Endocrine Abstracts

Annexin I, a phospholipid and calcium binding protein, plays a well demonstrated role in the early delayed inhibitory feedback actions of glucocorticoids in the pituitary. Annexin I is localized in folliculostellate (FS) cells and glucocorticoids act on these cells to externalize annexin I at specific foci on FS cell processes (Chapman et al, 2002 Endocrinology, in press). Annexin I lacks a signal sequence to direct it into the secretory pathway and the foci of externalized annexin I suggest these represent sites for a specific transport mechanism. We have recently demonstrated by inhibitor studies a role for ATP-binding cassette (ABC) transporters in the export of annexin I from anterior pituitary tissue and a FS (TtT/GF) cell line (Epton et al, 2002, Endocrine Abstracts 3). The aim of the present study was to investigate by immunofluorescence labeling and confocal microscopy the localization of ABCA1 in intact and permeabilised TtT/GF cells. ABCA1 immunoreactivity was localized to foci on intact TtT/GF cell processes and in permeabilised cells was distributed throughout the cytoplasm. Pre-absorption of the antibody with the immunogenic ABCA1 peptide abolished ABCA1 immunoreactivity. Treatment of TtT/GF cells with dexamethasone (3h, 10 nM) significantly increased the number of ABCA1 and annexin I co-immunoreactive foci on the cell surface (P<0.05 vs medium alone, n=5). Co-labeling for annexin I, connexin 43 (a gap junction protein) or tubulin (a microtubule protein) with ABCA1 in intact cells revealed colocalisation at the membrane patches. Furthermore, labeling of follistatin and S100, proteins that also lack a signal sequence and are released by FS cells, were also co-localised with cell surface ABCA1 labeling. These data support the hypothesis that the immunoreactive foci are specific sites for the export of material by ABCA1, possibly anchored in place by microtubules and in the areas of cell-cell communication via gap junctions.