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 beta-catenin or a 'noncanonical' Ca(++) pathway which can increase protein kinase C (PKC) activity. Although components of both Wnt/beta-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 beta-catenin. Transient overexpression of Wnt-1 or beta-catenin in FRTL-5 cells increased active beta-catenin (ABC), decreased thyroperoxidase (TPO) mRNA, and suppressed TPO-promoter activity. The target of beta-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. beta-Catenin overexpression significantly increased complex formation between beta-catenin/TCF-1 and an oligonucleotide containing the TCF/LEF sequence, suggesting that the beta-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 beta-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 beta-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.
Kim
Won Bae
WB
Edison Biotechnology Institute and College of Osteopathic Medicine, Ohio University, The Ridges, Athens, Ohio 45701, USA.
Lewis
Christopher J
CJ
McCall
Kelly D
KD
Malgor
Ramiro
R
Kohn
Aimee D
AD
Moon
Randall T
RT
Kohn
Leonard D
LD
D000818:Animals; D015152:Blotting, Northern / Q000379:methods; D015153:Blotting, Western / Q000379:methods; D002453:Cell Cycle / Q000235:genetics; D002460:Cell Line; D024202:Electrophoretic Mobility Shift Assay; D005434:Flow Cytometry; D015870:Gene Expression; D007453:Iodide Peroxidase / Q000235:genetics* / Q000378:metabolism; D011401:Promoter Regions, Genetic; D051381:Rats; D015398:Signal Transduction / Q000502:physiology*; D013961:Thyroid Gland / Q000378:metabolism*; D013972:Thyrotropin / Q000494:pharmacology; D014158:Transcription, Genetic / Q000502:physiology*; D014162:Transfection; D051155:Wnt1 Protein / Q000378:metabolism*; D051176:beta Catenin / Q000235:genetics / Q000378:metabolism
D051155:Wnt1 Protein; D051176:beta Catenin; D013972:Thyrotropin; D007453:Iodide Peroxidase