Differential roles of Smad2 and Smad3 in the regulation of TGF-β1-mediated growth inhibition and cell migration in pancreatic ductal adenocarcinoma cells: control by Rac1

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 1induced 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 1induced growth inhibition, via upregulation of the cdk inhibitor p21WAF1, 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 signalling at the R-Smad level from a tumoursuppressive towards a tumour-promoting outcome. Hence, Rac1 might represent a viable target for therapeutic intervention to inhibit PDAC progression.