Pituitary tumourigenesis is a complex and poorly understood process. Crucial to the initiation and growth of such tumours is the oncogene PTTG, which stimulates FGF-2-mediated angiogenesis. We recently investigated expression of the angiogenic factor VEGF and its receptor KDR in 103 pituitary tumours. Non-functioning tumours demonstrated markedly raised VEGF mRNA (3.2-fold, P<0.05) and protein levels compared to normal pituitaries (N=10). KDR was also highly induced in NFTs, as well as in the whole cohort of pituitary tumour subtypes compared with normal pituitary samples (14-fold, P<0.0001). In the present study, we have sought to determine functional relationships between these genes in vitro. In fetal neuronal NT2 cells, which showed low endogenous PTTG levels, PTTG was able to stimulate VEGF mRNA expression (2.7-fold, P<0.001, N=8), but not KDR. A similar result was apparent in choriocarcinoma JEG-3 cells (P=0.0002, N=8). A PTTG mutant which cannot be phosphorylated (amino acid substitution S165A; 'phos-') showed similar VEGF upregulation (2.9-fold, P<0.001, N = 8) in NT2 cells to wild type PTTG. A further mutant with abrogation of the key protein:protein interaction domain of PTTG (substitutions P163A, P164A, P166A, P170A, P172A, P173A; 'SH3-'), however, resulted in a significant reduction in VEGF stimulation compared to wild type (0.37-fold reduction, P<0.001). KDR mRNA expression was unaffected by either mutation or by wild type PTTG. Overall, therefore, our findings suggest that protein:protein interaction, but not phosphorylation, is vital to PTTG stimulation of VEGF. As we previously found FGF-2 expression to be influenced in an identical manner as VEGF by phos- and SH3- mutations, and FGF-2 can upregulate VEGF, altered VEGF and KDR signalling in pituitary tumours may occur through PTTG stimulation of FGF-2, which subsequently induces VEGF. We propose that upregulated VEGF, in the presence of high KDR levels, may account for angiogenic growth and progression of human pituitary tumours.
08 - 11 Apr 2002
British Endocrine Societies