Endocrine Abstracts

PTTG has been implicated in the pathogenesis of pituitary, thyroid and other tumours. PBF was recently identified as a protein which interacts specifically with PTTG both in vitro and in vivo. PBF facilitates PTTG's nuclear localisation, and mediates its ability to transactivate basic fibroblast growth factor (FGF-2). We investigated PBF expression and function in vitro, and examined its relationship to PTTG action. Transient transfection of wild type PTTG induced a significant increase in PBF mRNA expression in MCF-7 cells compared with vector-only controls (2.2-fold, N=12, P=0.03). In primary thyroid cells, transient PTTG overexpression yielded a large induction in PBF expression (462-fold compared with vector-only, N=12, P<0.001). In contrast, several PTTG mutants with altered SH3-interacting domains failed to upregulate PBF expression in either primary thyroid or MCF-7 cells. As overexpression of PTTG is tumourigenic in vivo, and results in upregulation of its binding factor in vitro, we sought to examine the influence of PBF on cell transformation. Both wild type PTTG and PBF stable transfectants showed enhanced colony formation compared with control NIH-3T3 cells (PTTG: 176 plus/minus 12 colonies/well, N=12, P<0.001; PBF: 102 plus/minus 4 colonies/well, N=6, P=0.002). Deletion of the region of PTTG responsible for interacting with PBF entirely abrogated its ability to transform cells (1.7 plus/minus 0.74 colonies/well, P<0.001 compared with wild type). Further, ablation of PBF's nuclear localisation signal significantly reduced its transforming ability (59 plus/minus 20.7 colonies, N=18, P=0.027 compared with wild type PBF). We show therefore that PBF is itself a transforming gene, and is intimately involved in PTTG's ability to effect anchorage-independent growth in vitro. Given that deletion of the PBF interacting domain of PTTG prevented cell transformation, and that ablation of PBF's nuclear localisation signal reduced colony formation, we propose that PTTG requires PBF interaction and subsequent nuclear localisation to mediate its tumourigenic functions in vivo.