Dysregulation of the Wnt/ß-catenin pathway in ACTH-secreting pituitary adenomas and its inhibition through SRIF-14 in AtT-20 cells

Abstract

Introduction: Altered Wnt/ß-catenin signalling has recently been recognized to contribute to the development of various 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. Somatostatin (SRIH) has been proposed as a therapeutic agent in the medical treatment of adrenocorticotropin (ACTH)-secreting pituitary adenomas. The mechanisms, however, and the intracellular signalling pathways involved herein are only partly understood. The aim of the present study was to investigate a potential role of the Wnt/ß-catenin pathway in ACTH-secreting pituitary adenomas and its modulation through SRIH. Methods: The ACTH secreting mouse pituitary cell line AtT-20 and samples from 10 ACTH-secreting pituitary adenomas were studied by Western blot analyses and RT-PCR for the expression of ß-catenin, TCF-4 and cyclin D1. The effects of SRIF-14 and of corticotropin releasing hormone (CRH) on the expression of ß-catenin, TCF-4 and cyclin D1 were studied in AtT-20 cells. Results: In ACTH-secreting adenomas the expression of ß-catenin, TCF-4 and cyclin D1 was higher than in normal control tissue both at the RNA and protein level. In the ACTH-secreting cell line AtT20 treatment with SRIF-14 rapidly decreased beta-catenin, TCF-4 and cyclin D1 mRNA expression at 4–6h and caused a potent and long-lasting decrease at the protein level. In contrast, stimulation of AtT-20 cells with CRH significantly increased both the transcriptional and the translational responses of ß-catenin, TCF-4 and cyclin D1. Conclusions: We demonstrate that key elements of the Wnt/ß-catenin signalling pathway are expressed in ACTH-secreting pituitary cells. The expression of ß-catenin, TCF-4 and cyclin D1 is inhibited by SRIF-14 and stimulated by CRH, which suggests a functional role in corticotrope cell function. In human ACTH-secreting pituitary adenomas ß-catenin, TCF-4 and cyclin D1 expression is upregulated, which may contribute to their pathologic behaviour. Down-regulation of these genes by somatostatin or its analogs might exert beneficial effects corticotrope dysfunction Cushing's disease.