Id2 and Id3 define the potency of cell proliferation and differentiation responses to transforming growth factor beta and bone morphogenetic protein

Transforming growth factors beta (TGF-betas) inhibit growth of epithelial cells and induce differentiation changes, such as epithelial-mesenchymal transition (EMT). On the other hand, bone morphogenetic proteins (BMPs) weakly affect epithelial cell growth and do not induce EMT. Smad4 transmits signals from both TGF-beta and BMP pathways. Stimulation of Smad4-deficient epithelial cells with TGF-beta 1 or BMP-7 in the absence or presence of exogenous Smad4, followed by cDNA microarray analysis, revealed 173 mostly Smad4-dependent, TGF-beta-, or BMP-responsive genes. Among 25 genes coregulated by both factors, inhibitors of differentiation Id2 and Id3 showed long-term repression by TGF-beta and sustained induction by BMP. The opposing regulation of Id genes is critical for proliferative and differentiation responses. Hence, ectopic Id2 or Id3 expression renders epithelial cells refractory to growth inhibition and EMT induced by TGF-beta, phenocopying the BMP response. Knockdown of endogenous Id2 or Id3 sensitizes epithelial cells to BMP, leading to robust growth inhibition and induction of transdifferentiation. Thus, Id genes sense Smad signals and create a permissive or refractory nuclear environment that defines decisions of cell fate and proliferation.     

BMP-7 opposes TGF-beta1-mediated collagen induction in mouse pulmonary myofibroblasts through Id2

Mesenchymal cells, primarily fibroblasts and myofibroblasts, are the principal matrix-producing cells during pulmonary fibrogenesis. Transforming growth factor (TGF)-beta signaling plays an important role in stimulating the expression of type I collagen of these cells. Bone morphogenetic protein (BMP)-7, a member of the TGF-beta superfamily, has been reported to oppose the fibrogenic activity of TGF-beta1. Here, we have addressed the effects of BMP-7 on the fibrogenic activity of pulmonary myofibroblasts. We first established cell lines from the lungs of transgenic mice harboring the COL1A2 upstream sequence fused to luciferase. They displayed a spindle shape and expressed vimentin and alpha-smooth muscle actin, but not E-cadherin. COL1A2 promoter activity was dose dependently induced by TGF-beta1, which was further augmented by adenoviral overexpression of Smad3, but was downregulated by Smad7. Under the identical condition, adenoviral overexpression of BMP-7 attenuated the TGF-beta1-dependent COL1A2 promoter activity. By immunocytochemistry, the ectopic expression of BMP-7 led to the nuclear localization of phospho-Smad1/5/8 and suppressed that of Smad3. BMP-7 suppressed the expression of mRNAs for COL1A2 and tissue inhibitor of metalloproteinase-2 while increasing those of inhibitors of differentiation (Id) 2 and 3. Ectopic expression of Id2 and Id3 was found to decrease the COL1A2 promoter activity. Finally, BMP-7 and Id2 decreased TGF-beta1-dependent collagen protein secretion. In conclusion, these data demonstrate that BMP-7 antagonizes the TGF-beta1-dependent fibrogenic activity of mouse pulmonary myofibroblastic cells by inducing Id2 and Id3.     

A role for Id in the regulation of TGF-beta-induced epithelial-mesenchymal transdifferentiation

Epithelial-mesenchymal transdifferentiation (EMT) is a critical morphogenic event that occurs during embryonic development and during the progression of various epithelial tumors. EMT can be induced by transforming growth factor (TGF)-beta in mouse NMuMG mammary epithelial cells. Here, we demonstrate a central role of helix-loop-helix factors, E2A and inhibitor of differentiation (Id) proteins, in TGF-beta-induced EMT. Epithelial cells ectopically expressing E2A adopt a fibroblastic phenotype and acquire migratory/invasive properties, concomitant with the suppression of E-cadherin expression. Id proteins interacted with E2A proteins and antagonized E2A-dependent suppression of the E-cadherin promoter. Levels of Id proteins were dramatically decreased by TGF-beta. Moreover, NMuMG cells overexpressed Id2 showed partial resistance to TGF-beta-induced EMT. Id proteins thus inhibit the action of E2A proteins on the expression of E-cadherin, but after TGF-beta stimulation, E2A proteins are present in molar excess of the Id proteins, thus over-riding their inhibitory function and leading to EMT.     

