Cooperative effect of cAMP and TGFβ in aggressive breast cancer

Aberrant activation of the cAMP/CREB and TGFβ/Smad signalling cascades are involved in breast cancer progression. Modulation of TGFβ-mediated gene activation by cAMP has been shown for different cell types of mesenchymal origin such as human dermal and rat cardiac fibroblasts. We investigated the interplay of these pathways in breast cancer cells grown in 2-dimensional (2D) as well as in 3D cultures. In the presence of either cAMP or TGFβ, mRNA expression levels of many cancer-related genes were significantly altered. Simultaneous activation of both pathways affected genes differently. In 2D cultures, some genes showed no response to forskolin alone, but displayed increased TGFβ responsiveness in the presence of forskolin. However, this effect was not observed under 3D culture conditions. Searching for the reasons for this differential responses in 2D vs. 3D cultures, we found that, in 2D cultures, but not 3D cultures, forskolin increased TGFβ-dependent Smad3 phosphorylation. We could further show that in 2D, but not 3D cultures, forskolin upregulated TGFβ receptor I (TGFβRI) expression. This upregulation could be inhibited by actinomycin, but not by a CREB-specific siRNA suggesting that forskolin increases TGFβRI transcription in a CREB-independent manner. In 3D cultured cells, Smad3 and TGFβRI expression as well as cAMP levels are higher than in 2D cultured cells. The increased levels of these three factors may have prevented the forskolin effect on TGFβ signalling as observed in 2D cultures. Taken together, our data suggest that, under certain conditions, cAMP can interfere with TGFβ signalling by upregulation of TGFβRI expression.