Oxygen-Independent Stabilization of Hypoxia Inducible Factor (HIF)-1 during RSV Infection

Background: Hypoxia-inducible factor 1 (HIF)-1 alpha is a transcription factor that functions as master regulator of mammalian oxygen homeostasis. In addition, recent studies identified a role for HIF-1 alpha as transcriptional regulator during inflammation or infection. Based on studies showing that respiratory syncytial virus (RSV) is among the most potent biological stimuli to induce an inflammatory milieu, we hypothesized a role of HIF-1 alpha as transcriptional regulator during infections with RSV. Methodology, Principal Findings: We gained first insight from immunohistocemical studies of RSV-infected human pulmonary epithelia that were stained for HIF-1 alpha. These studies revealed that RSV-positive cells also stained for HIF-1 alpha, suggesting concomitant HIF-activation during RSV infection. Similarly, Western blot analysis confirmed an approximately 8-fold increase in HIF-1 alpha protein 24 h after RSV infection. In contrast, HIF-1 alpha activation was abolished utilizing UV-treated RSV. Moreover, HIF-alpha-regulated genes (VEGF, CD73, FN-1, COX-2) were induced with RSV infection of wild-type cells. In contrast, HIF-1 alpha dependent gene induction was abolished in pulmonary epithelia following siRNA mediated repression of HIF-1 alpha. Measurements of the partial pressure of oxygen in the supernatants of RSV infected epithelia or controls revealed no differences in oxygen content, suggesting that HIF-1 alpha activation is not caused by RSV associated hypoxia. Finally, studies of RSV pneumonitis in mice confirmed HIF-alpha-activation in a murine in vivo model. Conclusions/Significance: Taking together, these studies suggest hypoxia-independent activation of HIF-1 alpha during infection with RSV in vitro and in vivo.