High levels of Daxx due to low cellular levels of HSP25 in murine cancer cells result in inefficient adenovirus replication.

When the adenoviral protein E1B55K binds death domain-associated protein (Daxx), the proteasome-dependent degradation of Daxx is initiated, and adenoviral replication is effectively maintained. Here, we show that the cellular levels of Daxx differ between human and mouse cancer cell lines. Specifically, we observed higher cellular Daxx levels and the diminished replication of oncolytic adenovirus in mouse cancer cell lines, suggesting that cellular Daxx levels limit the replication of oncolytic adenoviruses that lack E1B55K in murine cells. Indeed, the replication of oncolytic adenoviruses that lack E1B55K was significantly increased following infection with oncolytic adenovirus expressing Daxx-specific shRNA. Cellular Daxx levels were decreased in mouse cells expressing heat shock protein 25 (HSP25; homolog of human HSP27) following heat shock or stable transfection with HSP25-bearing plasmids. Furthermore, Daxx expression in murine cell lines was primarily regulated at the transcriptional level via HSP25-mediated inhibition of the nuclear translocation of the signal transducer and activator of transcription 3 (stat3) protein, which typically upregulates Daxx transcription. Conversely, human HSP27 enhanced stat3 activity to increase Daxx transcription. Interestingly, human Daxx, but not mouse Daxx, was degraded as normal by ubiquitin-dependent lysosomal degradation; however, HSP27 downregulation induced the ubiquitin-independent proteasomal degradation of Daxx.

TGF-ß downregulation-induced cancer cell death is finely regulated by the SAPK signaling cascade.

Transforming growth factor (TGF)-ß signaling is increasingly recognized as a key driver in cancer. In progressive cancer tissues, TGF-ß promotes tumor formation, and its increased expression often correlates with cancer malignancy. In this study, we utilized adenoviruses expressing short hairpin RNAs against TGF-ß1 and TGF-ß2 to investigate the role of TGF-ß downregulation in cancer cell death. We found that the downregulation of TGF-ß increased the phosphorylation of several SAPKs, such as p38 and JNK. Moreover, reactive oxygen species (ROS) production was also increased by TGF-ß downregulation, which triggered Akt inactivation and NOX4 increase-derived ROS in a cancer cell-type-specific manner. We also revealed the possibility of substantial gene fluctuation in response to TGF-ß downregulation related to SAPKs. The expression levels of Trx and GSTM1, which encode inhibitory proteins that bind to ASK1, were reduced, likely a result of the altered translocation of Smad complex proteins rather than from ROS production. Instead, both ROS and ROS-mediated ER stress were responsible for the decrease in interactions between ASK1 and Trx or GSTM1. Through these pathways, ASK1 was activated and induced cytotoxic tumor cell death via p38/JNK activation and (or) induction of ER stress.