Inhibition of P21-activated protein kinase 4 suppresses the formation of invadopodia

Abstract

P21-activated protein (PAK) family members are important regulators for a wide variety of cellular processes, including cell migration, cell cycle progression and apoptosis. PAK4, which belongs to groups II PAK family member, was shown to be overexpressed in a number of cancers and overexpression of PAK4 is frequently associated with poorer prognosis in terms of the high incidence of metastasis. A recent study showed that INKA1 was an inhibitor of PAK4 and contained an inhibitory domain of PAK4 at the C-terminus, called I-Box. However, whether INKA1 can block the PAK4 functions in cells remains not completely characterised. To explore the role of INKA1 on the inhibition of PAK4, we have confirmed the PAK4 inhibitory domain of INKA1 on two I-Box domains at the C-terminus of INKA1 using in vitro PAK4 kinase assay. Unexpectedly, our data indicated that INKA1 was also a good substrate of PAK4. Furthermore, we demonstrated that INKA1 can also inhibit PAK4 within the cells. The previous reports have suggested that PAK4 activity is involved in the formation of invadopodia. Here, we observed that PAK4 is localised in invadopodia and ectopic expression of INKA1 suppressed the formation of invadopodia in melanoma cell line, A375. Taken together, our data indicated that PAK4 is important for invadopodia formation and inhibition of PAK4 blocks the formation of invadopodia.