Nerve Growth Factor Rapidly Induces Ornithine Decarboxylase mRNA in PC12 Rat Pheochromocytoma Cells

The mechanism by which nerve growth factor (NGF) stimulates ornithine decarboxylase (OrnDCase; EC 4.1.1.17) activity in the rat pheochromocytoma cell line PC12 was investigated. As demonstrated previously, NGF rapidly induces OrnDCase activity in a dose-dependent manner, with maximal enzymatic activity at 4-6 hr after exposure to NGF. Activity subsequently returns to near basal levels. A cloned OrnDCase cDNA was used to analyze the levels of OrnDCase RNA. In response to NGF administration, OrnDCase RNA levels were induced. The time course of the OrnDCase RNA induction paralleled that of the enzyme activity induction, and the magnitude of both inductions was quantitatively the same. Increased concentration of OrnDCase RNA was clearly detected at the earliest time point examined, 2 hr. No change was observed in the size of OrnDCase RNA. The dose--response curves for both RNA and enzyme activity inductions were also similar. Thus, increased OrnDCase RNA levels fully account for, and are responsible for, the induction of activity. Further, one-third of the OrnDCase RNA induction was unaffected by cycloheximide treatment but was fully blocked by actinomycin D treatment, suggesting that NGF acts through at least two mechanisms to mediate the OrnDCase induction. The first mechanism is cycloheximide insensitive and the second is mediated through an event requiring ongoing protein synthesis. Both mechanisms require ongoing transcription, as evidenced by the complete sensitivity of the induction process to actinomycin D.