Endocrine Abstracts

The basic mechanisms underlying thyroid growth and proliferation in thyroid cancer are still incompletely characterized. Lithium inhibits GSK3beta and affects via phosphorylation the degradation of free beta-catenin, a potent transcription factor. Hence, using lithium as a model to modulate the wnt/beta-catenin pathway, we investigated its potential activation in-vitro in neoplastic human thyroid cell lines (FTC133, NPA).

Our results show that lithium functionally activates wnt/beta-catenin signalling by at least four mechanisms. First, lithium dose dependently inactivates the proteasome by more than fifty percent. Second, it inhibits GSK3beta via phosphorylation at serine-9 and stabilizes the free form of the active beta-catenin in a dose (1-20 millimolar) and time (0-24 hours) dependant manner. Third, E-cadherin expression is dose-dependently down-regulated by lithium (1-5 millimolar). Fourth, proliferation of the cells is significantly increased (MTT-proliferation assay) which appears to be mediated via nuclear translocation of stabilized beta-catenin (Reporter gene assay) and upregulation of cyclin D1 levels.

To characterize the specificity of a beta-catenin dependent effect on thyrocyte proliferation we transiently transfected FTC-133 cells with dominant negative TCF-4 (dnTCF-4) and dominant negative CREB (dnCREB) to block the wnt/beta-catenin and cAMP dependent pathways respectively. As lithium dependent stimulation of proliferation was still present in dnCREB FTC-133 cells and was reduced drastically in dnTCF-4 FTC133 cells, it seems plausible that thyrocyte proliferation is regulated by the wnt/beta-catenin signalling pathways and is not entirely dependent on cAMP signalling pathways. We further characterized in permanently transfected dnTCF4 cells the effects of recombinant human TSH, bovine TSH, EGF, IGF-1 and insulin on the thyroid growth, invasion and migration to elucidate the relevance of an active wnt/beta-catenin signalling pathway. Thus, our data indicate that regulation of wnt/beta-catenin plays a central role in the thyroid growth and lithium enhances the proliferative potency of the human thyroid cancer cell lines in-vitro by activiating its signalling.