Mechanisms of action of human interferons. Induction of 2'5'-oligo(A) polymerase.

Interferons induce an enzymatic activity that polymerizes ATP into 2'5'-oligoadenylate. This enzyme, designated 2'5'-oligo(A) polymerase, is induced in HeLa cells by type I interferon with a faster kinetics than by type II (immune) interferon. Cells treated with type I interferon in the presence of cycloheximide show elevated levels of polymerase when the inhibitor or protein synthesis is removed and actinomycin D is added to the cultures to prevent further synthesis of mRNA. This suggests that, in the presence of cycloheximide, interferon can activate transcription of mRNA for the 2'5'-oligo(A) polymerase, which is subsequently translated upon removal of this inhibitor. In contrast, cells treated with type II interferon in the same way do not show increased levels of polymerase. This indicates that type II interferon cannot activate transcription of a stable mRNA for the polymerase when protein synthesis is inhibited. This observation and the different kinetics of induction of 2'5'-oligo(A) polymerase suggest major differences in the mechanism of action of the different types of interferon, with type I being a "direct" inducer of the polymerase and type II an "indirect" inducer. The commitment of HeLa cells to the induction of 2'5'-oligo(A) polymerase declines if the cells are exposed to type I interferon in the presence of an inhibitor of RNA synthesis. This suggests that transcription of specific mRNAs is transiently activated by interferon and that repeated interaction of interferon with its receptor may be required for prolonged transcription of the mRNA for 2'5'-oligo(A) polymerase.