Differential effect of activin A and BMP-7 on myofibroblast differentiation and the role of the Smad signaling pathway.

PURPOSE: To investigate myofibroblast differentiation and signal transduction induced by TGF-beta family members activin A and bone morphogenetic protein (BMP)-7.

METHODS: Transcription of activin and receptors (ActR) for activin A and BMP-7 was detected by RT-PCR. Levels of marker proteins for differentiation and phosphorylation of similar to mothers against decapentaplegics (Smads) were quantified by Western blot analysis in response to BMP-7, activin A and follistatin. Transfection with antisense Smad2/3 was performed to evaluate signal transduction.

RESULTS: Activin A and receptors (ActR-I, ActR-IB, ActR-II) are transcribed in corneal fibroblasts. Compared with TGF-beta1 or serum, activin A but not BMP-7 increased alpha-smooth muscle (SM) actin and actin-binding proteins such as SM myosin, alpha-actinin, and vinculin. Talin, paxillin, and desmin were not induced and vimentin was downregulated by activin. Activin also induced extracellular matrix proteins fibronectin and integrin beta1. Activin-dependent accumulation of proteins was blocked by follistatin. Regarding signal transduction, activin A induced phosphorylation of Smad 2, and BMP-7 induced Smad 1, both of which were inhibited by follistatin. Transfection with antisense Smad 2/3 prevented activin-induced expression and accumulation of alpha-SM actin.

CONCLUSIONS: TGF-beta proteins have different functions in the cornea. Activin A and TGF-beta1, but not BMP-7, are regulators of corneal keratocyte differentiation and may play a role during myofibroblast transdifferentiation. Smad 2/3 signal transduction seems to be important in the regulation of muscle-specific genes. Further investigation of Smad signaling may help to better understand the function of TGF-beta family members in the cornea.