Targeted hPTTG overexpression in vivo reduces thyroidal growth and thyroid cell proliferation.

The human pituitary tumor transforming gene is overexpressed in thyroid cancers; inducing genetic instability through its role as a securin and propagating growth through induction of growth factors. We have demonstrated reduced cellular proliferation following hPTTG overexpression in neuronal cells. We hypothesised targeted hPTTG overexpression in mouse thyroids will result in hyperplastic/neoplastic growth and that stimulation of thyroid cell growth through standard methods of goitre induction may be altered in these mice. PTTG-Tg transgenic mice exhibited significant reductions in thyroid size at 6 weeks (n=50, 0.83-fold reduction, P=<0.001) and 6 months of age (n=8, 0.70-fold reduction, P=0.001) compared with age- and gender matched WT mice. Serum concentrations of total T3, total T4 and TSH were within the normal range and unchanged compared with WT mice. Cyclin D1 (n=4, 0.59-fold reduction, P=0.015) and PCNA expression (n=8, 0.54-fold reduction, P=<0.005) were significantly reduced compared to WT mice. To determine if reduced thyroid cell proliferation was related to dysregulated growth factor signalling, we determined expression of mitogenic factors implicated in thyroidal growth. A 1.4-fold induction of mIGF-1 mRNA (n=5, P<0.05) was observed but expression of mEGF (1.0-fold, n=5, P=NS), mTGFα (1.2-fold, n=5, P=NS) and mTGFβ (1.0-fold, n=5, P=NS) mRNA was similar in WT and PTTG-Tg mice. ELISA assays of primary mouse thyrocytes demonstrated no significant difference in mIGF-1 secretion (170.43±20.0 pg/ml, n=26), compared to wild-type controls (160.86±23.5 pg/ml, n=28). There was, however, a significant increase in mEGF secretion by hPTTG transgenic thyrocytes (20.91±3.4 pg/ml, n=12, P<0.005), compared to wild-type controls (9.5±2.0 pg/ml, n=17). To stimulate thyroid cell growth, mice were fed an iodine deficient diet containing 0.15% PTU and drinking water containing 0.5% sodium perchlorate and 0.02% methimazole. After 4 weeks we observed a 2-fold increase in thyroid size in WT mice (n=5) compared with a 1.75-fold increase in PTTG-Tg mice (n=8, P=NS). In conclusion: Targeted thyroidal hPTTG overexpression results in reduced cellular proliferation, potentially through PTTG’s role as a securin. Preliminary results indicate that the goitrogenic response to suppression of endogenous thyroid hormone synthesis, is unaltered in PTTG-Tg mice.