To catch a killer : on the mechanisms of interferon alpha induced apoptosis

Abstract

A major clinical problem regarding treatment of malignant tumours is primary or secondary resistance to therapy. Anti-tumour drugs act primarily by induction of apoptosis. However, the knowledge of how various substances induce apoptosis is still incomplete, and so is the reason for the great variation in cellular sensitivity to these drugs. The aim of this thesis was to characterize the pro-apoptotic signalling mechanisms induced by IFNalpha and to investigate the importance of the underlying genotype on the cellular sensitivity to IFNalpha-treatment.

ITN can exert prominent anti-cancer activities in some malignancies. However, the mechanism(s) of IFN's and-tumour activity is not clear, but induction of apoptosis has become a commonly accepted putative mechanism. In this thesis the molecular background to IFNalpha-induced apoptosis in malignant cell fines was investigated. Apoptosis induced by ITNalpha depends on activation of caspases, and activation of caspase-8 was found to be a triggering event in the caspase cascade. Furthermore, we show involvement of the mitochondrial pathway as demonstrated by activation of the pro-apoptotic Bcl-2 family members Bak and Bax, mitochondrial inner membrane depolarization and release of cytochrome c.

We have also shown that IFNalpha activates the PI3K/mTOR pathway. Signalling through PI3K/mTOR has been shown to primarily mediate survival. However, in the case of IFNalphamediated activation, this pathway is crucial for the apoptotic response. Inhibition of P13K as well as mTOR completely abrogates IFNalpha-induced apoptosis. No effect from inhibition of PI3K/mTOR is observed on the IFNalpha-induced classical Jak-STAT signalling pathway, indicating that Jak-STAT signalling alone is not sufficient to induce the apoptotic response to this cytokine. Furthermore, the antiviral effects of 117Na-treatment are unaffected by inhibition of PI3K/mTOR, hence this signalling pathway is crucial for induction of specific effects, such as apoptosis.

The impact of activated oncogenes on the apoptotic response to IFNalpha was also investigated. Introduction of a constitutive active form of the STAT3 oncogene (STAT3C) was shown to inhibit IFN's pro-apoptotic activity. The result of STAT3C expression is sustained STAT3/3 dimerization and nuclear translocation. STAT3C also rescued from the IFNinduced downregulation of STAT3/3 dimers, possibly explaining its ability to interfere with IFN-induced apoptosis. Furthermore, the presence of the HPV-16 E7 oncogene was shown to sensitize cells to apoptosis induced by IFN.

Delineation of the molecular background to IFN-induced apoptosis, and the impact of oncogene activation on the cellular sensitivity to this effect, may aid in an optimized use of IFNalpha in the treatment of patients with cancer.