Smad transcription factors mediate the growth inhibitory effect of transforming growth factor-b (TGF-b) in many cell types. Mutational inactivation of Smads has been correlated with loss of responsiveness to TGF-b-mediated signal transduction. In this study, we compare the contribution of individual Smads to TGF-b-induced growth inhibition and endogenous gene expression in isogenic cellular backgrounds. Smad2, Smad3 and Smad4 expression were selectively inhibited in differentiationcompetent cells by using improved antisense molecules. We found that TGF-b mediates its inhibitory effect on HaCaT keratinocyte cell growth predominantly through Smad3. Inhibition of Smad3 expression was sufficient to interfere with TGF-b-induced cell cycle arrest and to induce or suppress endogenous cell cycle regulators. Inhibition of Smad4 expression exhibited a partial effect, whereas inhibition of Smad2 expression had no effect. By gene expression profiling, we identified TGF-b-dependent genes that are differentially regulated by Smad2 and Smad3 under regular growth conditions on a genome-wide scale. We show that Smad2, Smad3 and Smad4 contribute to the regulation of TGF-b responses to varying extents, and demonstrate, in addition, that these Smads exhibit distinct roles in different cell types.

The study of signal transduction processes using antisense oligonucleotides is often complicated by low intracellular stability of the antisense reagents or by nonspecific effects that cause toxicity. Here, we introduce a new class of antisense molecules, so-called GeneBlocs, which are characterized by improved stability, high target RNA specificity, and low toxicity. GeneBlocs allow for efficient downregulation of mRNA expression at nanomolar concentrations, and they do not interfere with cell proliferation. We demonstrate these beneficial properties using a positive readout system. GeneBloc-mediated inhibition of tumor suppressor PTEN (phosphatase and tension homologue detected on chromosome 10) expression leads to hyperactivation of the phosphatidylinositol (PI) 3-kinase pathway, thereby mimicking the loss of PTEN function and its early consequences observed in mammalian cancer cells. Specifically, cells treated with PTEN GeneBlocs show functional activation of Akt, a downstream effector of PI 3-kinase signaling, and exhibit enhanced proliferation when seeded on a basement membrane matrix. In addition, GeneBlocs targeting the catalytic subunit of PI 3-kinase, p110, specifically inhibit signal transduction of endogenous or recombinant PI 3-kinase. This demonstrates that GeneBlocs are powerful tools to analyze and to modulate signal transduction processes and, therefore, represent alternative reagents for the validation of gene function.