Regulation of epithelial to mesenchymal transition by bone morphogenetic proteins.

Epithelial to mesenchymal transition (EMT) is a process in which fully differentiated epithelial cells lose many of their epithelial characteristics and adopt features typical of mesenchymal cells, thus allowing cells to become migratory and invasive. EMT is a critical process in development and its role in cancer and fibrosis is becoming increasingly recognised. It is also becoming apparent that EMT is not just restricted to embryonic development and disease in adults, but in fact may be an important process for the maintenance and regeneration of adult tissue architecture. While transforming growth factor-ß (TGF-ß) is considered a prototypic inducer of EMT, relatively little is known about other signalling molecules that regulate EMT. Bone morphogenic proteins (BMPs) are members of the TGF-ß superfamily and 20 different human BMPs have been identified. Originally named for their effects on bone, these proteins are now considered to be key morphogenetic signals that orchestrate tissue architecture throughout the body. BMP2, -4 and -7 are the best studied to date. There are disparate reports of the roles of BMPs in EMT during development, cancer and fibrosis. Here, we present an overview of this literature as well as the emerging role of EMT in tissue regeneration and the involvement of BMPs in regulating this process.

Bone morphogenetic proteins enhance an epithelial-mesenchymal transition in normal airway epithelial cells during restitution of a disrupted epithelium.

BACKGROUND: Mechanisms of airway repair are poorly understood. It has been proposed that, following injury, progenitor populations such as club cells (Clara) become undifferentiated, proliferate and re-differentiate to re-epithelialise the airway. The exact phenotype of such cells during repair is unknown however. We hypothesised that airway epithelial cells (AECs) undergo some degree of epithelial-mesenchymal transition (EMT) in order to migrate over a denuded airway and effect re-epithelialisation. Furthermore, based on our previous findings that BMP signalling is an early event in AECs following injury in vivo and that BMP4 down-regulates E-cadherin expression and enhances migration in AECs in vitro, we hypothesised that BMPs could play a role in inducing such a phenotypic switch. METHODS: Normal AECs were isolated from mouse lungs and analysed in a model of a disrupted epithelium. EMT marker expression and BMP signalling were examined by immunofluorescence, Western blotting and RT-PCR. RESULTS: Following generation of a wound area, AECs at the wound edge migrated and acquired a mesenchymal-like morphology. E-cadherin expression was reduced in migrating cells while vimentin and α-smooth muscle actin (α-SMA) expression was increased. Re-expression of membrane E-cadherin was subsequently observed in some cells in the wound area following re-establishment of the monolayer. A transient increase in the incidence of nuclear phosphorylated Smad1/5/8 was observed in migrating cells compared with confluent cells, indicating active BMP signalling during migration. BMP antagonists noggin and gremlin inhibited cell migration, confirming the involvement of BMP signalling in migration and indicating autocrine signalling, possibly involving BMP7 or BMP4 which were expressed in AECs. Exogenous BMP2, BMP4 and BMP7 induced a mesenchymal-like morphology in AECs, enhanced the rate of cell migration and increased α-SMA protein expression in AECs. CONCLUSIONS: Following disruption of an intact epithelium, migrating AECs at the wound edge acquire an EMT-like phenotype involving altered expression of E-cadherin, vimentin and α-SMA. BMP signalling is involved in AEC migration and is likely to mediate the switch towards an EMT-like phenotype by altering protein expression to facilitate cell migration and wound closure. We propose therefore that acquisition of an EMT-like phenotype by AECs is a normal aspect of wound repair. Furthermore, we suggest that diseases involving fibrosis may arise because the EMT phase of repair is prolonged by chronic injury/inflammation, rather than being caused by it, as is the current paradigm.

Bone morphogenetic protein signalling in airway epithelial cells during regeneration.

Mechanisms of lung regeneration after injury remain poorly understood. Bone morphogenetic protein 4 (BMP4) is critical for lung morphogenesis and regulates differentiation of the airway epithelium during development, although its mechanism of action is unknown. The role of BMPs in adult lungs is unclear. We hypothesised that BMP signalling is involved in regeneration of damaged adult airways after injury. Our aims were to characterise the regeneration process in 1-nitronaphthalene (1-NN) injured airways, to determine if and when BMP signalling is activated during this process and investigate the effects of BMP4 on normal adult airway epithelial cells (AECs). Rats were injected with 50 mg/kg 1-NN and protein expression in AECs was examined by Western blotting of lung lysis lavage, and by immunofluorescence, at 6, 24, 48 and 96 h post injection. Expression of signalling molecules p-ERK-1, p-ERK-2 and p-Smad1/5/8 in AECs peaked at 6 h post injection, coincident with maximal inflammation and prior to airway denudation which occurred at 24 h. While airways were re-epithelialised by 48h, AEC proliferation peaked later at 96 h post 1-NN injection. In vitro, BMP4 induced a mesenchymal-like morphology in normal AECs, downregulated E-cadherin expression and increased migration in a wound closure assay. Thus, following acute injury, increased BMP signalling in AECs coincides with inflammation and precedes airway denudation and re-epithelialisation. Our data indicate that, similar to its role in controlling tissue architecture during development, BMP signalling regulates regeneration of the airways following acute injury, involving downregulation of E-cadherin and induction of migration in AECs.