Human Adenovirus Type 5 Induces Cell Lysis through Autophagy and Autophagy-Triggered Caspase Activity

Abstract

Oncolytic adenoviruses, such as Delta-24-RGD, are promising therapies for patients with brain tumor. Clinical trials have shown that the potency of these cancer-selective adenoviruses should be increased to optimize therapeutic efficacy. One potential strategy is to increase the efficiency of adenovirus-induced cell lysis, a mechanism that has not been clearly described. In this study, for the first time, we report that autophagy plays a role in adenovirus-induced cell lysis. At the late stage after adenovirus infection, numerous autophagic vacuoles accompany the disruption of cellular structure, leading to cell lysis. The virus induces a complete autophagic process from autophagosome initiation to its turnover through fusion with the lysosome although the formation of the autophagosome is sufficient for virally induced cell lysis. Importantly, downmodulation of autophagy genes (ATG5 or ATG10) rescues the infected cells from being lysed by the virus. Moreover, autophagy triggers caspase activity via the extrinsic FADD/caspase 8 pathway, which also contributes to adenovirus-mediated cell lysis. Therefore, our study implicates autophagy and caspase activation as part of the mechanism for cell lysis induced by adenovirus and suggests that manipulation of the process is a potential strategy to optimize clinical efficacy of oncolytic adenoviruses.We reported previously the antiglioma effect of the E1A mutant oncolytic adenovirus Delta-24, which is targeted to the aberrant Rb/E2F1 pathway in cancer cells (10). In subsequent reports, we described how Delta-24-RGD, a version of Delta-24 whose infectivity in cancer cells is enhanced through insertion of an RGD-4C motif in the HI loop of the adenoviral fiber protein (37), showed oncolytic potency in intracranial models of human glioma xenografts derived from both malignant glioma cell lines (9) and brain tumor stem cells (15). On the basis of these preliminary data and toxicity studies performed under the guidance of the National Cancer Institute, we have translated Delta-24-RGD to the clinical setting, and it is currently being tested for toxicity in patients with recurrent malignant glioma at The University of Texas M. D. Anderson Cancer Center. Although we anticipate that the studies will show negligible toxicity, we expect that further improvements in the oncolytic potency of Delta-24-RGD will be necessary to induce optimal therapeutic effect. One of the potential avenues to increasing adenovirus potency that remains understudied is the mechanism by which adenoviruses induce cell lysis. In this regard, we along with other groups have shown that adenovirus infection results in macroautophagy (here referred to as autophagy) (12,15,41). Although it has been suggested that this cellular process might be linked to adenovirus-induced cell lysis (17), the underlying mechanism is still largely unknown.

Autophagosomal Membrane Serves as Platform for Intracellular Death-inducing Signaling Complex (iDISC)-mediated Caspase-8 Activation and Apoptosis

“…In conclusion, the present findings, together with those of recent studies in T cell clonal expansion (15), proteasome inhibition (29), and oncolytic adenovirus (28), indicate that the autophagosomal membrane may serve as a platform for iDISC formation, which activates caspase-8 and the caspase cascade, leading to autophagy-dependent apoptosis. As many anticancer therapies induce both apoptosis and autophagy, establishment of an autophagy-dependent mechanism of caspase activation reveals a novel strategy to enhance therapeutic efficacy in tumor cells.…”

“…To support this concept, forced membrane localization and self-association of caspase-8 has been shown to dramatically promote apoptosis (43). Furthermore, it has recently been suggested that caspase-8 is recruited to the Atg12-Atg5 complex through FADD and that autophagic machinery is required for activation of caspase-8 (6,15,28). In this study, we show that the caspase-8⅐FADD complex associates with Atg5 on Atg16-and LC3-positive structures, indicating that the autophagosomal membrane serves as a platform for this interaction.…”

“…lysosomal degradation) attenuates SKI-I-induced apoptosis. It has recently been reported that autophagy induction plays a role in regulating caspase-8 activation and subsequent cell death, although the precise role of autophagy in this process is not well defined (6,15,28,29). We therefore next investigated whether activation of caspase-8 is involved in SKI-I-induced cell death.…”

Oncolytic Adenovirus Loaded with L-carnosine as Novel Strategy to Enhance the Antitumor Activity

“…Indeed, it has been reported that Ad5 induces cell lysis through autophagy and that viruses may also use autophagy-related vacuoles as a means for new progeny to exit the infected cell (27). In addition, the combination of an adenovirus with an autophagy inducer correlates positively with virus replication, improving the efficacy of Ad-mediated oncolysis.…”

Targeting glioblastoma with oncolytic adenovirus delta 24

“…[25] Autophagic induced cell death and induction of apoptosis are well-characterized results of Ad-Δ24 infection giving the erratum for combination treatments. [26] ANTITUMOR IMMUNE RESPONSE Another important aspect of oncolytic virotherapy is the induction of augmented antitumor immune response. [27,28] Ad-Δ24-RGD has been shown to induce antiglioma immunity and to enhance the presentation of tumor-associated antigens to immune cells.…”