PTTG promotes mitogenic mechanisms in thyroid cells through autocrine pathways of interaction with TGF-α, EGF and IGF1

The human pituitary tumor transforming gene (hPTTG) is overexpressed in thyroid cancers; it induces genetic instability and propagates growth through the induction of growth factors. We set out to investigate the autocrine and paracrine pathways of interaction between hPTTG and epidermal growth factor (EGF), transforming growth factor-α (TGF-α) and insulin-like growth factor 1 (IGF1) in vitro and in vivo. Synchronised K1 papillary thyroid carcinoma cells were treated with TGF-α (5 nM), EGF (5 nM), or IGF1 (10 ng/ml) and hPTTG protein expression was determined by western blotting at 24, 36 and 48 h. Treatment with TGF-α resulted in a ~3-fold upregulation of hPTTG at all 3 time points and dose–response experiments confirmed significant hPTTG induction with 5 and 50 nM TGF-α. EGF treatment induced a ~4-fold increased expression of hPTTG at 36 and 48 h. Treatment with IGF1 resulted in a 2-fold upregulation of hPTTG at 24 and 36 h. To investigate if hPTTG in turn results in increased expression of these growth factors, we determined TGF-α, EGF and IGF1 mRNA expression through TaqMan RT-PCR following transient transfection of primary human thyrocytes with hPTTG. EGF (1.7-fold, n=6, P=0.006) and IGF1 mRNA (1.6-fold, n=6, P=0.03) were significantly upregulated by hPTTG, whereas TGF-α mRNA was not significantly induced (1.6-fold, n=6, P=NS). To investigate these findings in vivo, we subsequently evaluated mRNA expression of these mitogenic factors in our recently generated transgenic mouse model of targeted hPTTG overexpression in the thyroid gland. Upregulation of mEGF (2.7-fold, n=3, P=0.012) and mIGF1 (2.0-fold, P=0.02) was confirmed when comparing 6-week old PTTG+/+ mice to age-matched WT.

Conclusion: These results indicate that PTTG is involved in autocrine signalling mechanisms with growth factors such as TGF-α, EGF and IGF1 in the thyroid. We propose that aberrant control of these pathways may enhance tumour development and that further elucidation of these pathways may provide novel therapeutic targets for the prevention of thyroid tumour progression.