LPAR1 and pulmonary emphysema: This study demonstrates that [1] acute lung injury in adulthood induces an aggravated second hit response in rats with neonatal hyperoxia-induced BPD, [2] LPAR1-deficiency results in emphysema and reduced vascularization in adult rats, and [3] LPAR1-deficient rats with BPD are protected against the exacerbated second hit response induced by LPS.