1‐Methyl‐4‐Phenyl‐1,2,3,6‐Tetrahydropyridine‐Resistant, Flat‐Cell PC12 Variants Having a Partial Loss of Transformed Phenotype

We have cloned and characterized two variants of PC12 cells. MPT1 cells were selected by their resistance to 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP), and variant 2068 was isolated nonselectively as a large, flat‐cell variant commonly occurring in PC12 cultures. Variant 2068 cells also exhibit resistance to MPTP. Karyotype analysis revealed that these variants are true derivatives of wild‐type PC12 cells; however, each variant is tetraploid, whereas the wild‐type parent is diploid. The two variants contain an altered level and composition of lactate dehydrogenase isoenzymes, which could account for a previously described difference in lactate metabolism. Both variants exhibit a partial loss of transformed phenotype in culture in that they are nonrefractile, grow in monolayers, and fail to multiply in soft agar. We suggest that this alteration in transformed phenotype may result in altered mitochondria and lactate dehydrogenase and thus account for their resistance to MPTP. Compared with wild‐type PC12 cells, MPT1 cells have a decreased level of fos mRNA and an increased level of myc mRNA; the latter results from an increased level of transcription of exon 1 of the myc gene. Studies with hybrid cells obtained by fusing MPT1 cells with wild‐type‐like cells show that most, but not all, of the parameters of the MPT1 phenotype predominate.