Contrasting effects of activating mutations of GαS and the thyrotropin receptor on proliferation and differentiation of thyroid follicular cells

The cyclic AMP pathway is a major regulator of thyrocyte function and proliferation and, predictably, its inappropriate activation is associated with a sub-set of human thyroid tumours, Activating mutations are, however, more common in the thyrotropin receptor (TSHR) than in its downstream transducer, G alpha s. To investigate whether this reflects an inherent difference in their oncogenic potency, we compared the effects of retro retrovirally-transduced mutant (A623I) TSI-IR or (Q227L) G alpha s (GSP), using the rat thyroid cell line FRTL5 and primary human thyrocytes, In FRTL5, expression of GSP or mutant (m) TSHR induced a 2-3-fold increase in basal levels of cAMP. This was associated with TSH-independent proliferation (assessed by both cell number and DNA synthesis) and function las shown by increased expression of thyroglobulin (Tg) and the sodium/iodide symporter). In primary cultures, expression of mTSHR, but not GSP, consistently induced formation of colonies with epithelial morphology and thyroglobulin expression, capable of 10-15 population doublings (PD) compared to less than three in controls. Thus, while mTSHR and GSP exert similar effects in FRTL5, use of primary cultures reveals a major difference in their ability to induce sustained proliferation in normal human thyrocytes, and provides the first direct evidence that mTSHR is sufficient to initiate thyroid tumorigenesis.