Molecular details on BMP-7 activity on TGF-beta1-mediated CTGF expression in myofibroblasts

Background: In the setting of fibrogenic lesions, TGF-β1 drives cells to the myofibroblast phenotype and thereby causes excessive extracellular matrix (ECM) deposition. In the liver – as well as in other organs–BMP-7 has been described as an antifibrotic ligand abrogating profibrogenic TGF-β1 responses [1]. However, the exact molecular details of this antagonism have not been elucidated so far. To do so we employed a model system – L₆E₉ myoblasts – which transit into a myofibroblastic phenotype upon treatment with TGF-β1 [2]. In addition, we have already shown that in L₆E₉- myoblasts TGF-β1 responses are counteracted by BMP-7 [3]. Methods and Results: We have shown before that BMP-7-signaling is mediated via both BMP-receptors ALK2 and ALK3 to govern the phosphorylation of Smad1 and Smad5 and subsequent transcriptional upregulation of Id1 and Id2. To evaluate the role of the above mentioned components in antagonizing TGF-β1-responses we took advantage of the TGF-β1-regulated immediate-early gene CTGF. Unexpectedly we found that in L₆E₉ TGF-β1-mediated CTGF expression is independent of ALK5/Smad and p38 signalling but requires ERK-activation. Nevertheless, CTGF mRNA and protein were downregulated in the presence of BMP-7. This stimulatory effect was blocked by addition of cycloheximide suggesting the involvement of BMP-7 target gene expression. Knock down of receptors (ALK2 and ALK3) as well as Smads (Smad1 and Smad5) showed a slight reduction of the BMP-7-mediated inhibition whereas knock down of Id2 was ineffective. Conclusions: We could show that BMP-7 potently inhibits TGF-β1-mediated CTGF expression, which is regulated by MAP-kinases independently of ALK5-signaling. The inhibition most likely involves activation of ALK2/ALK3 and Smad1/Smad5 but the role of Id2 so far is unresolved. In conclusion, BMP-7-signaling alters MAP-kinase activity rather than classical Smad3 function in attenuation of TGF-β1-mediated CTGF expression in L₆E₉ cells.

Literature:

[1] Weiskirchen R, Meurer SK, Gressner OA, Herrmann J, Borkham-Kamphorst E, Gressner AM. Front Biosci. (2009) 14, 4992-5012.[2] Ignotz RA, Endo T and Massague J. J Biol Chem. (1987) 262, 6443-6446.[3] Scherner O, Meurer SK, Tihaa L, Gressner AM, and Weiskirchen R. J Biol Chem. (2007) 282, 13934-13943.

BMP-7 - L6E9 - MAP-kinases - TGF-beta - myoblasts - myofibroblasts