Direct cytotoxicity produced by adenoviral-mediated interferon α gene transfer in interferon-resistant cancer cells involves ER stress and caspase 4 activation

Over the past several years we have obtained considerable evidence indicating that adenoviruses-expressing interferon α (Ad-IFNα) can overcome resistance to the IFNα protein itself. Since cancer cells infected with Ad-IFNα also show high perinuclear cytoplasmic IFNα expression, we were interested in whether endoplasmic reticulum (ER) stress and cleavage of caspase 4 could have a major role in Ad-IFNα-produced cancer cell death. Indeed, procaspase 4 was upregulated and cleaved as early as 12 h after Ad-IFNα infection of the cancer cells, which co-localized with IFNα staining and ER tracker. In contrast, immortalized normal human urothelial cells, although exhibiting similar perinuclear IFNα staining, showed no cleaved caspase 4. Caspase 4 cleavage was not blocked by the caspase 8 specific inhibitor zIETD, indicating that caspase 4 activation was independent of caspase 8 activation. Blocking caspase 4 also inhibited activation of caspase 3 in Ad-IFNα containing cells. Finally, the cleaved form of caspase 4 (p10) was detected in Ad-IFNα-positive cancer cells from the urine of a patient following intravesical Ad-IFNα/Syn3 treatment. Therefore, ER stress and activation of caspase 4 appears to be an important mechanism involved in the direct cancer cell death produced by Ad-IFNα and also occurs in the clinical setting.