Induction of an oligodendroglial enzyme in C‐6 glioma cells maintained at high density or in serum‐free medium

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Abstract
The relationship between cell density and the activity of 2′:3′‐cyclic nucleotide 3′‐phosphohydrolase (CNP), an enzyme believed to be specific to oligodendroglial cells and myelin in the brain, has been studied in cultured C‐6 glioma cells. Over a 12‐day period, the specific activity of CNP underwent a 4‐fold increase in conjunction with an increase in the cell density (total protein/flask) and a decline in the growth rate of the cultures. In contrast, the specific activity of Na+, K+‐ATPase was not influenced by cell density. Experiments with cultures seeded at different initial densities indicated that the increase in CNP activity coincided with the attainment of a specific cell density rather than with the length of time that the cells were maintained in culture. Arrest of cell proliferation in non‐confluent C‐6 cells by means of thymidine blockade was not sufficient to cause an increase in the activity of CNP; however, removal of serum from the culture medium resulted in a 3‐fold induction of the enzyme in the absence of a high degree of cell contact. The induction of CNP in cells maintained in serum‐free medium paralleled the development of a series of distinct morphological changes reminiscent of glial differentiation, which occurred within 48 hours after removal of the serum. Inhibition of protein synthesis by cycloheximide prevented the induction of CNP in serum‐free cultures. The demonstration that an enhancement of an oligodendroglial characteristic in C‐6 glioma cells can be obtained by growing the cells to high density or by removing serum from the medium, provides further support for the suggestion that these cells may be analogous to the glial stem cells present in the developing brain.