Downregulation of Midkine gene expression and its response to retinoic acid treatment in the nitrofen-induced hypoplastic lung

Nitrofen-induced congenital diaphragmatic hernia (CDH) model has been widely used to investigate the pathogenesis of pulmonary hypoplasia (PH) in CDH. Recent studies have suggested that retinoids may be involved in the molecular mechanisms of PH in CDH. Prenatal treatment with retinoic acid (RA) has been reported to improve the growth of hypoplastic lung in the nitrofen CDH model. Midkine (MK), a RA-responsive growth factor, plays key roles in various organogenesis including lung development. In fetal lung, MK mRNA expression has its peak at E13.5–E16.5 and is markedly decreased during mid-to-late gestation, indicating its important role in early lung morphogenesis. We designed this study to investigate the hypothesis that the pulmonary MK gene expression is downregulated in the early lung morphogenesis in the nitrofen-induced PH, and to evaluate the effect of prenatal RA treatment on pulmonary MK gene expression in the nitrofen-induced CDH model.

Pregnant rats were exposed to either olive oil or nitrofen on day 9 of gestation (D9). Fetal lungs were harvested on D15, D18, and D21 and divided into control, nitrofen with or without CDH [CDH(+) or CDH(−)]. In addition, RA was given on days D18, D19, and D20 and fetal lungs were harvested on D21, and then divided into control + RA and nitrofen + RA. The pulmonary gene expression levels of MK were evaluated by real-time RT-PCR and statistically analyzed. Immunohistochemistry was also performed to examine protein expression/distribution of MK in fetal lung.

The relative mRNA expression levels of MK were significantly downregulated in nitrofen group compared to controls at D15 (^§ p < 0.01), whereas there were no significant differences at D18 and D21. MK gene expression levels were significantly upregulated in nitrofen + RA (0.71 ± 0.17) compared to the control (0.35 ± 0.16), CDH(−) (0.24 ± 0.15), CDH(+) (0.39 ± 0.19) and control + RA (0.47 ± 0.13) (*p < 0.05). Immunoreactivity of MK was also markedly decreased in nitrofen lungs compared to controls on D15, and increased in nitrofen + RA lungs compared to the other lungs on D21.

Downregulation of MK gene on D15 may contribute to primary PH in the nitrofen CDH model by disrupting early lung morphogenesis. Upregulation of MK gene after RA treatment in the nitrofen-induced hypoplastic lung suggests that RA may have a therapeutic potential to rescue PH in CDH through RA-responsive growth factor signaling.