Colorectal cancer (CRC) frequently metastasizes to the liver, a phenomenon that involves the participation of transforming-growth-factor-beta(1) (TGF beta(1)). Blockade of the protumorigenic effects elicited by TGF beta(1) in advanced CRC could attenuate liver metastasis. We aimed in the present study to assess the antimetastatic effect of TGF beta(1)-blocking peptides P17 and P144, and to study mechanisms responsible for this activity in a mouse model. Colon adenocarcinoma cells expressing luciferase were pretreated with TGF beta(1) (Mc38-luc(TGF beta 1) cells), injected into the spleen of mice and monitored for tumor development. TGF beta(1) increased primary tumor growth and liver metastasis, whereas systemic treatment of mice with either P17 or P144 significantly reduced tumor burden (p < 0.01). In metastatic nodules, mitotic/apoptotic ratio, mesenchymal traits and angiogenesis (evaluated by CD-31, as well as circulating endothelial and progenitor cells) induced by TGF beta(1) were consistently reduced following injection of peptides. In vitro experiments revealed a direct effect of TGF beta(1) in Mc38 cells, which resulted in activation of Smad2, Smad3 and Smad1/5/8, and increased invasion and transendothelial migration, whereas blockade of TGF beta(1)-signaling reverted these features. Because TGF beta(1)-mediated epithelial -mesenchymal transition (EMT) has been suggested to induce a cancer stem cell (CSC) phenotype, we analyzed the ability of this cytokine to induce tumorsphere formation and the expression of CSC markers. In TGF beta(1)-treated cells, tumorspheres were enriched in CD44 and SOX2, which were diminished in the presence of P17. Our data provide a preclinical rationale to evaluate P17 and P144 as potential therapeutic options for the treatment of metastatic CRC. (c) 2012 Elsevier Inc. All rights reserved.