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Biological Activity of Activating Thyroid-Stimulating Hormone Receptor Mutants Depends on the Cellular Context

Department of Medicine (Endocrinology, Metabolism and Diabetes Section) (D.F., M.D.L., F.A., K.S., M.L.) and Department of Pathology (D.W.-T.), University of Wales College of Medicine, Heath Park, Cardiff CF14 4XN, United Kingdom; Medizinische Klinik III (R.P.), Universität Leipzig, Philipp-Rosenthal Strasse 27, 04103 Leipzig, Germany; and Department of Medicine (M.E.), University of Birmingham, Queen Elizabeth Hospital, Birmingham B15 2TH, United Kingdom

Address all correspondence and requests for reprints to: Marian Ludgate, Ph.D., Department of Medicine (Endocrine Section), University of Wales College of Medicine, Cardiff, Heath Park, Cardiff CF14 4XN, United Kingdom. E-mail: ludgate{at}cf.ac.uk.

Activating TSH receptor (TSHR) mutations are a major cause of toxic thyroid adenoma and familial hyperthyroidism, and more than 37 such mutations have been described. Previously their functional activity had been assessed in terms of cAMP and inositol phosphate production and predominantly in transiently transfected COS-7 (monkey embryonic kidney cells), a model that does not reflect effects on thyrocyte proliferation and function. Here we have performed a systematic comparison of wild-type and seven gain-of-function TSHR mutants, introduced into rat FRTL-5 and human thyrocytes, using retroviral vectors. Our results show that 1) biological potency of TSHR mutants in thyroid cells does not correlate with their cAMP levels in transfected COS cells, highlighting the importance of cellular context and level of expression when assessing biological effects of oncogenic mutations; 2) dissociation between stimulation of function and growth occurs with thyrocyte differentiated functions more readily stimulated than growth; 3) TSHR mutants show a similar order of potency in FRTL-5 cells and human thyrocytes; 4) mutants inducing the highest stimulation of adenylyl cyclase may paradoxically fail to induce proliferation; and 5) biological effects of cAMP activating TSHR mutants are attenuated by complex counterregulatory mechanisms at least at the level of phosphodiesterases and cAMP regulatory element modulator isoforms.

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