PKCα-dependent activation of RhoA by syndecan-4 during focal adhesion formation

Syndecan-4 is a ubiquitously expressed transmembrane heparan sulphate proteoglycan acting in concert with integrins in the formation of focal adhesions and stress fibres. Signalling events studied thus far suggest the formation of a ternary complex between syndecan-4, phosphatidylinositol 4,5-bisphosphate and protein kinase C alpha (PKC alpha). Syndecan-4 clustering at the cell surface has also been associated with RhoA-dependent signalling, but the relationship between PKC alpha and RhoA has not been resolved. Here we present evidence that syndecan-4, PKC alpha and RhoA are in a linear pathway necessary for the formation and maintenance of stress fibres in primary rat embryo fibroblasts. Inhibition of PKC alpha activity through the use of specific pharmacological inhibitors, a dominant-negative construct, or siRNA downregulation of protein levels, attenuated focal adhesion formation and the maintenance of stress fibres. However, these effects could be bypassed through independent activation of RhoA with lysophosphatidic acid, but not by clustering of syndecan-4 with ligand. Furthermore, inhibition of PKC alpha could block the increase in the GTP levels of RhoA induced by clustering of syndecan-4 at the cell surface. All these data point to a mechanism whereby syndecan-4 signals to RhoA in a PKC alpha-dependent manner and PKC alpha directly influences RhoA activity.