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

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