Mitogenic signaling by receptor tyrosine kinases that involve increased activity of phosphatidylinositol-3-kinase (PI3K) and over-activation of protein kinase B (PKB/Akt) triggers a cascade of responses that drive tumour progression in a variety of human cancers. Some of these events have been associated with diminished expression of the cell cycle inhibitor p27 through inhibition of a Forkhead transcription factor (FKHR-L1) by Akt, while some others have recently been reported to be due to Akt-dependent phosphorylation of p27 on Thr157 and modulation of its cellular localization.
The aim of the present study was to assess Akt expression in pituitary tumours as a possible altered pathway to explain our previous findings on low levels of p27 expression in pituitary tumours. Transcriptional expression of the two isoforms Akt1 and Akt2, was assessed by 'real-time' quantitative RT-PCR in 45 pituitary tumours and 9 normal pituitaries and protein expression of total Akt and phosphorylated Akt and p27 was assessed in 40 pituitary tumours and 10 normal pituitaries by Western blotting and immunohistochemistry. Ethical Committee approval has been obtained for studies on human samples. Akt kinase activity was measured in rat somatotroph GH3 cells before and after treatment with regulators of PI3K.
Our results showed that Akt1 and Akt2 mRNA expression was statistically higher in tumours as a group versus normal pituitaries and this was in inverse correlation with the expression of p27. Phosphorylated Akt was also over-expressed in pituitary tumours. Akt kinase activity has been higher in EGF treated GH3 cells versus serum starved or wortmannin treated cells.
Over-expression and over-activation of Akt may be one feature impairing the cell cycle regulatory machinery and sustaining proliferation in pituitary tumours.
22 - 24 Mar 2004
British Endocrine Societies