Abstract
Interferon (IFN)alpha induces apoptosis via Bak and Bax and the mitochondrial pathway. Here, we investigated the role of known IFNalpha-induced signaling cascades upstream of Bak activation. By pharmacological and genetic inhibition of the kinases protein kinase C (PKC)delta, extracellular signal-regulated kinase (ERK), and c-Jun NH(2)-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 IFNalpha, 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 PKCdelta/ERK-dependent manner. Inhibition of these kinases did not affect the canonical IFNalpha-stimulated Janus tyrosine kinase-signal transducer and activator of transcription signaling or expression of IFN-responsive genes. Therefore, enucleated cells (cytoplasts) were examined for IFNalpha-induced apoptosis, to test directly whether this process depends on gene transcription. Cytoplasts were found to undergo apoptosis after IFNalpha 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 IFNalpha-induced apoptosis in cytoplasts. In conclusion, IFNalpha-induced apoptosis requires activation of ERK1/2, PKCdelta, and JNK downstream of PI3K and mTOR, and it can occur in a nucleus-independent manner, thus demonstrating for the first time that IFNalpha induces apoptosis in the absence of de novo transcription.