BMP-7 Does Not Protect against Bleomycin-Induced Lung or Skin Fibrosis

Bone morphogenic protein (BMP)-7 is a member of the BMP family which are structurally and functionally related, and part of the TGFβ super family of growth factors. BMP-7 has been reported to inhibit renal fibrosis and TGFβ1-induced epithelial-mesenchymal transition (EMT), in part through negative interactions with TGFβ1 induced Smad 2/3 activation. We utilized in vivo bleomycin-induced fibrosis models in the skin and lung to determine the potential therapeutic effect of BMP-7. We then determined the effect of BMP-7 on TGFβ1-induced EMT in lung epithelial cells and collagen production by human lung fibroblasts. We show that BMP-7 did not affect bleomycin-induced fibrosis in either the lung or skin in vivo; had no effect on expression of pro-fibrotic genes by human lung fibroblasts, either at rest or following exposure to TGFβ1; and did not modulate TGFβ1 -induced EMT in human lung epithelial cells. Taken together our data indicates that BMP-7 has no anti-fibrotic effect in lung or skin fibrosis either in vivo or in vitro. This suggests that the therapeutic options for BMP-7 may be confined to the renal compartment.     

BMP-7/TGF-β1 signalling in myoblasts: components involved in signalling and BMP-7-dependent blockage of TGF-β-mediated CTGF expression

We and others have recently described the antagonistic role of Bone morphogenetic protein-7 (BMP-7) in TGF-β signalling and myogenic differentiation. To specify the underlying mechanism(s), we here analysed the expression and function of the individual components mediating TGF-β1 and BMP-7 responses. We found that BMP-7 at a concentration of 25 ng/ml induces signalling exclusively via ALK2 and ALK3 leading to the activation of Smad1 and Smad5 and subsequent expression of Id proteins. In contrast, low doses of TGF-β1 (0.1 ng/ml) lead to an exclusive activation of ALK5 and phosphorylation of Smad2 and Smad3 that regulate specific target genes including connective tissue growth factor (CTGF). CTGF is rapidly induced by TGF-β1 already 1h after stimulation and reduced by BMP-7 application. Smad1/Smad5 or Id1/2 overexpression reduced the TGF-β1-mediated expression of CTGF. However, although siRNA-mediated knock down of Alk2/3 or Smad1/5 counteracts the BMP-7 effect on basal CTGF expression there was no consistent reversion of the observed BMP-7 effect on TGF-β1-mediated CTGF expression. Moreover, ALK5 inhibition using the SB431542 inhibitor significantly affected CTGF expression only at later time points whereas ERK1/2 inhibition completely abrogated CTGF expression. These findings point towards a regulatory role of BMP-7 that relies on modulation of Mitogen-activated protein kinases rather than mechanisms that are exclusively driven by differential Smad activation.     

Endoglin differentially modulates antagonistic transforming growth factor-beta1 and BMP-7 signaling

Transforming growth factor-beta1 (TGF-beta1) 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-beta1 signaling and the role of endoglin for both pathways in endoglin-deficient L(6)E(9) cells. In this myoblastic cell line, TGF-beta1 and BMPs are opposing cytokines, interfering with myogenic differentiation. Both induce specific target genes of which Id1 (for BMPs) and collagen I (for TGF-beta1) are two examples. TGF-beta1 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-beta1-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-beta1-induced ALK-5/Smad3 signaling, enhanced the BMP-7/Smad1/Smad5 pathway.     

MicroRNA-486-5p suppresses TGF-β<Subscript>2</Subscript>-induced proliferation,invasion and epithelial–mesenchymal transition of lens epithelial cells by targeting Smad2

