Extracellular ATP stimulates proliferation of vascular smooth muscle cells (VSMC) through activation of G protein coupled P2Y purinergic receptors. We have previously shown that ATP stimulates a transient activation of protein kinase A (PKA), which, together with the established mitogenic signaling of purinergic receptors, promotes proliferation of VSMC (Hogarth et al, Am. J. Physiol. Cell. Physiol. 2004, 287:C449-56). We also have shown that PKA can phosphorylate beta-catenin at two novel sites (Ser-552 and Ser-675) in vitro and in overexpression cell models (Taurin et al, J. Biol. Chem. 2006, 281:9971-6). Beta-catenin promotes cell proliferation by activation a family of T cell factor (TCF) transcription factors, which drive the transcription of genes implicated in cell cycle progression including cyclin D1. In the present study, using the phospho-specific antibodies against phospho-Ser552 or phospho-Ser675 sites of beta-catenin, we show that ATP can stimulate PKA-dependent phosphorylation of endogenous beta-catenin at both of these sites without affecting its expression levels in VSMC. This translates to a PKA-dependent stimulation of TCF transcriptional activity through an increased association of phosphorylated (by PKA) beta-catenin with TCF-4. Using the PKA inhibitor (PKI), or dominant negative TCF-4 mutant, we show that ATP-induced cyclin D1 promoter activation, cyclin D1 protein expression and proliferation of VSMC are all dependent on PKA and TCF activities. In conclusion, we show a novel mode of regulation of endogenous beta-catenin through its phosphorylation by PKA, and we demonstrate the importance of this mechanism for ATP-induced proliferation of VSMC.