On the pro-apoptotic signaling induced by interferon-alpha

Abstract

Interferon-alpha (IFNalpha) is a pleiotropic cytokine which has been used as an active drug in the treatment of malignant diseases for almost five decades. It has shown beneficial results in several hematological malignancies, as well as solid tumors, including hairy cell leukemia, multiple myeloma, chronic myeloid leukemia, renal carcinoma and malignant melanoma. However, many tumors do not respond or develop resistance to the effects of IFNalpha. Understanding of the molecular mechanisms of the anti-tumor effects of IFNalpha is crucial for the development of efficient treatment regimens and functional combination treatments.

In this thesis the molecular mechanism of IFNalpha stimulated apoptosis was investigated. Upstream signals which lead to the activation of mitochondrial events in apoptotic cell death were studied in detail to delineate the requirement of classical JAK/STAT signaling as well as neoclassical stress induced signaling.

By using chemical inhibitors and/or over-expression of dominant negative mutant proteins, we showed that PI3K/mTOR is required for the pro-apoptotic action of IFNalpha. We identified a small set of five genes, which require both STAT- and P13K signaling to be up-regulated by IFNalpha. Therefore we suggest that the products of these genes may play a role in the apoptotic response to IFNalpha.

Furthermore, we established that PKCdelta, ERK and JNK are sequentially activated by IFN, downstream of PI3K/mTOR. Inhibition of these kinases reduced the signs of apoptosis, as measured by phosphatidyl serine exposure, loss of deltapsimit, release of cytochrome c and caspase activation. Despite efficient inhibition of cell death, no alterations could be observed in STAT phosphorylation, translocation or overall transactivation. By studying enucleated cells (cytoplast) we found that IFNalpha induced cell death can occur also in the absence of IFNalpha induced gene activation.

In conclusion, we have shown that the pro-apoptotic mechanisms of IFNalpha depend mainly on direct activation of latent cytoplasmic signaling molecules, independent of de novo transcription of IFN regulated genes. We also suggest that STAT proteins are required for the onset of IFNalpha stimulated apoptosis, and that nuclear events may potentiate the response.