The pathological development of lens epithelial cells (LECs) leads to posterior capsular opacification (PCO). This study was undertaken to investigate the effects of microRNA-486-5p (miR-486-5p) on TGF-β₂-induced proliferation, invasion and epithelial-mesenchymal transition (EMT) in the lens epithelial cell line SRA01/04, and to explore the underlying molecular mechanisms. The expression of miR-486-5p in TGF-β₂-induced SRA01/04 cells was down-regulated, and the expression of Smad2, p-Smad2 and p-Smad3 was up-regulated. A dual-luciferase reporter assay revealed that miR-486-5p directly targets the 3′-UTR of Smad2. MiR-486-5p mimic transfection markedly down-regulated the expression levels of Smad2, thus inhibiting the expression of p-Smad2 and p-Smad3. MiR-486-5p overexpression in SRA01/04 cells markedly suppressed TGF-β₂-induced proliferation and invasion, inhibited protein expression of CDK2 and CDK4, down-regulated fibronectin, α-SMA and vimentin and up-regulated E-cadherin; these effects were partly reversed by Smad2 overexpression. In short, these data show that miR-486-5p overexpression can inhibit TGF-β₂-induced proliferation, invasion and EMT in SRA01/04 cells by repressing Smad2/Smad3 signalling, implying that miR-486-5p may be an effective target to interfere in the progression of PCO.     

Rho activation is required for transforming growth factor-beta-induced epithelial-mesenchymal transition in lens epithelial cells

Lens epithelial cells undergo epithelial-mesenchymal transition (EMT) after injury as in cataract extraction, leading to fibrosis of the lens capsule. We have recently shown that TGF-beta-induced EMT in lens epithelial cells depends on PI3 kinase/Akt signal pathway. In this report, we suggest Smad3 is necessary for TGF-beta-induced EMT by showing that the expression of dominant-negative Smad3 blocks the expression of alpha-smooth muscle actin (alpha-SMA) and morphological changes. We also show that TGF-beta induces a biphasic change in Rho activity, and that Y27632, a selective inhibitor of Rho effector ROCK, inhibits TGF-beta-induced EMT in vitro and in vivo. We finally show that Smad3 activation and Rho signal activation is independent each other. All of these findings suggest that Rho/ROCK activation together with Smad3 is necessary for TGF-beta-induced EMT in lens epithelial cells.     

BMP-7 counteracts TGF-beta1-induced epithelial-to-mesenchymal transition and reverses chronic renal injury

Bone morphogenic protein (BMP)-7 is a 35-kDa homodimeric protein and a member of the transforming growth factor (TGF)-beta superfamily. BMP-7 expression is highest in the kidney, and its genetic deletion in mice leads to severe impairment of eye, skeletal and kidney development. Here we report that BMP-7 reverses TGF-beta1-induced epithelial-to-mesenchymal transition (EMT) by reinduction of E-cadherin, a key epithelial cell adhesion molecule. Additionally, we provide molecular evidence for Smad-dependent reversal of TGF-beta1-induced EMT by BMP-7 in renal tubular epithelial cells and mammary ductal epithelial cells. In the kidney, EMT-induced accumulation of myofibroblasts and subsequent tubular atrophy are considered key determinants of renal fibrosis during chronic renal injury. We therefore tested the potential of BMP-7 to reverse TGF-beta1-induced de novo EMT in a mouse model of chronic renal injury. Our results show that systemic administration of recombinant human BMP-7 leads to repair of severely damaged renal tubular epithelial cells, in association with reversal of chronic renal injury. Collectively, these results provide evidence of cross talk between BMP-7 and TGF-beta1 in the regulation of EMT in health and disease.     

Snail is required for transforming growth factor-beta-induced epithelial-mesenchymal transition by activating PI3 kinase/Akt signal pathway

Lens epithelial cells undergo epithelial-mesenchymal transition (EMT) after injury as in cataract extraction, leading to fibrosis of the lens capsule. We have previously shown that EMT of primary lens epithelial cells in vitro depends on TGF-beta expression and more specifically, on signaling via Smad3. In this report, we suggest phosphatidylinositol 3-OH kinase (PI3K)/Akt signaling is also necessary for TGF-beta-induced EMT in lens epithelial cells by showing that LY294002, an inhibitor of the p110 catalytic subunit of PI3K, blocked the expression of alpha-smooth muscle actin (alpha-SMA) and morphological changes. We also identify Snail as an effector of TGF-beta-induced EMT. Snail has been shown to be a mediator of EMT during metastasis of cancer. We show that Snail is an immediate-early response gene for TGF-beta and the proximal Snail promoter is activated by TGF-beta through the action of Smad2, 3, and 4. We show that antisense inhibition of Snail expression blocks TGF-beta-induced EMT and furthermore Akt activation. All of these findings suggest that Snail participates in TGF-beta-induced EMT by acting upstream of Akt activation.