Thyroid hormones feed back directly on anterior pituitary cells to suppress the secretion of thyrotropin stimulating hormone, but the mechanism of feedback regulation has remained unclear. Here we report that thyroid hormone rapidly stimulates a specific class (KCNH2) of voltage-dependent potassium channels, encoded by the ether-a-go-go-related gene (ERG), through a Rac-dependent signalling cascade in a rat anterior pituitary cell line (GH4C1). GH4C1 cells in equimolar potassium solutions were voltage-clamped to -40 mV in the whole-cell recording mode. ERG channels were activated by a 2 s depolarisation to +20 mV. The peak ERG currents were measured (means ± S.E.M. normalized for cell capacitance) upon repolarisation to -100 mV as the slowly decaying tail currents (¦20 pA pF^-1) that were blocked by the class III anti-arrhythmic methane-sulfonanilide, E-4031. Bath application of the active derivative of thyroid hormone, triiodothyronine (T3, 50 nM), stimulated the ERG current with a rapid time course (72 ± 24 %, within 2 min, n = 10). We reported previously (Storey et al. 2001) that thyrotropin releasing hormone (TRH) inhibits ERG channels through a Rho-dependent pathway. Because Rac and Rho are known to antagonize each other in their roles in neurite formation in neurons, we investigated the role of Rac in T3 action with either constitutively active (63L) or dominant negative (17N) forms of the Rac protein. Addition of constitutively active Rac protein to the pipette solution mimicked T3 stimulation, increasing ERG current by 42 ± 9 % (n = 6), whereas dominant negative Rac protein in the pipette solution had no effect on basal current (9 ± 5 % decrease, n = 8). The T3 signalling was blocked in GH4C1 cells transiently transfected (24 h) with dominant negative Rac (17N) (T3; 12 ± 21 % decrease, n = 4), whilst TRH signalling was unaltered (TRH; 37 ± 8 % decrease, n = 8). Thus Rac activation is both necessary and sufficient for stimulation of the ERG channel potassium current by T3 in GH4C1 cells. T3 receptors along with other members of the nuclear receptor superfamily activate phosphatidylinositol-3-OH kinase (PI3-K) (Simoncini et al. 2000), which in other cells stimulates the Rac guanine exchange factor, vav. Inhibition of PI3-K (wortmannin, 50 nM) blocked the T3-induced increase in ERG channel currents (wortmannin, 38 ± 3 % current decrease, n = 3; control, 41 ± 20 % current increase, n = 3). This implicates PI3-K in the stimulation of Rac by T3 receptors.

Simoncini, T., Hafezi-Moghadam, A., Brazil, D.P., Ley, K.L., Chin, W.W. & Liao, J.K. (2000). Nature 407, 538-541.

Storey, N.M., O’Bryan, J.P. & Armstrong, D.L. (2001). FASEB Abstract 15, A842.