The human thyrotropin receptor: a heptahelical receptor capable of stimulating members of all four G protein families.

Abstract

Thyrotropin is the primary hormone that, via one heptahelical receptor, regulates thyroid cell functions such as secretion, specific gene expression, and growth. In human thyroid, thyrotropin receptor activation leads to stimulation of the adenylyl cyclase and phospholipase C cascades. However, the G proteins involved in thyrotropin receptor action have been only partially defined. In membranes of human thyroid gland, we immunologically identified a subunits of the G proteins Gs short, Gs long, Gil, GM2, Gu, Go (Go2 and another form of G., presumably Go,), Gq, Gli, G12, and G13.Activation of the thyrotropin (TSH) receptor by bovine TSH led to increased incorporation of the photoreactive GTP analogue [a-32P]GTP azidoanilide into immunoprecipitated a subunits of all G proteins detected in thyroid membranes. This effect was receptor-dependent and not due to direct G protein stimulation because it was mimicked by TSH receptorstimulating antibodies of patients suffering from Grave disease and was abolished by a receptor-blocking antiserum from a patient with autoimmune hypothyroidism. The TSH-induced activation of individual G proteins occurred with EC50 values of 5-50 milliunits/ml, indicating that the activated TSH receptor coupled with similar potency to different G proteins. When human thyroid slices were pretreated with pertussis toxin, the TSH receptor-mediated accumulation of cAMP increased by -35% with TSH at 1 milliunits/ml, indicating that the TSH receptor coupled to Gs and GI. Taken together, these findings show that, at least in human thyroid membranes, in which the protein is expressed at its physiological levels, the TSH receptor resembles a naturally occurring example of a general G protein-activating receptor.

Oncogenic potential of a mutant human thyrotropin receptor expressed in FRTL-5 cells

“…According to the ®rst hypothesis, and in agreement with the demonstration that the human TSHR can couple with similar potencies to di erent G proteins (Laugwitz et al, 1996), one could expect that TSHR* could activate more than one signal transduction pathway, when Gsp activates only one. However, it is unlikely that, in the case of the TSHR mutant form we have used, polyphosphoinositide hydrolysis by a PLC b isoform could contribute to signal transduction, because the M453T TSHR* expressed in COS-7 cells was showed to stimulate adenylyl cyclase but not PLC b isoform activity (De Roux et al, 1996).…”

Construction of Structural Mimetics of the Thyrotropin Receptor Intracellular Domain

“…We next used a microscale thermophoresis technique (35) to investigate the ability of the TSHR ICD mimetics to bind recombinant purified Ga subunits. TSHR is remarkable in its ability to activate all known G protein isoforms (21)(22)(23). Fluorescently labeled Ga subunits (Ga-(s), Ga-(i2), and Ga-(q)) were titrated with increasing concentrations (0.1-100 mM) of unlabeled mimetic proteins.…”

“…In cell-based assays, the TSHR exhibited a broad G protein-coupling profile (21)(22)(23). In this study, however, the 6-Helix-based TSHR ICD mimetics were able to recognize and activate the G-(s) isoform, but failed to do so with the G-(q) and G-(i) isoforms.…”

“…As a model GPCR, we used thyroid-stimulating hormone receptor (TSHR), a GPCR that has well-documented promiscuous G protein-coupling behavior and is able to activate all known G protein isoforms (21)(22)(23). The respective physiological/pathological consequences of the activation of individual isoforms of G proteins by TSHR, however, remain unclear.…”

Cooperative transformation of NIH3T3 cells by Gα12 and Rac1

“…Although the physiological role of Ga12 remains to be determined, growing evidence suggests that it participates in multiple cellular functions. Thyrotropin, a primary hormone regulating thyroid cell functions is shown to stimulate the uptake of photoreactive GTP analogue into immunoprecipitated Ga12 (Laugwitz et al, 1996). Similarly, in human platelet, Ga12 has been shown to be activated via thromboxane A2 and thrombin receptors and may therefore be involved in platelet activation (O ermans et al, 1994).…”