Ramachandran B, Houben K, Rozenberg Y Y, Haigh J R, Varpetian A, Howard B D
Department of Biological Chemistry, School of Medicine, University of California, Los Angeles 90024.
J Biol Chem. 1993 Nov 15;268(32):23891-7.
Wild type PC12 pheochromocytoma cells express a Na(+)-dependent norepinephrine transporter that operates in the uptake of catecholamines, including dopamine. This transporter is not expressed in two spontaneously occurring flat cell variants of PC12 or in two other flat cell variants whose phenotype was induced by expression of the Wnt-1 oncogene. However, each of the flat cell variants, including those that express Wnt-1, exhibit a Na(+)-dependent, Cl(-)-independent glutamate/aspartate transporter activity that is not present in wild type PC12 cells. The flat cell variants took up glycine by a Na(+)-dependent process as well as did wild type cells. All of the flat cell variants have decreased levels of norepinephrine transporter mRNA but normal levels of glycine transporter mRNA. Glutamate/aspartate transporter mRNA was detected only in the variants that exhibited glutamate/aspartate transporter activity, and the nucleotide sequence of a partial glutamate/aspartate transporter cDNA from these cells demonstrated that it was the glial form of the transporter that was expressed. These variants were more sensitive than was wild type PC12 to alanosine, a toxic aspartate analog that enters cells by a transporter-mediated system such as the glutamate/aspartate transporter; however, these variants were as sensitive as wild type cells to another toxic aspartate analog, N-(phosphonacetyl)-L-aspartic acid, which is believed to enter cells by endocytosis. We suggest that the Wnt-1 gene product, or a homolog, may be involved in glial differentiation and that the mechanisms that alter the expression of the norepinephrine and glutamate/aspartate transporters in wild type and variant PC12 cells may also operate to regulate neurotransmitter transporter expression in vivo.
野生型PC12嗜铬细胞瘤细胞表达一种钠依赖性去甲肾上腺素转运体，该转运体参与儿茶酚胺（包括多巴胺）的摄取。在PC12的两种自发出现的扁平细胞变体以及另外两种由Wnt-1癌基因表达诱导出表型的扁平细胞变体中，这种转运体不表达。然而，每种扁平细胞变体，包括那些表达Wnt-1的变体，都表现出一种野生型PC12细胞中不存在的钠依赖性、氯非依赖性谷氨酸/天冬氨酸转运体活性。扁平细胞变体通过钠依赖性过程摄取甘氨酸，野生型细胞也是如此。所有扁平细胞变体的去甲肾上腺素转运体mRNA水平降低，但甘氨酸转运体mRNA水平正常。仅在表现出谷氨酸/天冬氨酸转运体活性的变体中检测到谷氨酸/天冬氨酸转运体mRNA，并且来自这些细胞的部分谷氨酸/天冬氨酸转运体cDNA的核苷酸序列表明所表达的是该转运体的神经胶质形式。这些变体比野生型PC12对丙氨酸（一种有毒的天冬氨酸类似物，通过转运体介导的系统如谷氨酸/天冬氨酸转运体进入细胞）更敏感；然而，这些变体对另一种有毒的天冬氨酸类似物N-（膦酰乙酰基）-L-天冬氨酸的敏感性与野生型细胞相同，据信该类似物通过内吞作用进入细胞。我们认为Wnt-1基因产物或其同源物可能参与神经胶质细胞分化，并且在野生型和变体PC12细胞中改变去甲肾上腺素和谷氨酸/天冬氨酸转运体表达的机制也可能在体内调节神经递质转运体的表达。
