Thyrotropin regulates c-Jun N-terminal kinase (JNK) activity through two distinct signal pathways in human thyroid cells

c-Jun N-terminal kinases (JNK) participate in cellular responses to mitogen ic stimuli and environmental stresses. We investigated whether and how TSH, which promotes the proliferation and differentiation of thyroid cells, reg ulates JNK activity in primary cultured human thyroid cells. TSH stimulated JNK activity in cytosolic fractions of thyroid cells measured by in vitro kinase assay. A low concentration of TSH (10(-11) M) stimulated JNK activit y but at a higher dose (10(-8)-10(-7) M), TSH suppressed JNK activity witho ut any change of JNK protein level. Activation of JNK by TSH was also obser ved in CHO cells stably transfected with TSH receptor complementary DNA (cD NA), suggesting a ligand-receptor specific interaction. TSH stimulated JNK activity through a pertussis toxin-sensitive pathway. We next elucidated th e signal transduction pathways in TSH-induced JNK activation by examining t he involvement of four distinct intracellular signal molecules; protein kin ase C (PKC), cAMP, Ca2+, and PI3-kinase. The stimulation of JNK by TSH was blocked by two PKC inhibitors and suppressed by 8-bromo-cAMP or forskolin. These findings demonstrate that TSH regulates JNK activity biphasically in human thyroid cells through an interaction between Gi-PKC and cAMP-PKA path ways.