Cooney Kimberly A, Molden Brent M, Kowalczyk Nicholas S, Russell Susan, Baldini Giulia
From the Department of Biochemistry and Molecular Biology University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205-7199.
From the Department of Biochemistry and Molecular Biology University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205-7199
J Biol Chem. 2017 Oct 27;292(43):17731-17745. doi: 10.1074/jbc.M117.785758. Epub 2017 Sep 6.
Melanocortin-4 receptor (MC4R) is a G-protein-coupled receptor expressed in the brain's hypothalamus where it regulates energy homeostasis. MC4R agonists function to lower food intake and weight. In this respect, although obesity promotes hyperlipidemia and hypothalamic injury, MC4R agonists are nevertheless more effective to reduce food intake within hours of administration in overweight, rather than lean, mice. MC4R undergoes constitutive internalization and recycling to the plasma membrane with agonist binding inducing receptor retention along the intracellular route and, under prolonged exposure, desensitization. Here, we found that, in neuronal cells, lipid stress by exposure to elevated palmitate leaves unchanged the rate by which MC4R and transferrin receptor are constitutively excluded from the cell surface. However, lipid stress disrupted later steps of MC4R and transferrin receptor internalization to endosomes as well as traffic of agonist-occupied MC4R to lysosomes and MC4R desensitization. In the lipid-stressed cells, MC4R and clathrin were redistributed to the plasma membrane where they colocalized to sites that appeared by super-resolution microscopy to be modified and to have higher clathrin content than those of cells not exposed to elevated palmitate. The data suggest that lipid stress disrupts steps of endocytosis following MC4R localization to clathrin-coated sites and exclusion of the receptor from the extracellular medium. We conclude that increased effectiveness of MC4R agonists in obesity may be an unexpected outcome of neuronal injury with disrupted clathrin-dependent endocytosis and impaired receptor desensitization.
黑皮质素-4受体（MC4R）是一种G蛋白偶联受体，表达于大脑下丘脑，在那里调节能量平衡。MC4R激动剂具有降低食物摄入量和体重的作用。在这方面，尽管肥胖会促进高脂血症和下丘脑损伤，但MC4R激动剂在给药数小时内对超重而非瘦小鼠减少食物摄入量的效果更佳。MC4R会进行组成型内化并循环至质膜，激动剂结合会诱导受体沿细胞内途径滞留，长时间暴露后会导致脱敏。在此，我们发现，在神经元细胞中，暴露于升高的棕榈酸所产生的脂质应激不会改变MC4R和转铁蛋白受体从细胞表面组成型排出的速率。然而，脂质应激会破坏MC4R和转铁蛋白受体内化至内体的后续步骤，以及激动剂占据的MC4R向溶酶体的转运和MC4R脱敏。在脂质应激细胞中，MC4R和网格蛋白重新分布至质膜，在那里它们共定位于超分辨率显微镜下显示已被修饰且网格蛋白含量高于未暴露于升高棕榈酸的细胞的位点。数据表明，脂质应激会破坏MC4R定位于网格蛋白包被位点并从细胞外介质中排出后发生的内吞步骤。我们得出结论，MC4R激动剂在肥胖症中有效性增加可能是神经元损伤的意外结果，伴有网格蛋白依赖性内吞作用破坏和受体脱敏受损。
