Interferon α Induces Nucleus-independent Apoptosis by Activating Extracellular Signal-regulated Kinase 1/2 and c-Jun NH2-Terminal Kinase Downstream of Phosphatidylinositol 3-Kinase and Mammalian Target of Rapamycin

Abstract

Interferon (IFN)α induces apoptosis via Bak and Bax and the mitochondrial pathway. Here, we investigated the role of known IFNα-induced signaling cascades upstream of Bak activation. By pharmacological and genetic inhibition of the kinases protein kinase C (PKC)δ, extracellular signal-regulated kinase (ERK), and c-Jun NH2-terminal kinase (JNK) in U266-1984 and RHEK-1 cells, we could demonstrate that all three enzymes are critical for the apoptosis-associated mitochondrial events and apoptotic cell death induced by IFNα, at a step downstream of phosphatidylinositol 3-kinase (PI3K) and mammalian target of rapamycin (mTOR). Furthermore, the activation of JNK was found to occur in a PKCδ/ERK-dependent manner. Inhibition of these kinases did not affect the canonical IFNα-stimulated Janus tyrosine kinase-signal transducer and activator of transcription signaling or expression of IFN-responsive genes. Therefore, enucleated cells (cytoplasts) were examined for IFNα-induced apoptosis, to test directly whether this process depends on gene transcription. Cytoplasts were found to undergo apoptosis after IFNα treatment, as analyzed by several apoptosis markers by using flow cytometry, live cell imaging, and biochemical analysis of flow-sorted cytoplasts. Furthermore, inhibition of mTOR, ERK, and JNK blocked IFNα-induced apoptosis in cytoplasts. In conclusion, IFNα-induced apoptosis requires activation of ERK1/2, PKCδ, and JNK downstream of PI3K and mTOR, and it can occur in a nucleus-independent manner, thus demonstrating for the first time that IFNα induces apoptosis in the absence of de novo transcription.