Externalization of annexin I from a folliculo-stellate-like cell line

Our recent studies on rat pituitary tissue suggest that the annexin I-dependent inhibitory actions of glucocorticoids may not be exerted directly on endocrine cells but indirectly via folliculo-stellate (FS) cells. FS cells contain glucocorticoid receptors and abundant annexin I. We have studied the localization of annexin I in FS cells and the ability of dexamethasone to induce annexin I secretion by an FS (TtT/GF) cell line, using Western blotting and immunofluorescence microscopy. Exposure of TtT/GF cells to dexamethasone (0.1 mum, 3 h) caused an increase in the amount of annexin I protein in the intracellular compartment and attached to the surface of the cells. In nonpermeabilized cells, immunofluorescence label- ing revealed that annexin I immunoreactivity was associated with the cell surface and concentrated in focal patches on the ends of cytoplasmic processes; dexamethasone (0.1 mum, 3 h) increased both the number and intensity of these foci. Immunogold electron microscopy confirmed in anterior pituitary tissue the presence of immunoreactive-annexin at the surface of FS cell processes contacting endocrine cells. These data support our hypothesis that annexin I is released by FS cells in response to glucocorticoids to mediate glucocorticoid inhibitory actions on pituitary hormone release via a juxtacrine mechanism.