Endocrine Abstracts

We have previously shown Pituitary Tumor Transforming Gene (PTTG) and basic Fibroblast Growth Factor (FGF-2) to be upregulated in thyroid cancer. PTTG requires interaction with the protein PBF (PTTG Binding Factor) to stimulate FGF-2 secretion, and hence promote tumour angiogenesis and growth. We now report that PBF is also upregulated in hyperplastic and neoplastic thyroid tissues compared with normal thyroid (N=11) (2.6-fold in multinodular goitre (N=25, P=0.002), and 3.3-fold in thyroid cancer (N=27, P<0.001)). In addition, PBF mRNA expression was significantly elevated in recurrent (N=6) versus non-recurrent (N=18) thyroid cancers (3-fold, P<0.001) and was an independent predictor of recurrence when known prognostic indicators of age, gender, tumour size and type were taken into account (P=0.002, R2 = 0.499). As we recently reported that PBF is a transforming gene in vitro, we sequenced PBF in 27 thyroid cancers. No mutations were apparent suggesting that PBF mutation is an infrequent event in thyroid cancer. To further evaluate the implications of raised PBF expression in thyroid cancer we examined its ability to transactivate FGF-2 in vitro. Whilst over-expression of PBF did not stimulate FGF-2 expression directly in primary cultures of human thyroid cells, co-transfection of PBF with PTTG potentiated PTTG upregulation of FGF-2. We have recently reported that PTTG lacking the binding domain for PBF loses the ability to stimulate proliferation. We therefore examined PBF's role in cell turnover. In MTT assays, stable overexpression of wild type PBF in NIH3T3 cells did not affect cell proliferation. However, disruption of the gene's nuclear localisation sequence (NLS-), necessary for shuttling PTTG into the nucleus, resulted in significantly reduced cell turnover (Control = 100% proliferation, N= 4; PBF = 92.9%, P=NS, N=4; NLS- mutant = 88.3%, P = 0.033, N=4). We conclude, therefore, that PBF is a novel transforming gene which predicts recurrence in thyroid cancer, and we hypothesise that it functions by augmenting the multiple actions of PTTG in thyroid tumourigenesis.