Influence of oxidatively modified LDL on monocyte-macrophage differentiation

Transendothelial migration of peripheral blood mononuclear cells (PBMCs) and their subsequent interaction with the subendothelial matrix lead to their differentiation to macrophages (m[Greek Phi symbol]s). To study whether preexposure of monocytes in circulation to modified proteins influences their differentiation to m[Greek Phi symbol]s, an in vitro model system using isolated PBMC in culture was used. The effect of modified proteins such as oxidatively modified LDL (ox-LDL), acetylated and non-enzymatically glycated-BSA (NEG-BSA) on the differentiation process was studied by monitoring the upregulation of m[Greek Phi symbol] specific functions such as endocytosis, production of matrix metalloproteinases (MMPs), expression of surface antigen, activity of β-glucuronidase and down regulation of monocyte specific myeloperoxidase activity. Rate of endocytosis, production of MMPs and β-glucuronidase activity were significantly greater in cells treated with modified proteins irrespective of the nature of modification. Both CuSO₄ ox-LDL and HOCl ox-LDL increased the rate of expression of the m[Greek Phi symbol] specific functions. FACS analysis showed that the rate of upregulation of m[Greek Phi symbol] specific CD71 and down regulation of monocyte specific CD14 were high in cells supplemented with modified proteins. Studies using PPARγ antagonist and agonist suggest its involvement in CuSO₄ ox-LDL induced monocyte-macrophage (mo-m[Greek Phi symbol]) differentiation whereas the expression of macrophage specific functions in cells exposed to other modified proteins was independent of PPARγ. PBMC isolated from hypercholesterolemic rabbits in culture expressed m[Greek Phi symbol] specific functions at a faster rate compared to normal controls indicating that these observations are relevant in vivo. These results indicate that preexposure of monocytes to modified proteins promote their differentiation to m[Greek Phi symbol]s and may serve as a feed forward type control for clearing modified proteins.