Endocrine Abstracts

Introduction: The involvement of the Wnt/ß-catenin pathway is apparent different epithelial human cancers. Wnt stabilizes ß-catenin (free ß-catenin) by preventing its phosphorylation which targets ß-catenin for degradation. Free ß-catenin is translocated to the nucleus where it stimulates a number of genes that modulate proliferation and differentiation upon binding to TCF/LEF-1. We have previously shown that Wnt/ß-catenin pathway is dysregulated in adrenocorticotropin (ACTH)-secreting pituitary adenomas. The therapeutic options for this tumor entity are limited. In this context, SRIF has been proposed as a therapeutic agent in the treatment of ACTH-secreting pituitary adenomas. 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-14 and its effect on the expression of ß-catenin, GSK-3ß, TCF-4 and Cyclin D1 was analyzed both at the RNA and protein levels by RT-PCR and Western blotting, respectively. Additionally, the relative values of phospho-ß-catenin to total ß-catenin and phospho-GSK-3ß to total GSK-3ß were studied under the effects of SRIF-14 and forskolin (activator of cyclic AMP).

Results: In the ACTH-secreting pituitary cell line AtT-20 treatment with SRIF-14 rapidly decreased ß-catenin, TCF-4 and cyclin D1 mRNA expression at 4–6 h and caused a potent and long-lasting decrease also at the protein level. The downregulation of ß-catenin was blocked by forskolin. Furthermore, the phospho-ß-catenin/total ß-catenin ratio was higher in SRIF treated cells as compared to untreated controls and also to forskolin stimulated cells. These findings correlated well with the values of phospho-GSK-3ß, an inactive form of GSK-3ß, which was higher in untreated controls and forskolin treated cells comparative to SRIF treated cells.

Conclusions: Our data indicate that SRIF downregulates the Wnt/ß-catenin pathway and this effect can be blocked by the activation of adenylate-cyclase pathway in ACTH-secreting pituitary cell line AtT-20. Inhibition of Wnt/ß-catenin pathway by SRIF or its analogs might exert beneficial effects on corticotrope dysfunction in Cushing’s disease.