Reevaluation of the role of gangliosides in the binding and action of thyrotropin.

A cloned line of normal rat thyroid cells, FRTL, contained a small number of high-affinity binding sites for thyrotropin (TSH) when measured under physiological conditions. The cells also bound small amounts of cholera toxin, and both hormone and toxin stimulated cyclic AMP production by the cells. The major ganglioside of FRTL cells was N-acetylneuraminylgalactosylglucosylceramide (GM3), with minor amounts of gangliosides corresponding to galactosyl-N-acetylgalactosaminyl-[N-acetyl-neuraminyl]-galactosylglucosylceramide (GM1) and N-acetylneuraminylgalactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosylglucosylceramide (GD1a). Treatment of these cells with neuraminidase (acylneuraminyl hydrolase, EC 3.2.1.18) converted most of the GD1a to GM1. After neuraminidase treatment, the binding of cholera toxin, which binds to GM1, was increased, but there was no change in the binding of TSH. Preincubation of neuraminidase-treated FRTL cells with the B (binding) component of cholera toxin completely prevented cholera toxin binding but had no effect on the binding of TSH. Neuraminidase treatment also somewhat enhanced, rather than decreased, the cyclic AMP response to TSH. Pretreatment of FRTL cells with mixed brain gangliosides resulted in a 10-fold increase in cholera toxin binding. Again there was no enhancement of TSH binding or adenylate cyclase stimulation. Finally, prolonged exposure of FRTL cells to TSH induced down-regulation of TSH receptors but had no effect on gangliosides or cholera toxin receptors. The results indicate that more complex gangliosides do not serve as a component of the TSH receptor nor are they involved in the transmission of the hormone signal across the cell membrane of these cultured rat thyroid cells.