Interleukin-7 and transforming growth factor-beta play counter-regulatory roles in protein kinase C-delta-dependent control of fibroblast collagen synthesis in pulmonary fibrosis

Transforming growth factor-beta (TGF-beta) is a potent fibrogenic factor responsible for promoting synthesis of extracellular matrix. Interleukin-7 (IL-7) inhibits TGF-beta signaling by up-regulating Smad7, a major inhibitor of the Smad family. In a variety of cells, TGF-beta-mediated activation of target genes requires active protein kinase C-delta (PKC-delta) in addition to Smads (1). We determined the role of PKC-delta in the regulation of pulmonary fibroblast collagen synthesis in response to TGF-beta and IL-7 stimulation. Here we show that TGF-beta and IL-7 have opposing effects on PKC-delta; TGF-beta stimulates, while IL-7 inhibits, PKC-delta activity. IL-7 inhibits TGF-beta-induced PKC-delta phosphorylation at Ser-645 and Thr-505. Inhibition of PKC-delta with specific small inhibitory RNA restores TGF-beta-mediated induction of Smad7 and in parallel significantly reduces TGF-beta-mediated collagen synthesis. Thus, PKC-delta may play a critical role in the pathogenesis of pulmonary fibrosis and may serve as a molecular target for therapeutic intervention to suppress fibrosis.     

Ectopic expression of Smad7 inhibits transforming growth factor-beta responses in vascular smooth muscle cells.

Vascular injury stimulates the cytokine-growth factor network in the vascular wall, including transforming growth factor-beta (TGF-beta). Reportedly, the intracellular signaling of TGF-beta is mediated by Smad proteins. We tested the effects of the ectopic expression of inhibitory Smads in cultured rat smooth muscle cells (SMC) to identify the role of TGF-beta/Smad signaling on the phenotypic modulation of SMC. The cells exposed to human recombinant TGF-beta1 (10 ng/ml) were stimulated Smad2 phosphorylation. Infection with the replication-deficient adenovirus vector expressing Smad7, but not bacterial beta-galactosidase or Smad6, was found to inhibit TGF-beta-induced Smad2 phosphorylation in a dose-dependent manner. TGF-beta suppressed the serum-induced proliferation of SMC from 36.3% to 51.0% (p<0.01), as measured by hand-counting, and this inhibition was attenuated by the ectopic expression of Smad7 (from 30.7% to 74.8% of the reduction of TGF-beta-response, p<0.05), but not Smad6. A BrdU incorporation assay also showed that TGF-beta-mediated growth inhibition was attenuated by exogenous Smad7 and that this inhibition can be reversed by an additional expression of exogenous Smad2. TGF-beta increased the expression of alpha-smooth muscle actin and myosin heavy chain by 1.3-fold and 1.6-fold in comparison to the control, respectively, and these increases were attenuated by exogenous Smad7, but not Smad6. Our data indicate that Smads mediate TGF-beta responses on SMC phenotypes. Smad7, but not Smad6, may specifically act as an inhibitor of TGF-beta responses.     

The integrin-coupled signaling adaptor p130Cas suppresses Smad3 function in transforming growth factor-beta signaling

Reciprocal cooperative signaling by integrins and growth factor receptors at G1 phase during cell cycle progression is well documented. By contrast, little is known about the cross-talk between integrin and transforming growth factor (TGF)-beta signaling. Here, we show that integrin signaling counteracts the inhibitory effects of TGF-beta on cell growth and that this effect is mediated by p130Cas (Crk-associated substrate, 130 kDa). Adhesion to fibronectin or laminin reduces TGF-beta-induced Smad3 phosphorylation and thus inhibits TGF-beta-mediated growth arrest; loss of p130Cas abrogates these effects. Loss and gain of function studies demonstrated that, once tyrosine-phosphorylated via integrin signaling, p130Cas binds to Smad3 and reduces phosphorylation of Smad3. That in turn leads to inhibition of p15 and p21 expression and facilitation of cell cycle progression. Thus, p130Cas-mediated control of TGF-beta/Smad signaling may provide an additional clue to the mechanism underlying resistance to TGF-beta-induced growth inhibition.     

