Uptake mechanism and intracellular destiny of liposomes in RES

本研究的目的是阐明脂质体被网状内皮系统（RES）摄取机制与脂质体在细胞内的命运之间的关系。用动力学方法研究了大鼠肝脂质体摄取的饱和方式。肝脂质体的饱和特性可以用血浓度曲线下的面积而不是血浓度本身来解释。提出了饱和动力学的新模型“饱和模型”。动力学分析表明肝内至少有两种摄取途径，一种是高清除率/低容量途径，一种是低清除率/高容量途径。在离体灌注的肝脏系统下，研究了脂质体被肝脏摄取的机制，并显示了补体受体介导的吞噬作用。补体系统的激活依赖于脂质体的大小，而脂质体的大小依赖性调理作用则增强了肝脏对补体的摄取。这种补体受体介导的摄取途径对应于高清除率/低容量途径。研究了脂质体剂量对RES中脂质体胞内降解的影响。肝脏中至少有两种降解过程。脂质体降解的这种异质性可以用假设异质性降解过程的“分选模型”和假设异质性运输过程的“交通堵塞模型”来解释。利用ph敏感染料分析脂质体在腹腔巨噬细胞下向酸性腔室的细胞内转运，阐明脂质体的剂量影响脂质体与溶酶体融合前的转运过程，支持“交通堵塞模型”。

The objective of this study is to clarify the relationship between uptake mechanism of liposomes by the reticuloendothelial system (RES) and the intracellular destiny of liposomes. The saturation manner of liposome uptake by the liver was examined kinetically in rats. The saturation characteristics of liposomes by the liver was explained well with the area under the curve of blood concentration and not by the blood concentration itself. New model was postulated for this saturation kinetics as "satiated model". The kinetic analysis revealed that there were at least two kinds of uptake pathways in liver, one is the high clearance/low capacity and low clearance/high capacity pathway. The uptake mechanism of liposomes by the liver was then examined under the isolated perfused liver system, and the contribution of complement receptor mediated phagocytosis was shown. The activation of complement system was dependent on the size of liposomes and the enhanced uptake by the liver resulted from the size dependent opsonization of liposomes. This complement receptor mediated uptake pathway corresponded to the high clearance/low capacity pathway. The effect of liposome dose on the intracellular degradation of liposomes in RES was also investigated. There were at least two kinds of degradation processes in the liver. This heterogeneity in the degradation of liposomes can be explained by both "Sorting Model" which assumes the heterogenous degradation processes and "Traffic Jam Model" which assumes the heterogenous transport processes. The intracellular transport of liposomes into acidic compartment was analyzed under peritoneal macrophages using pH-sensitive dye and it was clarified that the dose of liposomes influenced the transport processes of liposomes before fusion with lysosomes, which supported the "Traffic Jam Model".