Persistent cAMP signaling by thyrotropin (TSH) receptors is not dependent on internalization

Evidence was presented that thyrotropin [thyroid-stimulating hormone (TSH)]-stimulated persistent cAMP signaling is dependent on receptor (with G-protein α subunits and adenylyl cyclase) internalization. Because it is not clear whether G proteins and adenylyl cyclase internalize with receptors, we tested whether persistent cAMP signaling by TSH receptor (TSHR) is dependent on internalization. We measured persistent TSHR signaling as an accumulation of cAMP in HEK-EM293 cells permanently expressing human TSHRs incubated with isobutylmethylxanthine for 30 min after washing the cells to remove unbound TSH, and TSHR internalization by fluorescence microscopy using Alexa-tagged TSH and binding assays using (125)I-TSH. TSHRs, but not the closely related lutropin or follitropin receptors, exhibit persistent cAMP signaling. TSHRs were not internalized by 30 min incubation with unlabeled TSH; however, expression of β-arrestin-2 promoted TSHR internalization that was inhibited by dynasore, a dynamin inhibitor. Expression of β-arrestin-2 had no effect on TSHR cAMP signaling, dynasore inhibited TSHR cAMP signaling in the absence or presence of TSHR internalization, and expression of a dominant-negative mutant dynamin, which inhibited internalization, had no effect on persistent cAMP signaling. Persistent cAMP signaling was specifically inhibited by a small molecule TSHR antagonist. We conclude that TSHRs do not have to be internalized to exhibit persistent cAMP signaling.