Rho GTPases RhoA and Rac1 mediate effects of dietary folate on metastatic potential of A549 cancer cells through the control of cofilin phosphorylation.

Folate, an important nutrient in the human diet, has been implicated in cancer, but its role in metastasis is not established. We have shown previously that the withdrawal of medium folate leads to the inhibition of migration and invasion of A549 lung carcinoma cells. Here we have demonstrated that medium folate regulates the function of Rho GTPases by enabling their carboxyl methylation and translocation to plasma membrane. Conversely, the lack of folate leads to the retention of these proteins in endoplasmic reticulum. Folate also promoted the switch from inactive (GDP-bound) to active (GTP-bound) GTPases, resulting in the activation of downstream kinases p21-activated kinase and LIM kinase and phosphorylation of the actin-depolymerizing factor cofilin. We have further demonstrated that in A549 cells two GTPases, RhoA and Rac1, but not Cdc42, are immediate sensors of folate status: the siRNA silencing of RhoA or Rac1 blocked effects of folate on cofilin phosphorylation and cellular migration and invasion. The finding that folate modulates metastatic potential of cancer cells was confirmed in an animal model of lung cancer using tail vein injection of A549 cells in SCID mice. A folate-rich diet enhanced lung colonization and distant metastasis to lymph nodes and decreased overall survival (35 versus 63 days for mice on a folate-restricted diet). High folate also promoted epithelial-mesenchymal transition in cancer cells and experimental mouse tumors. Our study provides experimental evidence for a mechanism of metastasis promotion by dietary folate and highlights the interaction between nutrients and metastasis-related signaling.

Virtual screening approach for the identification of new Rac1 inhibitors.

Rac1 protein is implicated in several events of atherosclerotic plaque development and represents a new potential pharmacological target for cardiovascular diseases. In this paper we describe a pharmacophore virtual screening followed by molecular docking calculations leading to the identification of five new Rac1 inhibitors. These compounds were shown to be more effective than the reference compound NSC23766 in reducing the intracellular levels of Rac1-GTP, thus supporting this approach for the development of new Rac1 inhibitors.

Trp(56) of rac1 specifies interaction with a subset of guanine nucleotide exchange factors.

Signaling specificity of Rho GTPase pathways is achieved in part by selective interaction between members of the Dbl family guanine nucleotide exchange factors (GEFs) and their Rho GTPase substrates. For example, Trio, GEF-H1, and Tiam1 are a subset of GEFs that specifically activate Rac1 but not the closely related Cdc42. The Rac1 specificity of these GEFs appears to be governed by Rac1-GEF binding interaction. To understand the detailed mechanism underlying the GEF specificity issue, we have analyzed a panel of chimeras made between Rac1 and Cdc42 and examined a series of point mutants of Rac1 made at the switch I, switch II, and beta(2)/beta(3) regions for their ability to interact with and to be activated by the GEFs. The results reveal that Rac1 residues of both the switch I and switch II regions are involved in GEF docking and GEF-mediated nucleotide disruption, because mutation of Asp(38), Asn(39), Gln(61), Tyr(64), or Arg(66)/Leu(67) into Ala results in the loss of GEF binding, whereas mutation at Tyr(32), Asp(65), or Leu(70)/Ser(71) leads to the loss of GEF catalysis while retaining the binding capability. The region between amino acids 53-72 of Rac1 is required for specific recognition and activation by the GEFs, and Trp(56) in beta(3) appears to be the critical determinant. Introduction of Trp(56) to Cdc42 renders it fully responsive to the Rac-specific GEF in vitro and in cells. Further, a polypeptide derived from the beta(3) region of Rac1 including the Trp(56) residue serves as a specific inhibitor for Rac1 interaction with the GEFs. Taken together, these results indicate that Trp(56) is the necessary and sufficient determinant of Rac1 for discrimination by the subset of Rac1-specific GEFs and suggest that a compound mimicking Trp(56) action could be explored as an interfering reagent specifically targeting Rac1 activation.