Endoglin differentially modulates antagonistic transforming growth factor-β1 and BMP-7 signaling

Transforming growth factor-beta 1 (TGF-beta 1) and BMP-7 (bone morphogenetic protein-7; OP-1) play central, antagonistic roles in kidney fibrosis, a setting in which the expression of endoglin (CD105), an accessory TGF-beta type III receptor, is increased. So far, endoglin is known as a negative regulator of TGF-beta/ALK-5 signaling. Here we analyzed the effect of BMP-7 on TGF-beta 1 signaling and the role of endoglin for both pathways in endoglin-deficient L6E9 cells. In this myoblastic cell line, TGF-beta 1 and BMPs are opposing cytokines, interfering with myogenic differentiation. Both induce specific target genes of which Id1 (for BMPs) and collagen I (for TGF-beta 1) are two examples. TGF-beta 1 activated two distinct type I receptors, ALK-5 and ALK-1, in these cells. Although the ALK-5/Smad3 signaling pathway mediated collagen I expression, ALK-1/Smad1/Smad5 signaling mediated a transient Id1 up-regulation. In contrast, BMP-7 exclusively activated Smad1/Smad5 resulting in a more prolonged Id1 expression. Although BMP-7 had no impact on collagen I abundance, it antagonized TGF-beta 1-induced collagen I expression and (CAGA) 12-MLP-Luc activity, effects that are mediated by the ALK-5/Smad3 pathway. Finally, we found that the transient overexpression of endoglin, previously shown to inhibit TGF-beta 1-induced ALK-5/Smad3 signaling, enhanced the BMP-7/Smad1/Smad5 pathway.