length of time (between 5 min and 2 h) that GH4C1 cells were incubated with 3 nM-TRH-IASA. Using either a very short (5 s) photolysis interval or low TRH-IASA concentrations, only the 76 kDa band was labelled. Minor bands appeared only after extended photolysis and use of high TRH-IASA concentrations. We conclude that the TRH receptor from rat pituitary GH4C1 cells is a single peptide with an apparent molecular mass of 76 kDa. Details of the chemical synthesis of TRH-ASA are given in Supplementary Publication SUP 50167 (5 pages), which has been deposited at the British Library Document Supply Centre, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1992) 281, 5.

Recognition by anti-idiotypic anti-thyrotropin-releasing hormone (TRH) antibody of rat TRH receptors

We asked whether anti-idiotypic antibodies raised against anti-TRH antibody could bind to TRH receptors in the rat anterior pituitary and brain. Six rabbits were immunized with IgG from a rabbit anti-TRH antiserum. One anti-idiotypic antibody caused strong, dose-dependent inhibition in anti-TRH antibody-binding to [125]I-TRH. This inhibition was not observed after treatment with goat anti-rabbit IgG antibody. The anti-idiotypic anti-TRH antibody significantly immunoprecipitated digitonin-solubilized pituitary TRH receptors. When eluates of digitonin-solubilized membranes which were adsorbed by either an anti-idiotypic anti-TRH IgG-, normal rabbit IgG-linked affinity column or control column were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis and visualized by silver stain, only the former eluate showed two bands under nonreducing conditions; one corresponded to a molecular weight marker of 200K, the other to 100K. Western blotting analysis with an anti-idiotypic anti-TRH antiserum showed a single band of molecular weight 56K under reducing conditions. The present study indicates that one can make anti-idiotypic antibodies that specifically recognize TRH receptors.

2-Diazohistidinyl thyrotropin-releasing hormone (TRH): a photosensitive TRH analog

We have synthesized a new thyrotropin releasing hormone (TRH) analog, pGlu-2-diazo-His-Pro-NH2 5, which is a potential photoaffinity label for the TRH receptor. Its precursor, pGlu-2-amino-His-Pro-NH2 4, has been synthesized through successive coupling of a 2-aminohistidine derivative 1 with proline and pyroglutamic acid derivatives. Diazotization of the fully deprotected tripeptide 4 gave the photoactivatable TRH analog 5. Compounds 4 and 5 exhibit IC50 values of 2 and 8 microM respectively, to be compared to the 0.02 microM value for native TRH.