Lipolysis products from triglyceride‐rich lipoproteins induce endothelial cell apoptosis by stimulating TGF‐beta1 release and Smad signaling

Abstract

Several studies have demonstrated that a wide variety of stimuli can induce programmed cell death (apoptosis) in endothelial cells, and have suggested that apoptosis could be an important mechanism in the development of atherosclerosis. TGF-β1 has been shown to induce apoptosis in endothelial cells by activating members of the Smad family. Lipolysis products generated from lipoprotein lipase (LpL)-mediated hydrolysis of triglyceride-rich lipoproteins (TGRL) are recently described as independent risk factors for the development of atherosclerosis. We hypothesize that TGRL lipolysis products can induce endothelial cell apoptosis by stimulating the release of TGF-β1, initiating Smad signaling in human aortic endothelial cells (HAEC). To test this hypothesis we treated HAECs with lipolysis products generated from co-incubation of 150 mg/dL of TGRL plus 2 units/mL of LpL. Analysis of cell supernatants by ELISA showed a rise in TGF-β1 release from cells treated with lipolysis products compared to controls. Western Blotting showed a significant increase in expression of phospho-Smad2 after 1 hour of treatment with lipolysis products. Immunofluorescent staining for Smad4 demonstrated significant translocation of Smad4 to the nucleus after 1 hour. Fluorimetric analysis revealed significant activation of caspase-3 after 2 hours of treatment. Together these findings reveal for the first time a possible signaling mechanism by which lipolysis products can induce apoptosis in HAECs. This work was supported by the Nora Eccles Treadwell Foundation and NIH HL 55667.