Transforming growth factor-β (TGF-β) plays complex roles in carcinogenesis, as it may exert both tumor suppressor and pro-oncogenic activities depending on the stage of the tumor. SMAD proteins transduce signals from the TGF-β receptors to regulate the transcription of specific target genes. Crosstalks with other signaling pathways may contribute to the specificity of TGF-β effects. In this report, we have investigated the effects of cyclic adenosine 3′, 5′-monophosphate (cAMP), a key second messenger in the cellular response to various hormones, on SMAD-dependent signaling in human HaCaT keratinocytes. Using either an artificial SMAD3/4-dependent reporter construct or the natural TGF-β target, plasminogen activator inhibitor-1, we show that membrane-permeable dibutyryl cAMP, and other intracellular cAMP-elevating agents such as the phosphodiesterase inhibitor isobutyl-methylxanthine, the adenylate cyclase activator forskolin, or exogenous prostaglandin E 2 (PGE 2), interfere with TGF-β-induced SMAD-specific gene transactivation. Inhibition of protein kinase A (PKA), the main downstream effector of cAMP, with H-89, suppressed cAMP-dependent repression of SMAD-driven gene expression. Inversely, coexpression of either an active PKA catalytic subunit or that of the cAMP response element (CRE)-binding protein (CREB) blocked SMAD-driven gene transactivation. cAMP-elevating agents did not inhibit nuclear translocation and DNA binding of SMAD3/4 complexes, but abolished the interactions of SMAD3 with the transcription coactivators CREB-binding protein (CBP) and p300 in a PKA-dependent manner. These results suggest that suppression of TGF-β/SMAD signaling and resulting gene transactivation by cAMP-inducing agents occurs via PKA-dependent, CREB-mediated, disruption of SMAD–CBP/p300 complexes.