Smad7 and protein phosphatase 1α are critical determinants in the duration of TGF-β/ALK1 signaling in endothelial cells
1 Dept. of Biochemistry and Molecular Biology, Faculty of Medicine, University of Iceland, Vatnsmyrarvegur 16, 101 Reykjavik, Iceland
2 Heart Lung Center, Department of Cardiology, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands
3 Graduate School of Comprehensive Human Sciences, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 305–8575, Japan
4 4Ludwig Institute for Cancer Research, Uppsala University, Biomedical Center, S-751 24, Uppsala, Sweden
5 Molecular Cell Biology, Leiden University Medical Center, Postbus 9600, 2300 RC Leiden, The Netherlands
BMC Cell Biology 2006, 7:16 doi:10.1186/1471-2121-7-16Published: 29 March 2006
In endothelial cells (EC), transforming growth factor-β (TGF-β) can bind to and transduce signals through ALK1 and ALK5. The TGF-β/ALK5 and TGF-β/ALK1 pathways have opposite effects on EC behaviour. Besides differential receptor binding, the duration of TGF-β signaling is an important specificity determinant for signaling responses. TGF-β/ALK1-induced Smad1/5 phosphorylation in ECs occurs transiently.
The temporal activation of TGF-β-induced Smad1/5 phosphorylation in ECs was found to be affected by de novo protein synthesis, and ALK1 and Smad5 expression levels determined signal strength of TGF-β/ALK1 signaling pathway. Smad7 and protein phosphatase 1α (PP1α) mRNA expression levels were found to be specifically upregulated by TGF-β/ALK1. Ectopic expression of Smad7 or PP1α potently inhibited TGF-β/ALK1-induced Smad1/5 phosphorylation in ECs. Conversely, siRNA-mediated knockdown of Smad7 or PP1α enhanced TGF-β/ALK1-induced signaling responses. PP1α interacted with ALK1 and this association was further potentiated by Smad7. Dephosphorylation of the ALK1, immunoprecipitated from cell lysates, was attenuated by a specific PP1 inhibitor.
Our results suggest that upon its induction by the TGF-β/ALK1 pathway, Smad7 may recruit PP1α to ALK1, and thereby control TGF-β/ALK1-induced Smad1/5 phosphorylation.