Rho GTPases: promising cellular targets for novel anticancer drugs.

Ras-homologous (Rho) GTPases play a pivotal role in the regulation of numerous cellular functions associated with malignant transformation and metastasis. Rho GTPases are localized at membranes and become activated upon stimulation of cell surface receptors. In their GTP-bound (=active) state, Rho proteins bind to effector proteins, thereby triggering specific cellular responses. Members of the Rho family of small GTPases are key regulators of actin reorganization, cell motility, cell-cell and cell-extracellular matrix (ECM) adhesion as well as of cell cycle progression, gene expression and apoptosis. Each of these functions is of importance for the development and progression of cancer. Furthermore, Rho guanine exchange factors (GEFs) are often oncogenic and the expression level of Rho GTPases frequently increases with malignancy. Rho proteins also affect cellular susceptibility to DNA damaging agents, including antineoplastic drugs and ionizing radiation (IR). Thus, modulation of Rho driven mechanisms may influence the therapeutic efficiency and/or the side effects of conventional antineoplastic therapy. Because of their pleiotropic functions, Rho proteins appear to be promising targets for the development of novel anticancer drugs. Experimental approaches to inhibit Rho (and Ras) have focused on the attenuation of their C-terminal isoprenylation. This is because C-terminal lipid modification is required for correct intracellular localization and function of Rho/Ras. Inhibitors of farnesyltransferase (FTI), geranylgeranyltransferase (GGTI) as well as of HMG-CoA-reductase (i. e. statins) have been investigated with respect to their usefulness in tumor therapy. The studies showed that these compounds affect tumor progression and furthermore have impact on the frequency of cell death induced by tumor therapeutics. A possible drawback of inhibitors of isoprenylation is their poor selectivity for individual Rho GTPases. Therefore, specific inhibitors of individual Rho functions (notably RhoA-, RhoB-, Rac1- or Cdc42-related functions) are predicted to be of great therapeutic benefit. Indeed, compounds developed as specific inhibitors of the RhoA-effector molecule Rho-kinase (ROK) have been demonstrated to exert anti-metastatic activity in vivo.