Oncolytic adenovirus-induced autophagy: tumor-suppressive effect and molecular basis.

Autophagy is a catabolic process that produces energy through lysosomal degradation of intracellular organelles. Autophagy functions as a cytoprotective factor under physiological conditions such as nutrient deprivation, hypoxia, and interruption of growth factors. On the other hand, infection with pathogenic viruses and bacteria also induces autophagy in infected cells. Oncolytic virotherapy with replication-competent viruses is thus a promising strategy to induce tumor-specific cell death. Oncolytic adenoviruses induce autophagy and subsequently contribute to cell death rather than cell survival in tumor cells. We previously developed a telomerase-specific replication-competent oncolytic adenovirus, OBP-301, which induces cell lysis in tumor cells with telomerase activities. OBP-301-mediated cytopathic activity is significantly associated with induction of autophagy biomarkers. In this review, we focus on the tumor-suppressive role and molecular basis of autophagic machinery induced by oncolytic adenoviruses. Addition of tumor-specific promoters and modification of the fiber knob of adenoviruses supports the oncolytic adenovirus-mediated autophagic cell death. Autophagy is cooperatively regulated by the E1-dependent activation pathway, E4-dependent inhibitory pathway, and microRNA-dependent fine-tuning. Thus, future exploration of the functional role and molecular mechanisms underlying oncolytic adenovirus-induced autophagy would provide novel insights and improve the therapeutic potential of oncolytic adenoviruses.