• actin cytoskeleton
• breast cancer
• cell movement
• progesterone receptor

Exposure to progesterone is associated with increased risk of breast cancer in women. However, the role of progesterone receptor (PR) signaling on breast cancer development and progression is still largely unclear. In particular, the mechanistic basis of PR action on cancer cell movement and invasion have not been explored. In the present work, we characterized the rapid signaling events that follow the recruitment of PR in T47-D breast cancer cells that lead to morphological changes of cell shape and of the actin cytoskeleton which are implicated in the initiation of cell movement and invasion. In particular, we identified a set of extra-nuclear signaling events initiated by PR directed to the actin-regulatory protein, moesin. The PR-initiated activation of moesin is associated with a shift from globular to fibrillar actin and with a remodeling of the actin cytoskeleton toward the cell membrane in T47-D breast cancer cells. These processes lead to marked changes in the morphology of the cell membrane, particularly to the development of pseudopodia and ruffles, which are implicated in cell movement. Moesin is necessary for the cytoskeletal and cell membrane remodeling, as well as for T47-D cell horizontal movement and invasion of three-dimensional matrices initiated by PR. The signaling of PR to moesin requires the rapid extra-nuclear activation of the small GTPase RhoA and of its downstream effector Rho-associated kinase (ROCK-2), that is responsible for moesin phosphorylation. However, PR recruits RhoA via different signaling cascades depending on the ligand engaging its binding pocket. Indeed, in the presence of progesterone, progesterone receptor A (PRA) (but not PRB) is driven to interact with the G protein Gá13, leading to RhoA activation. However, in the presence of the synthetic progestin medroxyprogesterone acetate (MPA) both PRA and PRB interact with the tyrosine kinase c-Src which thus signals to phosphatidylinositol-3 kinase and then to RhoA. These results identify new mechanisms of action of PR in breast cancer cells that are linked to migration and invasion, and that may thus be important for breast cancer progression. This also identifies the actin cytoskeleton as an important target of regulation by PR in breast cancer cells. PR acts on these targets through an original, extra-nuclear set of signaling events converging on the actin-regulatory protein, moesin, that differ in part depending on the PR ligand. These results characterize new targets to potentially interfere with the PR-dependent promotion of breast cancer progression.