Growth stimulating antibody, as another predisposing factor of Graves' disease (GD): analysis using monoclonal TSH receptor antibodies derived from patients with GD

TSH receptor antibody (TRAb) is clinically classified into thyroid stimulating antibody (TSAb) and thyroid-stimulation blocking antibody (TSBAb). Although the former is considered to cause Graves' disease (GD), its activity does not necessarily reflect hormone production and goiter size. Moreover, uptake of 99mTcO4(-), the best indicator for GD, is correlated with activity of TSH binding inhibitor immunoglobulin better than activity of TSAb. Because uptake of 99mTcO4(-) reflects thyroid volume, these observations suggest that there exist TRAb with thyrocyte growth stimulating activity (GSA) other than TSAb. In this study, we analyzed GSA of monoclonal TRAb established from patients with GD or idiopathic myxedema (IME). GSA was measured as the degree of FRTL-5 cell growth stimulated by each TRAb. The signaling pathways of the cell growth were pharmacologically analyzed. The cell growth stimulated by TSH was strongly suppressed by protein kinase A (PKA) inhibitor, but was not affected by extracellular signal regulated kinase kinase (MEK) inhibitor. Although TSAb from GD stimulated the cell growth, both inhibitors suppressed it. Surprisingly, the cell growth was also induced by TSBAb from GD and was only suppressed by MEK inhibitor. TSBAb from IME did not have GSA and attenuated the cell growth stimulated by TSH. We concluded that 1; in GD, not only TSAb but some TSBAb could stimulate thyrocyte growth. 2; TSBAb might be classified with respect to their effects on thyrocyte growth; i.e., thyrocyte growth stimulating antibody and thyrocyte growth-stimulation blocking antibody.