Sphingosine interacts directly with the receptor complex to inhibit thyrotropin-releasing hormone binding

Sphingosine inhibition of [3H] [N3-Me-His] TRH (MeTRH) binding, previously shown to be independent of its effects on protein kinase-C, has been further characterized in GH3 cell membranes and in a partially purified, digitonin-solubilized receptor preparation. In membranes, as in intact cells, sphingosine inhibited [3H]MeTRH binding by decreasing receptor affinity, but, in contrast to its effect in intact cells, did not affect the number of available binding sites. The inhibition of binding was linear up to 75 microM sphingosine (in the presence of 100 microM BSA at 0.1 mg membrane protein/ml), yielding an apparent Ki of 51 microM. Since GTP decreases the affinity for MeTRH binding in GH3 cell membranes, we studied interactions between GTP and sphingosine. While the effects of low concentrations of GTP gamma S and sphingosine were additive, sphingosine inhibition of MeTRH binding surpassed and was not affected by the addition of maximally inhibitory concentrations of GTP gamma S. Also, sphingosine (75 microM) did not affect the ability of a maximally effective dose of TRH to stimulate the low Km GTPase (vehicle, +35 +/- 5%; sphingosine, +32 +/- 10%); there was a 25% decrease in total GTPase activity in the presence of sphingosine. MeTRH binding to digitonin-solubilized receptors, which had properties similar to those described previously by others, including no effect of GTP on binding, was inhibited by sphingosine. In solubilized receptors, as in membranes, sphingosine caused a decrease in apparent affinity without changes in the number of binding sites. These data suggest that sphingosine interacts directly with the TRH receptor [or an associated factor(s) in the receptor complex] to decrease affinity by a mechanism that does not involve uncoupling of G-proteins.