Expression of mutationally activated G alpha s stimulates growth and differentiation of thyroid FRTL5 cells

The alpha subunit of the GTP-binding protein Gs mediates hormonal stimulation of adenylyl cyclase. Human pituitary and thyroid tumours harbour mutations of G alpha s that constitutively activate the protein by inhibiting its intrinsic GTPase activity. We have investigated the mitogenic action of mutationally activated alpha s in thyroid FRTL5 cells, a cell line dependent upon thyroid-stimulating hormone (TSH) for both growth and differentiation. Introduction of alpha s carrying the substitution of glutamine-227 with leucine (Q227L alpha s) by retroviral infection of FRTL5 cells resulted in the expected stimulation of membrane adenylyl cyclase activity and in increased intracellular accumulation of cAMP. Measurements of cytosolic Ca2+ levels did not detect any concomitant effect on the polyphosphoinositide-Ca2+ signalling pathway. Expression of Q227L alpha s conferred to FRTL5 cells the ability to synthesize DNA in the absence of TSH, as revealed by [3H]thymidine incorporation experiments, and to proliferate independently of the mitogenic hormone, although with a rate of growth slower than that observed with TSH stimulation. The effect of Q227L alpha s on cell proliferation was associated with the constitutive activation of iodide uptake. The results indicate that expression of mutationally activated G alpha s is sufficient to bypass the requirement for TSH and promotes autonomous growth and activation of thyroid-specific differentiated functions in FRTL5 cells.