Decoding the quantitative nature of TGF-beta/Smad signaling

How transforming growth factor-beta (TGF-beta) signaling elicits diverse cell responses remains elusive, despite the major molecular components of the pathway being known. We contend that understanding TGF-beta biology requires mathematical models to decipher the quantitative nature of TGF-beta/Smad signaling and to account for its complexity. Here, we review mathematical models of TGF-beta superfamily signaling that predict how robustness is achieved in bone-morphogenetic-protein signaling in the Drosophila embryo, how changes in receptor-trafficking dynamics can be exploited by cancer cells and how the basic mechanisms of TGF-beta/Smad signaling conspire to promote Smad accumulation in the nucleus. These studies demonstrate the power of mathematical modeling for understanding TGF-beta biology.     

TGF-beta and Smad3 signaling link inflammation to chronic fibrogenesis

Transient adenovirus-mediated gene transfer of IL-1beta (AdIL-1beta), a proinflammatory cytokine, induces marked inflammation and severe and progressive fibrosis in rat lungs. This is associated with an increase in TGF-beta1 concentration in bronchoalveolar lavage (BAL) fluid. TGF-beta1 is a key cytokine in the process of fibrogenesis, using intracellular signaling pathways involving Smad2 and Smad3. In this study we investigate whether inflammation induced by IL-1beta is able to independently induce lung fibrosis in mice deficient in the Smad3 gene. Seven days after AdIL-1beta administration, similar levels of IL-1beta transgene are seen in BAL in both wild-type (WT) and knockout (KO) mice, and BAL cell profiles demonstrated a similar marked neutrophilic inflammation. Phospho-Smad2 staining was positive in areas of inflammation in both WT and KO mice at day 7. By day 35 after transient IL-1beta expression, WT mice showed marked fibrosis in peribronchial areas, quantified by picrosirius red staining and morphometry. However, there was no evidence of fibrosis or collagen accumulation in IL-1beta-treated KO mice, and peribronchial areas were not different from KO mice treated with the control adenovector. TGF-beta1 and phospho-Smad2 were strongly positive at day 35 in fibrotic areas observed in WT mice, but no such staining was detectable in KO mice. The IL-1beta-induced chronic fibrotic response in mouse lungs is dependent on Smad3. KO and WT animals demonstrated a similar inflammatory response to overexpression of IL-1beta indicating that inflammation must link to the Smad3 pathway, likely through TGF-beta, to induce progressive fibrosis.     

Roles of autocrine TGF-beta receptor and Smad signaling in adipocyte differentiation

TGF-beta inhibits adipocyte differentiation, yet is expressed by adipocytes. The function of TGF-beta in adipogenesis, and its mechanism of action, is unknown. To address the role of TGF-beta signaling in adipocyte differentiation, we characterized the expression of the TGF-beta receptors, and the Smads which transmit or inhibit TGF-beta signals, during adipogenesis in 3T3-F442A cells. We found that the cell-surface availability of TGF-beta receptors strongly decreased as adipogenesis proceeds. Whereas mRNA levels for Smads 2, 3, and 4 were unchanged during differentiation, mRNA levels for Smads 6 and 7, which are known to inhibit TGF-beta responses, decreased severely. Dominant negative interference with TGF-beta receptor signaling, by stably expressing a truncated type II TGF-beta receptor, enhanced differentiation and decreased growth. Stable overexpression of Smad2 or Smad3 inhibited differentiation and dominant negative inhibition of Smad3 function, but not Smad2 function, enhanced adipogenesis. Increased Smad6 and Smad7 levels blocked differentiation and enhanced TGF-beta-induced responses. The inhibitory effect of Smad7 on adipocyte differentiation and its cooperation with TGF-beta was associated with the C-domain of Smad7. Our results indicate that endogenous TGF-beta signaling regulates the rate of adipogenesis, and that Smad2 and Smad3 have distinct functions in this endogenous control of differentiation. Smad6 and Smad7 act as negative regulators of adipogenesis and, even though known to inhibit TGF-beta responses, enhance the effects of TGF-beta on these cells.     

Osx transcriptional regulation is mediated by additional pathways to BMP2/Smad signaling

Bone morphogenetic protein (BMP)-2 induces Osterix (Osx) in mouse C2C12 cells and chondrocytes. Genetic studies place Osx downstream to the BMP-2/Smad/Runx2 signaling pathway; however, limited information is available on the mediators of Osx expression in osteoblast lineage commitment. Several lines of research implicate the presence of Runx2-independent ossification. Therefore, the purpose of this study was to identify possible mediators of Osx expression beyond the BMP-2/Smad pathway. Using real-time RT-PCR, we showed upregulation of Osx in response to BMP-2 in human mesenchymal stem cells (hMSC). Insulin-like growth factor (IGF)-I upregulated Osx, but not Runx2. Further, IGF-I in combination with BMP-2 was synergistic for Osx, suggesting a pathway beyond Smad signaling. MAPK was tested as a common mediator across BMP-2 and IGF-I signaling pathways. Inhibition of MAPK component ERK1/2 did not affect Runx2 gene expression, but inhibited Osx expression and matrix mineralization. BMP-2-mediated Osx expression was downregulated in response to p38 inhibition. We therefore conclude that during osteogenic lineage progression, in addition to the BMP-2/Smad pathway, IGF-I and MAPK signaling may mediate Osx.     

Asiatic acid inhibits liver fibrosis by blocking TGF-beta/Smad signaling in vivo and in vitro

Liver fibrosis is a major cause of liver failure, but treatment remains ineffective. In the present study, we investigated the mechanisms and anti-hepatofibrotic activities of asiatic acid (AA) in a rat model of liver fibrosis induced by carbon tetrachloride (CCl(4)) and in vitro in TGF-beta1-stimulated rat hepatic stellate cell line (HSC-T6). Treatment with AA significantly attenuated CCl(4)-induced liver fibrosis and functional impairment in a dosage-dependent manner, including blockade of the activation of HSC as determined by inhibiting de novo alpha smooth muscle actin (a-SMA) and collagen matrix expression, and an increase in ALT and AST (all p<0.01). The hepatoprotective effects of AA on fibrosis were associated with upregulation of hepatic Smad7, an inhibitor of TGF-beta signaling, thereby blocking upregulation of TGF-beta1 and CTGF and the activation of TGF-beta/Smad signaling. The anti-fibrosis activity and mechanisms of AA were further detected in vitro in HSC-T6. Addition of AA significantly induced Smad7 expression by HSC-T6 cells, thereby inhibiting TGF-beta1-induced Smad2/3 activation, myofibroblast transformation, and collagen matrix expression in a dosage-dependent manner. In contrast, knockdown of Smad7 in HSC-T6 cells prevented AA-induced inhibition of HSC-T6 cell activation and fibrosis in response to TGF-beta1, revealing an essential role for Smad7 in AA-induced anti-fibrotic activities during liver fibrosis in vivo and in vitro. In conclusion, AA may be a novel therapeutic agent for liver fibrosis. Induction of Smad7-dependent inhibition of TGF-beta/Smad-mediated fibrogenesis may be a central mechanism by which AA protects liver from injury.