MAPK3 and bronchopulmonary dysplasia: Expression and activation of ERK1 and ERK2 in HULEC-5a at baseline and following hyperoxia exposure: We initially sought to determine the relative expression and activation of ERK1 and ERK2 protein at baseline and following hyperoxia exposure in transformed human lung microvascular endothelial cells derived from a neonate (HULEC-5a) because hyperoxia and pulmonary microvascular endothelial cells play crucial roles in BPD pathogenesis.