Arrestin-β-1 Physically Scaffolds TSH and IGF1 Receptors to Enable Crosstalk

Endogenously expressed TSH receptors (TSHRs) on orbital fibroblasts of patients with Graves ophthalmopathy (GO) use crosstalk with IGF1 receptors (IGF1R) to synergistically stimulate secretion of hyaluronan (HA), a major component of GO pathology. We previously showed crosstalk occurred upstream of mitogen-activated protein kinase (ERK) phosphorylation. Because other G protein-coupled receptors engage arrestin-β-1 (ARRB1) and ERK, we tested whether ARRB1 was a necessary component of TSHR/IGF1R crosstalk. HA secretion was stimulated by the TSHR-stimulating monoclonal antibodies M22 and KSAb1, or immunoglobulins from patients with GO (GO-Igs). Treatment with M22, as previously shown, resulted in biphasic dose-response stimulation of HA secretion. The high-potency phase was IGF1R dependent, and the low-potency phase was partly IGF1R independent. KSAb1 produced a monophasic dose-response stimulation of HA secretion, whose potency was lowered >20-fold after IGF1R knockdown. ARRB1 knockdown abolished M22's high-potency phase and lowered KSAb1's potency and efficacy. ARRB1 knockdown inhibited GO-Ig stimulation of HA secretion and of ERK phosphorylation. Last, ARRB1 was shown to be necessary for TSHR/IGF1R proximity. In contrast, ARRB2 knockdowns did not show these effects. Thus, TSHR must neighbor IGF1R for crosstalk in GO fibroblasts to occur, and this depends on ARRB1 acting as a scaffold. Similar scaffolding of TSHR and IGF1R by ARRB1 was found in human osteoblast-like cells and human thyrocytes. These findings support a model of TSHR/IGF1R crosstalk that may be a general mechanism for G-protein-coupled receptor/receptor tyrosine kinase crosstalk dependent on ARRB1.