GPR40/FFA1 and neutral sphingomyelinase are involved in palmitate-boosted inflammatory response of microvascular endothelial cells to LPS

Objectives: Increased levels of both saturated fatty acids (SFAs) and lipopolysaccharide (LPS) are associated with type 2 diabetes. However, it remains largely unknown how SFAs interact with LPS to regulate inflammatory responses in microvascular endothelial cells (MIC ECs) that are critically involved in atherosclerosis as a diabetic complication. In this study, we compared the effects of LPS, palmitic acid (PA), the most abundant saturated fatty acid, or the combination of LPS and PA on interleukin (IL)-6 expression by MIC ECs and explored the underlying mechanisms. Methods: Human cardiac MIC ECs were treated with LPS, PA and LPS plus PA and the regulatory pathways including receptors, signal transduction, transcription and post-transcription, and sphingolipid metabolism for IL-6 expression were investigated. Results: G protein-coupled receptor (GPR)40 or free fatty acid receptor 1 (FFA1), but not toll-like receptor 4, was involved in PA-stimulated IL-6 expression. PA not only stimulated IL-6 expression by itself, but also remarkably enhanced LPS-stimulated IL-6 expression via a cooperative stimulation on mitogen-activated protein kinase and nuclear factor kappa B signaling pathways, and both transcriptional and post-transcriptional activation. Furthermore, PA induced a robust neutral sphingomyelinase (nSMase)-mediated sphingomyelin hydrolysis that was involved in PA-augmented IL-6 upregulation. Conclusion: PA boosted inflammatory response of microvascular endothelial cells to LPS via GPR40 and nSMase.