Chemical induction of unfolded protein response enhances cancer cell killing through lytic virus infection

Abstract

Cancer cells are susceptible to oncolytic viruses, albeit variably. Human adenoviruses (HAdVs) are widely used oncolytic agents, engineered to produce progeny within the tumor, and elicit bystander effects. We searched for host factors enhancing bystander effects, and conducted a targeted RNA-interference screen against guanine-nucleotide exchange factors (GEFs) of small GTPases. We show that unfolded protein response (UPR), which is readily inducible in aggressive tumor cells, enhances melanoma or epithelial cancer cell killing upon HAdV infection. UPR was triggered by knock-down of Golgi Brefeldin-A resistant guanine-nucleotide-exchange factor-1 (GBF-1), or the GBF-1 inhibitor Golgicide A (GCA), and stimulated HAdV infection. GBF-1 is a GEF for ADP-ribosylation factors (Arfs) regulating ER to Golgi and intra-Golgi transport. Cells treated with GCA enhanced HAdV-induced cytopathic effects in epithelial and melanoma cancer but not normal cells, if the drug was applied several hours prior to HAdV inoculation. This was shown by real-time label-free impedance measurements using xCELLigence™. GCA-treated cells contained fewer incoming HAdV than control cells, but boosted HAdV titers and spreading in cancer cells. GCA enhanced viral gene expression, or transgene expression from the cytomegalovirus promoter of B- or C-species HAdVs, but did not enhance viral E1A expression in uninfected cell lines, or cells transfected with plasmid reporter DNA. The UPR-enhanced cell killing required the nuclease activity of the UPR-sensor inositol-requiring enzyme 1 (IRE-1), and X-box binding protein 1 (XBP-1), which alleviate ER stress. The collective results show that chemical UPR induction and viruses boost tumor cell killing by enhancing oncolytic viral efficacy.

IMPORTANCE

Cancer is difficult to combat. A wide range of oncolytic viruses show promise for killing cancer cells. Yet, the efficacy of oncolytic killing is low. We searched for host factors enhancing adenovirus cancer cell killing, and found that the knock-down of GBF-1 (Golgi Brefeldin-A resistant guanine-nucleotide-exchange factor-1) or chemical inhibition of GBF-1 enhanced adenovirus infection by triggering the IRE- 1/XBP-1 branch of the unfolded protein response (UPR). IRE-1/XBP-1 promote cell survival, and enhanced the levels of the adenoviral immediate early gene product E1A, virus spreading, and killing of cancer cells. Aggressive tumor cells depend on a readily inducible UPR, and hence present prime targets for a combined strategy involving adenoviruses and small chemicals inducing UPR.