Stabilization of Invariant Chain mRNA by 12- O -Tetradecanoylphorbol-13-Acetate Is Blocked by IFN-γ in a Murine B Lymphoma Cell Line

Activation of protein kinase C (PKC) by 12-O-tetradecanoylphorbol-13-acetate (TPA) increased steady-state levels of mRNA encoding the major histocompatibility complex (MHC) class II antigen I-A_β and the class II antigen-associated invariant chain (Ii, CD74) in A20 B lymphoma cells and in normal mouse B cells. The increase in Ii mRNA levels appeared to be due to a slight increase in the rate of gene transcription and an increase in the stability of Ii mRNA. The half-life of Ii mRNA increased from 12 h to >24 h following treatment with TPA, as determined by Northern blot analysis following actinomycin D treatment or by the [³H]-uridine pulse-chase method. Interferon-γ (IFN-γ), which has been well characterized as a cytokine that induces class II antigens and the Ii, increased Ii expression slightly in A20 cells. However, cotreatment of cells with TPA and IFN-γ resulted in a block in the TPA-induced increase in Ii expression. Transcription of the Ii gene was minimally affected following treatment with IFN-γ alone, and cells treated with both TPA and IFN-γ had the same transcription rate as the control cells. IFN-γ did, however, block stabilization of Ii mRNA by TPA. Activation of PKC by TPA, which was previously shown to lead to membrane translocation and downregulation, was not inhibited by IFN-γ. Therefore, IFN-γ appeared to block a downstream signal transduction pathway activated by PKC that controls stability of Ii mRNA.