Effect of removal of glutamine and addition of dexamethasone on the activities of glutamine synthetase, ornithine decarboxylase and lactate dehydrogenase in primary cultures of forebrain and cerebellar astrocytes.
The regulation of glutamine synthetase (GS) and ornithine decarboxylase (ODC) was studied in primary cultures of two types of astrocytes derived from either newborn forebrain or 8-day-old cerebellum of the rat. In the 14-day-old cultures the specific activities of both these enzymes were about twice as great in forebrain astrocytes as in cerebellar astrocytes. Treatment with dexamethasone or removal of glutamine from the culture medium caused a marked increase in the specific activity of GS. The glutamine-mediated relative increase in GS activity was similar in both types of astrocytes. Removal of glutamine caused a transient reduction in ODC activity in the forebrain astrocytes, while in cerebellar astrocytes the activity remained markedly decreased throughout the period of glutamine deprivation. The severe reduction in ODC activity had relatively little effect on the cell numbers of protein content of the astrocyte cultures. The increase in GS activities, involving protein synthesis de novo, caused by removal of glutamine and by addition of dexamethasone, were additive and therefore probably mediated by different mechanisms. The induction of GS after glutamine removal was blocked by cycloheximide but not by alpha-amanitin, suggesting regulation at the post-transcriptional level. In contrast, the dexamethasone-mediated induction of GS appeared to be regulated at the transcriptional level, as it was markedly reduced by alpha-amanitin. None of these conditions had any effect on lactate dehydrogenase activity. Treatment with alpha-amanitin resulted in a complete suppression of the activity of ODC (a protein with a very short half life), in both the control and dexamethasone treated cultures. However, this enzyme activity was reduced only partially in astrocytes cultured in glutamine deficient medium, suggesting that under these experimental conditions the mRNA may be markedly stabilized in astroglial cells.
Patel
A J
AJ
Hunt
A
A
Faraji-Shadan
F
F
D000818:Animals; D001253:Astrocytes / Q000201:enzymology*; D002478:Cells, Cultured; D002531:Cerebellum / Q000201:enzymology; D003907:Dexamethasone / Q000494:pharmacology*; D004027:Diencephalon / Q000201:enzymology; D005974:Glutamate-Ammonia Ligase / Q000378:metabolism*; D005973:Glutamine / Q000502:physiology*; D007770:L-Lactate Dehydrogenase / Q000378:metabolism*; D009955:Ornithine Decarboxylase / Q000378:metabolism*; D051381:Rats; D013687:Telencephalon / Q000201:enzymology
D005973:Glutamine; D003907:Dexamethasone; D007770:L-Lactate Dehydrogenase; D009955:Ornithine Decarboxylase; D005974:Glutamate-Ammonia Ligase