Smad7 and Smad6 differentially modulate transforming growth factor beta -induced inhibition of embryonic lung morphogenesis

Transforming growth factors beta (TGF-beta) are known negative regulators of lung development, and excessive TGF-beta production has been noted in pulmonary hypoplasia associated with lung fibrosis. Inhibitory Smad7 was recently identified to antagonize TGF-beta family signaling by interfering with the activation of TGF-beta signal-transducing Smad complexes. To investigate whether Smad7 can regulate TGF-beta-induced inhibition of lung morphogenesis, ectopic overexpression of Smad7 was introduced into embryonic mouse lungs in culture using a recombinant adenovirus containing Smad7 cDNA. Although exogenous TGF-beta efficiently reduced epithelial lung branching morphogenesis in control virus-infected lung culture, TGF-beta-induced branching inhibition was abolished after epithelial transfer of the Smad7 gene into lungs in culture. Smad7 also prevented TGF-beta-mediated down-regulation of surfactant protein C gene expression, a marker of bronchial epithelial differentiation, in cultured embryonic lungs. Moreover, we found that Smad7 transgene expression blocked Smad2 phosphorylation induced by exogenous TGF-beta ligand in lung culture, indicating that Smad7 exerts its inhibitory effect on both lung growth and epithelial cell differentiation through modulation of TGF-beta pathway-restricted Smad activity. However, the above anti-TGF-beta signal transduction effects were not observed in cultured embryonic lungs with Smad6 adenoviral gene transfer, suggesting that Smad7 and Smad6 differentially regulate TGF-beta signaling in developing lungs. Our data therefore provide direct evidence that Smad7, but not Smad6, prevents TGF-beta-mediated inhibition of both lung branching morphogenesis and cytodifferentiation, establishing the mechanistic basis for Smad7 as a novel target to ameliorate aberrant TGF-beta signaling during lung development, injury, and repair.     

Antagonistic effects of Smad2 versus Smad7 are sensitive to their expression level during tooth development

Members of the transforming growth factor-beta (TGF-beta) superfamily regulate cell proliferation, differentiation, and apoptosis, controlling the development and maintenance of most tissues. TGF-beta signal is transmitted through the phosphorylation of Smad proteins by TGF-beta receptor serine/threonine kinase. During early tooth development, TGF-beta inhibits proliferation of enamel organ epithelial cells but the underlying molecular mechanisms are largely unknown. Here we tested the hypothesis that antagonistic effects between Smad2 and Smad7 regulate TGF-beta signaling during tooth development. Attenuation of Smad2 gene expression resulted in significant advancement of embryonic tooth development with increased proliferation of enamel organ epithelial cells, while attenuation of Smad7 resulted in significant inhibition of embryonic tooth development with increased apoptotic activity within enamel organ epithelium. These findings suggest that different Smads may have differential activities in regulating TGF-beta-mediated cell proliferation and death. Furthermore, functional haploinsufficiency of Smad2, but not Smad3, altered TGF-beta-mediated tooth development. The results indicate that Smads are critical factors in orchestrating TGF-beta-mediated gene regulation during embryonic tooth development. The effectiveness of TGF-beta signaling is highly sensitive to the level of Smad gene expression.     

Adenovirus-mediated gene transfer of dominant-negative Smad4 blocks TGF-beta signaling in pancreatic acinar cells

Transforming growth factor-beta (TGF-beta) is a potent inhibitor of pancreatic acinar cell growth. Smad4 is a central mediator in the TGF-beta signaling pathway. To study the effect of Smad4 on pancreatic growth, cell cycle protein expression, and the expression of a TGF-beta-responsive promoter in vitro, we constructed an adenovirus containing dominant-negative COOH terminal truncated Smad4 (AddnSmad4) downstream of the rat elastase promoter. Acinar cells expressed dominant-negative Smad4 within 8 h after infection, and expression persisted for 72 h. Mouse pancreatic acini were infected with either AddnSmad4 or control adenovirus expressing green fluorescent protein, and TGF-beta was added 8 h after infection. Acinar cells were then incubated for 1, 2, or 3 days, and [(3)H]thymidine incorporation was determined. AddnSmad4 significantly reduced TGF-beta inhibition of [(3)H]thymidine incorporation, with maximal effects on day 3. AddnSmad4 also completely blocked TGF-beta-mediated growth inhibition in the presence of basic fibroblast growth factor. We next examined the effects of AddnSmad4 on TGF-beta-induced expression of the cell cycle regulatory proteins p21(Cip1) and p27(Kip1). TGF-beta induced upregulation of p21(Cip1), which was completely blocked by AddnSmad4. AddnSmad4 also inhibited TGF-beta-induced expression of the TGF-beta-responsive luciferase reporter 3TP-Lux. These results show that Smad4 is essential in TGF-beta-mediated signaling in pancreatic acinar cells.