ZAK negatively regulates RhoGDIβ-induced Rac1-mediated hypertrophic growth and cell migration

The mitogen-activated protein kinase (MAPK) signaling pathway consists of the sequentially acting upstream kinases MAPK kinase kinase (MAP3K) and MAPK kinase (MAP2K), and the downstream MAPKs, p38MAPK, extracellular signal-regulated kinase (ERK1/2), and c-jun N-terminal kinase (JNK). The mixed lineage kinases are a family of serine/threonine kinases, all of which are classified as MAP3Ks. The seven mixed lineage kinases cloned over the past several years can be classified into three subfamilies based on domain organization and sequence similarity: the MLKs (MLK1–4), the dual leucine zipper-bearing kinases (DLK and LZK), and the zipper sterile-α-motif (SAM) kinases (ZAKα and ZAKβ) [1,2]. ZAK can activate the JNK pathway and the nuclear factor κB (NFκB) pathway [3], and it induces JNK activation through a dual phosphorylation kinase, JNKK2/MKK7 [4]. Overexpression of wild-type ZAK induced apoptosis in a hepatoma cell line [3], and a recent report indicated that ZAK expression in a rat cardiac cell line, H9c2, induced hypertrophic growth and re-expression of atrial natriuretic factor (ANF) [5]. ZAK also mediates TGF-β-induced cardiac hypertrophic growth via a novel TGF-β signaling pathway [6]. In our previous study [5], we showed that the leucine zipper of ZAK mediates homodimerization and promotes autophosphorylation and JNK activation.We identified RhoGDIβ(Rho GDP dissociation inhibitor beta) as a ZAK effector. RhoGDIβ, also known as Ly-GDI or D4-GDI, belongs to a family of Rho GDP dissociation inhibitors, and it is thought to regulate the activity and localization of Rho family proteins [7-10]. The RhoGTPase family includes Rho, Rac, and Cdc42, which differentially regulate the actin cytoskeleton [11-13] and function as molecular switches in cellular signal transduction by alternating between an inactive GDP-bound form that is maintained in cytosolic complexes with GDIs and a GTP-bound form that usually associates with the plasma membrane and interacts with downstre