FGF2 translationally induced by hypoxia is involved in negative and positive feedback loops with HIF-1alpha

Background: Fibroblast growth factor 2 (FGF2) is a major angiogenic factor involved in angiogenesis and arteriogenesis, however the regulation of its expression during these processes is poorly documented. FGF2 mRNA contains an internal ribosome entry site (IRES), a translational regulator expected to allow mRNA expression during cellular stress. Methodology/Principal Findings: In the present study, we have developed a skin ischemia model in transgenic mice expressing a reporter transgene under the control of the FGF2 IRES. The results reveal that FGF2 is induced at the protein level during ischemia, concomitant with HIF-1a induction and a decrease in FGF2 mRNA. In addition, the FGF2 IRES is strongly activated under these ischemic conditions associated with hypoxia, whereas cap-dependent translation is repressed by 4E-BP hypophosphorylation. We also show that up-regulation of FGF2 protein expression in response to hypoxia correlates with the increase of FGF2 IRES activity in vitro, in human retinoblasts 911. The use of siRNAs targeting HIF or FGF2 indicates that FGF2 and HIF-1a reciprocally regulate their expression/accumulation, by a negative feedback loop in early hypoxia, followed by a positive feedback loop in late hypoxia. Conclusion/Significance: FGF2 expression is up-regulated in vivo and in vitro in response to hypoxia. Strikingly, this upregulation is not transcriptional. It seems to occur by an IRES-dependent mechanism, revealing new mechanistic aspects of the hypoxic response. In addition, our data show that FGF2 interacts with HIF-1a in a unique crosstalk, with distinct stages in early and late hypoxia. These data reveal the physiological importance of IRES-dependent translation during hypoxic stress and underline the complexity of the cellular response to hypoxia, suggesting a novel role of FGF2 in the regulation of HIF-1a during the induction of angiogenesis. Citation: Conte C, Riant E, Toutain C, Pujol F, Arnal J-F, et al. (2008) FGF2 Translationally Induced by Hypoxia Is Involved in Negative and Positive Feedback Loops with HIF-1a. PLoS ONE 3(8): e3078. doi:10.1371/journal.pone.0003078 Editor: Thomas Preiss, Victor Chang Cardiac Research Institute, Australia Received June 6, 2008; Accepted August 6, 2008; Published August 27, 2008 Copyright: 2008 Conte et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by grants from the Agence Nationale pour la Recherche, Association Française contre les Myopathies, Association pour la Recherche sur le Cancer, Cancéropole Grand Sud-Ouest, Conseil Régional Midi-Pyrénées, Fondation de l’Avenir, Ligue Contre le Cancer. Competing Interests: The authors have declared that no competing interests exist. * E-mail: [email protected]