Rapid and transient reorganization of the cytoskeleton in GH3B6 cells during short-term exposure to thyroliberin

The cytoskeletal organization of the rat pituitary tumor cell line GH3B6 was analysed using immunofluorescence, in basal conditions and after stimulation by thyroliberin (TRH). Under basal conditions, a dense and entangled cytoplasmic microtubule network, a perinuclear cage of cytokeratin fibers, and a diffuse distribution of F-actin were revealed. Short-term stimulation of these cells by TRH induces a first early phase of PRL release (0-2 min), concomitant with a rarefaction of cytoplasmic PRL-containing granules, followed by a second plateau phase (5-30 min), concomitant with modifications of the Golgi zone. We show that TRH induced early and transient modifications in the cytoskeletal distribution during these short periods of stimulation. First, after 2 min of stimulation, small fluorescent tubulin blebs appeared under the plasma membrane. Then, after 5 min they disappeared, and a thin actin network, accentuated by thicker fibers, organized transiently in the cytoplasm. After 30 min, the microtubules and cytokeratin networks had extended throughout the cytoplasm and the actin distribution was diffuse again. So, in this study, we have shown the existence of a parallelism between the redistribution of intracellular PRL compartments and the reorganization of cytoskeletal elements, during exposure to TRH. We could not clearly correlate these modifications with transduction mechanisms involved in TRH action.