Rac, a member of the Rho family GTPases, has been implicated in the re
gulation of a wide range of biological processes including actin remod
eling, cell transformation, G1 cell cycle progression, and gene expres
sion. To determine whether Rac GTPase activity is required for epiderm
al growth factor-induced mitogenesis, Rat-2 stable cells expressing a
dominant-negative Rac1 mutant, RacN17, were prepared, Exposure to EGF
exhibited a significantly restricted growth response in Rat-2cRacN17 c
ells compared to Rat-2 parental cells, suggesting an essential role of
Rac in EGF-induced mitogenesis. In contrast, addition of lysophosphat
idic acid exerted the same level of growth in Rat-2 and Rat-2-RacN17 c
ells. To gain further evidence for the essential role of Rac in EGF-in
duced mitogenesis, we performed the microinjection experiment. EGF-ind
uced DNA synthesis was significantly blocked by microinjection of reco
mbinant RacN17 protein, and not control IgG, Our further study to anal
yze the downstream mediator of Rac in EGF-signaling to mitogenesis dem
onstrated that Rac-activated phospholipase A, plays a critical role. T
aken together, our results suggest that the ''Rac and Rac-activated PL
A(2)'' cascade is one of the major mitogenic pathways induced by EGF.