Cyclic Amp Enhances Tgf Beta Responses Of Breast Cancer Cells By Upregulating Tgf Beta Receptor I Expression

Cellular functions are regulated by complex networks of many different signaling pathways. The TGF beta and cAMP pathways are of particular importance in tumor progression. We analyzed the cross-talk between these pathways in breast cancer cells in 2D and 3D cultures. We found that cAMP potentiated TGF beta-dependent gene expression by enhancing Smad3 phosphorylation. Higher levels of total Smad3, as observed in 3D-cultured cells, blocked this effect. Two Smad3 regulating proteins, YAP (Yes-associated protein) and T beta RI (TGF beta receptor 1), were responsive to cAMP. While YAP had little effect on TGF beta-dependent expression and Smad3 phosphorylation, a constitutively active form of T beta RI mimicked the cAMP effect on TGF beta signaling. In 3D-cultured cells, which show much higher levels of T beta RI and cAMP, T beta RI was unresponsive to cAMP. Upregulation of T beta RI expression by cAMP was dependent on transcription. A proximal T beta RI promoter fragment was moderately, but significantly activated by cAMP suggesting that cAMP increases T beta RI expression at least partially by activating T beta RI transcription. Neither the cAMP-responsive element binding protein (CREB) nor the T beta RI-regulating transcription factor Six1 was required for the cAMP effect. An inhibitor of histone deacetylases alone or together with cAMP increased T beta RI expression by a similar extent as cAMP alone suggesting that cAMP may exert its effect by interfering with histone acetylation. Along with an additive stimulatory effect of cAMP and TGF beta on p21 expression an additive inhibitory effect of these agents on proliferation was observed. Finally, we show that mesenchymal stem cells that interact with breast cancer cells can simultaneously activate the cAMP and TGF beta pathways. In summary, these data suggest that combined effects of cAMP and TGF beta, as e.g. induced by mesenchymal stem cells, involve the upregulation of T beta RI expression on the transcriptional level, likely due to changes in histone acetylation. As a consequence, cancer cell functions such as proliferation are affected.