Department of Physiology, University of Kentucky College of Medicine, Lexington, KY, USA.
Department of Physiology, University of Kentucky College of Medicine, Lexington, KY, USA.
J Lipid Res. 2023 Oct;64(10):100435. doi: 10.1016/j.jlr.2023.100435. Epub 2023 Aug 26.
Obesity-associated diabetes is linked to the accumulation of ceramide in various organs, including the liver. The exact mechanisms by which ceramide contributes to diabetic pathology are unclear, but one proposed scenario is that ceramide accumulation may inhibit insulin signaling pathways. It is unknown however whether the excess ceramide is generated proximal to the insulin receptor, that is, at the plasma membrane (PM), where it could affect the insulin signaling pathway directly, or the onset of insulin resistance is due to ceramide-induced mitochondrial dysfunction and/or lipotoxicity. Using hepatic cell lines and primary cultures, gain- and loss- of function approach, and state-of-the art lipid imaging, this study shows that PM-associated neutral sphingomyelinase 2 (nSMase2) regulates ceramide homeostasis in fat-loaded hepatocytes and drives the onset of insulin resistance. Our results provide evidence of a regulated translocation of nSMase2 to the PM which leads to local generation of ceramide and insulin resistance in cells treated with palmitic acid (PAL), a type of fat commonly found in diabetogenic diets. Oleic acid, which also causes accumulation of lipid droplets, does not induce nSMase2 translocation and insulin resistance. Experiments using the acyl-biotin exchange method to quantify protein palmitoylation show that cellular PAL abundance regulates the rate of nSMase2 palmitoylation. Furthermore, while inhibition of nSMase2 with GW4869 prevents PAL-induced insulin resistance, the overexpression of wild type nSMase2 but not palmitoylation-defective mutant protein potentiates the suppressive effect of PAL on insulin signaling. Overall, this study identifies nSMase2 as a novel component of the mechanism of insulin resistance onset in fat-loaded hepatocytes, that is, cell-autonomous and driven by PAL.
肥胖相关的糖尿病与包括肝脏在内的各种器官中神经酰胺的积累有关。神经酰胺导致糖尿病病理的具体机制尚不清楚，但有一种假设是，神经酰胺的积累可能会抑制胰岛素信号通路。然而，目前尚不清楚多余的神经酰胺是在胰岛素受体附近产生的，即在质膜（PM）处，它可以直接影响胰岛素信号通路，还是胰岛素抵抗的发生是由于神经酰胺诱导的线粒体功能障碍和/或脂毒性。本研究使用肝细胞系和原代培养物、增益和缺失功能方法以及最先进的脂质成像技术，表明 PM 相关的中性鞘磷脂酶 2（nSMase2）调节脂肪负荷肝细胞中的神经酰胺稳态，并导致胰岛素抵抗的发生。我们的研究结果提供了证据，表明 nSMase2 被调节易位到 PM，导致细胞用棕榈酸（PAL）处理时局部产生神经酰胺和胰岛素抵抗，PAL 是一种常见于致糖尿病饮食中的脂肪。同样导致脂滴积累的油酸不会诱导 nSMase2 易位和胰岛素抵抗。使用酰基辅酶 A 交换方法定量蛋白质棕榈酰化的实验表明，细胞内 PAL 丰度调节 nSMase2 棕榈酰化的速率。此外，虽然用 GW4869 抑制 nSMase2 可预防 PAL 诱导的胰岛素抵抗，但野生型 nSMase2 的过表达而不是棕榈酰化缺陷突变蛋白增强了 PAL 对胰岛素信号的抑制作用。总的来说，这项研究确定 nSMase2 是脂肪负荷肝细胞中胰岛素抵抗发生机制的一个新组成部分，即细胞自主和由 PAL 驱动。
