Hussaini I M, Brown M D, Karns L R, Carpenter J, Redpath G T, Gonias S L, Vandenberg S R
Department of Pathology (Neuropathology), University of Virginia Health Sciences Center, Charlottesville 22908, USA.
Glia. 1999 Jan;25(1):71-84.
Low density lipoprotein receptor-related protein (LRP) is a multifunctional endocytotic receptor that may modify the biological activity of reactive astrocytes in neuroplasticity and neurodegeneration and of malignant astrocytes in brain invasion. In this study, the regulation of LRP by epidermal growth factor receptor (EGFR) ligands in both cultured human fetal astrocytes and astrocytic tumor cell lines (U-251 MG and U-1242 MG) was investigated. All astrocytic cell types expressed LRP, as determined by the binding of activated alpha2-macroglobulin (alpha2M*) on intact cells and by Western and Northern blot analyses of cell extracts. Primary cultured astrocytes expressed the highest levels of alpha2M*-binding capacity (Bmax = 30 fmol/mg protein). This was twofold higher than for the U-1242 MG astrocytoma cells (Bmax = 15 fmol/mg protein) and fourfold greater than for the glioblastoma U-251 MG cells (7.0 fmol/mg protein). Receptor affinity (K(D)) ranged from 0.25 to 0.6 nM in all the astroglial cell types. Functional LRP at the surface was down-regulated by EGF, compared with controls, as indicated by a reduction of both Bmax and LRP-mediated endocytosis by approximately 50% and 60%, respectively. In comparison, EGF treatment of primary astrocytes did not down-regulate LRP expression or LRP-mediated endocytosis. Treatment of the tumor cells with EGF or TGFalpha (25 ng/ml) significantly down-regulated total cellular LRP. Receptor-associated protein (RAP) mRNA expression was not affected by EGF in either tumor cells or primary astrocytes. The reduction of LRP in the tumor cells resulted from a specific decrease in LRP mRNA transcription, as determined by Northern blot and nuclear run-on experiments. These data suggest that EGF mediates a functional down-regulation of LRP endocytotic activity in astrocytic tumor cells and that LRP expression is differentially regulated in neoplastic and non-neoplastic astrocytes.
低密度脂蛋白受体相关蛋白（LRP）是一种多功能内吞受体，它可能会改变反应性星形胶质细胞在神经可塑性和神经退行性变中的生物学活性，以及恶性星形胶质细胞在脑侵袭中的生物学活性。在本研究中，我们调查了表皮生长因子受体（EGFR）配体对培养的人胎儿星形胶质细胞和星形胶质细胞瘤细胞系（U-251 MG和U-1242 MG）中LRP的调节作用。通过活化的α2-巨球蛋白（α2M*）与完整细胞的结合以及对细胞提取物的蛋白质免疫印迹和Northern印迹分析确定，所有星形胶质细胞类型均表达LRP。原代培养的星形胶质细胞表达的α2M*结合能力最高（Bmax = 30 fmol/mg蛋白质）。这比U-1242 MG星形细胞瘤细胞（Bmax = 15 fmol/mg蛋白质）高两倍，比胶质母细胞瘤U-251 MG细胞（7.0 fmol/mg蛋白质）大四倍。所有星形胶质细胞类型的受体亲和力（K(D)）范围为0.25至0.6 nM。与对照组相比, 表皮生长因子（EGF）使表面功能性LRP下调，Bmax和LRP介导的内吞作用分别降低约50%和60%。相比之下，EGF处理原代星形胶质细胞并未下调LRP表达或LRP介导的内吞作用。用EGF或转化生长因子α（TGFα，25 ng/ml）处理肿瘤细胞可显著下调总细胞LRP。受体相关蛋白（RAP）mRNA表达在肿瘤细胞或原代星形胶质细胞中均不受EGF影响。通过Northern印迹和细胞核连续转录实验确定，肿瘤细胞中LRP的减少是由于LRP mRNA转录的特异性降低。这些数据表明，EGF介导星形胶质细胞瘤细胞中LRP内吞活性的功能性下调，并且LRP表达在肿瘤性和非肿瘤性星形胶质细胞中受到不同调节。
