The present paper proposes a novel role of the thyrotropin receptor that belongs to the class A of cell membrane located G-protein coupled receptors. Its activation both from hormone stimulation and from cellular swelling leads to release of the osmolyte taurine. It is suggested that the TSH-receptor protein detects cell volume changes based on its Na+-sensitivity. Activation of this class of hormone receptors appear to involve release of a sodium ion bound in the receptor, and such a release may occur also during swelling activation, associated with a reduced cytosol Na+-concentration. The thyrotropin receptor induced signal that stimulates taurine efflux comprises adenylyl cyclase, cAMP and Epac. Additional strong modulations of signaling elements appeared to occur at locations downstream of cAMP that affects taurine efflux, conditions that were related to cellular volume.
Department of Biosciences, University of Oslo, POBox 1066, Blindern, 0316, Oslo, Norway. [email protected].
Cellular osmolyte release is important in preventing water accumulation and swelling. However, the signaling pathways that detect volume increase and activate solute efflux are still not fully understood. We investigated efflux activation of the osmolyte taurine which is actively accumulated in rat thyrocytes (FRTL-5). Efflux of accumulated [3H]taurine was stimulated by cellular swelling and thyrotropin (TSH). These effects were significantly diminished in cells having reduced TSH receptor concentrations. Phosphodiesterase inhibitors (IBMX, Rolipram) enhanced both responses. An analog of forskolin (FSK; 7-deacetyl-7-[O-(N-methylpiperazino)-γ-butyryl] dihydrochloride) and an analog of cAMP, specific for activating exchange protein activated directly by cAMP (Epac; 8-(4-chlorophenylthio)-2′-O-methyladenosine-3′,5′-cyclic monophosphate, acetoxymethyl ester), significantly stimulated [3H]taurine efflux. A cAMP analog specific for activating protein kinase A (PKA; N6-benzoyladenosine-3′,5′-cyclic monophosphate, acetoxymethyl ester) had no significant stimulatory effect on [3H]taurine efflux rate. The amiloride analog, 5-(N-ethyl-N-isopropyl)-amiloride, which inhibits a TSH-stimulated Na+/H+ exchanger, enhanced (100 %) and ouabain inhibited (50 %) the TSH-stimulated [3H]taurine efflux rate. The effect of FSK on efflux was strongly potentiated by Na+-free iso-osmotic conditions and by osmolality/cell volume that affected also the db-cAMP-stimulated efflux. The TSH receptors and downstream elements of the signaling pathway comprising adenylyl cyclase, cAMP and Epac appeared to mediate the hormone-induced signal for [3H]taurine efflux from FRTL-5 cells. With less evidence, the cell volume/osmolality-induced [3H]taurine efflux cascade appeared to share some of the hormone signaling elements and to modulate the hormone signaling pathway at two levels through cellular Na+.Go To Amino Acids