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Thyrotropin regulates c-Jun N-terminal kinase (JNK) activity through two distinct signal pathways in human thyroid
1Department of Nature Medicine, Nagasaki University School of Medicine, Japan.
Abstract:
c-Jun N-terminal kinases (JNK) participate in cellular responses to mitogenic stimuli and environmental stresses. We investigated whether and how TSH, which promotes the proliferation and differentiation of thyroid cells, regulates 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 activity but at a higher dose (10(-8)-10(-7) M), TSH suppressed JNK activity without any change of JNK protein level. Activation of JNK by TSH was also observed in CHO cells stably transfected with TSH receptor complementary DNA (cDNA), suggesting a ligand-receptor specific interaction. TSH stimulated JNK activity through a pertussis toxin-sensitive pathway. We next elucidated the signal transduction pathways in TSH-induced JNK activation by examining the involvement of four distinct intracellular signal molecules; protein kinase 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 pathways.
Insights
Thyroid-stimulating hormone (TSH) biphasically regulates c-Jun N-terminal kinases (JNK) activity in human thyroid cells. Low TSH doses activate JNK, while higher doses suppress it via Gi-PKC and cAMP-PKA pathways.
Area of Science:
- Endocrinology
- Molecular Biology
- Cell Signaling
Background:
- c-Jun N-terminal kinases (JNK) are crucial for cellular responses to stress and mitogens.
- Thyroid-stimulating hormone (TSH) regulates thyroid cell proliferation and differentiation.
- The precise mechanisms of TSH regulation on JNK activity in thyroid cells remain incompletely understood.
Purpose of the Study:
- To investigate the regulation of JNK activity by TSH in primary human thyroid cells.
- To elucidate the signal transduction pathways involved in TSH-mediated JNK modulation.
- To determine if TSH receptor interaction specifically mediates JNK activation.
Main Methods:
- In vitro kinase assays were used to measure JNK activity in cytosolic fractions of thyroid cells.
- TSH was applied at various concentrations (10(-11) M to 10(-7) M) to assess dose-dependent effects.
- Signal transduction pathways were examined using specific inhibitors and modulators for protein kinase C (PKC), cAMP, Ca2+, and PI3-kinase.
- Experiments were conducted in primary cultured human thyroid cells and CHO cells expressing the TSH receptor.
Main Results:
- TSH stimulated JNK activity at a low concentration (10(-11) M) but suppressed it at higher concentrations (10(-8)-10(-7) M) without altering JNK protein levels.
- TSH-induced JNK activation was observed in cells expressing the TSH receptor, indicating ligand-receptor specificity.
- The TSH-mediated pathway involved a pertussis toxin-sensitive component.
- PKC inhibitors blocked TSH-stimulated JNK, while 8-bromo-cAMP or forskolin suppressed it, implicating Gi-PKC and cAMP-PKA pathways.
Conclusions:
- TSH exerts a biphasic regulation on JNK activity in human thyroid cells.
- The regulation involves a complex interplay between Gi-PKC and cAMP-PKA signaling cascades.
- These findings provide novel insights into the molecular mechanisms by which TSH influences thyroid cell function.