| Abstract: | The cellular actions of the thyroid hormones L-thyroxine and L-triiodothyronine are mediated by the association of hormone with a chromatin-associated receptor. In cultured GH1 cells, a hormone-responsive rat pituitary cell line, thyroid hormone decreases the concentration of its receptor at early incubation times by reducing the accumulation of newly synthesized receptor. In this study, we demonstrate that cholera toxin also reduces the amount of nuclear receptor in GH1 cells in a time- and dose-dependent fashion, without altering the affinity of the receptor for hormone. The reduction of receptor mediated by cholera toxin is not secondary to a generalized inhibition of cell protein synthesis or cell replication rates and this effect can be abolished by pretreatment of the cholera toxin with soluble ganglioside II3-alpha-N- acetylneuraminosylgangliotetraosylceramide . This effect requires an intact cholera toxin molecule and does not occur at similar concentrations of the membrane-binding B subunit of cholera toxin. In order to study the influence of cholera toxin on thyroid hormone receptor turnover, we have used a dense amino acid-labeling technique. The results indicate that cholera toxin does not change the half-life of receptor, but decreases the rate of appearance of newly synthesized receptor. This decreased rate completely accounts for the lowered steady state receptor levels. The extent of cAMP stimulation by cholera toxin does not correlate with the extent of receptor reduction and forskolin, which stimulates cAMP 25- to 500-fold, does not decrease thyroid hormone receptor abundance. These studies suggest that cholera toxin modulates receptor levels by a mechanism(s) that is not mediated by cAMP in GH1 cells. |
