The Involvement of the ERK Pathway in the Activation of the cAMP Response-Element-Binding Protein by Thyroxine in TT Cells

Abstract

It has been demonstrated that thyroxine stimulates calcitonin (CT) secretion from TT cells (a model human thyroid C cells) via the cAMP-dependent protein kinase A (PKA) pathway. The nongenomic effects of thyroid hormone including activation of cAMP-dependent protein kinase and mitogen-activated protein kinase (MAPK) pathway have also been reported in TT cells. Therefore, thyroid hormones may regulate the phosphorylation of cAMP response element-binding protein (CREB) related to the MAPK pathway, but further work is needed to confirm this possibility. TT cells, were incubated in medium containing the vehicle or thyroxine. The levels of intracellular total-extracellular signal-regulated kinases (T-ERK1/2) and phospho- ERK1/2 (P-ERK1/2) were measured. The thyroxine effects on CT secretion were examined through the cAMP-dependent PKA with ERK pathways from TT cells. TT cells were treated by vehicle, thyroxine plus KT5720 (a PKA inhibitor) or U0126 (a MAP kinase inhibitor). The amount and localization of P-CREB and P-ERK were analyzed by immunofluorescence. The results indicated that thyroxine increased nuclear P-CREB expression and intracellular levels of T-ERK1/2 and P-ERK1/2 and those effects were inhibited by KT5720 and U0126. These results suggested that the thyroxine stimulates CREB phosphorylation from TT cells via the expression of P-ERK.