iloprost and Corneal-Neovascularization

We have previously demonstrated that iloprost, a stable prostacyclin (PGI(2)) analogue, induces angiogenesis in vivo, through a vascular endothelial growth factor (VEGF)-dependent mechanism. In this study, we demonstrate that iloprost-induced angiogenesis and VEGF upregulation are modulated by peroxisome proliferator-activated receptor-alpha (PPARalpha), a ligand-inducible transcription factor that belongs to the nuclear hormone receptor superfamily and plays multiple biological activities in the vascular system. We show that iloprost is unable to induce angiogenesis in mice lacking the PPARalpha gene (PPARalpha-/- mice). Likewise, iloprost-induced VEGF upregulation is absent in PPARalpha-/- mice. In contrast, iloprost induces a robust angiogenic response in wild-type mice, along with local upregulation of VEGF. Importantly, mice lacking the PPARalpha gene exhibit a normal angiogenic response to VEGF, indicating that the absence of PPARalpha does not result in a general impairment of angiogenesis, but specifically affects the ability of iloprost to induce angiogenesis. Our data demonstrate unexpected functional relationships between the PGI(2) system and the PPAR signaling pathway and shed new light on the molecular mechanisms involved in iloprost-induced angiogenesis.

Topics: Angiogenesis Inducing Agents; Animals; Corneal Neovascularization; Iloprost; Mice; Mice, Knockout; PPAR alpha; RNA, Messenger; Signal Transduction; Transfection; Up-Regulation; Vascular Endothelial Growth Factor A; Vascular Endothelial Growth Factor Receptor-1

Until recently, prostacyclin (PGI2) biological activities were thought to be exclusively mediated by cell surface receptors named IP. Recent studies have instead identified a novel pathway of PGI2 signaling, occurring through activation of peroxisome proliferator-activated receptors (PPARs) located in the nucleus. The availability of stable PGI2 analogs with different affinity for IP receptors and PPARs provides the possibility to test the importance and function of this dual pathway in vitro and in vivo. In this study, the in vivo angiogenic properties of different PGI2 analogs and the potential relationship between PPAR-mediated pathways, vascular endothelial growth factor (VEGF), and angiogenesis were investigated.. By using the murine corneal model of angiogenesis, we found that PGI2 analogs able to act on nuclear PPARs, such as iloprost and carbaprostacyclin (cPGI), induce angiogenesis in vivo. In contrast, cicaprost, a PGI2 analog that only acts on IP receptors, has no in vivo angiogenic activity. Interestingly, angiogenesis induced by iloprost and cPGI does not differ in extent and morphology from that induced by VEGF and is associated with local increment of VEGF mRNA expression and protein levels. Finally, iloprost-induced angiogenesis is significantly decreased by systemic inhibition of VEGF activity, obtained by gene transfer of a soluble form of the VEGF receptor Flt-1.. These data demonstrate that stable PGI2 analogs may have angiogenic properties in vivo, depending on their ability to act on PPARs. The resulting angiogenic process appears to be mediated by VEGF. These findings indicate that important physiological activities in the cardiovascular system, such as angiogenesis and VEGF induction, may be modulated by PGI2 through specific activation of the PPAR signaling pathway in vivo, with potentially important fundamental and clinical implications.

Topics: Animals; Antineoplastic Agents; Corneal Neovascularization; Epoprostenol; Extracellular Matrix Proteins; Iloprost; Mice; Myosin Heavy Chains; Nonmuscle Myosin Type IIB; Platelet Aggregation Inhibitors; Receptors, Cytoplasmic and Nuclear; RNA, Messenger; Transcription Factors; Vascular Endothelial Growth Factor A; Vascular Endothelial Growth Factor Receptor-1