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Fate-determining mechanisms in epithelial-myofibroblast transition: major inhibitory role for Smad3.

Masszi A, Speight P, Charbonney E, Lodyga M, Nakano H, Szászi K, Kapus A - J. Cell Biol. (2010)

Bottom Line: Because the SMA promoter harbors both MRTF-responsive CC(A/T)-rich GG element (CArG) boxes and TGF-beta-responsive Smad-binding elements, we hypothesized that the myogenic program is mobilized by a synergy between MRTF and Smad3.Furthermore, Smad3 is degraded under two-hit conditions, thereby liberating the myogenic program.Thus, Smad3 is a critical timer/delayer of MF commitment in the epithelium, and EMyT can be dissected into Smad3-promoted (mesenchymal) and Smad3-inhibited (myogenic) phases.

View Article: PubMed Central - HTML - PubMed

Affiliation: Keenan Research Centre, Li Ka Shing Knowledge Institute, University of Toronto, Toronto, Ontario M5B 1W8, Canada.

ABSTRACT
Epithelial-myofibroblast (MF) transition (EMyT) is a critical process in organ fibrosis, leading to alpha-smooth muscle actin (SMA) expression in the epithelium. The mechanism underlying the activation of this myogenic program is unknown. We have shown previously that both injury to intercellular contacts and transforming growth factor beta (TGF-beta) are indispensable for SMA expression (two-hit model) and that contact disruption induces nuclear translocation of myocardin-related transcription factor (MRTF). Because the SMA promoter harbors both MRTF-responsive CC(A/T)-rich GG element (CArG) boxes and TGF-beta-responsive Smad-binding elements, we hypothesized that the myogenic program is mobilized by a synergy between MRTF and Smad3. In this study, we show that the synergy between injury and TGF-beta exclusively requires CArG elements. Surprisingly, Smad3 inhibits MRTF-driven activation of the SMA promoter, and Smad3 silencing renders injury sufficient to induce SMA expression. Furthermore, Smad3 is degraded under two-hit conditions, thereby liberating the myogenic program. Thus, Smad3 is a critical timer/delayer of MF commitment in the epithelium, and EMyT can be dissected into Smad3-promoted (mesenchymal) and Smad3-inhibited (myogenic) phases.

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Smad3 is a critical delayer/timer of the MF commitment in epithelial cells. (A) Potential or tested consequences of various bilateral interactions among Smad3, MRTF, and SRF. (B) New model dissecting the process of EMyT into a Smad3-dependent (early/mesenchymal) and -independent/inhibited (late/myogenic) phase, and the proposed role of the Smad3–MRTF interaction or the lack thereof in various key events of EMyT.
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fig9: Smad3 is a critical delayer/timer of the MF commitment in epithelial cells. (A) Potential or tested consequences of various bilateral interactions among Smad3, MRTF, and SRF. (B) New model dissecting the process of EMyT into a Smad3-dependent (early/mesenchymal) and -independent/inhibited (late/myogenic) phase, and the proposed role of the Smad3–MRTF interaction or the lack thereof in various key events of EMyT.

Mentions: We identified a 7-aa segment within the B1 region of MRTF-B, which is critical both for the MRTF–Smad3 binding and for the efficient inhibition of the MRTF-triggered SMA promoter by Smad3. The simplest interpretation of our data is that direct binding between Smad3 and MRTF inhibits the interaction between MRTF and the CArG box–SRF complex (Fig. 9 A). Consistent with such mechanism (a), the binding sites for Smad3 and SRF on MRTF are adjacent, (b) the MRTF–SRF association inversely correlates with Smad3 expression (Fig. 7), and (c) Smad3 down-regulation enhances MRTF binding to the CArG boxes of the endogenous SMA promoter (Fig. 5). A possible additional mechanism invokes that SRF can directly bind to Smad3 (Lee et al., 2007), which may also inhibit the SRF–MRTF association (Fig. 9).


