Constitutive activation of cyclic AMP but not phosphatidylinositol signaling caused by four mutations in the 6th transmembrane helix of the human thyrotropin receptor

Four different somatic mutations (F631C, T632I, D633E, and D633Y) in the putative 6th transmembrane helix of the human thyrotropin receptor (TSHR) were recently described in hyperfunctioning thyroid adenomas [Porcellini et al. (1994) J. Clin. Endocrinol. Metab. 79, 657-661]. We transiently expressed these mutant receptors in Cos-7 cells and measured [125I]TSH binding, basal and TSH-stimulated cAMP production, and phosphatidylinositol hydrolysis. The concentration of receptors expressed at the cell surface was lower for the mutants than for the wild type (WT) TSHR. Compared to the WT, all four mutant receptors caused a marked increase in basal cAMP levels, but did not increase basal production of inositol phosphates. This suggests that autonomous thyroid function and adenoma formation may be related to constitutive activation of the cAMP pathway alone. A cluster of conserved residues at the base of the 6th transmembrane helix of the TSHR and other glycoprotein hormone receptors appears important for maintaining an inactive receptor conformation.