Differential expression of transporters for norepinephrine and glutamate in wild type, variant, and WNT1-expressing PC12 cells

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.