Solubilization of pituitary receptors for thyrotropin-releasing hormone

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.

Subcellular distribution of protein kinase C of GH3 cells: quantitation and characterization by polyacrylamide gel electrophoresis

Quantitative estimation of cytosolic Ca2+- and phospholipid-dependent protein kinase (PKC) activity was performed by polyacrylamide gel electrophoresis under nondenaturating conditions (PAGE). With this method less than 50 micrograms of cytosol protein can be accurately quantitated for PKC activity. The amount of cytosolic PKC activity recovered after PAGE was comparable to the amount obtained by DEAE-cellulose chromatography. Homogenization of GH3 cells in the presence of 2 mM EGTA/EDTA revealed that 80% of the total cellular PKC activity resided in the cytosol. However, omission of the ion chelator during cell disruption followed by subcellular fractionation and extraction of subcellular fractions by EDTA/EGTA showed that 80% of the total PKC was found in the lysosomal-mitochondrial and microsomal extracts. Detailed analysis of PKC activities demonstrated that cytosolic PKC was identical with the PKC solubilized by EDTA/EGTA from subcellular fractions. In conclusion, GH3 cells appear to contain one species of PKC with an apparent molecular weight of 90,000 which seems to be associated with membranes via a calcium-dependent mechanism (or mechanisms).

Sulfhydryl groups in receptor binding of thyrotropin-releasing hormone to rat amygdala

Binding of [3H]-[3-Me-His2]thyrotropin-releasing hormone [( 3H]MeTRH) to TRH receptors in rat amygdala was decreased by sulfhydryl reagents in a time-, temperature-, and concentration-dependent manner. A pronounced reduction in receptor density, with little or no change in binding affinity, was apparent following disulfide bond reduction by dithiothreitol (DTT), alkylation of thiol groups by N-ethylmaleimide (NEM), and their oxidation by 5,5'-dithiobis (2-nitrobenzoic acid). Heavy metals (Cd2+, Hg2+), which complex with reactive -SH residues, also potently inhibited binding. The pharmacological specificity of residual [3H]MeTRH binding in chemically modified amygdala membranes was the same as that in control preparations. Sequential exposure to thiol reagents, in the presence or absence of cations, revealed possible additive effects. Pretreatment of membranes with TRH (10(-8) - 10(-6) M), and its continued presence during modification, afforded protection against DTT and NEM. These results indicate the possible importance of thiol groups in the maintenance of TRH receptor conformation.

Purification and characterization of plasma membrane fractions from cultured pituitary glands

Highly purified plasma membrane fractions have been prepared from GH3 pituitary cells grown in suspension cultures. These membrane fractions have been obtained by differential and sucrose gradient centrifugation and were characterized in terms of their lipid content, marker enzyme analysis and the binding of 3H-labelled thyrotropin-releasing hormone (TRH) to its receptor. Alkaline phosphatase and 5'-nucleotidase activities were enriched 12-to 15-fold in the plasma membrane fraction with somewhat greater enrichment (28-fold) of the specific binding component for [3H]TRH, with a specific activity of 2286 fmol [3H]TRH bound per mg protein. A single class of binding sites for TRH was observed with an apparent dissociation constant of 18 nM, a value similar to that observed for intact cells. No detectable TRH binding to the nuclear fraction was observed that could not be ascribed to residual plasma membrane contamination. By electron microscopy, these fragments appeared to be sealed vesicles with an average diameter of approximately 1800 A. The binding of 125I-labelled wheat germ agglutinin was used as a marker for plasma membrane purification. In addition to specific binding to this membrane fraction, specific binding was also observed in the nuclear fraction. Studies with fluorescein-labelled wheat germ agglutinin revealed that, in fixed cells, fluorescence was restricted to the plasma membrane. However, if the cells were treated with Triton before labelling, most of the fluorescence was then associated with the cell nucleus. Hence, the use of wheat germ agglutinin binding as a specific membrane marker must be reevaluated.

Direct visualization of receptors for thyrotropin-releasing hormone with a fluorescein-labeled analog

Thyrotropin-releasing hormone (TRH) binds to specific receptors on GH4C1 pituitary tumor cells. A fluorescently labeled analog of TRH was synthesized by coupling pGlu-His-ProNH(CH2)6NH2 to fluorescein isothiocyanate. The fluorescein-labeled peptide (FL-TRH) stimulated prolactin synthesis and release by GH4C1 cultures and bound to TRH receptors with an apparent Kd of 400 nM. Binding of FL-TRH to unfixed, viable GH4C1 cells was followed by fluorescence microscopy. After incubation with 1.4 microM FL-TRH for 1 hr at 37 degrees C, the surface of all cells was fluorescent and patches of intense fluorescence were evident. Control cultures incubated with FL-TRH and excess TRH were not fluorescent, and a line of pituitary tumor cells which lacks TRH receptors displayed little fluorescence after incubation with FL-TRH. When GH4C1 cells were incubated with FL-TRH for 1 hr at 37 degrees C and then with excess TRH for an additional 1 hr, the fluorescence associated with the cells was diminished to control levels. The results demonstrate that the fluorescein-labeled peptide labels specific TRH receptors.