title: Nuclear function of Smad7 promotes myogenesis.
abstract: In the "canonical" view of transforming growth factor beta (TGF-beta) signaling, Smad7 plays an inhibitory role. While Smad7 represses Smad3 activation by TGF-beta, it does not reverse the inhibitory effect of TGF-beta on myogenesis, suggesting a different function in myogenic cells. We previously reported a promyogenic role of Smad7 mediated by an interaction with MyoD. Based on this association, we hypothesized a possible nuclear function of Smad7 independent of its role at the level of the receptor. We therefore engineered a chimera of Smad7 with a nuclear localization signal (NLS), which serves to prevent and therefore bypass binding to the TGF-beta receptor while concomitantly constitutively localizing Smad7 to the nucleus. This Smad7-NLS did not repress Smad3 activation by TGF-beta but did retain its ability to enhance myogenic gene activation and phenotypic myogenesis, indicating that the nuclear, receptor-independent function of Smad7 is sufficient to promote myogenesis. Furthermore, Smad7 physically interacts with MyoD and antagonizes the repressive effects of active MEK on MyoD. Reporter and myogenic conversion assays indicate a pivotal regulation of MyoD transcriptional properties by the balance between Smad7 and active MEK. Thus, Smad7 has a nuclear coactivator function that is independent of TGF-beta signaling and necessary to promote myogenic differentiation.
authors.affiliation: Department of Biology, York University, St., Toronto, Ontario M3J 1P3, Canada.
authors.forename: Tetsuaki
authors.initials: T
authors.lastname: Miyake
authors.forename: Nezeka S
authors.initials: NS
authors.lastname: Alli
authors.forename: John C
authors.initials: JC
authors.lastname: McDermott
mesh_terms: D000818:Animals; D002454:Cell Differentiation / Q000502:physiology; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism*; D016207:Cytokines / Q000378:metabolism; D020930:MAP Kinase Kinase Kinases / Q000378:metabolism; D051379:Mice; D024510:Muscle Development / Q000502:physiology*; D009132:Muscles / Q000378:metabolism; D017570:MyoD Protein / Q000378:metabolism; D019913:Nuclear Localization Signals / Q000378:metabolism*; D018125:Receptors, Transforming Growth Factor beta / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D051900:Smad3 Protein / Q000378:metabolism; D051906:Smad7 Protein / Q000378:metabolism*; D014162:Transfection; D016212:Transforming Growth Factor beta / Q000378:metabolism*
chemical_list: D016207:Cytokines; D017570:MyoD Protein; C057363:MyoD1 myogenic differentiation protein; D019913:Nuclear Localization Signals; D018125:Receptors, Transforming Growth Factor beta; D011993:Recombinant Fusion Proteins; D051900:Smad3 Protein; C497791:Smad3 protein, mouse; D051906:Smad7 Protein; C497807:Smad7 protein, mouse; D016212:Transforming Growth Factor beta; C092073:cardiotrophin 1; D020930:MAP Kinase Kinase Kinases