Overexpression of Wnt-1 in thyrocytes enhances cellular growth but suppresses transcription of the thyroperoxidase gene via different signaling mechanisms

Wnt binding to cell surface receptors can activate a ‘canonical’ pathway that increases cellular β-catenin or a ‘noncanonical’ Ca++ pathway which can increase protein kinase C (PKC) activity. Although components of both Wnt/β-catenin-signaling pathways exist in thyrocytes, their biological role is largely unknown. In evaluating the biological role of Wnt signaling in differentiated FRTL-5 thyroid cells, we showed that TSH increased canonical Wnt-1 but, surprisingly, decreased the active form of β-catenin. Transient overexpression of Wnt-1 or β-catenin in FRTL-5 cells increased active β-catenin (ABC), decreased thyroperoxidase (TPO) mRNA, and suppressed TPO-promoter activity. The target of β-catenin suppressive action was a consensus T cell factor/lymphoid enhancing factor (TCF/LEF)-binding site 5′-A/T A/T CAAAG-3′, −137 to −129 bp on the rat TPO promoter. β-Catenin overexpression significantly increased complex formation between β-catenin/TCF-1 and an oligonucleotide containing the TCF/LEF sequence, suggesting that the β-catenin/TCF-1 complex acts as a transcriptional repressor of the TPO gene. Stable over-expression of Wnt-1 in FRTL-5 cells significantly increased the growth rate without increasing β-catenin levels. Increased growth was blunted by a PKC inhibitor, staurosporin. Wnt-1 overexpression increased serine phosphorylation, without affecting tyrosine phosphorylation, of signal transducers and activators of transcription 3 (STAT3) protein. In addition, these final results suggest that TSH-induced increase in Wnt-1 levels in thyrocytes contributes to enhanced cellular growth via a PKC pathway that increases STAT3 serine phosphorylation and activation, whereas TSH-induced decrease in activation of β-catenin simultaneously relieves transcriptional suppression of TPO. We hypothesize that Wnt signaling contributes to the ability of TSH to simultaneously increase cell growth and functional, thyroid-specific, gene expression.