Epidermal growth factor stimulation of the ACK1/Dbl pathway in a Cdc42 and Grb2-dependent manner

The tyrosine kinase ACK1 phosphorylates and activates the guanine nucleotide exchange factor Dbl, which in turn directs the Rho family GTP-binding proteins. However, the regulatory mechanism of ACK1/Dbl signaling in response to extracellular stimuli remains poorly understood. Here we describe that epidermal growth factor stimulates the ACK1/Dbl pathway, leading to actin cytoskeletal rearrangements. The role of the two ACK1-binding proteins Cdc42 and Grb2 was assessed by overexpression of the Cdc42/Rac interactive binding domain and a dominant-negative Grb2 mutant, respectively. Specific inhibition of the interaction of ACK1 with Cdc42 or Grb2 by the use of these constructs diminished tyrosine phosphorylation of both ACK1 and Dbl in response to EGF. Therefore, the activation of ACK1 and subsequent downstream signaling require both Cdc42-dependent and Grb2-dependent processes within the cell. In addition, we show that EGF transiently induces formation of the focal complex and stress fibers when ACK1 was ectopically expressed. The induction of these structures was totally sensitive to the action of botulinum toxin C from Clostridium botulinum, suggesting a pivotal role of Rho. These results provide evidence that ACK1 acts as a mediator of EGF signals to Rho family GTP-binding proteins through phosphorylation and activation of GEFs such as Dbl.