Θετική και αρνητική ρύθμιση των μικρών GTPασών Rho και του κυτταροσκελετού ακτίνης από τις BMP και TGF-­β

Abstract

Actin cytoskeleton dynamics support and coordinate a large number of signaling events that control cell proliferation, differentiation and migration. Cytoskeletal plasticity is based on multi-??protein complexes that regulate actin polymerization, stability and assembly with myosin. Growth factors provide essential signals that govern cytoskeletal dynamics. One such growth factor network is the transforming growth factor β (TGF-??β) that includes secreted polypeptides such as bone morphogenetic proteins (BMP) and activins. Here, it is demonstrated that BMP-??7 signals through Smad1, p38 and p44/42 (Erk1/2) MAPKs and induces actin polymerization and focal adhesion remodeling in starved fibroblasts and various other cell lines as potently as TGF-??β. Moreover, BMP-??7 induces migration of Swiss3T3 fibroblasts letting at the same time proliferation and apoptosis unaffected. An important regulator of actin remodeling, the kinase ROCK1 mediates changes of actin dynamics by BMP-??7, while this growth factor induces rapid activation of RhoA and RhoB GTPases with concomitant inactivation of Cdc42. In parallel, BMP-??7 induces RhoB and Smad6 expression providing with a positive and a negative signalling feedback simultaneously. Activation of Rho GTPases and the downstream ROCK1 kinase by BMP-??7 correlate well with induction of phosphorylation on Ser19 of the myosin light chain, but not with LIMK1 kinase activation. ROCK inhibitor Y-??27632 blocks actin polymerization and reduces migration capacity induced by BMP-??7. Furthermore, the same ROCK inhibitor blocks the myosin light chain phosphorylation of Ser19 during BMP-??7 signalling. Depletion of endogenous myosin light chain also inhibits the actin remodeling induced by BMP-??7. Thus, in a next step, this novel pathway regulates fibroblast migration without affecting cell proliferation. Taking everything into consideration, here it is established not only that BMP signals through Smad and, the so-??called, non-??Smad pathway of MAPKs, but also through Rho/ROCK pathway which targets myosin light chain during actin remodeling and migration of Swiss3T3 fibroblasts.