RhoGDIβ-induced hypertrophic growth in H9c2 cells is negatively regulated by ZAK

We found that overexpression of RhoGDI beta, a Rho GDP dissociation inhibitor, induced hypertrophic growth and suppressed cell cycle progression in a cultured cardiomyoblast cell line. Knockdown of RhoGDI beta expression by RNA interference blocked hypertrophic growth. We further demonstrated that RhoGDI beta physically interacts with ZAK and is phosphorylated by ZAK in vitro, and this phosphorylation negatively regulates RhoGDI beta functions. Moreover, the ZAK-RhoGDI beta interaction may maintain ZAK in an inactive hypophosphorylated form. These two proteins could negatively regulate one another such that ZAK suppresses RhoGDI beta functions through phosphorylation and RhoGDI beta counteracts the effects of ZAK by physical interaction. Knockdown of ZAK expression in ZAK- and RhoGDI beta-expressing cells by ZAK-specific RNA interference restored the full functions of RhoGDI beta.