Initiation of cell locomotility is a morphogenetic checkpoint in thyroid epithelial cells regulated by ERK and PI3-kinase signals

Epithelial locomotility is a fundamental determinant of tissue patterning t hat is subject to strict physiological regulation. The current, study sough t to identify cellular signals that initiate cell migration in cultured thy roid epithelial cells. Porcine thyroid cells cultured as 3-dimensional foll icles convert to 2-dimensional monolayers when deprived of agents that stim ulate cAMP/PKA signaling. This morphogenetic event is driven by the activat ion of cell-on-substrate locomotility, providing a convenient assay for eve nts that regulate the initiation of locomotion. In this system, the extrace llular signal regulated kinase (ERK) pathway became activated as follicles converted to monolayer, as demonstrated by immunoblotting for activation-sp ecific phosphorylation and nuclear accumulation of ERK. Inhibition of ERK a ctivation using the drug PD98059 effectively prevented cells from beginning to migrate. PD98059 inhibited cell spreading, actin filament reorganizatio n and the assembly of focal adhesions, cellular events that mediate the ini tiation of thyroid cell locomotility. Akt (PKB) signaling was also activate d during follicle-to-monolayer conversion and the phosphoinositide 3-kinase (PI3-kinase) inhibitor, wortmannin, also blocked the initiation of cell mo vement. Wortmannin did not, however, block activation of ERK signaling. The se findings, therefore, identify the ERK and PI3-kinase signaling pathways as important stimulators of thyroid cell locomotility. These findings are i ncorporated into a model where the initiation of thyroid cell motility cons titutes a morphogenetic checkpoint regulated by coordinated changes in stim ulatory (ERK, PI3-kinase) and tonic inhibitory (cAMP/PKA) signaling pathway s. Cell Motil. Cytoskeleton 49:93-103, 2001. (C) 2001 Wiley-Liss, Inc.