Recombinant adenoviruses are presently being tested clinically as a new strategy for the treatment of cancer. An important determining factor for the successful entry of such adenoviruses into target cells is expression of the coxsackievirus and adenovirus receptor (CAR) at the cell surface. Recent observations suggest that expression of this receptor, which physiologically participates in formation of cell-cell adhesions, is frequently reduced in highly malignant cancer cells. This raises the possibility that those tumors representing the greatest therapeutic challenge might be the least susceptible to infection with therapeutic adenoviruses. We explored the role of the Raf-MEK-ERK pathway on CAR expression in a panel of cancer cells because this pathway is frequently up-regulated in cancer cells and is known to down-regulate cell-cell adhesion molecules. We found that disruption of signaling through the Raf-MEK-ERK pathway by inhibition of MEK up-regulated CAR expression, which was accompanied by increased representation of the protein at the cell surface. After Raf-MEK-ERK inhibition, adenovirus entry into cells was increased and cell killing by replication competent adenoviruses was enhanced in a CAR-dependent manner. Conversely, induction of Raf-1 resulted in reduction and disruption of CAR expression at the cell surface. We conclude that loss of CAR expression in cancer cells is, at least in part, mediated through the Raf-MEK-ERK signal transduction pathway and that pharmacological restoration of CAR at the cell surface could improve adenovirus-based treatments of cancer.
