Diversity of TGF-β-signaling in rat hepatic stellate cells and in myofibroblasts

Aims: TGF-β is the major cytokine regulating hepatic stellate cell (HSC) function in the course of liver fibrogenesis. We priviously have shown that HSC express beside the „classical“ TGF-β sensors, i.e. ALK5 (type I receptor), RII (type II receptor) and betaglycan (type III receptor), endoglin an alternative type III receptor [1]. Endoglin is able to modulate TGF-β responses either via the classical pathway [2] or an accessory TGF-β pathway [3]. Here we show that the components of the accessory signaling branche, i.e. ALK1 (type I receptor) as well as the "BMP"-Smads1, 5 and 8, are expressed in HSC and transdifferentiated myofibroblasts. Furthermore we show that this signaling pathway is activated in a time and dose dependent manner which is different from the classical pathway.

Methods: The expression of the alternative type I receptor ALK1 and BMP-Smad1, 5 and 8 could be displayed by RT-PCR, Northern blot, western blot and immunocytochemistry in rat HSC, MFB and human MFB. The activation of the „classical“ and the „accessory“ TGF-β signaling pathway was differentiated by using an ALK5 inhibitor, which inhibits type I receptors with Smad2/3 specificity [4] and pSmad specific antibodies, which discriminate between TGF-β and BMP Smads.

Results: In HSC TGF-β1 binds and activates two different type I receptors as determined by a specific inhibitor and by cross linking studies. As expected „BMP“-Smads are actived by ALK1 but with a different time and dose dependency as compared to the „TGF-β“-Smads activated by ALK5 [5]. Moreover "BMP"-Smads show a more complex dependency on ALK5 activity in the course of transdifferentiation then has previously shown for endothelial cells [6].

Conclusions: In conclusion TGF-β activates distinctive signaling pathways depending on ligand concentration and transdifferentiation state, offering the opportunity to interfere with HSC function in a selective manner.

Literatur: References: [1] Meurer, Tihaa, Lahme, Gressner, Weiskirchen. J Biol Chem (2005) 280, 3078-87 [2] Guo, Slevin, Li, Parameshwar, Liu, Kumar, Bernabeu, Kumar. Anticancer Res (2004) 24, 1337-45. [3] Lebrin, Goumans, Jonker, Carvalho, Valdimarsdottir, Thorikay, Mummery, Arthur, ten Dijke. EMBO J (2004) 23, 4018-28 [4] Inman, Nicolas, Callahan, Harling, Gaster, Reith, Laping, Hill. Mol Pharmacol. (2002) 62, 65-74 [5] Liu, Gaca, Swenson, Vellucci, Reiss, Wells. J Biol Chem (2003) 278, 11721-8 [6] Goumanns, Valdimarsdottir, Itoh, Rosendahl, Sideras, ten Dijke. EMBO J (2002) 21, 1743-53