The molecular genetic events underlying thyroid carcinogenesis are poorly understood. Mice harboring a knock-in dominantly negative mutant thyroid hormone receptor b (TRb PV/PV mouse) spontaneously develop follicular thyroid carcinoma similar to human thyroid cancer. Using this mutant mouse, we tested the hypothesis that the peroxisome proliferator-activated receptor c (PPARc) could function as a tumor suppressor in thyroid cancer in vivo. Using the offspring from the cross of TRb PV/ þ and PPARc þ /À mice, we found that thyroid carcinogenesis progressed significantly faster in TRb PV/PV mice with PPARc insufficiency from increased cell proliferation and reduced apoptosis. Reduced PPARc protein abundance led to the activation of the nuclear factor-jB signaling pathway, resulting in the activation of cyclin D1 and repression of critical genes involved in apoptosis. Treatment of TRb PV/PV mice with a PPARc agonist, rosiglitazone, delayed the progression of thyroid carcinogenesis by decreasing cell proliferation and activation of apoptosis. These results suggest that PPARc is a critical modifier in thyroid carcinogenesis and could be tested as a therapeutic target in thyroid follicular carcinoma.

Inactivation and silencing of PTEN have been observed in multiple cancers, including follicular thyroid carcinoma. PTEN (phosphatase and tensin homologue deleted from chromosome 10) functions as a tumour suppressor by opposing the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT) signalling pathway. Despite correlative data, how deregulated PTEN signalling leads to thyroid carcinogenesis is not known. Mice harbouring a dominant-negative mutant thyroid hormone receptor β (TRβPV/PV mice) spontaneously develop follicular thyroid carcinoma and distant metastases similar to human cancer. To elucidate the role of PTEN in thyroid carcinogenesis, we generated TRβPV/PV mice haploinsufficient for Pten (TRβPV/PVPten+/− mouse). PTEN deficiency accelerated the progression of thyroid tumour and increased the occurrence of metastasis spread to the lung in TRβPV/PVPten+/− mice, thereby significantly reducing their survival as compared with TRβPV/PVPten+/+ mice. AKT activation was further increased by two-fold in TRβPV/PVPten+/− mice thyroids, leading to increased activity of the downstream mammalian target of rapamycin (mTOR)–p70S6K signalling and decreased activity of the forkhead family member FOXO3a. Consistently, cyclin D1 expression was increased. Apoptosis was decreased as indicated by increased expression of nuclear factor-κB (NF-κB) and decreased caspase-3 activity in the thyroids of TRβPV/PVPten+/− mice. Our results indicate that PTEN deficiency resulted in increased cell proliferation and survival in the thyroids of TRβPV/PVPten+/− mice. Altogether, our study provides direct evidence to indicate that in vivo, PTEN is a critical regulator in the follicular thyroid cancer progression and invasiveness.

Aberrant expression and mutations of thyroid hormone receptor genes (TRs) are closely associated with several types of human cancers. To test the hypothesis that TRs could function as tumor suppressors, we took advantage of mice with deletion of all functional TRs (TRα1−/− TRβ−/−mice). As these mice aged, they spontaneously developed follicular thyroid carcinoma with pathological progression from hyperplasia to capsular invasion, vascular invasion, anaplasia and metastasis to the lung, similar to human thyroid cancer. Detailed molecular analysis revealed that known tumor promoters such as pituitary tumor-transforming gene were activated and tumor suppressors such as peroxisome proliferator-activated receptor γ and p53 were suppressed during carcinogenesis. In addition, consistent with the human cancer, AKT–mTOR–p70S6K signaling and vascular growth factor and its receptor were activated to facilitate tumor progression. This report presents in vivo evidence that functional loss of both TRα1 and TRβ genes promotes tumor development and metastasis. Thus, TRs could function as tumor suppressors in a mouse model of metastatic follicular thyroid cancer.

Correlative data suggest that thyroid hormone receptor-β (TRβ) mutations could increase the risk of mammary tumor development, but unequivocal evidence is still lacking. To explore the role of TRβ mutants in vivo in breast tumor development and progression, we took advantage of a knock-in mouse model harboring a mutation in the Thrb gene encoding TRβ (ThrbPV mouse). Although in adult nulliparous females, a single ThrbPV allele did not contribute to mammary gland abnormalities, the presence of two ThrbPV alleles led to mammary hyperplasia in ~36% ThrbPV/PV mice. The ThrbPV mutation further markedly augmented the risk of mammary hyperplasia in a mouse model with high susceptibility to mammary tumors (Pten+/− mouse), as demonstrated by the occurrence of mammary hyperplasia in ~60% of Thrbpv/+Pten+/− and ~77% of ThrbPV/PV Pten+/− mice versus ~33% of Thrb+/+Pten+/− mice. The ThrbPV mutation increased the activity of signal transducer and activator of transcription (STAT5) to increase cell proliferation and the expression of the STAT5 target gene encoding β-casein in the mammary gland. We next sought to understand the molecular mechanism underlying STAT5 overactivation by TRβPV. Cell-based studies with a breast cancer cell line (T47D cells) showed that thyroid hormone (T3) repressed STAT5 signaling in TRβ-expressing cells through decreasing STAT5-mediated transcription activity and target gene expression, whereas sustained STAT5 signaling was observed in TRβPV-expressing cells. Collectively, these findings show for the first time that a TRβ mutation promotes the development of mammary hyperplasia via aberrant activation of STAT5, thereby conferring a fertile genetic ground for tumorigenesis.

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