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

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 α subunits of the G proteins G(s short), G(s long), G(i1), G(i2), G(i3), G(o) (G(o2) and another form of G(o), presumably G(o1)), G(q), G₁₁, G₁₂, and G₁₃. Activation of the thyrotropin (TSH) receptor by bovine TSH led to increased incorporation of the photoreactive GTP analogue [α- ³²P]GTP azidoanilide into immunoprecipitated α 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 receptor- stimulating 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 G(s) and G(i). 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.