The effect of cobalt chloride on adenovirus gene expression
Objective: To investigate the effect of cobalt chloride (CoCl2) on transgene expression and viral particle titers in tumor cells infected by conditionally replicating adenovirus expression vector with hypoxia response elements (HRE)-regulated E1AE1B expression (Ad-5HRE-E1AE1B-RFP) and non-HRE regulated replication-deficient adenovirus expression vector (Ad-EGFP, Ad-Luc) in vitro and in vivo. Methods: Ad-5HRE-E1AE1B-RFP had five duplicated HRE and mini CMV acted as a promoter to drive E1AE1B expression and constitutive expression of RFP as reporter. The hypoxia model was optimized by exposing tumor cells to different concentrations of CoCl2. The hypoxia-inducible factor 1 alpha (HIF-1α) protein expression was determined by Western blotting. Under the optimized hypoxia model, the positive expression of exogenous gene and virus replication of Ad-5HRE-E1AE1B-RFP or Ad-EGFP-infected tumor cells were examined by conversed microscopic observation, FACS analysis and plaques formation test. Furthermore, transgene expression induced by combined application of hypoxia-inducible replicative adenovirus and replication deficient adenovirus (Ad-Luc) was also evaluated by examining the lucifererse activity in xenografted tumor models in nude mice by micro PET. Results: Western blotting results showed that CoCl2 at 0.4 and 0.08 μg/mL could stabilize and acumulate HIF-1α protein in gastric cancer SGC7901 cells, which could better mimic hypoxia condition. The microscopic observation and FACS analysis showed that CoCl2 at 0.4 μg/mL could remarkably increase the transduction efficacy of Ad-5HRE-E1AE1B-RFP, which was verified by significant increase in the percentage of positive expression of exogenous gene RFP and fluorescence intensity. But plaques formation test showed that Ad-5HRE-E1AE1B-RFP had no replication. CoCl2 0.4 μg/mL augmented the tranduction efficacy and expression levels of non-HRE regulated replication deficient adenovirus Ad-EGFP and Ad-Luc. Combined intratumoral injection of Ad-5HRE-E1AE1B-RFP and Ad-Luc significantly increased the expression of Ad-Luc in nude mice. Conclusion: CoCl2 markedly enhances transgene expression of recombinant adenovirus. However, the underlying mechanism is not only related to the CoCl2-induced hypoxia, but also probably related to regulation of gene transcription.
