MOLECULAR MECHANISMS FOR REGULATION OF MAMMALIAN GLUCOSE TRANSPORTER EXPRESSION AND ITS FUNCTION

哺乳动物细胞的葡萄糖摄取是由一种称为葡萄糖转运蛋白的膜蛋白介导的，该蛋白是一种分子量约为50 kDa的N-连接糖蛋白。我们以前已经证明，哺乳动物细胞的葡萄糖摄取是由脑型葡萄糖转运蛋白GLUT 1的生长因子依赖性表达及其糖基化变化调节的，该糖基化变化调节对D-葡萄糖的亲和力。在本研究中，我们已经使用人细胞杂交体研究了人肿瘤细胞中GLUT 1表达的变化，所述人细胞杂交体提供了宫颈癌HeLa的致瘤性受11号染色体中推定的肿瘤抑制因子控制的证据，并且发现GLUT 1中的肿瘤相关糖基化变化。非致瘤性HeLa x成纤维细胞杂交体表达50-55 kDa GLUT 1，而在致瘤性分离体中，GLUT 1糖基化发生改变，其分子量约为70 kDa，对寡糖的进一步研究表明，GLUT 1糖基化的改变主要是由于寡糖中N-乙酰乳糖胺重复序列的增加，与糖基化改变一致，在致瘤杂交细胞中，对2-脱氧葡萄糖的亲和力增加了2倍，但Vmax几乎没有变化。这些结果表明，在人类肿瘤细胞的致瘤行为中，GLUT糖基化的调节可能具有功能性作用。对这些人类细胞杂交体的后续研究表明，另一种膜蛋白小窝蛋白，在致瘤性杂交细胞中，质膜小窝结构的主要成分小窝蛋白的表达减少，并进一步研究了这种小窝蛋白的表达在HeLa细胞致瘤性和抑癌功能中的作用。

Glucose uptake in mammalian cells is mediated by an integral membrane protein called as a glucose transporter which is an N-linked glycoprotein with molecular mass of about 50 kDa.We have previously demonstrated that glucose uptake in mammalian cells is regulated by growth factors-dependent expression of a brain-type glucose transporter GLUT1 as well as its glycosylation change which modulates affinity to D-glucose.In the present research we have studied changes in GLUT1 expression in human tumor cells using human cell hybrids which have provied an evidence that the tumorigenicity of a cervical carcinoma HeLa is under the control of a putative tumor suppressor in chromosome 11, and found a tumor-associated glycosylation change in GLUT1.An non-tumorigenic HeLa x fibroblast cell hybrid expressed the 50-55 kDa GLUT1, whereas in a tumorigenic segregant, GLUT1 glycosylation was altered and its molecular mass was about 70 kDa.Further studies on the oligosaccharides revealed that this glycosylation change in GLUT1 was mainly due to the increase in N-acetyl-lactosamine repeats in the oligosaccharides.In accordance with altered glycosylation, affinity for 2-deoxyglucose in the tumorigenic hybrid cells increased 2-fold, but there was little change in the Vmax.These results suggest that there may be a functional role for the modulation of GLUT glycosylation in the tumorigenic behavior of human tumor cells.Subsequent studies on these human cell hybrids indicated that expression of another integral membrane protein caveolin, which is a major component of a caveolae structure in the plasma membrane, decreased in a tumorigenic hybrid cells.Further studies on the role of this caveolin expression in tumorigenicity and tumor suppressor function of HeLa cells are carried out.