Effects of thyrotropin and thyroid hormones in vivo on thyroid responsiveness to thyrotropin in vitro.

The thyroid gland of rats fed propylthiouracil is known to be unresponsive in vitro to thyrotropin; to investigate further the underlying mechanism groups of rats were variously treated with propylthiouracil and thyroid hormone or subjected to hypophysectomy. In vitro responsiveness of the thyroids was tested by measuring an increase in the concentration of c AMP when thyrotropin or prostaglandin E1 was added to the medium. Results showed that responsiveness to thyrotropin partially returned with rats fed prophylthiouracil and hypophysectomized 5, but not 2, days before death; hypophysectomy of normal rats led to increased in vitro responsiveness to thyrotropin and this was partially reversed by injections of thyrotropin for a week before death. Administration of thyroid hormone had little effect in these investigations and in vitro responsiveness to prostaglanding E1 was not consistently influenced by any of the in vivo regimens. From this experience we conclude that, at least as studied in vitro, circulating thyrotropin has a significant role in modulating responsiveness of the thyroid to thyrotropin.     

Carbamylcholine and thyrotropin activate distinctive pathways of protein phosphorylation in dog thyroid

The role of protein phosphorylation in the regulation of thyroid function by carbamylcholine was investigated using dog thyroid slices incubated in the presence of [³²P]phosphate and two-dimensional electrophoresis. In these intact cells, carbachol increased the phosphorylation of three polypeptides with M_r values of 21 500, 24 000 and 29 000. Maximal [³²P]phosphate incorporation occurred within 5 min of addition of carbamylcholine for 10 min increased the phosphorylation of 11 polypeptides whcih were identical to those observed previously after 2 h of hormone action (Lecocq, R., Lamy, F. and Dumont, J.E. (1979) Eur. J. Biochem. 102, 147–152). All three polypeptides whose phosphorylation is increased by carbamylcholine were different from those whose phosphorylation is increased by thyrotropin. Under our experimental conditions, the calcium ionophore A23187 did not stimulate significantly [³²P]phosphate incorporation in these three polypeptides. In conclusion, our results show that carbamylcholine and thyrotropin, which have some antagonist and some similar effects on dog thyroid, do not act through the phosphorylation of the same proteins. Although we have, in our previous chapter, established that in a rise in intracellular cyclic AMP could accout for the effect of thyrotropin on protein phosphorylation, the nature of the intracellular mediator of carbamylcholilne action on this parameter is still uncertain.     

Effect of thyrotropin on glycosaminoglycans synthesized by primocultured thyroid cells

The synthesis of glycosaminoglycans (GAGs) was investigated in porcine thyroid cells under the influence or not of thyrotropin. After labelling with [³H] glucosamine and [³⁵S] $\text{SO}{}_4{}^{\mathrm{2?}}$, enriched GAG-fractions prepared from culture media, cells, and eventually substrate adhering materials, were analyzed by cellulose acetate electrophoresis combined with specific degradations. They comprised heparan sulfate and hyaluronic acid together with an unknown sulfated component labile to endo-β-galactosidase. Whereas global labellings of newly made GAGs were not significantly modified by thyrotropin, we reproducibly observed with the hormone a substantial increase in the proportion of hyaluronic acid [³H] label and, when cells organized into follicles, of the proportion of cell-associated [³H] GAGs. This system thus offers an interesting model to study how the responsiveness to an hormone and the reorganization that follows might implicate specific glycoconjugates.     

Effect of denaturation on the susceptibility of proteins to enzymic phosphorylation.

Heat-denatured chicken egg white lysozyme and the reduced carboxymethylated maleylated derivative of this protein were found to serve as substrates for rabbit skeletal muscle cyclic AMP-dependent protein kinase. The native form of the protein was not a substrate. Two phosphoryl groups per mole of lysozyme were incorporated in the reaction. It was determined that the phosphoryl moieties were bound to serine 24 and serine 50 in the modified protein. Serine 24 was phosphorylated approximately 3 times as fast as serine 50. Reduced carboxymethylated maleylated derivatives of bovine serum albumin, phosphorylase b, and creatine kinase also served as substrates for the protein kinase whereas their native forms did not. The reduced carboxymethylated maleylated derivative of the inhibitory subunit of troponin was a poorer substrate than the native form of the protein. Maleylated histones F1 and F2b were also poorer substrates than the nonderivatized forms. The significance of these experiments with reference to the specificity of cyclic AMP-dependent protein kinase is discussed.     

