Small GTP-binding protein Rho stimulates the actomyosin system, leading to invasion of tumor cells.

We have shown previously that Rho plays a pivotal role in 1-oleoyl-lysophosphatidic acid (LPA)-dependent invasion of rat hepatoma cells (MM1). Herein we made stable transfectants of MM1 expressing active and Botulinum exoenzyme C3 (C3)-sensitive (Val14), or active and C3-insensitive (Val14/Ile41) forms of human RhoA. Both transfectants showed greatly promoted invasive ability in vitro in the absence of LPA as well as in vivo, adherence to the dish with scattered shape, and enhanced phosphorylation level of 20-kDa myosin light chain (MLC20). A specific MLC kinase inhibitor (KT5926) could inhibit their invasion and the phosphorylation level of MLC20. Stable active RhoA transfectants of W1 cells (low invasive counterpart of MM1) also demonstrated promoted invasive ability in vitro and in vivo, and enhanced phosphorylation level of MLC20. C3 treatment inhibited the invasiveness of the Val14 RhoA transfectant but not that of the Val14/Ile41 RhoA transfectant. LPA enhanced the invasiveness of both transfectants, and this enhancement was abolished by the C3 treatment. These results suggested that 1) the Rho signaling pathway and actomyosin system were linked in the transmigration of tumor cells, and 2) expressed active RhoA enhanced LPA-induced tumor cell invasion via the activation of endogenous RhoA pathway, indicating a positive feedback mechanism in the activation of RhoA.