gms | German Medical Science

Objective: The Wnt/β-catenin signalling pathway plays a critical role in normal development as well as in tumorigenesis of various tissues including endocrines. We have shown previously that the Wnt/β-catenin pathway is dysregulated in adrenocorticotropin (ACTH)-secreting pituitary adenomas. In this context, SRIF has been shown to inhibit ACTH secretion and proliferation of an ACTH-secreting pituitary cell line AtT-20. The mechanisms remain largely unclear. The aim of the present study was to investigate the effect of SRIF on the Wnt/ß-catenin pathway in ACTH-secreting pituitary cells.

Methods: The ACTH secreting mouse pituitary cell line AtT-20 was treated with SRIF (3 µM) and its effect on the expression of β-catenin, GSK-3β, TCF-4 and Cyclin D1 was analyzed both at RNA and protein levels by RT-PCR and Western blotting, respectively. The ratio of phospho-β-catenin to total β-catenin and phospho-GSK-3β to total GSK-3β were studied by Western blotting. The nuclear expression of β-catenin was investigated by immunoflourescence.

Results: In the AtT-20 cells, SRIF inhibited basal activation of the Wnt/β-catenin pathway as demonstrated by the decrease of β-catenin, TCF-4 and cyclin D1 expression. SRIF decreased the phosphorylation of GSK-3β at Ser9, increased phosphorylation of β-catenin at Ser33/37 and as a result accelerated degradation of β-catenin. It indicates that SRIF may decelerate Wnt/β-catenin signalling through its action on the phosphorylation status of GSK-3β. We also tested the functional importance of Wnt/β-catenin signalling for the control of AtT-20 cell proliferation. The AtT-20 cells were transfected with dominant-negative TCF-4, that lacks the β-catenin binding domain. The transfection reduced both mRNA and protein expression of cyclin D1 and inhibited cell proliferation, which demonstrates the functional significance of the Wnt/β-catenin pathway for the proliferation of AtT-20 cells. This was further supported by the decrease in β-catenin nuclear expression under SRIF treatment.

Conclusions: Our data demonstrate the important role of e Wnt/β-catenin signalling in proliferation control of pituitary corticotrophs and describe a mechanism for its regulation by SRIF. Inhibition of the Wnt/β-catenin pathway by SRIF or its analogs may be one potential approach to inhibit proliferation and excessive hormone secretion in the ACTH-secreting pituitary cells.