Uptake of Adenoviral Vectors via Fibroblast Growth Factor Receptors Involves Intracellular Pathways That Differ from the Targeting Ligand

Target-specific delivery of adenoviral gene therapy vectors has been achieved by introducing basic fibroblast growth factor (FGF2) onto viral capsids. FGF2-retargeted vectors enter the cell through high-affinity FGF receptors while normal adenoviral receptor interactions are ablated. In addition, FGF2-mediated targeting permits a higher level of transgene expression and in vivo efficacy. We now present studies on the intracellular pathways and mechanisms of transduction by FGF2-retargeted adenovirus. FGF2 retargeting results in increased virion entry. Nuclear delivery is also increased, but to a level that is directly proportional to virion entry. In addition, after entry, the retargeted particle rapidly localizes to the nucleus in a time frame similar to that of adenovirus alone. Transgene expression is always enhanced with FGF2-mediated delivery, whether overall transduction of the population is increased, equivalent, or decreased relative to nontargeted adenoviral vectors. However, the increase in transgene expression does not correlate quantitatively with enhanced cellular entry, indicating that other factors may influence transgene expression levels. The increase in transgene expression occurs only when the FGF2-retargeting moiety is physically complexed with the adenoviral vector, indicating a requirement for a spatial link between the ligand and the virus particle. The FGF2–adenoviral complex activates the FGF receptor-mediated proliferative signaling cascade, but this signal transduction is not required for the enhanced level of gene expression observed after FGF2-mediated delivery. These findings emphasize that, in addition to altering receptor tropism, the influence of FGF2 retargeting extends to intracellular adenoviral trafficking pathways. Although the increased delivery of virions into the cell and nucleus contributes to the enhanced transgene expression observed with FGF2 retargeting, other as yet undefined cellular mechanisms also contribute to this process.