Charakterisierung des BMP-7-Signalwegs und der TGF-beta-1-BMP-7-Wechselwirkungen in myofibroblasten-ähnlichen Zellen

It has been shown that the majority of the cellular alterations and changes in gene expression in several fibrotic diseases is caused by the cytokine TGF-beta1. Furthermore, it was found that in some cases another member of the TGF-beta superfamily of cytokines, BMP-7, can antagonize the profibogenic effects of TGF-beta1. One feature of liver fibrosis is the transdifferentiation of hepatic stellate cells to a myofibroblast-like phenotype. These cells contribute to the development of the fibrosis by massive expression of proteins of the extra-cellular matrix. During this study, the myoblast cell line L6E9 served as a model for this process, as they also transdifferentiate to myofibroblast-like cells when treated with TGF-beta1. Furthermore, the transduction of TGF-beta1 signals is well known in this cell line and can be easier distinguished from BMP-7 signaling compared to hepatic stellate cells. It was shown that BMP-7 signals are detected via the BMP-type I-receptors Alk2 and Alk3 in L6E9. Intracellulary, the signals are mediated to the nucleus via Smad1 and Smad5, where they lead to the upregulation of the expression of Id1, Id2 and Id3 in a concentration-dependent manner. Additionally, the activation of the MAP-kinase pathway by BMP-7 and TGF-beta1 was validated with a probable selectivity for the MAP-kinase p38. The stimulation of L6E9 with BMP-7 leads to the inhibition of the expression of Ctgf, a fibrosis-associated TGF-beta1 target gene. This effect is not based on a direct inhibition of TGF-beta1 signal transduction or transcription of Ctgf, but is mediated by the upregulation of a BMP-7 target gene. The specific inhibition of the TGF-beta1/Smad3 pathway had no influence on the expression of Ctgf, whereas the inhibition of the MAP-kinases p38 and p44/42 led to a down-regulation of Ctgf expression. In conclusion the Ctgf expression is regulated by TGF-beta1 via the MAP-kinase pathway in L6E9, while the antagonistic effect of BMP-7 is mediated via an indirect mechanism which is based on the described Smad1/5 pathway.