A role for
heterotrimeric G proteins in the regulation of Golgi function and formation of secretory granules is generally accepted. We set out to study the effect of activation of
heterotrimeric G proteins by
aluminum fluoride on secretory granule formation in AtT-20 corticotropic
tumor cells and in melanotrophs from the rat pituitary. In AtT-20 cells, treatment with
aluminum fluoride or
fluoride alone for 60 min induced complete dispersal of Golgi, ER-Golgi intermediate compartment and Golgi matrix markers, while betaCOP immunoreactiviy retained a juxtanuclear position and TGN38 was unaffected. Electron microscopy showed compression of Golgi cisternae followed by conversion of the Golgi stacks into clusters of tubular and vesicular elements. In the melanotroph of the rat pituitary a similar compression of Golgi cisternae was observed, followed by a progressive loss of cisternae from the stacks. As shown in other cells,
brefeldin A induced redistribution of the
Golgi matrix protein GM130 to punctate structures in the cytoplasm in AtT-20 cells, while
mannosidase II immunoreactivity was completely dispersed.
Fluoride induced a complete dispersal of
mannosidase II and GM130 immunoreactivity. The effect of
fluoride was fully reversible with reestablishment of normal
mannosidase II and GM130 immunoreactivity within 2 h. After 1 h of recovery, showing varying stages of reassembly, the patterns of
mannosidase II and GM130 immunoreactivity were identical in individual cells, indicating that Golgi matrix and cisternae reassemble with similar kinetics during recovery from
fluoride treatment. Instead of a specific
aluminum fluoride effect on secretory granule formation in the trans-Golgi network, we thus observe a unique form of Golgi dispersal induced by
fluoride alone, possibly via its action as a
phosphatase inhibitor.