Fate-determining mechanisms in epithelial-myofibroblast transition: major inhibitory role for Smad3.

Masszi A, Speight P, Charbonney E, Lodyga M, Nakano H, Szászi K, Kapus A - J. Cell Biol. (2010)

Smad3 is a critical delayer/timer of the MF commitment in epithelial cells. (A) Potential or tested consequences of various bilateral interactions among Smad3, MRTF, and SRF. (B) New model dissecting the process of EMyT into a Smad3-dependent (early/mesenchymal) and -independent/inhibited (late/myogenic) phase, and the proposed role of the Smad3–MRTF interaction or the lack thereof in various key events of EMyT.
© Copyright Policy - openaccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC2819691&req=5

fig9: Smad3 is a critical delayer/timer of the MF commitment in epithelial cells. (A) Potential or tested consequences of various bilateral interactions among Smad3, MRTF, and SRF. (B) New model dissecting the process of EMyT into a Smad3-dependent (early/mesenchymal) and -independent/inhibited (late/myogenic) phase, and the proposed role of the Smad3–MRTF interaction or the lack thereof in various key events of EMyT.
Mentions: We identified a 7-aa segment within the B1 region of MRTF-B, which is critical both for the MRTF–Smad3 binding and for the efficient inhibition of the MRTF-triggered SMA promoter by Smad3. The simplest interpretation of our data is that direct binding between Smad3 and MRTF inhibits the interaction between MRTF and the CArG box–SRF complex (Fig. 9 A). Consistent with such mechanism (a), the binding sites for Smad3 and SRF on MRTF are adjacent, (b) the MRTF–SRF association inversely correlates with Smad3 expression (Fig. 7), and (c) Smad3 down-regulation enhances MRTF binding to the CArG boxes of the endogenous SMA promoter (Fig. 5). A possible additional mechanism invokes that SRF can directly bind to Smad3 (Lee et al., 2007), which may also inhibit the SRF–MRTF association (Fig. 9).

Bottom Line: Because the SMA promoter harbors both MRTF-responsive CC(A/T)-rich GG element (CArG) boxes and TGF-beta-responsive Smad-binding elements, we hypothesized that the myogenic program is mobilized by a synergy between MRTF and Smad3.Furthermore, Smad3 is degraded under two-hit conditions, thereby liberating the myogenic program.Thus, Smad3 is a critical timer/delayer of MF commitment in the epithelium, and EMyT can be dissected into Smad3-promoted (mesenchymal) and Smad3-inhibited (myogenic) phases.

View Article: PubMed Central - HTML - PubMed

Affiliation: Keenan Research Centre, Li Ka Shing Knowledge Institute, University of Toronto, Toronto, Ontario M5B 1W8, Canada.

ABSTRACT
Epithelial-myofibroblast (MF) transition (EMyT) is a critical process in organ fibrosis, leading to alpha-smooth muscle actin (SMA) expression in the epithelium. The mechanism underlying the activation of this myogenic program is unknown. We have shown previously that both injury to intercellular contacts and transforming growth factor beta (TGF-beta) are indispensable for SMA expression (two-hit model) and that contact disruption induces nuclear translocation of myocardin-related transcription factor (MRTF). Because the SMA promoter harbors both MRTF-responsive CC(A/T)-rich GG element (CArG) boxes and TGF-beta-responsive Smad-binding elements, we hypothesized that the myogenic program is mobilized by a synergy between MRTF and Smad3. In this study, we show that the synergy between injury and TGF-beta exclusively requires CArG elements. Surprisingly, Smad3 inhibits MRTF-driven activation of the SMA promoter, and Smad3 silencing renders injury sufficient to induce SMA expression. Furthermore, Smad3 is degraded under two-hit conditions, thereby liberating the myogenic program. Thus, Smad3 is a critical timer/delayer of MF commitment in the epithelium, and EMyT can be dissected into Smad3-promoted (mesenchymal) and Smad3-inhibited (myogenic) phases.

Show MeSH
Related in: MedlinePlus