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 investigated the pathways of interaction between hPTTG and epidermal growth factor (EGF), transforming growth factor-α (TGF-α), insulin-like growth factor 1 (IGF1) and basic fibroblast growth factor (FGF2) in vitro and in vivo. Synchronised K1 and TPC-1 papillary thyroid carcinoma cells were treated with TGF-α (5 nM), EGF (5 nM), IGF1 (10 ng/ml), or FGF-2 (5 nM) and hPTTG protein expression was determined by western blotting at 24 h. EGF treatment in both K1 and TPC-1 cells induced a 2-fold upregulation of hPTTG, an effect abrogated by treatment with the specific MAPK inhibitor PD98059 (30 μM), but not with the specific PKC inhibitor BIS-I (50 nM). TGF-α treatment upregulated hPTTG in K1 (3.6-fold) and TPC-1 cells (4-fold), with similar abrogation by treatment with PD98059 but not with BIS-I. IGF1 treatment induced hPTTG in K1 (2.3-fold) and TPC-1 cells (2.6-fold), where treatment with either of the specific PI3-kinase inhibitors Wortmannin (20 μM) and LY294002 (50 μM) abrogated this effect. FGF2 had no effect in K1 cells but significantly upregulated hPTTG in TPC-1 cells (7-fold). To investigate if hPTTG in turn induces 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=4, P=0.004), IGF1 (1.6-fold, n=5, P=0.002) and TGF-α mRNA (1.6-fold, n=3, P=0.024) were significantly upregulated by hPTTG. To investigate these findings in vivo, we evaluated mRNA expression of these mitogenic factors in our transgenic mouse model with thyroid-targeted hPTTG overexpression. Upregulation of mEGF (2.7-fold, n=3, P=0.012) and mIGF1 (2.0-fold, n=3, P=0.02) was confirmed when comparing 6-week-old hPTTG+/+ mice to age-matched WT mice.
Conclusion: PTTG is involved in autocrine signalling mechanisms with growth factors including TGF-α, EGF, IGF1 and FGF2 in the thyroid. Aberrant control of these pathways may enhance tumour development and further elucidation of these pathways may provide novel therapeutic targets for the prevention of thyroid tumour progression.