Differential roles of Smad2 and Smad3 in the regulation of TGF-[beta]1-mediated growth inhibition and cell migration in pancreatic ductal adenocarcinoma cells: control by Rac1

• Published in: Molecular cancer 2011-05, Vol.10, p.67
• Main Authors: Ungefroren, Hendrik, Groth, Stephanie, Sebens, Susanne, Lehnert, Hendrik, Gieseler, Frank, Fändrich, Fred
• Format: Article
• Language: English
• Subjects: Adenocarcinoma; Cancer cells; Genetic aspects; Physiological aspects; Polymerase chain reaction; Transforming growth factors
• ISSN: 1476-4598; 1476-4598
• DOI: 10.1186/1476-4598-10-67

Background Progression of pancreatic ductal adenocarcinoma (PDAC) is largely the result of genetic and/or epigenetic alterations in the transforming growth factor-beta (TGF-[beta])/Smad signalling pathway, eventually resulting in loss of TGF-[beta]-mediated growth arrest and an increase in cellular migration, invasion, and metastasis. These cellular responses to TGF-[beta] are mediated solely or partially through the canonical Smad signalling pathway which commences with activation of receptor-regulated Smads (R-Smads) Smad2 and Smad3 by the TGF-[beta] type I receptor. However, little is known on the relative contribution of each R-Smad, the possible existence of functional antagonism, or the crosstalk with other signalling pathways in the control of TGF-[beta]1-induced growth inhibition and cell migration. Using genetic and pharmacologic approaches we have inhibited in PDAC cells endogenous Smad2 and Smad3, as well as a potential regulator, the small GTPase Rac1, and have analysed the consequences for TGF-[beta]1-mediated growth inhibition and cell migration (chemokinesis). Results SiRNA-mediated silencing of Smad3 in the TGF-[beta] responsive PDAC cell line PANC-1 reduced TGF-[beta]1-induced growth inhibition but increased the migratory response, while silencing of Smad2 enhanced growth inhibition but decreased chemokinesis. Interestingly, siRNA-mediated silencing of the small GTPase Rac1, or ectopic expression of a dominant-negative Rac1 mutant largely mimicked the effect of Smad2 silencing on both TGF-[beta]1-induced growth inhibition, via upregulation of the cdk inhibitor p21.sup.WAF1.sup., and cell migration. Inhibition of Rac1 activation reduced both TGF-[beta]1-induction of a Smad2-specific transcriptional reporter and Smad2 C-terminal phosphorylation in PDAC cells while Smad3-specific transcriptional activity and Smad3 C-terminal phosphorylation appeared increased. Disruption of autocrine TGF-[beta] signalling in PANC-1 cells rendered cells less susceptible to the growth-suppressive effect of Rac1 inhibition, suggesting that the decrease in "basal" proliferation upon Rac1 inhibition was caused by potentiation of autocrine TGF-[beta] growth inhibition. Conclusions In malignant cells with a functional TGF-[beta] signalling pathway Rac1 antagonizes the TGF-[beta]1 growth inhibitory response and enhances cell migration by antagonistically regulating Smad2 and Smad3 activation. This study reveals that Rac1 is prooncogenic in that it can alter TGF-[beta