Pattern of protein phosphorylation in intact stimulated cells: thyrotropin and dog thyroid

Two-dimensional, high-resolution electrophoretic technique of O'Farrell has been adapted to the analysis of thyroid phosphorylated proteins. Proteins were extracted from dog thyroid slices which had been incubated in the presence of [32P]phosphate with thyrotropin or with different agents which enhance the intracellular accumulation of cyclic AMP. About 350 phosphorylated polypeptides have been separated. Thyrotropin stimulates the phosphorylation of at least eight of these polypeptides. An increase in the phosphorylation of the same polypeptides was observed was observed when dog thyroid slices were incubated with dibutyryl adenosine 3':5'-monophosphate, cholera toxin or prostaglandin E1 instead of thyrotropin. Our results confirm that most of dog thyroid protein phosphorylation is independent of cyclic AMP. They offer a first link between the action of cyclic AMP on protein kinase and the physiological effects of thyrotropin. They strongly substantiate the hypothesis that most thyrotropin effects are mediated by cyclic AMP.     

Effect of gelatin on the cyclic AMP response of primocultured hog thyroid cells to acute thyrotropin stimulation.

The cyclic AMP response of cultured hog thyroid cells to acute thyrotropin stimulation was shown to be under a dual regulatory control by thyrotropin: both positive and negative regulation have been described. When added to the culture medium, gelatin (0.25%) promoted the reorganization of the cells into folicle-like structures, as does thyrotropin. Unlike thyrotropin, gelatin did not induce an increase in intracellular cyclic AMP but enhanced the acute cyclic AMP response to thyrotropin in cells cultured in gelatin-containing medium. When both gelatin and thyrotropin were present, the positive effect of low concentrations of hormone (less than 50 microU/ml) was increased whereas the refractory process observed in the presence of higher concentrations of hormone (greater than 50 microU/ml) was unchanged. These effects of gelatin might be mediated by interaction of the denatured collagen molecules with external proteins of the plasma membrane of thyroid cells.     

Effect of thyrotropin on the properties of messenger RNA from cultured human thyroid cells

When added to the culture medium of thyroid cells isolated from diffuse nontoxic goiter, thyrotropin increased the poly(adenylic acid) content and the template activity of the unfractionated RNA. This increase was correlated with higher thyroglobulin messenger activity, as demonstrated by specific immunoprecipitation of the labeled peptides synthesized in two heterologous cell-free systems. When RNAs were separated in a sucrose gradient, thyrotropin was shown to enhance the poly(adenylic acid) content and template activity of fractions with sedimentation coefficients of 34, 23 and 15 S. Specific immunoprecipitation showed that a thyroglobulin messenger activity was present in these three fractions. Another way by which thyrotropin regulates the thyroid protein synthesis is suggested by the shift of poly(adenylic acid)-containing RNA to large polysomes when thyroid cells were cultured in the presence of the hormone.     

Effects of thyrotropin on thyroid chromatin: Enhanced sensitivity to micrococcal nuclease and increased nuclear protein phosphorylation

Thyroid slices were incubated with or without TSH for 2 or 5 h. Nuclei were then prepared, subjected to mild digestion with micrococcal nuclease, and centrifuged at 1200 × g. The amount of DNA in 1200 × g supernatants was increased by TSH at 5 h, but not at 2 h. In parallel studies, thyroid slices were incubated with ³²P_i and labeling of acid-soluble nuclear proteins was examined. TSH-dependent increases in labeling of histones H1 and H3, and of the high mobility group protein HMG 14, were observed at 2 h; however, there were no apparent changes in TSH-dependent labeling between 2 and 5 h, in nuclease-sensitive or in bulk chromatin. These results suggest that the observed TSH-dependent changes in the micrococcal nuclease-sensitivity of thyroid nuclear chromatin were not induced directly by changes in the phosphorylation of the histones or HMG 14.     

Noninactivation of thyrotropin by cultured porcine thyroid cells.

Freshly isolated porcine thyroid cells were cultured in the presence of highly purified porcine thyrotropin. Cells associate into follicles between the second and tenth day of culture and later form a monolayer. The biological and immunological activity of thyrotropin was measured daily in the media. Thyrotropin concentration and biological activity remained unchanged from the onset of the culture up to day 14. Limiting factors influencing thyroglobulin biosynthesis do not appear before day 13. The loss of follicular organization at day 10 cannot be explained by thyrotropin degradation in the medium. Considering the number of receptors per cell and the half life of the thyrotropin . receptor complex in the two dissociation compartments previously demonstrated, it appears in terms of both biological activity and affinity for the receptors that the thyrotropin molecules released from the first compartment do not differ from native molecules. It can be calculated that at least 31% of the molecules released from the second compartment are not inactivated. Thus, it is probable that the catabolism of thyrotropin on the receptor, or near the receptor site, does not play an important role in the regulation of thyroid cell function in vitro.