Deficiency or inhibition of lysophosphatidic acid receptor 1 protects against hyperoxia-induced lung injury in neonatal rats

RATIONALE. Blocking of lysophosphatidic acid (LPA) receptor (LPAR) 1 may be a novel therapeutic option for premature infants at risk of bronchopulmonary dysplasia (BPD) by preventing the LPAR1 mediated adverse effects of its ligand LPA, consisting of pulmonary inflammation, pulmonary arterial hypertension (PAH) and fibrosis. METHODS. To advance our knowledge on LPAR1 signaling in neonatal cardiopulmonary disease in vivo, we studied the beneficial effects of LPAR1 blocking in neonatal Wistar rats with experimental BPD, induced by exposure to hyperoxia for 10 days, using two different models: (I) LPAR1 mutant rats with a loss of function phenotype, and (II) pharmacological treatment with the LPAR1 and -3 inhibitor Ki16425 (5 mg/kg/day). Parameters investigated included survival, lung and heart histopathology, fibrin and collagen deposition, vascular leakage, and differential mRNA expression in the lungs of key genes involved in LPA signaling and BPD pathogenesis. LPAR1 mutant rats have a substitution of methionine 318 by arginine in helix 8 of the receptor. To test the effect of LPAR1M318Ron the LPA-induced MAP kinase-signaling cascade, which is the main mediator of the stimulatory effect of LPA on cell proliferation, we isolated primary rat embryonic fibroblasts (REFs) that endogenously express LPAR1 and tested activation of the MAP kinase-signaling cascade in these cells. RESULTS. Homozygous LPAR1M318Rmutant REFs showed a strongly reduced LPA-induced ERK1/2 phosphorylation response compared to wild type REFs, demonstrating that the M318R mutation in LPAR1 diminishes LPA signaling in primary REFs. The lack of function phenotype that was predicted by computational analyses is further supported by a similar craniofacial malformation in mutant rats as observed in LPAR1 knockout mice. LPAR1 mutant rats were protected against experimental BPD and mortality with reduced alveolar septal thickness, lung inflammation (a reduced influx of macrophages and neutrophils), and hyperoxia-induced collagen III deposition. However, LPAR1 mutant rats were not protected against alveolar enlargement, increased medial wall thickness of small arterioles, fibrin deposition, and vascular alveolar leakage. Treatment of experimental BPD with Ki16425 confirmed the data observed in LPAR1 mutant rats, but did not reduce the influx of neutrophils to the lung and mortality in rats with experimental BPD. In addition, Ki16425 treatment protected against hyperoxia-induced PAH and right ventricular hypertrophy (RVH). CONCLUSION. LPAR1 deficiency attenuates pulmonary injury by reducing pulmonary inflammation and fibrosis, thereby reducing mortality, but does not affect alveolar and vascular development and, unlike Ki16425 treatment, does not prevent PAH-induced RVH in neonatal rats with experimental BPD.