Oxidized LDL regulates vascular endothelial growth factor expression in human macrophages and endothelial cells through activation of peroxisome proliferator-activated receptor-γ

Vascular endothelial growth factor (VEGF) has been recognized as an angiogenic factor that induces endothelial proliferation and vascular permeability. Recent studies have also suggested that VEGF can promote macrophage migration, which is critical for atherosclerosis. We have reported that VEGF is remarkably expressed in activated macrophages, endothelial cells, and smooth muscle cells within human coronary atherosclerotic lesions, and we have proposed the significance of VEGF in the progression of atherosclerosis. To clarify the mechanism of VEGF expression in atherosclerotic lesions, we examined the regulation of VEGF expression by oxidized low density lipoprotein (Ox-LDL), which is abundant in atherosclerotic arterial walls. A recent report has revealed that peroxisome proliferator-activated receptor-γ (PPARγ) is expressed not only in adipocytes but also in monocytes/macrophages and has suggested that PPARγ may have a role in the differentiation of monocytes/macrophages. Furthermore, 9- and 13-hydroxy-(S)-10,12-octadecadienoic acid (9- and 13-HODE, respectively), the components of Ox-LDL, may be PPARγ ligands. Therefore, we investigated the involvement of PPARγ in the regulation of VEGF by Ox-LDL. PPARγ expression was detected in human monocyte/macrophage cell lines, human acute monocytic leukemia (THP-1) cells, and human coronary artery endothelial cells (HCAECs). Ox-LDL (10 to 50 μg/mL) upregulated VEGF secretion from THP-1 dose-dependently. VEGF mRNA expression in HCAECs was also upregulated by Ox-LDL. The mRNA expression of VEGF in THP-1 cells and HCAECs was also augmented by PPARγ activators, troglitazone (TRO), and 15-deoxy-Δ^12,14-prostaglandin J₂ (PGJ2). In contrast, VEGF expression in another monocyte/macrophage cell line, human histiocytic lymphoma cells (U937), which lacks PPARγ expression, was not augmented by TRO or PGJ2. We established the U937 cell line, which permanently expresses PPARγ (U937T). TRO and Ox-LDL augmented VEGF expression in U937T. In addition, VEGF production by THP-1 cells was significantly increased by exposure to 9-HODE and 13-HODE. In conclusion, Ox-LDL upregulates VEGF expression in macrophages and endothelial cells, at least in part, through the activation of PPARγ.