Midkine Deficiency Attenuates Lipopolysaccharide-induced Pulmonary Inflammation

Midkine (MDK) is a multifunctional heparin-binding growth factor, and has been shown to regulate cell growth, survival and migration. It also plays important roles in several inflammatory diseases such as sepsis. However, the role of MDK in the lungs has not yet been elucidated. In the present study, we investigated the role of MDK in pulmonary inflammation experiments using a mouse lipopolysaccharide (LPS)-induced pneumonia model and human bronchial cells. Wild-type and MDK-deficient mice were injected intratracheally with LPS, and several inflammatory parameters were analyzed. In the wild-type mice, mRNA expression of MDK in lung tissues was significantly increased after intratracheal LPS injection. The MDK-deficient mice showed significantly lower counts of total cells and neutrophils, as well as lower concentrations of neutrophil chemokines, KC and MIP-2 in bronchoalveolar lavage fluid, compared to wild-type mice. Moreover, mRNA expressions of TNF-α, KC and MIP-2 in lung tissues, as well as histopathological lung inflammation score, were significantly lower in the MDK-deficient mice. Furthermore, in in vitro experiments using bronchial epithelial cells, LPS stimulation increased mRNA expression of MDK, and MDK knockdown by siRNA decreased LPS-induced TNF-α and CXCL8 upregulation. These findings suggest that deficiency of MDK attenuates LPS-induced pulmonary inflammation.