Abstract
Atherosclerosis is a chronic inflammatory disease characterised by the accumulation of monocytic cells and lipids within the sub-endothelial space by direct monocyte to endothelial cell contact through gap junctions (GJs). Both cell types express connexin 43 (Cx43) isoforms that permit the formation of GJs. This is enhanced by adhesion molecules in the presence of pro-inflammatory stimuli, such as tumour necrosis factor α (TNF-α). TNF-α is suggested to have a role in Cx43 expression mainly mediated through MAPK pathways over other intercellular pathways; however, to date the mechanism remains unclear. Experiments were carried out in the absence and presence of 25ng/ml TNF-α and the functional integrity of human umbilical vein endothelial cell (HUVEC) monolayers was assessed by measuring the trans-endothelial electrical resistance (TEER). The trans-endothelial migration (TEM) assay used as a model for the transmigration of monocytes to the sub-endothelial space. Monocytes were added to HUVEC monolayers, and cells which passed from one chamber to another were collected, marked with CD14+ and measured by flow cytometry. The efficiency of TEER and TEM was measured in: 1) the presence of the GJ inhibitor, 43GAP27; 2) HUVECs transfected with Cx43-siRNA; and 3) MAPK inhibitors, chelerythrine chloride (CHE). TEER measurements showed a reduction in the presence of TNF-α compared to the control cells (13.58±2.82Ω.cm2, 43.94±3.32Ω.cm2; n=6); however, when the protein kinase C (PKC) inhibitor CHE was added, the resistance was the same as in the control cells (48.64±3.62Ω.cm2, 45.76±4.83Ω.cm2; n=6). TNF-α enhanced the migration of monocytes (66.3±2.1%; n=3) compared to the control cells (9.9±4.5%; n=3; P<0.001), but this was prevented in monolayer also treated with CHE (13.70±2.45%; n=3; P<0.001). Migration of monocytes was attenuated by the GJ blocker 43GAP27 and was comparable to HUVECs transfected with Cx43-siRNA (8.33±4.66%; n=3). High expression of total PKC in the HUVECs treated with TNF-α was shown compared to the control cells, but this effect was reduced in HUVECs treated with TNF-α when CHE was applied. These results show that GJs are formed between ECs and monocytes, and that TNF-α increased TEM, reduced the TEER, and had no effect in HUVECs transfected with Cx43-siRNA. The effect of TNF-α is mediated through PKC as its effect was attenuated by the PKC inhibitor CHE. This study has demonstrated the important role of GJ communication as an underlying mechanism during the early stages of atherosclerosis development, enhanced by the action of TNF-α.