Epithelium-specific adenoviral transfer of a dominant-negative mutant TGF-β type II receptor stimulates embryonic lung branching morphogenesis in culture and potentiates EGF and PDGF-AA
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Although exogenous transforming growth factor-beta (TGF-beta) is known to inhibit branching morphogenesis in mouse embryonic lungs in culture, whether the principal negative function of endogenous TGF-beta signaling resides in lung epithelium or mesenchyme remains unresolved. A recombinant adenovirus was constructed, containing a mutated human TGF-beta type II receptor with a truncated cytoplasmic kinase domain. We examined whether this dominant-negative receptor could abolish epithelium-specific endogenous TGF-beta signaling. We introduced the recombinant adenovirus into lung explants via intra-tracheal micro-injection. This resulted in over-expression of exogenous truncated TGF-beta type II receptor only in airway epithelium, not in mesenchyme, as assessed by mRNA level and protein localization. Blockade of endogenous TGF-beta receptor signaling in epithelial endoderm by the mutated dominant-negative TGF-beta type II receptor resulted in significant (65%) stimulation of epithelial branching morphogenesis, while exogenous TGF-beta no longer downregulated epithelial PCNA immunoreactivity and surfactant protein C (SP-C) expression. Additionally, the mitogenic responses to epidermal growth factor (EGF) and platelet-derived growth factor, PDGF-AA were potentiated by 33 and 31%, respectively. We conclude that epithelium-specific adenovirus-mediated over-expression of a dominant-negative TGF-beta type II receptor completely and specifically abolished the anti-proliferative effects of both endogenous and exogenous TGF-beta. Therefore, epithelium-specific TGF-beta signaling is sufficient to negatively regulate embryonic lung-branching morphogenesis in culture. We speculate that abrogation of TGF-beta signaling stimulates lung morphogenesis by potentiating the inductive and permissive effects of other endogenous peptide growth factors such as EGF and PDGF-AA.