The mechanism of cross talk between the Wnt signaling and cyclic AMP (cAMP)-dependent protein kinase (protein kinase A [PKA]) pathways was studied. Prostaglandin E 1 (PGE 1), isoproterenol, and dibutyryl cAMP (Bt 2 cAMP), all of which activate PKA, increased the cytoplasmic and nuclear β-catenin protein level, and these actions were suppressed by a PKA inhibitor and RNA interference for PKA. PGE 1 and Bt 2 cAMP also increased T-cell factor (Tcf)-dependent transcription through β-catenin. Bt 2 cAMP suppressed degradation of β-catenin at the protein level. Although PKA did not affect the formation of a complex between glycogen synthase kinase 3β (GSK-3β), β-catenin, and Axin, phosphorylation of β-catenin by PKA inhibited ubiquitination of β-catenin in intact cells and in vitro. Ser675 was found to be a site for phosphorylation by PKA, and substitution of this serine residue with alanine in β-catenin attenuated inhibition of the ubiquitination of β-catenin by PKA, PKA-induced stabilization of β-catenin, and PKA-dependent activation of Tcf. These results indicate that PKA inhibits the ubiquitination of β-catenin by phosphorylating β-catenin, thereby causing β-catenin to accumulate and the Wnt signaling pathway to be activated.