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Oncolytic viruses represent an emerging approach to cancer therapy. However, better understanding of their interaction with the host cancer cell and approaches to enhance their efficacy are needed. Efficient protein folding is essential for normal cell function but is perturbed in many cancer types and in cells suffering viral infections, resulting in endoplasmic reticulum (ER) stress and the consequent induction of the unfolded protein response (UPR). However, the interactions between viral infection and UPR signalling are poorly understood and could potentially be exploited for enhancing therapeutic effect. Here, we investigate the effect of chemically induced endoplasmic reticulum (ER) stress on the activity of the chimeric group B oncolytic adenovirus Enadenotucirev, its closely related parental virus Ad11p, and the archetypal group C wild-type adenovirus Ad5. We find that EnAd and Ad11p induce ER stress much more powerfully than Ad5 infection. We show that treatment of colorectal and ovarian cancer cell lines with thapsigargin or ionomycin caused an influx of Ca2+, leading to an upregulation in E1A transcript and protein levels. Increased E1A protein levels, in turn, increased levels of expression of the E2B viral DNA polymerase, genome replication, late viral protein expression, infectious virus particle production and cell killing during Enadenotucirev and Ad11p but not Ad5 infection. The engineering of an EnAd virus armed with the ability to genetically knockdown the target of thapsigargin inhibition however did not result in enhanced replication. We then show that the effect of thapsigargin and ionomycin treatment was not due to the induction of ER stress, but rather the influx of extracellular Ca2+ and consequent increase in protein kinase C activity. These results underscore the importance of Ca2+ homeostasis during adenoviral infection, indicate a novel signalling pathway between protein kinase C and E1A, and raise the possibility of using Ca2+ flux-modulating agents in the manufacture and potentiation of oncolytic virotherapies. |
