Fatty acids induce apoptosis in human smooth muscle cells depending on chain length, saturation, and duration of exposure

Objective: Plasma free fatty acid (FFA) concentrations are increased in states of insulin resistance. Therefore, this study evaluated apoptosis and underlying mechanisms induced by selected nutritional FFAs, a defined FFA-mix, and human plasma containing high FFA concentrations in human smooth muscle cells (HSMCs). Research design and methods: HSMCs were incubated (24-72 h) with selected FFAs (100-300 mu mol/l), an FFA-mix (palmitic-/stearic-/oleic-/linoleic-/alpha-linolenic acid =2.6/1/3.6/9/1; 300-900 mu mol/l), or with high FFA-plasma (600 mu mol/l) versus respective control cultures. Apoptosis, caspase acitvation, and protein expression were determined by DNA-fragmentation assays, flow cytometry, and Western blots, respectively. Results: Exposure (24 h) of HSMCs to 300 mu mol/l stearic-, oleic-, linoleic-, alpha-linolenic-, and arachidonic acid induced apoptosis, correlating (p < 0.01) with the FFAs' chain length (r=0.602) and number of FFA double bonds (r=0.956). After 48 h, 100 mu mol/l of all tested FFAs-including palmitic acid - were already sufficient to trigger HSMCs' cell death. FFA-exposure resulted in activation of caspases and apoptosis was completely abolished by co-incubation with caspase inhibitors and negatively correlated) < 0.01) with the base-excision repair protein XRCC1 (r=-0.765) and with c-myc's antagonist mad (r=-0.916), whereas positive correlations (p < 0.01) were found for protein expression of the proto-oncogene c-myc (r=0.972) and the transcription factor E2F-1 (r=0.971). Exposure of HSMCs to the defined FFA-mix and to plasma samples from individuals with elevated plasma FFAs supported the results obtained by defined FFA stimulation. Conclusions: Since smooth muscle cells surround the macrophage/foam cell/lipid-laden artheromatous core of atherosclerotic lesions with a protective fibrous cap, their FFA-induced HSMC apoptosis could contribute to progression of atherosclerosis by thinning of the fibrous cap and subsequent plaque destabilization. (C) 2008 Elsevier Ireland Ltd. All rights reserved.