Laboratory of Molecular Cell Biology, Zoology Department, Assiut University, Assiut, Egypt.
Cell Signal. 2012 Mar;24(3):677-84. doi: 10.1016/j.cellsig.2011.10.016. Epub 2011 Nov 9.
Transferrin receptor mediates internalization of transferrin with bound ferric ions through the clathrin-dependent pathway. We found that binding of transferrin to the receptor induced rapid generation of cell surface ceramide which correlated with activation of acid, but not neutral, sphingomyelinase. At the onset of transferrin internalization both ceramide level and acid sphingomyelinase activity returned to their basic levels. Down-regulation of acid sphingomyelinase in cells with imipramine or silencing of the enzyme expression with siRNA stimulated transferrin internalization and inhibited its recycling. In these conditions colocalization of transferrin with clathrin was markedly reduced. Simultaneously, K(+) depletion of cells which interfered with the assembly of clathrin-coated pits inhibited the uptake of transferrin much less efficiently than it did in control conditions. The down-regulation of acid sphingomyelinase activity led to the translocation of transferrin receptor to the raft fraction of the plasma membrane upon transferrin binding. The data suggest that lack of cell surface ceramide, generated in physiological conditions by acid sphingomyelinase during transferrin binding, enables internalization of transferrin/transferrin receptor complex by clathrin-independent pathway.
转铁蛋白受体通过网格蛋白依赖途径介导结合铁离子的转铁蛋白的内化。我们发现，转铁蛋白与受体的结合诱导细胞表面神经酰胺的快速产生，这与酸性鞘磷脂酶（而非中性鞘磷脂酶）的激活相关。在转铁蛋白内化开始时，神经酰胺水平和酸性鞘磷脂酶活性均恢复到基础水平。用阿米替林下调细胞中的酸性鞘磷脂酶或用 siRNA 沉默酶的表达可刺激转铁蛋白内化并抑制其回收。在这些条件下，转铁蛋白与网格蛋白的共定位明显减少。同时，细胞内 K+耗竭（干扰网格蛋白包被小窝的组装）对转铁蛋白摄取的抑制作用远不及对照条件下有效。酸性鞘磷脂酶活性的下调导致转铁蛋白结合后转铁蛋白受体向质膜筏区的易位。数据表明，在生理条件下，酸性鞘磷脂酶在结合转铁蛋白时产生的细胞表面神经酰胺缺失，使转铁蛋白/转铁蛋白受体复合物能够通过网格蛋白非依赖途径内化。
