The pituitary tumor-transforming gene promotes angiogenesis in a mouse model of follicular thyroid cancer

Overexpression of the pituitary tumor-transforming gene (PTTG) has been associated with tumorigenesis. In a mouse model that spontaneously develops follicular thyroid cancer (FTC) with distant metastasis (TR beta PV mouse), PTTG is overexpressed, similar to human thyroid cancer. To evaluate the role of PTTG in thyroid carcinogenesis, we studied the offspring of TR beta PV mice with mice lacking PTTG (PTTG(-/-) mice). The thyroids of TR beta(PV/PV) PTTG(-/-) mice were significantly smaller than TR beta(PV/PV) mice. Ki-67 staining showed a decrease in thyroid proliferation in TR beta(PV/PV) PTTG(-/-) mice. Our evaluation of the Rb-E2F pathway, a central mediator of cell growth, found that TR beta(PV/PV) PTTG(-/-) mice exhibited a decrease in protein levels of phosphorylated Rb along with an elevation of the cdk inhibitor p21. Histological examination documented no difference in FTC occurrence between TR beta(PV/PV) and TR beta(PV/PV) PTTG(-/-) mice, which indicates that PTTG removal does not prevent the initiation of FTC. However, TR beta(PV/PV) PTTG(-/-) mice had a significant decrease in vascular invasion and less development of lung metastasis as they progressively aged. CD31 staining also showed a decrease in vessel density in TR beta(PV/PV) PTTG(-/-) versus TR beta(PV/PV) thyroids. Given the decreased vascular invasion in the PTTG knockout mice, we studied genes involved in angiogenesis. Real-time reverse transcription-polymerase chain reaction showed a consistent decrease in pro-angiogenic factors, fibroblast growth factor (FGF2), its receptor FGFR1 and vascular endothelial growth factor. Our results highlight the dual roles of PTTG as a regulator of thyroid growth and contributor to tumor progression. The separation of the pathways regulating cell proliferation, tumor initiation and tumor progression should direct future therapeutic options.