Paired Box Gene 8-Peroxisome Proliferator-Activated Receptor-γ Fusion Protein and Loss of Phosphatase and Tensin Homolog Synergistically Cause Thyroid Hyperplasia in Transgenic Mice

Abstract

Approximately 35% of follicular thyroid carcinomas and a small fraction of follicular adenomas are associated with a t(2;3)(q13;p25) chromosomal translocation that fuses paired box gene 8 (PAX8) with the peroxisome proliferator-activated receptor-gamma gene (PPARG), resulting in expression of a PAX8-PPARgamma fusion protein, PPFP. The mechanism by which PPFP contributes to follicular thyroid neoplasia is poorly understood. Therefore, we have created mice with thyroid-specific expression of PPFP. At 1 yr of age, 25% of PPFP mice demonstrate mild thyroid hyperplasia. We bred these mice to mice with thyroid-specific single-allele deletion of the tumor suppressor Pten, denoted ThyPten(+/-). In humans, PTEN deletion is associated with follicular adenomas and carcinomas, and in mice, deletion of one Pten allele causes mild thyroid hyperplasia. We found that PPFP synergizes with ThyPten(+/-) to cause marked thyroid hyperplasia, but carcinomas were not observed. AKT phosphorylation was increased as expected in the ThyPten(+/-) thyroids, and also was increased in the PPFP thyroids and in human PPFP follicular cancers. Staining for the cell cycle marker Ki-67 was increased in the PPFP, ThyPten(+/-), and PPFP;ThyPten(+/-) thyroids compared with wild-type thyroids. Several genes with increased expression in PPFP cancers also were found to be increased in the thyroids of PPFP mice. This transgenic mouse model of thyroidal PPFP expression exhibits properties similar to those of PPFP thyroid cancers. However, the mice develop thyroid hyperplasia, not carcinoma, suggesting that additional events are required to cause follicular thyroid cancer.

The Putative PAX8/PPAR Fusion Oncoprotein Exhibits Partial Tumor Suppressor Activity through Up-Regulation of Micro-RNA-122 and Dominant-Negative PPAR Activity

“…6 In neither case did PPFP induce carcinoma. PPFP did inhibit mammary epithelial cell PPARγ activity, increasing these cells' susceptibility to carcinogen-induced tumorigenesis.…”

“…2 Two PPFP transgenic models have been developed, one targeting PPFP to mammary epithelial cells 27 and the other targeting it to the thyroid. 6 In neither case did PPFP induce carcinoma. PPFP did inhibit mammary epithelial cell PPARγ activity, increasing these cells' susceptibility to carcinogen-induced tumorigenesis.…”

“…Furthermore, our group and others have confirmed that PPFP acts as an oncogene in vitro, with increased rates of cell growth and attachmentindependent growth and decreased rates of apoptosis of several cell lines in vitro. 4,5 However, PPFP fails to transform normal thyrocytes in a transgenic animal, 6 suggesting that other oncogenic events are required to elicit its oncogenic actions. Consequently, the precise role of PPFP in the pathogenesis of FTC remains uncertain.…”

“…27 In the thyroid-specific transgenic mouse model, a small proportion (25%) of mice expressing PPFP exhibited mild benign hyperplasia, which appeared to be enhanced in the presence of PTEN haploinsufficiency. 6 These studies suggest that PPFP is not a primary oncogene, capable of cell transformation, but do not exclude the possibility that it …”

Lessons from Mouse Models of Thyroid Cancer

“…It is poorly understood on how the fusion protein causes FTC (61). A mouse model with the PAX8-PPARg under the control of the bovine Tg promoter does not develop FTC and only 25% develop hyperplasia (61,62). Another signaling pathway implicated in FTC is the phosphatidylinositol-3-kinase (PI3K)=AKT pathway, which is highlighted below and in Table 2.…”