Gueddari N, Favre G, Marmouget C, Soula G, Le Gaillard F
Laboratoire de Ciblage en Therapeutique. EA/UPRES. Université Paul Sabatier (Toulouse), Faculté des Sciences, Pharmaceutiques, France.
Biochimie. 1996;78(10):874-81. doi: 10.1016/s0300-9084(97)84341-6.
In common with other tumour cell lines but in contrast to normal cells, the human adenocarcinoma cell line A549 showed a biphasic regulation of the LDL receptor activity during growth both LDL binding and metabolism (sum of internalised and degraded LDL) increased during the log exponential growth phase and decreased when the cells approached confluence. This period of increasing LDL receptor activity coincided with a high resistance to cholesterol down-regulation which suggested a sterol-independent pathway of stimulation. Since A549 cells have an autocrine loop of growth factors, two of which have tyrosine kinase activity, the LDL receptor activity was tested in the presence of the tyrosine kinase inhibitor, genistein. When cells were incubated in the absence of cholesterol (LPDS medium), the inhibition that occurred was two-fold higher during the exponential growth phase than during the confluent phase. Moreover, the residual LDL binding and metabolism after genistein inhibition were completely resistant to down-regulation by cholesterol only during the growth phase. When cholesterol was present (FCS medium). inhibition was observed only during the growth phase. The inhibition of LDL receptor activity by genistein was found to be the result of a loss in the number of LDL binding sites, while the dissociation constant was not affected. This loss was accompanied by a disappearance of mRNA as shown by RNase mapping. By comparison, LDL receptor activity of normal cells (fibroblasts) was also affected by genistein during the exponential growth phase but was much more cholesterol-dependent. Taken together, these results suggest that the tyrosine kinase pathway is essential to up-regulate LDL receptor expression in highly dividing cells and particularly in tumour cells in which the sterol regulation is deficient.
与其他肿瘤细胞系相同，但与正常细胞不同的是，人腺癌细胞系A549在生长过程中对低密度脂蛋白（LDL）受体活性呈现双相调节，在对数指数生长期，LDL结合和代谢（内化和降解的LDL总和）均增加，而当细胞接近汇合时则下降。LDL受体活性增加的这段时期与对胆固醇下调的高抗性同时出现，这表明存在一条不依赖固醇的刺激途径。由于A549细胞具有生长因子的自分泌环，其中两种具有酪氨酸激酶活性，因此在酪氨酸激酶抑制剂染料木黄酮存在的情况下测试了LDL受体活性。当细胞在无胆固醇的情况下孵育（LPDS培养基）时，指数生长期发生的抑制作用比汇合期高两倍。此外，染料木黄酮抑制后残留的LDL结合和代谢仅在生长期对胆固醇下调完全具有抗性。当存在胆固醇时（FCS培养基），仅在生长期观察到抑制作用。发现染料木黄酮对LDL受体活性的抑制是LDL结合位点数量减少的结果，而解离常数不受影响。如核糖核酸酶图谱所示，这种减少伴随着mRNA的消失。相比之下，正常细胞（成纤维细胞）的LDL受体活性在指数生长期也受到染料木黄酮的影响，但对胆固醇的依赖性更强。综上所述，这些结果表明酪氨酸激酶途径对于在高度分裂的细胞中，特别是在固醇调节缺陷的肿瘤细胞中上调LDL受体表达至关重要。
