Calcium-dependent viral internalization is required for adenovirus type 7 induction of IL-8 protein

The host response to adenovirus (Ad) infection involves induction of cytokines in lung epithelia. We have demonstrated induction of the lung neutrophil chemokine interleukin-8 (IL-8) by Ad7, a major lung pathogen, in A549 lung epithelial cells and lung tissue through activation of the Erk signaling pathway. However, the mechanism of IL-8 induction is still unclear. In this paper, we first showed that Ad7 viral gene expression is not essential for IL-8 induction as psoralen-UV inactivation of Ad7 did not affect IL-8 mRNA induction or IL-8 protein induction in A549 cells. We then inhibited internalization of Ad7 by treatment of A549 cells with EGTA in calcium-free medium during exposure to Ad7. We verified that this treatment inhibited Ad internalization by confocal microscopy, FACS analysis and Ad E1A and fiber mRNA expression. Preventing internalization by calcium depletion did not inhibit Erk activation by Ad7. However, calcium-dependent internalization was required for IL-8 protein production in Ad7 exposed cells. This is not likely due to an effect of calcium depletion on downstream Erk signaling or IL-8 protein production since calcium depletion did not block IL-8 protein production stimulated by PMA, and because addition of EGTA subsequent to Ad7 internalization also did not prevent Ad induction of IL-8. These studies indicate that Ad7 internalization is calcium-dependent and is required for IL-8 protein induction upon Ad7 infection. Ad7 induction of Erk is independent of calcium and does not require virus internalization.