CD36 Induction on Human Monocytes upon Adhesion to Tumor Necrosis Factor-activated Endothelial Cells

Abstract

Cell adhesion between circulating monocytes and the endothelium is a critical component of vascular thromboregulation and atherogenesis. The biochemical and genetic consequences of adhesion are poorly understood. We have found that monocyte surface expression of CD36, an integral membrane receptor for thrombospondin, collagen, and oxidized low density lipoprotein, increased dramatically upon adhesion to tumor necrosis factor-activated human umbilical vein endothelial cells (HUVEC). Expression was assessed by indirect immunofluorescence microscopy and immunoblotting using monoclonal antibodies to CD36. Steady-state CD36 mRNA levels, detected by RNase protection assay, also showed a similar pattern of up-regulation. To verify the adhesion dependence of the observed phenomenon, monocytes were co-cultured with tumor necrosis factor-activated HUVEC in a transwell apparatus that physically separated monocytes from the endothelial cells. Under these conditions, no increase in CD36 expression was detected, demonstrating that the enhanced monocyte CD36 expression observed is not due to soluble factors released by HUVEC. To characterize the specific adhesion molecules involved in the process, co-culture assays were performed on murine L cells transfected with either human E-selectin or intercellular adhesion molecule-1 cDNAs. A dramatic increase in CD36 mRNA was seen upon monocyte adhesion to E-selectin-transfected L cells compared with adhesion to intercellular adhesion molecule-1 or control transfectants. Furthermore, monoclonal antibodies to E-selectin inhibited the adhesion-dependent up-regulation of CD36 mRNA induced by transfected L cells or cytokine-activated endothelial cells. These findings demonstrate adhesiondependent gene regulation of monocyte CD36 and suggest the possible involvement of E-selectin in initiating this process. Cell adhesion between circulating monocytes and the endothelium is a critical component of vascular thromboregulation and atherogenesis. The biochemical and genetic consequences of adhesion are poorly understood. We have found that monocyte surface expression of CD36, an integral membrane receptor for thrombospondin, collagen, and oxidized low density lipoprotein, increased dramatically upon adhesion to tumor necrosis factor-activated human umbilical vein endothelial cells (HUVEC). Expression was assessed by indirect immunofluorescence microscopy and immunoblotting using monoclonal antibodies to CD36. Steady-state CD36 mRNA levels, detected by RNase protection assay, also showed a similar pattern of up-regulation. To verify the adhesion dependence of the observed phenomenon, monocytes were co-cultured with tumor necrosis factor-activated HUVEC in a transwell apparatus that physically separated monocytes from the endothelial cells. Under these conditions, no increase in CD36 expression was detected, demonstrating that the enhanced monocyte CD36 expression observed is not due to soluble factors released by HUVEC. To characterize the specific adhesion molecules involved in the process, co-culture assays were performed on murine L cells transfected with either human E-selectin or intercellular adhesion molecule-1 cDNAs. A dramatic increase in CD36 mRNA was seen upon monocyte adhesion to E-selectin-transfected L cells compared with adhesion to intercellular adhesion molecule-1 or control transfectants. Furthermore, monoclonal antibodies to E-selectin inhibited the adhesion-dependent up-regulation of CD36 mRNA induced by transfected L cells or cytokine-activated endothelial cells. These findings demonstrate adhesiondependent gene regulation of monocyte CD36 and suggest the possible involvement of E-selectin in initiating this process.