Upregulation of aldose reductase by homocysteine in type II alveolar epithelial cells

Homocysteine (Hcy) has recently been recognized as an integral component of several disorders. However, the association between hyperhomocysteinemia (HHcy) and pulmonary disease is not well understood. The combination of two-dimensional electrophoresis and tandem mass spectrometry detected and identified proteins that are differentially expressed in human type II alveolar epithelial cells (A549 cells) treated by Hcy. We found that aldose reductase (AR) showed more abundant expression in the cells. Further, Hcy (100-500 μM) could induce a time- and dose-dependent upregulation of AR protein levels. Immunohistochemical staining of cross-sections from HHcy mice lungs also revealed increased expression of AR protein. Intracellular levels of reactive oxygen species (ROS) were remarkably elevated in A549 cells treated with Hcy. Pretreatment of A549 cells with catalase and SOD significantly suppressed the Hcy-induced AR expression, which suggests the involvement of ROS in this process. The major signaling pathway mediating the upregulation of AR was demonstrated to be the Ras/Raf/ERK1/2 pathway. In addition, Hcy might reduce surfactant protein B (SP-B) expression in the cells, which could be significantly attenuated by Alrestatin, an AR inhibitor, indicating a damaging role of Hcy-induced AR elevation in the lung. These results show a novel and unanticipated link between HHcy and AR upregulation that may be a risk factor in pulmonary disease of patients with HHcy.