MEK-1/2 inhibition reduces branching morphogenesis and causes mesenchymal cell apoptosis in fetal rat lungs

The roles of the mitogen-activated protein (MAP) kinases extracellular signal-regulated kinases-1 and -2 (ERK-1/2) in fetal lung development have not been extensively characterized. To determine if ERK-1/2 signaling plays a role in fetal lung branching morphogenesis, U-0126, an inhibitor of the upstream kinase MAP ERK kinase (MEK), was added to fetal lung explants in vitro. Morphometry as measured by branching, area, perimeter, and complexity were significantly reduced in U-0126-treated lungs. At the same time, U-0126 treatment reduced ERK-1/2, slightly increased p38 kinase, but did not change c-Jun NH(2)-terminal kinase activities, indicating that U-0126 specifically inhibited the ERK-1/2 enzymes. These changes were associated with increased apoptosis as measured by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling and immunofluorescent labeling of anti-active caspase-3 in the mesenchyme of explants after U-0126 treatment compared with the control. Mitosis characterized by immunolocalization of proliferating cell nuclear antigen was found predominantly in the epithelium and was reduced in U-0126-treated explants. Thus U-0126 causes specific inhibition of ERK-1/2 signaling, diminished branching morphogenesis, characterized by increased mesenchymal apoptosis, and decreased epithelial proliferation in fetal lung explants.