Karlin J B, Johnson W J, Benedict C R, Chacko G K, Phillips M C, Rothblat G H
J Biol Chem. 1987 Sep 15;262(26):12557-64.
The bidirectional flux of unesterified cholesterol between cells and high density lipoprotein (HDL) was studied in relationship to the binding of HDL to cells. At 100 micrograms at HDL protein/ml, the rate constant for cholesterol efflux from rat Fu5AH hepatoma cells is 3 X 10(-3)/min (t1/2 for efflux of 3.9 h), whereas efflux from GM3468 human fibroblasts is 0.075/4 h (equivalent to a t1/2 for efflux of 37 h). The relatively slow efflux of cholesterol from fibroblasts in comparison to rat hepatoma cells was observed previously with micellar and vesicular phospholipid-containing acceptors, which promote efflux by a mechanism involving the diffusion of cholesterol in the aqueous phase between the plasma membrane and the acceptor particles. When plotted against the logarithm of HDL concentration, the isotherms for efflux are centered at 300 and 100 micrograms of HDL protein/ml with the hepatoma cells and fibroblasts, respectively. These concentrations are 8-150 times greater than the corresponding values for Kd of specific HDL binding (2 and 12 micrograms of protein/ml, for hepatoma cells and fibroblasts, respectively). The treatment of HDL with tetranitromethane reduces the lipoprotein's affinity for specific cell-surface binding sites by 80-90%. However, at HDL concentrations of 5-60 micrograms of protein/ml, this treatment does not significantly inhibit cholesterol efflux from hepatoma cells, and inhibits efflux from fibroblasts an average of about 15%. Over the same range of concentrations, nitration alters influx by amounts less than 30% in the two cell types. These effects on flux do not parallel the reduced affinity of nitrated HDL for specific cell-surface binding sites. In summary, the present results do not support the concept that cholesterol transfer is facilitated by the specific cell-surface binding of HDL, but are consistent with the aqueous diffusion model of cholesterol transfer between cells and lipoproteins.
研究了细胞与高密度脂蛋白（HDL）之间未酯化胆固醇的双向通量，并探讨了HDL与细胞结合的关系。在HDL蛋白浓度为100μg/ml时，大鼠Fu5AH肝癌细胞胆固醇流出的速率常数为3×10⁻³/min（流出半衰期为3.9小时），而GM3468人成纤维细胞的胆固醇流出速率为0.075/4小时（相当于流出半衰期为37小时）。与大鼠肝癌细胞相比，成纤维细胞胆固醇流出相对较慢，此前在含胶束和囊泡磷脂的受体中也观察到这一现象，这些受体通过一种涉及胆固醇在质膜和受体颗粒之间水相扩散的机制促进胆固醇流出。以HDL浓度的对数作图时，肝癌细胞和成纤维细胞胆固醇流出的等温线分别以300μg/ml和100μg/ml的HDL蛋白为中心。这些浓度分别比HDL特异性结合的Kd值（肝癌细胞和成纤维细胞分别为2μg/ml和12μg/ml）高8至150倍。用四硝基甲烷处理HDL可使脂蛋白对特定细胞表面结合位点的亲和力降低80%至90%。然而，在HDL浓度为5至60μg/ml时，这种处理不会显著抑制肝癌细胞的胆固醇流出，而成纤维细胞的胆固醇流出平均仅受约15%的抑制。在相同浓度范围内，硝化作用使两种细胞类型的胆固醇流入量改变不到30%。这些对通量的影响与硝化HDL对特定细胞表面结合位点亲和力的降低并不平行。总之，目前的结果不支持HDL特异性细胞表面结合促进胆固醇转移的概念，但与细胞和脂蛋白之间胆固醇转移的水相扩散模型一致。
