Uptake mechanism and intracellular destiny of liposomes in RES

本研究的目的是阐明网状内皮系统(Res)对脂质体的摄取机制与脂质体在细胞内的命运之间的关系。对大鼠肝脏摄取脂质体的饱和方式进行了动力学考察。用血药浓度曲线下面积可以很好地解释脂质体在肝脏的饱和特性，而不能用血药浓度本身解释。该饱和动力学的新模型被假定为“饱和模型”。动力学分析表明，肝脏至少存在两种摄取途径，一种是高清除/低容量途径，另一种是低清除/高容量途径。在离体灌流肝系统下，研究了脂质体在肝脏的摄取机制，并证明了补体受体介导的吞噬作用的贡献。补体系统的激活依赖于脂质体的大小，而肝脏对补体的摄取增强是由于脂质体的大小依赖调理所致。补体受体介导的摄取途径对应于高清除/低容量途径。此外，还考察了脂质体剂量对脂质体在RES中细胞内降解的影响。肝脏中至少存在两种降解过程。脂质体降解的异质性可以用假设异质降解过程的“分选模型”和假设异质运输过程的“交通堵塞模型”来解释。用pH敏感染料分析了脂质体在巨噬细胞内向酸性隔室的转运，明确了脂质体在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".