Endocrine Abstracts

Our previous studies have shown that the acute regulatory actions of glucocorticoids (GCs) on the secretion of corticotrophin (ACTH) are dependent upon the translocation of a 37 kDa protein, annexin 1 (ANXA1 [1]) from within the cell to the outer cell surface where it is retained by a Ca²⁺-dependent mechanism (2). The mechanisms by which ANXA1 is exported from pituitary cells in response to a steroid challenge and acts on the endocrine cells to suppress peptide release are unknown. Phosphorylation of key regulatory proteins is one of the primary mechanisms by which cells respond to extracellular signals. The aim of this study was to determine the potential role of protein kinase C- (PKC) and tyrosine kinase-mediated phosphorylation in the ANXA1-dependent inhibition of pituitary peptide release by glucocorticoids. Utilising an established in vitro pituitary system (2) we determined that the inhibitory actions of dexamethasone on the forskolin-evoked release of ACTH were abolished by the PKC inhibitor, PKC_19-36 and ANXA5 (which sequesters PKC in other systems). In contrast, the tyrosine kinase inhibitors, p60 ^v-src (137-157) and genistein were ineffective in this regard. Measures of ANXA1 by immunoprecipitation and western blot analysis showed that ANXA1 translocated from the cytoplasm to the cell surface in response to a dexamethasone challenge is both serine- and tyrosine-phosphorylated. However, the cellular exportation of the protein is abolished by PKC_19-36 but not by the tyrosine kinase inhibitors, and thus appears to be dependent upon PKC. These results provide important novel evidence to suggest that PKC-dependent mechanisms are essential for both the cellular exportation and the biological activity of ANXA1.

1.Buckingham JC, Flower RJ 1997 Lipocortin 1: a second messenger of glucocorticoid action in the hypothalamo-pituitary-adrenocortical axis. Mol Med Today 3:296-302

2.Taylor AD, Cowell AM, Flower RJ, Buckingham JC 1993 Lipocortin 1 mediates an early inhibition action of glucocorticoids on the secretion of ACTH by the rat anterior pituitary gland in vitro. Neuroendocrinology 58:430-439